Pacific - Atlantic Mollusc Migration: Pliocene Inter-Ocean Gateway Archives on Tjörnes, North Iceland (Topics in Geobiology, 52) 3030596621, 9783030596620

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Topics in Geobiology 52

Jón Eiríksson Leifur A. Símonarson  Editors

Pacific Atlantic Mollusc Migration Pliocene Inter-Ocean Gateway Archives on Tjörnes, North Iceland

Topics in Geobiology Volume 52 Series Editors Neil H. Landman Department of Paleontology, American Museum of Natural History, New York, NY, USA Peter J. Harries Department of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, NC, USA

The Topics in Geobiology series covers the broad discipline of geobiology that is devoted to documenting life history of the Earth. A critical theme inherent in addressing this issue and one that is at the heart of the series is the interplay between the history of life and the changing environment. The series aims for high quality, scholarly volumes of original research as well as broad reviews. Geobiology remains a vibrant as well as a rapidly advancing and dynamic field. Given this field’s multidiscipline nature, it treats a broad spectrum of geologic, biologic, and geochemical themes all focused on documenting and understanding the fossil record and what it reveals about the evolutionary history of life. The Topics in Geobiology series was initiated to delve into how these numerous facets have influenced and controlled life on Earth. Recent volumes have showcased specific taxonomic groups, major themes in the discipline, as well as approaches to improving our understanding of how life has evolved. Taxonomic volumes focus on the biology and paleobiology of organisms – their ecology and mode of life – and, in addition, the fossil record – their phylogeny and evolutionary patterns – as well as their distribution in time and space. Theme-based volumes, such as predator-prey relationships, biomineralization, paleobiogeography, and approaches to high-resolution stratigraphy, cover specific topics and how important elements are manifested in a wide range of organisms and how those dynamics have changed through the evolutionary history of life. Comments or suggestions for future volumes are welcomed. More information about this series at http://www.springer.com/series/6623

Jón Eiríksson  •  Leifur A. Símonarson Editors

Pacific - Atlantic Mollusc Migration Pliocene Inter-Ocean Gateway Archives on Tjörnes, North Iceland

Editors Jón Eiríksson Institute of Earth Sciences University of Iceland Reykjavík, Iceland

Leifur A. Símonarson Institute of Earth Sciences University of Iceland Reykjavík, Iceland

ISSN 0275-0120 Topics in Geobiology ISBN 978-3-030-59662-0    ISBN 978-3-030-59663-7 (eBook) https://doi.org/10.1007/978-3-030-59663-7 © The Editor(s) (if applicable) and The Author(s) 2021 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Foreword: The Paleoclimatic and Paleobiogeographic Significance of the Tjörnes Basin, Northern Iceland

Since the mid-nineteenth century, geologists and paleontologists have recognized the scientific importance and unique nature of the richly fossiliferous sediments exposed along the Tjörnes Peninsula in Northern Iceland. In the following century and a half, Tjörnes has attracted the attention of an international “who’s who” in Cenozoic paleontology, as well as many paleoclimatologists unraveling the complex climatic history of the North Atlantic and Arctic Oceans. In a seminal meeting, sponsored by the Royal Society of London in 1984, and published in Philosophical Transactions of the Royal Society of London, Series B, volume 318 (“The past three million years: evolution of climatic variability in the North Atlantic region”), an international group of experts addressed climatic history of the last 3 million years. Notably, one of the main invited participants was Iceland’s Dr. Thorleifur Einarsson, who literally wrote the book “Geology of Iceland” (1994, 1999) and was also known for his expertise in Tjörnes paleoclimatology. Einarsson’s key contribution was linking the marine history of Tjörnes to the rapidly growing paleoclimate records from deep-sea marine sediment cores and improving chronology of climate evolution. This work was closely linked to the dating of Pliocene-Pleistocene glacial sediments and volcanics in Iceland and on Tjörnes in particular, based on paleomagnetic data and biostratigraphic work which was presented jointly with a group from the U.S. Geological Survey at the 1965 INQUA meeting in Boulder, Colorado. The current volume – Pacific–Atlantic Mollusc Migration: Inter-Ocean Gateway Archives on Tjörnes, North Iceland, edited by two other Icelandic geologists, Jón Eiríksson and Leifur A. Símonarson, – is a comprehensive treatise that will remain a legacy to the central role Tjörnes has played in Cenozoic paleoclimatology and marine faunal evolution during the last 6 million years. Its 14 chapters capture decades of research by the editors and their colleagues, as well as major up-to-date reviews of wide-ranging topics in paleontology and paleoclimatology. Summarizing such an iconic and unique sedimentary basin such as Tjörnes is a daunting task given the complex tectonic, volcanic, and sedimentary history, and the editors must be commended for their highly successful efforts. I’ll mention just a few of the highlights readers will find in these review chapters.

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Foreword: The Paleoclimatic and Paleobiogeographic Significance …

First, chronology is the key to any geological field study, and here we find improved Tjörnes chronology of the Barmur (Pliocene) and Breiðavík (Quaternary) Groups, tectonic and volcanic history of northern Iceland, and stratigraphic correlation of Tjörnes sediments to European and North American Miocene-Quaternary sequences. There are also new insights into taxonomy, paleobiogeography, and ecology of the rich diversity of Tjörnes fossil mollusks and foraminifera. Substantial new data on mollusk and foraminifera systematics and biostratigraphy are presented as well as improved stratigraphic control of sampling through lithostratigraphic mapping. At least as far back as the 1960s, Tjörnes has provided the best data on one of the greatest faunal migrations, the Pacific–Arctic–Atlantic migration of marine species. The Tjörnes volume has clarified in detail this step-wise migration and its relationship between climatic evolution and opening (Bering Strait) and closing (Central American Seaway, Emergence of Isthmus of Panama) of oceanic gateways of the past 5 million years. Excellent discussions on sedimentary lithofacies, ranging from classic marine facies to lignites, fluvial and lacustrine sediments, diamictites, and tillites, provide the field geologist with an invaluable understanding of Iceland’s natural geological history. In many ways, this is a unique volume due to the breadth of topics covered, ranging from Miocene-Quaternary stratigraphy, paleobiogeography, climate evolution, tectonics, and ocean gateways. It will remain an invaluable future resource for anyone interested in the climate history and biological evolution of the last 5 million years. Senior Research Geologist United States Department of the Interior U.S. Geological Survey Eastern Geology and Paleoclimate Science Center 926A National Center 12201 Sunrise Valley Drive Reston, VA 20192, USA

Thomas M. Cronin

Acknowledgements

A great many people have contributed to the present volume in addition to the editors and authors of individual chapters. Nearly half a century of research devoted to Tjörnes is compiled here, having been instigated by Professor Þorleifur Einarsson†, University of Iceland, and we dedicate the work to his sustained support and encouragement. The fieldwork for this study was supported by the Science Institute, University of Iceland, and the Department of Geoscience, Aarhus University, as well as the Icelandic Council of Science and the Danish Natural Science Research Council. We are grateful to the Tjörnes Community for logistical assistance over the years and are, in particular, indebted to the families at Ytri-Tunga, Hallbjarnarstaðir, and Breiðavík for invaluable help and friendship. Many dozens of geology students of the University of Iceland have taken part in data collection and observations during field courses on Tjörnes in sedimentology and palaeontology, supervised by the editors. Their enthusiastic contribution is significant and highly appreciated. The field reports are too numerous to be listed, and only very few are mentioned in respective chapters. We are grateful to the Icelandic Museum of Natural History for access to fossil material and permission to photograph samples. Kristján Jónasson is thanked for his helpful and smooth cooperation in this matter. Svend V.  Funder, curator at the Geological Museum, University of Copenhagen, Section for Geogenetics, is thanked for help in accessing and allowing photography of material from Tjörnes. Hjördís Erna Sigurðardóttir, The Árni Magnússon Institute for Icelandic Studies, Department of Name Studies, is thanked for valuable assistance in procuring place names on Tjörnes. We have received many useful comments and suggestions from persons who undertook reviewing the chapters. These include Haukur Jóhannesson, Jón Ólafsson, Guðmundur Guðmundsson, Freysteinn Sigmundsson, Ólafur Ingólfsson, Helgi Torfason, Björn S. Harðarson, Frank P. Wesselingh, Ronald Pouwer, Karen Luise Knudsen, Thomas M. Cronin, Steffen Mischke, Anders Schomacker, Marit-Solveig Seidenkrantz, Ívar Ö. Benediktsson, and Páll Einarsson. We thank them for their

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vital contributions, but the chapter authors, naturally, remain responsible for any shortcomings, mistakes or errors that might still be present in the papers. We are grateful to Arnþór Garðarsson for giving us access to his excellent aerial photographs of the Tjörnes coastline, and allowing us to include a few of these as illustrations. Photographs were also provided by geology students Ásgeir Guðmundsson and Vignir Njáll Bergþórsson, Alfred Geptner, Friðgeir Grímsson, photographer Daníel Bergmann, Bryndís Brandsdóttir, and Þorleifur Einarsson†. These are acknowledged in appropriate chapters. All other photographs and plates in the chapters were photographed by Jón Eiríksson. The TanDEM-X digital elevation model used in Chap. 3 is from the German Space Agency (DLR), under the project of IDEM_GEOL0123. In relation to the foraminiferal studies, we would like to thank Rolf W. Feyling-­ Hanssen† and Thomas M. Cronin for permission to include some of their samples from Tjörnes and Breiðavík in the present work. Our thanks also to Svend Meldgaard Christiansen†, who processed the foraminiferal samples, and to Svend Meldgaard Christiansen† and Annette Mønsted Pedersen, who picked a major part of the specimens for foraminiferal analyses. The colleagues and directors at the Institute of Earth Sciences, University of Iceland, are thanked for invaluable access to facilities and fruitful discussions. Special thanks are due to Leó Kristjánsson† for discussions of the age model for the Tjörnes sequence and for his contributions to palaeomagnetic work on the sediments and volcanics. Jón Eiríksson acknowledges access to facilities and highly valued help concerning the research on Tjörnes during numerous visits to the Department of Geosciences, Aarhus University, Denmark. Thanks are also due to valuable access to facilities made available by Kurt H. Kjær and colleagues during his stay at the Section for GeoGenetics, University of Copenhagen. We are very grateful to Sheresta Saini at Springer for assistance in the early stages of preparing the volume, and to Silembarasan Panneerselvam as well as Clement Wilson Kamalesh and Zachary Romano at Springer for guidance and help during the later stages. In particular, we appreciate the support and patience of our families during the compilation of the Tjörnes volume.

Contents

1 A Brief Resumé of the Geology of Iceland��������������������������������������������    1 Jón Eiríksson and Leifur A. Símonarson 2 The Marine Realm Around Iceland: A Review of Biological Research������������������������������������������������������������������������������   13 Leifur A. Símonarson, Jón Eiríksson, and Karen Luise Knudsen 3 The Evolution of the Tjörnes Sedimentary Basin in Relation to the Tjörnes Fracture Zone and the Geological Structure of Iceland������������������������������������������������   37 Jón Eiríksson, Andrés I. Guðmundsson, Leifur A. Símonarson, Páll Einarsson, Ásta Rut Hjartardóttir, and Bryndís Brandsdóttir 4 A Review of the Research History of the Tjörnes Sequence, North Iceland��������������������������������������������������������������������������   57 Jón Eiríksson and Leifur A. Símonarson 5 Lithostratigraphy of the Tjörnes Sequence in Barmur and Höskuldsvík on the West Coast of Tjörnes, North Iceland������������������������������������������������������������������������   93 Jón Eiríksson, Leifur A. Símonarson, and Karen Luise Knudsen 6 An Age Model for the Miocene to Pleistocene Tjörnes Sequence, North Iceland ����������������������������������������������������������  213 Jón Eiríksson, Leifur A. Símonarson, and Karen Luise Knudsen 7 Systematic Overview of the Pliocene Molluscs and Barnacles of the Barmur Group on Tjörnes, North Iceland��������������������������������������������������������������������������������������������  237 Leifur A. Símonarson and Jón Eiríksson ix

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  8 Foraminifera in the Pliocene Barmur Group on Tjörnes, North Iceland ����������������������������������������������������������������������  443 Karen Luise Knudsen, Jón Eiríksson, and Leifur A. Símonarson   9 Reconstructing the Paleoenvironments of the Pliocene Barmur Group in the Tjörnes Basin, North Iceland��������������������������������������������������������������������������������  525 Leifur A. Símonarson, Karen Luise Knudsen, and Jón Eiríksson 10 Lithostratigraphy of the Upper Part of the Tjörnes Sequence in Furuvík, Breiðavík, Öxarfjörður, and Central Tjörnes Mountains, North Iceland ����������������������������������  567 Jón Eiríksson, Andrés I. Guðmundsson, Leifur A. Símonarson, and Karen Luise Knudsen 11 Systematic Overview of the Molluscs and Barnacles of the Quaternary Breiðavík Group North Iceland ������������������������������������������������������������������������������  667 Leifur A. Símonarson and Jón Eiríksson 12 Foraminifera in the Early Pleistocene Part of the Breiðavík Group, North Iceland��������������������������������������������������  757 Karen Luise Knudsen, Jón Eiríksson, and Leifur A. Símonarson 13 Reconstructing the Paleoenvironments of the Quaternary Tjörnes Basin, North Iceland����������������������������������  803 Jón Eiríksson, Karen Luise Knudsen, and Leifur A. Símonarson 14 Migration of Pacific Marine Mollusc Fauna into the North Atlantic Across the Arctic Ocean in Pliocene and Early Pleistocene Time ������������������������������������������������  841 Leifur A. Símonarson and Jón Eiríksson Index������������������������������������������������������������������������������������������������������������������  869

Chapter 1

A Brief Resumé of the Geology of Iceland Jón Eiríksson and Leifur A. Símonarson

Abstract  The current divergent plate boundary between North America and Eurasia across Iceland and the Iceland shelf is expressed by several segments between the submarine Reykjanes Ridge in the southwest and the Kolbeinsey Ridge in the north. Major elements of the present plate margin configuration were established at about 25 Ma, and Iceland occupies a complex plate boundary between the Reykjanes Ridge and the Kolbeinsey Ridge. Volcanic productivity is higher in Iceland than along the spreading axes to the south and north, indicating a mantle anomaly beneath the island, a hot spot. The area of the shelf around Iceland is larger than the subaerial island. Miocene lava sequences predominate in the far east and far west of Iceland, with Quaternary rocks occupying the central island. The geological structure is broadly symmetrical with gentle regional dips toward the volcanic spreading axes. Since the onset of the last Ice Age at about 2.6 Ma, Iceland has been periodically covered by an ice cap extending to the shelf. Today, Iceland is located directly in the path of highaltitude westerly jet streams. The climate is cold-­temperate and maritime, and at present, c. 10% of the island’s area of 103,000 km2 is covered by glaciers. Keywords  Plate tectonics · Iceland · Quaternary · Ice age · Neogene · Marine sediments · Submarine ridges

1.1  Introduction The geological structure of Iceland is controlled by plate boundary processes. Geographically, Iceland is located at the intersection of two oceanic ridges, the Mid-­ Atlantic Ridge separating the North American and the Eurasian lithospheric plates on one hand, and the series of ridges connecting Greenland and Iceland, Iceland and the Faroe Islands, and continuing to the United Kingdom. Volcanism within the North Atlantic igneous province was initiated at c. 61 Ma, with massive volcanic activity extending from northern Canada to Scotland between 61 and 56 Ma (Storey et al., 2007). Until c. 25 Ma, the active spreading between Greenland and Scandinavia J. Eiríksson (*) · L. A. Símonarson Institute of Earth Sciences, University of Iceland, Reykjavík, Iceland e-mail: [email protected]; [email protected] © The Author(s) 2021 J. Eiríksson, L. A. Símonarson (eds.), Pacific - Atlantic Mollusc Migration, Topics in Geobiology 52, https://doi.org/10.1007/978-3-030-59663-7_1

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took place along the Ægir Ridge (Fig. 1.1), which is now extinct. A segment of East Greenland was separated from the North American plate by a new, more westerly rift system and spreading axis, which has been active for the last 25 Ma. The Jan Mayen Ridge represents this isolated segment of continental crust between Iceland and Jan Mayen to the north. This westward jump of the plate boundary reflects the nonstationary nature of the rifting axis. Currently, the axis appears to be moving northwestward with respect to the mantle below. Features of the geological structure of Iceland indicate that this movement is still active, leading to repeated eastward shifts of the spreading axis across Iceland. From east to west, the geological structure of Iceland is broadly symmetrical, with the youngest rocks in the central part (Fig.  1.2). Factors contributing to the

Fig. 1.1  Map of the northern North Atlantic showing the geographical location of Iceland and its relationship to major plate tectonic features including submarine ridges and magnetic anomalies, with age boundaries, on the seafloor. (Modified after Olesen et al. (2007). AeR Ægir Ridge, KR Kolbeinsey Ridge, MR Mohn’s Ridge, RR Reykjanes Ridge, WJMFZ West Jan Mayen Fracture Zone, EJMFZ East Jan Mayen Fracture Zone)

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Fig. 1.2  Geological map of Iceland with major volcanic and seismic zones superimposed. WVZ Western Volcanic Zone, SISZ South Iceland Seismic Zone, EVZ Eastern Volcanic Zone, NVZ Northern Volcanic Zone, TFZ Tjörnes Fracture Zone. (Modified after Jóhannesson & Sæmundsson, 1998; Thordarson & Höskuldsson, 2002; Einarsson, 2008; Hardarson et al., 2008)

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exceptionally thick Miocene, Pliocene, and Quaternary sequences in Iceland include the relatively constant extrusion of volcanics along the plate margin, coupled with subsidence and tilting of flanking rock units toward the axial zones of rifting and volcanism. These elements are significant for the preservation potential of the geological record because the resistant lava flows form a shield against subsequent erosion and weathering of any underlying sediment. When the plates move away from the rifting zones, erosion and weathering have led to the development of exposures of tilted rock sequences in distal fjords and valleys. Today, Iceland is located within a sensitive climatic zone in the North Atlantic directly in the path of high-altitude westerly jet streams. The climate is c­ old-­temperate and maritime, and c. 10% of the island’s area of 103,000 km2 is covered by glaciers. Any changes in the path of cyclones across the Northern Hemisphere, or changes in the ocean circulation and configuration of the water masses, are likely to express themselves rapidly in climatically controlled environmental conditions such as geomorphological processes, vegetation, the mass balance of glaciers, or ice sheets in Iceland. The late Cainozoic sections in Iceland comprise a substantial data archive on the variations in ice caps and glaciers in the North Atlantic area.

1.2  Plate Tectonic Setting The topography of Iceland forms a large bulge on the North Atlantic Ocean floor (Fig. 1.3), which is generally considered to reflect an anomaly in the mantle below. This anomaly represents a stationary hot spot beneath Iceland, which was initiated at c. 61 Ma as a major temperature anomaly in the mantle, a mantle plume, with an

Fig. 1.3  Topographic profile along the axis of the Mid Atlantic Ridge (c. 20–80°N), across the Azores and Iceland highs and into the Arctic Basin. (Modified after Vogt & Jung, 2005). The topographic peak in Iceland coincides with the center of the assumed mantle plume beneath Iceland

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initial radius of c. 1000 km (Richards et al., 1989). The current plume is seismically depicted as a c. 150 km wide anomaly (Bijwaard & Spakman, 1999; Wolfe et al., 1997), rooted at a depth of over 660 km under Iceland (Shen et al., 1998). The relatively well-defined divergent plate margin along the North Atlantic breaks up into a series of segments on crossing the Iceland platform leading to the formation of independently moving blocks, as well as two-direction conservative transform zones offsetting the main divergence boundary eastward relative to the offshore Reykjanes and Kolbeinsey submarine spreading ridge segments (cf. Einarsson, 2008). These transform zones, the Tjörnes Fracture Zone in the north and the South Iceland Seismic Zone in the south (Fig. 1.2), connect the North Atlantic submarine spreading ridges to the main spreading axis across Iceland. This complex configuration of plate and microplate boundaries in Iceland is most probably related to the mantle anomaly beneath the Iceland platform, and the northwestward movement of the spreading system across the hot spot. This movement amounts to c. 0.3 cm/year and is independent of the divergence at the plate margin in the northern North Atlantic, which amounts to c. 2 cm/year (1 cm/year in each direction, see overview in Thordarson & Höskuldsson, 2002). In response to the movement of the plate boundary away from the hot spot, readjustment tends to take place and a new spreading axis develops above it. Plate margins are defined on the basis of seismic and volcanic activity, and in Iceland, there are several zones of crustal deformation and volcanic activity. There is no general consensus on naming these deformation segments, but the main north– south trending spreading axis across the Iceland hot spot is generally called NVZ, the Northern Volcanic Zone (Fig. 1.2). It appears to be propagating in both directions (Einarsson, 2008), and the southern continuation, which extends to the South Iceland shelf is commonly called the EVZ (Eastern Volcanic Zone). This zone is parallel to the WVZ (Western Volcanic Zone), which is located in Southwest Iceland and forms an oblique continuation of the Reykjanes Ridge spreading axis. The accumulation of volcanic material within the rift zones results in sagging of the crust, and in tilting the adjacent rock units toward the main, active volcanic zones. In South Iceland, the crust between the WVZ and the EVZ assumes an anticlinal form. In addition to the major volcanic zones, there are two active intraplate volcanic zones or belts in Iceland where young (< 2 million years old) volcanic rocks rest unconformably on older bedrock, indicating renewed magma extrusion after a significant interval of quiescence (Thordarson & Höskuldsson, 2002). These are the Öræfi Volcanic Belt to the east of the plume center and the current plate margin (EVZ), and the Snæfellsnes Volcanic Belt in West Iceland, which is situated on the mantle plume trail and is in part superimposed on an extinct volcanic zone, precursor to the WVZ (Thordarson & Höskuldsson, 2002). It is possible that the westward migration of the entire Iceland rift system is forcing the plume to establish a new volcanic zone by melting its way through the crust of East Iceland. Thus the Öræfi Volcanic Belt may indicate an embryonic eastward shift in the location of the spreading axis across Iceland (Thordarson & Höskuldsson, 2002).

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1.3  Geological History 1.3.1  A  ccumulation of Volcanics and Sediments at a Terrestrial Divergent Plate Margin The geological structure of Iceland is broadly symmetrical, with the youngest rocks in the central part (Fig. 1.2). Key elements in retaining the Miocene as well as the exceptionally thick Pliocene and Quaternary sequences in Iceland are the relatively constant extrusion of volcanics along the plate margins, coupled with subsidence and tilting of flanking rock units toward the axial zones of rifting and volcanism. The average rate of subsidence is of the order of 1  mm/year (Pálmason, 1973). These elements are significant for the preservation potential of the geological record because the resistant lava flows provide a shield against subsequent erosion and weathering of any underlying sediment. When the plates move away from the rifting zones, erosion and weathering supersede the constructive processes and thick, tilted rock sequences are exposed in mountainous sections in distal fjords and valleys. In general terms, the geological units become younger toward the center of the island. But, as the rift zones in Iceland continually become inactive and new rift zones are formed closer to the mantle plume, older successions are broken up, tilted, eroded, and separated by younger geological units and unconformities. Such readjustments of the spreading axis across Iceland are considered to be manifested in the Neogene and Quaternary lava sequences, which display gentle, linear synclines reflecting former positions of the spreading axes and associated subsidence. The main episodes of this geological story were summarized by Denk et al. (2011). At 24–15  Ma, the main spreading activity on land was located in the so-called Northwest Iceland Rift Zone, now presumed to be located submarine on the shelf off the Vestfirðir coast, and around 15 Ma a new rift zone, the Snæfellsnes-Húnaflói Rift Zone, evolved to the east (Hardarson et al., 1997, 2008). At 7–6 Ma, the southern part of the Snæfellsnes-Húnaflói Rift Zone became extinct and the presently active Western Volcanic Zone (also known as the Western Rift Zone) developed. Then, at about 3–2 Ma, the northern part of the Snæfellsnes-Húnaflói Rift Zone also became extinct and the presently active Northern Volcanic Zone was formed (Jóhannesson, 1980). The repeated rift relocations have had an important effect on the geology of Iceland. Denk et al. (2011), among others, have suggested that they caused massive erosion and deposition, forming extensive sedimentary formations, often containing plant and, in some rare cases, animal fossils. However, it is doubtful whether the present state of geological mapping and the available scale of geological maps of Iceland does allow us to resolve the nature of the sedimentary horizons in the Neogene and Quaternary successions, and it has to be considered an open question whether the sedimentary horizons were formed across major structural and erosional unconformities or in active spreading systems with developing subsiding basins.

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1.3.2  Climatic Conditions in Iceland The modern climate of Iceland is cold-temperate and oceanic, and characterized by the geographic location in the northern North Atlantic (Einarsson, 1984). The Irminger Current branch of the Gulf Stream flows clockwise around Iceland and separates the region from the Arctic realm (cf. Símonarson et al., 2020). The boundary, however, is unstable and fluctuations have, in the geological past, repeatedly transferred Iceland to and from arctic climate conditions. An overview of the climate history of Iceland, based on vegetation record preserved in 11 sedimentary horizons in the volcanic succession has recently been published by Denk et  al. (2011). From 15 to 12 Ma, the climate was subtropical, warm, and humid with no dry seasons and relatively hot summers, becoming somewhat cooler from 12 to 10 Ma, with disappearance of many warmth-loving plant species. No major changes are seen from 10 to 8 Ma, but a gradual cooling is indicated between 8  Ma and 5.5 Ma. A climate shift to cooler climate was suggested between c. 5.5 and 4.4 Ma, but relatively warm conditions are indicated between 4.4 Ma and 3.6 Ma. Another shift, and now toward the present cool to cold conditions, occurred between 3.6 Ma and 2.4 Ma.

1.3.3  Glaciation History of Iceland Recent overviews of the glaciation record of Iceland show that a long record of repeated glacial-interglacial cycles has been preserved (Einarsson & Albertsson, 1988; Eiríksson, 2008; Geirsdóttir, 2011). Reconnaissance studies of the geology of Iceland at the turn of the nineteenth century showed that glacial processes began to affect the rock facies during the Pliocene before the onset of the so-called Glacial Period, and that the Ice Age glaciations were repeatedly separated by interglacials (Pjetursson, 1905; Thoroddsen, 1906). The introduction of paleomagnetic techniques in the mapping of rock sequences in Iceland during the 1950s and 1960s and an increasing number of absolute radiometric dates for lava flows and intrusions have led to the accumulation of considerable data on the initiation and frequency of glaciations in Iceland. The first phase of this research started in the 1960s and dealt mainly with isolated sections. A second phase of the research history began in the 1970s with regional mapping of Pliocene and Pleistocene areas coupled with K/Ar dating. The results have been reviewed by Sæmundsson (1979) and by Einarsson and Albertsson (1988). Based on this mapping effort, a widespread late Pliocene onset of glaciation in Iceland was suggested at just over 3 Ma ago. The origin of many of the diamictites and volcaniclastic breccias was a controversial issue in the geological literature for most of the twentieth century, but the application of detailed sedimentological studies and facies analyses to these deposits in the last three decades has led to a new appraisal of the nature and regional extent of glaciation events in Iceland (Eiríksson & Geirsdóttir, 1991; Geirsdóttir, 1991; Geirsdóttir & Eiríksson, 1994).

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1.3.4  Terrestrial Sedimentary Basins and Sequences The great majority of Pliocene and Pleistocene rock sequences in terrestrial Iceland are exposed along the flanks of the presently active volcanic zones in Iceland, broadly outlined by the distribution of Late Ice Age lavas and hyaloclastites (Fig. 1.2). Most of the sections have been carved into the gently dipping volcanic successions by glacial and fluvial valley erosion or coastal abrasion. Quite continuous sequences have become available by combined symmetrical tilting due to subsidence in the volcanic zones leading to a synclinal structure and the gradual movement of newly created crust transversely away from these zones. However, the sections within the volcanic zones, which contain the Brunhes glacial record from the last 0.8 Ma or so, remain poorly exposed and the rocks are largely hidden by young volcanics. A compilation of glacial events in Iceland shows that glaciation of high relief mountainous areas and volcanic centers had already started at 7 Ma (Albertsson, 1981; Geirsdóttir, 2011). The frequency of preserved glacial events after the first regional expansion of local highland glaciers close to the Gauss/Matuyama Chron boundary exceeds one event per 100,000 years. These events amounted to massive glacial events covering most of Iceland. Each event was separated by ice-free conditions.

1.3.5  Sedimentary Basins Offshore The topographic anomaly created by Iceland is considerably larger than indicated by the present coastline of the island. In fact, the submarine shelf around Iceland covers an area larger than the terrestrial part (Fig. 1.4). Data on the geology of the shelf are rather limited. Outside the submarine plate tectonic Reykjanes and Kolbeinsey spreading ridges, which are characterized by volcanism, there are two deep drillings available, i.e., on the islands of Heimaey, South Iceland, and Flatey, North Iceland (Fig. 1.2). Both these boreholes indicate the presence of thick sediments on the shelf. A major gravity anomaly on the North Iceland shelf has been interpreted as reflecting a sedimentary basin with up to 4 km thick sediments within the Tjörnes Fracture Zone (Pálmason, 1974; Flóvenz & Gunnarsson, 1991). The Flatey borehole is located within this gravity anomaly, and the Pliocene and Pleistocene sediments exposed on the uplifted Tjörnes horst reflect shallow marine and coastal sedimentation (Albertsson & Eiríksson, 1989; Eiríksson et al., 1990; Ólafsson et al., 1992). Another uplifted segment of shallow marine sediments is exposed on Snæfellsnes peninsula in West Iceland (Fig. 1.2), and the borehole on Heimaey, South Iceland, as well as numerous fossiliferous xenoliths in volcanoes in the coastal and offshore South Iceland indicate shallow marine sedimentation on the shelf in that region (Áskelsson, 1960; Símonarson, 1979). Sedimentary basins in a shelf setting have thus formed along the Tjörnes Fracture Zone and the South

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Fig. 1.4  Bathymetric map delineating the Icelandic shelf. (Modified after Jakobsson et al., 2012)

Iceland Seismic Zone which currently offset the spreading axis across Iceland. The uplifted Snæfellsnes marine sediments, as well as an isolated occurrence of marine fossiliferous sediments in Southeast Iceland (Akhmetiev et al., 1978) indicate that sediments may form a substantial part of the shelf around Iceland, and the age estimates of these deposits indicate that the shelf sedimentary basins were active during the Pleistocene and the Pliocene, at least.

References Akhmetiev, M. A., Geptner, A. R., Gladenkov, Y. B., Milanovsky, E. E., & Trifonov, V. G. (1978). Iceland and Mid-Ocean ridge. Stratigraphy. Lithology. Moscow: Nauka. 204 pp. Albertsson, K.  J. (1981). On Tertiary tillites in Iceland. In M.  J. Hambrey (Ed.), Earth’s pre-­ Pleistocene glacial record (pp. 556–562). London: Cambridge University Press. Albertsson, K. J., & Eiríksson, J. (1989). K/Ar ages of rocks from the Flatey borehole in the offshore Skjálfandi Basin, North Iceland. Jökull, 38, 55–60. Áskelsson, J. (1960). Fossiliferous xenoliths in the Móberg Formation of South Iceland. Acta Naturalia Islandica, 2(3), 1–30. Bijwaard, H., & Spakman, W. (1999). Tomographic evidence for a narrow whole mantle plume below Iceland. Earth and Planetary Science Letters, 166, 121–126. Denk, T., Grímsson, F., Zetter, R., & Símonarson, L. A. (2011). Late Cainozoic Floras of Iceland. 15 Million Years of Vegetation and Climate History in the Northern North Atlantic (Vol. 35, p. 854). Dordrecht, The Netherlands/Heidelberg, Germany/London/New York: Springer.

