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The Event Horizon: Homo Prometheus and the Climate Catastrophe Andrew Y. Glikson
Yildirim Dilek, Department of Geology and Environmental Earth Science, Miami University, Oxford, OH, U.S.A
Franco Pirajno, Geological Survey of Western Australia, and The University of Western, Australia, Perth, Australia
M.J.R.Wortel, Faculty of Geosciences, Utrecht University, The Netherlands
More information about this series at http://www.springer.com/series/7377
Andrew Y. Glikson • Colin Groves
Climate, Fire and Human Evolution
The Deep Time Dimensions of the Anthropocene
Andrew Y. Glikson
School of Archaeology and Anthropology
Australian National University
Canberra, ACT, Australia
Responsible Series Editor: F. Pirajno
Colin Groves
School of Archaeology and Anthropology
Australian National University
Canberra, ACT, Australia
This book represents an expansion of the book by Andrew Y. Glikson, Evolution of the Atmosphere, Fire and the Anthropocene Climate Event Horizon (Springer, 2014).
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, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made.
Printed on acid-free paper
Springer International Publishing AG Switzerland is part of Springer Science+Business Media (www. springer.com)
Bushfire smoke covering the sun in Tasmania’s Southern Midlands, looking west towards Lake Repulse, Friday, Jan. 4, 2013 (Kim Foale; AAP Image, by permission)
In honor of Sir David Attenborough
Foreword
Andrew Y. Glikson and Colin Groves’ new book Climate, Fire, and Human Evolution traces the fascinating and complex history of the Earth over the past 4 billion years. It explores the fundamental context of the Earth’s climate system; the cycles of carbon, oxygen and nitrogen and the crucial role of fire, to provide the critical baseline for our understanding of how a single species, Homo sapiens, has changed the atmosphere, oceans and biosphere.
The fate of our species, and all the others with which we share this planet, is now in peril from the unintended consequences of our development, and especially our use of energy. I commend this scholarly yet readable work as a vital reference for understanding our past and present, and hopefully for saving our future.
North Ryde, Sydney, NSW, Australia Lesley Hughes
Prologue
On Earth – a unique habitable planet in the solar system and possibly beyond – the evolution of the atmosphere, oceans and life are intimately intertwined, where life depends critically on the presence of liquid water allowed by the Earth’s unique orbital position around the sun and evolving atmosphere–ocean processes which regulate surface temperatures in the range of ~ −90° to +58 °C (Fig. 1). The carbon
Fig. 1 Earth, Mars and Venus comparison and the terrestrial evolutionary chain, showing a ~4.4 Ga zircon crystal (Peck et al. 2001; Elsevier, by permission); water, DNA-RNA chains; Pilbara stromatolite (courtesy J.W. Schopf) and Nanobe organisms (Uwins et al. 1998) (courtesy P. Uwins)
cycle is intrinsically linked to Earth’s mantle and tectonic activity, releasing carbon through volcanic eruptions and returning it to the mantle via tectonic subduction. The atmosphere, mediating the carbon, oxygen and nitrogen cycles, acts as lungs of the biosphere, allowing the existence of an aqueous medium where metabolic microbiological processes are performed in aqueous, land, air and extreme environments, including by chemo-bacteria around volcanic fumaroles, microbes and algae within and below ice and nanobes in deep crustal fractures and near-surface phototrophs. In this book, we trace milestones in the evolution of the atmosphere, oceans and biosphere through natural evolution and cataclysms from about ~3.8 billion years ago [Ga] all the way to the Anthropocene – an era triggered by a genus which uniquely mastered fire, a source of energy allowing it to release energy orders of magnitude higher than its own physiological capacity, facilitating its domination over the composition of the terrestrial atmosphere and large part of the biosphere.
Based on evidence from 4.4 billion–year-old zircons, hydrous activity has already been in existence at the earliest stages of the development of the Earth crust, in spite of solar luminosity being some 28 % lower than at present (Sagan and Mullen 1972), consistent with concepts regarding homeostasis in the Earth’s atmosphere–ocean–biosphere system (Lovelock and Margulis 1974). According to this hypothesis, life acquired a homeostatic influence over the planetary environment where the physical and chemical conditions of most of the planetary surface adjusted to conditions mostly favourable for life through maintenance of liquid water with pH not far from neutral, despite major changes including increase in insolation, escape of hydrogen to space and enrichment of the atmosphere in oxygen. Maintenance of liquid water occurred through variations in the greenhouse gas levels of the atmosphere and distribution of infrared-absorbing oceans. From an initial Venus-like atmosphere dominated by CO, CH4, CO2, SO2, N2O, H2 and H2S, the sequestration of CO2 and build-up of nitrogen – a stable non-reactive gas – has led to intermittent ice ages from at least as early as ~3.0 Ga, succeeded by multi-stage variations in the level of atmospheric photosynthetic oxygen produced by phytoplankton and from the Silurian ~420 Ma ago by land plants. The advent of land plants under an oxygen-rich atmosphere resulted in a flammable carbon-rich biosphere. Shifts in state of the climate caused by re-arrangement of continent–ocean patterns through plate tectonics and changes in atmospheric composition associated with erosion and weathering processes led to changes in the carbon cycle. Abrupt events such as volcanism, asteroid impacts, possible supernovae and episodic release of methane and hydrogen sulphide were superposed on longer-term trends, triggering amplifying feedback processes. Changes in atmospheric chemistry resulted in variations in alkalinity, acidity (pH) and oxidation/reduction state (Eh) of the hydrosphere and thereby of the marine food chain.
