Cold Breath of Dormant Volcanoes: The Unknown World of CO2 Mofettes 3662653745, 9783662653746

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Hardy Pfanz

Cold Breath of Dormant Volcanoes The unknown world of CO2 mofettes

Cold Breath of Dormant Volcanoes

Hardy Pfanz

Cold Breath of Dormant Volcanoes The unknown world of CO2 mofettes

Prof. Dr. Hardy Pfanz Angewandte Botanik & Vulkanbiologie Universität Duisburg-Essen Essen, Nordrhein-Westfalen, Germany

ISBN 978-3-662-65374-6    ISBN 978-3-662-65375-3 (eBook) https://doi.org/10.1007/978-3-662-65375-3 © Springer-Verlag GmbH Germany, part of Springer Nature 2023 The translation was done with the help of artificial intelligence (machine translation by the service DeepL.com). A subsequent human revision was done primarily in terms of content. 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-Verlag GmbH, DE, part of Springer Nature. The registered company address is: Heidelberger Platz 3, 14197 Berlin, Germany

Mofettes are dangerous sites, the exploration of which requires precise knowledge of the geological as well as physicochemical and climatic factors. We expressly warn against independent search and exploration of mofettes! Above all, avoid the vicinity of the ground. Do not kneel or lie down under any circumstances. CO2 gas is heavier than air. There is danger to life, since CO2 in increased concentrations has a toxic effect on all living organisms.

Dedicated to my friend and colleague Priv. Doz. Dr. Bohumir Lomský, who left us much too early.

Preface to the Second Edition

The almost exponentially increasing knowledge of cold CO2 emissions around volcanoes and tectonic faults necessitated an expanded description of this interesting geo-biological phenomenon. Many “mofettologists” have left behind the purely descriptive, phenomenological phase of mofette research and have meanwhile provided quantitative statements and descriptions that allow much more insight into this fantastic phenomenon of geogenic CO2 degassing. It should be mentioned that because of this fascinating biological contribution to volcanology, many a dyed-in-the-wool eu-geologist now takes note of the green weeds (connoisseurs call it vegetation) on the bare soil, and even uses them analytically. It is a pleasure to observe how even the most classical volcanologist now uses the plant genus Carex as a matter to point out the indicator effect of these plants for geogenic CO2 emissions to his interested geo-students. It would be nice if this work could again convince people of the beauty, rarity and diabolical charm of mofettes. Perhaps this would also contribute to the fragile mofette sites of northwest Czechia or those of the East Eifel (Laacher See) finally getting the protection they deserve as geotopes or better geo-biotopes. I wish you many new insights and mephitic fun while reading. Dülmen, Germany Summer 2019

Hardy Pfanz

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Preface to the First Edition

Plants and their reactions and adaptations are much more exciting than one might commonly believe. Although “Lieschen Müller” still imagines the classic botanist in scuffed corduroy trousers, protected by a headdress that has gone completely out of fashion and roaming through swamps and bogs with the characteristic vasculum (botanical vessel), botanical science has developed enormously in recent decades. In addition to the purely qualitative description of plants and their environment, one has now moved on to highly quantitative procedures and sophisticated techniques. For a long time, researchers have been investigating the causal relationships between the organic and inorganic environment, both in the laboratory and in the field. In this context, the examination of the phenomena of plant adaptation to extreme sites is particularly exciting. One such site that has recently received increasing attention, and to which botanical sciences are contributing significantly, is the site of volcanic gaseous CO2 exhalations – the mofettes. In close collaboration with many geologists, volcanologists, sedimentologists, pedologists, geochemists and geophysicists, biology is currently working on these exciting sites – and knowledge is increasing accordingly. The findings and explanations, and the unanswered questions of this collaboration are the content of this booklet. May the reading be instructive and exciting as well as motivating. If, after reading the book, the reader should show botanophilic as well as vulcanophilic affections, the author would have achieved his set goal. Dülmen, Germany Spring 2008

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Acknowledgements

I owe a debt of gratitude to my father, Gerhard Pfanz, for making it legible and for the nerve-racking translation of the handwritten court file in Sütterlin on gas drilling in the Brohl Valley/Eifel. Prof. Dr. Dominik Vodnik and Prof. Dr. Franc Batič (Universität Ljubljana, Slovenia) I owe a great debt of gratitude for the introduction to field mofettology. Dir. Dr. Antonio Raschi (IBIMET, Florence) is thanked for the great cooperation in working on Italian mofettes. I would like to thank Dr. Saßmannshausen for the excellent identification of mofette plants including the exact determination of some delicate Carex species during our measuring campaigns. I would like to thank Dr. Annika Thomalla (she holds by far the world record of borehole drilling with far more than 900,000 holes drilled for CO2 soil measurements) for the preparation of the pretty gas emission maps, the help with the evaluation of difficult data as well as the editing of illustrations. Mr. Heinz Lempertz (DVG Mendig) is to be thanked for organisation and support of many measuring campaigns in the Eastern Eifel. Many thanks to Father Basilius Sandner of the Benedictine Order of Maria Laach and the entire monastery for the access to the archives and the many aids during the preparation of the volume. Dr. Horst Kämpf (GFZ Potsdam) is thanked for the guided tours through German and Czech mofette areas. Dr. Wittmann did an excellent job with the ecophysiological measurements in Slovenia and the Czech Republic despite the permanent attacks of vicious mosquitoes. We would like to thank the company Carbo and especially its former managing director, Dr. Krause, as well as the new managing director, Mr. Oliver Kik, for a factory tour and the provision of old photographs. The colleagues Professor

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Dr. K.  Heide, Prof. Dr. G.  Büchel (all University of Jena), Dr. Christina Flechsig (Universität Leipzig), PD Dr. Volker Tank (Deutsche Luft- und Raumfahrttechnik DLR, Oberpfaffenhofen), Dr. U.  Koch (Sächsische Akademie der Wissenschaften), and Dir. Dr. Bohumir Lomsky (VULHM, Prague) for their active assistance, their valuable information but also for their initial scepticism towards botanists poaching in foreign territories. Many thanks to the assistants Christa Kosch and Sabine Kühr for superb help in the field and excellent data processing. Many thanks also to Mrs. Kerstin Werner (Roermond, NL) for the great ideas and help with the tectonic cornfield research. Thanks to the students Olivier Ruiz and Laura Siegert for the professional identification of the not always handsome and truly not good smelling animal corpses. Further thanks are due to Dr. Simon Plank (DLR Oberpfaffenhofen) for providing the aerial photo data, Dr. Michael Tercek and John King (Yellowstone), Dr. John (Jack) Lockwood (Hawaii) and Dr. Bill Evans (USDA, San Francisco). Dr. Fatima Viveiros, Dr. Catarina P.P. Silva (Ponta Delgada, Azores), Prof. Dr. Georgos Papatheodorou, Prof. Dr. Maria Geragas, Mr. Xenophon Dimas (all Universität Patras), and Prof. Dr. Kostas Kyriakopoulos (Universität Athen) are thanked for their guidance and help in the respective field. Prof. Dr. Galip Yüce (Haceteppe University of Ankara) and his team, Prof. Dr. Antoine Kies (Universität Luxemburg), Dr. Giovanni Chiodini (Osservatorio Vesuviano, Naples), Prof. Dr. Thilo Rennert (Universität Hohenheim), Prof. Dr. Wolfgang Bilger (Universität Kiel), Prof. Dr. Dominik Begerow (Universität Bochum), Prof. Dr. Hans-Ulrich Schmincke (Geomar, Kiel), Dr. Heiko Woith (GFZ Potsdam), forester Karl-Hermann Gräf (Forestry Office Koblenz), Mrs. Uhl from the Department of Nature Conservation, Struktur und Genehmigungsdirektion Nord, Koblenz, and Mrs. K.  Flick, Untere Landschaftsschutzbehörde des Kreises Siegen-­ Wittgenstein, for their valuable support during field visits. For the permission to make annual measurements in the Bad Pyrmont haze cave, I thank Mr. Dirk Langhammer and the deputy spa director André Schubert. Thanks to my son Benny Pfanz and my wife Elfi Pfanz for the preparation of some illustrations and the perfect proofreading. The essential help and nicely formulated criticism during proofreading by Dr. Walter D’Alessandro (INGV, Palermo) and Dr. Jens Heinicke (Bergakademie Freiberg) must definitely be mentioned here: both colleagues made an effort to give me an understanding of the geological and geophysical background of geogenic gases. I

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would also like to thank Walter DʼAlessandro for the many excursions and measuring aids and for his never-ending patience in explaining the world of volcanoes to a biologist. Finally, I would like to thank Springer Verlag, who managed to satisfy even a volcanic biologist in the end. Of course, this book will again contain errors and inaccuracies. This cannot be completely ruled out as to the abundance of information. Here I ask for direct communication, so that I can weed out these errors for subsequent editions. Also, as is actually always the case, I will have forgotten some colleagues in the acknowledgements. I deeply apologize for this and vow to do better in the third edition of the Mofette Book.

Contents

1 I ntroduction  1 2 Magmatic,  Volcanic and Other Geogenic Gas Exhalations  3 2.1 Solfataras   4 2.2 Fumaroles   4 2.2.1 Hot Geysers   5 2.3 Methane Emissions   7 2.3.1 Mud Volcanoes   8 2.3.2 Eternal Fire   9 2.3.3 Submarine Methane Sources   9 2.4 Black and White Smokers  10 2.5 CO2 Emissions  11 3 M  ofettes 13 3.1 What Are Mofettes  13 3.2 Eu-Mofettes – Genuine Mofettes  20 3.2.1 Dry or Wet Mofettes?  21 3.3 Aquatic Mofettes  22 3.4 Gas-Powered Cold-Water Geysers or Bubblers  23 3.5 Acid Springs and Wells  24 3.6 Iron Ochre  25 3.7 Biogenic Decalcification  26 3.7.1 Carbonate Can Be Hard  27 3.7.2 The Stony Grooves or Gutters  28 3.8 Wandering Mofettes  28 3.9 Pseudo-Mofettes  28 xvii

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3.9.1 Degraded Peatlands – Sauerland  29 3.9.2 “Seismo-Tectonically” Induced Changes in Geogenic CO2 Release  31 3.9.3 Biogenic Soil Respiration  32 3.10 Is It Possible to Search for Mofettes?  34 4 G  eological-Volcanological Basics 37 5 Carbon  Dioxide: The Decisive Parameter for Mofettes 41 5.1 Nomenclature  41 5.2 The Physico-Chemical Properties of CO2  42 5.2.1 Odour and Taste  42 5.2.2 Water Solubility  42 5.2.3 The Acid Effect  42 5.2.4 Absorption of Thermal Radiation  46 5.2.5 The Specific Density  47 5.2.6 Visibility of the Gas  47 5.2.7 The Displacement of Oxygen  48 5.3 The Toxicity of Carbon Dioxide  50 6 M  ofettes as Habitats 53 6.1 Plants in Mofettes  53 6.1.1 Plants Perish  55 6.1.2 Growth and Habitus of Plants in Mofettes  57 6.2 Adapted Plants Can Grow – The Azonal Vegetation  59 6.2.1 Clear Boundaries  60 6.3 Physiological Adaptations of Plants  63 6.3.1 Nutrient Elements in Mofette Plants  66 6.4 Mofettophilia and Mofettophobia  67 6.5 CO2 and Trees  69 6.5.1 The Problem of the Swale  72 6.6 The Hot ‘Fumarolic’ Mofettes of the Azores  72 6.7 Mofettes Seen from Above  75 7 B  acteria and Fungi 77 7.1 Bacteria and Archaea  77 7.2 Fungi 78 7.2.1 Umbelopsis  78 7.2.2 Mushrooms Fungi in Mofette Soils  79 7.2.3 The Soil Yeasts  79

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8 A  nimals 81 8.1 Animals Die  82 8.2 Thanatocoenoses – Animal Cemetries  84 8.2.1 Why Do So Many Different Animals End Up in the CO2 Death Trap?  85 8.3 Animal Life in the CO2-Gas Gradient  89 8.3.1 Springtails and Nematodes  89 8.4 Animals Mark CO2-Emission Borders  90 8.4.1 Moles  90 8.4.2 Swallows  92 8.4.3 Bats  93 8.4.4 Wild Boar  94 8.5 The Deliberate Killing of Animals  95 8.5.1 Grotta del Cane  95 8.5.2 La Grotte du Chien a Royat  97 9 M  ofettes and Climate 99 9.1 Mofettes and the Microclimate  99 9.2 The Bossoleto CO2 Gas Valley 100 9.2.1 The CO2 Diurnal Cycle 100 9.2.2 Cloud formation in Bossoleto 102 9.3 Bad Pyrmont 103 9.4 Mofettes and the Global Climate 105 10 The  Bottoms in Mofettes107 10.1 The Gas within the Soil 107 10.1.1 The Transect Display 108 10.1.2 The Area Depiction 109 10.1.3 The Cube Depiction of Soil Monoliths 110 10.2 The CO2 to Oxygen Ratio 111 10.3 pH Values in Mofette Soils 111 10.4 Buffering of the Soil 112 10.5 The Humus Content of Mofette Soils 113 10.6 The Mineral Content 114 11 The  Importance and Use of Mofettes115 11.1 Economic Use 115 11.1.1 The Production of Mineral Waters 116 11.1.2 Production of Chemicals 117 11.1.3 The Production of Dry Ice 118

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11.1.4 CO2 in the Production, Processing and Preservation of Foodstuffs 11.1.5 CO2 in Pest Control 11.1.6 CO2 in Fire Fighting 11.1.7 CO2 and Plant Fertilization 11.2 The Drilling and Production of CO2 Gas 11.2.1 Fighting the Greenhouse Effect 11.3 The Medical Use of Mofettes 11.3.1 Cures and Balneology 11.3.2 Poorly Healing Wounds 11.4 Mofettes and Tourism: Geo-Biotopes 11.5 Italy: Campi Flegrei 11.5.1 Czech Republic: Soos 11.6 Germany: The Volcanic Eifel

118 118 119 119 120 120 123 123 125 125 127 129 130

12 The  Mythical-Ethnological Significance of CO2-degassing135 12.1 Mythical Places and Sacrificial Sites 135 12.1.1 Oracle Sites 136 12.1.2 The Ancient Entrances to Hell 137 13 D  anger from CO2: Human Tragedies143 13.1 Mammoth: Horseshoe Lake 144 13.2 Djeng Plateau 146 13.3 African Volcanic Lakes: Monoun and Nyos in Cameroon 147 13.4 Nyiragongo and Nyamulagira 147 13.5 Two Gruesome Stories from the Laacher Lake 148 13.5.1 The Death of the Young Monks 148 13.5.2 The Question of Plausibility 151 13.5.3 The Bayer Hole at Lake Laach 154 14 Mofexotic  Stories and Mofette Pareidolia155 14.1 Trapped Gas Bubbles 155 14.1.1 Ice Sheet and CO2 Gas 155 14.1.2 The Carbonate Crust 156 14.2 The Splash Stone 157 14.3 Bubble Games 157 14.3.1 Floating of Soap Bubbles 157 14.3.2 Changes in the Bubble Layer 158 14.4 The Sinking of the Greek Meadow Island Micro-­ Mephitos (μικρό μεφίτος)159