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Einarsson, M. Á. (1984). Climate of Iceland. In H. V. Loon (Ed.), Climates of the oceans (pp. 673– 697). Amsterdam: Elsevier. Einarsson, P. (2008). Plate boundaries, rifts and transforms in Iceland. Jökull, 58, 35–58. Einarsson, T., & Albertsson, K. J. (1988). The glacial history of Iceland during the past three million years. Philosophical Transactions of the Royal Society of London. B, 318, 637–644. Eiríksson, J. (2008). Glaciation events in the Pliocene – Pleistocene volcanic succession of Iceland. Jökull, 58, 315–329. Eiríksson, J., & Geirsdóttir, Á. (1991). A record of Pliocene and Pleistocene glaciations and climatic changes in the North Atlantic based on variations in volcanic and sedimentary facies in Iceland. Marine Geology, 101, 147–159. Eiríksson, J., Gudmundsson, A. I., Kristjánsson, L., & Gunnarsson, K. (1990). Paleomagnetism of Pliocene-Pleistocene sediments and lava flows on Tjörnes and Flatey, North Iceland. Boreas, 19, 39–55. Flóvenz, Ó. G., & Gunnarsson, K. (1991). Seismic crustal structure in Iceland and surrounding area. Tectonophysics, 189, 1–17. Geirsdóttir, Á. (1991). Diamictites of late Pliocene age in western Iceland. Jökull, 40, 3–25. Geirsdóttir, Á. (2011). Pliocene and Pleistocene glaciations of Iceland: A brief overview of the glacial history. In J. Ehlers, P. L. Gibbard, & P. D. Hughes (Eds.), Quaternary glaciations – Extent and chronology. A closer look (pp. 199–210). Amsterdam: Elsevier. Geirsdóttir, Á., & Eiríksson, J. (1994). Growth of an intermittent ice sheet in Iceland during the late Pliocene and early Pleistocene. Quaternary Research, 42, 115–130. Hardarson, B. S., Fitton, J. G., Ellam, R. M., & Pringle, M. S. (1997). Rift relocation—A geochemical and geochronological investigation of a palaeo-rift in Northwest Iceland. Earth and Planetary Science Letters, 153(3–4), 181–196. Hardarson, B. S., Fitton, J. G., & Hjartarson, Á. (2008). Tertiary volcanism in Iceland. Jökull, 58, 161–178. Jakobsson, M., Mayer, L. A., Coakley, B., Dowdeswell, J. A., Forbes, S., Fridman, B., et al. (2012). The international bathymetric chart of the Arctic Ocean (IBCAO) version 3.0. Geophysical Research Letters, 39, L12609. https://doi.org/10.1029/2012GL052219 Jóhannesson, H. (1980). Evolution of rift zones in western Iceland (in Icelandic with English summary). Náttúrufræðingurinn, 50, 13–31. Jóhannesson, H., & Sæmundsson, K. (1998). Geological map of Iceland. Bedrock geology. 1:500.000. Reykjavík, Iceland: Icelandic Institute of Natural History. Ólafsson, M., Friðleifsson, G. Ó., Eiríksson, J., Sigvaldason, H., & Ármannsson, H. (1992). Könnun á uppruna gass í Öxarfirði. Borun og mælingar á holu ÆR-04 við Skógalón. Reykjavík, Iceland: Orkustofnun OS-92031/JHD-03. 77 pp. Olesen, O. J. E., Lundin, E., Mauring, E., Skilbrei, J. R., Torsvik, T. H., et al. (2007). An improved tectonic model for the Eocene opening of the Norwegian-Greenland Sea: Use of modern magnetic data. Marine and Petroleum Geology, 24(1), 53–66. Pálmason, G. (1973). Kinematics and heat flow in a volcanic rift zone with application to Iceland. Geopysical Journal of the Royal Astronomical Society, 33, 451–481. Pálmason, G. (1974). Insular margins of Iceland. In C. A. Burk (Ed.), The geology of continental margins (pp. 375–379). New York: Springer. Pjetursson, H. (1905). Om Islands Geologi. Meddelelser fra Dansk Geologisk Forening, 2(11), 1–104. Richards, M.  A., Duncan, R.  A., & Courtillot, V.  E. (1989). Flood basalts and hot-spot tracks: Plume heads and tails. Science, 246(4926), 103–107. Sæmundsson, K. (1979). Outline of the geology of Iceland. Jökull, 29, 7–28. Shen, Y., Solomon, S. C., Bjarnason, I. T., & Wolfe, C. J. (1998). Seismic evidence for a lower-­ mantle origin of the Iceland plume. Nature, 395, 62–65. Símonarson, L. A. (1979). On climatic changes in Iceland. Jökull, 29, 44–46.

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Símonarson, L. A., Eiríksson, J., & Knudsen, K. L. (2020). The marine realm around Iceland – A review of biological research. In J. Eiríksson & L. A. Símonarson (Eds.), Pacific – Atlantic Mollusc migration. Cham, Switzerland: Springer. This volume, Chapter 2. Storey, M., Duncan, R. A., & Tegner, C. (2007). Timing and duration of volcanism in the North Atlantic Igneous Province: Implications for geodynamics and links to the Iceland hotspot. Chemical Geology, 241(3–4), 264–281. Thordarson, T., & Höskuldsson, Á. (2002). Iceland (3). Bodmin & King’s Lynn: Terra Publishing. 200 pp. Thoroddsen, T. (1906). Island. Grundriss der Geographie und Geologie. Petermanns Mitteilungen, Ergänzungshefte, 152–153, 1–358. Vogt, P.  R., & Jung, W.-Y. (2005). Paired basement ridges: Spreading axis migration across mantle heterogeneities? In G.  R. Foulger, J.  H. Natland, D.  C. Presnall, & D.  L. Anderson (Eds.), Plates, plumes, and paradigms (pp. 555–579). Boulder, CO: The Geological Society of America. Wolfe, C. J., Bjarnason, I. T., VanDecar, J. C., & Solomon, S. C. (1997). Seismic structure of the Iceland mantle plume. Nature, 385, 245–247.

Chapter 2

The Marine Realm Around Iceland: A Review of Biological Research Leifur A. Símonarson, Jón Eiríksson, and Karen Luise Knudsen

Abstract  Iceland is surrounded by a shelf with depths down to 200–500 m along its 6000 km long coastline. The tidal range is highest in the south and west, and the intertidal zone is relatively narrow in northern and eastern Iceland. Relative warm high-salinity North Atlantic Water is brought to the South and West Iceland shelf by the Irminger Current flowing clockwise around the island. The marine Polar Front with steep temperature and salinity gradients separates the Atlantic Water and the cold lower salinity Polar and Arctic waters of the East Greenland and East Icelandic Currents. The oceanography north of Iceland is highly unstable. A coastal water mass follows the coastline and shallow parts of the Icelandic shelf, flowing clockwise around Iceland. The modern marine fauna is composed of boreal and subarctic-­ boreal species, but also some arctic species. Several hundreds of species new to the Icelandic fauna and at least 29 mollusc species new to science were recorded when the BIOICE sampling was completed in 2004 based on sampling down to 3000 m. Up to then, sampling had been limited to c. 400 m, and most of the data had been published in issues of The Zoology of Iceland. Keywords  Iceland marine realm · Marine molluscs · barnacles · Marine foraminifera · Marine biota · Iceland oceanography · Polar Front

L. A. Símonarson (*) · J. Eiríksson Institute of Earth Sciences, University of Iceland, Reykjavík, Iceland e-mail: [email protected]; [email protected] K. L. Knudsen Department of Geoscience, Aarhus University, Aarhus C, Denmark e-mail: [email protected] © The Author(s) 2021 J. Eiríksson, L. A. Símonarson (eds.), Pacific - Atlantic Mollusc Migration, Topics in Geobiology 52, https://doi.org/10.1007/978-3-030-59663-7_2

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2.1  The Marine Environment Iceland, which is located in the northern North Atlantic Ocean, centered at 65°N, 20°W, has a coastline of about 6000 km and is surrounded by a shelf with varying depths down to 200–500 m. The shelf occupies an area of about 115,000 km2 and exceeds the area of 103,000 km2 covered by the island itself. Iceland is located at the junction of submarine ridges separating deep basins reaching depths of 1500–2500 m. The ocean around Iceland is divided into various sectors or regions (Fig. 2.1), the Irminger Sea to the southwest, the Iceland Sea to the north, the Norwegian Sea to the east, and the Iceland Basin to the south (Hansen & Østerhus, 2000). The bound-

Fig. 2.1  Map of the northern North Atlantic bathymetry showing the geographical location of Iceland, major submarine ridges, and ocean sectors around Iceland. Depth contour interval 1000 m. GIR Greenland-Iceland Ridge, FIR Iceland-Faroe Ridge, RR Reykjanes Ridge, KR Kolbeinsey Ridge, JMR Jan Mayen Ridge

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aries between these oceanic regions are defined by submarine ridges. The main ridges are the Greenland-Iceland Ridge and the Reykjanes Ridge to the west and southwest of Iceland, the Jan Mayen Ridge and the Kolbeinsey Ridge to the north, and the Iceland-Faroe Ridge to the east (Malmberg, 2004). These regions are all part of the North Atlantic Ocean, but the areas to the north and east of Iceland are parts of the Nordic Seas (Iceland, Greenland, and Norwegian Seas). The tidal wave reaches the Icelandic shelf and coast from southeast and runs westward along the south coast and then clockwise around the island. The tidal range from average spring tide to average neap tide changes along the coast from South Iceland, where it is 2.5 m in Vestmannaeyjar and 3.1 m in Grindavík, to 3.8 m in Reykjavík. It is highest in Breiðafjörður in western Iceland where it reaches 4.1 m in Stykkishólmur (Fig. 2.2). Then it decreases to 3.2 m in Ísafjörður in northwestern Iceland and to 1.3 m in Akureyri in North Iceland and 1.7 m in Fáskrúðsfjörður in East Iceland (Stefánsson, 1999). The tidal range is thus considerably higher in southern and western Iceland than in the northern and eastern parts of the island.

Fig. 2.2  Location map of Iceland and the shelf around Iceland. Bathymetric contour interval 1000 m

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Fig. 2.3  Surface ocean circulation in the northern North Atlantic (modified after Map A in Hurdle, 1986). The Icelandic Coastal Current is shown with thin, black arrows (modified after Logemann et al., 2013; Valdimarsson et al., 2012)

This significantly affects the width of the tidal zone, which is also related to the coastal topography, and the intertidal zone is thus narrower in northern and eastern Iceland than in the western and southern parts of the country. Relatively warm high-salinity North Atlantic Water is brought to South and Southwest Iceland by the Irminger Current flowing clockwise along the Icelandic shelf (Fig. 2.3). The Irminger Current reaches down to the sea floor in shallow parts of the Iceland shelf. This water mass is over 500 m thick on approaching Iceland, tapering off en route around the island (Eiríksson et al., 2011; Knudsen & Eiríksson, 2002; Stefánsson, 1962, 1999). The sea floor of the deeper part of the continental slope off Southeast Iceland is probably affected by water from the Nordic Seas (Eiríksson et al., 2011; Eldevik et al., 2009; Jónsson & Valdimarsson, 2004). Arctic Water fills the Iceland Sea basin below 400 m water depth northwest and north of Iceland. Source waters for the Denmark Strait overflow water is part of the North Atlantic Deep Water and the Global Thermohaline Circulation (Stefánsson, 1962).

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The relative strength of the Irminger Current inflow to the North Iceland shelf is of paramount importance for the environmental conditions there (Eiríksson et al., 2011), being related to the global thermohaline conveyor belt (Broecker, 1991). The marine Polar Front (Fig. 2.3) with steep temperature and salinity gradients separates the relatively warm high-salinity Atlantic Water, brought by the Irminger Current and the cold low-salinity Polar Water of the East Icelandic Current or Arctic Surface Water (Johannessen, 1986). In terms of glacial-interglacial cyclicity, the position of the Polar Front probably shifted across Iceland (Ruddiman & McIntyre, 1973). The relative strength of the Irminger Current and the East Icelandic Current, as well as the composition of the Arctic Surface Water of the East Icelandic Current, is very important for the environmental conditions on the North Icelandic shelf (Eiríksson et al., 2011). Generally, the strength of the Irminger Current is related to deep-water formation in the Nordic Seas, and it is expected to be strong during active d­ eep-­water formation and weaker during periods of freshening of the surface waters of the Nordic Seas (Eiríksson et  al., 2011; Malmberg & Jónsson, 1997; Stefánsson & Olafsson, 1991; Våge et al., 2011; Våge et al., 2015). When the Irminger Current dominates, relatively warm, high-salinity and high-­productivity conditions prevail, but cold, low-salinity, and low-productivity conditions prevail, when cold Polar Water from the East Greenland Current reaches the Iceland Shelf and the East Icelandic Current (Eiríksson et  al., 2011; Johannessen, 1986; Johannessen et al., 1994). In recent years, the Arctic Surface Water north of Iceland consisted of Polar waters intermittently directly from the cold East Greenland Current, mixed with Atlantic waters (Malmberg & Jónsson, 1997). Atmospheric conditions in the region can amplify or reduce the surface circulation strength depending on prevailing wind directions (Hátún et al., 2005; Ólafsson, 1999). The boundary region between North Atlantic and Arctic waters north of Iceland is variable and the position of the Polar Front has shifted considerably, even within the last century. Thus, sea-surface temperature records for the twentieth century indicate that the Polar Front was shifted northward during the first decades of the twentieth century (Hanna et al., 2004), and another example is a marked southward shift during the sea-ice years and the Great Salinity Anomaly of the 1960s (Malmberg & Jónsson, 1997). The North Atlantic Water comprises different water masses with regional hydrographic properties. Depending on origin and degree of mixing, as well as interchange with other water masses, it has salinities above 35‰ and up to 35.4‰ and temperatures from 5 to 9 °C (Malmberg, 2004). In the Icelandic Basin, the highest values are found in the eastern part and the lowest in the western part, where the water mass is named Modified Atlantic Water (Malmberg, 2004). The water mass in the East Icelandic Current has relatively lower salinity and is colder, typically with temperatures from 0 to 4 °C and a salinity from less than 30 to 34.7‰ (Gíslason et al., 2016; Stefánsson, 1962). Therefore, this water is not very dense and will freeze more easily. The final water mass in Iceland is the coastal water on the shallower parts of the Icelandic shelf, from the coastline and generally down to depths about 100 m during the summer (Ólafsson et al., 2008). The coastal water flows clockwise around the

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island (Fig.  2.3), driven by the barotropic pressure field related to a freshwater induced density front (Logemann et al., 2013). Wind stress is also an important factor, and the coastal current has higher velocity and lower salinity (>30‰) than the current further out on the shelf (Ólafsson et al., 2008). The annual temperatures of the coastal water are between 1 and 8 °C, highest in South and West Iceland, but lower in the northern and eastern parts of the island (Logemann et  al., 2013; Stefánsson, 1999). The freshwater discharge along the coast produces low-­salinity near-shore water enriched by river-borne silicate (Ólafsson et al., 2008). The nutrients in the coastal water together with the stratifying effect of the fresh water on the water column are thought to be important elements of the spring algal bloom in Icelandic waters (Þórðardóttir, 1986). The flow also plays a crucial role in the recruitment processes of several fish species in Icelandic waters (Marteinsdóttir & Ástþórsson, 2005; Ólafsson, 1985).

2.2  The Marine Life Around Iceland: A Historical Review A map with the present biogeographical zones for the northern North Atlantic region is shown in Fig. 2.4. Because of the ongoing instability of the water-mass distributions north of Iceland, leading to oscillations of the Polar Front, we have indicated the boundary between the boreal and subarctic realms with a broken line close to Iceland. This chapter provides a brief introduction to the marine life around Iceland as a background for understanding the fossiliferous formations in the Tjörnes area in a climatic context. References to the modern marine fauna of Iceland reveal that it is mainly composed of boreal and subarctic-boreal species, but some arctic species are also present. Most of the species are common in northwestern European waters, but some North American species occur as well.

2.2.1  Invertebrates: Early Studies 2.2.1.1  Foraminifera: Early Studies (Before c. 1990) Terquem and Terquem (1886) listed about 150 species of foraminifera from three localities off Iceland. However, according to Nørvang (1945), the list is uncertain, partly because it includes several synonyms, and partly because it apparently contains many forms only known from “Tertiary formations” and some are reported of more southerly distribution, even though none of the finding localities were in North Iceland or near the Pliocene Tjörnes beds. Nørvang (1945) recorded 87 species of foraminifera, that is, 83 benthic and four planktonic species, occurring off the coasts of Iceland down to a water depth of about 400 m. He mentioned that his survey most probably only included the most

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Fig. 2.4  Biogeographical zones for the northern North Atlantic (modified after Dinter, 2001; Feyling-Hanssen, 1955; Funder et al., 2002). Because of the ongoing instability of the water-mass distributions north of Iceland, leading to oscillations of the Polar Front, the boundary between the boreal and subarctic realms is indicated with a broken line close to Iceland

abundant forms and that future investigations would most likely increase his number considerably. According to the species distribution, Nørvang (1945) divided the Icelandic coasts into a northern area, covering the north and east coast with an arctic fauna, and a southern area covering the west and south coasts with a boreal fauna. Later, Jarke (1958) described a few foraminiferal assemblages off East Iceland, and Adams and Frampton (1965) noted a few recent benthic foraminifera from littoral sediments in three fjords of Northwest Iceland. They recorded 17 species, of which seven were not previously recorded from Icelandic waters, and they determined two different faunal associations. More detailed studies by Mackensen (1987) and Wagener (1988) described the benthic foraminiferal distribution across the Iceland-Faroe Ridge off East Iceland. Mackensen (1987) recorded both the living and the dead foraminifera in 20 samples from a transect across the Iceland-Faroe Ridge at water depths down to more than 2000  m. He found a marked difference in species composition in the relatively warm water masses on the southern slope and in the colder waters (25; Alve & Murray 1999).

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Fossil occurrence: Elphidium margaritaceum was recorded from the Lower Pleistocene of the Netherlands (van Voorthuysen, 1950a, b), and it is widely distributed, particularly in the interglacial deposits throughout the Quaternary of NW Europe (e.g., van Voorthuysen, 1957; Lafrenz, 1963; Konradi, 1976; Knudsen et al., 2009), as well as in the Holocene (Feyling-Hanssen et al., 1971).

12.5  Environmental and Stratigraphical Summary Iceland is located at the boundary between the Arctic and Atlantic climate regions in the northern North Atlantic, an ideal area for tracking even minor change in ocean circulation. An example of the results of such climatic changes is represented by the Svarthamar Member of the Breiðavík Group, in which the foraminiferal assemblages reflect a glacial-interglacial transition in a shallow inner-shelf environment. The following section presents a short description of the foraminiferal contents in the Breiðavík Group and an overview of the environmental interpretations, as well as some stratigraphical remarks (see also Eiríksson et al., 1992, 1993). The combined environmental development, including sedimentological results and mollusc indications in the Breiðavík Group, is described by Eiríksson et al. (2020b), who also compared the record with the benthic oxygen isotope stack curve of Lisiecki and Raymo (2005).

12.5.1  The Hörgi Formation (Unit 2) The sparse content of extremely badly preserved foraminiferal tests in samples from the Hörgi Formation only give a vague indication of the ecology, but the dominant species Elphidium clavatum and Haynesina orbiculare would point to an arctic shallow-water environment (Fig. 12.5). An arctic environment is also indicated by the mollusc assemblages (Vilhjálmsson, 1985; Símonarson & Eiríksson, 2020).

12.5.2  The Þrengingar Formation (Units 8–12) Foraminiferal tests are preserved in four of the units in the Þrengingar Formation (i.e., units 9, 10, 12x, and 12; Fig. 12.5). Unit 9 only contained very few specimens and not enough for an environmental interpretation. Some of the samples from units 10, 12x, and 12 are relatively rich in foraminifera, and the species composition reflects an interesting environmental and stratigraphical development. Unit 10: The dominant foraminiferal species in unit 10 are Elphidium clavatum, E. albiumbilicatum, and E. magellanicum (Fig. 12.5). Some indeterminate specimens of the genus Elphidium in sample 1402 (Elphidium spp.) presumably also

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belong in one or more of these three species, but they cannot be determined with certainty because of their bad preservation state. Islandiella helenae and Cassidulina teretis are characteristic elements of the fauna. A remarkable feature in unit 10, especially in the lower part, is a relatively high content of Miliolida, mainly Ouinqueloculina stalkeri, which indicates close to normal-marine salinity (Murray, 1991). The content of Stainforthia feylingi, Cassidulina reniforme, and Islandiella helenae points to similar environmental conditions (i.e., Nagy, 1965; Elverhøi et al., 1980; Osterman & Nelson, 1989) with a relatively open oceanic connection and a water depth of presumably more than 25 m. There is a decrease in the amount of Cassidulina reniforme and Miliolida through unit 10, and a few percentages of the shallower water species Buccella frigida appear towards the top of this unit. The unfossiliferous sample 1404 was taken from a thin ash layer within unit 10 (Fig. 12.4). The assemblages in unit 10 indicate an arctic to boreal-arctic (subarctic) marine environment (cf. Fig. 12.6) with a relatively fast marine transgression followed by a regression towards the end. Most of the foraminifera are infaunal species indicating low-energy environments (cf. Murray, 1991). Units 12x and 12: The assemblages in unit 12x and unit 12 are generally rich both in species and in specimens (Fig. 12.5). It should be mentioned, however, that the sparse foraminiferal content in the lowermost sample (1400) is suggested to represent reworked specimens from the underlying unit 10, and this sample will, therefore, not be included in the following description. The dominant species throughout units 12x and 12 is Cibicides lobatulus, and Elphidium clavatum, E. albiumbilicatum, and E. hallandense are common as well. A large amount of badly preserved indeterminate specimens of the genus Elphidium (Elphidium spp.) in sample 1064 presumably also represents one or more of these three species. The group Miliolida is still present (e.g., Ouinqueloculina seminulum), but Quinqueloculina stalkeri is no longer present. Only few specimens of Cassidulina reniforme still occur in the lower part of units 12x and 12, and new elements in the assemblages, although in low frequencies, are taxa such as Astrononion gallowayi, Islandiella inflata, Nonionella pulchella, Elphidium karenae, E. margaritaceum, E. incertum, Haynesina germanica, Stainforthia fusiformis, Gavelinopsis praegeri, Glabratella wrightii, and Rosalina sp. Together, these taxa constitute an important and characteristic faunal element indicating ameliorated temperature conditions, i.e., a boreal-arctic (subarctic) or even a boreal environment (cf. Gudina & Evserov, 1973; Feyling-Hanssen, 1980a, 1983, 1990c; Kelly et al., 1999). Cibicides lobatulus is an epifaunal, attached species indicating high-energy conditions (Mackensen, 1987; Wagener, 1988), and a similar environment is also reflected by taxa such as Gavelinopsis praegeri, Glabratelle wrightii, Rosalina sp., and some of the Miliolida (Ouinqueloculina seminulum and Miliolinella subrotunda) (cf. Murray, 1991). The assemblages are pointing to shelf environments, shallowing towards the top of unit 12. The uppermost samples (1191–1193) are, however, collected at the location of Þrengingar a few kilometers further inland in a southeastern direction (Fig. 12.2), which is closer to the former coastline (Eiríksson et al., 2020b), and they may represent a relatively shallower facies rather than a younger age than samples 1413–1415.

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In general, the assemblages of units 12x and 12 reflect temperature conditions warmer than at present around Iceland, even warmer than found on the south coasts of Iceland today, indicating a northward shift of the Polar Front (cf. Fig. 12.1). In summary, the foraminiferal assemblages in units 10, 12x, and 12 indicate a climatic change through a glacial-interglacial cycle. The regressive trend through the cold unit 10 may be a result of isostatic uplift of the area after deglaciation, while the assemblages at the base of units 12x and 12 indicate a transgression, presumably caused by the subsequent eustatic rise in sea level at the beginning of a following relatively warmer interglacial period. This corresponds to similar patterns of regression and a subsequent transgression through glacial-interglacial sequences from areas along the eastern margin of the North Atlantic during the Late Pleistocene. The fast transgression after deglaciation is supposed to be local and isostatically controlled, while the regression took place concurrently with the isostatic rebound, perhaps before a global interglacial eustatic rise of sea level began (cf. discussion in Boulton, 1990). Mollusc studies of the units 10, 12x, and 12 indicate a similar glacial-interglacial environmental change from arctic to boreal-arctic (subarctic) and boreal conditions (Vilhjálmsson, 1985; Eiríksson et al., 1992, 2020b; Símonarson & Eiríksson, 2020). In Cronin’s (1991) study of the ostracods from the Þrengingar Formation, only samples from unit 12x and unit 12 were represented, and only one of his samples contained enough ostracod shells for temperature estimates, i.e., his sample 59-I (foraminiferal sample 1415) from unit 12x (see above). This sample would represent the initial transgression of the interglacial as indicated by the foraminiferal assemblages. Cronin (1991) described the climate indication of sample 59-I as cool, with a mean winter and summer temperature estimate of −0.8 and 3.0 °C, respectively, never exceeding 8 °C during the warmest months. A development from cold to warmer foraminiferal assemblages during the accumulation of the Svarthamar Member thus indicates an Early Pleistocene glacial-­ interglacial cycle of a range comparable to the Late Pleistocene cycles (see also Eiríksson et al. (2020b).

12.5.3  The Máná Formation (Unit 14) The few foraminiferal tests found in one fossiliferous sample from the Máná Formation (Fig.  12.5) points to a relatively shallow, arctic environment. A more precise environmental interpretation is possible based on mollusc contents (Vilhjálmsson, 1985; Símonarson & Eiríksson, 2020; Eiríksson et al., 2020b), which show clearly ameliorated temperature conditions during an interglacial period.

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12.5.4  Remarks on Stratigraphy and Immigration Most of the foraminiferal taxa found in the Breiðavík Group sediments are also living today in the North Atlantic region. However, an Early Pleistocene age for the present glacial-interglacial cycle is supported by the occurrence of Cassidulina teretis in assemblages of unit 10. This species is common in Pliocene and Early Pleistocene deposits of the Arctic and the North Atlantic region (i.e., Feyling-­ Hanssen, 1980a, b; Diester-Haass & Schnitker, 1989; Jansen et al., 1990; Knudsen & Ásbjörnsdóttir, 1991; Knudsen & Sejrup, 1993; Seidenkrantz, 1995). The species Cassidulina teretis occurs until slightly above the Matuyama-­Brunhes boundary both in the North Sea (Sejrup et al., 1987) and in the Norwegian Sea (Jansen et al., 1990; Seidenkrantz, 1995). A possible immigration of Pacific species into the Atlantic Ocean is suggested by the sparse occurrences of the three species Cassidulina limbata, Islandiella inflata, and Nonionella pulchella in the Breiðavík material (see also descriptions for each of the species above). These three species are not usually found in assemblages from the Atlantic Ocean. Cassidulina limbata was described from Pliocene deposits in California. It occurs both in Pliocene and Pleistocene deposits in that area (i.e., Galloway & Wissler, 1927; Cushman & Todd, 1947; Bandy, 1950), and it is also recorded in recent nearshore assemblages from western North America (Lankford & Phleger, 1973). Islandiella inflata was originally described from the Quaternary of NW Siberia, and it occurs both in the Early and Late Pleistocene deposits in the Siberian Arctic (Gudina, 1966; Gudina & Evserov, 1973). Feyling-Hanssen (1976, 1980a, b) found Islandiella inflata to be frequent in the Upper Pliocene and lowermost Pleistocene deposits in Baffin Island, Arctic Canada. Nonionella pulchella was originally described from recent shallow waters off Japan, and it has not previously been reported from the Atlantic. An Early Pleistocene migration of Pacific species into the North Atlantic was strongly supported by the study of ostracods in the Breiðavík Group sediments (Cronin 1991). Thus, Cronin found several Pacific ostracod species which had their first North Atlantic stratigraphic appearance in the Þrengingar Formation (sample 59-I/1415).

12.6  Conclusions A systematic description of the foraminiferal taxa found in the Breiðavík Group sediments is presented for the first time, including notes on their ecological preferences and stratigraphical distributions. The assemblage development through the Early Pleistocene (about 1.5 Ma old) Þrengingar Formation (Svarthamar Member) reflects a change from an arctic glacier-proximal environment (unit 10) to boreal-­ arctic (subarctic) or even boreal conditions (units 12x and 12). The deposits represent a full glacial-interglacial cycle comparable to those known from Late Pleistocene glacial-interglacial deposits along the margins of the North Atlantic.

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Thus, the Polar Front must have shifted northwards across Iceland into the Norwegian-Greenland Seas during warm stages in Middle Matuyama time. As previously described for ostracods from the same section (Cronin, 1991), an Early Pleistocene migration of Pacific species into the North Atlantic is also indicated by some of the foraminiferal species.

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Heron-Allen, E., & Earland, A. (1911). On the recent and fossil foraminifera from the shore sands of Selsey Bill, Sussex. Journal of the Royal Microscopical Society, 8, 436–448. Heron-Allen, E., & Earland, A. (1913). The Foraminifera of the Clare Island District, Co. Mayo, Ireland, Clare Island Survey, Part 64 - Foraminifera. Proceedings of the Royal Irish Academy, 31, 1–188. Heron-Allen, E., & Earland, A. (1932). Foraminifera. Part I, the icefree area of the Falkland Islands and adjacent seas. Discovery Report, 4, pp. 291–460. Cambridge: Cambridge University Press. Hessland, I. (1943). Marine Schalenablagerungen Nord-Bohusläns. Bulletin of the Geological Institute of the University of Upsala, 31, 1–348. Höglund, H. (1947). Foraminifera in the Gullmar Fjord and Skagerak. Zoologiska Bidrag från Uppsala, 26, 1–328. Hurdle, B. G. (1986). The Nordic seas (777 pp). New York: Springer Verlag. Jansen, E., Sjøholm, J., Bleil, U., & Erichsen, A. (1990). Neogene and Pleistocene glaciations in the northern hemisphere and late Miocene-Pliocene global ice volume fluctuations: Evidence from the Norwegian Sea. In J.  Thiede & J.  Bleil (Eds.), Geological history of the Polar Oceans: Arctic versus Antarctic (pp. 677–705). Dordrecht, The Netherlands: Kluwer Academic Publishers. Jennings, A. E., & Helgadottir, G. (1994). Foraminiferal assemblages from the fjords and shelf of eastern Greenland. Journal of Foraminiferal Research, 24, 123–144. Jennings, A. E., Weiner, N. J., Helgadóttir, G., & Andrews, J. T. (2004). Modern foraminiferal faunas of the southwestern to northern Iceland shelf: Oceanographic and environmental controls. Journal of Foraminiferal Research, 34, 180–207. Kelly, M., Funder, S., Houmark-Nielsen, M., Knudsen, K.  L., Kronborg, C., Landvik, J., et  al. (1999). Quaternary glacial and marine environmental history of Northwest Greenland: A review and reappraisal. Quaternary Science Reviews, 18, 373–392. Kihle, R., & Løfaldli, M. (1974). Stratigraphic Atlas of Foraminifera from unconsolidated sediments on the Norwegian Continental Shelf. IKU Petroleum Research, Oslo. The Continental Shelf Division (NTNF), Publication, 35, King, C., & Hughes, M. J. (1983). Caenozoic Micropalaeontological biostratigraphy of the North Sea. Institute of Geological Sciences, London, Report, 82(7), 1–40. Knudsen, K. L. (1978). Middle and Late Weichselian marine deposits at Nørre Lyngby, northern Jutland, Denmark, and their foraminiferal faunas. Geological Survey of Denmark, Series II, 112, 1–44. Knudsen, K. L. (1982). 14. Foraminifers. In E. Olausson (Ed.), The Pleistocene/Holocene boundary in South-Western Sweden (pp. 148–177). Sveriges Geologiska Undersökning, Serie C794, Årsbok, 76, 148–177. Knudsen, K. L., & Ásbjörnsdóttir, L. (1991). Plio-Pleistocene foraminiferal stratigraphy and correlation in the Central North Sea. Marine Geology, 101, 113–124. Knudsen, K. L., Ditlefsen, C., Penney, D. N., Kristensen, P., Kronborg, C., & Eiríksson, J. (2014). Elsterian-Holsteinian deposits at Kås Hoved, northern Denmark: Sediments, foraminifera, ostracods and stable isotopes. Boreas, 43, 251–271. Knudsen, K.  L., Jiang, H., Jansen, E., Eiríksson, J., Heinemeier, J., & Seidenkrantz, M.-S. (2004). Environmental changes off North Iceland during the deglaciation and the Holocene: Foraminifera, diatoms and stable isotopes. Marine Micropalaeontology, 50, 273–305. Knudsen, K. L., Kristensen, P., & Larsen, N. K. (2009). Marine glacial and interglacial stratigraphy in Vendsyssel, northern Denmark: foraminifera and stable isotopes. Boreas, 38, 787–810. Knudsen, K. L., & Seidenkrantz, M.-S. (1994). Stainforthia feylingi new species from arctic to subarctic environments, previously recorded as Stainforthia schreibersiana Czjzek. Cushman Foundation for Foraminiferal Research, Special Publication, 32, 5–13. Knudsen, K. L., & Sejrup, H. P. (1993). Early to late Pleistocene in the Devils Hole area, the Central North Sea: Foraminiferal and amino acid stratigraphy. Journal of Quaternary Science, 8, 1–14. Knudsen, K. L., Stabell, B., Seidenkrantz, M.-S., Eiríksson, J., & Blake Jr., W. (2008). Deglacial and Holocene conditions in northernmost Baffin Bay: Sediments, foraminifera, diatoms and stable isotopes. Boreas, 37, 346–376.