Born on a combustible Earth surface under increasingly unstable climates shifting from the relatively warm Pliocene (5.2–2.6 Ma) to the deep ice ages of the Pleistocene, the arrival of humans depended on both biological adaptations and cultural evolution. The mastering of fire as a necessity allowed the genus Homo to increase entropy in nature by orders of magnitude. Gathered around campfires during long nights for hundreds of thousands of years, captivated by the flickering life-
like dance of the flames, humans developed insights, imagination, cravings, hope, premonitions of death and aspiration for immortality, omniscience, omnipotence and concepts of spirits and gods. Inherent in pantheism is the reverence of Earth, its rocks and its living creatures, contrasted by the subsequent rise of monotheistic skygod creeds, many of which regard Earth as but a corridor to heaven. Once the climate stabilized in the early Holocene, from about 7000 years ago production of excess grain and animal husbandry by Neolithic civilizations, in particular along the great river valleys, allowed human imagination and dreams to express themselves through the construction of monuments to immortality and through genocidal tribal and national wars in the name of gods and of ideologies. Further to burning large parts of the forests, the discovery of combustion and exhumation of carbon deposits derived from the Earth’s ~420 million–year-old fossil biospheres set the stage for an anthropogenic oxidation event, leading to an abrupt shift in state of the atmosphere–ocean–cryosphere system. The consequent progressive mass extinction of species is tracking towards levels commensurate with those of the past five great mass extinctions of species, constituting a geological event horizon in the history of planet Earth.
Acknowledgements
This book represents an expansion of Evolution of the Atmosphere, Fire and the Anthropocene Climate Event Horizon (Glikson 2014), in connection with which we wish to thank the following people: Brenda McAvoy kindly helped with proof corrections. Helpful comments were obtained from Wallace Ambrose, Barrie Pittock, Hugh Davies, Leona Ellis, Miryam Glikson-Simpson, Victor Gostin, Clive Hamilton, Edward Linacre, Tony McMichael, Reg Morrison, Bruce Radke and Colin Soskolne. I thank Petra Van Steenbergen and Corina Van der Giessen for editorial help. I am grateful to Reg Morrison for contributing special figures and photographs, Jim Gehler for Ediacara photos, Mary White and John Laurie for fossil plant photos and Gerta Keller for reproduction of figures. The following people gave permission to reproduce figures in the book: John Adamek, Anita Andrew, Annemarie Abbondanzo, Robert Berner, Tom Boden, David Bowman, Karl Braganza, Pep Canadell, Randall Carlson, Giuseppe Cortese, Peter deMenocal, Gifty Dzah, Alexey Fedorov, Jim Gehling, Kath Grey, Jeanette Hammann, James Hansen, Paul Hoffman, John Johnson, Jean Jouzel, Barry Lomax, Petra Löw, Cesca McInerney, Michele McLeod, Yvonne Mondragon, Jennifer Phillips, Miha Razinger, Dana Royer, Elizabeth Sandler, Bill Schopf, Appy Sluijs, Phillipa Uwins, John Valley, Simon Wilde and James Zachos.