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14.5 Mofety Gouláš160 14.6 Coot, Wild Boar and Wapiti Deer 161 14.7 The Maize La Ola-Wave163 14.8 CO2 Makes You Fat 163 14.9 CO2 Makes You Sexy 164 14.10 The Pope and the Mofettes 165 14.11 Blasphemous Carbon Dioxide 165 14.12 Pareidolia 166 14.12.1 Mephitic Devilspotting 166 15 Epilogue: Protection of Mofettes169 Literature Cited and Further Reading171 I ndex195

1 Introduction

When strolling through formerly volcanic areas, such as northwestern Czechia, northeastern Slovenia or the German Eifel Mountains, one can discover mofettes everywhere (Fig.  1.1). Small dead animals, the absence of a plant cover or unusual growth forms of plants are often unmistakable signs of these volcanic degassing phenomena. If such sites are found, caution is advised. High concentrations of CO2 can build up in soil depressions, which can lead to asphyxiation. But don’t panic, mofettes are only really dangerous if you don’t know what phenomenon you are dealing with and how to behave. Mofettes are “dry” CO2 emissions; the gas flows from the ground along fissure zones. Mofettes they are not fed by spring water containing dissolved CO2, as is the case with acid springs. Mofettes on slopes are usually harmless because the gas, which is heavier than air, can run off downhill. In flat terrain, the gas is dispersed by air movements and thereby diluted. However, emissions with a high degassing rate (high gas flow) in valley locations are extremely dangerous, as the gas can form “dry gas lakes”. Depending on the CO2 concentration, this can lead to death by asphyxiation. In the following, the exciting phenomenon of geogenic degassing of carbon dioxide and other fluids will be discussed in more detail and the links to chemical and physical changes in the soil will be explained. The micro- and mesoclimate altered specifically by CO2 gas will be mentioned, as well as the many influences on – and adaptations of – organisms. The benefits of mofettes as well as the potential harm and hazards of degassing CO2 will be covered.

© Springer-Verlag GmbH Germany, part of Springer Nature 2023 H. Pfanz, Cold Breath of Dormant Volcanoes, https://doi.org/10.1007/978-3-662-65375-3_1

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Fig. 1.1  The mofette on the Hartoušov meadow in the Czech Plesná valley. The lack of vegetation in the main degassing areas and the mofette-specific vegetation in the degassing fields are clearly visible. (© H. Pfanz 2019)

Finally, the historical influence of CO2 gas on the history and myths of the Greeks will be touched. Our review is rounded off by clarification of wellknown everyday phenomena and hitherto little-known exotic phenomena.

2 Magmatic, Volcanic and Other Geogenic Gas Exhalations

Gas emissions of volcanoes are known for a long time. Both pre- and post-­ eruptively, volcanic gases can reach the earth surface from the depths of the magma chambers. And of course, especially during a volcanic eruption, huge amounts of gases of CO2 and water vapour escape from the volcanic craters. Tectonic events, such as strong earthquakes, open cracks and fissures in the Earth’s crust that act as new transport pathways for the fluids to rise and exhale. Many authors consider mofettes and other gaseous exhalations as unmistakable signs of an aging volcanism that is coming to an end. However, recent studies in particular show that the release of gases of magmatic origin can also be a sign of new or renewed volcanic activity. Due to their poorer solubility in magmas and the pressure relief during magma ascent, CO2 and water vapour escape earlier than other volcanic gases (e.g. SO2, H2S). They are often visible as high rising columns of “smoke” (water vapour). At Etna and at many other volcanoes (Nyiragongo), magma chambers with carbon dioxide and water vapour fill up at a depth of a few kilometres up to six months before the actual eruption. From here, these fluids escape through open channels, fissures and former volcanic vents to the earth’s surface. Then often also an increased mofette activity at the edge of the volcanoes is to be stated. This increased release of CO2 is, in addition to increased seismicity and geodetically measurable ground movements, an indication of the expected activity of the volcano. There is time to take precautions. The release of toxic gas clouds accompanying the eruption is also frequently reported. As mentioned earlier, eruption clouds from volcanoes release large amounts of sulfur dioxide (SO2), hydrogen chloride (HCl), hydrogen fluoride

© Springer-Verlag GmbH Germany, part of Springer Nature 2023 H. Pfanz, Cold Breath of Dormant Volcanoes, https://doi.org/10.1007/978-3-662-65375-3_2

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(HF), carbon monoxide (CO), and hydrogen sulfide (H2S) in addition to CO2 and water vapor (Schmincke 2000; Simper 2005). We will discuss this point in more detail when we discuss the last days of Pompeji and Herculaneum due to the eruption of Mount Vesuvius.

2.1 Solfataras According to the type and chemical reaction of the volcanic gases, a distinction is made between different types of degassing at the Earth’s surface. In addition to mofettes, other gaseous exhalations can be found in or near volcanoes. Natural gas emissions that emit sulphur-containing compounds are called solfataras (sulfur, sulphur = Latin for sulfur), regardless of whether the sulphur is emitted in its oxidised form as sulphur dioxide (SO2) or in its reduced form as hydrogen sulphide (H2S). The nose, however, very quickly tells passers-­by which form of sulphur it is (hydrogen sulphide smells very unpleasant like rotten eggs). Precipitates of elemental sulphur can be easily recognised by the mostly yellow deposits of very fine crystals on the bottom (Fig. 2.1). Solfataras can be found, for example, on the crater rims of Mount Etna, on the Aeolian island of Vulcano and on the New Zealand volcanic island of Whakaari (White Island). One of the most famous places for solfataras, where elemental sulfur is also broken in large quantities by sheer man power and carried up the crater rim, is the volcano Ijen on Java Timur. The sulphur deposits shine widely in yellow, orange-red to red hues, but can also appear whitish. The warm colors do not originate from the gas (H2S) itself, but from the crystalline deposits, which appear in different colors depending on the crystal structure of the sulfur.

2.2 Fumaroles If hot (water) steam is released from hydrothermal sources into the atmosphere, this is referred to as fumaroles (fumus  =  Latin for smoke, steam, vapour) (Fig. 2.2) or steam emissions. Here, temperatures of over 100–700 °C are reached (Paonita et  al. 2002). Fumaroles are formed when circulating groundwater in the deep rock is heated by geothermal processes and flows upward as water vapour. The pressure should not be too high. Only then the exhaling point remains a fumarole. However, if the water is under high pressure or if very high pressure is generated by the gas transfer of the liquid water

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Fig. 2.1  Solfataras at the rim of a minor crater of Etna with yellow sulphur efflorescence. (© H. Pfanz 2019)

at high temperatures, then real phreato-volcanic eruptions can occur. Usually, the fumarolic water is of ombrogenic origin. Sometimes, however, seawater penetrating even into hot deep zones is found near the coast (e.g. Stromboli; D’Alessandro pers. comm.). Well known for many fumaroles are the Yellowstone National Park, as well as Iceland, Kamchatka, New Zealand but also some volcanic fields in the Andes.

2.2.1 Hot Geysers Periodic eruptions of very hot, liquid water are called geysers. The name comes from the Icelandic and stands for geysa = to swell, to move violently or to flow forth. Alexander von Humboldt (1845, resp. 2004) also called these phenomena the “Isländische Kochbrunnen (Icelandic boiling fountains)”. The principle of hot water geysers is the presence of a water reservoir which is fed by

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Fig. 2.2  Fumaroles at Pisciarelli near the Campi Flegrei (Phlegraean Fields) near Pozzuoli. (© H. Pfanz 2019)

groundwater or rain and heated by a magma chamber. In the eruption channel above, there must be at least one bottleneck, which acts as a flow obstacle. This bottleneck prevents the water vapour pressure from escaping unhindered via convection at the earth’s surface. Pressure builds up due to the water column above. If the pressure becomes too strong due to the temperature increase below the neck of the bottle, the resistance at the constriction point is overcome by hot steam bubbles and hot water is thrown upwards out of the channel in an eruption column. The eruptions can be regular or irregular (Fig. 2.3). During these eruptions, hot water can be driven up to 130 m into the air (e.g. Steamboat Geyser, Old Faithful, Yellowstone Natl. Park). These hot water geysers can be found in Iceland, New Zealand, Yellowstone National Park and Alaska, the African Rift Valley, the South American Andes (especially Chile), but also in the volcanic regions of Kamchatka (Bryan 2008). Large quantities of gas and water are also erupted during the actual eruptions of volcanoes. In eruption clouds from volcanoes, the average gas composition consists of about 35–90% water vapour by volume, 5–50% CO2, 2–30% SO2, and varying concentrations of hydrogen chloride (HCl),

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Fig. 2.3  Up to 9  m high hot water geyser “Prince of Wales Feathers” erupts in the Whakarewarewa valley near Te Puia/Rotorua on New Zealand. (© H. Pfanz 2019)

hydrogen fluoride (HF), carbon monoxide (CO) and hydrogen sulphide (H2S) (Schmincke 2000; Simper 2005).

2.3 Methane Emissions Methane emissions are found, characterized and quantified worldwide (Etiope 2015; Daskalopoulou et al. 2018a, b, 2019). Natural emissions and anthropogenic releases are distinguished. Natural CH4 emissions are by far dominated by wetlands (mud volcanoes, swamps, bogs; mainly in tundra) with about 220  Tg CH4 per year. They are followed by geological and aquatic emissions (60 and 30 Tg CH4 a-1,, respectively). Termites, fire, and thawing permafrost soils also contribute to the emission balance (Ciais et al. 2013). Among anthropogenic sources, methane release from drilling activities, digestive processes of cattle and other ruminants, and landfill leaks (90, 80, and 75 Tg CH4 a−1, respectively) are nearly equal. Rice cultivation also contributes significantly to the global methane release with about 30 Tg CH4 a−1. The release of methane from dead biomass is of the same order of magnitude (Ciais et al. 2013; Etiope 2015).

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2.3.1 Mud Volcanoes A distinction must be made between magmatically driven emissions, such as mofettes or solfataras and fumaroles, and mud volcanoes, which are mainly found above the areas of hydrocarbon deposits (Martinelli and Panahi 2005; Etiope 2015). Here, methane (CH4) and some other hydrocarbons diffuse to the Earth’s surface. A liquid, grey-black, usually muddy-clayish mass periodically swells and releases a methane bubble to the atmosphere. The continuous ejection of clay minerals and water leads to the buildup of a crater-like cone, the shape of which eventually led to the somewhat misleading name “mud volcano”. Such a mud-producing “volcano” is found at the Vulcanii Noroiosi in Romania (Fig. 2.4a). The bursting, clayish methane bubbles produce sometimes spectacular photographic images (Fig. 2.4b). Mud volcanoes can be found all over the world. Azerbaijan, Indonesia, Taiwan, but also the Po Valley in Italy are characterized by the occurrence of such mud volcanoes. However, the best known European mud volcanoes are in Romania (Vulcanii Noroiosi; Buzău Mountains; Eastern Carpathians). There, in a vegetationless lunar landscape, several hectares of land are strewn with mud volcanoes. Methane emission from the ground is not uncommon in Germany either. This gas escapes continuously from underground coal seams (Spalding and Ziegler 1998; Thielemann 2000). In subtropical-tropical areas, significant climate-relevant quantities of methane escape from swampy soils and especially from rice fields (Minami and Neue 1994).

Fig. 2.4  Mud volcanoes are not real volcanoes. In Romania there is the mud volcano “Paclele de la Beciu” near Buzau as part of the Vulcanii Noroiosi (a). At irregular intervals, the muddy-clayish mass is lifted by methane gas and shortly afterwards bursts into a wide variety of forms (b). (© H. Pfanz 2019)

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2.3.2 Eternal Fire Since methane burns with a bluish flame at a certain methane/air ratio (or explodes if confined!), such a phenomenon is also known as eternal fire (ever lasting fire). Eternal fires occur quite frequently in areas with methane emissions. These are areas above oil deposits, such as in Azerbaijan or the Turkish Chimaera (Etiope 2015), or near mud volcanoes. A widely known fire, which has been extinct for some time, occurred on the Lippewiesen near Hamm in North Rhine-Westphalia (Spalding and Ziegler 1998; Thielemann 2000). A well-known methane fire in Asia is the Suei Ho Tong Yuan fire near the Chu-You fault zone in Taiwan (Fig. 2.5) or the burning fields of Chimaera (Hosgörmez et al. 2008; Etiope et al. 2011). Methane emission also occurs as marsh gas in bogs and swamps (bog lights). According to several authors, methane is formed biogenically by methanogenic bacteria and archaea, which can reduce geogenic carbon dioxide (CO2) to methane (CH4) level (Takai et al. 2006; Reay et al. 2010). The spontaneous ignition is still unknown.

2.3.3 Submarine Methane Sources In some areas, many so-called “pockmarks” can be seen on the seafloor (Papatheodorou et  al. 1993; Etiope et  al. 2005). These are volcano-like

Fig. 2.5  Eternal fire over a methane well; Suei-Ho-Tong-Yuan, Taiwan. (© H. Pfanz 2019)

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bottom funnels from which submarine methane degasses. As was said above, methane degassing is never of true volcanic origin, but always associated with deep petroleum deposits. Pockmarks are found in the German Bight, the Mediterranean Sea, the Atlantic Ocean and many other ocean locations around the world. The gas emission in the oceans is estimated at 8–65 Tg methane per year. About 0.4–48 Tg of methane per year is then released to the atmosphere from seawater (Gentz 2013; Hovland and Judd 1988; Judd 2004; Kvenvolden and Rogers 2005). Mention must also be made of the methane deposits fixed on the seabed under certain temperature and pressure conditions, the so-called gas hydrates. In the event of further global warming, these could also lead to an additional continuous release of gas.