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Konradi, P.  B. (1976). Foraminifera in Eemian deposits at Stensigmose, southern Jutland. Geological Survey of Denmark, Series II, 105, 1–57. Korsun, S., & Hald, M. (1998). Modern benthic foraminifera off Novaya Zemlya tidewater glaciers, Russian Arctic. Arctic and Alpine Research, 30, 61–77. Korsun, S., Hald, M., Golikova, E., Yudina, A., Kuznetsov, I., Mikhailov, D., et  al. (2014). Intertidal foraminiferal fauna and the distribution of Elphidiidae at Chupa Inlet, western White Sea. Marine Biology Research, 10, 153–166. Korsun, S. A., Pogodina, I. A., Forman, S. L., & Lubinski, D. J. (1995). Recent foraminifera in glaciomarine sediments from three arctic fjords of Novaja Zemlja and Svalbard. Polar Research, 14, 15–32. Kristensen, P., & Knudsen, K.  L. (2006). Palaeoenvironments of a complete Eemian sequence at Mommark, southern Denmark: Foraminifera, ostracods and stable isotopes. Boreas, 35, 349–366. Kubischta, F., Knudsen, K. L., Ojala, A. E. K., & Salonen, V.-P. (2011). Holocene benthic foraminiferal record from a high-arctic fjord, Nordaustlandet, Svalbard. Geografiska Annaler: Series A, Physical Geography, 93, 227–242. Lafrenz, H. R. (1963). Foraminiferen as dem marinen Riss-Würm-Interglacial (Eem) in Schlecwig-­ Holstein. Meyniana, 13, 10–46. Lankford, R. R., & Phleger, F. B. (1973). Foraminifera from the nearshore turbulent zone, western North America. Journal of Foraminiferal Research, 3(3), 101–132. Leslie, R. J. (1965). Ecology and paleoecology of Hudson Bay foraminifera. Bedford Institute of Oceanography, Dartmouth, N.S. Report, 65–6. 1–92. Linné, C. von (1758). Systema naturae . . . 10th Edition, Lipsiae, 1: 1–824. Lisiecki, L. E., & Raymo, M. E. (2005). A Pliocene-Pleistocene stack of 57 globally distributed benthic δ18O records. Paleoceanography, 20, PA1003. https://doi.org/10.1029/2004PA001071 Lloyd, J. M. (2006). Modern distribution of benthic foraminifera from Disko Bugt, West Greenland. Journal of Foraminiferal Research, 36, 315–331. Loeblich, A. R., & Tappan, H. (1953). Studies of Arctic foraminifera. Smithsonian Miscellaneous, Collections, 121(7), 1–150. Loeblich, A.  R., & Tappan, H. (1987). Foraminiferal genera and their classification (970 pp). New York: Van Nostrand Reinhold Compagny. Loeblich, A.  R., & Tappan, H. (1992). Present status of foraminiferal classification. In Y. Takayanagi, & T. Saito (Eds), Studies in Benthic foraminifera (pp. 93–102). Proceedings of the Fourth Symposium on benthic foraminifera, Sendai, 1990 (Benthos ‘90). Tokyo: Tokai University Press. Lutze, G. F. (1965). Zur Foraminiferen-Fauna der Ostsee. Meyniana, 15, 75–147. Mackensen, A. (1987). Bentishe Foraminiferen auf dem Island-Scottland Rücken: Umwelt-­ Anzeiger an der Grenze zweier ozeanischer Räume. Paläontologische Zeitschrift, 61, 14–176. Madsen, H. B., & Knudsen, K. L. (1994). Recent foraminifera in shelf sediments of the Scoresby Sund fjord, East Greenland. Boreas, 23, 495–504. Möller, P., Federov, G., Seidenkrantz, M.-S., & Sparrenbom, C. (2008). Glacial history of the Cape Chelyuskin area, Arctic Russia. Polar Research, 27, 222–248. Montagu, G. (1803). Testacea Brittanica, or, Natural History of British shells, marine, land, and fresh-water, including the most minute: Systematically arranged and embellished with figures (Vol. 2, 606 pp). Romsey, England: Hollis. Mudie, P. J., Keen, C. E., Hardy, I. A., & Vilks, G. (1984). Multivariate analysis and quantitative paleoecology of benthic foraminifera in surface and Late Quaternary shelf sediments, northern Canada. Marine Micropaleontology, 8, 283–313. Murray, J.  W. (1965). Two species of British recent Foraminiferida. Cushman Foundation for Foraminiferal Research, Contribution, 16, 148–150. Murray, J.  W. (1971). An atlas of British recent foraminifera (244 pp). London: Heinemann Educational Books. Murray, J. W. (1991). Ecology and paleoecology of benthic foraminifera (397 pp). Harlow, Essex: Longmen Scientific and Technical.

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Murray, J. W. (2003). An illustrated guide to the benthic foraminifera of the Hebridean shelf, west of Scotland, with notes on their mode of life. Palaeontologia Electronica, 5(1), 1–31. Murray, J.  W. (2006). Ecology and application of benthic foraminifera (426 pp). Cambridge: Cambridge University Press. Murray, J. W., & Alve, E. (2016). Benthic foraminiferal biogeography in NW European fjords: A baseline for assessing future change. Estuarine, Coastal and Shelf Science, 181, 218–230. Nagy, J. (1965). Foraminifera in some bottom samples from shallow waters in Vestspitsbergen. Norsk Polarinstitutt, Årbok, 1963, 10–128. Nørvang, A. (1945). The zoology of Iceland, Foraminifera 2 (2) (79 pp). Copenhagen/Reykjavik, Iceland: E. Munksgaard. Nørvang, A. (1958). Islandiella n.g. and Cassidulina d’Orbigny. Videnskabelige Meddelelser fra Dansk Naturhistorisk Forening, 120, 25–41. Nyholm, K.-G. (1961). Morphogenesis and biology of the foraminifer Cibicides lobatulus. Zoologiska Bidrag från Uppsala, 33, 157–196. Ogg, J. G., & Smith, A. G. (2004). The geomagnetic polarity time scale. In F. Gradstein, J. Ogg, & A. Smith (Eds.), A geological timescale (pp. 63–86). Cambridge: Cambridge University Press. Osterman, L. E., & Nelson, A. R. (1989). Latest Quaternary and Holocene paleoceanography of eastern Baffin Island continental shelf, Canada: Benthic foraminiferal evidence. Canadian Journal of Earth Sciences, 26, 2236–2248. Pedersen, A. M. (1995a). The Lower Pleistocene in the North Sea, paper 1: Foraminiferal biozonation in the Early Pleistocene in the Central North Sea. Danmarks Geologiske Undersøgelse, Serie C, 13, 1–56. Pedersen, A. M. (1995b). The Lower Pleistocene in the North Sea, paper 2: Pliocene – Middle Pleistocene biostratigraphy in the Central Danish North Sea wells E-1, P-1 and TWB-12. Danmarks Geologiske Undersøgelse, Serie C, 13, 1–28. Polodova, I., Nikulina, A., Schönfeld, J., & Dullo, W.-C. (2009). Recent benthic foraminifera in the Flensburg Fjord (Western Baltic Sea). Journal of Micropaleontology, 28, 131–142. Polyak, L., Korsun, S., Febo, L. A., Stanovoy, V., Khusid, T., Hald, M., et al. (2002). Benthic foraminiferal assemblages from the southern Kara Sea, a river-influenced arctic marine environment. Journal of Foraminiferal Research, 32, 252–273. Reuss, A.  E. (1850). Neue Foraminiferen aus den Schichten des östereichischen Tertiärbeckens. Denkschriften der Kaiserliche Akademie der Wissenschaften, Mathematisch-Naturwissens­ chaftliche Classe, 1, 365–390. Risdal, D. (1964). Foraminiferfaunaens relasjon til dybdeforholdene i Oslofjorden, med en diskusjon av de senkvartære foraminifersoner. Norges Geologiske Undersøkelse, 226, 1–142. Rytter, F., Knudsen, K. L., Seidenkrantz, M.-S., & Eiríksson, J. (2002). Modern distribution of benthic foraminifera on the North Icelandic shelf and slope. Journal of Foraminiferal Research, 32, 217–244. Seidenkrantz, M.-S. (1992). Plio-Pleistocene paleoecology and stratigraphy in the northernmost Norrth Sea. Journal of Foraminiferal Research, 22, 363–378. Seidenkrantz, M.-S. (1995). Cassidulina teretis Tappan and Cassidulina neoteretis new species (Foraminifera): Stratigraphic markers for deep sea and outer shelf areas. Journal of Foraminiferal Research, 14, 145–157. Seidenkrantz, M.-S. (2013). Benthic foraminifera as palaeo sea-ice indicators in the subarctic realm – examples from the Labrador Sea - Baffin Bay region. Quaternary Science Reviews, 79, 135–144. Sejrup, H.  P., Aarseth, I., Ellingsen, K.  L., Reither, E., Jansen, E., Løvlie, R., et  al. (1987). Quaternary stratigraphy of the Fladen area, Central North Sea: A multidisciplinary study. Journal of Quaternary Science, 2, 35–58. Sejrup, H. P., & Guilbault, J.-P. (1980). Cassidulina reniforme and C. obtusa (Foraminifera), taxonomy, distribution and ecology. Sarsia, 65, 79–85. Sen Gupta, B. K. (1999). Modern Foraminifera (371 pp). Dordrecht: Kluwer. Símonarson, L. A., & Eiríksson, J. (2020). Systematic overview of the Molluscs and Barnacles of the Quaternary Breiðavík Group on Tjörnes. In J. Eiríksson & L. A. Símonarson (Eds.), Pacific – Atlantic Mollusc migration. Cham, Switzerland: Springer. This Volume, Chapter 11.

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Steinsund, P.  I., Polyak, L., Hald, M., Mikhailov, V., & Korsun, S. (1994). Distribution of Calcareous Benthic Foraminifera in Recent Sediments of the Barents and Kara Seas. In P. I. Steinsund (Ed.), Benthic foraminifera in surface sediments of the Barents and Kara seas: modern and late Quaternary applications (pp. 61–102). PhD thesis, University of Tromsø, Tromsø. Tappan, H. (1951). Northern Alaska index foraminifera. Cushman Foundation for Foraminiferal Research, Contribution, 2, 1–8. ten Dam, A., & Reinhold, T. (1942). Die stratigraphische Gliederung des niederländischen Oligo-­ Miozäns nach Foraminiferen (mit Ausnahme von Süd Limburg). Mededeelingen van der Geologische Stichtung, Ser C-V, 2, 1–106. Todd, R., & Low, D. (1967). Recent foraminifera from the Gulf of Alaska and southeastern Alaska. United States Geological Survey, Professional Paper, 573-A, 1–46. van Voorthuysen, J.  H. (1949). Foraminifera of the Icenian (Oldest marine Pleistocene) of the Netherlands. Verhandelingen van het Nederlandsch Geologisch-Mijnbouwkundig Genootschap, Geologische Serie, 15, 63–69. van Voorthuysen, J. H. (1950a). The quantitative distribution of the Plio-Pleistocene foraminifera of a boring at the Hague (Netherlands). Mededelingen van de Geologische Stichting, Neuwe Serie, 4, 31–49. van Voorthuysen, J.  H. (1950b). The quantitative distribution of the Pleistocene, Pliocene and Miocene foraminifera of Boring Zaandam (Netherlands). Mededelingen van de Geologische Stichting, Neuwe Serie, 4, 51–72. van Voorthuysen, J. H. (1957). Foraminiferen aus dem Eemian (Riss-Würm-Interglazial) in den Bohrung Amersfoort I (Locus Typicus). Mededelingen van de Geologische Stichting, Nieuwe Serie, 11, 27–38. Verhoeven, K., Louwye, S., Eiríksson, J., & De Schepper, S. (2011). A new age model for the Pliocene-Pleistocene Tjörnes section on Iceland: Its implication for the timing of North AtlanticPacific paleoceanographic pathways. Paleogeography, Paleoclimatology, Paleoecology, 309, 33–52. Vilhjálmsson, M. (1985). The lower Pleistocene mollusc fauna of the Breiðavík Beds, Tjörnes, North Iceland. MSc thesis, University of Copenhagen, Copenhagen. 207 pp. Wagener, M. (1988). Quartäre und rezente benthische Foraminiferen der Island-Färöer-Schwelle. Facies, 19, 97–127. Wagner, F. J. E. (1968). Faunal study, Hudson Bay and Tyrell Sea. In P. J. Hood (Ed.), Earth science symposium on Hudson Bay (pp. 7–48). Geological Survey of Canada, Paper, 68–53. Walker, G., & Boys, W. (1784). Testacea minuta rariora . . . A collection of the minute and rare shells, lately discovered in the sand of the sea shore near Sandwich (25 pp). London: J. March. Walker, G., & Jacob, E. (1798). A description and arrangement of minute and rare shells. In G.  Adams (Ed.), Essays on the Microscope. 2nd Edition, with considerable additions and improvements by F. Kanmacher (pp. 633–645). London: Dillon and Keating. Weiss, L. (1954). Foraminifera and origin of the Gardiners Clay (Pleistocene), eastern Long Island, New York. United States Geological Survey, Professional Paper, 254-G, 143–163. Williamson, W.  C. (1858). On the recent foraminifera of Great Britain (107 pp). London: Ray Society Publications.

Plate 12.1  Light microscope multifocus images of specimens from the Svarthamar Member of the Þrengingar Formation. 1. Quinqueloculina seminula (Linné), from sample 1415 (unit 12x). 2. Quinqueloculina stalkeri Loeblich and Tappan, from sample 1401 (unit 10). 3–4. Cassidulina limbata Cushman and Hughes, from sample 1399 (unit 10). 5. Cassidulina reniforme Nørvang, from sample 1399 (unit 10). 6–7. Cassidulina teretis Tappan, from sample 1399 (unit 10). 8–9. Islandiella helenae Feyling-Hanssen and Buzas, from samples 1401 (unit 10) and 1400 (unit 12x), respectively. 10. Islandiella inflata (Gudina), from sample 1415 (unit 12x). 11–12. Stainforthia feylingi Knudsen and Seidenkrantz, from sample 1401 (unit 10). 13–16. Cibicides lobatulus (Walker and Jacob), from sample 1415 (unit 12x). Scale bar = 0.1 mm. Lithological units refer to Bárðarson (1925)

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Plate 12.2  Light microscope multifocus images of specimens from the Svarthamar Member of the Þrengingar Formation. 1–2. Astrononion gallowayi Loeblich and Tappan, from sample 1415

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Plate 12.2 (continued) (unit 12x). 3. Haynesina orbiculare (Brady), from sample 1415 (unit 12x). 4. Nonionella pulchella Hada, from sample 1415 (unit 12x). 5–6. Buccella frigida (Cushman), from sample 1399 (unit 10). 7–8. Elphidium albiumbilicatum Weiss, from sample 1415 (unit 12x). 9–11. Elphidium clavatum (Cushman), from samples 1415 (unit 12x), 1401 (unit 10), and 1091 (unit 12), respectively. 12. Elphidium hallandense Brotzen, from sample 1415 (unit 12x). 13–14. Elphidium karenae Ásbjörnsdóttir, from sample 1413 (unit 12). 15. Elphidium magellanicum Heron-Allen and Earland, from sample 1401 (unit 10). 16. Elphidium margaritaceum Cushman, from sample 1414 (unit 12). Scale bar = 0.1 mm. Lithological units refer to Bárðarson (1925)

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Plate 12.3  Scanning electron micrographs (SEM) of specimens from the Svarthamar Member of the Þrengingar Formation (modified from Eiríksson et  al., 1993, with updated taxonomy).

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Plate 12.3 (continued)  1. Elphidium karenae Ásbjörnsdóttir, from sample 1413 (unit 12). 2. Elphidium clavatum (Cushman), from sample 1191 (unit 12). 3. Elphidium clavatum (Cushman), from sample 1092 (unit 12). 4. Elphidium hallandense Brotzen, from sample 1092 (unit 12). 5. Elphidium albiumbilicatum Weiss, from sample 1399 (unit 10). 6. Elphidium margaritaceum Cushman, from sample 1415 (unit 12x). 7–8. Cibicides lobatulus (Walker and Jacob), from sample 1413 (unit 12). 9–11. Nonionella pulchella Hada, from sample 1415 (unit 12x). 12. Stainforthia feylingi Knudsen and Seidenkrantz, from sample 1401 (unit 10). 13. Quinqueloculina seminula (Linné), from sample 1415 (unit 12x). 14. Quinqueloculina stalkeri Loeblich and Tappan, from sample 1401 (unit 10). 15. Astrononion gallowayi Loeblich and Tappan, from sample 1415 (unit 12x). 16. Cornuspira involvens (Reuss), from sample 1415 (unit 12x). Scale bar  =  0.1  mm. Lithological units refer to Bárðarson (1925)

Chapter 13

Reconstructing the Paleoenvironments of the Quaternary Tjörnes Basin, North Iceland Jón Eiríksson, Karen Luise Knudsen, and Leifur A. Símonarson

Abstract  Environmental history of the Breiðavík Group on Tjörnes, North Iceland, is reconstructed based on sedimentary facies, foraminifera, and molluscs. The lithological record is characterized by repeated diamictites reflecting advances of glacial ice over the region, and during the early Pleistocene, accumulation of volcanics and sediments deposited close to sea level. The Breiðavík Group contains a unique record of Cainozoic glacier variations in the North Atlantic reflected in lithological variations where marine and terrestrial sediments are intercalated between lava flows and pyroclastic rocks. As a detailed example of the development from a full glaciation through deglaciation and interglacial conditions, we present the 1.5 Ma old Svarthamar Member which shows a lithological cycle from glacial to proglacial and then to shallow marine sedimentation coinciding with a faunal succession reflecting a change from arctic to boreal-arctic or even boreal conditions in the sea north of Iceland. The age model for the sequence indicates that this occurred just after the Olduvai event of the Matuyama chron. The amplitude of this climatic cycle is comparable to the Late Pleistocene glacial-interglacial cycles in the North Atlantic. Most of the upper part of the Breiðavík Group accumulated in a terrestrial environment. Keywords  Glacial-interglacial sedimentary facies · Shallow marine faunas · Transgressions-regressions · Quaternary sea level variability · Quaternary molluscs · Quaternary foraminfera

J. Eiríksson (*) · L. A. Símonarson Institute of Earth Sciences, University of Iceland, Reykjavík, Iceland e-mail: [email protected]; [email protected] K. L. Knudsen Department of Geoscience, Aarhus University, Aarhus, Denmark e-mail: [email protected] © The Author(s) 2021 J. Eiríksson, L. A. Símonarson (eds.), Pacific - Atlantic Mollusc Migration, Topics in Geobiology 52, https://doi.org/10.1007/978-3-030-59663-7_13

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13.1  Introduction The present chapter deals with environmental changes in North Iceland during the Quaternary ice age. This was a period of dramatic environmental changes in Iceland with periodic growth of glaciers in the central highlands to form an ice sheet repeatedly extending beyond the coastline into the North Atlantic Ocean (Eiríksson, 2008; Geirsdóttir, 2011; Geirsdóttir & Eiríksson, 1994). The Early Pleistocene geological data archive preserved in the Tjörnes sequence, North Iceland, is unique in Iceland as it contains both volcanics and marine fossiliferous sediments. The sedimentary and volcanic facies enable us to reconstruct significant parts of the history of glacial-­ interglacial cyclicity and sea level variability in the region, and the fossil material carries a paleoceanographic signal which is a key factor in evaluating the scale of climate variability in Iceland during the first 1.5 million years of the Pleistocene. The age model for the record is based on available radiometric dates of volcanics (Albertsson, 1976, 1978; Camps et  al., 2011) and paleomagnetic stratigraphy (Einarsson et al., 1967; Eiríksson et al., 1990). The geographic location of Iceland close to the boundary between the Arctic and Atlantic climate realms contributes to sensitivity of the Icelandic geological record to climate change. This watchdog location where Iceland – and the Tjörnes sedimentary basin – has been alternating between cold Arctic and milder Atlantic climate came into focus when data on the shift from the last, Weichselian glaciation into the modern, Holocene climate were published (Ruddiman and McIntyre, 1973). One of the purposes of the present study is to use geological data on glaciation history from Tjörnes to evaluate local, terrestrial responses to past climate changes reflected by the stacked global benthic isotope record of the world oceans (Lisiecki & Raymo, 2005, 2007). While the global record does contain information on ice volume changes, it does not contain clues about the geographic location of ice masses. The youngest lithostratigraphic group of the Tjörnes sequence, the Breiðavík Group, was defined by Eiríksson (1981). The group is named after an embayment on the northern tip of Tjörnes Peninsula (Fig. 13.1). The Breiðavík Group is characterized by lithologies which range from basaltic lava flows, volcanic tuffs, and phreatomagmatic eruptives to mudrocks, sandstones, conglomerates, and diamictites. The Breiðavík Group contains a remarkably detailed history of Upper Cainozoic glaciations in North Iceland. A total of 14 glaciations have been recorded and radiometric dates and paleomagnetic correlations indicate that the earliest glaciation took place close to the base of the Quaternary (Eiríksson et al., 2020d). The lower part of the Breiðavík Group displays distinct cyclicity, where a bed of tillite is typically followed by kame conglomerates and glacio-­lacustrine mudrocks and then by marine sediments with fossil faunas, sometimes reflecting deglacial conditions indicating glacial retreat developing into a full interglacial. These deglacial sediments interfinger with and are replaced in the vertical sense by fluvial sediments and lava flows indicating regression and ice-free ­conditions. The upper part of the Breiðavík Group was formed during a phase of tectonic uplift on Tjörnes Peninsula, and the

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Fig. 13.1  Tjörnes Peninsula – Location Map

reduction in the retained record of glaciations may reflect a reduced preservation potential rather than an overall change in the frequency of climatic cyclicity. The molluscan and foraminiferal assemblages found in the Pleistocene Breiðavík Group are quite different from those in the Pliocene Barmur Group of the Tjörnes

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sequence (Eiríksson et al., 2020e). Arctic and subarctic taxa are more frequent, and the faunas have more species in common with Pleistocene faunas than Pliocene assemblages. Numerous thermophilic species found in the Barmur Group have disappeared and the faunal diversities are considerably lower. Extinct species are not as numerous, and the faunas have a more modern appearance and do not deviate significantly from other Pleistocene faunal assemblages in the northern North Atlantic region. Thus, marine microfossil data from the lower part of the Breiðavík Group strongly indicate Pleistocene age and the available stable isotope data from molluscs indicate lower water temperatures than obtained for the Pliocene Barmur Group.

13.2  Review of the Breiðavík Group Paleoenvironments Conflicting views on the age of the Breiðavík Group, in particular the fossiliferous sediments exposed in the Breiðavík embayment, are reviewed by Eiríksson and Símonarson (2020a) and as mentioned, the whole group is now considered to be of Quaternary age. The mollusc fauna of the Breiðavík Group was studied in detail by Vilhjálmsson (1985), who described several species from these beds for the first time. Based on quantitative analyses, Vilhjálmsson grouped the mollusc fauna into five assemblages, four infaunal and one epifaunal assemblage, and the ecological implications of these were discussed. A separate study of one of the Breiðavík Group member units, the Svarthamar Member, included detailed mollusc faunal analyses in an integration of sedimentological, microfossil, and macrofossil data (Eiríksson et al., 1992, 1993). New, detailed taxonomic descriptions of foraminifera and molluscs in the Early Pleistocene of the Breiðavík Group are presented by Knudsen et al. (2020) and Eiríksson and Símonarson (2020), respectively. The present chapter summarizes all these results. The ecological implications of the fauna and integration with lithological data are discussed separately for each formation unit in the following. The ecological and sedimentological zonation of coastal to shallow-marine environments and the terms used in the present chapter are illustrated in Fig. 13.2.

13.2.1  S  edimentary Facies and Mollusc Assemblages of the Furuvík Formation At the base of the oldest lithostratigraphic unit of the Breiðavík Group, the Furugerði Member of the Furuvík Formation (Fig. 13.3), there is evidence of glacial erosion at an unconformity cut across the Höskuldsvík Group, and the lowest sedimentary unit is interpreted as a subglacial traction tillite. This is followed by outwash conglomerates and sandstones, which are widely found resting on the tillite. A glacio-­lacustrine

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Fig. 13.2  Ecological zonation of the shallow marine environment. MHW Mean High Water, MLW Mean Low Water, MFWB Mean Fair Weather Wave Base, MSWB Mean Storm Weather Wave Base

environment followed with dropstones and lenses of ice-rafted coarse material. Subsequently, this evolved into a shoreline environment as the area became submerged beneath sea level by a eustatic transgression. A massive siltstone facies with a mollusc fauna follows. The sparse bivalve fauna found in the siltstone of the Furugerði Member is characteristic for a Mya truncata-Hiatella assemblage. In East Greenland, the young Mya truncata live attached to algae in large numbers, whereas the adults belong to the Arctic Macoma calcarea community of the infauna (Ockelmann, 1958). The few specimens of Mya truncata are here preserved as cores without shell material, but apparently, they were large enough to live infaunally. In East Greenland, the species Hiatella rugosa belongs to the epifauna, the young animals found in large numbers attached to algae in shallow water, but the adults are mainly attached to stones or small irregularities on the sea floor (Ockelmann, 1958). Thus, the faunal assemblage in the Furugerði Member was a mixture of infaunal and epifaunal species. The specimens are not found in the position of life (life assemblage), and most probably they were transported with the seawater into a sheltered area when the sea transgressed the proglacial area. The water depth was apparently only a few metres when the marine unit in the Furugerði Member was deposited. Deposition took place in quiet water, as indicated by the fine-grained and massive structure of the siltstone facies (Eiríksson, 1985; Eiríksson et al., 2020c). No juvenile specimens were found, which indicates deposition outside the tidal zone. During the transgression, saline seawater entered the lake which resulted in increasing salinity. Initially, the conditions were not fully marine, first it was probably oligohaline and finally it may have reached mesohaline conditions. The next sedimentary facies is an upwards coarsening sequence, starting with sandstone and grading into conglomerate. In the eastern part of Furuvík, where the sedimentary sequence disappears below sea level, the conglomerate grades into a

Fig. 13.3  Stratigraphic column with indication of fossiliferous intervals and paleomagnetic time scale. (Ogg & Smith, 2004; modified after Eiríksson et al., 1990)

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trough cross-bedded sandstone facies. At Stangarhorn (Fig. 13.1), where the top of this sequence crops out again, a non-fossiliferous laminated siltstone facies, interpreted as lacustrine sediments, is seen below a conglomerate with sandstone lenses at the top of the sedimentary sequence. These sedimentary facies are taken to indicate an initial intensification of currents, and subsequently, the sedimentary structures and the presence of lacustrine sediments indicate a regression, probably due to an emergence of the area above sea level. The upwards coarsening trend of the upper part of the sequence is thus thought to reflect a regressional environment, and the development of an alluvial coastal plain studded with lakes. The sediments were eventually covered by a lava flow which is seen in both the Furuvík and Stangarhorn sections. The lava flow is columnar jointed and forms the topmost stratum of the Furugerði Member (Fig. 13.3). It completes the first cycle of the Breiðavík Group. The cycle started with a glaciation of the area followed by deglaciation, marine transgression, regression, and finally, development of an alluvial/lava plain during ice-free conditions. The upper unit of the Furuvík Formation the Miðnef Member (Fig. 13.3) features a tillite above the Furugerði lava which has clearly been smoothed and eroded. The tillite most probably indicates a second glaciation of the area and is followed by a series of thin lava flows in Furuvík, indicating an ice-free, terrestrial environment during emplacement.

13.2.2  S  edimentary Facies, Mollusc Assemblages, and Foraminifera of the Hörgi Formation The lowest stratum of the Hörgi Formation (Fig. 13.3) is a diamictite which rests upon a glacially striated erosional surface. In the coastal section between Hörgi and Breiðavík, this surface cuts across rocks of the Furuvík Formation. In the mountains Grasafjall and Búrfell (Fig. 13.1), it cuts across the Miocene Kaldakvísl Group (cf. Eiríksson et al., 2020e). In both cases, the erosional substratum reveals a shallow valley topography, which coincides with a tectonic graben structure (Eiríksson et al., 2020c). The basal diamictite reflects a glacier advancing across the Miðnef Member lava plain, initiating valley erosion and leaving northward-trending glacial striae and subglacial traction till on the erosional unconformity at the base of the Hörgi Formation (Figs. 13.3 and 13.4). As the glacier retreats, masses of ice stagnate and supraglacial outwash sediments accumulate in meltwater channels and pools. Lobes of debris slump down the snout of the glacier and to and fro on the dead-ice topography. Mud is locally and intermittently deposited adjacent to or along with the outwash sediments in pools of standing water. Melting of ice beneath the supraglacial sediments and removal of lateral support leads to sagging, faulting, and deformation of the sedimentary bodies. The glacier continues to recede and the buried ice gradually melts away. Kettle holes and depressions in the kame topography form traps for mud, initially carried by meltwater streams.

Fig. 13.4  Composite lithostratigraphic column for the lower part of the Breiðavík Group. The unit column shows unit numbers used by Bárðarson (1925), and bed numbers refer to lithostratigraphic units for the respective members and formations (cf. Eiríksson et al., 2020c)

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A transgression now submerges the area in the wake of the retreating glacier. Deposition of silt continues, however, containing an infaunal mollusc assemblage characterized by Portlandia arctica and several Yoldiella species (upper part of Bárðarson’s (1925) unit 2; Fig. 13.4). The only epifaunal species found is the small pectinid Similipecten greenlandicus. The shallow burrowing bivalves are almost all with united valves and in the position of life. Furthermore, size-frequency distribution of 25 measurable specimens with united valves from one sample in the lower part of the Hörgi Formation indicates a low death rate among juveniles (Vilhjálmsson, 1985). Another size-frequency distribution of 26 specimens of Yoldiella lenticula with paired valves from the lowermost part of the Hörgi Formation is very similar to that observed for Portlandia arctica (Vilhjálmsson, 1985). The species were probably living in sheltered and quiet environment where the juveniles had rather good possibilities to survive. Low mortality among juvenile animals can certainly help the larvae to settle successfully and to form a life assemblages (biocoenosis) preserved in the sediment (Fig. 13.5). The coastal outcrops of the kame topography and adjacent kettle depressions show an elevation difference of over 40 m. Conglomerate lenses extending from the kame conglomerates into and interfingering with the kettle hole mud rocks (Fig. 13.6) contain more epifaunal species such as Erginus rubellus, Mytilus edulis, and Balanus balanus. They were apparently living on the sides of the kames in the Fig. 13.5 Size-frequency distribution of 25 measureable specimens of Portlandia arctica (Gray) in the Hörgi Formation in Breiðavík

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close vicinity and washed downslope during erosional events. Portlandia, Yoldiella or Similipecten have never been found in these conglomerates. As a result of the downslope transport towards the silty bottom away from the kames, the shells are generally crushed, not in the position of life and the bivalves generally disarticulated. This strongly indicates a death assemblage (thanatocoenosis). Judging from the mollusc fauna, the minimal water depths vary from 0 m (several species) to 9 m (Thracia septentrionalis). The presence of Portlandia arctica and Yoldiella intermedia in the mudrocks indicates a shallow sublittoral environment and water depths exceeding 9 m. In East Greenland, P. arctica prefers depths between 10 and 50 m, and in the Jørgen Brønlund Fjord area, North Greenland, it has been found living at

Fig. 13.6  Conglomerate lenses and mudrocks in depression between kame deposits in the Hörgi Formation. (a) View of the marine mudrocks. (b) Conglomerate lenses with epifauna. (c) Dropstone in inter-kame sediment

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depths between 3 and 18 m (Ockelmann, 1958; Schiøtte, 1989). The mollusc fauna includes species with low-salinity tolerance. The presence of Portlandia arctica in the mudrock strongly indicates reduced salinity as the species is mainly found off the mouth of rivers and glacial fronts where large quantities of mud and silt are deposited (Lubinsky, 1980; Ockelmann, 1958). Further indications are two δ13C analyses made on Macoma calcarea shells with very low values, −0.91 and −0.46, respectively (Vilhjálmsson, 1985). They indicate strongly reduced salinity and mesohaline conditions. Boulders and pebbles of plutonic and metamorphic composition are found in the conglomerate lenses indicating release from icebergs or sea-­ ice rafting material from distant petrographic provinces. The brackish, silty facies is covered by a sandstone with conglomerate lenses and marine fossils of a fauna that has been reworked, but it does not contain the infaunal assemblage species of the mud rocks below. Reworking of the upper regions of kames is intensified, and the contact with the marine mud beneath is erosional. The coarsegrained, cross-beds, and fossils of the sandstone are compatible with a bar environment, initially transgressing over the dead-ice topography, but later receding again. Landwards of the shoreline, an alluvial coastal plain was characterized by fluvial gravels and lacustrine sediments. A tuff horizon is preserved in the lacustrine sediments but is not found elsewhere. A regressional sequence is reflected in the coastal outcrops in Breiðavík by the disappearance of marine fossils and deposition of flat-bedded sandstones with channels and lag gravels. Subsequently, a lava flows across the plain (Fig. 13.4). It is at present found exposed at Hörgi, in the Fossgil and Tröllagil gullies, and in Búrfell (Fig. 13.1), and does not indicate eruption or emplacement in contact with water or ice. To summarize, the interpretation of the Hörgi sequence indicates a new cycle of glaciation, deglaciation, and transgression. A prograding alluvial plain environment with subaerial lava flows during interglacial conditions then completes the cycle.