Chapter 1 Early Earth Systems
Ancient Water
No one
Was there to hear
The muffled roar of an earthquake
Nor anyone who froze with fear
Of rising cliffs, eclipsed deep lakes
And sparkling comet-lit horizons
Brighter than one thousand suns
That blinded no one’s vision
No one
Stood there in awe
Of an angry black coned volcano
Nor any pair of eyes that saw Red streams eject from inferno
Plumes spewing out of Earth
And yellow sulphur clouds
Choking no one’s breath
No one
Was numbed by thunder
As jet black storms gathered
Nor anyone was struck asunder By lightning, when rocks shuttered Engulfed by gushing torrents
That drowned the smouldering ashes
Which no one was to lament
In time
Once again an orange star rose Above a sleeping archipelago
A.Y. Glikson, C. Groves, Climate, Fire and Human Evolution, Modern Approaches in Solid Earth Sciences 10, DOI 10.1007/978-3-319-22512-8_1
(By Andrew Glikson)
Abstract The development of isotopic age determination methods and stable isotopic tracers to paleo-climate investigations, including oxygen (δ18O), sulphur (δ33S) and carbon (δ13C), integrated with Sedimentological records and organic and biological proxies studies, allows vital insights into the composition of early atmosphere–ocean-biosphere system, suggesting low atmospheric oxygen, high levels of greenhouse gases (CO2, CO, CH4 and likely H2S), oceanic anoxia and high acidity, limiting habitats to single-cell methanogenic and photosynthesizing autotrophs. Increases in atmospheric oxygen have been related to proliferation of phytoplankton in the oceans, likely about ~2.4 Ga (billion years-ago) and 0.7–0.6 Ga. The oldest recorded indirect traces of biogenic activity are provided by dolomite and banded iron formation (BIF) from ~3.85 Ga-old Akilia and 3.71–3.70 Ga Isua greenstone belt, southwest Greenland, where metamorphosed banded ironstones and dolomite seawater-like REE and Y signatures (Bolhar et al. Earth Planet Sci Lett 222:43–60, 2004; Friend et al. Contrib Miner Petrol 183(4):725–737, 2007) were shown to be consistent with those of sea water (Nutman et al. Precamb Res 183:725–737, 2010). Oldest possible micro-fossils occur in ~3.49 Ga black chert in the central Pilbara Craton (Glikson. Aust J of Earth Sci 55:125–139, 2008; Glikson. Icarus 207:39–44, 2010; Duck et al. Geochim Cosmochim Acta 70:1457–1470, 2008; Golding et al. Earliest seafloor hydrothermal systems on earth: comparison with modern analogues. In: Golding S, Glikson MV (eds) Earliest life on earth: habitats, environments and methods of detection. Springer, Dordrecht, pp 1–15, 2010) and in 3.465 Ga brecciated chert (Schopf et al. Precamb Res 158:141–155, 2007). Possible stromatolites occur in ~3.49 and ~3.42 carbonates. The evidence suggests life may have developed around fumaroles in the ancient oceans as soon as they formed. The evidence indicates extended atmospheric greenhouse periods interrupted by glacial periods which led to an increase in oxygen solubility in water, with implications for enhanced life. Intermittent volcanic eruptions and asteroid and comet impacts, representing continuation of the Late Heavy Bombardment as recorded on the Moon, resulted in major crises in biological evolution.
1.1 Archaean and Proterozoic Atmospheres
Terrestrial climates are driven through the exposure of the Earth surface to solar insolation cycles (Solanki 2002; Bard and Frank 2006), variations in the gaseous and aerosol composition of the atmosphere, the effects of photosynthesis on CO2 and O2 cycles, microbial effects on methane levels, volcanic eruptions, asteroid and comet impacts and other factors. By contrast to the thick CO2 and SO2 blankets on Venus, which exert an extreme pressure of 93 bar at the surface, and unlike the thin 0.006 bar atmosphere of Mars, the presence in the Earth’s atmosphere of trace concentrations of well-mixed greenhouse gases (GHG) (CO2, CH4, N2O, O3) modulates surface temperatures, allowing the presence of liquid water (Fig. 1.1) and thereby
life. During the Holocene surface temperatures ranged between −89 °C and +58 °C, with a mean of about +14 °C.
During early stages of terrestrial evolution low solar luminosity (The Faint Young Sun) lower luminosity than at present representing early stages in the fusion of hydrogen to helium (Sagan and Mullen 1972), is thought to have been compensated by high greenhouse gas (GHG) levels (Fig. 1.2), allowing surface temperature to remain above freezing (Kasting 1993). This author suggested that to warm the oceans above freezing point the atmosphere would have needed CO2 levels some 100–1000 times the present atmospheric level. Alternative hypotheses were proposed by Longdoz and Francois (1997) in terms of albedo and seasonal variations on the early Earth. Rosing et al. (2010) pointed out a high ocean to continent surface area ratio in the Archaean would have led to a lower albedo and due to absorption of infrared radiation by open water. Temporal fluctuations in atmospheric GHG levels constituted a major driver of alternating glacial and greenhouse states (Kasting and Ono 2006). Rosing et al. 2010 suggested the Archaean atmosphere was less clouded and more transparent than later atmospheres due to a paucity of condensation nuclei such as occur above land, including dust particles, soot and sulphuric acid released by plant photosynthesis (Kreidenweis and Seinfeld 1988). The relations between GHG, clouds and surface albedo during the Archaean remains a subject of continuing debate (Goldblatt and Zahnle 2011).