2.4 Black and White Smokers In this context  the submarine hydrothermal vents, commonly known as “Black or White Smokers”, should not be forgotten. These very hot hydrothermal fluids are a mixture of highly mineralised waters and various gases containing sulphur and carbon dioxide, which form insoluble black precipitates. They are the starting point for the formation of hydrothermal deposits, such as manganese ores. Such emissions are released from submarine volcanoes, such as the Japanese volcano Eifuku or the youngest Hawaiʻi volcano Lōʻihi, which is just in statu nascendi still about 1000 m below the sea surface. However, such gas emissions are also described from the mid-ocean ridge of the Atlantic and other plate tectonic divergence zones at 3000 m ocean depth (e.g. Lost City; Two Boats or Sister Peaks; Colín-García et al. 2016). In this completely lightless environment, an incredibly interesting, partly still unknown fauna and bacterial flora has developed on the seafloor at the degassing vents (Kiel 2010). Chemoautotrophic bacteria growing as rock cover can synthesize organic molecules from inorganic precursors such as (the actually highly toxic) hydrogen sulfide (H2S) and CO2 with the help of the temperatures prevailing there (Lazcano 2010). Since this process occurs without sunlight, it is called chemosynthesis – in contrast to the better known sunlight-driven photosynthesis of green plants. These chemolithotrophic bacteria thus represent the autotrophic producers in the deep-sea food chain (Rogers et al. 2012). Both, the deep-sea snail (Gigantopelta aegis) and the sulfide worm (Paralvinella sulfincola), as well as crustaceans such as the sulfide copepod (Stygiopontius quadrispinosis) and the smaller shrimp (Rimicaris

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kairei and Mirocaris indica), graze the bacteria directly from the surfaces of the smokers. Certain tubeworms (Olavius algarvensis), which have no mouth, no stomach, nor intestines, also rely on such bacteria; however, the bacteria grow inside the tubeworm. The hot fumes rising from the fissures and the CO2 are thus the essential basis of life for a rich and very specialized ecosystem of the deep sea. Whether and to what extent these systems were involved in the origin of life has been discussed for some time in the relevant publications.

2.5 CO2 Emissions Geogenic exhalations, in which the main gas component consists of carbon dioxide, can have various sources in the Earth’s mantle and lower crust. Besides the already mentioned magmatic and eventually volcanic sources, thermo-­ metamorphic processes are another important cause for this gas release. An example of this are the areas in eastern Italy or Greece. Analyses of CO2 isotope ratios provide information on the genesis of these gases. Such gas emissions can contain up to 99% pure CO2 gas (Kämpf et al. 2013; Bräuer et al. 2018; Saßmannshausen 2010; Thomalla 2015). However, since the mephitic phenomenon is the main focus of this book, geogenic degassing of CO2 in all its facets will now be reviewed and discussed more intensively.

3 Mofettes

In the literature, mofettes are often incorrectly referred to as fumaroles. Especially in the older literature, the term fumarole is applied to all fluid-­ exhaling, geogenic manifestations. Meanwhile, the term mofette is applied only to small- or larger-scale gaseous exhalations, that emit fairly pure carbon dioxide (up to 99%) and which are not mineral water sources. The degassing temperature of mofettes is generally close to the ambient temperature (ambient, normal or cold mofettes), but in exceptional cases in thermal regions it may also be 90–100 °C (New Zealand, Azores). Let us start with mofettes emitting at normal-temperature, the “ambient mofettes”. High-­ temperature mofettes in New Zealand or the Azores will be discussed later in Sect. 6.6.

3.1 What Are Mofettes In many encyclopedias, mofettes are defined as “sites of carbon dioxide emissions in volcanic areas” (e.g. Hermann et al. 1985). And indeed, mofettes are more or less pure CO2 exhalations of geogenic origin. The name mofette is derived from the Italian word mofeta. This comes from the Latin or Oscian/Samnite word mefitis or mephitis (Fig.  3.1) and means as much as “harmful exhalation of the earth” (Stowasser 1994). In ancient Pompeii, in Rome, and especially in the territory of the Samnites and Oscians in southern Italy, the goddess Mefitis, a Dea odoris gravissimi et

© Springer-Verlag GmbH Germany, part of Springer Nature 2023 H. Pfanz, Cold Breath of Dormant Volcanoes, https://doi.org/10.1007/978-3-662-65375-3_3

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Fig. 3.1  The goddess of earth smells and exhalations  – Mefitis. After a statuette from San Pietri di Cantoni. Redrawn by Lisann Fisch (a). The name “Mefitis“carved in stone (b).

pestiferi i.e. a “goddess of foul and noxious odors” was worshipped (Ziegler and Sontheimer 1979; Fink 1993).1 Mefitis was also the goddess of springs and of women in childbirth. The Oscians and Samnites lived in a period around 600–82 BC in an area in Italy that today includes parts of Molise, Basilicata, Apulia and Campania.

 Other terms for mofettes, sometimes incorrectly used (e.g. fumaroles, solfataras) are “natural CO2 degassing“(natural carbon dioxide springs NCDS), geogenic CO2 exhalations, geothermal degassing, diffuse CO2 emissions, mephitic gas degassing, dry carbon dioxide degassing, carbonic acid springs, and volcanic gas degassing.  The word mofette is also found in Goethe’s drama Faust in the form of Mephisto or Mephistopheles and means devilish natures.

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One of the most famous Italian mofette areas “Mefite D’Ansanto“near Rocca San Felice is named after this goddess (Fig. 3.2). One of the world’s largest, but also most dangerous CO2-emitting site is located here (Chiodini et  al. 2010). A well-known temple dedicated to the goddess Mefitis is said to have existed in a dry valley filled with CO2 gas (Rainini 1985); various foundation stone walls still exist. The name mephitis has also been adopted in zoology; namely for a handsome, black-and-white animal that sprays extremely strong-smelling thiol compounds (including 3-methyl-1-butanethiol, quinoline-2-methanethiol) from its anal gland when threatened (Wilson and Reeder 2005). The scientific name of the striped skunk is therefore Mephitis mephitis. However, it must be critically noted with regard to the nomenclature of zoologists that true mofettes (Eu-mofettes) cannot be recognized by their olfactory emissions  – they normally don’t smell at all. If one nevertheless smells something strange in mofette areas, then it is usually traces of swamp gases such as hydrogen sulphide, ammonia and methane derivatives.

Fig. 3.2  Mefite D’Ansanto in Italy is an area of extreme CO2 emission. On the lower left, in front of the wooded hill, was a sacred grove with a sanctuary where sacrifices were made to the goddess Mefitis. (© H. Pfanz 2019)

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A currently acceptable scientific definition for mofettes is: Mofettes are locally confined areas of geogenic CO2 degassing (exhalations). They always show a relation to recent or postvolcanic phenomena or to metamorphic processes in carbonate rocks (Pfanz 2008; see also Chiodini et al. 2010).

Mofettes are found near deep fault zones, tectonic plate boundaries, and in areas of recent, incipient, or subsiding volcanic activity (e.g., northwestern Bohemia or northern Slovenia; Geissler et al. 2005; Kämpf et al. 2005; Vodnik et al. 2006) or in the Eifel (Pfanz 2008; Goepel et al. 2014). The carbon dioxide released to the atmosphere originates from magma chambers of the Earth’s mantle or crust, or is formed during metamorphism by extreme heating of limestone rocks (Spera and Bergman 1980; Spera 1981, 1984). Down to a depth of about 800 m it occurs as a supercritical fluid, which is most easily described physically by a certain similarity to water. The gas then penetrates upward from the site of genesis through fractures and crevices. Once at the top, the gas then emerges dry to the earth’s surface through the finest cracks in the ground. The gas emissions themselves are hardly warmer than the ambient temperature (ambient mofettes), especially since the gas temperature is cooled by the gas relaxation during ascent. Exceptions in the form of warm or hot mofettes occur in thermal regions (Azores, New Zealand). Mofettes are very difficult to recognize as such in the field without the appropriate gas measuring equipment. The mofette specialist can, however, detect possible gas leaks on the basis of the azonal vegetation, the occurrence of certain animals/animal corpses or changes in the soil texture and chemistry. However, if the gas emerges through surface waters (puddles, pools, springs, streams, lakes), it is easy for anyone to recognize it by the many gas bubbles (cf. mofettes on the eastern shore of Lake Laach; Fig. 3.3). The CO2 flux to the earth’s surface but also the CO2 concentrations in the soil can fluctuate seasonally or due to weather conditions (cf. also Dressel 1871; Thomalla 2015); however, they are often relatively stable over the long term. CO2 concentrations measured at soil depths of up to 100  cm vary between 2% and 100%, with a corresponding decrease in soil oxygen content with increasing CO2 (Vodnik et al. 2006). The contribution of biogenic CO2 formation in the soil will be discussed later. For animals and humans, even a small percentage increase in CO2 concentration is life-threatening. However, some plants thrive quite well at mofette sites and even have competitive advantages. Certain sedge (Carex) and rush (Juncus) species as well as reed (Phragmites) even tolerate up to 100% CO2 in near-surface soil layers and sometimes even in the atmosphere (Kies et  al. 2015; Fig. 3.4).

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Fig. 3.3  CO2 bubbles and surging water on the eastern shore of Laacher See. (© H. Pfanz 2019)

In some areas (Eifel, Tuscany, NW Czech Republic) mofettes can be recognized from some distance by the vegetation. Due to the specific absorption of infrared radiation by CO2 and its greater density compared to air, very interesting greenhouse effect studies can be performed on some mofettes located in valley depressions (Tank et al. 2003, 2005, 2008; Pfanz 2008; Kies et al. 2015; see also Chap. 9). In degassing soils, the concentration of vital oxygen correlates well with the concentration of geogenic CO2. The higher the CO2 concentration of the soil air, the lower its oxygen content (Vodnik et al. 2006). Organismic life on and in mofettes is therefore very limited. Plants and bacteria that occur in mofette areas must therefore have special adaptations in order to survive in this hostile environment. Most animals may not enter these areas. If animals nevertheless venture to the degassing sites, they die within a few minutes (lack of oxygen, acidification of the blood). Their corpses, which characteristically remain undecomposed for a long time, are often found near mofettes (Fig. 3.5). However, research has made progress in characterizing animals in mofettes in recent years. For example, distinct animal species have been described that can serve as bioindicators to show the boundaries between highly CO2-­ degassing and non-degassing sites (Pfanz 2008, 2019;  Russell et  al. 2011; Hohberg et  al. 2015; Balkenhol et  al. 2016). Other species are suitable to

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Fig. 3.4  Overview of the Bossoleto CO2 gas valley. The valley is a collapsed sinkhole about 10 m deep. The non-vegetated areas indicate the sites of extreme CO2 emission from the ground. The whitish stones and boulders are of travertine origin. The stands of reed (Phragmites australis) growing at different levels on the valley floor and on the slope rise can be seen. (© H. Pfanz 2019)

serve as indicator organisms for specific soil concentrations of carbon dioxide. Very special are the endemic mofette animals, which apparently occur exclusively in mofette areas (Schulz and Potapov 2010). The degassing of CO2 on the mofette surface occurs neither uniformly nor evenly. In some places the emission is increased, in others it is less or not measurable. This inhomogeneity of CO2 degassing within a mofette field is shown in Fig.  3.6 for a mofette in the northwest of the Czech Republic (Saßmannshausen 2010; Thomalla 2015; Nickschick et al. 2015). The actual degassing centers (vents) are clearly visible. However, the release of CO2 can be much lower directly next to extremely gassy areas. The CO2 can degas through clearly visible holes in the ground (Fig.  3.7) or, less spectacularly, reach the surface through small cracks or fissures in the ground. What looks harmless in dry weather, betrayed only by some faint whistling, may turn out to be a mofette field degassing nearly everywhere after a downpour. The gas must now perfuse the water layer formed near the ground by the rain in the form of bubbles and is thus easily detected.

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Fig. 3.5  Dead cat and dead pigeon on top in an extremely strong CO2-emitting valley-­ mofette. The dead bodies remain undecomposed for longer because of the oxygen-­ poor atmosphere. (© H. Pfanz 2019)

No less a person than Alexander von Humboldt describes such phenomena in his Cosmos (Volume 4, 1845) under „…II. Reaction des Inneren der Erde gegen die Oberfläche; sich offenbarend: c) durch den Ausbruch elastischer Flüssigkeiten, zu Zeiten von Erscheinungen der Selbstentzündung begleitet (Gas  - und Schlammvulkane, Naphta - Feuer, Salsen) “...II. Reaction of the interior of the earth against the surface; manifesting itself: c) by the eruption of elastic fluids, at times accompanied by phenomena of spontaneous combustion (gas and mud volcanoes, naphtha fires, brines) ...”.

He describes the mineral springs and acidic gases along with the geogenic release of carbonic acid gas and speculates on whether these phenomena could be directly or indirectly related to volcanism. He also mentions experiments in which he investigated the fire-extinguishing effect of volcanic gases with the aid of small wax candles. Further scientific mention of mofettes can be found in the works of Dechen (1864), Nöggerath (1870) as well as Dressel (1871) and Steinbach (1880 resp.

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Fig. 3.6  Inhomogeneous degassing of CO2 in a mofette area in the Czech Vogtland. The highest degassing can be seen at the vegetation-free site in the center. Mosses show the sites with increased CO2 release, while the white flowering meadow chervil (Anthriscus sylvestris) in the background grows exclusively on non-gassing control areas. (© H. Pfanz 2019)

2002). However, a detailed discussion of the actual mofette gas, the “mephitic gas”, can already be found in Gehler (1789): … “Von Natur aus findet sich die fixe Luft in Gruben, Hölen, Brunnen und anderen Plätzen, denen der Luftzug mangelt, wo sie durch natürliche Gährung oder Verbrennung, z. B. in der Nachbarschaft der Vulkane, Kiese u. dgl. entstehen kann. Schon seit Jahrhunderten kennt man die Hundsgrotte (Grotta del cane) bey Neapel …”. ... “By nature, the fixed air is found in pits, caves, wells and other places that lack air, where it can be produced by natural germination or combustion, e.g. in the vicinity of volcanoes, gravels and the like. The cave of dogs (Grotta del cane) near Naples has been known for centuries ...”.

3.2 Eu-Mofettes – Genuine Mofettes Eu-mofettes or true mofettes are those in which the CO2 concentration in the gaseous effusate is at least 98% (v/v). True mofettes are rare, but are the rule in the Eifel Mountains/Germany, in parts of Slovenia and in NW Czechia. In

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Fig. 3.7  Large quantities of carbon dioxide escape from cracks and smaller fissures as well as a central spit hole in a Tuscan field near San Gimignano. (© H. Pfanz 2019)

the Mediterranean mofettes, H2S or water vapour is very often admixed. But even there, the concentration of the gaseous admixtures is usually less than 1–2%.

3.2.1 Dry or Wet Mofettes? There is still disagreement among mofettologists about the nomenclatural classification of mofettes. Mofettes should be called “dry mofettes” if the gas has moved upwards through the rocks to the earth’s surface in a dry state, i.e. not dissolved in water. Whether it then degasses at the Earth’s surface through dry or rain-­ soaked soil or through bodies of water (puddles, pools, streams, lakes, seas) is not relevant. A bubbling puddle of mofette would therefore be considered a dry mofette if the CO2 gas was added to the overlying body of water directly below the puddle. However, only a close examination will reveal a clear differentiation between a dry mofette and a mineral spring. To illustrate what has been said, Fig. 3.8 shows a red-backed shrike (Lanius collurio) asphyxiated by CO2 in a small trench. The animal died by suffocation in a “dry“mofette (Fig. 3.8a); when later, after a heavy downpour, the gully

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Fig. 3.8  A red-backed shrike (Lanius collurio) suffocated in a small CO2 gas lake in dry weather (a). A little later, it rained (b). Gully in the meadow near Hartoušov. (© H. Pfanz 2019)

filled with rainwater, the bird’s body looks as lying in a “wet“mofette (Fig. 3.8b). However, Fig. 3.8b is also a dry mofette according to our definition above; the rain puddle on top is only flowed through by the CO2 emission underneath. A really “wet mofette” doesn’t exist. However, if the CO2 rises together with deep waters or meets groundwater aquifers in the uppermost rock horizons and soil zones, the gas will mix with the water in this area, according to the pressure and temperature conditions. In this case, we speak of acidic or sour springs, because of the lower pH value due to the dissolved carbonic acid.