13.2.3  S  edimentary Facies, Mollusc Assemblages, and Foraminifera of the Þrengingar Formation Outcrops along the coast of central and eastern Breiðavík Bay between Fossgil and Höfðaskarð and sections along the brook gullies Bæjargil, Tröllagil, and Fossgil expose fossiliferous sediments belonging to the Þrengingar Formation (Figs. 13.1 and 13.4). The Þrengingar Formation corresponds approximately to Bárðarson’s (1925) units 3–12, but it contains strata omitted by Bárðarson and additional strata from inland exposures (Eiríksson et al., 2020c). The formation is divided into two member units, the Fossgil Member and the Svarthamar Member (Fig. 13.4).

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13.2.3.1  S  edimentary Facies and Mollusc Assemblages of the Fossgil Member The coastal section in Breiðavík Bay shows an erosional unconformity forming a fairly steep slope dipping towards east. The unconformity cuts off the Hörgi Formation laterally. A renewed glaciation of the Tjörnes area, probably coupled with a lowering of sea level, is indicated by a tillite bed and glacial striae at the base of the Fossgil Member (Fig. 13.4). An extensive proglacial lake is then indicated by laminated to thinly bedded mudrocks with erratics, conformably overlying the uneven substratum. This is considered to reflect a proglacial environment in the wake of a retreating glacier. The glacier retreats further, and sea level rises gradually. The lake becomes saline, and a massive mudrock (Bárðarson’s (1925) unit 4) with marine fossils was deposited (Eiríksson, 1985; Eiríksson et al., 2020c). The lowermost marine part of the member was inhabited by a Portlandia arcticaYoldiella assemblage similar to the one in the lowermost part of the Hörgi Formation. The bivalves are generally with united valves, in the position of life, and their distribution in the sediment strongly indicates a life assemblage (biocoenosis). The infaunal assemblage in the mudrock facies is a low-diversity one and contains arctic molluscs such as Portlandia arctica, which indicates low sea temperature (for modern faunal provinces, see Fig.  13.7). In East Greenland, Portlandia arctica lives mainly associated with the Polar Current water at sea temperatures from 0 to −1.7 °C, but sometimes it is also associated with the Fjord water with slightly higher temperatures (Ockelmann, 1958; Ussing, 1934). In the White Sea, there is an isolated stock of P. arctica, most probably preserved as a relict from the last glacial stage. There, it lives in water with temperature from 0.5 to −1.4 °C, except in an isolated locality in the western part where the temperature in July is 3.6 °C at a depth of 23 m (Jensen, 1942). When the low water depth of the mudrock facies is taken into consideration, it is tempting to suggest that the sea temperature was closer to 0 °C than the highest level in the White Sea. The δ18O value found in Macoma calcarea from the Fossil Member is +2.19, which indicates higher temperature (Vilhjálmsson, 1985). However, low salinity may have affected the oxygen analyses to show too high temperature. The fossil assemblages include species with low-salinity tolerance, and the presence of Portlandia arctica strongly indicates mesohaline conditions. This is further indicated by a δ13C analysis on shell of Macoma calcarea from Tröllagil, which showed a value of −0.68 (Vilhjálmsson, 1985). The sedimentation was then interrupted by multiple tuff layers found in the middle part of the Fossgil Member (unit 5, Fig. 13.4). The tuff material was probably erupted by explosive volcanism during shallow-water submarine eruptions (Eiríksson, 1985). The thick-bedded tuffs with siltstone interbeds were apparently deposited in the sea as indicated by marine molluscs. This part of the Fossgil Member is characterized by a Macoma calcarea assemblage.

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Fig. 13.7  Biogeographical zones for the northern North Atlantic. (Modified after Dinter, 2001; Feyling-Hanssen, 1955; Funder et al., 2002)

The marine, low-salinity mud facies is followed by a sand facies. The contact is gradational, but locally there is evidence of reworking, which has penetrated into the tuff beds. An increase in current velocity or a change of water depth is inferred. Locally, the tuff layers have been reworked and contain a death assemblage (thanatocoenosis). A transgressing bar environment is reflected by coarse sand with marine fossils, which encroaches upon the lagoon mud as sea level continues to rise. The infaunal assemblage found in the sandstone facies of the Fossgil Member is dominated by Macoma calcarea, Mya truncata, and Serripes groenlandicus, whereas Portlandia arctica has disappeared. This indicates rise in summer sea temperature, which probably became as high as 3–4 °C in the area and which probably was combined with increasing salinity. The sea level rise eventually comes to a halt and the situation becomes reversed. An alluvial coastal plain environment progrades over the bar sands. Lake sediments, flat-bedded sands and channel sequences involving gravels and sands characterize the environment at the top of the Fossgil Member. Two lava flows now reach the area and cover the sediments at Fossgil (Fig. 13.4).

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13.2.3.2  S  edimentary Facies, Mollusc Assemblages, and Foraminifera of the Svarthamar Member The sedimentary facies and faunas of the Svarthamar are discussed in detail below under a separate heading (Sect. 13.3) and will therefore only be outlined briefly here. Further details of the stratigraphy and steps in the environmental changes are presented by Eiríksson et al. (2020c). A discontinuous diamictite resting on an erosional unconformity at Fossgil marks the base of the Svarthamar Member of the Þrengingar Formation in the coastal section in the Breiðavík Bay (Fig. 13.1). Immediately above the diamictite, which is interpreted as a subglacial traction tillite, there is a thick conglomerate (Fig. 13.4). In the coastal section east of Fossgil, the conglomerate becomes finer-­ grained towards the top, and sand and siltstone lenses increase in volume. This finer-grained unit becomes fossiliferous east of Bæjargil until it disappears beneath sea level. It reappears in the vicinity of the Svarthamar cliff and becomes increasingly silty towards the top, which is sometimes marked by a tuff bed (although silt continued to accumulate after the eruption of the tuff). Elsewhere, the unit is marked by an erosional unconformity, and its thickness is variable. This is reflected by the undulating attitude of the tuff bed. A second erosional surface indicates that the tuff layer has been swept away by an erosional event that has penetrated into the fine-­ grained, fossiliferous unit at Svarthamar, and locally along the Bæjargil. The topography, which developed during the continued accumulation of marine mudrocks, was eventually levelled by the continuing accumulation of the marine muds. A thin, discontinuous tuff band occurs in this mudstone both in Breiðavík and Bæjargil. Four almost vertical sandstone dykes cut through the mudrock units. The facies sequence of the Svarthamar Member starts with an erosional surface with glacial striae, subglacial traction till, and kames. These are the products of a glacier which advanced over the Tjörnes area and receded again, leaving stagnant ice with accumulation of supraglacial outwash sediments followed by a transgression. A delta begins to form at the shoreline, but due to the rise of the sea level, an upwards fining sequence is formed. Marine fauna colonizes distal parts of the delta slope and the prodelta. In the lowermost marine part of the Svarthamar Member (Bárðarson’s (1925) unit 8), there is a low-diversity infauna with Portlandia arctica and Macoma calcarea as dominating species indicating reduced salinity and mesohaline conditions and similar water depth relations as found in the lowermost part of the Hörgi Formation and the Fossgil Member. The mudrock facies in the middle part of the Svarthamar Member corresponding to unit 10 of Bárðarson (1925: Fig. 13.4) contains marine fossils, as well as erratics. The mollusc assemblage is dominated by the infaunal bivalves Macoma calcarea and Ennucula tenuis. These species, together with the Buccinum-Lepeta caeca epifauna, indicate that not only the temperature was increasing but the salinity as well. However, the Buccinum-Lepeta caeca epifauna with low diversity seems to emphasize that the sea temperature has hardly reached the present temperature in the area, but it may have been 3–4 °C.

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The mudrock facies (unit 10) is characterized by the Islandiella helenae-­ Cassidulina teretis foraminiferal zone. The dominant foraminiferal species are the Elphidium excavatum, E. albiumbilicatum, and E. magellanicum. The assemblages indicate that there must have been a relatively good connection to the open ocean (see details below), a water depth of presumably more than 25 m, and an arctic to boreal-arctic marine environment similar to present-day cold-water conditions in the northernmost Atlantic (cf. Fig. 13.7). The uppermost unit of the Svarthamar Member, unit 12 according to Bárðarson (1925), consists of a fossiliferous sandstone facies. The Arctica islandica-­Cyrtodaria angusta infauna and the Mytilus edulis-Nucella lapillus epifauna are of considerably higher diversity than those in underlying layers of the formation. The faunal assemblages are the most boreal in character of all assemblages found in the Breiðavík Group. It is concluded that the sea temperature was at least as high as the present one in the Breiðavík area and the salinity also rose considerably. The trough-cross-bedded sandstone facies (unit 12) contains a Cibicides lobatulus-­Elphidium hallandense foraminiferal assemblage and is generally rich both in species and in specimens. Cibicides lobatulus is the dominant species throughout unit 12, and Elphidium excavatum, E. albiumbilicatum, and E. hallandense are frequent. The assemblages indicate ameliorated temperature conditions, that is, a boreal-arctic or even a boreal environment (see details below), corresponding to temperature conditions even warmer than found on the south coasts of Iceland today, and a relatively shallow water depth, which is shallowing further towards the top of the zone.

13.2.4  S  edimentary Facies, Mollusc Assemblages, and Foraminifera of the Máná Formation Two units of marine fossiliferous sandstone are exposed in the Breiðavík coastal section to the north of Höfðaskarð (Fig. 13.1). The stratigraphically lower unit is exposed in Stapavík and rests on an erosional contact. The two marine units are laterally separated by a conglomerate, but both are overlain by the same reversed polarity pillowy lava flow (Figs. 13.3 and 13.4). The Stapavík marine unit is truncated by the unconformity beneath the Torfhóll Member units (Eiríksson et  al., 2020c). Consequently, the Stapavík Member contains sedimentary facies and fauna that represent a separate transgression. The sedimentary facies and the mollusc fauna also indicate different environmental conditions for these two units. In earlier stratigraphical nomenclature and classification (e.g., Einarsson et al., 1967), the so-called Breiðavík beds were defined to reach up to reversely magnetized lava flows at Voladalstorfa (unit 15, Bárðarson, 1925). These lava flows are placed within the Máná Formation in the present study, and they constitute the top of the Torfhóll Member, which then contains Bárðarson’s units 13–15 (Eiríksson et al., 2020c). The Stapavík Member units were not recognized as separate units by these authors.

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13.2.4.1  S  edimentary Facies and Mollusc Assemblages of the Stapavík Member The base of the Stapavík Member is defined by a diamictite bed resting on an erosional unconformity exposed in Torfhóll and reaching sea level at Skarfaflös in Stapavík (Fig. 13.1). The diamictite is overlain by a thick conglomerate (Fig. 13.4). The internal structure of the conglomerate (severe deformation and irregular dip directions) leads to the interpretation as kames, formed during the retreat phase of a glaciation causing erosion and deposition of a subglacial traction tillite (Eiríksson, 1985). The conglomerate is followed by lenses of lacustrine mud with erratics and then by cross-bedded sandstones and conglomerates, both facies with marine fauna. The marine molluscs occur both in the sandstone and interbedded siltstone beds, with an Arctica islandica-Macoma calcarea infaunal assemblage and a Chlamys breidavikensis-Balanus balanus epifaunal assemblage, but the faunal diversity is rather low. A tuff bed is intercalated in the marine sequence. This reflects transgression and a shallow-marine, high-energy environment. A series of reversed polarity non-porphyritic basaltic, olivine tholeiitic lava flow units on the northeast coast of Tjörnes Peninsula is tentatively assigned to the Stapavík Member (Eiríksson et al., 2020c). The lavas do not show any signs of contact with water, and their origin and emplacement are interpreted as reflecting subaerial effusive volcanism in an ice-free environment. 13.2.4.2  S  edimentary Facies, Mollusc Assemblages, and Foraminifera of the Torfhóll Member The Torfhóll Member rocks rest on an erosional surface across the Stapavík and Svarthamar Members in Breiðavík (Figs. 13.1 and 13.4). A cross-bedded conglomerate is the lowest facies, followed by mudrocks. They contain marine fossils in the upper part and then grade into a cross-bedded, very fine-grained sandstone which is predominantly made up of volcanic tuff. The Torfhóll Member sediments are overlain by a pillowy lava flow, the so-called Máná basalt. On the northeast side of Tjörnes (200 m south of Skarfaflös, cf. Eiríksson, 1981), a subglacial traction tillite is found intercalated between an erosional unconformity below and the Máná basalts above. This tillite forms the lowest unit of the Torfhóll Member. The Torfhóll facies sequence reflects a glacier covering the area once again, depositing subglacial traction till upon an erosional surface. The glacier then retreats leaving a proglacial outwash plain. This plain is subsequently submerged by a proglacial lake, in which poorly sorted mud accumulates along with erratics dropped from icebergs calved off the glacier margin. Local deformation of the cross-bedded outwash gravels is taken to indicate a temporary readvance of the glacier front, or it might have been caused by scouring icebergs. The appearance of marine fauna in the mud facies coincides with a marine transgression in the wake of glacier retreat and development of a brackish lagoon environment (unit 14, Fig.  13.4). The mudrock contains infaunal molluscs belonging to the Macoma calcarea-­

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Ciliatocardium ciliatum assemblage. The faunal diversity is very low and no epifaunal species were found. The sediment is generally massive, but faintly laminated near the base and frequently with erratics. About 70% of the specimens of M. calcarea have united valves, and the bivalve shells are generally uncrushed, but they have not been found in the position of life. Therefore, they seem slightly reworked and most probably represent a death assemblage. A transgressing bar environment with increased wave action and current velocities then brings cross-bedded sands with marine shells across the lagoon mud. The bulk of the sandstone consists almost entirely of volcanic glass, which is extremely hard and breaks into conchoidal fragments. Apparently, this unit features the youngest unit (LAD) with Cyrtodaria angusta, well known from the Barmur Group of the Tjörnes sequence. The faunal diversity in the Torfhóll Member is considerably lower than in the Svarthamar Member. However, there are no arctic taxa such as Portlandia arctica in the assemblages, and species like Arctica islandica and C. angusta indicate seawater temperatures almost as high as the present one in the area. Further indications are from two oxygen isotope analyses of bivalves from Torfhóll. Macoma calcarea showed δ18O value of +3.35 and Serripes groenlandicus + 3.04 and 2.53 (Vilhjálmsson, 1985: Table 1). The salinity was probably somewhat reduced as indicated by three δ13C carbon isotope analyses from Torfhóll (δ13C 0.35, δ13C − 1.25, δ13C, 0.58). Only very few shallow-water foraminifera were preserved in the Torfhóll Member (unit 14) but not enough for a precise environmental interpretation (Knudsen et al., 2020). Landwards of the shoreline, the coastal plain became covered by numerous lava flows, extending as pillow lavas into the tidal and subtidal regions. The Torfhóll Member rocks crop out in several sections on the east coast of Tjörnes, and these sections were analysed and interpreted by Birgisdóttir (1984). She identified a sedimentary horizon intercalated between the lavas of the Stapavík and Torfhóll Members (‘Sedimentschicht I’) as a subglacial traction tillite, ice-contact and glaciofluvial deposits, and fluvial deposits. 13.2.4.3  S  edimentary and Volcanic Facies of the Dimmidalur and Búrfellsá Members The geological record of the remaining member units of the Máná Formation, above the reversed polarity Máná basalts of the Torfhóll Member (Fig. 13.3), is much simpler than below. The most complete outcrops are located in mountains on central Tjörnes, Búrfell, and Grasafjall, and the lithologies consist of extensive units of subaerial lava flows, diamictites, and conglomerates in the Dimmidalur and Búrfellsá Members. Details of correlations and stratigraphic relationships are presented in Eiríksson et al. (2020c). A conglomerate at the base of the Dimmidalur Member is interpreted as a fluvial deposit, perhaps deposited in a proglacial environment in front of an advancing glacier. It is overlain by a diamictite, which is interpreted as a subglacial traction

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tillite indicating glaciation of the region, and lava flows concluding the member reflect ice-free conditions after deglaciation. The lavas of the Dimmidalur Member are truncated by an erosional unconformity indicating a period of erosion, followed by deposition of a subglacial traction tillite at the base of the Búrfellsá Member, and, finally, an ice-free environment and emplacement of several lava flow units. These two cycles of erosion, till deposition, and volcanism in ice-free conditions are evidence of glacier variation. The duration and extent of these variations cannot be determined from the data preserved in the Tjörnes sequence, but the simplest and most straightforward interpretation is that they reflect two glacial-interglacial cycles. According to the current age model (Eiríksson et  al., 2020d), the cycles occurred towards the end of the Matuyama chron (Fig. 13.3). At that time, the plate tectonic spreading axis in Öxarfjörður east of Tjörnes Peninsula had become active after a northward propagation of the axis, probably leading to border uplift of the Tjörnes horst (Eiríksson et al., 2020b). This is probably the reason why transgression-regression cycles associated with isostatic adjustments to variable ice load are not recorded in most units of the upper part of the Quaternary Tjörnes sequence. This part of the exposed sequence was probably formed at elevation exceeding the maximum range of isostatic sea level adjustments.

13.2.5  S  edimentary and Volcanic Facies of the Grasafjall Formation The entire Grasafjall Formation consists of alternating volcanics and glacial sediments. No marine sediments have been identified. There are three levels of diamictites, one in each of the three members, the Miðlækur Member, the Skeiðsöxl Member, and the Bangastaðir Member (Eiríksson et al., 2020c; Fig. 13.3). The Miðlækur Member consists of a diamictite bed interpreted as a subglacial traction tillite followed by normal polarity lava flows reflecting an initial ice cover followed by ice-free conditions and uninhibited flow of effusive lavas. The Skeiðsöxl Member begins with a diamictite that is interpreted as a subglacial traction tillite, which is followed in the sequence by lava flows reflecting ice-free, terrestrial conditions. After a period of glacial erosion of the Skeiðsöxl Member lavas, reflected by an erosional surface, sediments interpreted as a subglacial traction tillite were deposited, and the overlying conglomerates and sandstones are interpreted as proglacial outwash fan deposits. These sedimentary facies belong to the Bangastaðir Member. In central Tjörnes, a subglacial eruption took place during the glaciation, producing a table mountain which forms the upper part of Búrfell Mountain. Two sets of lavas then flowed across the lowland part of the area during subsequent ice-free conditions, separated by a period of deposition of fluvial gravel and sand, possibly eolian sand towards the top.

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13.2.6  S  edimentary and Volcanic Facies, and Mollusc Assemblages of the Húsavík Formation and Younger Deposits A shield volcano named Grjótháls is located in southern Tjörnes to the southeast of Húsavík village (Fig. 13.1), and the lavas formed in the Grjótháls eruption represent an important marker horizon in the area. The lavas belong to the Bakkaá Member of the Húsavík Formation (cf. Eiríksson et al., 2020c). To the north of Húsavík, the Bakkaá river gully exposes a 10 m thick diamictite above and separated from the Miocene Kaldakvisl Lava Group rocks by an erosional unconformity. The diamictite is overlain by the Grjótháls lava, which locally seems to have intruded the diamictite as evidenced by intricate sills and veins. In coastal sections south of Húsavík, the lava is observed as a pillow breccia, locally overlying marine sediments. This indicates that the Grjótháls lava flowed into the sea in that area. The development based on these observations (cf. review and references in Eiríksson et al., 2020c) started with glacial erosion and deposition of the Bakkaá diamictite as a subglacial traction till, followed by deposition of marine mud, and subsequently the area was covered by the extensive Grjótháls lava. The age of the Grjótháls lava is not known, but as it has been subjected to glacial erosion it cannot be younger than the last interglacial (Eemian). It is considered to have flowed into the sea while the relative sea level was some tens of metres higher than at present. Glacial striae, fluted till deposits, and fluvial gravels deposited as deltaic deposits at raised beach levels on Tjörnes Peninsula are evidence of a full glaciation after the Grjótháls eruption took place, and it is overwhelmingly likely that this happened during the Weichselian. Molluscs that probably belong to the last transgression in the Tjörnes area are present at a few localities (Eiríksson et  al., 2020c). At the top of the section in Höfðaskarð in Breiðavík, there are local remnants of sedimentary horizons that discordantly overlie sediments of the Torfhóll Member. The sequence above the unconformity begins with a diamictite with shear planes. Fossils of the burrowing molluscs Mya truncata and Hiatella arctica are found in living positions in the top of the diamictite, which is covered by a sandy conglomerate in which a granite pebble was found. In coastal outcrops in the Húsavík village, Emilsson (1929) reported marine macrofossils of assumed Lateglacial age, and finally, fossils have been collected from unconsolidated sediments (also of ­presumed Lateglacial age) at the mouth of the river Hallbjarnarstaðaá (cf. Eiríksson et al., 2020c). During the Holocene, soil accumulation, delta deposits in the Öxarfjörður and Skjálfandi troughs, as well as post-glacial lava flows both to the west and east of Tjörnes conclude this last member unit of the Tjörnes sequence, the Kelduhverfi Member (Eiríksson et al., 2020c).

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13.3  T  he Svarthamar Member: An Early Pleistocene Glacial-Interglacial Cycle The Svarthamar Member is one of the most detailed glacial-interglacial cycles of the Breiðavík Group. We present an interpretation of the vertical sedimentary facies sequence and marine fossil fauna, comprising glacial erosion, deposition of subglacial sediments, and deformation of the substratum followed by glacial retreat hand in hand with increased explosive volcanism and rising sea level and ice-cold water temperatures. This is followed by regression and more temperate water masses similar to the modern waters around Iceland. The data and results are digested from previously published papers (Eiríksson, 1985; Eiríksson et al., 1992). Details of the stratigraphic relationships and an age model are presented by Eiríksson et al. (2020c, d), and the faunal contents by Símonarson and Eiríksson (2020) and Knudsen et al. (2020).

13.3.1  S  tratigraphy, Sedimentary Facies, Molluscs, and Foraminifera of the Svarthamar Member A detailed lithological column of the sedimentary facies of the Svarthamar Member is shown in Fig. 13.8, where the sedimentary facies and their stratigraphic distribution are tabulated. A comprehensive description and interpretation of the facies sequence and fossils in terms of physical processes, ecology, and stratigraphy is presented below. Erosional unconformity. The Svarthamar Member is separated from the underlying Fossgil Member (Fig. 13.4) by an erosional unconformity which is, for example, seen at the Fossgil Fall headcut (Fig. 13.9). Parallel sets of north-northeast trending linear striations, interpreted as glacial striae, were observed at the unconformity in the coastal section on each side of the Fossgil gully. These are considered to be related to glacial erosion during advance and maximum stage of glaciation. Up to 3 m deep east-west trending fractures extend from the unconformity into the underlying tuff beds. The fractures are filled with diamictite and are probably extensional faults related to shear stress from the overriding glacier. The pre-glaciation bedrock and landscape was characterized by marine sandstones, fluvial deposits, and lava flows (belonging to the Fossgil Member, cf. Eiríksson et  al., 2020c; Fig.  13.4). Glacial erosion penetrated locally into lower units such as tuff beds and marine sands. Massive diamictite (VI:E, Fig. 13.8). The unconformity is locally covered by a unit of massive diamictite with a silty-sandy matrix separating angular to subrounded pebble- to cobble-sized clasts. Many of the cobble-sized clasts originate from the tuff beds below. A fabric analysis of pebble and cobble long axes in the diamictite shows a strong subhorizontal north-northeast trending pole distribution with a minor nearly transverse component. The clast fabric and texture of the diamictite facies, as well as the abundance of angular clasts from the local basement

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Fig. 13.8  Detailed stratigraphic log and facies table for the Svarthamar Member, after Eiriksson et al. (1992). Bárðarson’s (1925) units are shown on the right-hand side of the log. Horizontal lines depict bed boundaries. (Legend, see Fig. 13.4)

mixed with subrounded basaltic clasts, are considered to favour a subglacial traction tillite interpretation (Eiríksson et al., 1992). The erosional unconformity below and the diamictite reflect a full glacial with an ice-cap centre south of Tjörnes, and advancing beyond the coastline of North Iceland. Tabular cross-bedded coarse conglomerate (XIII:A, Fig.  13.8). Locally, the unconformity at the base of the Svarthamar Member is succeeded by a very coarse-­ grained conglomerate with large boulders. The framework is locally open. The ­lowest part of the unit forms a 10 m thick tabular set of north-east dipping crossbeds in the coastal sections. Northeasterly dips up to 28° are observed in Svarthamar and 27° to the east of Fossgil (Fig. 13.1) where the facies are coarsest. Laterally, the conglomerate has an interfingering contact with sandstones and mudrocks. In the nearly vertical coastal section, the facies occurs in two units with different dip directions. The facies is considered to be a fluvial sediment deposited on a proximal delta slope. The dip directions indicate that the sediment transport was towards northeast. Near the top of the section the dips level out, and this is thought to mark the transition to delta platform sedimentation in braided channels. As a whole, the conglomerate facies features a distinct fining upwards sequence (Bárðarson’s (1925) units

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Fig. 13.9  Coastal section A-B in Breiðavík from Fossgil towards Voladalstorfa (view from coast, cf. Fig.  13.1). The inserted numbers 5–13 refer to Bárðarson’s (1925) units (his unit 11 corresponds to unit 6/7, see text). For details of the lithostratigrapy see Eiríksson et al. (2020c)

6/7, Fig. 13.9). The deposition of the delta must therefore have coincided with a relative sea level rise and transgression. The margin of the ice sheet had retreated, and the subglacially formed diamicton is locally washed away by fluvial processes. The newly exposed isostatically depressed proglacial environment is now partly submerged by a eustatic rise of sea level. Graded sandstone and laminated siltstone facies (XIII:B, Fig. 13.8). This facies has a laterally interfingering contact with the coarse conglomerate facies. The ­contact rises towards southeast across the delta slope and platform sediments and shows marked lateral variation. The proximal part is characterized by coarse sandstone with sharp-based conglomerate lenses which grade diffusely upwards into sandstone and laminated siltstone. One bouldery horizon occurs at this level, which becomes distinctly more fine-grained upwards in the section. The distal part is predominantly sandy with sharp-based sand beds which grade upwards into a laminated siltstone facies. Many of the units display soft sediment deformation structures such as recumbent folds, normal faults, and convolute bedding. The structures have clearly formed at the water-sediment interface and indicate rapid sedimentation on a sloping bottom. There is a lateral decrease in overall grain size along the coastal

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section towards northeast from Fossgil. To the east of Bæjargil (Fig. 13.9), the facies is predominantly silty. Fossil molluscs are found in the mudrock, but no foraminifera were found in samples from this unit, which corresponds to Bárðarson’s (1925) unit 8. At Svarthamar, it displays proximal characteristics again. Its top is sometimes marked by a tuff bed (although silt continued to accumulate after the eruption of the tuff, cf. Fig. 13.9). Elsewhere, the top of the unit is marked by an erosional unconformity, and its thickness is variable. This is reflected by the undulating attitude of the tuff bed. Within the graded sandstone and laminated siltstone facies, there are two levels of Fine-grained tuff facies (XIII:C, XIII:G, Fig. 13.8). This is a very fine-grained (silt) black tuff of basaltic composition with molluscs and occurs in two units interbedded in massive, fossiliferous siltstone facies. The facies is interpreted as volcanic ash layers deposited on the prodelta sea bed. The maximum observed thickness is 2 m, and locally there are signs of reworking of the ash. In and close to the lower occurrence of the tuff facies (Bárðarson’s unit 9), there is a low-diversity infauna with Portlandia arctica and Macoma calcarea as dominating species. The bivalves are generally with articulated valves, in the position of life, and their distribution in the sediment strongly indicates a life assemblage (biocoenosis). However, locally the sandstone is deformed and contains slightly reworked life assemblage. The dominating molluscs strongly indicate reduced salinity and mesohaline conditions in an arctic marine environment. Further indication is a δ13C analysis on shell of Serripes groenlandicus from Bæjargil, which showed a value of −1.76 (Vilhjálmsson, 1985). The mollusc species in the lowermost part of the marine Svarthamar Member indicate similar depth relations as found in the lowermost part of the Hörgi Formation and the Fossgil Member, with estimated depth range 3–50 m. One foraminiferal sample collected from unit 9 contained a few badly preserved foraminferal tests. This is presumably a residual fauna and too sparse to give any paleoecological information. The origin of the graded sandstone and laminated siltstone facies is considered to be mixed, the graded units representing deposition from turbidity currents from the proximal delta slope, while the laminated siltstone facies accumulated from settling out of a turbid meltwater wedge off the river mouth. Massive siltstone facies with dropstones (XIII:F, Fig.  13.8). There is no well-­ defined boundary with the graded sandstone and laminated siltstone facies below. The contact appears to be gradational. The facies (Bárðarson’s (1925) unit 10) is not or only very faintly laminated and contains erratic pebbles and sand grains, as well as numerous molluscs and foraminifera. The mud is thought to have settled from suspension in the prodelta region of the delta, where flocculation and bioturbation may have eliminated all signs of fluctuating sediment input. Vigorous slumping may also have liquidized and remobilized some of the sediments, which may have entered the area as turbidity currents. The erratics are interpreted as ice-rafted debris from seasonal ice cover or icebergs. The mollusc assemblage is dominated by the infaunal bivalves Macoma calcarea, Mya truncata, Hiatella rugosa, and Ennucula tenuis. M. calcarea burrows deeper in the sediment than E. tenuis, but generally those species were found in the

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position of life. More than 80% of the specimens were found with articulated valves, and the disarticulated right and left valves were found in almost equal amounts (Vilhjálmsson, 1985). Therefore, this assemblage is considered to represent a life assemblage. The low-diversity faunal assemblage was infaunal, and the most arctic species such as Portlandia arctica had disappeared. Together with the Buccinum-­ Lepeta caeca epifauna, this indicates that not only the temperature was increasing but the salinity as well. Thus, the infauna indicates rise in sea temperature, whereas the Buccinum-Lepeta caeca epifauna with low diversity seems to emphasize that it has hardly reached the present sea temperature in the area, but it may have been 3–4 °C. An oxygen isotope analysis on Serripes groenlandicus from the mudrock in Svarthamar revealed a δ18O value of +2.42 (Vilhjálmsson, 1985). However, as was the case in the Fossgil Member, low salinity may have affected the oxygen analysis to show too high temperature. Foraminifera in this facies were defined as the Islandiella helenae-Cassidulina teretis assemblage by Eiríksson et al. (1992). A characteristic feature, especially in the lower part of the zone, is a relatively high content of the miliolids (mainly of the species Quinqueloculina stalkeri), which indicates close to normal marine salinities (Murray, 1991). The presence of Stainforthia feylingi, Cassidulina reniforme, and Islandiella helenae points to similar environmental conditions in an arctic environment (i.e., Elverhøi et al., 1980; Nagy, 1965; Osterman & Nelson, 1989). There must have been a relatively good connection to the open ocean, and the water depth was presumably more than 25 m. There is a decrease in the amount of Cassidulina reniforme and of miliolids towards the top of the assemblage zone, and a few percentages of the shallower water species Buccella frigida and Cibicides lobatulus appear in the upper fossiliferous sample. In general, the foraminiferal assemblages of unit 10 indicate an arctic to boreal-­ arctic marine environment. The faunal development points to a relatively fast marine transgression followed by a regression towards the top of the zone. Most of the foraminifera are infaunal species indicating low-energy environments (Murray, 1991). To summarize the environmental changes during the deposition of the massive siltstone facies with dropstones (unit 10), the shoreline retreats southwards in the basin, hand in hand with rising sea level. Sedimentation rates are high due to an intensive runoff from the waning glacier. A delta sequence continues to be built out northeastwards into the bay, but as the rise of sea level is relatively rapid, the typically regressional delta model is reversed here, leading to an upwards fining sequence. Marine fauna colonizes distal parts of the delta slope and prodelta. The delta platform sediments are locally very coarse and contain abundant intraformational sedimentary pebbles and boulders. Some of these are fossiliferous and are probably derived from the Tjörnes sedimentary zone (Bárðarson, 1925). The extremely coarse grain size and steep large-scale cross-beds, which occur locally in the platform sediments, and the presence of intraformational angular boulders may indicate that sheet floods took place during the development. This may reflect the existence of valleys in the interior, leading to the formation of ice-dammed lakes as the ice sheet thinned, and to occasional bursts. Active volcanism is manifested in