Planetary evolution transpires through gradual changes as well as major upheavals. The former include plate tectonics, crustal accretion, crustal subduction, rise and erosion of mountain belts. The latter include abrupt magmatic and tectonic events and extra-terrestrial impact-triggered cratering. Geochronological age sequences, geochemistry and isotopic indices point to secular evolution from a
Fig. 1.1 Natural mortar and pestle – Komati River, Barberton Mountain Land, Swaziland (Photograph by Andrew Glikson)
The Faint Young Sun Paradox
Even though the Sun was about 30% dimmer than it is now, the temperature on Earth has been more or less stable.
Fig. 1.2 The faint young sun paradox according to Sagan and Mullen (1972), suggesting compensation of the lower solar luminosity by high atmospheric greenhouse gas levels at early stages of terrestrial evolution. (From the Habitable Planet: A system approach to environmental science, produced by the Harvard-Smithsonian Center for Astrophysics, Science Media Group and used with permission by the Annenberg Learner (Courtesy Michele McLeod) www.learner.org; http:// www.learner.org/courses/envsci/unit/text.php?unit=1&secNum=4)
Atmospheric CO2 levels are buffered by the oceans (at present ~37,000 GtC) which contain about 48 times the atmospheric CO2 inventory (currently ~800 GtC). The solubility of CO2 in water decreases with higher temperature and salinity and the transformation of the CO3[−2] ion to carbonic acid (HCO3[−1]) retards the growth of calcifying organisms, including corals and plankton. Plants and animals work in opposite directions of the entropy scale, where plants synthesize complex organic compounds from CO2 and water, producing oxygen, whereas animals burn oxygen and expel CO2. Disturbances in the carbon and oxygen balance occur when changes occur in the extent of photosynthetic processes, CO2 solubility in the oceans, burial of carbon in carbonate and from remains of plants and oxidation of carbon through fire and combustion.
Forming a thin breathable veneer only slightly more than one thousandth the Earth’s diameter, evolving both gradually as well as through major perturbations, the atmosphere acts as lungs of the biosphere, facilitating an exchange of carbon and oxygen with plants and animals (Royer et al. 2004, 2007; Siegenthaler et al. 2005; Berner 2006; Berner et al. 2007; Beerling and Royer 2011). In turn biological activity continuously modifies the atmosphere, for example through production of methane in anoxic environments, release of photosynthetic oxygen from plants and
of dimethyl sulphide from marine phytoplankton. Long term chemical changes in the air-ocean system are affected by changes in plate tectonic-driven geomorphic changes including subduction of oceanic and continental plates (Ruddiman 1997, 2003, 2008), weathering, volcanic and methane eruptions, and variations in marine and terrestrial photosynthetic activity (Broecker 2000; Zachos et al. 2001; Hansen et al. 2007; Glikson 2008). The range of paleo-climate proxies used in these studies are reviewed in detail by Royer et al. (2001).
Early terrestrial beginnings are interpreted in terms of a cosmic collision ~4.5 billion years-ago between an embryonic semi-molten Earth and a Marsscale body – Theia – determined from Pb isotopes (Stevenson 1987). The consequent formation of a metallic core, inducing a magnetic field which protects the Earth from cosmic radiation, and a strong gravity field which to a large extent prevents atmospheric gases from escaping into space, resulted in a haven for life at the Earth surface (Gould 1990). Relict ~4.4 Ga and younger zircons, representing the Hadean era, a term coined by Preston Cloud in 1972, signify vestiges of granitic and felsic volcanic crustal nuclei, implying the presence of a water component in the melt and low- temperature surface conditions (Wilde et al. 2001; Mojzsis et al. 2001; Knauth 2005, Valley et al. 2002) (Figs. 1.3 and 1.4 ). However, Pidgeon et al. (2013) and Pidgeon (2014) attributed the δ 18O values of zircon to secondary radiation damage and associated with hydrous alteration. Precambrian terrains contain relict ~4.1–3.8 Ga-old rocks, including volcanic and sedimentary components, exposed in Greenland, Labrador, Slave Province, Minnesota, Siberia, northeast China, southern Africa, India, Western Australia and Antarctica (Van Kranendonk 2007). These formations, formed parallel to the Late Heavy Bombardment (LHB) on the Moon (~3.95–3.85 Ga) (Ryder 1991), are metamorphosed to an extent complicating recognition of primary impact shock features, which to date precluded an identification of signatures of the LHB on Earth.