3.3 Aquatic Mofettes Mofette gases escape from the ground wherever cracks and fissures facilitate gas mobility. Degassing CO2 sources are therefore to be expected both on land and in water bodies. And, of course, many such underwater mofettes are known. They are found in abundance on the eastern shore of Laacher See (Eifel, Germany), in the small river Plesná in the western Czech Republic or in the river Stavnica in northern Slovenia. It is therefore not surprising that CO2 vents can also be found on the seabed. If one travels by ship through the Aeolian Islands north of Sicily, one can observe upwellings of the sea surface between the islands of Vulcano and Stromboli at about the level of the small island of Panarea. Surface turbulence is best seen when the sea is calm. A dive to a depth of about 15 m gives clarity. One finds oneself in an old submarine volcanic crater, whose not eroded parts of the crater rim rise as small rock

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Fig. 3.9  Submarine CO2 degassing off Panaraea; Aeolian Islands, Sicily. (© Franco Italiano)

towers out of the sea. At the bottom of the crater, magmatic gases are still escaping. As on land, CO2 gas escapes from the floor and bubbles upwards between the Posidonia plants to find its way to the sea surface (Fig. 3.9).

3.4 Gas-Powered Cold-Water Geysers or Bubblers In addition to the well-known hot water geysers, there are also cold water geysers. The pressure required for the eruption and the outflow of the water is in this case not achieved by high temperatures, but by a high CO2 gas concentration. Geogenically rising gas dissolves, e.g. in the groundwater of a borehole, up to a critical concentration (Heinrich 1910; Glennon and Pfaff 2004). The pressure drop of the rising fluid causes the carbon dioxide to dissolve. In the process, countless individual bubbles become coalescing gas bubbles that drive the overlying water column out of the borehole (slug flow) – similar to the effect when opening a champagne bottle. The geyser at Andernach/Namedy (Rhein river/Germany) is an exquisite example of the gas pressure that can build up to eventually produce a column of water up to 60 m high (Figs. 11.5 and 3.10).

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Fig. 3.10  In the French Massif Central, many small fountains can be found. Shown here is the Source de Ile; Sainte Marguerite/Auvergne. (© H. Pfanz 2019)

On a smaller scale, a cold, CO2-driven water ejection also takes place at the Wallender Born (Brubbel) near Wallenborn in the western Eifel Mountains. This small spring also raises its water level periodically, but only by approximately two metres. Here, too, CO2 is released from underground waters. The water is passively carried upward. May (2002a, b) measured the gas flow during such an eruption cycle and calculated the total amount of gas emitted. During a cycle of about one hour about 11 m3 of gas were emitted. The main eruption took place within only about 20 min. A particularly graceful example of a cold-water geyser is the gargouillère (gargoyle) near Lignat at Saint Georges on the Allier in the Puy de Dôme/ Massif Central area (Fig. 3.10; see also Gal et al. 2018).

3.5 Acid Springs and Wells Acidic springs are the surface seeps of waters that have been loaded with natural CO2 at greater depths along the upwelling zones. In this case, the CO2 bubbles through the water body and dissolves according to the respective temperature and pressure conditions. In this process, the pH value of the water is lowered from neutral 7 to pH 6 - 5, i.e. it gets acidified by the carbonic acid formed.

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Due to the acidity of the carbonic acid (see later), the acidified water also dissolves cationic minerals from the rocks on its way up; a natural mineral water is formed. The East and West Eifel are prime examples of regions with mineral springs of volcanic origin (Carle 1975; Stoffels and Thein 2000). The waters known far beyond Germany are extracted and bottled in this region.

3.6 Iron Ochre As a concomitant of mofettes and acidic springs, ochre precipitates can frequently be observed on the surface of the soil. Rust-coloured coatings in stream beds, on stones, plant parts and all objects that are placed in such waters for a longer period are clear signs of this iron ochre. Ochre consists mainly of iron(III) oxide hydrates together with iron hydroxides (e.g. Fe(OH)2) and iron oxides (e.g. Fe2O3); sometimes black manganese oxides or organic components are also involved in the formation of ochre. The reduced iron ions dissolved out in the deeper layers of the rock by the acidic action of carbonated waters were transported upward with the CO2 water flow. Once on the soil surface, they came into contact with atmospheric oxygen, and the iron ions were oxidized to oxihydrates. They acquire their rust-red colour and become insoluble in water (Fig. 3.11). On stagnant waters, thin skins may

Fig. 3.11  Ochre precipitates in a swamp near Oldrussov, north-west of the Czech Republic. (© H. Pfanz 2019)

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form on the surface, shining in bright rainbow colours depending on the sunlight and sometimes look like an oil film. Ochre deposits also occur on organic material, whether dead or still alive. In this case, leaves that have fallen into the solution or parts of plants hanging into the water are petrified. Beautiful examples of such precipitations can be found in the nature reserve of the Wehrer Kessel, at Wassenach (Roman well and horse well) very close to the Laacher See or in the Czech Soos reserve. A good place for spotting petrified parts of water plants is also Mammoth Hotsprings at Yellowstone Natl. Park. In Karlovy Vary (Czech Republic), the “rust sintering“is exploited for tourism and petrified rust roses (made of paper) and small rust amphorae are sold to tourists.

3.7 Biogenic Decalcification If carbon dioxide dissolves in the groundwater, carbonic acid is formed (see Eq. 5.1, page 45). If alkali or alkaline earth ions are present at the same time, poorly soluble salts are formed. In the case of calcium, these are the poorly soluble calcium carbonate (CaCO3) and the more readily soluble calcium hydrogen carbonate or calcium bicarbonate (Ca(HCO3)2). Both forms precipitate over time and form crusts. Calcium bicarbonate, however, remains in solution when there is an excess of carbonic acid (or bicarbonate or CO2) in the solution. This is naturally the case in mofette waters. The actual precipitation does not occur within the lithosphere, but only at the earth’s surface. Here, the excess CO2 degasses from the solution according to the lower atmospheric pressure and all wetted surfaces are covered with a whitish crust of calcium carbonate. Equation of sintering: (3.1)

Photosynthetically active plants (often mosses; Palustriella (Cratoneuron) commutatum) can also actively remove carbon dioxide from fluids containing CO2 (they reduce it photosynthetically to the sugar level) and thus contribute to the rapid precipitation of carbonates. This process is called biogenic decalcification of waters or biogenic lime sintering. In this process, lime tuff or travertine is formed (Pentecost 2005). The processes of decalcification are actually a bit more complicated than shown above, as pH, temperature and the level of CO2 concentration play a crucial role in the physico-chemical

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processes that take place in the solutions. A fine example of such biogenic decalcification is the waterfall at Nohn in the West Eifel Mountains (Fig. 3.12b).

3.7.1 Carbonate Can Be Hard When CO2, which is gaseous at room temperature, forms into salts in liquid media, it can create quite hard structures – the carbonates. Very often, these are calcium and magnesium carbonates, which can be seen in stalactite caves, travertine waterfalls and lime sinter terraces (e.g. Mammoth Hotsprings, YNP). In the household, such “undesirable” carbonates are also known from calcified coffee machines, washing machines and boilers (Peter et al. 1995). The actual calcification proceeds according to the following scheme: 

(3.2)

or 

(3.3 )

Fig. 3.12  Travertine, calcareous sinter or calcareous tuff formation by water containing high concentrations of hydrogen carbonate. Newly sintered rock is formed at the dripping edge and when the drops impact; Tete de Griffon (a). Detail of dripping at the moss Cratoneuron commutatum (Palustriella commutata), typical for such cascades, at the Dreimühlenwasserfall near Nohn in the Westeifel (b). (© H. Pfanz 2019)

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In this process, anionic hydrogen carbonate (HCO3−) dissolved in water binds the cationic calcium, forming water-insoluble calcium carbonate (CaCO3) and CO2 gas (2) or carbonic acid (3) dissolved in water.

3.7.2 The Stony Grooves or Gutters Precipitation of carbonates can also result in a very atypical construction of the bed of small streams. Thus, in travertine areas with very high carbonate contents in the water, the channel of a stream is sometimes shifted significantly upward above the ground surface, instead of being dug downward into the bedrock by erosion. Wonderful rivulets of carbonates can be admired in the ancient city of Hierapolis in Pamukkale, Turkey. In Viterbo, Italy, one can also see such elevated water channels in the famous Terme del Bullicame. And even in Germany one can find travertine or tuff walls up to 1.6 m high on which a stream flows in a channel. A beautiful example of such a limestone tufa creek is the “Steinerne Rinne” near Wolfsbronn on the Hahnenkamm (Middle Franconia; Fig. 3.13).

3.8 Wandering Mofettes Not yet fully understood is the mechanism of the migrating mofettes of Caprese Michelangelo (Tuscan Apennines). There, the phenomenon of apparently “mobile mofettes“has been studied for years. In a few years, the degassing sites had “migrated” several meters up or down the slope. Film recordings impressively documented this. The movement of the mofettes seems to be activated by local earthquakes (Heinicke et al. 2006) and also depends on soil properties and precipitation amounts and intensities. Presumably, changes in the fracturing of the surrounding rock are responsible for these processes.

3.9 Pseudo-Mofettes Not all CO2 in the soil is magmatic or volcanogenic in origin. Some CO2 degassing is linked to tectonic fault zones, the amount of which can be activated by seismic events (Etiope 1999; Rogie et al. 2003). However, the main cause of elevated CO2 in normal soil is the natural respiration of soil

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Fig. 3.13  The stony gutter near Wolfsbronn in the Altmühltal Nature Park. (© H. Pfanz 2019)

organisms (Sect. 3.9.3 and Chap. 10). However, degraded peatlands can obviously also have extremely elevated soil CO2 and CH4 concentrations.

3.9.1 Degraded Peatlands – Sauerland In the “Eicherwald nature preserve” in the Rothaargebirge, a partially destroyed peat bog has been re-planted with non-native woody plants. Due to the peat mining within the bog, soil chemism was dramatically changed. Meanwhile, extreme gas concentrations can be measured in the subsoil. In a soil depth of 60 cm often up to 30% CO2 can be found. In other places, the methane concentration in the soil becomes as high as 70% (Fig. 3.14a, b). The formation of these gases is biogenic and due to the decomposition of organic matter by anaerobic microorganisms within the bog soil. Improper

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Fig. 3.14  a and b: Unusually high carbon dioxide and methane concentrations at a soil depth of 60 cm in the Eicherwald nature reserve in the Rothaargebirge. Soil gas investigations were carried out on an area of 11 × 13 m in a drained and reforested bog. (© H. Pfanz 2019)

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treatment of peatland (peat mining and especially drainage and reforestation) has allowed oxygen access to certain soil depths. Oxygen is otherwise excluded in intact peatland soils; they are strictly anaerobic. Bacteria and soil fungi now decompose the existing organic material and release methane and, as an oxidation product, CO2. This leads to the enormous, unnatural gas increases within the topsoil. This fact is even more alarming, as methane releases promote the global greenhouse effect twenty times more intensively than carbon dioxide increases. For these reasons, further destruction of peatlands must be strictly avoided.

3.9.2 “Seismo-Tectonically” Induced Changes in Geogenic CO2 Release Near the Dutch town of Roermond there are large tectonic fault systems with extensive NW-SE trending fault zones, such as the Peelboundary fault near the German-Dutch border. These faults are caused by active crustal dynamics in the Lower Rhine Graben. A strong earthquake with a magnitude of M = 5.9 occurred along this fault zone in 1992 and affected not only Roermond but also other cities like Mönchengladbach and Köln (Cologne; van Eck and Davenport 1995). The near-surface disturbances are still to be seen from the air by a linear displacement on the ground. In one particular section, this fault can be traced in the terrain along the edge of a forest, through cornfields and meadows, and even through a residential house whose roof has been deformed like a staircase. In a maize field, the plants to the right and left of the disturbance grow differently well (Fig. 3.15). Maize plants on one side of the disturbance grew up to 1.2 m high, while half a meter distant on the opposite side they were only 50  cm high. The vegetation in the meadow next to field was similar. Soil investigations have shown that the water content, the pH-value and the soil conductivity are different on both sides of the disturbance. This causes the better or worse growth of the plants (Pfanz and Werner unpublished). The reason for this is a clay wall formation parallel to the fissure, which prevents a free water exchange between both sides of the fault. The clay formation is the result of innumerable earthquakes and solution processes, that transformed the rubbing rock plates into clayey or loamy deposits (van Eck and Davenport 1995). Along the fault zone various gases can be measured. Oxygen, CH4, CO2 and CO escape from the fault zone in measurable quantities. Figure  3.16 shows the distribution of soil CO2 within this meadow. Excitingly, the

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Fig. 3.15  Cornfield near Roermond, Netherlands. A tectonic fault moves from approximately right front to left back. The fault can be seen in the maize field as well as in the adjacent meadow. The maize plants reach different heights on both sides of the fault. (© Kerstin Werner)

common bluegrass (Poa trivialis) grows exactly on this fissure and cannot be found at a distance of a few decimeters to the left and right of the fissure (Fig. 3.16a, b).

3.9.3 Biogenic Soil Respiration In addition to geogenic CO2 releases, there is also a biogenic CO2 release within the soil. The latter is the common basis for CO2 increases in the soil. All soil organisms (the edaphon) respire, consuming oxygen and releasing corresponding amounts of carbon dioxide. This is true for all plant roots, for soil insects, soil arachnids, soil worms and for soil mammals, but also for soil algae, soil bacteria and soil fungi. Due to the respiration of the edaphon, the oxygen content of the soil is depleted and oxygen is therefore always lower within the soil than in the overlying atmosphere. The deeper one digs into the soil, the less oxygen is present.

3 Mofettes 

a

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33

0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0

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Longitudinal [m]

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8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42

Cross transect [m] 42 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

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30 27 24 21 18 15 12 9 6 3 0

0

3

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9

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15

18

21

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27

30

33

36

39

Cross transect (m)

Fig. 3.16  Roermond; Soil CO2 concentration within a meadow (a) and growth of Poa trivialis (b) directly on the linear disturbance of the Peelboundary Fault

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The situation for CO2 is reversed. CO2 is always more concentrated within the soil than within the atmosphere. While the atmospheric CO2 concentration is about 0.04%, soil CO2 is always in the percentage range. 1–10% CO2 can be measured in extremely humus-rich and organism-rich soils (Raich and Potter 1995; Lipson et al. 2005).