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extensive basaltic ash layers that were probably erupted in phreatomagmatic eruptions in the sea. Numerous dropstones and the arctic mollusc fauna in the lowest part of the prodelta muds indicate that the sea temperature was initially very low and that ice was still calving into the sea in the vicinity. Foraminiferal assemblages indicate that shortly after the biggest ash fall, the water depth had reached at least 25 m, and that normal salinity had been attained. The mollusc fauna is predominantly infaunal and the arctic assemblage disappears. Gradually, the sedimentation rate is reduced, the mollusc fauna indicates a stiffer substrate, and the uppermost foraminiferal assemblages indicate regression. Low-energy environments prevailed throughout. In distal parts of the delta, marine mud continues to accumulate rapidly in relatively quiet water. The mud is unstable, however, and large depressions (scars) form locally, either through slumps, or erosion by turbidity currents initiated in the delta slope. Lag gravels are observed at some of these ‘channels’. The scars created by the slumps (or turbidity currents) were yet again filled with marine mud and occasional thin tuff layers. Siltstone-pebbly sandstone facies (XIII:E, Fig. 13.8). Flat-bedded coarse sandstone with numerous intraformational pebbles of laminated siltstone composition. Occasional sets of cross-beds. No fossils were found. The facies indicates a period of increased energy on the delta slope with concurrent erosion of silts. It was probably a local event, perhaps a storm with local erosion of the nearshore sediments, or an abrupt change in the river system. Shelly conglomerate facies (XIII:H, Fig. 13.8). Moderately sorted pebble conglomerate with a sandy matrix. Up to 50% of the sediment consists of basaltic pebbles, about 15% of pebbles of sedimentary origin, the remainder consists of shell fragments and sandy matrix. A trace of rhyolitic pebbles, as well as plutonic and metamorphic clasts, is present. The latter most probably reflect ice-rafted debris from a source outside Iceland, carried towards the North Iceland coastline with sea ice or icebergs. The facies is distinctly bedded with alternating low-angle lenses of sandstone and conglomerate dipping towards north and northeast. This facies occurs in a lenticular unit (unit 12x, Bárðarson, 1925) which grades upwards into a cross-­ bedded sandstone facies. The base is sharp and erosional except where the unit rests on the massive siltstone with dropstones facies. The grain size, sorting, and shell fragments indicate that the facies was formed in very shallow water, in the tidal zone. The mollusc fauna of the top of this unit clearly represents reworked s­ pecimens and contains a very mixed fauna, and the foraminifera at the base of the unit are apparently reworked from the underlying deposits (see also description below). Cross-bedded sandstone facies (XIII:I, Fig. 13.8). Medium-grained trough cross-­ bedded sandstone with abundant molluscs often arranged in bands of shells and shell fragments. Thin bands of very fine siltstone occur. This facies indicates fairly high energy with relatively strong currents and was probably deposited in a shallow-­ marine subtidal environment, above or at the storm-wave base. The shells are strongly oriented and clearly belong to a death assemblage that has suffered some transport. The maximum projection plane of most of the shells is parallel with the bedding planes. The shell fabric probably indicates transport along northeast-­

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southwest oriented flow directions (Eiríksson et al., 1992). The shell fabric data and dip directions in the shelly conglomerate facies below are considered consistent with intermittent sediment transport back and forth, roughly perpendicular to an east-west trending coastline. This sandstone facies represents the uppermost unit of the Svarthamar Member (unit 12 according to Bárðarson (1925)). There are two mollusc faunal assemblages, the Arctica islandica-Cyrtodaria angusta infauna and the Mytilus edulis-Nucella lapillus epifauna, both with considerably higher diversity than those in underlying layers of the Svarthamar Member. The faunal assemblages are the most boreal in character of all assemblages found in the Breiðavík Group. Even though the littoral Nucella lapillus has not been found in great numbers in the unit, its occurrence is remarkable. The species lived at the coasts of northern Iceland in the mid-Holocene, but after that it disappeared and did not return until the beginning of the twentieth century, when the climate ameliorated and the seawater temperatures rose at about AD 1920 (Áskelsson, 1935). In these years, the sea temperature off the north coast rose enough (1–2 °C) for the species to migrate to the Tjörnes area, where it still inhabits the coastal area. The bivalve Zirfaea crispata also reached the north coast of Iceland after about AD 1920 (Áskelsson, 1935). Thus, it is concluded that the sea temperature was at least as high as the present one in the Breiðavík area, even about 5–6 °C. This is also supported by two oxygen isotope analyses made on shells of Arctica islandica from the infaunal assemblage. They showed δ18O values of +2.54 and  +  2.63, respectively (Vilhjálmsson, 1985). Strauch (1963) compared the sea temperature to the present one off North Iceland (‘eine Fauna gefunden, die mindestens diejenigen Temperaturen voraussetzt, die sich heute an Nordislands Küsten eingestellt haben’). Furthermore, Gladenkov et al. (1975) reached to even higher? summer temperature, between 6.7 and 7.1 °C. In addition to relatively high temperature, the salinity rose also considerably. This is indicated by a δ13C analyses on shell of Arctica islandica from Svarthamar, which showed values of +2.01 and +1.46 (Vilhjálmsson, 1985). Thus, fully marine conditions cannot be excluded. The cross-bedded sandstone facies contains a new foraminiferal assemblage, that is, the Cibicides lobatulus-Elphidium hallandense assemblage. This assemblage is found in the uppermost part of Bárðarson’s (1925) unit 12x and in the entire unit 12 (for details and references to ecological factors, see Eiríksson et al., 1992; Knudsen et al., 2020). The assemblages are generally rich both in species and in specimens. Cibicides lobatulus is the dominant species, and Elphidium excavatum, E. albiumbilicatum, and E. hallandense occur frequently. Miliolids are still present in the assemblages, but now mainly as the species Ouinqueloculina seminulum. A small amount of the arctic Cassidulina reniforme still occurs in the lower part of unit 12. New elements in the assemblages compared to those in unit 10 are species such as Astrononion gallowayi, Islandiella inflata, Nonionella pulchella, Elphidium karenae, E. margaritaceum, E. incertum, Haynesina germanica, Stainforthia fusiformis, Gavelinopsis praegeri, Glabratella wrightii, and Rosalina sp. Each of these taxa occurs with a low frequency, but together they constitute an important and characteristic faunal element. They are indicators of ameliorated temperature conditions, that is, a boreal-arctic or even a boreal environment.

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Cibicides lobatulus is an epifaunal attached species indicating a high-energy environment, and species such as Gavelinopsis praegeri, Glabratelle wrightii, Rosalina sp., and some of the miliolids (Ouinqueloculina seminulum and Miliolinella subrotunda) point to a similar environment. The water must have been relatively shallow during deposition, and even shallower towards the top of the unit than in the lower part (cf. Knudsen et  al., 2020). This fits with the fact that the uppermost samples are from Þrengingar, a few kilometres further inland in a southeastern direction (Fig. 13.1), that is, closer to the former coastline. Therefore, they probably represent a shallower facies rather than a younger age than samples taken from the coastal profile. In general, the foraminiferal assemblages in the cross-bedded sandstone facies clearly reflect temperature conditions warmer than at present around Iceland, even warmer than found on the south coasts of Iceland today. Graded bouldery conglomerate facies (XV:B, Fig.  13.8). Sharp-based unit of angular to subrounded intraformational boulders of tuff and siltstone composition grading upwards into flat-bedded and cross-bedded sandstone. No fossils were found. This facies is present in the uppermost unit of the Svarthamar Member in the coastal section in Breiðavík (Figs. 13.1 and 13.4) and is interpreted as a proximal turbidite resulting from a slump from the nearshore region. This may indicate increased energy due to a regression marking the end of marine sedimentation in the area during this cycle. Erosional unconformity. The top of the Svarthamar Member in the coastal section in Breiðavík is marked by an erosional unconformity, which is followed by a diamictite facies (Fig. 13.4). The origin of the unconformity and the sediment above has been ascribed to a succeeding glacial erosion and deposition of subglacial traction till (Eiríksson et al., 2020c). The uppermost unit of the Svarthamar Member, the cross-bedded sandstone facies (unit 12), shows signs of subhorizontal deformation along internal shear planes with north-trending lineations parallel with flute moulds on the sole of the subglacial traction tillite above.

13.3.2  The Svarthamar Glacial-Interglacial Cycle: A Synthesis Data from the Svarthamar Member enables the reconstruction of environmental changes through a full glacial-interglacial cycle in a coastal region in Iceland during the Early Pleistocene. The reconstruction is based on the facies types, fossil contents, and on field relationships. The reconstruction is schematic where a north-­ south trending tectonic rift valley is envisaged, comprising a sedimentary basin extending from land onto the shelf. Along the western margin, a horst structure was already active, and part of the Pliocene Tjörnes sediments were exposed to erosion, as evidenced by fossiliferous clasts in Breiðavík (Símonarson et al., 2020). In harmony with Sæmundsson’s (1974) interpretation, the basin is thought to have been related to the Tjörnes Fracture Zone plate margin.

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A schematic illustration of the sequence of events reflected by the Svarthamar facies sequence is presented in Fig. 13.10, which shows four steps in the accumulation history of the Svarthamar Member: 1. Glacial. An erosional surface with glacial striae and a bed of lodgement till are produced by a glacier which advances from the south over the Tjörnes area. The pre-glaciation landscape was characterized by fluvial deposits and lava flows (belonging to the Fossgil Member, Fig. 13.4). Glacial erosion penetrated locally into lower units such as tuff beds and marine sands. 2. Outwash delta. The glacier retreats and the till are locally washed away by fluvial processes. The newly exposed isostatically depressed proglacial environment is now partly submerged by a eustatic rise of sea level. 3. Transgression. The shoreline migrates southwards in the basin, hand in hand with rising sea level, and a transgressive fining upwards delta sequence which is built out northeastwards into the bay. Marine fauna colonizes distal parts of the delta slope and prodelta, the mollusc fauna consisting of a Portlandia arctica assemblage, corresponding to Bárðarson’s, 1925 units 8–9 (Fig.  13.11). This assemblage is named after the high-arctic, infaunal, deposit feeding bivalve Portlandia arctica. The most common taxon in this assemblage is the mobile infaunal deposit feeder Nucula tenuis. The lowermost sample with preserved foraminiferal tests was at the same level as the Portlandia arctica mollusc assemblage, containing a residual fauna with no paleoecological information. In the central Svarthamar Member, corresponding to the transgressive phase of the Svarthamar cycle (Bárðarson’s (1925) unit 10), a foraminiferal assemblage was identified, the Islandiella helenae-Cassidulina teretis assemblage, indicating an arctic to boreal-arctic environment. This is indicated by the dominant species Elphidium clavatum together with i.a. Cassidulina reniforme and Islandiella helenae. The faunal development points to a relatively fast marine transgression followed by a regression towards the top of the zone. Most of the foraminifera in this assemblage are infaunal species indicating low-energy environments, with water depth probably exceeding 25 m. The mollusc fauna in unit 10 consists of the Macoma calcarea assemblage is dominated by infaunal, deposit-feeding species such as Macoma calcarea and Nucula tenuis. The faunas generally indicate a low-energy environment with a high sedimentation rate and soft bottom conditions. The species in this assemblage have their main distribution in the arctic region (Fig. 13.7), although also found further south in boreal-­ arctic environments. 4. Shoreline. A sheet of coarse fossiliferous sand is now deposited on the mud. A lens of gravel, which is seen at the base of the sand sheet, contains abundant marine shell fragments and complete shells. The gravel was deposited after erosion of a soft substratum; the contact is locally intricate and bands of mud stretch upwards into the gravel. These facies are interpreted as a high-energy beach or nearshore bar deposits, indicating a full regression. The gravel may partly be derived from the emerging delta platform deposits, and it is taken to represent the basal part of a migrating bar environment. Vertically, the gravel grades into

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Fig. 13.10  Block diagram sequence showing steps in the environmental history of the Svarthamar Member. The four steps are explained in the text. (Legend for lithological facies, see Fig. 13.4. text. Modified after Eiríksson et al., 1993)

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Fig. 13.11  Mollusc assemblages and climate indicators of the Svarthamar Member. The biostratigraphic units refer to Bárðarson (1925). (Legend for sediment log, see Fig. 13.4. Modified after Eiríksson et al., 1993)

fossiliferous sands, which indicates transgression (unit 12). In the macrofauna, the Mya truncata assemblage characterizes the gravel unit (Bárðarson’s (1925) unit 12x), although the deep burrowing species Mya truncata is also represented by single samples from the top part of unit 10. The species is found as fragmented close to the base of unit 12, where they clearly represent reworked specimens. In the sand sheet above, the Arctica islandica-Cyrtodaria angusta assemblage corresponds to Bárðarson’s (1925) unit 12. The assemblage is generally rich both in species and in specimens. The most prominent species are large individuals of the low arctic-boreal bivalve Arctica islandica and the extinct bivalve Cyrtodaria angusta. The faunal composition indicates ­shallow-­water and high-energy conditions and ameliorated temperature conditions. As to the foraminifera, the Cibicides lobatulus-Elphidium hallandense assemblage, covering Bárðarson’s (1925) units 12x and 12 (Fig. 13.12), is generally rich both in species and in specimens. The assemblage composition at the base of unit 12x, however, appears to have been reworked from the underlying unit. Cibicides lobatulus, an epifaunal attached species indicating high-energy environments, is the dominant species throughout. The assemblage composition indicates that water must have been relatively shallow during deposition, and even shallowing towards the top. The assemblages clearly reflect temperature conditions warmer than at present around Iceland, even warmer than found on the south coasts of Iceland today.

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Landwards of the shoreline, an alluvial plain develops. Fluvial channels are cut into the substratum, elsewhere non-fossiliferous sands accumulate, for example, in the Búrfell area (central Tjörnes Peninsula, Fig. 13.1). Probably, lava flows entered the southern reaches of the area and covered parts of the alluvial plain. This is assumed by analogy with other member units in Breiðavík. Towards the end of the history of the Svarthamar accumulation, proximal turbidite sediments show increased energy in the environment, perhaps due to regression. The record ends abruptly where the sequence is cut by an erosional unconformity followed by a tillite bed (Fig. 13.4).

13.4  Discussion The location of Iceland at the boundary between the Arctic and Atlantic climate regions in the northern North Atlantic and the current position in the path of the high-altitude westerly jet streams and low-depressions across the North Atlantic Ocean contributes to a cold-temperate maritime climate with over 10% of the island’s area of 103,000 km3 being covered by glaciers (Björnsson 2017). Changes in the position and shape of the frontal gradients between the Polar and Atlantic air and water masses are therefore likely to be reflected in Icelandic geological data archives. Thus, the mass balance of glaciers, vegetation, and marine ecology of

Fig. 13.12  Foraminiferal assemblages and climate indicators of the Svarthamar Member. The biostratigraphic units refer to Bárðarson (1925). (Legend for sediment log, see Fig. 13.4. Modified after Eiríksson et al., 1993)

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water masses around Iceland are likely to react strongly to switching between the two main climate regions. The front between the surface water masses of the Labrador and Greenland-Norwegian Seas on one hand and the warm Atlantic Water on the other shifted northwards across Iceland during the Weichselian deglaciation (Ruddiman & McIntyre, 1973), and it is generally assumed that a corresponding shift of the air masses and low depression track across the North Atlantic took place during the Pliocene and Early to Middle Pleistocene. A reference stack isotope record for the world oceans has been presented by Lisiecki and Raymo (2005) and used with a common timescale (Ogg & Smith, 2004), and it serves as a reference for paleoclimate and sea level studies. While the deep-sea record contains a wealth of evidence about global climate, it does not contain direct evidence about the location or extent of glacial ice. This must be obtained from land and shelf sections. The periodic growth and recession of Pliocene and Pleistocene ice sheets in the Northern Hemisphere is generally related to rhythmic climatic responses to orbital insolation changes (Ruddiman & McIntyre, 1984; Shackleton & Opdyke, 1973). Deep-sea cores from the North Atlantic reveal a record of changes in δ18O, δ13C, and CaCO3 concentration that indicate linkages between ice volumes and shifting fronts between water masses and deep-water formation (Ruddiman & McIntyre, 1973; Raymo et al., 1990; Jansen et al., 1990). A shift from a 41,000-year periodicity for the Late Pliocene and Early Pleistocene to a 100,000-year periodicity for the Middle and Late Pleistocene has been inferred (Ruddiman et al. 1986). The glaciation events recorded in the Tjörnes sequence are shown in Fig. 13.13 together with the benthic oxygen isotope stack of Lisiecki and Raymo (2005). The Breiðavík Group glacial-interglacial cycles dating from the Matuyama chron (2.58–0.78 Ma) are unique in Iceland as they carry information on oceanography and sea level changes. The rocks accumulated in volcanically active areas on terrestrial Iceland do not commonly contain fossil evidence. However, there is some evidence that the Early and Middle Pleistocene flora contained elements that disappeared later, notably Pinus and Alnus, which have been found in lacustrine sediments in western, northern, and southern Iceland along with Betula, Salix, and grasses, which are typical for all Pleistocene pollen and plant impression sites in Iceland (Denk et al., 2011). This indicates that at least some of the ice-free intervals were as warm or warmer than today. Generally, the lithological data in the predominantly terrestrial sections yield on-off glaciation ­signals only, however, with alternations between facies associations related to glacial processes and ones related to subaerial conditions. Detailed analyses of the mollusc and foraminiferal faunas of the Breiðavík Group have shown that the Matuyama-age glacial-interglacial cycles show a development from ice-cold, deglacial phase faunas to faunas typical for the seas around Iceland during the present interglacial (Eiríksson et  al., 1992; Knudsen et  al., 2020; Símonarson & Eiríksson, 2020; Vilhjálmsson, 1985). Therefore, it is suggested that the sea temperature was periodically very low, even lower than 0 °C, and the area more or less covered with ice. Cronin (1991) concluded from studies of ostracods that low water temperatures occurred in the Hörgi and Þrengingar Formations, dur-

13  Reconstructing the Paleoenvironments of the Quaternary Tjörnes Basin, North… Fig. 13.13 Glaciation events (black triangle triplets) in the Tjörnes sequence and the benthic marine isotope stack LR04 (Lisiecki & Raymo, 2005), which reflects temperature and global ice-volume changes. High values correspond to increased global ice-volume (glacial) stages

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ing deposition of the Cythere-Finmarchinella ostracod zone which contained assemblages indicating mean winter and summer temperatures of −1.5  °C and 3–4 °C, respectively, never exceeding 8 °C during the warmest months. A development from cold to warmer foraminiferal assemblages during the accumulation of the Svarthamar Member supports the conclusion that the glacial-interglacial cycles during the Early Pleistocene in North Iceland were of a range comparable to the Late Pleistocene cycles (Knudsen et  al., 2020). Apparently, the range of oceanographic changes and sea level changes associated with glacial-interglacial cyclicity during the Matuyama chron was more amplified than the global benthic isotope stack record would indicate, being comparable to the Eemian-Weichselian-­ Holocene cycle.

13.5  Conclusions The Breiðavík Group documents a remarkably detailed history of Quaternary glaciations from the Tjörnes Peninsula. The ideas about the origin of the Breiðavík Group rocks and the interpretation of the lithological cycles presented in this chapter may be summed up as follows: 1. Subglacial traction tillite beds in the sequence were deposited by an ice sheet which is assumed to have covered most of Iceland and the surrounding shelf. 2. Outwash and kame conglomerates and glaciolacustrine mudrocks were formed during the retreat of the ice sheet. 3. Marine transgressions in the wake of retreating ice are evident by the presence of marine sediments with fossils in close association with deglaciation sediments including dropstones. The faunas indicate arctic marine environments in the sediments immediately above glacial deposits. 4. Marine sediments with interglacial fauna cover the tillites and deglaciation sediments in Breiðavík. These sediments interfinger with and are followed in the vertical sense by fluvial sediments and lava flows indicating regression and ice-­ free conditions. Such a sequence of facies constitutes an ideal Breiðavík Group cycle and is considered to have accumulated during a climatic glacial-­interglacial cycle. The oceanographic contrasts between deglacial and interglacial phases are comparable to those of the last glacial-interglacial of the Quaternary in the region. 5. The frequency of the cyclicity of the Breiðavik Group is generally similar to that of the oxygen isotope stages defined for the Quaternary deep-sea sediments. According to palaeomagnetic and K/Ar data, the first seven of the lodgement tillites were deposited between ca. 2.6 Ma and c. 1.25 Ma. 6. The upper part of the Breiðavík Group was formed during reduced rate of subsidence and later during uplift on Tjörnes, which today represents a horst structure. The apparently lower frequency of glacial events is suggested not to reflect

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overall changes in the rate of climatic cyclicity, but rather a reduced preservation potential of the rocks on Tjörnes. 7. The oceanographic Polar Front was shifted northwards across Iceland into the Nordic Seas during warm stages in Middle Matuyama time, indicating that the Greenland-Iceland and the Iceland-Faroe ridges did not form barriers for the North Atlantic Current at that time.

References Albertsson, K. J. (1976). K/Ar ages of Pliocene-Pleistocene glaciations with special reference to the Tjörnes sequence, northern Iceland. Ph.D. thesis, University of Cambridge, Cambridge, p. 268. Albertsson, K. J. (1978). Um aldur jarðlaga á Tjörnesi. Náttúrufræðingurinn, 48, 1–8. Áskelsson, J. (1935). Some remarks on the distribution of the species Zirphaea crispata L. and Purpura lapillus L. on the north-coast of Iceland. Videnskabelige Meddelelser fra Dansk Naturhistorisk Forening, 99, 65–72. Bárðarson, G. G. (1925). A stratigraphical survey of the Pliocene deposits at Tjörnes, in northern Iceland. Det Kongelige Danske Videnskabernes Selskab. Biologiske Meddelelser, 4(5), 1–118. Birgisdóttir, L. (1984). Sedimentschichten des östlichen Teils der Tjörnes-Halbinsel, Fjallahöfn-­ Knarrarbrekkutangi, Nord-Island. Diplom thesis, Christian-Albrechts Universität zu Kiel, Kiel. p. 82. Björnsson, H. (2017). The glaciers of Iceland. A historical, cultural and scientific overview. Atlantis Press. p. 613. Camps, P., Singer, B.  S., Carvallo, C., Goguitchaichvili, A., Fanjat, G., & Allen, B. (2011). The Kamikatsura event and the Matuyama-Brunhes reversal recorded in lavas from Tjörnes Peninsula, northern Iceland. Earth and Planetary Science Letters, 310(1–2), 33–44. Cronin, T.  M. (1991). Late Neogene marine Ostracoda from Tjörnes, Iceland. Journal of Paleontology, 65(5), 767–794. Denk, T., Grímsson, F., Zetter, R., & Símonarson, L. A. (2011). Late Cainozoic Floras of Iceland. 15 Million Years of Vegetation and Climate History in the Northern North Atlantic (35) (p. 854). Dordrecht, The Netherlands/Heidelberg, Germany/London/New York: Springer. Dinter, W. P. (2001). Biogeography of the OSPAR maritime area: Synopsis and synthesis of biogeographical distribution patterns described for the north-East Atlantic (p. 167). Bonn, Germany: Federal Agency for Nature Conservation. Einarsson, T., Hopkins, D.  M., & Doell, R.  R. (1967). The stratigraphy of Tjörnes, northern Iceland, and the history of the Bering Land Bridge. In D. M. Hopkins (Ed.), The Bering Land Bridge (pp. 312–325). Stanford, CA: Stanford University Press. Eiríksson, J. (1981). Lithostratigraphy of the upper Tjörnes sequence, North Iceland: The Breidavík Group. Acta Naturalia Islandica, 29, 1–37. Eiríksson, J. (1985). Facies analysis of the Breidavík Group sediments on Tjörnes. Acta Naturalia Islandica, 31, 1–56. Eiríksson, J. (2008). Glaciation events in the Pliocene – Pleistocene volcanic succession of Iceland. Jökull, 58, 315–329. Eiríksson, J., & Símonarson, L.  A. (2020a). A review of the research history of the Tjörnes Sequence, North Iceland. In J. Eiríksson & L. A. Símonarson (Eds.), Pacific – Atlantic mollusc migration. Cham, Switzerland: Springer. This volume, Chapter 4. Eiríksson, J., Gudmundsson, A. I., Kristjánsson, L., & Gunnarsson, K. (1990). Paleomagnetism of Pliocene-Pleistocene sediments and lava flows on Tjörnes and Flatey, North Iceland. Boreas, 19, 39–55.

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Eiríksson, J., Knudsen, K. L., & Vilhjálmsson, M. (1992). An Early Pleistocene glacial-interglacial cycle in the Breiðavík Group on Tjörnes, Iceland: Sedimentary Facies, Foraminiffera, and molluscs. Quaternary Science Reviews, 11, 733–757. Eiríksson, J., Knudsen, K.  L., & Vilhjálmsson, M. (1993). Áhrif loftslagsbreytinga á lífríki og setlög við norðurströnd Íslands á fyrri hluta ísaldar. Náttúrufræðingurinn, 63(3–4), 159–199. Eiríksson, J., Guðmundsson, A.  I., Símonarson, L.  A., Einarsson, P., Hjartardóttir, Á. R., & Brandsdóttir, B. (2020b). The evolution of the Tjörnes sedimentary basin in relation the Tjörnes Fracture Zone and the geological structure of iceland. In J. Eiríksson & L. A. Símonarson (Eds.), Pacific – Atlantic mollusc migration. Cham, Switzerland: Springer. This volume, Chapter 3. Eiríksson, J., Guðmundsson, A. I., Símonarson, L. A., & Knudsen, K. L. (2020c). Lithostratigraphy of the upper part of the Tjörnes sequence in Furuvík, Breiðavík, Öxarfjörður, and central Tjörnes mountains, North Iceland. In J. Eiríksson & L. A. Símonarson (Eds.), Pacific – Atlantic mollusc migration. Cham, Switzerland: Springer. This volume, Chapter 10. Eiríksson, J., Símonarson, L. A., & Knudsen, K. L. (2020d). An age model for the Miocene to Pleistocene Tjörnes sequence, North Iceland. In J.  Eiríksson & L.  A. Símonarson (Eds.), Pacific – Atlantic mollusc migration. Cham, Switzerland: Springer. This volume, Chapter 6. Eiríksson, J., Símonarson, L.  A., & Knudsen, K.  L. (2020e). Lithostratigraphy of the Tjörnes sequence in Barmur and Höskuldsvík on the West Coast of Tjörnes, North Iceland. In J. Eiríksson & L. A. Símonarson (Eds.), Pacific – Atlantic mollusc migration. Cham, Switzerland: Springer. This volume, Chapter 5. Elverhøi, A., Liestøl, O., & Nagy, J. (1980). Glacial erosion, sediments and microfauna in the inner part of Kongsfjorden, Spitsbergen. Norsk Polarinstitutt, Skrifter, 172, 33–61. Emilsson, S. (1929). Beitrage zur Geologie Islands. Vorlaufige Mitteilung. Centralblatt für Mineralogie, Geologie und Paläontologie, 1929B, 1–4. Feyling-Hanssen, R. W. (1955). Late-Pleistocene deposits at Kapp Wijk, Vestspitsbergen. Norsk Polarinstitutt Skrifter, 108, 1–21. Funder, S., Demidov, I., & Yelovicheva, Y. (2002). Hydrography and mollusc faunas of the Baltic and White Sea-North Sea seaway in the Eemian. Palaeogeography, Palaeoclimatology, Palaeoecology, 184, 275–304. Geirsdóttir, Á. (2011). Pliocene and Pleistocene glaciations of Iceland: A brief overview of the glacial history. In J. Ehlers, P. L. Gibbard, & P. D. Hughes (Eds.), Quaternary glaciations – extent and chronology. A closer look (pp. 199–210). Amsterdam: Elsevier. Geirsdóttir, Á., & Eiríksson, J. (1994). Growth of an intermittent ice sheet in Iceland during the late Pliocene and Early Pleistocene. Quaternary Research, 42, 115–130. Gladenkov, Y. B., Krasnov, E. V., Ignatev, A. V., & Scheigus, V. E. (1975). On temperature conditions in the late Cainozoic habitat of Mollusca in the North Atlantic. Doklady of the Academy of Sciences USSR, 223(1), 176–177. Jansen, E., Sjøholm, J., Bleil, U., & Erichsen, A. (1990). Neogene and Pleistocene glaciations in the Northern Hemisphere and Late Miocene – Pliocene global ice volume fluctuations: Evidence from the Norwegian Sea. In A. Jessen (Ed.), Geological history of the polar oceans: Arctic versus Antarctic (pp. 677–705). Dordrecht, The Netherlands: Kluwer Academic Publishers. Jensen, A. S. (1942). Two West Greenland localities for deposits from the Ica age and the Post-­ glacial warm period. Det kongelige Danske Videnskabernes Selskab. Biologiske Meddelelser, 17(4), 1–35. Knudsen, K. L., Eiríksson, J., & Símonarson, L. A. (2020). Foraminifera in the Early Pleistocene part of the Breiðavík Group on Tjörnes, North Iceland. In J. Eiríksson & L. A. Símonarson (Eds.), Pacific  – Atlantic mollusc migration. Cham, Switzerland: Springer. This volume, Chapter 12. Lisiecki, L. E., & Raymo, M. E. (2005). A Pliocene-Pleistocene stack of 57 globally distributed benthic δ18O records. Paleoceanography, 20, PA1003. https://doi.org/10.1029/2004PA001071 Lisiecki, L. E., & Raymo, M. E. (2007). Plio-Pleistocene climate evolution; trends and transitions in glacial cycle dynamics. Quaternary Science Reviews, 28, 56–69.

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Lubinsky, I. (1980). Marine bivalve molluscs of the Canadian central and eastern Arctic: Faunal composition and zoogeography. Canadian Bulletin of Fisheries and Aquatic Sciences, 207, 1–111. Murray, J. W. (1991). Ecology and Palaeoecology of benthic foraminifera (Vol. 397). Harlow, UK: Longman Scientific and Technical. Nagy, J. (1965). Foraminifera in some bottom samples from shallow waters in Vestspitsbergen. Norsk Polarinstitutt, Årbok, 1963, 109–128. Ockelmann, W. K. (1958). The zoology of East Greenland. Marine lamellibranchiata. Meddelelser om Grønland, 122(4), 1–256. Ogg, J.  G., & Smith, A.  G. (2004). The geomagnetic polarity time scale. In F.  M. Gradstein, J. G. Ogg, & A. G. Smith (Eds.), A Geologic Time Scale 2004 (pp. 63–86). Cambridge, UK: Cambridge University Press. Osterman, L. E., & Nelson, A. R. (1989). Latest Quaternary and Holocene paleoceanography of eastern Baffin Island continental shelf, Canada: Benthic foraminiferal evidence. Canadian Journal of Earth Sciences, 26, 2236–2248. Raymo, M. E., Ruddiman, W. F., Shackleton, N. J., & Oppo, D. W. (1990). Evolution of Atlantic-­ Pacific δ13C gradients over the last 2.5 m.y. Earth and Planetary Science Letters, 97, 353–368. Ruddiman, W. F., & McIntyre, A. (1973). Time-transgressive deglacial retreat of polar waters from the North Atlantic. Quaternary Research, 3, 117–130. Ruddiman, W. F., & McIntyre, A. (1984). Ice-age thermal response and climatic role of the surface Atlantic Ocean, 40°N to 63°N. Bulletin of the Geological Society of America, 95, 381–396. Ruddiman, W. F., Raymo, M. & McIntyre, A. (1986). Matuyama 41,000-year cycles: NorthAtlantic Ocean and northern hemisphere ice sheets. Earth and Planetary Science Letters, 80, 117–129. Sæmundsson, K. (1974). Evolution of the axial rifting zone in northern Iceland and the Tjörnes Fracture Zone. Bulletin of the Geological Society of America, 85, 495–504. Schiøtte, T. (1989). Marine Mollusca from Jørgen Brønlund Fjord, North Greenland, including the description of Diaphana vedelsbyae n. sp. Meddelelser om Grønland, Bioscience, 28, 1–24. Shackleton, N. J., & Opdyke, N. D. (1973). Oxygen isotope and palaeomagnetic stratigraphy og equatorial core V28-238:Oxygen isotope temperatures and ice volumes on a 105 and 106 year scale. Quaternary Research, 3, 39–55. Símonarson, L. A., & Eiríksson, J. (2020). Systematic overview of the molluscs and barnacles of the Quaternary Breiðavík Group on Tjörnes, North Iceland. In J. Eiríksson & L. A. Símonarson (Eds.), Pacific  – Atlantic mollusc migration. Cham, Switzerland: Springer. This volume, Chapter 11. Símonarson, L. A., Knudsen, K. L., & Eiríksson, J. (2020). Reconstructing the Paleoenvironments of the Pliocene Barmur Group in the Tjörnes Basin, North Iceland. In J. Eiríksson & L. A. Símonarson (Eds.), Pacific – Atlantic mollusc migration. Cham, Switzerland: Springer. This volume, Chapter 9. Strauch, F. (1963). Zur Geologie von Tjörnes (Nordisland). Sonderveröffentlichungen des Geologischen Instituts der Universität Köln, 8, 1–129. Ussing, H. (1934). Contribution to the animal ecology of the Scoresby Sound Fjord complex (East Greenland). III. The hydrographical conditions in the Scoresby Sound Fjord complex. Meddelelser om Grønland, 100(3), 10–17. Vilhjálmsson, M. (1985). The Lower Pleistocene mollusc fauna of the Breidavík beds, Tjörnes, North Iceland (p. 207). Copenhagen, Denmark: MSc thesis, University of Copenhagen.