Knauth and Lowe (2003) and Knauth (2005) measured low δ18O values in ~3.5–3.2 Ga cherts of the Onverwacht Group, Barberton Greenstone Belt (BGB), Kaapvaal Craton, suggesting extremely high ocean temperatures in the range of 55–85 °C (Figs. 1.3, and 1.4). The maximum δ18O value in Barberton chert (+22‰) is lower than the minimum values (+23‰) in Phanerozoic sedimentary cherts, precluding late diagenesis as the explanation of the overall low δ18O values. Regional metamorphic, hydrothermal, or long-term resetting of original δ18O values is also precluded by preservation of δ18O across different metamorphic grades. According to Knauth (2005) high-temperature conditions extended beyond submarine fumaroles and the Archaean oceans were characterized by high salinities 1.5–2.0 times the modern level. In this interpretation ensuing evaporite deposits were removed by subduction, allowing lower salinities. However, well preserved Archaean sedimentary sequences contain little evidence of evaporite deposits. The low-oxygen levels of the Archaean atmosphere and hydrosphere limited marine life to extremophile cyanobacteria. Microbial methanogenesis involves reactions of CO2 with H2 or acetate (CH3CO2 ) produced from fermenta-
Fig. 1.3 A compilation of oxygen isotope data for cherts. The overall increase in δ18O with time is interpreted as global cooling over the past 3500 Ma. The variation in δ18O for cherts at any given time is caused by the presence of low δ18O meteoric waters during burial at elevated temperatures (Knauth 2005; Elsevier, by permission). Insets: (1) Columbia Glacier (NASA). (http://www. google.com.au/search?q=nasa+glacier&hl=en&tbm=isch&tbo=u&source=univ&sa=X&ei=hkm EUbCDBonGkgXDg4C4CQ&sqi=2&ved=0CEYQsAQ&biw=1360&bih=878); (2) http://www. flickr.com/photos/anieto2k/8636213185/sizes/l/in/photostream/ (anieto2k’s photo stream)
Fig. 1.4 δ18O ratio of igneous zircons from 4.4 Ga to recent, displaying an increase in abundance of low-temperature effects with time from approximately ~2.3 Ga (Courtesy J.W. Valley)
tion of photo-synthetically produced organic matter. Photolysis of methane may have created a thin atmospheric organic haze.
By contrast to the high temperature estimates by Knauth and Lowe (2003) and Knauth (2005), combined relationship of δ18O and δD (deuterium) suggests chert precipitated from ocean water at temperatures no warmer than 40 °C (Hren et al. 2009). A subsequent study of the δ18O of Barberton phosphates (Blake et al. 2010) placed Archaean ocean temperatures at 26–35 °C. High temperature low pH water can be expected to have resulted in extensive corrosion and syngenetic dissolution of Archaean cherts and quartz grains in arenites, for which there is little evidence.
Studies of oxygen isotopes of Hadean zircons indicate little difference between the maximum δ18O values of ~4.4–4.2 Ga zircons and those of younger ~3.6–3.4 Ga zircons (Fig. 1.4), which suggests presence of relatively low temperature water near or at the surface (Valley et al. 2002). However, secondary penetration of meteoric OH−1 molecules associated with radiation damage zircons must be borne in mind (Pidgeon et al. 2013; Pidgeon 2014).
An overall increase with time in δ18O shown by terrestrial sediments (Valley 2008) (Fig. 1.4) reflects a long term cooling of the hydrosphere, consistent with an overall but intermittent temporal decline in atmospheric CO2 shown by plant leaf pore (stomata) studies (Berner 2004, 2006; Beerling and Berner 2005; Royer et al. 2001, 2004, 2007). This long-term decline may have been associated with increased rates of weathering-sequestration of CO2 related to erosion of rising orogenic belts, including the Caledonian, Hercynian, Alpine, Himalayan and Andean mountain chains (Ruddiman 1997, 2003). Such a trend is consistent with suggested increase in the role of plate tectonics through time (Glikson 1980), which led to an increase in sequestration of CO2 by weathering of orogenic belts and subduction of CO2-rich carbonate and carbonaceous shale.