3.10 Is It Possible to Search for Mofettes? One can search for mofettes and of course also find mofettes. Prerequisites are either good geological knowledge of the region or exquisite knowledge in botany or zoology. Let’s start with the latter. An accumulation of animal corpses in volcanic areas could be a first indication for such CO2 emissions. As already mentioned, animals die within a few minutes at CO2 concentrations above 5–8%. Therefore, one can roam olfactorily, using the nose, through mofette areas to locate the degassing sites via the smell of decaying animal corpses.The absence of molehills (Pfanz 2008) or a dramatic reduction in ground fauna may also be indicative (Hohberg et al. 2015; Balkenhol et al. 2016). Geological knowledge helps much better to locate faults in volcanic areas where degassing of CO2 occurs. With suitable maps and an eye for geomorphological features, the trained geologist is usually able to identify tectonic faults in the terrain. If one is trained in botany and vegetation science or has a fair amount of knowledge of plant species, one can infer “geological disturbances” from the azonal vegetation, i.e. the “botanical disturbances”. The method is very well suited to locate geogenic CO2 emissions by means of vegetation disturbances, and has already proven successful in Central Europe and some non-European sites (Saßmannshausen 2010; Thomalla 2015; Pfanz et al. 2019c). Also aerial photographs are now being used to find botanical markers in plant distribution in order to locate disturbances and thus identify regions of volcanic CO2 exhalation. Meanwhile, various plant species have been identified that either occur frequently in the vicinity of mofettes or, in contrast, strictly avoid degassing sites. With the help of such mofettophilic and mofettophobic indicator plants, one can infer the soil gas concentration of CO2 and O2 from the pure presence of the plants. This is called bioindication or, in our special case, phyto-­ geoindication, since plants can be used to describe the physical-chemical conditions of the soil.

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The increased fluid pressure at the degassing points is also indicative. In areas with guaranteed snow, particularly strongly degassing mofettes can be detected in winter by means of circular disturbances in ice layers on ponds (Fig. 3.17). After heavy rain, degassing sites can even be found acoustically. Hissing and gurgling reveal the locations of high gas flows. For lovers of such mofette sounds, a “mofette CD” has been released in Poland (Nacher and Styczyński 2005). Finally, thermographic studies can also be used to detect geogenic degassing (Tank et al. 2008). Recently, drone and satellite technology has been used to map mofette areas worldwide to quantify the magnitude of degassing (cf. Sect. 6.7).

Fig. 3.17  Ice holes in the ice of a peatland pond in Soos, Czech Republic. The permanent mechanical movement of the water surface by the bubbling CO2 gases prevented uniform freezing. (© H. Pfanz 2019)

4 Geological-Volcanological Basics

The global carbon reservoir amounts to about 106 × 1015 t carbon and is distributed to over 80% in carbonate rocks (Table 4.1). The remainder is made up of inorganically bound carbon on land and in the sea and carbon bound in organic matter (living organisms, plant and animal corpses), as well as gaseous CO2 in the atmosphere. CO2 has been released from the Earth’s interior since its formation. Calculations have been made as to which proportion of the carbon dioxide comes from volcanic eruptions or from the degassing of the ocean crust (Bredehoeft and Ingebritsen 1990). The worldwide geogenic carbon dioxide releases were compiled by Barnes et al. (1978). The authors showed that CO2 is mainly released along continental plate boundaries and also from deep fault zones within the continents (so-­called intraplate boundaries). These are specifically (1) the circumpacific arc (Ring of Fire) and (2) the mountain ranges extending across central and southern Europe to Asia Minor. Furthermore, they found that the locations of the emissions are always also locations of recent tectonic faults, i.e. potential earthquake zones. The carbon isotope provides information on the actual reservoir of carbon dioxide. Especially the content of the carbon isotope 13C tells something about the origin of the carbon from the earth’s mantle (magmatic) or the earth’s crust (often metamorphic or biogenic). This is still fiercely debated among geologists and volcanologists, since due to fluid mixing during the ascent of the gases to the Earth’s surface, the isotope value can also lose its significance. However, analysis of helium isotopes (3He/4He) can be used to

© Springer-Verlag GmbH Germany, part of Springer Nature 2023 H. Pfanz, Cold Breath of Dormant Volcanoes, https://doi.org/10.1007/978-3-662-65375-3_4

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Table 4.1 Percentage of different carbon reservoirs in the total global carbon in 1015 t. (After Sundquist 1985; from Bredehoeft and Ingebritsen 1990)

Reservoir

Carbon in 1015 t

Atmosphere Oceans Continents Carbonate rocks: Oceans Continents Metamorphic rocks Total

0.036 6.4 7.3 28 54 10 106

substantiate the significance of carbon isotopes (Kämpf et al. 2007; Bräuer et al. 2013). The noble gas helium can migrate undisturbed from its reservoir rock to the Earth’s surface. In doing so, it often uses the same transport pathways as CO2. It is assumed that there is almost no 3He left in the crust; 3He originates almost exclusively from the Earth’s mantle. If this proportion is extremely high, similar to that at an active volcano, one speaks of a high fluid proportion from the Earth’s mantle. In our case it may be assumed that the major part of the carbon dioxide, which is found in mofettes, is originally of magmatic origin. In the East Eifel, mofettes and acid mineral springs are only found together with pre (?) – or post volcanic phenomena (Laacher See; Wehrer and Riedener Kessel). In the Western and Eastern Eifel, more than 750,000 tons of CO2 are thus released per year (cf. May 1994, 2001, 2002a, b). According to Stoffels and Thein (2000), however, cooling magma chambers alone cannot explain this enormous amount of released gas. Recent publications by Ritter (2007) and Hensch et  al. (2019) show that there are also recent upwelling fluids in the Eifel. With the help of helium isotope analysis, the origin of CO2 was investigated and attributed to the formation of basaltic magmas from the Earth’s upper mantle (at depths of 100–150 km) (Bräuer et al. 2018; Kämpf et al. 2013). These magmas contain fluids and can ascend due to their slightly lower density. During ascent, cooling occurs at the boundary between mantle and crust, and the gaseous fluids start to escape from the melt due to increasing concentration and a lowering pressure. From here, the fluids diffuse through fissures, crevices and cracks towards the Earth’s surface and then degas through the ground into the atmosphere. According to Degen (2001). “... large fractions of CO2, which have been bound to mineral water for a long time or escape as free gas, are fed by large gas reservoirs contained in the magma of the

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Earth’s mantle. These molten rocks, on account of their lower density, rise to the boundary between the earth’s mantle and the crust, cool in the process, and lose the gaseous constituents in the temperature range between 200 and 400 °C, owing to the decrease in solubility...” (see also Dressel 1871; von Dechen 1864).

5 Carbon Dioxide: The Decisive Parameter for Mofettes

5.1 Nomenclature What exactly is this compound of one carbon and two oxygen atoms? A wonderful description of carbonic acid can be found in Gehler (1789): ... “According to Bergmann, the fixed air is in a ratio of 3:2, according to Lavoisier in a ratio of 561:455, specifically heavier than the atmospheric air, and therefore sinks to the ground in the latter. This gives the opportunity for very interesting experiments, such as the Düc de Chaulnes (Mem. des Sav. etrangers 1778) performed before the Paris Academy. One can pour invisible fixed air from one vessel into another, like water or like any visible fluid, and thereby extinguish a light, kill a mouse, and so on. To the eye, one pours nothing from one cup in which there is nothing into another in which there is also nothing, with great care not to spill anything, and yet one can thereby kill animals, extinguish lights, crystallize salts, and the like.”

Chemically, carbon dioxide is the highest possible oxidation state that carbon (C) can reach, while methane (CH4) is the highest reduced form of carbon. From this it can be deduced that methane is easily oxidizable and thus combustible, while CO2 is absolutely no longer oxidizable and thus incombustible. Therefore, CO2 can be used to extinguish fires. Further properties of carbon dioxide are its odourlessness and tastelessness, its specific density and its absorption of infrared radiation.

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5.2 The Physico-Chemical Properties of CO2 Carbon dioxide is a colourless and odourless gas at room temperature and normal pressure. In the “Physical Dictionary” by Gehler (1789) it is still to be found under “mephitic gas”. Other names for the gas, which is not yet fully understood, are: Wild or vinified gas, fixed or artificial air, aerial acid, acid air, gas mephiticum, gas vinosum, mephitis vinosa, mephitis acidula, aër fixus, and acidum mephiticum.

5.2.1 Odour and Taste CO2 is considered an odorless and tasteless gas (Sax and Lewis 1989); quite unlike the strong smelling or highly irritating volcanic gases such as HBr, SO2, or H2S. In dilute form, CO2 is really not odorous. But when it is enriched or presented in a highly concentrated form, it can be easily detected olfactorily or gustatorily. So if you carefully sip a small sample at the surface of a dry CO2 gas lake, you will very clearly sense the smell and taste of mineral water or even champagne. But be careful! Too much of the gas is fatal! It is easier and much safer to sniff with the nose at the opening of a just opened beer or champagne bottle (see Sect. 5.2.3.3). The smell effect is the same as in the CO2 gas lake, but the amount of gas is so small that you (usually) survive.

5.2.2 Water Solubility CO2 is very soluble in water. The CO2 solubility in water is 3346 mg/L at 273.15 K (O°C) and P CO2 = 1.013 hPa. At 20 °C (293 K) but the same pressure, it is still about 1700 mg/L (Wiebe and Caddy 1940). Dissolved in water, it forms a weak acid – carbonic acid.

5.2.3 The Acid Effect If gaseous CO2 comes into contact with aqueous solutions, carbonic acid is formed according to Eq. (5.1). In a first step, the gas is physically dissolved and then chemically bound to water. The unstable carbonic acid is formed. Thereafter, however, the acid dissociates rapidly, depending on the pH of the solution, to form hydrogen carbonate or carbonate and protons. The dissolution of CO2 in water as well as the two-step dissociation of carbonic acid takes place according to Eq. (5.1):

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(5.1)

However, the equation also shows that gaseous CO2 can be released again from its salts. In this case, the equation is shifted to the left side (in the case of acidification). In the case of alkalization of the medium, the equation would shift more or less to the right side. The strength of the dissociation and the equilibrium setting can be calculated well with the Henderson-Hasselbalch equation, in which the respective pH value and the pK values for hydrogen carbonate (bicarbonate HCO3¯) and carbonate (CO32¯) enter. From the above equation it can be seen that carbonic acid releases gaseous CO2 in acidic solutions, whereas in neutral and alkaline solutions it is dissociated as hydrogen carbonate or carbonate and thus remains dissolved in water. These properties are of outstanding importance for understanding the effect of carbon dioxide (Wollmann 1942). For example, the chemical binding (and “disposal”) of CO2 in alkaline waters is of immense importance in CO2 buffering of the world’s oceans (and will need to be considered in plans to dispose of carbon dioxide in oceans; see Caldeira and Wickett 2003, 2005).

5.2.3.1 Buffering of CO2 in the Ocean Since seawater is usually slightly alkaline, atmospheric CO2 dissolves in seawater not only physically but also chemically. In the process, the carbonic acid dissociates and releases protons. This CO2-induced acidification will successively change the pH value in the world’s oceans towards the neutral point and further towards slightly acidic pH values. These changes will have an immense impact on many marine species. Already today, such changes can be observed in marine animals at certain locations. Because acidic pH values make it harder for marine organisms to access cations, the uptake of calcium (Ca2+) and magnesium (Mg2+) becomes more difficult for many mussels and clams. These cations are used to fix carbonates (CO32−) for shell construction. Under laboratory experiments, partial dissolution of the shells of pteropods (winged snails or marine butterflies) has been demonstrated in Clio pyramidata (Orr et al. 2005). Corals and certain plankton species are also affected. Hall-Spencer et al. (2008) showed the effects of submarine volcanic CO2 degassing on mussels and reached similar conclusions. The snails Osilinus turbinata (dice turban snail), Patella caerulea (common limpet) and Hexaplex trunculus (blunt spiny snail) showed severely eroded shells in seawater acidified by CO2 emissions (pH around 7.3). The

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Mediterranean Neptune grass, Posidonia oceanica, which is heavily overgrown with Corallinaceae (“coral moss”) at slightly alkaline seawater pH (pH 8.2), hardly shows any epiphytic growth in CO2 acidified water (pH 7.6) (Hall-­ Spencer et al. 2008). The ability of dissolved carbon dioxide to form intermediate carbonic acid (H2CO3) and therefore to acidify aqueous solutions is used in the production of carbonated mineral waters. However, CO2 in the air (currently 0.041%) naturally acidifies not only mineral waters but also fog and raindrops, turning normal rain into slightly acidified rain with pH values around 5.6–5.8 (Fellenberg 1997). This is the reason why even completely unpolluted rainwater always reacts slightly acidic. In the case of anthropogenically more polluted “acid rain”, whose pH is much lower (pH from 5.0 to 1.8  in some cases), sulphur dioxide (SO2) and/or nitrogen oxides (NOx) are also dissolved and have formed the strong acids sulphuric acid and nitric acid after oxidation.

5.2.3.2 The Human Mofette: The Stomach as a Reaction Chamber The fact that CO2 rapidly degasses from the solution at low pH can be demonstrated very impressively in a self-experiment: Quickly drink larger amounts of preferably cool mineral water (which should contain a lot of dissolved carbonic acid) and wait a few seconds. A pressure in the stomach area, which by the way cannot be withstood for a long time, unmistakably indicates the formation of gaseous carbon dioxide. The difference in temperature (mineral water approx. 15 °C, stomach approx. 37 °C) and especially the difference in pH (mineral water pH 3–5, stomach approx. pH 1–2) have abruptly brought the physically and chemically dissolved carbon dioxide to the side of the undissociated gas (cf. equation 5.1), which now occupies a significantly larger volume and seeks its way out, following the least resistance (Fig. 5.1).