Chapter 14

Migration of Pacific Marine Mollusc Fauna into the North Atlantic Across the Arctic Ocean in Pliocene and Early Pleistocene Time Leifur A. Símonarson and Jón Eiríksson

Abstract  The Tjörnes sequence documents an exchange of molluscs between the North Pacific and the Arctic and North Atlantic Oceans. Before the Pliocene, the main open passage between the Atlantic and the Pacific was through the Central American Seaway. A Beringian land bridge prevented exchange of marine biota in the northern region. The Bering Strait was probably opened towards the termination of the Miocene at 5.3 Ma, and the final closure of the Central American Seaway took place at about 2.7 Ma. At least 34 molluscan species of Pacific ancestry reached Iceland during the Pliocene and three species reached Iceland, while the Lower Pleistocene part of the Breiðavík Group on Tjörnes was deposited. The major migration of Pacific species into the Tjörnes area is manifested at the base of the Serripes biozone of the Barmur Group of the Tjörnes sequence at about 3.8 Ma. This was preceded by earlier appearances of mollusc species close to the Miocene-Pliocene boundary. Mollusc migration into the Tjörnes area was not a single, abrupt event, but occurred at various times during the Pliocene and Lower Pleistocene. Some of these species of Pacific origin have subsequently dominated boreal, subarctic, and arctic molluscan assemblages in the North Atlantic. Keywords  Mollusc migration · Bering Strait pathway · Iceland · Tjörnes · Atlantic-Pacific interoceanic connection · Molluscs

L. A. Símonarson (*) · J. Eiríksson Institute of Earth Sciences, University of Iceland, Reykjavík, Iceland e-mail: [email protected]; [email protected] © The Author(s) 2021 J. Eiríksson, L. A. Símonarson (eds.), Pacific - Atlantic Mollusc Migration, Topics in Geobiology 52, https://doi.org/10.1007/978-3-030-59663-7_14

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14.1  Introduction Marine gateways between the world oceans are critically important for ocean circulation and distribution of heat on a global scale. The uppermost four metres of today’s oceans carry as much heat as the whole atmosphere, so the heat budget of the Earth is highly dependent on ocean currents (Wang et al., 2004). Opening and closing of ocean gateways and pathways for density-driven ocean ­currents in the past have had both local and regional impacts on environments through time. During the time interval covered by the present study, two major changes of past ocean gateways have taken place, the closure of the Panama Strait and the opening of the Bering Strait. Results of these changes are reflected in the fossil faunas and sedimentary record of the Tjörnes sequence, North Iceland. The Tjörnes Basin, North Iceland (Fig. 14.1), began to develop within the Tjörnes Fracture Zone (TFZ) during the Miocene and began to accumulate alternating marine and terrestrial sediments, the Tjörnes sequence, at the termination of the Miocene (Eiríksson et al., 2020a, c). Thick shallow-marine and coastal sediments from the Pliocene have been preserved in the Tjörnes Basin as the Barmur Group, as well as a younger, Quaternary unit, the Breiðavík Group. The Barmur Group has

Fig. 14.1  Location map of the Northern Hemisphere

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a rich fossil marine fauna, and the sequence, which also contains volcanics, is accessible in coastal and mountain sections in the uplifted Tjörnes horst. The geological record of the Tjörnes sequence contains data that are considered relevant for the climate history and paleogeography of the Northern Hemisphere (cf. e.g., Cronin, 1991a). The long record of Pliocene and Pleistocene faunas and floras, as well as cyclical facies sequences from this boundary region between the Arctic and the North Atlantic realms, contribute to the climate history and sea level changes in the region. The Tjörnes sequence also records the appearance of marine invertebrates of Pacific origin, and it was suggested by Hopkins et al., (1965) and Einarsson et  al., (1967) that this event was related to the opening of the Bering Strait. Traditionally, the Pliocene part of the Tjörnes sequence has been subdivided into three biozones based on mollusc assemblages, that is, the Tapes Zone, the Mactra Zone, and the Serripes Zone (Bárðarson, 1925). Two distinct faunal changes have been observed in the molluscan assemblages of the Tjörnes sequence (Símonarson & Eiríksson, 2008). The first one occurs in the middle part of the Mactra Zone and is apparently connected to environmental changes reflecting a change from a relatively sheltered infralittoral environment to a more open sublittoral one (Símonarson et al., 2020). The species of Paphia and Cerastoderma, which had been common in older units of the sequence almost disappeared, while Arctica islandica (Linné) and Spisula arcuata (Sowerby) became increasingly more common after the change which took place at about 4 Ma. The second faunal change is of quite different character and does not appear to be related only to environmental changes in the Tjörnes area. This change involves the migration of a number of species of North Pacific origin into the North Atlantic, which have since been among dominants in arctic, subarctic, and even boreal assemblages of marine faunas in the northern part of the Atlantic Ocean (Durham & MacNeil, 1967; Símonarson & Eiríksson, 2008). Some of these species had already reached the Tjörnes area when the oldest part of the beds were deposited, being of terminal Miocene age (Eiríksson et al., 2020c; Verhoeven et al., 2011). However, the tide of the migration seems to have taken place during the deposition of the lowermost part of the Serripes Zone at about 3.8 Ma (Durham & MacNeil, 1967; Símonarson & Eiríksson, 2008). The Pacific species include Neptunea species, Buccinum undatum Linné, Mytilus edulis Linné, Serripes groenlandicus (Mohr), Ciliatocardium ciliatum (Fabricius), Macoma species, and species belonging to the genus Mya. This faunal change, involving the migration of the Pacific species into the North Atlantic in the late Cainozoic, will be further discussed in a later section of the present chapter. At least 36 species of molluscs preserved in the Tjörnes sequence seem to be of Pacific origin, but about 10 species are apparently of uncertain origin (Durham & MacNeil, 1967; Símonarson & Eiríksson, 2020a). The remaining species probably originated in the Atlantic region. The opening of the Bering Strait and closure of the Central American Seaway resulted in strengthening of trans-Arctic Late Cainozoic oceanographic interchange in response to a reorganization of the Northern Hemisphere ocean circulation caused by these events (De Schepper et  al., 2015; Gladenkov et al., 2002; Haug & Tiedemann, 1998; Marincovich, 2000; Marincovich & Gladenkov, 1999). Migration

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of mollusc species was predominantly one-­directional, from the North Pacific and through the Arctic Ocean towards the North Atlantic. This probably reflects a prevailing clockwise surface current pattern in the Arctic, or it may in part be a function of the richer biotas in the Pacific (Durham & MacNeil, 1967). The first steppingstone in the North Atlantic for many of these migrating species was the Tjörnes area, reflected by a prominent faunal change in the Barmur Group. The Tjörnes Basin sediments represent the only accessible marine fossiliferous sequence in the northern part of the North Atlantic deposited during the time when these important events took place, that is, the opening of the Bering Strait and the closure of the Central American Seaway. The fossil Tjörnes fauna, especially the molluscan assemblages, is therefore an important data source for understanding the consequences of these paleogeographic and oceanographic events.

14.2  L  ate Cainozoic Changes of Gateways Between the Pacific and North Atlantic Oceans Major changes in the Earth’s climate system have taken place during the Late Cainozoic. A dramatic example is the onset of widespread glaciation at the beginning of the Quaternary and subsequent glacial-interglacial variability. However, this had been preceded by a cooling trend throughout most of the Cainozoic. The role of ocean circulation for distribution of heat and precipitation on Earth enhances the importance of geological data on changes in past oceanic gateways and tectonic development of continents. The opening of the Bering Strait and closure of the Central American Seaway represent two very important paleogeographic and biogeographic events in the late Cainozoic (Marincovich, 2000). An overview of these events and their implications is presented in the following.

14.2.1  The Opening of the Bering Strait The modern Beringia is considered to cover western Alaska, northeastern Siberia, and the shallow parts of Bering and Chukchi Seas, that is, the region that extends east from the Kolyma River in Siberia to the Mackenzie River in Canada (Hopkins, 1967b, 1996, Fig. 14.2). There are strong implications for a Cretaceous origin of Beringia, but throughout most of the Cainozoic, the region joined Eurasia and North America into one supercontinent and allowed interchange of terrestrial fauna and flora, but at the same time scaling down or preventing any marine connection between North Pacific and the Arctic and the North Atlantic Oceans (Fiorillo, 2008; Hopkins, 1967b).

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Fig. 14.2  Geography of the modern Beringia region

Beringia remained above sea level throughout most of the early and middle Cainozoic time and plants, as well as terrestrial animals such as mammals and reptiles were exchanged readily between Eurasia and North America (Fiorillo, 2008; Simpson, 1947). During these times, tectonic downwarping created local basins where alluvial sediments and peat accumulated, and adjacent uplift created local highlands that became sources of coarser sediments (Hopkins, 1967b). Subsidence of the southern margin of Beringia moved the shoreline northward at least as far as the Pribilof Islands, and a sedimentary sequence, now gently deformed, that includes marine sediments of Miocene age was formed (Scholl et al., 1966). The lack of molluscs of Atlantic ancestry in the faunal assemblages of the Pestsov Suite in the southern Gulf of Anadyr and the St. Lawrence Island (Fig.  14.2) suggests that a marine embayment reached far northward from the continental margin well before the first opening of the Bering Strait (Petrov, 1967). Tectonic activity in the recent Gulf of Anadyr seems to have begun well before the end of the Miocene Epoch and

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further large-scale tectonic events strongly affected all of Alaska (Hopkins, 1967b; Marincovich, 2000). In the late Cainozoic, tectonic deformation in Alaska was mainly expressed by ongoing interaction between the Pacific plate and the southern margin of Alaska, especially involving underthrusting of the continental block of the Yakutat terrane (Marincovich, 2000; Scholl, 1999). It has been argued that the consequent ­compression of Alaska between the northwest moving Yakutat terrane and the immobile Arctic composite terrane (composing about 25% of northern Alaska) led to acceleration of the westward extrusion of western and southwestern Alaska towards the Bering Sea (Plafker & Berg, 1994). The extrusion most probably follows the path of south-west trending strike-slip faults observed across western Alaska (Marincovich, 2000). Increased tectonism in the Bering Strait region about 6 Ma ago or later seems to be associated with the formation of the Bering tectonic block (Mackey et al., 1997). Apparently, the enhanced extension resulted in crustal thinning and subsidence of the Bering Strait region, which probably contributed to the initial submergence of the Bering Strait (Marincovich, 2000; Scholl, 1999). The Bering Strait today is rather shallow with a maximum depth about 50  m (Schaffer & Bendtsen, 1994). Flooding of this gateway between the Pacific and Arctic Ocean was most probably initiated by a eustatic rise of sea level that peaked at +75 m about 5 Ma ago and remained at about +60 m in the period from about 5.5 to 4.1  Ma (Haq et  al., 1987). Apparently, the eustatic rise closely followed the crustal extension in the Bering Strait region, but the relative influence of the crustal thinning and the eustacy are not known (Marincovich, 2000). Beringia, or the Bering Land Bridge, certainly constituted an effective barrier to marine trans-Arctic migration during most of the Cainozoic. However, as early as the Pontian Stage (6.25–5.3 Ma, according to Snel et al., 2006), a few thermophilic marine mollusc genera and species migrated by a northern route from the Pacific to the North Atlantic (Durham & MacNeil, 1967; Einarsson et  al., 1967; MacNeil, 1965). Some of these molluscs have been found in the Tapes and Mactra Zones of the Tjörnes sequence, but the most effective migration to the Tjörnes area took place during the deposition of the lowermost parts of the Serripes Zone, the youngest biozone of the Barmur Group of the Tjörnes sequence (Durham & MacNeil, 1967; Einarsson et  al., 1967; Marincovich, 2000; Símonarson & Eiríksson, 2008; Símonarson & Eiríksson, 2020a). Since the Late Pliocene, at least nine intervals with sea level high enough to flood the Bering Strait have been recorded in western Alaska (Hopkins, 1967a). Biostratigraphically and chronostratigraphically important diatoms from the Milky River Formation, southwestern Alaska, and the occurrence of the marine bivalve genera Astarte in southern Alaska, yielded an age range of 5.5–4.8  Ma for the earliest opening of the Bering Strait (Marincovich & Gladenkov, 1999; Marincovich, 2000). Later, Gladenkov et al. (2002) presented a refined age for the earliest opening very near the end of the Miocene at 5.32 Ma. Astarte is supposed to have migrated from the North Atlantic into the North Pacific during the earliest opening of the Bering Strait (Durham & MacNeil, 1967; Marincovich & Gladenkov, 1999).

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The water flow through the Bering Strait is mainly sustained by a sea level difference and salinity contrast between the Pacific and Arctic Oceans (Aagaard et al., 2006; Rudels et al., 2013). The transfer of low-salinity water to the Arctic Ocean through the Bering Strait is one of the main sources of low-salinity waters in that ocean (Weyl, 1968). Other fresh water sources for the Arctic Ocean are limited to river runoff and ice melt (Rudels et al., 2013). On the other hand, the main inflow of warm Atlantic water into the Arctic Ocean is through the Barents Sea and is ­estimated to be 10 times greater in volume than the Pacific inflow (BeszczynskaMöller et al., 2011). The Pacific water flows eastward from the Bering Strait along the Beaufort slope south of the Beaufort Gyre, and part of it probably merges into the Beaufort Gyre water mass (Woodgate, 2013). This cold, low-salinity flow continues partly through the complex channels of Canadian Archipelago as the Labrador Current, an outflow to Baffin Bay from the Arctic Ocean. The greatest part of the Pacific water, however, flows north of the Beaufort Gyre into the Arctic Ocean, continuing towards the Fram Strait and joining the cold, low-salinity East Greenland Current southward along the east coast of Greenland (Woodgate, 2013). The opening of the Bering Strait certainly affected the water circulation in the Arctic Ocean, as well as the northernmost part of the Bering Sea. Initially, the appearance of Astarte genera in southern Alaska at 5.5–4.8 Ma indicates southward flow through the strait (Marincovich & Gladenkov, 1999). Widespread occurrence of North Pacific molluscs in the North Atlantic area is somewhat younger and reflects a reversal to northward flow through the Bering Strait (Marincovich & Gladenkov, 1999). As mentioned, a refined age for the earliest opening of the strait indicates strongly that the Bering Strait first opened near the end of the Miocene at 5.32 Ma (Gladenkov et al., 2002). At present, relatively fresh North Pacific water is transported through the strait into the Arctic Ocean and subsequently into the North Atlantic (Hu et al., 2015). The reversal in the water flow direction through the strait seems to have taken place sometimes after 4.6 Ma, when a critical threshold in the closure history of the Central American Seaway took place (Marincovich & Gladenkov, 1999). However, southward flow through the Bering Strait can occur periodically in connection with northerly winds (Rudels, 2015). Apparently, the closure of the Central American Seaway caused greater reorganization of the Northern Hemisphere ocean circulation than the opening of the Bering Strait.

14.2.2  The Closing of the Central American Seaway Until the late Miocene there was no land connection between North and South America, and no exchange of plants or terrestrial animals was possible because the Central American Seaway separated the two continents (cf. Montes et al., 2015). The distribution of mid-Miocene to late Pliocene marine sediments along the tectonic boundary of the South American plate and the Panama Arc in Central America indicates that the seaway was located in the present-day Panama-Costa Rica region

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(Haug & Tiedemann, 1998) and is well known as the Central American Seaway (Fig. 14.3). The closing of the Central American Seaway and formation of the Isthmus of Panama involved subduction of the Farallon Plate beneath the Caribbean and South American plates driving the formation of a volcanic arc (Panama Arc), on the edge of the Caribbean Plate (Buchs et al., 2010; O’Dea et al., 2016). The latest development in the complex Paleogene and the Neogene history of plate movement and relative sea level changes began at around 6 Ma when the Panama Arc began to rise. This has continued to the present day (O’Dea et al., 2016). The arc uplift combined with fall in sea level, driven by the expansion of ice sheets around 3 Ma and repeated Pleistocene glaciation beginning at 2.6  Ma, resulted in the final closure of the Central American Seaway (Bartoli et al., 2005; O’Dea et al., 2016). The formation of the Isthmus of Panama certainly reorganized the water circulation in the eastern Pacific and the western Atlantic, as well as in the Arctic Ocean. In the deep ocean between Pacific and Caribbean sites, the divergences in neodymium isotopes (Newkirk & Martin, 2009), benthic foraminiferal δ13C (Keigwin, 1982; Lear et al., 2003), and an abrupt increase in the Pacific carbonate compensation depth indicate strongly that deep-water connection was brought to an end between 12 and 9.2 Ma. Then the deepest part of the sill of the Panama Arc must have shoaled to less than 1800  m or even as shallow as 1200  m (Osborne et  al., 2014). However, until about 4 Ma there was little taxonomic or ecological difference in shelf benthic and neritic assemblages between the eastern tropical Pacific and the Caribbean (Aguilera et al., 2011; Jackson et al., 1993; Schwarzhans &

Fig. 14.3  Geography of Central America showing the Cainozoic gateways, the Canal Seaway and the Atroto Seaway

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Aguilera, 2013; Woodring, 1966). Most probably, easy movement of water carried larvae and adults between the oceans through the seaway (Landau et al., 2009). Ocean circulation models suggest that the constriction of the interoceanic seaway caused the western Atlantic to become saltier because Trade Winds transported moisture into the Pacific (Lunt et al., 2008). The models predict that western Atlantic surface-water salinity began to diverge from eastern Pacific values at about 4.6 Ma and reached the recent Caribbean values at about 4.2  Ma (Haug et al., 2001; Keigwin, 1982). Shallow seawater continued to be exchanged through the seaway until about 4  Ma, when the number of diverging marine species peaked and the Caribbean underwent a profound environmental, ecological, and evolutionary transformation because of significant restriction of the interoceanic seaway (O’Dea et  al., 2016). Molecular divergence between eastern Pacific and western Atlantic shallow-water marine organisms provides evidence of the last interoceanic connections which demonstrate interoceanic gene flow until approximately 3.0–3.2  Ma, suggesting rather strong currents through the strait into the Caribbean (O’Dea et al., 2016). Several factors constrain the timing of the final closure of the Central American Seaway. These include an absence of further gene flow after c. 3.2 Ma, the end of surface water exchange between the oceans at 2.76 Ma based on marine plankton assemblages and surface salinity contrasts, and the acceleration of the dispersal rate of terrestrial mammals between North and South America just before 2.7  Ma (O’Dea et  al., 2016). Thus, the final closing almost coincides with or is slightly older than the first Upper Cainozoic regional glaciations of Iceland and Greenland followed by those in Alaska, Eurasian Arctic, and Northeast Asia (Eiríksson, 2008; O’Dea et al., 2016). During the shallowing and after the closure of the Central American Seaway, trade winds carried water vapour from the east across the low isthmus of Panama and deposited fresh water in the eastern Pacific through rainfall. As a result, the Pacific became relatively fresher, while salinity slowly but steadily increased in the Caribbean on the Atlantic side (Haug & Keigwin, 2004; Lunt et al., 2008). About 4.6 Ma, a rather strong carbonate dissolution started in the North Pacific in response to the freshening and cooling of the water (Haug & Tiedemann, 1998). Therefore, the intensification of the northward flowing relatively warm current along the western coasts of North America and up to the Bering Sea brought fresher and cooler Pacific water up to the Bering Strait and northward through the strait into the Arctic Ocean (cf. Backman, 1979; Marincovich, 2000). This current is disturbed by the upwelling of cooler, nutrient-rich waters of the California Current that rises up from the depths between Baja in California and the edge of British Columbia where breezes often blow from land to the sea and push the ocean surface waters away from the coast (Checkley & Barth, 2009). However, this northward flowing current brought additional heat and moisture farther to the north and into the Arctic Ocean (Haug & Tiedemann, 1998). The moisture was most probably provided by an increased thermohaline circulation since 4.6  Ma (Haug & Tiedemann, 1998). Apparently, the Arctic Ocean was then sea-ice free and a few degrees warmer than today (Buchardt & Símonarson, 2003; Durham & MacNeil, 1967). As mentioned, the final closing of the Central American Seaway probably occurred at about 2.7 Ma

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(O’Dea et al., 2016), and shortly after that the atmospheric moisture content and the decreasing temperature were necessary precondition for ice-sheet growth (Haug & Tiedemann, 1998). The elevation of the Central America Isthmus is considered to have intensified the Gulf Stream from the Caribbean Sea and brought warm and high saline water masses and humid air to high northern latitudes in the North Atlantic as the North Atlantic Current (Berggren & Hollister, 1974, 1977; Haug & Tiedemann, 1998). With a branch of the North Atlantic Current, relative warm high-salinity Atlantic Water was brought to South and Southwest Iceland by the Irminger Current (Eiríksson et al., 2004, 2011; Knudsen et al., 2008; Stefánsson, 1962). The Irminger Current is now flowing clockwise along the Icelandic shelf and reaches down to the sea floor in shallow parts of the shelf, as this water mass is about 300–400 m thick (Stefánsson, 1962). West of Iceland, the Irminger Current divides into two branches, one proceeding north- and eastward around Iceland, and the other flowing westward and southwestward, merging with the East Greenland Current and eventually also the West Greenland Current. Entering the Norwegian Sea, the evaporative cooling of the warm, high-salinity North Atlantic Current waters produced dense surface waters to sink and form great quantities of North Atlantic Deep Water (Backman, 1979; Haug & Tiedemann, 1998; Worthington, 1970). The evaporative cooling during formation of the North Atlantic Deep water is supposed to have increased atmospheric moisture content which is necessary precondition for ice-sheet growth (Haug & Tiedemann, 1998).

14.3  The Late Cainozoic Trans-Arctic Migration According to Durham and MacNeil (1967), a considerable number of marine molluscs migrated through the Bering Strait in the late Cainozoic, mainly from the Pacific to the Arctic Ocean and then to the North Atlantic. They found 125 species, mainly gastropods and bivalves, of Pacific origin that entered the Arctic-North Atlantic area but pointed out that another 33 species were of uncertain affinity, and some of them may be of Pacific origin. On the other hand, they were not able to recognize more than 16 species of Atlantic origin that reached the Pacific during this time. They considered the dominance of Pacific migrants to be due to a prevailing eastward current pattern in the Arctic Ocean and in part a function of the richer biotas of the Pacific (cf. Ekman, 1935). Durham and MacNeil concluded that 16 gastropod species and 12 species of bivalves of Pacific ancestry reached Iceland while the Tjörnes sequence was deposited. The appearance of marine fossil genera and species of Pacific origin in the Tjörnes sequence is well known from previous studies (Cronin, 1991a; Durham & MacNeil, 1967; Einarsson et al., 1967). Increased diversity of the marine fauna in the lowest part of the Serripes Zone has been taken as evidence of changed ocean circulation and changed paleogeography in the Northern Hemisphere. In the present chapter, we review the evidence from the Tjörnes sequence, incorporating new data

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on the distribution and age of fossil material of Pacific genera and species in the Pliocene and Pleistocene lithostratigraphic units of the sequence (Eiríksson et al., 2020c; Símonarson & Eiríksson, 2020a, b). Our evaluation of the late Cainozoic trans-Arctic migration is mostly based on molluscan groups. Most of the migrating species of bivalves and gastropods have larval development, and their ability to migrate is significantly depending on the duration of the pelagic larva stage. Durham and MacNeil (1967) recorded one brachiopod species and three species of echinoderms of North Pacific origin that appeared in Arctic and European areas in late Cainozoic. They have not been recorded from the Tjörnes or Breiðavík beds except the echinoid species Strongylocentrotus droebachiensis (Müller) which was found in the Lower Pleistocene Svarthamar and Stapavík Members of the Breiðavík Group (Vilhjálmsson, 1985). The absence of Pacific ostracod taxa in the Tapes and Mactra Zones may have been due to a climatic barrier (Cronin, 1991a; Durham & MacNeil, 1967). More cryophilic species such as those found in the Lower Pleistocene Breiðavík deposits may not have been able to tolerate the warm water that was present while the lower part of the Tjörnes sequence was deposited. In contrast to the absence of Pacific ostracod species in the lower Tjörnes sequence, the first appearance of Pacific species is in the Cytheridea ostracod zone of the Pliocene Serripes Zone and later in the Lower Pleistocene Cythere-Finmarchinella zones in the Breiðavík beds (Threngingar and Máná Formations). Thus, Cronin found the following species of Pacific ancestry in the Cytheridea Zone: Acanthocythereis dunelmensis (Norman), Finmarchinella cf. rectangulata Tabuki, Kotoracythere? sp., Palmenella limicola (Norman), and Semicytherura undata (Sars). In the Cythere-Finmarchinella zones he reported Cythere lutea Müller, Elofsonella concinna (Jones), E. neoconcinna (Bassiouni), Finmarchinella angulata (Sars), F. finmarchia (Sars), F. logani (Brady & Crosskey), Munseyella mananensis Hazel & Valentine, and Robertsonites tuberculata (Sars). Furthermore, Cronin described eight new species from Tjörnes and concluded that the trans-Arctic migration was not necessarily an abrupt event but that Pacific species migrated into the North Atlantic at different times during the Pliocene and Lower Pleistocene. Three foraminiferal species identified in the Early Pleistocene part of the Breiðavík Group, Cassidulina limbata (Cushman and Hughes, Islandiella inflata (Gudina), and Nonionella pulchella Hada support the idea of Pacific influence in the Early Pleistocene foraminiferal assemblages of Iceland (Knudsen et al., 2020). The most significant faunal change in the Tjörnes area during the deposition of the Tjörnes sequence is directly connected to the migration of North Pacific species into the Arctic and North Atlantic Oceans (Durham & MacNeil, 1967; Marincovich, 2000; Símonarson & Eiríksson, 2008). These species and their descendants have since been among dominants in arctic, subarctic, and even boreal assemblages within marine faunas in the northern part of the Atlantic. The migration must have taken place after the opening of the Bering Strait when the Arctic Ocean was still ice free and warmer than at present as some of the migrating species no longer range that far north (Buchardt & Símonarson, 2003). However, the sea temperatures in the

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Arctic Ocean during the migration were apparently a few degrees lower than those in the North Pacific and North Atlantic, but the differences were not so striking as today. The Arctic Ocean probably acted as a filter to the migration, allowing the species best adapted to the conditions in the Arctic to move along and reach the North Atlantic first (Buchardt & Símonarson, 2003). Thus, the migrating species are not considered to reflect changes in sea temperature in the Tjörnes area while the tide of the migration reached the area, but rather the temperature condition in the Arctic Ocean during the migration (Buchardt & Símonarson, 2003).

14.3.1  P  acific Molluscs in the Tapes and Mactra Zones of the Barmur Group, Tjörnes Sequence The first species of Pacific affinity that reached North Iceland and the Tjörnes area before 3.8 Ma are mainly littoral. They are represented by species as Testudinalia (Collisella) testudinalis (Müller), Littorina squalida Broderip & Sowerby, Mytilus edulis Linné, and Modiolus modiolus (Linné), all found in unit 9 (cf. Bárðarson, 1925) in the Pliocene Mactra Zone (Table 14.1). According to the most recent age model for the Tjörnes sequence (Eiríksson et al., 2020c), the first transgression and marine sedimentation (Tapes Zone) within the Barmur Group took place close to the Miocene-Pliocene boundary. The Tapes-Mactra boundary was estimated at 4.4 Ma, the Mactra-Serripes boundary at 3.8 Ma, and the top of the Serripes Zone at 3.2 Ma. In the following, we review the species and genera of Pacific ancestry that appear in the two lowest biozones of the Barmur Group, the Tapes and Mactra Zones. Mytilus edulis Linné. The first molluscan species with well-documented Pacific history that reached the Tjörnes area during the trans-Arctic migration is Mytilus edulis Linné. It has been found in all units of the oldest biozone of the Tjörnes sequence, the Tapes Zone, and occurs in almost all units of the younger Mactra and Serripes Zones. It is especially abundant in sediments reflecting marine transgression close to the Miocene-Pliocene boundary in unit 1 (cf. Bárðarson, 1925) of the Tapes Zone. The species has not been found in Miocene deposits in the Atlantic area, and it probably became distributed during the Pliocene southward to the Bay of Biscay. There are no signs of differentiation within the species during this long migration. Testudinalia. The genus Testudinalia (or Acmaea) is present in Oligocene deposits in the North Pacific, but in the North Atlantic it first occurs in the Pliocene Tjörnes sequence (Durham & MacNeil, 1967). Testudinalia testudinalis (Müller) is by some authors referred to the subgenus Collisella Dall (Malatesta & Zarlenga, 1986), and one species of this subgenus is represented in the North Pacific Pliocene and nine in the Pleistocene (Grant & Gale, 1931; Malatesta & Zarlenga, 1986). The occurrence of this species in North Atlantic Pliocene sediments indicates that the species migrated through the Bering Strait into the Arctic Ocean and the North Atlantic while the Mactra Zone of the Tjörnes sequence was deposited. Most

Table 14.1  Mollusc genera and species of Pacific origin and their levels of appearance in the Tjörnes area. Breiðavík Group 0–2.58 Ma, Serripes Zone 3.2–3.8 Ma, Mactra Zone 3.8–4.4 Ma, Tapes Zone (marine part) 4.4–5.3 Ma Lithostratigraphic unit Species

Biostratgraphic /chronostratigraphc unit

Mytilus edulis Linné Testudinalia (Collisella) testudinalis (Müller) Margarites groenlandicus (Gmelin) Margarites costalis (Gould) Littorina squalida Broderip & Sowerby Cryptonatica affinis (Gmelin) Euspira pallida (Broderip & Sowerby) ? Euspira montagui (Forbes) Buccinum finmarkianum Verkrüzen Searlesia costifera (Wood) ? Cytharella sp. Musculus niger (Gray) Modiolus modiolus (Linné) ? Hiatella arctica (Linné) ? Zirfaea crispata (Linné) Solariella sp. Velutina velutina (Müller) Boreotrophon clathratus (Linné) Buccinum undatum Linné Buccinum cyaneum Bruguière Neptunea lyratodespecta Strauch Admete viridula (Fabricius) Admete couthouy (Jay) Oenopota borealis (Reeve) Oenopota decussata tjoernesensis (Schlesch) Curtitoma trevelliana (Turton) Propebela harpularia (Couthouy) Propebela nobilis (Møller) Boreoscala greenlandica (Perry) Cylichna alba (Brown) Ciliatocardium ciliatum (Fabricius) Serripes groenlandicus (Mohr) Macoma calcarea (Gmelin) Macoma obliqua (Sowerby) Mya truncata Linné Mya truncata gudmunduri Strauch Mya schwarzbachi Strauch Panomya trapezoidis Strauch ? Panomya obliquelongata Strauch Nucella lapillus (Linné) Ennucula tenuis (Montagu) Mya truncata pseudoarenaria Schlesch

Barmur Group Tapes Mactra Zone Zone X X X X X X X X X X X X X X X X

Serripes Zone X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X

Breiðavík Group Holocene X X X X

X X X X

X X

X X X

X

X X

X X

X X X

X X X X

X

?