Studies of the nature of the early terrestrial atmosphere–biosphere system hinge on the sulphur, carbon and oxygen stable isotopes (Kasting 1993; Pavlov and Kasting 2002; Holland 2006; Kasting and Ono 2006). The geochemical behaviour of multiple sulphur isotopes is a key proxy for long-term changes in atmospheric chemistry (Mojzsis 2007; Thiemens 1999). The identification of mass-independent fractionation of sulphur isotopes (MIF-S) in pre-2.45 Ga sediments has been correlated with ultraviolet radiation effects on the δ33S values, with implications for an ozone and oxygen-poor Archaean atmosphere (Fig. 1.5) (Farquhar et al. 2000, 2007), whereas other authors suggested heterogeneous Archaean oxygen levels (Ohmoto et al. 2006). Development of photosynthesis, and thereby limited release of oxygen as early as about 3.4 Ga, is suggested by identification of heliotropic stromatolite reefs in the Pilbara Craton (Allwood et al. 2007). The abrupt disappearance of positive MIF-S anomalies at ~2.45 Ga poses a problem, as atmospheric enrichment in oxygen due to progressive photosynthesis could, perhaps, be expected to result in a gradual rather than an abrupt decline in MIF-S signatures. MIF-S (δ33S) anomalies (Fig. 1.5) overlap mid-Archaean impact periods (~3.26–3.24 Ga) and Late Archaean impact periods (~2.63, ~2.56, ~2.48 Ga) (Lowe et al. 1989, 2003; Simonson and Hassler 1997; Simonson et al. 2000; Simonson and Glass 2004; Glikson 2001, 2004, 2005, 2006; Glikson et al. 2004; Glikson and Vickers
Stratigraphic Units associated with (preceding or postdating) periods of asteroid bombardment
~ 2.63 Ga Jeerinah Fm shale
~ 3.26 Ga Gorge Creek Fm
~ 3.46 Ga Rhyolite / Warrawoona volcanics
Periods not known to be associated with asteroid
Fig. 1.5 Plots of mass independent fractionation values for Sulphur isotopes (∆33S – MIF-S) vs. Age. The high ∆33S values for pre-2.45 Ga sulphur up to about ∆33S = 11 is interpreted in terms UV-induced isotopic fractionation, allowed by a lack of an ozone layer. Periods of major asteroid impacts during which ozone may have been destroyed are indicated (From Glikson 2010; Elsevier, by permission)
2006, 2007), though no specific age correlations are observed. Estimates of projectile diameters derived from mass balance calculations of iridium levels, 53Cr/52Cr anomalies and size-frequency distribution of fallout impact spherules (microkrystites) (Melosh and Vickery 1991), suggest projectiles of the ~3.26–3.24, ~2.63, ~2.56 and ~2.48 Ga impact events reached several tens of kilometre in diameter (Byerly and Lowe 1994; Shukolyukov et al. 2000; Kyte et al. 2003; Glikson et al. 2004; Glikson 2005, 2013). Impacts on this scale would have led to major atmospheric effects, including large scale ejection of carbon and sulphur-bearing materials, effects on the ozone layer and isotopic fractionation of sulphur. Archaean impact ejecta units in the Pilbara and Kaapvaal Cratons are almost invariably overlain by ferruginous shale and banded iron formations (BIF) (Glikson 2006; Glikson and Vickers 2007). The origin of BIFs is interpreted, alternatively, in terms of oxidation of ferrous to ferric iron under oxygen-poor atmospheric and hydrospheric conditions (Cloud 1968, 1973; Morris 1993), direct chemo-lithotropic or photoferrotropic oxidation of ferrous iron, and UV-triggered photo-chemical reactions (Cairns-Smith 1978). The relations between sulphur, oxygen and carbon isotopes, atmospheric oxygen levels, photosynthesis, banded iron formations and glaciations
with implications for evolution of the early atmosphere remain the subject of current investigations.
During much of its early history Earth was dominated by an oxygen-poor, CO2, CO and methane-rich atmosphere of several thousand to tens of thousands ppm CO2, resulting in low-pH acid oceans. High temperatures of ocean waters (Knauth and Lowe 2003; Knauth 2005) allowed little sequestration of the CO2 accumulated in the atmosphere from episodic volcanism, impact cratering, metamorphic release of CO2, dissociation of methane from sediments and microbial activity and, following the advent of plants on land in the Silurian (~420 Ma), from decomposed vegetation (Berner 2004; Berner 2006; Beerling and Berner 2005; Royer et al. 2004, 2007; Royer 2006; Glikson 2008). The low solubility of CO2 in the warm water of the early hydrosphere and little weathering-capture of CO2, due to a low continent/ ocean crust ratio and limited exposure of land surface, ensured long term at ~2.9 Ga, 2.4–2.2 Ga, 575–543 Ma, the late Ordovician (~446–443 Ma), CarboniferousPermian (~326–267 Ma), Jurassic (187–163 Ma) and post-Eocene, signifies episodic large-scale CO2 sequestration events (Ruddiman 2003).