5.2.3.3 Opening Beer and Champagne Bottles If you open a bottle of cold beer, you will hear a hissing and whistling. This is due to the overpressure, which can now escape to the outside. During this process and due to the pressure relief by opening, also carbon dioxide gas escapes, which was previously in supersaturated form (also as carbonate and hydrogen carbonate salt) in the beer. Particularly in the upper part of the

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Fig. 5.1  The human mofette. Noisy belching of carbon dioxide after drinking mineral water. Detailed instructions in the text. (© H. Pfanz 2019)

bottle neck directly under the bottle cap, there is a highly concentrated CO2 water vapor mixture that has already been created during bottling. The degassing occurs extremely quickly because the previously existing pressure drops abruptly by opening the bottle cap. The system relaxes and the outflowing gas cools down considerably in the process. During this process, the carbon dioxide concentration in the upper neck of the bottle directly above the bottle contents is again significantly increased. Measurements taken when opening such bottles with a portable CO2/O2 analyzer showed up to 92% CO2 (and 5% O2) for chilled beers and almost 96% CO2 and 2% oxygen for chilled sparkling wine (own data). If one could measure faster and more accurately above the beverage surface, one would probably be able to detect 100% CO2. At the same time, the water vapor present is condensed due to the sharp drop in temperature. Highly concentrated CO2 and water vapor (or “water ice” as well as dry ice crystals) then result in the typical “fog streaks” when opening a carbonated beverage (Liger-Belair et al. 2017; Liger-Belair and Séon 2017). This process can be observed even more clearly when uncorking a champagne bottle. Here, too, it is the sudden pressure difference during uncorking that causes the overpressurized beverage bottle to foam and overflow. Inside the bottle there is a supersaturated CO2 solution. The ions of carbonic acid (hydrogen carbonate HCO3− and carbonate CO32−) are present next to

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undissociated carbonic acid (H2CO3) or physically dissolved carbon dioxide (H2O × CO2). Before closing, CO2 was added in excess during bottling – an overpressure was created (the gas overpressure in the different beverages was created slightly differently in each case; however, this is not the subject of this paper). The sudden drop in pressure when the bottle is opened causes CO2 molecules to degas. Tiny gas bubbles are formed on the rough walls of the vessels and especially on tiny particles of the vessel wall (Zhang and Xu 2008), which later detach and float upward by the buoyancy force. As they rise, they collide, merge, grow larger, rise faster, collide again, and thus steadily increase in volume. Having reached the vicinity of the opening of the vessel, the former bubbles have in the meantime become stately bubbles, which, moreover, have considerably accelerated their buoyancy speed. Due to their speed and size, the CO2 bubbles now pull the liquid in their vicinity upwards with them. The neck of the bottle forces the bubbles to unite transiently due to its constriction. The excess gas pressure now manifests itself in vehement effervescence.

5.2.4 Absorption of Thermal Radiation Carbon dioxide absorbs infrared radiation of wavelengths around 2.7  μm, 4.3 μm and 15 μm. Because of this property, CO2 can be measured very well in the gas phase (infrared absorption spectrometry). However, this property also causes its positive contribution to the air temperature. Without CO2 and water vapour in the atmosphere, the average temperature of the Earth’s atmosphere would be about −12 to −16  °C.  Fortunately, the current average air temperature is about +16 °C due to the absorption behaviour of gases, especially the content of water vapour, natural CO2 and methane; this is called the natural greenhouse effect. The property of CO2 to absorb infrared radiation causes nowadays, however, its fatal contribution to the anthropogenic greenhouse effect. The unchecked burning of fossil fuels by mankind leads to a significant increase in the CO2 concentration of the atmosphere. Since the industrial revolution, the CO2 concentration has increased from about 260 ppm (pars per millione) to today’s 410 ppm CO2. The infrared heat radiation emitted by the earth is now increasingly absorbed and partially reflected by the increased CO2 in the atmosphere; the atmosphere is visibly warming. We are already feeling the consequences of this reaction in everyday life.

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A wonderful example of how this greenhouse effect can be clearly demonstrated is the “Valle di Bossoleto” nature laboratory in Tuscany. Here we have been able to analyse and prove beyond doubt the connection between the increase in atmospheric carbon dioxide and the associated increase in atmospheric temperature (Kies et al. 2015; see also Chap. 9 ff.).

5.2.5 The Specific Density CO2 has a molecular weight of 44.01 g mol−1. With a density of 1.98 kg m−3 it is approx. 1.5 times heavier than air. In higher concentrations, it can therefore form so-called dry gas lakes in natural hollows or in cellars, when there is no wind.

5.2.5.1 Carbon Dioxide Forms Gas Lakes and Gas Streams Life-threatening dry CO2 gas lakes can form in poorly ventilated spaces (e.g., fermentation cellars and wells), volcanic caves, or in ground depressions (see lethal gas traps in Sect. 8.1 and Chap. 13; Raschi et al. 1997; Bettarini et al. 1999; Pfanz 2008). In calm weather conditions, such gas lakes can be quite stable and form a clear boundary between the heavy CO2 lake and the atmosphere above (Kies et al. 2015). Gas concentrations may well be 100% at the bottom of such a gas lake. Such gas lakes also form in CO2 caves, such as in the Aragonite Cave in Zbrasov/Czech Republic, but they are also found in cellars of residential buildings and garages in the Eastern Eifel (Degen 2001). Deadly gas lakes also play an important role in the ancient mythology of the Greeks and Romans (cf. Chap. 12). For the Sanctuary of Pluto at Hierapolis, as the entrance to Hell, this has just recently been published (Pfanz et  al. 2014, 2019a, b). At the Sousaki volcano in Greece (D’Alessandro et al. 2006), one can even observe a dry CO2 gas stream flowing out of small caves on the slope (Fig. 5.2a Gas lake and 5.2b Gas stream). The gas stream emerging from the caves is easily recognizable by its (artificial) coloration. Suffocated animals (mice, birds, insects) can be found in the stream bed.

5.2.6 Visibility of the Gas Carbon dioxide is a colourless gas. One can’t actually see it. If it occurs in high concentrations in valleys and hollows, it can sometimes be seen quite well

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Fig. 5.2  Whitish-colored CO2 gas lake in a small cave on the rim slope of a former crater of Sousaki volcano (a). A reddish colored dry CO2 gas stream flows down the slope from a gas cave of Sousaki volcano (b). Thanks to the help of Dr. F. Italiano and Dr. W. D’Alessandro, INGV-Palermo. (© H. Pfanz 2019)

when the sun is in a suitable position. However, it is not the actual gas lake that can be seen, but its surface, which differs from the atmosphere above, because of the different reflection of the sunlight. Using suitable pyro-torches, however, it is possible to stain a CO2-gas lake for a short time (see also Fig. 5.3). The deadly gas lake “Il Bossoleto” near Siena, more than 4  m high, was coloured in the morning with pyro-torches. In the lower part of the gas lake, one can see very clearly the stratified layering of the different gas levels.

5.2.7 The Displacement of Oxygen Because CO2 displaces atmospheric oxygen by its mere presence, leading to a decrease in oxygen concentration, it can cause oxygen deprivation, respiratory problems, and asphyxiation in aërobic organisms (Niel et al. 2007; Pilz et al. 2017 and references therein). In the presence of very high concentrations of mephitic CO2, the oxygen concentration of the air can be reduced from

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Fig. 5.3  Colouring of a 4.3 m high CO2 gas lake in the Bossoleto gas valley near Siena. The horizontal spread of the coloured particles of a yellow smoke flare in the CO2 gas is clearly visible. (© H. Pfanz 2019)

normally 20.8% to zero. This has been measured several times in mofette soils but also in the air of valley mofettes (Pfanz 2008; Vodnik et al. 2006; Kies et al. 2015). The displacement of oxygen and the fact that CO2 represents the highest possible oxidation state of carbon (and is therefore no longer flammable) makes it useful in extinguishing fires. Firefighters therefore also use it to extinguish flames where the use of water is not advisable. Even Dante Alighieri (1265–1321) wrote about the oxygen displacement by CO2. He describes the extinguishing of flames on the hot Bullicame stream near Viterbo (Inferno, Canto XII, XIV and XV). “Cosa non fu dagli occhi tuoi scorta Notabil come lo presente rio, Che sopra sè tutte fiammelle ammorta.” (L'Inferno, Canto XIV, 88-90)

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5.3 The Toxicity of Carbon Dioxide Every vintner knows about the danger of CO2 and therefore pays very close attention to the necessary precautions when going into the fermentation cellar. During the fermentation of grape must to wine, a lot of carbon dioxide is produced by the yeast fungi (Saccharomyces cerevisiae) during the conversion of sugar to alcohol. One molecule of CO2 is released per molecule of alcohol formed. This process can best be observed in the phase of the most violent fermentation (in wine: during the formation of “Federweißer” or “Sturm”). The gas escapes from the fermentation vats in considerable quantities. Since carbon dioxide gas is specifically heavier than air, it forms an invisible gas lake at the bottom of the fermentation cellar. Even a short stay of a few minutes in such a gas lake can be fatal. In Gehler (1789) we can read: “This type of gas quickly extinguishes the fire and strongly attracts the vapor of the candles. It is unsuitable for breathing, and animals cannot live in it. The warm-blooded die most quickly, later the amphibians, the insects are only half killed, the irritability is quickly destroyed, and the still warm heart of a thus killed animal shows no more movement.” Today, carbon dioxide is assigned to category IV as a hazardous working substance with a very weak potential. This may seem correct when comparing the toxicity limits with other known gases. Sulphur dioxide, the nitrogen oxides or ozone already have a harmful effect in concentrations of a few ppm (pars per millione = parts per million), whereas carbon dioxide only develops its harmful effect on health in concentrations of several percent. However, carbon dioxide is an “air-displacing gas”. In certain topographically lower locations (valleys, hollows, cellars, caves) and under certain climatic conditions (no wind), CO2 can accumulate in concentrations of several tens of percent and is then highly dangerous (cf. Chapters 8 and 13). The MAK value (maximum workplace concentration) for CO2 is 5000 ppm (or 0.5 vol%). For one hour, the concentration may increase to 1% (= 10,000 ppm). Toxicologists know that increased concentrations of CO2 can be dangerous for humans. From a concentration of 1% CO2 in the breathing air, human respiration is accelerated. Longer exposure times to this concentration causes headache (Krinninger 2001; Bundesamt für Arbeitsschutz und Unfallordnung 1982; Hansell and Oppenheimer 2004; D’Alessandro 2006). It should be noted, that during normal exhalation we release about 4.5% CO2 with the air we breathe. If we would not permanently exhale the CO2 formed in the body, we would very quickly poison ourselves through the CO2 accumulating in the blood (more on this later). At concentrations of 3 to 5% CO2 our respiration

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is increased by up to 400%; this concentration is therefore also used to stimulate the circulation and is also used in resuscitation measures after cardiac arrest (Krinninger 2001). Prolonged inhalation of CO2 concentrations greater than 10% usually causes people to faint. Short-term inhalation of 30% CO2 causes palpitations, confusion, visual disturbances, and sometimes death (Stupfel and Le Guern 1989; Hansell and Oppenheimer 2004; D’Alessandro 2006). The causal effect of CO2 is on the one hand the displacement of vital oxygen (hypoxia, anoxia), but on the other hand also the acidifying effect of CO2 in the blood (hypercapnia) and in the body cells. The acidifying effect of CO2 is also a known problem for divers. Hyperventilating before the dive not only increases the oxygen content of the blood, but also depletes existing CO2. The danger of acidification of the blood is thus postponed (Table 5.1). Midwives and gynaecologists are also aware of the potential acidity of carbon dioxide. During the birth process, women can influence the acidity of their blood by panting (increased intake of oxygen, increased exhalation of CO2) or “breathing into the hands held in front of the mouth” (reduced oxygen intake, rebreathing of CO2). Increased exhalation of CO2 alkalizes, increased inhalation of CO2 acidifies the blood. Hypercapnia is often a consequence of certain diseases of the lungs, in which there is a reduction in CO2 exhalation. The stinging sensation in the mouth and nose, which occurs when CO2 dissolves on the mucous membranes, was already used by the ancients as a warning system for carbonic acid eruptions in mines (Werne and Thiel 1914). But even in buildings without geogenic CO2 release, the CO2 concentration in the air can be dangerously elevated. With poor ventilation, the value Table 5.1  Toxicity levels of carbon dioxide and their consequences for humans. Slightly modified after D'Alessandro (2006), Kaye et  al. (2004), Hansell and Oppenheimer (2004), Fischedick et al. (2015) and Smith (1996) CO2 concentration [%] 0.041 0.15 0.5 4.3 5 8–10

20

Effect Current concentration in globally averaged tropospheric air Recommended maximum value indoors MAK value (maximum workplace concentration) Concentration in the breathing air (exhalation – expiration) First symptoms like dizziness and headache appear Further symptoms: Increase in blood pressure, shortness of breath, nausea, blue discoloration of the skin, weakness and even fainting; onset of death after approx. 30–60 min. Immediate fainting; onset of death after approx. 5–10 min

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of 1500  ppm CO2 can be significantly exceeded indoors. In bedrooms or crowded lecture halls, CO2 concentrations can even reach levels of 3000 ppm or more within hours. Although these concentrations are not directly harmful, these rooms should be ventilated regularly (Kim et  al. 2002; Grams et al. 2003).

6 Mofettes as Habitats

Despite the sometimes extremely hostile environment, mofettes are growing sites and habitats not only for anaerobic organisms. At the beginning  of mofette research, chlorotic plants and dying or dead animals were recognized. But today’s view of mofettes has changed significantly. The mofette has been recognized as a habitat or even an ecosystem. Meanwhile, the adaptations and strategies of various organisms to extreme factors such as very high CO2, low oxygen content or hyper-acidified soil are now also being studied. For many organisms normally found in low-oxygen habitats, such as marsh or bog soils, mofettes can provide both refugia and sites with less competitive pressure.

6.1 Plants in Mofettes Older publications already report on the positive and also very negative effects of CO2 on plants: “The fixed air is a true acid ... - ... According to Priestley's experiments, plants do not thrive in it, although, as D. Ingenhouß (Versuche mit Pflanzen rc.) has shown, they vegetate very well in water that is acidic with air, and take the acid from the same into themselves” ... (Gehler 1789).

Plants use atmospheric CO2 to reduce it to carbohydrates using solar energy. At the current CO2 concentration in the atmosphere (0.041%), plants can

© Springer-Verlag GmbH Germany, part of Springer Nature 2023 H. Pfanz, Cold Breath of Dormant Volcanoes, https://doi.org/10.1007/978-3-662-65375-3_6

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photosynthesize quite well, but the photosynthetic capacity can be increased significantly by increasing the CO2 concentration. For most plants, however, maximal photosynthetic saturation is reached at CO2 concentrations of 0.3–0.5% CO2.(Larcher 1994). Nevertheless, very high CO2 concentrations can lead to a transient inhibition of photosynthesis in plants (Fig. 6.1). When plants grow on mofette soils, it can happen that the geogenic CO2 is transported via the intercellulars of the roots upward to the leaves and thus the CO2 concentration available for photosynthesis is increased significantly above 0.5%. If mofette plants grow in an above-ground CO2 gas lake, the situation can become even more dramatic. In the Bossoleto Valley in Italy, reed plants (Phragmites australis) are exposed to nearly 100% CO2 in the morning sun. However, at these elevated CO2 concentrations, photosynthesis of the plants has long been inhibited (Pfanz, Wittmann, Bilger, unpublished). Therefore, an increase in substrate concentration no longer has a fertilizing and stimulating effect, but an inhibiting one. The main cause of the inhibition of photosynthesis by high CO2 is acidification within the chloroplasts (Pfanz and Heber 1986; Pfanz et al. 1987; Pfanz 1994).