X X X X

X X X

X X X

X

X

X X X

X X X

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p­ robably, the species evolved from Acmaea insessa/Discurria insessa/Lottia insessa (Hinds), the only Pliocene North Pacific Collisella. Thus, it is assumed to have originated in the Pacific, although the typical form has not been found living there, where a larger form, more rounded and almost smooth, has been separated as a subspecies T. (C). testudinalis scutum Eschscholtz (cf. Malatesta & Zarlenga, 1986). This form may have developed further into the more typical Atlantic form before it reached the Tjörnes area. As the species has not been found in Pliocene deposits in the British Isles or continental Europe, it is tempting to conclude that it first migrated southwards from Iceland after the formation of the Barmur Group, presumably during the Lower Pleistocene. Margarites groenlandicus (Gmelin) and M. costalis (Gould) are most probably of Pacific origin (Durham & MacNeil, 1967). They reached the Tjörnes area while the Pliocene Mactra Zone was deposited and have since been among the most common Margarites species in the North Atlantic. Malatesta and Zarlenga (1986) discussed the reliability of the record by Grant and Gale (1931) from Miocene beds in the Pacific area and emphasized that there are no records of Margarites from Pliocene deposits in the Pacific. This indicates that the evolution of these species found place in the Arctic Ocean during the migration. Littorina squalida Broderip & Sowerby. The species Littorina squalida Broderip & Sowerby is considered to be one of the North Pacific species that took part in the Pliocene migration into the Arctic Ocean and North Atlantic (Reid, 1996). It is known from the Pacific since mid-Miocene (at about 10  Ma), and it reached the Tjörnes area while the Mactra Zone was being deposited. The species also occurs frequently in the uppermost units of the Serripes Zone, but has not been found in younger deposits in the North Atlantic and does not live there at present (Reid). It probably did not survive in the Atlantic, but before disappearing, it became an ascendant to the well-known L. littorea (Linné). This indicates that the species separation took place during deposition of the uppermost Serripes Zone of Tjörnes, coinciding with a further migration of L. squalida southward to the North Sea area. It is tempting to suggest that this happened on the Iceland-Faroe Ridge before the littoral-living L. squalida ever reached the North Sea area and Britain. The oldest fossil occurrence of L. littorea in the Red Crag Formation in England, Britain, seems to be between 2.4 and 3.2 Ma (Reid, 1996). Cryptonatica and Euspira. The prosobranh gastropod genera Cryptonatica and Euspira are most probably of Pacific affinity and participated in the trans-Arctic migration through the Bering Strait while the Tapes and Mactra Zones were deposited in the Tjörnes area (Durham & MacNeil, 1967). Since then, species of these genera were distributed in the North Atlantic, and all of them reached the North Sea area in the south, although Cryptonatica smithii (Brown) first occurs there in the?Upper Pleistocene beds of Ardincaple, Tayport in Scotland, Britain (Harmer, 1921; Wood, 1857). The first and only Naticidae that reached the Tjörnes area while the Tapes Zone was deposited is the large-shelled Euspira catenoides (Wood) that already occurs in unit 5 (cf. Bárðarson, 1925). It is suggested that it evolved from the rather well-known E. pallida (Broderip & Sowerby) during the migration through the Arctic Ocean, and that this speciation was completed before the species

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reached the Tjörnes area. The species E. catenoides migrated southward to the North Sea area already in Pliocene time (Harmer, 1921; Marquet, 1998), but apparently it became extinct during the Lower Pleistocene. Buccinum finmarkianum Verkrüzen is considered of Pacific origin, and it reached the Tjörnes area during the deposition of the Pliocene Mactra Zone (Durham & MacNeil, 1967). The species is mainly subarctic, but living in the North Atlantic, while Searlesia costifera (Wood), another Pacific species, disappeared during Middle Pleistocene. Apparently, it could not survive some of the severe glaciations of the Middle Pleistocene. Cytharella sp. One specimen of Cytharella sp. has been recorded in the Mactra Zone (Símonarson & Eiríksson (2020a), that is, from the Brunngil Member of the Skeifá Formation (unit 9, Bárðarson, 1925). As the species identification is difficult, the direction of migration is not certain, but according to Grant and Gale (1931) the genus seems to be of Pacific origin. However, they stated that true species of Cytharella have not been reported as fossils in “California and Lower California,” but according to them, there are a number of living species of Cytharella that might appear in fossil state in the Pacific area at any time from Eocene to Recent. Therefore, it seems difficult to exclude Pacific origin of Cytharella and a Pliocene migration to the North Atlantic. Furthermore, Cytharella costata (Donovan) has been found in Pliocene deposits such as the Coralline Crag Formation in England, Britain (Harmer, 1915), and C. substriolata (Harmer) and C. vandewouweri (Gilbert) in the Pliocene Luchtbal Sand Member of the Antwerp region in Belgium (Marquet & Landau, 2006). If our species from unit 9 migrated in the Pliocene from the Pacific to the North Atlantic, it probably reached the North Sea Basin already in early Pliocene. Musculus niger (Gray). This species is considered to have originated in the Pacific, and it reached the Tjörnes area during the deposition of the Mactra Zone (Durham & MacNeil, 1967). This seems to be the oldest occurrence (FAD) of the species, and it might mark the earliest penetration of cold waters south to mid-­ latitudes (Símonarson et al., 1998). Modiolus modiolus (Linné). Apparently, Modiolus modiolus (Linné) evolved in the Pacific area in the?late Miocene most probably from M. capax (Soot-Ryen, in Malatesta & Zarlenga, 1986). Durham and MacNeil (1967) mentioned the species as being of Pacific origin, but did not report it from the Pacific Pliocene. It reached the Tjörnes area during the deposition of unit 9 of the Mactra Zone and has since been living around Iceland and extended its distribution to the North Sea area already in the Pliocene. It is now widespread in the boreal region of the North Atlantic with scattered outposts in the subarctic and lusitanian regions. Hiatella arctica (Linné). Durham and MacNeil (1967) considered Hiatella arctica (Linné) to have originated in the Atlantic before it migrated to the Pacific. On the other hand, Bernard (1979) and Gordillo (2001) regarded it as a Pacific species that migrated into the Arctic Ocean and the North Atlantic and suggested that it evolved from H. sahalinensis (Takeda). These authors supposed that the species migrated through the Bering Strait after its opening before 4.8 Ma ago, but after 5.5 or even 7.4 Ma ago (Marincovich, 2000; Marincovich & Gladenkov, 1999), an age which was later refined at 5.32 Ma, close to the end of the Miocene (Gladenkov

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et al., 2002). If the Oligocene specimens are correctly identified as H. arctica, then the species was already distributed in the North Sea Basin long before the opening of the Bering Strait (see Sorgenfrei, 1958; Gordillo, 2001). It is therefore tempting to follow Durham and MacNeil (1967) and consider H. arctica as an Atlantic form, because it has a distinctly earlier appearance in the Atlantic than in the Pacific. If it migrated into the Pacific before the opening of the Bering Strait, the easiest migration route was probably via the Central American Seaway, before the final closure at about 2.7 Ma (O’Dea et al., 2016). Even though H. arctica is an Atlantic species, it is not unlikely that the species migrated back to the North Atlantic via the Arctic Ocean, as it was probably widely distributed in the North Pacific while the Mactra Zone was deposited in the Tjörnes area. Thus, it is suggested to have migrated through the Bering Strait and participated in the trans-Arctic migration. Zirfaea crispata (Linné). Even though Durham and MacNeil (1967) listed this species as an Icelandic fossil with Pacific affinities, the shallow-water species Zirfaea crispata (Linné) is not known as fossil from the Pacific area and seems to have originated in the Atlantic. This species reached the Tjörnes area during the deposition of Barmur Group and is found in the Mactra Zone.

14.3.2  P  acific Molluscs Appearing in the Serripes Zone of the Barmur Group, Tjörnes Sequence The majority of molluscan species of Pacific ancestry appeared in the Tjörnes area during the deposition of the lower part of the Serripes Zone at about and slightly after 3.8 Ma. That was the time when the tide of the trans-Arctic migration reached the North Atlantic. Several well-known prosobranch genera such as Velutina, Buccinum, Colus, Neptunea,?Searlesia, Admete, Oenopota, Propebela, and Boreoscala arrived in North Iceland at this time (Table 14.1). The majority of the species is still living in the North Atlantic, with the exception of Neptunea lyratodespecta Strauch, which most probably became extinct in the Middle Pleistocene, and the Searlesia species that apparently disappeared in the North Sea area during the Middle Pleistocene, while Oenopota borealis (Reeve) already became extinct in Lower Pleistocene (Harmer, 1914, 1915; Strauch, 1972a). Oenopota decussata ­tjoernesensis (Schlesch) is only known in the Pliocene sediments of the Serripes Zone. Otherwise, we have no knowledge of its stratigraphical range. Durham and MacNeil (1967) recorded the subspecies among Icelandic fossil molluscs with Pacific affinity. In the following, we review the occurrence of mollusc species and genera of Pacific ancestry in the Serripes Zone of the Barmur Group. Solariella sp. The species found in the Tjörnes material seems to have higher spiral form than Solariella obscura (Couthouy), three spirals on the penultimate whorl, and stronger radial on the base. As these characters do not agree well with any of the known Solariella species, it may belong to another Pacific species or even a new one. S. obscura (Couthouy) and S. varicosa (Mighels & Adams) are now liv-

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ing in the Pacific, and Grant and Gale (1931) reported a number of Solariella species from the Eocene of California and Washington State, but no species has been reported there from the Pliocene. Did S. obscura migrate from the Pacific while the specimens found in the Tjörnes sequence reached the northern Iceland during the Pliocene, and did some of the S. obscura specimens evolve those characteristics found in the Solariella shells from Tjörnes? Unfortunately, further identification of this very limited and somewhat damaged material from the Tjörnes sequence seems hardly possible. Velutina velutina (Müller). The occurrence of Velutina velutina (Müller) in Pliocene sediments in northern Pacific is apparently of similar age as the occurrence in the Serripes Zone. The specimen in the Tjörnes sequence is probably the oldest one in the North Atlantic area, which may indicate a migration from the Pacific to the Atlantic while the Serripes Zone was deposited. The long-lasting pelagic larval stage might have facilitated the migration of the species. Boreotrophon clathratus (Linné). Grant and Gale (1931) mentioned at least ten species of Boreotrophon from the Pacific area, the oldest from Miocene sediments, and Durham and MacNeil (1967) mentioned the genus among Pacific Pliocene immigrants to the North Atlantic. Probably, Boreotrophon clathratus evolved from one of these Pacific groups during migration to the North Atlantic while the Serripes Zone of the Tjörnes sequence was deposited. Buccinum undatum Linné. Durham and MacNeil (1967) listed Buccinum undatum Linné among Pliocene immigrants from the Pacific to the North Atlantic. The genus Buccinum is of ancient origin, as it first appeared in the late Oligocene and Grant and Gale (1931) recorded at least three species from the Miocene of Alaska and Washington State. Today, B. undatum does not live off northern Alaska or in the Pacific (Macpherson, 1971), and presumably it evolved from the Buccinum plectrum stock during the migration through the Arctic Ocean or the northernmost part of the North Atlantic before it reached the Tjörnes area, while the Serripes Zone was deposited. Buccinum cyaneum Bruguière. This species seems to be of Pacific origin and probably it migrated into the North Atlantic during the deposition of the Serripes Zone (Durham & MacNeil, 1967). Neptunea. Fossil specimens of the genus Neptunea found in the Tjörnes sequence have been referred to different species: N. despecta Linné, N. antiqua Müller, N. carinata (Pennant), N. subantiquata Maton & Rackett, N. decemcostata (Say), and N. lyratodespecta Strauch. The variation within the Neptunea material from Tjörnes is not as striking as the variation within the recent Neptunea despecta (Linné) in Iceland. Therefore, we agree with Strauch (1972a) that all the specimens found in the Tjörnes sequence can be referred to one single species, which he named N. lyratodespecta. He further considered that this new species evolved from the Pacific stock of N. lyrata (Martyn) in the northernmost part of the North Atlantic during the trans-Arctic migration (Strauch). The species reached the Tjörnes area while the lowermost part of the Serripes Zone was deposited, and then it became distributed southward to the North Sea area, where it most probably became extinct in mid-Pleistocene time (Harmer, 1914; Strauch, 1972a).

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Admete viridula (Fabricius), A. couthouy (Jay), and several other gastropods found in the Serripes Zone. The two species of Admete are considered of Pacific origin according to Durham and MacNeil (1967). This was apparently also the case with Oenopota borealis (Reeve), Oenopota decussata tjoernesensis (Schlesch), Curtitoma trevelliana (Turton), Probebela harpularia (Couthouy), Popebela nobilis (Möller), Boreoscala greenlandica (Perry), and Cylichna alba (Brown) (cf. Durham & MacNeil). Since these prosobranchs reached the Tjörnes area, they have been rather common in the northern parts of the North Atlantic. However, Oenopota borealis (Reeve) already became extinct in Lower Pleistocene (Harmer, 1914, 1915; Strauch, 1972a), and Oenopota decussata tjoernesensis (Schlesch) is only known in the Pliocene sediments of the Serripes Zone. Ciliatocardium ciliatum (Fabricius) and Serripes groenlandicus (Mohr). Several bivalve species of Pacific ancestry appeared in the Tjörnes area during the deposition of the lower part of the Serripes Zone at about and slightly after 3.8  Ma (Table 14.1). According to Durham and MacNeil (1967), the two commonly found species of Cardiidae in the North Atlantic, that is, Ciliatocardium ciliatum (Fabricius) and Serripes groenlandicus (Mohr), are considered of Pacific affinity. The latter is an index fossil for the Serripes biozone at Tjörnes and has also given name to the Serripes zones in Britain and Holland. After these species reached Iceland during the Pliocene migration, they became distributed southward to the North Sea area, but today their southern limit in the North Atlantic seems to be in South Iceland. Macoma calcarea (Gmelin) is considered a Pacific species that migrated into the North Atlantic and the Tjörnes area while the Serripes Zone was deposited (Durham & MacNeil, 1967). Furthermore, Macoma obliqua (Sowerby) was probably among the first groups of boreal Pacific Macoma species to reach the North Atlantic during the trans-Arctic migration (cf. Coan, 1971). It probably migrated together with the closely related M. lyelli Dall and M. cookei Gardner, but these have both been found in late Miocene sediments on the North American east coast. However, Macoma obliqua first reached the Tjörnes area during the deposition of the Serripes Zone. It disappeared from the coasts of northwestern Europe in the Lower Pleistocene, but migrated as a boreal guest into the Mediterranean (Malatesta & Zarlenga, 1986). The occurrence in Italy indicates that this took place during the late Emilian and the latest Sicilian, but it has not been found in Italian sediments after the Sicilian (Bellomo & Raffi, 1991; Sami & Taviani, 1997). Subsequently the species ­disappeared from the North Atlantic, but kept on living in the North Pacific (cf. Coan, 1971). Mya. The bivalve genus Mya has a well-documented and long history in the Pacific, and the earliest record seems to be from the late Eocene or early Oligocene of Japan (Durham & MacNeil, 1967; MacNeil, 1965). Mya arenaria Linné was one of the first members of the Pacific Mya group to migrate into the North Atlantic as it has been reported from the Yorktown Formation of Virginia (Gardner, 1943; MacNeil, 1965). These authors considered the formation of late Miocene age. However, more recent biostratigraphic work based on ostracods and foraminifera shows Pliocene age of the Yorktown Formation (Cronin, 1991b). The species

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has not been found in the Barmur Group sediments on Tjörnes, although some sources have stated the opposite (cf. De Schepper et  al., 2015; Marincovich, 2000). It was first found alive in East Iceland in the mid-twentieth century, but it has never been found in Icelandic sediments. The specimens erroneously reported as Mya arenaria from Icelandic deposits have been identified as Mya truncata ovata A.  S. Jensen by Schlesch (1924), Mya truncata, L. f. ovata, Jensen by Bárðarson (1925), Mya pseudoarenaria by Schlesch (1931), Mya pseudoarenaria Schlesch by Durham & MacNeil (1967), and Mya cf. ovata Jensen by Gladenkov et al. (1980). After a thorough study of the genus Mya, Strauch (1972a) came to the conclusion that the specimens belong to a distinct species, which he named Mya schwarzbachi. We support his conclusion and are therefore using that name in this chapter. M. arenaria had reached the North Sea Basin already in the Pliocene, apparently bypassing Iceland, but eventually it became distributed northward to Iceland, where the first specimen was found near Hornafjörður in the southeastern part of the island in the year 1958 (Óskarsson, 1964; Strauch, 1972a). Mya truncata Linné is a typical example for the group of shallow-water benthic molluscs that originated in the Pacific and migrated into the Arctic Ocean and the North Atlantic while the Tjörnes sequence was deposited (Durham & MacNeil, 1967; Strauch, 1972a). The earliest appearance of M. truncata in Iceland and probably in Europe is in the Serripes Zone of the Tjörnes sequence. There, the species is extremely rare, and it is not commonly found until in the Lower Pleistocene part of the Breiðavík Group. While we have found about 100 valves of Mya schwarzbachi Strauch in the Barmur Group, we have only come across a single valve of M. truncata, but several valves of Mya truncata gudmunduri were reported by Strauch. We do not know about any other Pliocene deposits east of Arctic Canada where the species occurs, and several authors (cf. MacNeil, 1965; Strauch, 1972a) have indeed regarded it as characteristic for Pleistocene deposits in the North Atlantic area. M. truncata is widespread and well known in the Breiðavík Group on Tjörnes, as well as in Lower Pleistocene Greenlandic localities. It reached Meighen Island in Arctic Canada during the Pliocene and shortly after that, it appeared in the Tjörnes area, but apparently it did not arrive in the British Isles and continental Europe until the Lower Pleistocene. Mya truncata gudmunduri Strauch. Apparently, the subspecies Mya truncata gudmunduri reached Iceland at the same time as Mya truncata. If the specimens from Tugidak Island in the Gulf of Alaska described by MacNeil (1965) belong to this subspecies, as hinted by Strauch (1972b), it probably already evolved in the North Pacific before it migrated through the Bering Strait. Already in the Pliocene, it extended its distribution south to the North Sea area, as indicated by the occurrence in the British Isles and continental Europe (Strauch). Apparently, it became extinct during the Lower Pleistocene. Mya schwarzbachi Strauch. Strauch (1972a) regarded Mya schwarzbachi as having evolved from M. cuneiformis (Böhm) during the Pliocene when it migrated through the Arctic Ocean, but apparently, M. cuneiforms did never reach the North Atlantic or the Tjörnes area as did M. schwarzbachi. In the Pliocene, M. schwarzba-

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chi extended its distribution south to the North Sea area, as strongly indicated by the occurrence in the British Isles and continental Europe (Strauch). Most probably, it became extinct in the late Pliocene or in early Lower Pleistocene. Panomya. Strauch (1972a) revised fossil and recent species of the genus Panomya, and according to him, Panomya trapezoidis Strauch originated in the Pacific and migrated into the Arctic Ocean and the North Atlantic while the Serripes Zone was deposited. Furthermore, it migrated southward to the British Isles and continental Europe, where it has been found in Pliocene and Lower Pleistocene sediments (Strauch). When P. trapezoidis had reached the North Atlantic in Pliocene time, it probably evolved into P. obliquelongata Strauch, which has been found together with P. trapezoidis in the Serripes Zone (Strauch). According to Strauch (1972b), P. obliquelongata migrated into the Arctic Ocean and the Pacific, where it probably became extinct in Lower Pleistocene time (MacNeil et al., 1943; Strauch 1972b). P. obliquelongata is considered to have evolved via P. bivonae (Philippi) into the recent P. norvegica (Spengler) during the Lower or Middle Pleistocene (Strauch). According to Durham and MacNeil (1967), P. norvegica is also of Pacific origin.

14.3.3  P  acific Molluscs Appearing in the Lower Pleistocene Part of the Breiðavík Group, Tjörnes Sequence The Breiðavík Group on Tjörnes spans the Quaternary Era, with the base dating from 2.58 Ma according to the latest age model for the Tjörnes sequence (Eiríksson et al., 2020c). Marine sediments alternate with glacial sediments and volcanics in the lower, Early Pleistocene part of the Breiðavík Group (Eiríksson et al., 2020b). Durham and MacNeil (1967) recorded the prosobranch gastropod Boreotrophon truncatus (Strøm) with Pacific affinity and listed it among species that migrated to the North Atlantic and Iceland in Later (?Lower) Pleistocene after the deposition of the Barmur Group. Apparently, this species or its ancestor have never been found fossil in the Pacific area, and therefore, it is difficult to verify its Pacific origin. Grant and Gale (1931) and Durham and MacNeil (1967) considered the genus Nucella to be of North Pacific origin, and the latter authors listed N. lapillus (Linné) among species that migrated into the North Atlantic and reached Britain during the Pliocene. However, the oldest occurrence in Iceland seems to be in the Svarthamar Member in Breiðavík, deposited during the Lower Pleistocene (cf. Áskelsson, 1935; Bárðarson, 1925; Eiríksson et al., 1992; Vilhjálmsson, 1985). This leads us to conclude that N. lapillus arrived at Iceland during the Lower Pleistocene after the deposition of the Barmur Group, but according to Durham and MacNeil (1967), it already reached Britain during the Pliocene (Table 14.1). From all available information, the bivalve species Ennucula tenuis (Montagu) dates back to the Pliocene both in the Pacific and the Atlantic. However, Durham and MacNeil (1967) considered the species to have originated in the Pacific and

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migrated to the Tjörnes area in? Later (Lower) Pleistocene, but finally they listed it among those with uncertain direction of migration. Another bivalve, the subspecies Mya truncata pseudoarenaria Schlesch, reached Iceland and the Tjörnes area later than the species M. truncata, and occurs for the first time in the Svarthamar Member at about 1.5 Ma (Eiríksson et al., 1992). Apparently, the subspecies evolved from Mya truncata, when it migrated through the Bering Strait into the Arctic Ocean. At that time, some specimens became more adapted to decreased temperature and kept the juvenile form of the predecessor (cf. Strauch, 1972a). If the specimens from the British Isles are correctly identified, the subspecies extended its distribution to the North Sea area, but later it retreated almost entirely from the northwestern Atlantic area south of Iceland, and today it clearly has a more northern occurrence than typical M. truncata (cf. Strauch, 1972a).

14.4  Summary The exchange of marine animals, in particular the influx of molluscs of Pacific Ocean origin to the North Atlantic Ocean via the Arctic Ocean and the Nordic Seas, is a well-documented consequence of the opening of the Bering Strait at the Miocene-Pliocene boundary. The closing of the Central American Seaway following the formation of the Isthmus of Panama resulted in reorganized water circulation in the eastern Pacific and the western Atlantic, as well as in the Arctic Ocean. A density gradient between the two major oceans was enhanced with salinity slowly but steadily increasing in the Caribbean, while the eastern Pacific became fresher as winds brought moisture to the west across the low-lying Isthmus of Panama. The new gateway through the Bering Strait was a prerequisite for the exchange of marine fauna between the Pacific and Arctic Oceans. The presence of the Atlantic-­ Arctic bivalve genera Astarte in southern Alaska at 5.5–4.8 Ma seems consistent with an initial southward flow through the Bering Strait (Marincovich & Gladenkov, 1999). However, a refined age for the earliest opening of the strait indicates strongly that it first opened near the end of the Miocene at 5.32 Ma (Gladenkov et al., 2002). A widespread occurrence of North Pacific molluscs in the North Atlantic area is somewhat younger and reflects a reversal to northward flow through the Bering Strait (Marincovich & Gladenkov, 1999). The reversal seems to have taken place sometimes after 4.6  Ma, when a critical threshold in the closure history of the Central American Seaway took place (Marincovich & Gladenkov, 1999). However, southward flow through the Bering Strait can occur in modern times in connection with periods of persistent northerly winds (Rudels et al., 2013). According to our data, 23 species of gastropod species and 11 species of bivalves of Pacific ancestry reached the North Atlantic and the Tjörnes area during the deposition of the late Miocene and Pliocene Barmur Group, and one gastropod species and two bivalve species of Pacific origin migrated into the area during the deposi-

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tion of the Lower Pleistocene Breiðavík beds (Table 14.1). However, several molluscan genera and species of Pacific origin, that migrated into the North Atlantic while the Tjörnes sequence was deposited, have never been found in any Icelandic sediments. According to Durham and MacNeil (1967), most of these taxa reached the North Sea Basin already in Pliocene time, but apparently the bivalve Mya arenaria Linné already migrated into the Atlantic area in the late Miocene (Gardner, 1943; MacNeil, 1965). These taxa may have bypassed the island, but it cannot be excluded that they have not yet been found in Icelandic sediments in spite of rather extensive collections through decades and even centuries. These include some well-­ known gastropod species such as Amauropsis islandica (Gmelin), Trichotropis borealis Broderip & Sowerby, and Volutopsius norwegicus (Gmelin), as well as the bivalves Kellia suborbicularis (Montagu) and Pandora glacialis Leach. These species all live in Icelandic waters today, except the Arctic bivalve P. glacialis.

14.5  Conclusions 1. The oldest marine sediments of the Tjörnes sequence contain mollusc species of Pacific origin, dating from the Miocene-Pliocene boundary. This is closely synchronous with the opening of the Bering Strait between Alaska and Siberia at 5.32 Ma. 2. At least 23 species of gastropods and 11 species of bivalves of Pacific ancestry reached the coasts of North Iceland during the deposition of the Miocene to Pliocene Barmur Group sediments on Tjörnes. 3. At least one gastropod species and two bivalve species of Pacific origin migrated to the Tjörnes region during the deposition of the Early Pleistocene lower part of the Breiðavík Group sediments on Tjörnes. 4. The major migration of Pacific species of gastropods, bivalvia, and ostracods into the Tjörnes area came in the lowermost part of the Serripes biozone at about 3.8 Ma. 5. The Pacific molluscs found in the Tjörnes sequence have not been recorded from older deposits in the North Atlantic area. 6. Thermophilous species of Pacific origin in the Tjörnes sediments indicate a relatively warm Arctic Ocean allowing passage from the Pacific to the Nordic Seas. 7. The arrival of relatively colder-water molluscs of Pacific origin in the lowermost part of the Serripes Zone is not considered to reflect cooling in the Tjörnes area but may reflect changes in the Arctic Ocean.

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Index

A Abra alba, 354, 355 Acanthocardia echinata, 344, 431, 544 Aclis minor, 307, 308 Acteon noae, 311 Acteon tornatilis, 310 Admete couthouyi, 294, 295 Admete viridula, 293, 858 Ægir Ridge, 2 Aequipecten opercularis, 327, 328 Alvania patorfikensis, 674, 675, 729, 731 Alvania punctura, 261–263 Arctic Circle, 444 Arctic Surface Water, 569 Arctica islandica, 26, 177, 338–340, 429, 535, 538–542, 545, 622, 708, 709 Arthropoda B. hopkinsi, 380 Astarte basterotii, 332, 333 Astarte crenata, 331, 332 Astarte galeotti, 333, 334 Astrononion gallowayi, 777 Astrononion sp., 481 Atlantic meridional overturning circulation (AMOC), 526 Aubignyna perlucida, 486, 544 Axinopsida orbiculata, 709, 710 B Baculites sp., 379 Bagginidae, 477 Balanus balanus, 727, 728 Balanus hopkinsi, 380, 535, 538, 541 Bangastaðir Member, 650, 654

Barmur Group, 72, 75, 97, 102, 103, 107, 109, 110, 182, 198, 209, 444, 844 conglomerate facies, 105 diamictite facies, 104 Egilsgjóta, 223, 224 Eyvík Graben, 219 fossiliferous sandstones, 102 Grænhöfði, 221 Héðinshöfði Formation, 219 Höskuldsvík, 225 Kaldakvísl, 221 lithofacies types, 103 lithostratigraphical units, 102 Lynghöfði, 221 macroscopic field observations, 103 marine fossiliferous, 110 mudrock and Lignite Facies, 112 pebbly coarse sandstones, 105 sandstone bed, 102 sandstones, 108 sediments, 382 shelly conglomerate facies, 105–108 siltstone facies, 110 Skeifá, 223, 224 systematic sedimentological analyses, 103 Tapes Zone, 220 Tjörnes beds, 218, 219 Barmur sequence, 113 Basal basalts, 101 Bathyarca cf. glacialis, 698 Bathymetric map, 9 Benthic algae, 29 Benthic foraminifera, 24 Benthic oxygen, 498 Bering Sea, 849

© The Author(s) 2021 J. Eiríksson, L. A. Símonarson (eds.), Pacific - Atlantic Mollusc Migration, Topics in Geobiology 52, https://doi.org/10.1007/978-3-030-59663-7

869

870 Bering Strait, 844, 846, 847, 851 Arctic Ocean, 847 Chukchi Seas, 844 marine embayment, 845 origin, 844 Pacific water flows, 847 region, 846 tectonic activity, 845 water circulation, 847 Biocoenosis, 450 Biogeographical zones, 450 BIOICE project, 23, 27 Bivalvia A. alba, 354, 355 A. basterotii, 332, 333 A. crenata, 331, 332 A. echinata, 344 A. galeotti, 333, 334 A. islandica, 338–340 A. opercularis, 327, 328 C. angusta, 373, 374 C. chamaeformis, 342, 343 C. ciliatum, 345, 346 C. decorticata, 347, 348 C. tjornesensis, 328, 329 E. ensis, 359, 360 G. glycymeris, 320–322 H. arctica, 368–370 H. squamula, 330 H. squamula aculeata, 330, 331 L. complanatum, 366, 367 Lyonsia sp., 378 M. calcarea, 355, 356 M. edulis, 322, 323, 325 M. modiolus, 326, 327 M. niger, 325, 326 M. obliqua, 358 M. praetenuis, 357 M. schwarzbachi, 365, 366 M. truncata, 361–363 M. truncata gudmunduri, 363, 364 N. minuta, 318 N. nucleus, 316 N. pernula, 319 P. aurea, 348, 349 P. aurea senescens, 349, 350 P. obliquelongata, 372, 373 P. pinnulatum, 343 P. rhomboides, 350–352 P. rustica, 341 P. trapezoidis, 371, 372 S. arcuata, 353, 354 S. groenlandicus, 346, 347 T. borealis, 334, 335

Index T. convexa, 376, 377 T. elliptica, 337, 338 Teredo sp., 375, 376 T. montagui, 336 T. phaseolina, 377, 378 V. corrugata, 352, 353 Y. myalis, 319, 320 Z. crispata, 374, 375 Bolivinacea, 473 Bolivina sp., 473 Bolivinidae, 473 Boreoscala greenlandica, 305, 306 Boreotrophon clathratus, 276, 277, 857 Boreotrophon truncatus, 678, 679 Bouger gravity anomaly map, 42 Breiðavík beds, 817 Breiðavík Group, 46, 72, 99, 757, 758, 788, 804, 828, 860 aerial photographs, 572 alluvial conglomerates, 597 climatic cyclicity, 659 exposures, 570 glaciation, 659 glaciolacustrine mudrocks, 659 kame conglomerates anticlinal/synclinal forms, 585 Boulton’s classification, 585 clay and glaciofluvial material, 585 conglomerate horizons, 584 crevasses, 587 debris ridges, 587 deformation, 588 deglaciation processes, 585 eskers, 584 fluvioglacial material, 587 genetic classification, 584, 585 graded bedding, 588 grain size and sorting, 584 ice-contact conditions, 584 ice-contact washed deposits, 584 ice melts, 587 internal bedding structures, 584 sediments, 584 subchannel melting, 588 subglacial tunnels, 584 supraglacial and englacial models, 585 topography and drainage deposition, 584 vertical changes, 588 Kolbeinsey Ridge, 570 lava flows, 570, 597–599 lithofacies types, 577, 578 lithostratigraphic subdivision, 575 marine mudrocks, 595

Index marine sediments, 659 Matuyama-Brunhes transition, 229, 230 Matuyama Chron, 227–229 mudrocks, 588–593 outwash and lacustrine conglomerates, 588–593 proglacial associations, 588–593 research methods, 576 sandstones, 595, 597 sedimentological characteristics, 572, 575 shoreline bar and lagoon associations, 596 stratigraphy, 570 subaerial lava flows, 659 tillites, 578–580, 582 volcanic tuffs, 597–599 Breiðavík Group documents, 836 Breiðavík Group Paleoenvironments embayment, 806 epifaunal species, 811 foraminifera and molluscs, 806 Furuvík Formation, 806, 809 glacio-lacustrine environment, 807 sedimentary facies, 807 sparse bivalve fauna, 807 Breiðavík sediments, 67 Brunngil Member, 164 foraminifera, 166 lithological description, 164 marine macrofossils, 165 Brunngil region, 167 Buccella calida, 780 Buccella frigida, 485, 519, 779 Buccella spp., 486 Buccella tenerrima, 485, 486, 780 Buccinum cyaneum, 279, 280, 683, 857 Buccinum finmarkianum, 280, 281, 855 Buccinum undatum, 277–279, 681, 682, 857 Bulimina aculeata, 475, 476 Bulimina elongata, 476, 515, 550 Bulimina spp., 476, 775 Buliminacea, 475 Buliminida, 473–477 Búrfell Mountain, 820 Búrfellsá Member, 647, 648, 820 C Cainozoic changes geological data, 844 glaciation, 844 Cainozoic marine faunas Balanus hopkinsi, 382 bivalves, 381 Breiðavík sequence, 385

871 dinoflagellates, 381 faunal changes, 382 Greenlandic sites, 386, 387 North American sites, 387, 388 North Atlantic area, 381 North Sea area, 386 sedimentary xenoliths in Skammidalur, 383, 385 Serripes Zone, 382 Siberian and Russian sites, 388, 389 Tjörnes Beds Group, 381 Tjörnes sequence, 382 Cainozoic trans-Arctic migration, 851 Solariella species, 856 species, 850 Calyptraea chinensis, 267, 268, 549 Cancris auricula, 477 Capulus ungaricus, 267 Capulus unguis, 266, 267 Cassidulina laevigata, 474, 515, 536, 538 Cassidulina limbata, 474, 475, 515, 770, 771 Cassidulina reniforme, 475, 771 Cassidulina spp., 475 Cassidulina teretis, 771 Cassidulinacea, 473 Cassidulinidae, 474 Central America Isthmus, 850 Central American Seaway, 849, 861 factors, 849 Isthmus of Panama, 848 molecular divergence, 849 plants/terrestrial animals, 847 Pleistocene glaciation, 848 shallow seawater, 849 Cephalopoda Baculites sp., 379 Cerastoderma, 535 Cerastoderma decorticata, 347, 348, 536, 538 Chilostomellacea, 484 Chlamys breidavikensis, 700, 701 Chlamys tjornesensis, 328, 329 Chondrophors in myarian species, 365 Cibicides grossus, 479, 515 Cibicides lobatulus, 479, 480, 515, 541, 543, 549, 553–554, 777, 828, 829 Cibicides refulgens, 480, 517 Cibicides spp., 480 Cibicididae, 479 Cibicidoides limbatosuturalis, 480, 481, 517 Cibicidoides pachyderma, 481, 517 Ciliatocardium ciliatum, 345, 346, 713, 714, 858 Climatic conditions, 7 Colus altus, 286