The presence of siderite as crusts on river cobbles in 3.2 Ga conglomerates in the Archaean Barberton greenstone belt and of nahcolite within 3.4 Ga marine sedimentary rocks (Lowe and Tice 2004) requires CO2 levels 7–10 times the present atmospheric CO2 concentrations under temperatures of ~25 °C (Eugster 1966; Hessler et al. 2004). The presence of siderite (FeCO3) and nahcolite (NaHCO3) provides a constraint on the amount of CO2 available for the reactions (Hessler 2012):
Rosing et al. 2010 suggest an upper limit for Archaean CO2 within 10 times the present atmospheric level based on the presence of magnetite (Fe3O4) in equilibrium with siderite in Archaean shallow-marine sediments, as higher CO2 concentrations would result in higher FeO levels in the water. According to Hessler (2012) such CO2 level would not be enough to prevent the oceans from freezing under the faint early sun conditions. That liquid water existed in the early Archaean would have resulted from high levels of atmospheric methane released by methanogenic microbial activity (Kharecha et al. 2005), consistent with views regarding biogenic environments in the Barberton greenstone belt (Tice and Lowe 2006; Noffke et al. 2006; Heubeck 2009) and in the Pilbara (Lowe 1983; Hoffman et al. 1999, Allwood et al. 2007). In a low-oxygen Archaean atmosphere methane would have had a longer residence time (Zahnle 1986; Pavlov and Kasting 2002) reaching higher concentration than during later periods under oxygen-rich atmospheric conditions. High CH4 would lead to atmospheric hydrocarbon haze as on Saturn’s moon Titan (Trainer et al. 2006; Hessler 2012), higher albedo and a possible degree of cooling. According to Goldblatt et al. (2009) a high level of Nitrogen would have been associated with enhancement of a CO2− CH4 greenhouse atmosphere as N2 increases atmospheric
1 Early Earth Systems
pressure and the rate at which the greenhouse gases can absorb infrared radiation. Estimates of atmospheric CO2 and CH4 at 2.5 Ga suggest that doubling of the present atmospheric N2 concentration would have caused warming by 4.4 °C. The source of nitrogen has been attributed to volcanic degassing (Mather et al. 2004). The presence of fixed nitrogen is considered essential for biological synthesis. Following the development of photosynthesis nitrogen would have been removed from the atmosphere and sequestered through reactions such as N2 + 3 H2O → 2 NH3 + 1.5 O2, releasing oxygen to the oceans.
Central to studies of early atmospheres is the level of oxygen and its relation to photosynthesis. Sulphur isotopic analyses record mass-independent fractionation of sulphur isotopes (δ33S) (MIF-S) in sediments older than ~2.45 Ga, widely interpreted in terms of UV-triggered reactions under oxygen-poor ozone-depleted atmosphere and stratosphere (Farquhar et al. 2000, 2007) (Fig. 1.5). From about ~2.45–2.32 Ga – a period dominated by deposition of banded iron formations (BIF) (Figs. 1.6, and 1.7), high δ33S values signify development of an ozone layer shielding the surface from UV radiation (Farquhar et al. 2000; Kump 2009) (Fig. 1.5). According to Kopp et al. (2005) photosynthetic oxygen release from cyanobacteria
Fig. 1.6 Isua supracrustal belt: (a) Relict pillows in amphibolite facies ~3.8 Ga meta-basalts of arc-tholeiite to picrite compositions from the southern flank of the Isua Belt; (b) Low strain zone in ~3760 Ma Isua banded iron formations. Note the grading of the layering and the layering on a cm-scale; (c) Agglomerate clasts in a ~3.7 Ga Ma felsic intermediate unit from the eastern end of the Isua belt; (d) Original layering preserved in quartz + dolomite + biotite ± hyalophane (Na-Ba feldspar) metasediments, likely derived from marl and evaporite (Courtesy Allen Nutman)
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CHAPTER III
The third of the great periods into which the geological record is divided is known as the Tertiary Throughout most of its length it appears to have been characterized by remarkably mild and equable climatic conditions extending into comparatively high latitudes, so that the west coast of Greenland, for instance, had a flora of almost sub-tropical aspect. Since the plants in question—chiefly palms and cycads—are not of identical species with their present-day representatives, it is unsafe to base numerical estimates of the temperature upon them, but it is at least obvious that these regions were warmer than they are at present.
Let us glance for a moment at the geography of the Tertiary period. The most noticeable point is a great expanse of sea over south-eastern Europe, including the Mediterranean countries, extending away over the Black Sea and Caspian, and stretching in a great arm to the Arctic Ocean, south of Novaya Zemlya. The geology of the archipelago north of Canada is not yet well known, but it seems probable that there was a considerable area of Tertiary ocean there also. The sea further encroached on the present boundaries of North America, both east and west, and on the north-eastern coast of Asia. Bearing in mind the principles set out in the first chapter, we can infer from these changes a great increase in the winter temperature of the regions along the Arctic circle. The increase reached a maximum on the west of Greenland and in western Siberia, but the west coast of Alaska also had a decidedly warm climate in the late Miocene and Pliocene.
The basin of the Arctic Ocean, which already existed at that stage, was raised to a temperature considerably higher than the present by
three great streams of warm water flowing into it. If, as seems probable, the Bering Strait was deeper, and the submarine ridge across the North Atlantic less pronounced, the obstacles to the outflow of cold water along the ocean floor were much less than now. Finally, the winter temperatures of the land-masses to the south, and especially Siberia, being already very much less severe owing to the sea over Europe, the temperature of the water of the great rivers flowing into the Arctic was not so low. For these reasons the development of ice in the Arctic Ocean was very much diminished, and possibly entirely absent, allowing a great amelioration of the climate of Greenland, the rigor of which is at present much enhanced by the ice which flows down the Greenland Sea and round Cape Farewell.