Fig. 6.1  Light curve of photosynthesis of wood garlic (Allium ursinum). The plant was selected because it was not adapted to mofette sites. It can be seen that the control has its photosynthetic maximum at about 500 μmol photons m−2 s−1. After that, more light causes an inhibition of photosynthesis. 15% CO2 in the atmosphere inhibits photosynthesis. Above 28% CO2 a photosynthetic reaction is hardly measurable. Regeneration without CO2 shows control values after half an hour

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This leaves the question, how and why can plants grow on mofette sites at all, and why do some species even seem to prefer mofette sites? Some of these plant species can even be used indicatively as “positive” biological indicator plants for the occurrence of CO2 in soils because of their preferred occurrence on mofette soils. Other species, on the other hand, avoid CO2-gassing soils and are thus displaced from mofette areas. They even indicate the boundaries to degassing areas and can therefore also be used as (negative) bioindicators.

6.1.1 Plants Perish Many plants suffer the same fate as animals. They cannot thrive in mofette sites; they die sooner or later. Others cannot germinate at all; either their seeds cannot form radicles under the prevailing oxygen deficiency in the soil, or their young roots die and rot within a very short time (Geisler 1973; Raschi et  al. 1997, 1999; Pfanz et  al. 2004, 2005, 2007; Vodnik et  al. 2002a, b, 2005, 2018). At such sites, the soil is usually free of any vegetation (Fig. 6.2a). The damaging effect of significantly elevated CO2 concentrations can even be demonstrated in algal layers. The effect ranges from slight damage to cell death of the algae (Fig. 6.2b). In the Californian caldera of Long Valley (Sierra Nevada), degassing CO2 also had a devastating effect on vegetation. Between 1978 and 2003, several

Fig. 6.2  Rape field (Brassica napus) on a slope near Hartoušov. The inhomogeneous CO2 emission from the soil clearly affects the growth and flowering onset of oilseed rape. In addition to completely unvegetated areas, growth restrictions and flowering delays can be seen. In the background, optimal growth of oilseed rape indicates normal CO2 levels in the soil (a). Dead (white), dying (grey) and still vital algae of an algal cover (green) in a mofette puddle of the Smradioch mofette near Marienbad (b). (© H. Pfanz 2019)

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earthquakes occurred in a high-level valley near Mammoth (Horseshoe Lake). This altered the fracturing of the mountain, with increased amounts of carbon dioxide finding their way to the earth’s surface (Hill and Prejean 2005). Large areas of pine (Pinus contorta; lodgepole pine) forests on the flanks of the valley died (Fig. 6.3a). This was caused by the very high CO2 degassing and the resulting dramatic drop in soil oxygen concentrations. Unlike marsh plants, pines are unable to supply their underground organs with “surface-generated” oxygen. Due to the lack of oxygen in the soil, the roots died off and, as a consequence, forest areas died off over a wide area. In 2003, however, growth of young pines could already be observed again (Fig. 6.3b). The emission of CO2 had decreased significantly. It seems that the dry pine forest will again recolonize this area. A dramatic increase in dying trees has also been noticed at the edge of Lake Laach in the Eastern Eifel (Germany). In the rooting zone of many trees close to the shore, CO2 concentrations of up to 80% can be measured (Pfanz et al. unpublished). The high CO2 content of the riparian soils forces the oxygen-­ requiring roots of oaks, beeches, poplars, limes and chestnuts from the deeper soil layers into higher soil zones closer to the surface. This relocation of the roots is associated with two disadvantages for the trees. In hot and dry summers, the roots are much more directly exposed to the dry-hot soil and they also find less soil water. Weakening the trees naturally allows parasites and pathogens free rein. However, the near-surface displacement of the root mass serving as anchorage also leads to mechanical destabilization of the trees; wind pressure and snow load therefore cause the trees to topple over more quickly (Fig. 6.4).

Fig. 6.3  Dead lodgepole pine (Pinus contorta) on the flanks of Horseshoe Lake near Mammoth, Sierra Nevada in 2005. Extreme geogenic CO2 degassing had led to the death of entire forests in this volcanic area (a). Five years later (2010), young growth was already present again in some places due to decreasing emissions (b). (© H. Pfanz 2019)

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Fig. 6.4  Dead tree on the shore of the Laacher See. (© A. Thomalla)

6.1.2 Growth and Habitus of Plants in Mofettes Degassing sites of CO2 do not necessarily have to be completely devoid of vegetation. Besides well-adapted specialists (discussed below), there are many, hardly specialized plant species that thrive more or less well at mofette sites. In northern Slovenian and northwestern Czech meadow mofettes, but also in many places of the Volcanic Eifel, one can make such observations. It is often the case that the closer the respective plant species grow to the actual emission sites, the smaller they are (Turk et al. 2002; Pfanz et al. 2004, 2007; Vodnik et al. 2001, 2002a, b, 2005, 2018) (Fig. 6.5). Morphometric changes of plant organs under high CO2-stress can be taken from Table 6.1 for scorpionweed (Phacelia tanacetifolia). It can be seen very clearly that an increase in soil gas has a strong influence on the size of the plants as well as on the size and dry mass of the individual plant organs. Most plants growing near the actual emission sites are clearly smaller and show a sometimes significantly reduced rooting depth compared to plants from non-degassing control sites (Geisler 1973; Turk et al. 2002; Vodnik et al. 2002a, b; Maček et al. 2003; Pfanz et al. 2004). At high CO2 levels, leaves often remain smaller, number of flowers is reduced, and seed formation is decreased (Stubbe 2002). Characteristic is also a reduced chlorophyll content of the leaves as well as a poorer nutrient supply of the whole plant (Pfanz et al.

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Fig. 6.5  A field with scorpionweed (Phacelia tanacetifolia) in the Wehrer Kessel (Eastern Eifel). The sparsely vegetated strips in the field indicate linear CO2 emissions. (© H. Pfanz 2019) Table 6.1  Dry weight, leaf area, stomatal density and plant height of different organs of scorpionweed (Phacelia tanacetifolia) as a function of CO2 concentration at 30 cm soil depth in a field in the Wehrer Kessel. The mean values are given; the standard deviations have been omitted for reasons of clarity (from Stubbe 2002) (n = 10) Organ Leaf mass (total) Flower mass (total) Stem (total) Root (total) Total plant Plant height Leaf surfaces Stomatal density (number per mm2)

CO2 concentration in the soil 2.5% CO2

41.4% CO2

88.8% CO2

69.1 g 65.6 g 189.9 g 1.9 g 236.5 g Approx. 70 cm Approx. 40 cm2 107

5.4 g 6.7 g 14.7 g 1.5 g 28.2 g Approx. 58 cm Approx. 17 cm2 170

2.9 g 3.9 g 5.3 g 0.7 g 12.8 g Approx. 32 cm Approx. 6 cm2 243

2004, 2007; Ross et al. 2000). Leaf photosynthesis of timothy grass (Phleum pratense) and chicken millet (Echinochloa crus-galli) showed significant changes in Slovenian mofette fields, both in absolute photosynthetic rates, CO2 compensation point and carboxylation capacity (Vodnik et al. 2003; Pfanz et al.

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2004). The changes in transpiration caused by extremely high CO2 are shown in Vodnik et al. (2002a, b).

6.2 Adapted Plants Can Grow – The Azonal Vegetation If plants grow in places where they are not supposed to occur, they are called azonally growing. Thus, if a cottonsedge (Eriophorum spec.; Fig. 6.6), which normally grows in swamps and bogs, is found in large stands in the middle of a rather dry meadow, the reason for this azonality must be found. In the present case, the reason is a soil depleted in oxygen due to the presence of mofettic CO2 gas; the soil has become hypoxic to anoxic not due to the presence of groundwater, but due to oxygen displacement by CO2. In such a hypoxic site, cottonsedge has the advantage of possessing evolutionary adaptations that enable the plant to supply its own roots with the necessary oxygen. This fact gives cottonsedge significant competitive advantages over other plant competitors for root space and light. However, azonal vegetation is not always required to grow in mofette fields. Sometimes anatomical-morphological adaptations are sufficient to counter CO2 stress. For example, the expression of larger intercellulars (i.e., a large

Fig. 6.6  The white infructescences of the sheathed cottonsedge (Eriophorum vaginatum) shine in the sun. The cottonsedges belong to the positive gas-indicating (mofettophilic) plant species. (© H. Pfanz 2019)

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aërenchyma) can ensure that higher CO2 concentrations can be better dissipated by the plant and more easily released into the surrounding atmosphere (Turk et al. 2002). An increase in the density of lenticels in the periderm of young trees also serves the same purpose (cf. linden trees, Laacher See). Lifting the shoot is another way to avoid close contact to CO2 gassing from the soil (Fig.  6.7). Here, the side shoots of mouse-ear (Hieracium pilosella) grow on grasses; in doing so, an air space is created below them, which reduces the influence of geogenic CO2. Such behavior is ecologically known as stress avoidance (Osmond et al. 1987). Similar strategies are present in ivy (Hedera helix), blackberry (Rubus fruticosus agg.) and hedge bindweed (Falliopa dumetorum) at the U1 mofette on the shore of Lake Laach. Here, the plants grow with their roots mainly outside the gassy mofette area and only push their stolons onto the mofette (Pfanz et al. 2019c).

6.2.1 Clear Boundaries From what has just been said, it is clear that plants in mofettes can detect boundaries between CO2-gassing and non-gassing areas and indicate them by their pure  presence. In a wet, slightly nutrient-rich lowland in the Plesná Valley (NW Bohemia), the boundaries between the active mofette and the non-gassing control areas can be seen very clearly (Fig. 6.8). In the foreground,

Fig. 6.7  Normally, the plagiotropic stolons of mouse-ear (Hieracium pilosella) grow directly on the ground. On CO2 degassing sites, stolons can often be found lifted off the ground; this creates a windy space below the plant. (© H. Pfanz 2019)

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Fig. 6.8  The boundaries between high CO2 degassing and low degassing sites are often indicated by vegetation. In the foreground are clumps of Deschampsia cespitosa (turfgrass); medium CO2 is present in the soil. The white-fruited band of cotton grass clearly indicates high gassing areas (mofettophilia). Extremely mofettophobic stands of meadowsweet (Filipendula ulmaria) grow in front of and behind it. Here the CO2 concentrations of the soil are in the control range. (© H. Pfanz 2019)

hoards of Deschampsia cespitosa (turfgrass) can be seen. Low to medium concentrations of soil-CO2 are present here. The white-fruited band of cottonsedge in the centre of the picture clearly indicates high-gas areas (mofettophilic vegetation). In front of and behind the band of cottonsedge grow extremely mofettophobic stands of meadowsweet (Filipendula ulmaria). Here, the CO2 concentrations of the soil are clearly below 2–3% CO2, i.e. clearly within the control range.

6.2.1.1 The “Pathway Mofette” – The Smallest Known Mofette with Zoned Vegetation In a Czech mofette meadow, a small, almost circular mofette with a diameter of about one meter was discovered directly on a dirt road in 2005–2015. On windless days, 100% CO2 can be measured in the center of this mofette directly on the ground surface. And even on windy days, 90–100% carbon dioxide are seen at 10–20 cm soil depth. The central part of the mofette is free

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of any vegetation because of the high CO2 flux (up to 80 mol CO2 m−2 day−1). Animal corpses (insects, spiders) are often found. Two distinct rings of grasses are arranged concentrically around the vegetation-free core. The inner ring is formed by bent foxtail (Alopecurus geniculatus), while the outer ring is populated with meadow bluegrass (Phleum pratense). The maximum rooting depth of these grasses at this mofette is 10–20 cm, far shallower than the rooting depth of these grass species in normal meadow soils. A. geniculatus is a characteristic species of floodplain grasslands and indicates transient waterlogging as well as oxygen depletion (Oberdorfer 2001). Measurements showed that soil CO2 concentrations at 10–20 cm soil depth directly at the Alopecurus ring ranged from 24% to 60% CO2. Between the plants of the outer Phleum ring only 5–21% CO2 were measured. Thus, both grass species differ in their tolerance to different concentrations of CO2 in the soil. Outside the grass rings, soil concentrations at the same soil depths were only in the range of 2 and 4% CO2, and thus at control values of normal unaffected meadow soils (Fig. 6.9).

6.2.1.2 The “Onion Skin” Growth in Caprese Michelangelo In Caprese Michelangelo, i.e. very close to the village where the famous Italian painter and sculptor Michelangelo Buonarroti was born in 1475, one can find mofettes on certain hills whose CO2 fluxes and concentrations, starting from a central vent, continuously decrease towards the outside. In the photograph of Fig. 6.10, this looks almost like the shell structure of vegetable onions. It becomes clear that certain plant species can colonize different CO2 concentrations in the soil.

6.2.1.3 Cotton Grass and Meadow Sedge Belt Similar growth conditions were observed at a small but strongly degassing mofette swale in Hartoušov-South (NW Bohemia). This sinkhole is another, also circular structure, which shows high CO2 concentrations as well as a very high gas flux (Fig. 6.11). Since the swale is about 15–20 cm deeper than the surrounding terrain, a thin, ground-level CO2 gas lake forms, which still reveals 70% CO2 in the lower 10 cm on calm days. But even with wind, up to 20% CO2 can still be measured there during the day. At the bottom (40 cm depth) the CO2 concentrations reach almost 100%. The vegetation ring separating the completely vegetation-free zone from the adjacent meadow

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Fig. 6.9  The pathway mofette in the Plesná Valley is built concentrically around a highly gassy and therefore non-vegetated area. The area in the centre is completely devoid of vegetation because of the high gas flow. The ring-shaped structure as well as the two concentric grass circles can be seen. The inner ring is formed by bent foxtail (Alopecurus geniculatus), while the outer one is populated with meadow bluegrass (Phleum pratense). Unfortunately, the pathway mofette had been dramatically affected by heavy farm vehicles a few weeks before the photograph was taken and the exact scientific examination carried out. (© H. Pfanz 2019)

vegetation, consists of mofettophilic meadow sedge (Carex nigra). Adjacent grows the likewise mofettophilic grass Nardus stricta together with the mofettovague tussock grass (Deschampsia cespitosa).