Index

872 Colus elegans, 283, 284 Colus glaber, 285 Colus gracilis, 284 Colus imperspicuus, 283 Colus olavii, 282 Colus sp., 684 Conglomerate, 827 Conglomerate lenses, 811 Crag Formation in England, 214 Crag sediments, 101 Cryptonatica affinis, 270–272, 676, 677 Cryptonatica and Euspira, 854 Cryptonatica occlusa, 272, 273 Cryptonatica smithii, 271, 272, 543, 548, 550 Curtitoma cf. trevelliana, 684, 685 Curtitoma decussata tjoernesensis, 298, 299 Curtitoma trevelliana, 299, 300 Cyclocardia chamaeformis, 342, 343 Cylichna alba, 312 Cylichna cylindracea, 313 Cylichnoides occultus, 313–315, 538, 686, 687 Cyprina islandica horizon, 172 Cyrtodaria angusta, 373, 374, 553, 723, 724 Cytharella costata, 302, 303 Cytharella sp., 302, 303, 855 D Dalvík Lineament (DL), 48 Diameter, 238 Dimmidalur Member, 644, 647, 819 Discorbacea, 477 Discorbidae, 477 Discorbinellacea, 478 Discorbinellidae, 478 Discorbis spp., 477 Discorbitura cushmanni, 478, 515 Divergent plate margin, 5, 6 E Early Pleistocene geological data, 804 Earthquakes, 40, 80 Eastern Volcanic Zone (EVZ), 3, 5 Ecrobia ventrosa, 256–258 Eemian-Weichselian-Holocene cycle, 836 Egilsgjóta fault zone, 143 Egilsgjóta locality, 139 Egilsgjota Member lithological description, 141 marine macrofossils, 141 Ellipsolagenidae, 473 Elphidiella hannai, 487, 519 Elphidiella heteropora, 487, 488, 521

Elphidiella minuta, 488, 521 Elphidiella tumida, 488 Elphidiidae, 486 Elphidium, 469, 498 Elphidium albiumbilicatum, 488, 489, 521, 780 Elphidium asklundi, 781 Elphidium bartletti, 489, 490, 781 Elphidium clavatum, 490, 491, 521, 782 Elphidium gerthi, 491 Elphidium haagensis, 491, 492, 521, 543, 555 Elphidium hallandense, 492, 521, 783 Elphidium hauerinum, 492, 493, 521 Elphidium hughesi, 493, 521, 543, 555 Elphidium incertum, 493, 494, 521, 783 Elphidium karenae, 783 Elphidium macellum, 494, 523 Elphidium magellanicum, 784 Elphidium margaritaceum, 494, 495, 523, 784 Elphidium pseudolessoni, 495, 523 Elphidium selseyensis, 495, 496 Elphidium sp., 496, 523 Elphidium spp., 496, 497, 524 Emarginula crassa, 240, 241 Ennucula tenuis, 687–689 Ensis ensis, 359, 360 Epifaunal species, 471 Eponides spp., 477 Eponididae, 477 Erginus rubellus, 670, 729 Erosional unconformity, 829 Euspira catenoides, 275 Euspira montagui, 274, 275 Euspira pallida, 273, 274, 677, 678 Eyjafjarðaráll Rift (ER), 47 Eyvík Formation, 104, 116–119 Eyvík Graben, 219 F Faujasina subrotunda, 497, 498, 523 Fissurina danica, 473 Fissurina spp., 473 Flat-bedded coarse sandstone, 827 Flatey borehole, 8 Flekatá Member, 156 foraminifera, 154 lithological description, 154 marine macrofossils, 154 Foraminifera, 826 biogeographical zones, 450, 451 content, 763, 764 environmental/stratigraphical Hörgi Formation (Unit 2), 785 Máná Formation (Unit 14), 787

Index remarks/immigration, 788 Þrengingar Formation (Units 8–12), 785–787 fossiliferous samples, 469 Mactra Zone, 450, 452, 454–468 method, 448–450 Miliolinella spp., 453 morphotypes groups, 450 preservation, 450 Pyrgo spp., 453 Quinqueloculina spp., 453 sampling/methods, 448–450, 762 Scutoloris sp., 470 Serripes Zone, 450, 452, 454–468 Sigmoilina sp., 470 stratigraphy, 763 systematics, 764 Tapes Zone, 450, 452, 454–468 taxa, Breiðavík Group, 765 Tjörnes Peninsula, 758 Foraminiferal assemblages, 19, 826 Mactra Zone, 498, 500, 501 Serripes Zone, 498, 501, 502 Tapes Zone, 498, 499 Foraminiferal test, 451 Fossgil Member, 619, 814–816 alluvial coastal/lava plain, 624 Breiðavík Bay, 622 Búrfell Mountain, 617 diamictite/lava flow, 620 glacial-glaciolacustrine, 624 lacustrine sediment, 617 Máná Formation, 620 marine sedimentation, 617 paleoenvironments, 624 paleogeographical conditions, 625 shoreline, 624 subsequent mapping, 617 subunits, 620 transgressing bar environment, 815 Tröllagil, 620, 621 tuff layers, 814 Fossiliferous sand, 830 Fucus serratus, 671 Furugerði Member, 605 Furuvík Formation, 806 G Gastropoda Opisthobranchia (see Opisthobranchia) Prosobranchia (see Prosobranchia) Gavelinopsis praegeri, 775

873 Gavellinidae, 484 Geological research history, Tjörnes sequence economic resources, 59 features, 58 fossil evidence, 59 fossiliferous beds and lignites, 60 fossil plant, 61 geological literature, 59 glaciations, 59 Icelandic and foreign, 61 marine sediments, 58 mollusc species, 59 Tjörnes sequence, 59 Geomorphological processes, 4 Gibbula cineroides, 249 Gibbula tumida, 248 Glabratella wrightii, 776, 777 Glacial-interglacial cycles, 7, 829, 832 Glacial Period, 7 Glacial striae, 821 Glaciation events, 835 Glaciation record, 7 Glacio-lacustrine environment, 806–807 Glandulina laevigata, 770 Globulina spp., 470 Glycymeris glycymeris, 320–322, 544 Grænhöfði Formation, 131, 138 Hústorfa Member, 137 Klungur Member, 131 marine fossils, 131 Tungugerðiskambar Member, 133 Grænur Member, 170, 179 Grain size distribution, 580 Grasafjall Formations, 656–658, 820 Bangastaðir Member, 650, 654 diamictite bed, 820 Miðlækur Member, 650 Skeiðsöxl Member, 650 Gravity data, 42 Grjótháls, 821 Guttulina spp., 472 H Háibakki Member, 145 foraminifera, 144 lithological description, 144 marine macrofossils, 144 Hallbjarnarstaðaá river, 182 Hallbjarnarstaðaá River channel, 191 Hallbjarnarstaðakambur, 172, 179, 181, 183, 186–188 Hanzawaia boueana, 484, 485

874 Hauerinidae, 470, 769 Haynesina depressula, 778 Haynesina germanica, 778 Haynesina orbiculare, 483, 519, 778 Héðinshöfði Formation, 219 basal unit, 113 failures and mass movement, 115 terrestrial sediments, 113 Héðinshöfði promontory, 113 Heteranomia squamula, 330, 427, 703, 753 Heteranomia squamula aculeata, 330, 331, 704 Hiatella, 369 Hiatella arctica, 368–370, 855 Hiatella rugosa, 721–723 Highest occurrence (HO) levels, 225 Holocene, 568 Holocene climate, 804 Hörgi Formation, 613, 614, 809, 811 basal conglomerate unit, 608 basal sediments, 608 evolution of environments, 613 glacial, 613 glacio-lacustrine, 614 interface, 608 Króklækur, 609 marine macrofossils, 612 mudrocks, 612 sandstones, 612 shoreline, 614, 616 Þrengingar Formation, 609 Hörgi sequence, 813 Höskuldsvík Group, 46, 202, 206 coastal section, 202 Höskuldsvík lava, 206 type locality, 202 Hringver Formation, 164 Hvilft Member, 157 member, 156 Skeifárbás Member, 158 Hringvershvilft area, 152 Húsavík Faults, 40 Húsavík Formation, 654–656 Húsavík-Flatey fault zone, 45 Húsavík-Flatey Zone (HFF), 47 Hústorfa Member, 137 foraminifera, 138 marine macrofossils, 137 Hvilft Member, 157 lithological description, 157 marine macrofossils, 157 Hydrobia ulvae, 256, 257

Index I Ice Age, 7, 8 Iceland Arctic species, 386 Arthropoda, 380 (see also Arthropoda) Bivalvia (see Bivalvia) Cephalopoda, 379 climatic conditions, 7 geological map, 2, 3 geological structure, 1, 2 glaciation record, 7 North Atlantic, 4 Patella pellucida, 244 plate tectonics, 4–5 Plio-Pleistocene section, 443 sedimentary basins offshore, 8, 9 sedimentary xenoliths with marine molluscs, 383 sediments, 6 Skammidalur Formation, 381 terrestrial sedimentary basins and sequences, 8 Testudinalia (Collisella) testudinalis, 245 volcanics, 6 western Norway, 241 Iceland marine realm, 15 Iceland-Faroe Ridge, 15 Icelandic Economic Zone (IEZ), 23 Icelandic waters, 20 Ice-rafted debris (IRD), 528 Indicator species, 238 Insular Basalt Formation, 62 Invertebrates brachiopods, 22 crustaceans, 22 ctenophores, 21 foraminiferal, 19 Halacaridae, 22 modern studies foraminifera distributions, 23 foraminifera species, 24 infaunal animal communities, 27 Molluscan species, 25 pallial sinus, 26 planktonic species, 24 Tanaidacea, 27 molluscs, 20 Nucella lapillus, 20 Ostracoda, 22 sponge species, 21 Islandiella helenae, 772 Islandiella inflata, 773 Islandiella norcrossi, 773

Index J Jan Mayen Ridge, 2 Janthina exigua, 306, 307 K Kaldakvísl Group, 209 Kaldakvísl Lava Group, 100–102, 218 basaltic lava flows, 100 classic transaction, 101 lava flows and sediments, 102 stratigraphy, 100 Kaldakvísl locality, 119–122, 124 geological map, 121 Kambsgjá Member, 183–185, 193–196 Keeled group, 451, 471 Klungur basalts, 135 Klungur Member, 131, 133 Kolbeinsey Ridge (KR), 47, 78, 528 Kvíslarkambur, 126 Kvislarkambur Member, 129, 130 Kvíslarós Member, 129 L Lagena laevis, 470 Lagena spp., 470 Lagenida, 470–473 Laminaria, 671 Lentidium complanatum, 145, 171, 366, 367, 540, 541, 543, 545, 547, 553 Lepeta caeca, 247, 672 Light microscope multifocus images, 796, 798–799 Liomesus dalei, 281, 282 Lithification, 762 Lithological cycles, 660 Lithostratigraphical unit, 450 Lithostratigraphic group, 804 Lithostratigraphic Group units, 209 Lithostratigraphic work, 68, 94 Lithostratigraphy, 65 benthic stable isotopes, 568 characteristics, 568 climate history, 568 cold-temperate and maritime, 569 collision-related tectonic settings, 568 deep-sea sediments, 568 divergent plate margins, 568 elements, 568 erosion, 569 faunal distribution, 570 Furugerði Member, 600, 602, 605, 607

875 glacial and oceanographic variability, 569 glaciation, 568 ice-core stratigraphy, 568 intensive relief and erosion, 568 Miðnef Member, 607, 608 North Iceland shelf region, 570 Oxygen isotope curves, 568 plate tectonic processes, 568 processes, 568 Quaternary stratigraphy, 568 regional oceanography, 569 sedimentary particles, 570 Svarthamar Member, 625, 631, 633, 634 tectonic and volcanic processes, 569 Tjörnes, 569 Littorina islandica, 254 Littorina obtusata, 673, 674, 729 Littorina species, 255 Littorina squalida, 252–254, 542, 547, 854 Lynghöfði Formation, 126 Kvíslarkambur Member, 129 Kvíslarós Member, 129 Lyonsia cf. arenosa, 725, 726 Lyonsia sp., 378 M Macoma calcarea, 355, 356, 435, 622, 716, 717, 858 Macoma obliqua, 224, 358 Macoma praetenuis, 357, 547 Macrofossils, 105 Mactra Zone, 444, 445, 449, 450, 452, 454–468, 498, 500, 501 Máná basalt, 642 Máná Formation, 656–658, 819 Búrfellsá Member, 647, 648 Dimmidalur Member, 644, 647 erosional surface, 634 interpretation, 634 lava flows, 635 marine fossiliferous sandstone, 634 mollusc fauna, 634 sedimentary facies, 634 sediments, 634 Stapavík Member, 637–639 Torfhóll Member, 640, 642–644 Margarites costalis, 251, 252 Margarites groenlandicus, 251, 672, 673, 854 Marginulina spp., 470 Marine environment deep-water formation, 17 intertidal zone, 16

Index

876 Marine environment (cont.) Irminger Current, 16, 17 ridges, 15 sea floor, 16 sea-surface temperature, 17 tidal range, 15 tidal wave, 15 water mass, 17 wind stress, 18 Marine fossiliferous sediments Pleistocene age, 64 stratigraphy, 63 transgression, 63 Marine gateways, 842 Marine invertebrate systematics, 242 Marine life, Iceland biogeographical zones, 18 brachiopods, 22 crustaceans, 22 Echinoidea, 23 foraminifera, 18 Holothurioidea, 23 invertebrates (see Invertebrates) modern marine fauna, 18 molluscs, 20 octocorals, 21 polychaetan species, 22 species distribution, 22 sponge species, 21 Marine macrofossils, 181 climate data, 70 stratigraphy and paleontology, 70 Strauch, 70 Marine microfossils foraminifera, 76 ostracod study, 76 Marine molluscs, 20, 728 Marine sediments, 6, 8, 9 Marine trans-Arctic migration, 846 Marine transgressions, 836 Matuyama-Brunhes transition, 68 Melanella frielei, 304, 305 Melonis barleeanus, 482, 517 Metapolymorphina charlottensis, 472 Metaxia metaxa, 303, 304 Mid-Atlantic Ridge, 1, 4 Miðlækur Member, 650, 654 Miðnef Member, 607, 608 Miliolinella spp., 453 Miliolinella subrotunda, 768 Miocene, 6 Miocene glaciations, 62 Miocene-Pliocene bedrock, 99 Miocene-Pliocene boundary, 220, 526, 852

Modiolus modiolus, 326, 327, 855 Mollusc assemblages, 832 Mollusc migration Boreotrophon clathratus, 277 Cainozoic faunas (see Cainozoic marine faunas) Lunatia, 275 Macoma calcarea, 356 Naticidae, 272 Tjörnes beds, 281, 388 Molluscabase, 237 Molluscan species, 25 Molluscs, 21, 821 Morphotype groups, 454–468 Mudrock facies, 816, 817 Musculus niger, 325, 326, 425, 855 Myarian species, 364 Mya schwarzbachi, 365, 366, 547, 859 Mya sp., 360 Mya truncata, 361–363, 622, 717–719, 859 Mya truncata gudmunduri, 363, 364, 437 Mya truncata pseudoarenaria, 719, 720 Mysella bidentata, 712 Mytilus edulis, 322–325, 535, 536, 538, 540–541, 554, 699, 700 N Nassarius cf. consociatus, 417 Nassarius consociatus, 292 Nassarius reticosus, 291 Nassarius tjoernesensis, 290 Neogene, 6 Neogene/Quaternary boundary, 66 Neptunea lyratodespecta, 287, 288 Nodosariacea, 470 Nöf Member, 198 lithological description, 205 marine macrofossils, 205 Nonionacea, 481 Nonion crassesuturatus, 482 Nonion granosum, 482, 483, 517 Nonionella graciosa, 483, 519, 542, 550 Nonionella proloculata, 484, 519, 542, 544 Nonionella pulchella, 779 Nonionella spp., 484 Nonionellina labradorica, 484 Nonionidae, 481 Nonvolcanic processes, 38 North Atlantic Ocean floor, 4 North Iceland, 475, 486, 503 age model data, 214 biostratigraphic correlations, 214

Index field relations, 214 fossil material, 214 geological features, 214 geological mapping, 214 Höskuldsvík Group, 225, 226 Kaldakvísl Lava Group, 218 lithostratigraphic units, 214 marine deposits, 214 paleomagnetic measurements, 214 Tjörnes sequence, 230–232 volcanic and sedimentary rocks, 214 volcanic rocks, 214 Northern Volcanic Zone (NVZ), 3, 5, 41, 47, 49, 52, 528 Nucella lapillus, 20, 679–681 Nuculana minuta, 318, 689 Nuculana pernula, 319, 690 Nucula nucleus, 316, 317, 423, 545 Nuculana minuta, 318, 689 Nuculana pernula, 319, 690 O Obtusella tumidula, 260, 261 Oceanographic properties, 24 Oddi Formation lithostratigraphic logs, 122 Syðrihóll Member, 125 Tunguá Member, 124 Oenopota borealis, 296, 297 Oenopota pingeli, 297, 298 Oenopota pyramidalis, 295, 296 Omalogyra atomus, 265 Ondina divisa, 309, 421 Onoba aculeus, 675, 676 Onoba aculeus, 263 Onoba semicostata, 264, 539 Oolina spp., 473, 770 Oolona hexagona, 473 Oolona melo, 473, 770 Opisthobranchia A. noae, 311 A. tornatilis, 310 C. alba, 312 C. cylindracea, 313 C. occultus, 313–315 P Pacific marine mollusc fauna, 843 Pacific Molluscs Testudinalia, 852 Paleoceanography, 94 Paleoenvironments, 602

877 Paleomagnetic dating, 214, 219, 224, 227, 228, 230, 233 Paleomagnetic groups, 66 Paleomagnetic measurements, 69 Paleomagnetic research, 66, 67 Paleomagnetic stratigraphy, 804 Paleomagnetism, 67 Paleontology, 668, 669 Paleoecology, 237, 238 Paleoenvironmental development, 498 reconstructions and biostratigraphy, 444–449 Paleomagnetic techniques, 7 Paleotemperatures, 444 Panomya obliquelongata, 372, 373, 439 Panomya trapezoidis, 371, 372, 860 Paphia, 535 Paphia aurea, 348–350, 538, 539 Paphia rhomboides, 350–352, 433, 536 Parvicardium pinnulatum, 343 Patella aspera, 242, 243 Patella pellucida, 244, 671 Patella vulgata, 241, 242 Pebble morphology, 105 Phytoplankton, 30 Planispiral group, 471 Plano-convex group, 451, 471 Planorbulinacea, 478 Planulina ariminensis, 478, 515 Planulinidae, 478 Plate margins, 5 Plate tectonics, 2, 4, 5, 8, 40 Pleistocene age, 806 Pleistocene faunas Greenlandic sites, 731, 732 Icelandic sites, 730 North American sites, 732, 734 North Sea sites, 730 Russian site, 735 Siberian site, 735 Pliocene Barmur Group, 71, 729, 805, 806 AMOC, 526 Arctic/subarctic species, 533 benthic oxygen isotope stack, 556, 558 biozones, 533 coastal and shallow-marine environments, 529 Cyperaceae, 556 deep-sea records, 526 development, 558 ecological information, 531 paleoenvironmental development, 531 sedimentological zonation, 531

Index

878 Pliocene Barmur Group (cont.) environmental parameters, 533 first appearance datum, 532 fluvial depositional environment, 556 fossil marine fauna, 533 fossil molluscs, 553 fracture zone, 529 geological map, 551 glacial and periglacial processes, 529 Grænhöfði Formation, 558 grasses, 560 gymnosperms, 556 Icelandic waters, 533 ice-volume changes, 555 Ilex, 526 infaunal species, 533 IRD, 528 Late Miocene, 528 lignites, 554 Mactra Zone, 526, 540–546, 559, 561 marine environment, 554 marine fossils, 533 marine macrofossils, 532 marine sediments, 558 mollusc biozones, 526 mollusc fauna, 553, 554 ocean temperature, 555 paleoecology, 531 regressions, 528, 555 regression-transgression cycles, 551 Reká Formation, 558 sandstone, 554 sea-water temperatures, 533 sedimentary environments, 531 sedimentological and ecological changes, 561 sediments, 529, 553 seismic profiles, 528 Serripes Zone, 546–550, 559–561 shales, 529 shallow-marine environment, 526 siltstones, 529 Tapes Zone, 526, 535, 536, 538–540, 556 tectonic environment, 529 temperatures, 558 terrestrial and shallow marine deposits, 528 terrestrial mudrocks, 554 topographic elements, 529 transform deformation, 529 transgression, 529 transgressions, 528, 555 Viscum album, 526 Pliocene foraminiferal stratigraphy, 448–449, 498, 499, 502, 503

Pliocene Formation, 59 Pliocene paleoecology diameter, 238 Molluscabase, 237 paired, 238 standard nomenclature, 237 Polar Front, 17, 19 Polymorphinacea, 470 Polymorphinidae, 470, 472, 770 Polyplacophora, 20 Portlandia arctica, 670, 691–694, 731 Þrengingar Formation, 630, 813 Propebela harpularia, 300, 301 Propebela nobilis, 301, 302 Prosobranch gastropods, 383 Prosobranchia A. couthouyi, 294, 295 A. minor, 307 A. punctura, 261, 263 A. viridula, 293 B. clathratus, 276, 277 B. cyaneum, 279, 280 B. finmarkianum, 280, 281 B. greenlandica, 305, 306 B. undatum, 277–279 C. affinis, 270, 271 C. altus, 286 C. chinensis, 267, 268 C. elegans, 283, 284 C. glaber, 285 C. gracilis, 284 C. imperspicuus, 283 C. occlusa, 272, 273 C. olavii, 282 C. smithii, 271, 272 C. tjoernesensis, 298, 299 C. trevelliana, 299, 300 C. unguis, 266, 267 Cytharella sp., 302, 303 E. catenoides, 275 E. crassa, 240–241 E. montagui, 274, 275 E. pallida, 273, 274 E. ventrosa, 256–258 G. cineroides, 249 G. tumida, 248 H. ulvae, 256 J. exigua, 306, 307 L. caeca, 247 L. dalei, 281, 282 L. islandica, 254 L. squalida, 252–254 M. costalis, 251, 252 M. frielei, 304, 305

Index M. groenlandicus, 251 M. metaxa, 303, 304 N. consociatus, 292 N. lyratodespecta, 287, 288 N. reticosus, 291 N. tjoernesensis, 290 O. aculeus, 263 O. atomus, 265 O. borealis, 296, 297 O. divisa, 309 O. pingeli, 297, 298 O. pyramidalis, 295, 296 O. semicostata, 264 O. tumidula, 260, 261 P. aspera, 242, 243 P. harpularia, 300, 301 P. nobilis, 301, 302 P. pellucida, 244 P. sarsi, 259, 260 P. vulgata, 241, 242 R. obsoleta, 258, 259 S. costifera, 288, 289 S. lundgrenii, 289 Solariella sp., 249, 250 S. planorbis, 266 T. (Collisella) testudinalis, 245, 246 V. velutina, 269 Pseudopolymorphina spp., 472 Pusillina sarsi, 259, 260 Pygocardia rustica, 341, 539 Pyrgo spp., 453 Q Quaternary, 4, 6 Quaternary biostratigraphy, 214, 233 Quaternary Breiðavík Group, 68 Quaternary formations, 69 Quaternary ice age, 804 Quinqueloculina seminula, 769 Quinqueloculina spp., 453 Quinqueloculina stalkeri, 769 R Radiometric ages, 81 Radiometric dating, 214, 218, 219, 231 Regressional environment, 809 Reká Formation, 148 Flekatá Member, 155 Rekárfoss Member, 151 Skonsur Member, 150 type area, 148 Rekárfoss Member, 151, 152

879 lithological description, 150 marine macrofossils, 151 Reká River, 148 Retusa obtusa pertenuis, 685, 686 Reynisnes Peninsula, 79 Rifting axis, 2 Rifting zones, 4 Rissoa obsoleta, 258, 259 Rosalina spp., 776 Rosalina williamsoni, 478 Rosalinidae, 477 Rotaliacea, 486 Rotaliida, 477–498 Rounded planispiral group, 451 S Sæmundsson’s interpretation, 80 Sarsicytheridea-Thaerocythere, 554 Scanning electron micrographs (SEM), 765, 800–801 Scutoloris sp., 470 Sea-bird species, 29 Searlesia costifera, 288, 289 Searlesia lundgrenii, 289 Sedimentary basins offshore, 8, 9 Sedimentary processes, 592 Sediments, North Iceland Shelf lavas capping, 43 near-shore environments, 42 volcanic system, 43 Seismic reflection profiling, 42 Serripes groenlandicus, 224, 346, 347, 547, 714 Serripes Zone, 444, 445, 449, 450, 452, 454–468, 498, 501, 502 Shallow marine environment, 807 Shallow marine faunas, 531, 555 Sigmoilina sp., 470 Silty facies, 813 Similipecten greenlandicus, 701, 702 SKD-SK trough, 51 Skeiðsöxl Member, 650, 654 Skeifá Formation, 167 Brunngil Member, 164 coastal waterfall, 160 Ytri Svarthamar, 165 Skeifárbás Member, 158 lithological description, 161 marine macrofossils, 161 Skeneopsis planorbis, 266 Skonsubakki Member, 145 lithological description, 147 marine macrofossils, 147

880 Skonsur Member, 150 foraminifera, 149 lithological description, 149 marine macrofossils, 149 Snæfellsnes-Húnaflói Rift Zone, 6 Solariella obscura, 249, 250 Solariella sp., 249, 250 Spisula arcuata, 535, 541, 553 Spisula arcuata, 353, 354 Spisula elliptica, 715, 716 Stainforthia fusiformis, 475, 775 Stainforthia schreibersiana, 774 Stainforthiidae, 475 Stapavík Member, 635–639, 818 Stapi–Höskuldsvík region, 203 Stapi Member, 198, 206 Stórhöfði Formation, 192 Nöf Member, 198 Stapi Member sediments, 198 Tófugjá Member, 193 Stratigraphical log unit, 450 Stratigraphy, 209 coastal outcrops, 99 lithological units, 100 lithostratigraphy, 99 reconnaissance, 99 Submarine ridges, 2, 5, 6, 8 Surface ocean circulation, 16 Svarthamar accumulation, 833 Svarthamar facies sequence, 830 Svarthamar Member, 625, 629, 631, 633, 634, 816, 822, 828, 830 coarse-grained conglomerate, 823 conglomerate facies, 824 diamictite facies, 822 massive diamictite, 822 mollusc assemblage, 825 paleoecological information, 825 pre-glaciation bedrock, 822 sedimentary facies, 822 tuff facies, 825 volcanism, 822 Syðrihóll Member, 127 T Tapered group, 451, 471 Tapes Zone, 444, 445, 449, 450, 452, 454–468, 498, 499 Teredo sp., 375, 376 Terrestrial sedimentary basins and sequences, 8 Tertiary formations, 18 Tertiary Plateau Basalts, 66 Testudinalia, 852

Index Testudinalia (Collisella) testudinalis, 245, 246 The Zoology of Iceland, 23 Thraca convexa, 441 Thracia cf. septentrionalis, 724, 725 Thracia convexa, 376, 377, 539 Thracia phaseolina, 377, 378 Thyasira cf. sarsii, 710, 711 Tjomes sequence, 98 Tjörnes, 58, 732, 736 A. baserotii, 332 A. crenata, 332 Acteon, 310 A. galeotti, 334 Arctic Circle, 444 Baculites, 379 Barmur Group, 239 Barmur Group sediments, 382 basalts, 61 B. hopkinsi, 380 bivalve species, 383 breccias and conglomerates, 62 Cainozoic faunas (see Cainozoic marine faunas) C. borealis, 342 C. ciliatum, 345 C. decorticata, 348 Cibicidoides sp., 481 C. tjornesensis, 328, 329 distinctly arctic species, 386 Ensis ensis, 359 gastropod species, 381 geological account, 61 geological structure, 81 geology, 61 late Neogene Siberian deposits, 388 L. complanatum, 367 L. norwegia, 378 Mactra Zone, 251, 252 marine molluscs, 381 M. edulis, 322 molluscan assemblages, 382 M. schwarzbachi, 366 M. truncata, 361, 362, 388 M. truncata gudmunduri, 363 North Iceland, 503 northern Iceland, 443, 444 N. pernula, 319 oxygen isotope, 444 Panomya, 371 P. aurea, 350 Pliocene and Lower Pleistocene, 387 Pliocene biozones, 445 Pliocene Tjörnes fauna, 386 P. obliquelongata, 372 P. rhomboides, 352

Index prosobranch gastropods, 383 Prosobranchia (see Prosobranchia) S. arcuata, 354 Serripes Zone, 311, 385 Solariella species, 250 stratigraphic work, 63 Tapes Zone, 315 T. borealis, 335 tectonic disturbances and subsidence, 61 temperature changes, 62 Teredo sp., 376 T. montagui, 336 Tjörnes Basin (TB), 48, 51, 526 Tjörnes beds, 98 Tjörnes Breiðavík Group, 385 Tjörnes coal mine spoil heap, 76 Tjörnes Fracture Zone (TFZ), 5, 8, 233, 526, 842 concept, 77 earthquakes, 40, 41 faults and tectonic deformation, 41 HFF, 47 location map, 77 modern and postglacial activity, 41 Neogene bedrock, 39 NVZ, 47 ocean floor, 40 strike-slip movement, 77 structure and nature, 40 tectonic position, 40 transcurrent movement, 77 volcanic succession, 78 volcano-tectonic features, 80 volcano-tectonic zones, 47 Tjörnes geology, 81 Tjörnes horst geological map, 43 topography, 43 Tjörnes Peninsula, 44, 45, 95, 805 Tjörnes sedimentery basin seismic data, 50 TB sediments, 49 TFZ, 48 Tjörnes sequence, 59, 65, 67, 71, 73, 98, 526, 804, 821, 843, 850–852 Barmur group, 45 Breiðavík Group, 46 climatic history, 74 environmental changes, 45 fossil material, 72 geology, 94 Höskuldsvík Group, 46 isotopes in molluscs, 72 lignites, 75 marine invertebrates, 94

881 mining operations, 75 mollusc stratigraphy, 94 paleomagnetic data, 94 paleontology, 72, 74 palynology, 74 plant assemblages, 75 Pleistocene deposits, 74 sedimentary basin, 45 stratigraphy, 72 tectonics, 94, 95 Tófugjá Member, 193, 197, 198 Torfhóll facies sequence, 818 Torfhóll Member, 640–644, 818 Transform fault, 40 Transform zones, 5 Transgressing bar environment, 819 Transitional horizons, 444 Treatise on Invertebrate Paleontology, 668 Trichohyalidae, 485 Tridonta borealis, 334, 335, 704–706 Tridonta elliptica, 337, 338, 707, 708 Tridonta montagui, 336, 706, 707 Trifarina fluens, 476, 477, 515 Tunga Formation Hallbjarnarstaðakambur Member, 179 Tungukambur Member, 177 Tunguá Member, 127 Tungugerði Formation, 138 Egilsgjóta, 142 Háibakki section, 144 Tungugerðisbakkar, 145 Tungugerdiskambar Member, 134, 136 Tungugerðiskambar Member, 133, 136 Tungugrænur Formation Grænur Member, 170 Tungugrænur hill, 167 Tungugrænur locality, 177 Tungukambur Member, 177, 184–186 Turbellaria, 21 Turritelinacea, 475 U Uplift bordering plate tectonic rifts, 50 Uvigerinidae, 476 V Vaginulinidae, 470 Velutina velutina, 269 Venerupis corrugata, 352, 353 Verruca stroemia, 727 Vertebrates amphibians and reptiles, 27 commercial species, 28

Index

882 Vertebrates (cont.) Icelandic coasts, 28 marine fishes, 28 marine mammalian fauna, 28 Volcanic activity, 42 Volcaniclastic sediments, 576 Volcanism, 1, 80 Voluminous sand bodies, 108 Votalág Member, 169, 178 W Western Volcanic Zone (WVZ), 3, 5 Wind stress, 18

Y Yakataga Formation, 387 Yoldia hyperborea, 677, 678 Yoldia myalis, 319, 320 Yoldiella frigida, 697, 729 Yoldiella intermedia, 695, 696 Yoldiella lenticula, 694, 695 Ytri Svarthamar Member, 165, 172, 173 Z Zirfaea crispata, 374, 375, 548, 856 Zoogeography, 238, 240, 668