6.3 Physiological Adaptations of Plants Although there are no endemic plants (perhaps with one exception: Agrostis canina ssp. monteluccii; Fig. 6.12), i.e. plants found only in mofettes, a larger group of plants seems to have succeeded in exploiting certain site advantages and are therefore colonizing mofettes. However, they thrive in CO2-emission fields not because, but despite the fact that the circumstances are adverse. In Central Europe, they are often marsh plants (helophytes), some of which are excellently adapted to the conditions of mofettes. A main criterion seems to be the tolerance of oxygen-poor soils. These helophytes are able to produce oxygen in the above-ground organs via photosynthesis by means of certain physiological and anatomical-morphological characteristics and conduct it to

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Polygonum aviculare Bromus mollis Festuca arundinacea

Cynodon dactylon

Fig. 6.10  Highly gassing mofette on a hill near Caprese Michelangelo. On the left side, the high gas fluxes do not allow plant growth. On the right side, an “onion-skin” growth of several plant species can be seen. Bird’s-eye knotweed (Polygonum aviculare) grows in the upper right, with dogtooth grass (Cynodon dactylon) joining it below. Further out, soft trespass (Bromus mollis) and reed fescue (Festuca arundinacea) grow. The non-gassy outer area is dominated by several grass species as well as wild carrot (Daucus carota). (© H. Pfanz 2019)

the underground organs (roots, rhizomes). If the roots to be supplied are close to the soil surface, this can be done over short distances by normal diffusion. However, there is also the possibility that an accelerated diffusion, the so-­ called thermoosmosis, is set in motion by using temperature differences in roots lying deeper in the soil (Vartapetian and Jackson 1997). This technique is otherwise only used by marsh or floating-leaf plants (although alders use it). In the stems and roots of such plants one usually finds very large intercellular spaces, which serve the easier conduction of oxygen downwards. This tissue is also called aërenchyma (air tissue) according to its function. The partly huge gas spaces allow the oxygen to diffuse from places of higher concentration (leaf ) to places of consumption (root) without great resistance. Certain marsh plants, such as reed (Phragmites australis), have an additional trick for supplying oxygen to underground organs. This exploits a physical feature called Venturi ventilation, which can also work in non-living tube systems (Armstrong and Armstrong 1988, 1990; Armstrong et  al. 1992; Vartapetian and Jackson 1997). When wind blows over tubes that are open at the top, it creates a negative pressure in these tubes. This negative pressure

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Fig. 6.11  Hartoušov South. In the mofette swale no plant growth is possible due to the very high CO2 gas flux. The swale is bordered by a ring of mofettophilic meadow sedge (Carex nigra). In the outer area, the mofettophilous grass Nardus stricta joins in. (© H. Pfanz 2019)

causes air to be sucked out from inside the tube. The displaced volume of air is then replaced via a corresponding opening (e.g. another open end of the tube) by means of sucked-in outside air. In the case of reeds, broken off, dead, mostly previous year’s stems play the role of the open tubes. The wind passing over them extracts the used air from the tube, which is then replaced by unused, oxygenated air from other broken stems (or possibly surviving leaves). The intervening rhizomes and roots thus receive the oxygen necessary for respiration – the plants survive. It is also worth mentioning that during oxygen transport into subterranean organs, part of the oxygen also diffuses uncontrolled out of the roots into the soil space (the rhizosphere). The soil near the roots is therefore also supplied with certain amounts of oxygen. This supports the symbiotic root fungi (Maček et al. 2016), but also promotes animal life in the root layer. In the valley mofette “Il Bossoleto”, no living animals would be found at all in the completely anaërobic soil layers of the valley floor. However, as reed plants pump oxygen into their roots, they indirectly supply the soil with oxygen and so allow some collembola species to survive near the roots (Pfanz and Schulz unpublished).

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Fig. 6.12  The endemic mofette grass Agrostis canina ssp. monteluccii on the banks of the Solfatara River near Viterbo, Italy. (© H. Pfanz 2019)

6.3.1 Nutrient Elements in Mofette Plants Plant growth under elevated CO2 stress usually leads to a reduction of nutrient elements in the leaves and fruits of the plants (Cotrufo et al. 1998, 1999; Pfanz et al. 2004; Vodnik et al. 2005). This has also been found with elevated CO2 fumigations. Plants of a Slovenian meadow mofette contained significantly less N, S, P, K and Zn than plants from non-gassing sites (Pfanz et al. 2007). In appearance, these plants are smaller and show peinomorphosis (deficient growth; cf. Figure  6.2a) or chlorosis (yellowing; Fig.  6.13). The reasons for this are usually a lack of nutrients in mofette soils and a reduced uptake capacity of the plants. The latter is partly due to the hyopxic to anoxic conditions in mofette soils (cf. Marschner 1995; Ross et al. 2000), but on the other hand also from the damaged fine roots and root hairs (Maček et  al. 2003, 2005). These are the actual sites of water and inorganic nutrient uptake. In addition, the reduced redox potential of mofette soils has an adverse effct on nutrient uptake (Maček et al. 2016). In mofette soils, humification and mineralization of dead plant parts are usually also reduced (Cotrufo et  al. 1998, 1999; Ross et  al. 2000; Rennert et  al. 2011; Rennert and Pfanz 2016a, b).

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Fig. 6.13  The bog mofette in the Plesná valley. Around a CO2 vent yellowing (chlorosis) of the surrounding vegetation can be seen. Middle right: cottonsedge is fruiting. (© H. Pfanz 2019)

6.4 Mofettophilia and Mofettophobia A textbook example for the study of botanical mofettophilia is the forest mofette “U1” on the eastern shore of the Laacher See  (Eifel Mountains, Germany). The site is covered with grasses from the Poaceae family (grasses) and Cyperaceae (sedges), some of which have an extremely high tolerance to high carbon dioxide concentrations (as well as oxygen deficiency). In this context, the growth of the lesser pond-sedge (Carex acutiformis) precisely describes the sites where CO2 gas concentrations in the rooted soil are extremely high (see also Pfanz 2008; Pfanz et al. 2019c). Such plant species are called mofettophilous, i.e. high CO2-loving. Certain plants grow only where gas concentrations are somewhat lower. Still other plant species indicate nearly gas-free soil (e.g. Vinca minor). These species are called mofettophobic or CO2-avoiding. Figs. 6.14a-d show that the high CO2 degassing in the mofette moves from the top left diagonally through the 756 m2 area to the bottom right (Fig. 6.14a). The extremely high degassing areas are shown in yellow and red. At the depicted soil depth (20 cm), the high-gassing area at the top left is separated from the rest by a narrow channel with significantly lower CO2 emission. This channel is now the actual growth zone for mofettophobic plant species. The

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Cross transect [m]

a 14 12 10 8 6 4 2 0

CO2 20 cm

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54

Longitudinal transect [m]

Cross transect [m]

b

14 12 10 8 6 4 2 0

Carex acutiformis [%]

0 2 4 6 8 10 12 14 16 1820 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54

0-10 10-20 20-40 40-60 60-80 80-100

Longitudinal transect [m]

Cross transect [m]

c 14 12 10 8 6 4 2 0

Carex acutiformis [%]

0 2 4 6 8 10 12 14 16 1820 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54

Longitudinal transect [m]

Cross transect [m]

d 14 12 10 8 6 4 2 0

Number of species [%]

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54

0-6 6-8 8-10 10-12 12-14 14-16 >16

Longitudinal transect [m]

Fig. 6.14  The famous mofette U1 on the eastern shore of Lake Laach. The CO2 emissions of the 54 m long and 14 m wide mofette are shown in different colours. Grey and blue colours indicate control values and low CO2 degassing, respectively. Green, yellow and red colours represent continuously higher CO2 concentrations. Maximum CO2 concentrations are between 93% and 99% (a). The mofettophobic wood anemone (Anemone nemorosa) grows exclusively in the narrow gap between the gassing islands (c). The species number of plants occurring on the mofette surface is only really high where CO2 degassing is particularly low (d). The only truly CO2-indicating eu-­ mofettophilous species on the plot is lesser pond-sedge (Carex acutiformis) (b)

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wood anemone (Anemone nemorosa;) grows exclusively in this area and occurs only sporadically in the rest of the area (Fig. 6.14c). This growth pattern is also followed by white snowberry (Symphoricarpos albus), fingered larkspur (Corydalis solida), lesser celandine (Ranunculus ficaria), grove grass (Poa nemoralis), pearl grass (Melica mutica), and greater stitchwort (Stellaria holostea); they are strongly mofettophobic and grow exclusively in low to non-­ gassy areas. The number of different plant species per area and the degree of cover of plants above the soil also correlated with CO2 emission. High CO2 concentrations allowed only a few plant species (in some cases only one species per m2 was found) to grow; the vegetation cover was also clearly lower (5–6% of the soil was covered by plants). In contrast, low to no increases in CO2 gassing allowed many plant species (up to 17 species per m2) to occur while maintaining a high degree of cover (up to 84% of the soil was covered by plants) (Fig. 6.14d). Only one of the 69 species present, namely lesser pond-sedge (Carex acutiformis), proved to be reliably mofettophilic and grew exclusively on strongly CO2-degassing sites. However, as this sedge does not tolerate permanent shading, it is not found in the upper left part of the plot, which is shaded by the surrounding trees (Fig. 6.14a, b; Pfanz et al. 2019c). Annual measurements carried out between 2013 and 2021 provide a very accurate picture of the soil gases CO2 and O2, as well as soil pH values, water and humus content, and prevailing vegetation. The geogenic emission pattern of CO2 was clearly shown to be inhomogeneous, reflecting the unevenly distributed soil channels and fractures. CO2 concentrations of this mofette area ranged from 1% to 2% on the control sites to nearly 100% on some extremely high gassing sites (Fig. 6.14a). As will be shown, the oxygen concentration drops linearly with increasing soil CO2. This was first published by Vodnik et al. (2006). There is a weak positive correlation between soil CO2, soil pH and soil conductivity. Where soil water was elevated, humus contents were also slightly elevated (Pfanz et al. 2019c).

6.5 CO2 and Trees Geogenic carbon dioxide naturally also affects woody plants. However, forests with stands of trees growing directly on mofettes are very rare and valued objects of study. Most trees avoid areas of high CO2 gas. Except for a few tree species growing in water stagnant soils (alder Alnus glutinosa, willow Salix spec.), trees are not able to supply their roots with sufficient atmogenic or photosynthetically formed oxygen.

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Nevertheless, some locations are known where the response of trees to geogenic CO2 stress can be studied (Saurer et  al. 2003; Tognetti et  al. 2000; Vejpustkova et al. 2016; Vodnik et al. 2018). In the Czech Plesná valley, a 25-year-old wild pear (European wild pear; Pyrus pyraster) grew in the middle of a strongly gassing mofette. The soil measurements showed that the pear had chosen a growing site that had an area of about 400 cm2 and was a cylindrical tube in depth, in which no CO2 increase could be detected (Fig. 6.15a, b). Twenty-five years ago, many wild pear seeds had sprouted on this mofette. However, most pear seedlings had died shortly after emergence due to the high soil CO2 concentrations. Only outside the mofette on the control soils did some pear trees establish and grow to a handsome height of over 10 m. Within the mofette, the only establishing pear tree was able to survive but remained relatively small at about 1.3 m in height, despite its age of 25 years; its root space was very limited (Fig. 6.15b). From an age of about 15 years, it flowered and also bore fruit. After several earthquakes in the 2011 period, the tree started to ail, lost all its flowers and fruits and eventually its leaves. Within a few months it died. Gas measurements revealed that the quake had shifted some gas-bearing soil channels and cracks, and now CO2 had also entered previously non-gas-bearing areas. The death sentence for the mofettophobic wild pear tree had been pronounced. A dense aspen forest grows directly next to the  above mentioned pear mofette. Due to the vegetative dispersal behaviour of aspen (Populus tremula), the approximately 120 trees consist of only about four genetically separable individuals. Thus, they are genetically identical aspen clones. Investigations by Vejpustkova et al. (2016) revealed that the trees had been fertilized at a young age in both height growth and stem thickness by the escaping geogenic gas, thus promoting growth. This corresponds to the known CO2 fertilization of herbaceous plants (Larcher 1994). Only at an age of 25–30 years did the inhibiting effect of the high degassing become apparent. This is very interesting, as it means that, depending on the developmental stage of the trees, geogenic carbon dioxide can have a promoting effect in the first years of growth and then a clearly growth-inhibiting effect later  (Vejpustkova et al. 2016). At the Laacher See, directly on the well-known mofette U1, trees also grow which show clear growth disturbances due to the emitted CO2. Despite their advanced age, oaks, hawthorns, beeches and lime trees remain small, gnarled and the leaves in the treetops become chlorotic. They are all inhibited in root growth by the escaping soil gas. Water and nutrient uptake also does not function as it does for trees in non-gas areas. Tree-ring analyses clearly showed that

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Fig. 6.15  CO2 degassing patterns of the “pear mofette “at Hartoušov at four different soil depths (10, 20, 40, 60 cm). The CO2 concentrations are indicated in different colours. A yellow to red colouring indicates high to very high CO2 emission, blue colouring indicates low CO2 degassing. The growing site of the small pear tree is marked by the white circle (a). In the middle of the picture the meanwhile dead wild pear in seen (b). (© Pfanz, Baakes and Thomalla)

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the annual growth of these trees is far lower than that of control sites (Pfanz, Thomalla, Vejpustkova unpublished). The large-scale death of lodgepole pine (Pinus contorta) in a high mountain valley near Mammoth in autumn 1994 due to drastically increased CO2 concentrations in the soil (20–90%) has already been reported above (Hill and Prejean 2005). At that time, CO2 release was between 300 and 1200 t per day (Farrar et al. 1995). In Yellowstone National Park, investigations were carried out on a forest mofette (Kretec Vale). The age at death of the trees could be determined by tree ring analysis. This made it possible to calculate the rate at which CO2 degassing is progressing in the area (Pfanz, Tercek, King, in preparation). The data obtained are also used by scientists concerned about the possible eruption of the “super volcano” Yellowstone.

6.5.1 The Problem of the Swale Swales are depressions in the terrain that can reach depths of a few decimetres to metres. Some shallow depressions in mofette areas also appear as completely vegetation-free swales. Measurements revealed that the complete absence of vegetation is due to the degassing behavior of CO2. As explained above, some plant species are quite capable of tolerating high CO2 concentrations (cf. plant mofettophily). In most cases, however, their adaptation mechanisms are not designed to tolerate high CO2 fluxes. This means that high degassing rates (in μMol CO2 m−2 s−1) cannot be compensated even by the most mofettophilic plants. The acidic soil pH and the prevailing anoxia in the soil then also lead to acidification of the root cells and the formation of ethanol and lactic acid in the affected root cells; the cells die.

6.6 The Hot ‘Fumarolic’ Mofettes of the Azores So far, we had dealt exclusively with CO2 emissions, with ambient degassing temperatures. However, there are mofettes in Kamchatka, on New Zealand and above all on the Azores, which degas in hot thermal regions. Under these circumstances, the organisms in the mofettes are affected not only by an oversupply of carbon dioxide and an undersupply of oxygen, but also by extreme ground heat. In an Azorian mofette, temperatures measured at 40 cm soil depth were as high as 95.2  °C.  However, temperatures of 53.9  °C were still measured at

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