The cumulative effect of all these changes—greater water area, greater inflow of warm surface water, less inflow of cold river water, less ice-development—must have been a mild equable rainy climate, entirely suitable to a rich vegetation. The sub-tropical aspect of that vegetation should not be stressed, for it was probably as much an expression of the geological age of the period in question as of its climate.
The objection may be raised that at the present time the subantarctic islands in the great Southern Ocean have the most maritime climate in the world, but are not by any means places of opulent vegetation. The difference is entirely accounted for by the presence of the great ice-bearing Antarctic continent. Its effect is twofold. Firstly, the glaciers shed into the Southern Ocean an immense quantity of ice and ice-cold water annually, which must have an appreciable effect on temperature. Secondly, the presence of this ice-covered continent and the floating ice in its neighbourhood extending as far as the sixtieth parallel, by forming a marked contrast with the warmer waters further north, greatly intensifies the strength of the atmospheric circulation in these regions, resulting in the development of a great succession of severe storms which sweep the sub-antarctic islands. There are no great land-masses to break the force of the wind, and these latitudes are among the stormiest, windiest regions of the earth—gale succeeding gale, winter and
summer alike; and it is largely to the extraordinary power of the wind that we must attribute the desolate appearance of the islands.
The picture we have drawn of the high northern latitudes in early Tertiary times is vastly different. A great warm ocean occupied the Arctic regions, fed by three ocean currents analogous to the Gulf Drift, and the fall of temperature was gradual from the tropic to the pole. The return colder currents were mainly along the ocean floor and with little ice-formation the storms were few and not severe. On the western shores of the continents mild rain-bearing south-west winds prevailed, and a quiet moist warm atmosphere existed which was especially favourable to plant life. This favourable state of affairs lasted until well on in the Miocene, and then changes set in. The land and sea distribution underwent essential modifications. The great Tertiary continent or archipelago which is believed to have existed in the western Pacific, and whose last remaining summits now form the scattered islands of that ocean, gradually subsided, and in its place elevation began in higher latitudes. Bering Strait became narrow and shallow, and was probably for a time entirely closed, while the connexion between the Arctic and Indian Oceans was closed permanently, leaving in its lowest areas a chain of great inland seas and lakes, of which the Caspian and Aral Seas are now the greatest representatives. The Canadian Archipelago was probably raised above its present level, and formed a great northern extension of the American continental area. The changes in the Atlantic also were very extensive. The West Indies were the site of a large and lofty Antillean continent; further north a considerable landmass existed east of Newfoundland; Greenland was joined on the west to the extended American continent, and considerably enlarged to the south-east. Iceland, though it remained an island, was elevated and probably nearly doubled in area, and between Iceland and the north of Scotland was developed a great submarine ridge, which may or may not have risen above the sea in places. The British Isles became a solid block of land, united with continental Europe across the English Channel and the great plain which is now the North Sea. Scandinavia was elevated by more than a thousand feet, and the elevation extended at least as far as Spitzbergen. The
Murman area had a considerable extension. In eastern Asia the Sea of Okhotsk was land and Japan was united to the mainland.
In the southern hemisphere our knowledge is not nearly so detailed. The presence of marine Middle-Tertiary beds with temperate mollusca in Graham Land and of plant-bearing beds in Seymour Island point to a smaller Antarctic continent and very much warmer conditions at this time in the South as well as in the North Polar regions. For the close of the Tertiary, however, we have strong grounds in the distribution of animals and plants for assuming that the Antarctic continent was greatly increased in size, with promontories uniting it to Australia on the one hand and to South America on the other. New Zealand was largely increased in area, and South Africa probably extended further polewards. The subantarctic islands attained a much greater area. Conditions were ripe for the Ice Age in the southern as well as the northern hemisphere.
BIBLIOGRAPHY
Kerner von Marilaun, F “Synthese der morphogenen Winterklimate Europas zur Tertiärzeit ” Wien, SitzBer K Akad Wiss, 122, 1913, pp 233-98
Osborn, H. F. “The age of mammals in Europe, Asia and North America.” 8vo. New York, 1910.
Nathorst, A. G. “On the fossil floras of the Arctic regions as evidence of geological climates.” London, Bot. J. 2, 1913, pp. 197-202; and Washington, Report Smithsonian Inst., 1911.
Dall, W H “On climatic conditions at Nome, Alaska, during the Pliocene ” Amer J Sci , ser 4, Vol 23, 1907, p 457
Nansen, F. “The bathymetrical features of the North Polar seas, with a discussion of the continental shelves and previous oscillations of the shore-line.” Norwegian N. Polar Exped., 1893-96. Scientific Results, Vol. 4.
Spencer, J W “Reconstruction of the Antillean continent ” Bull Amer Geol Soc , 6, 1895, pp 103-40
Wilckens, D. “Die Mollusken der antarktischen Tertiärformation.” Wiss. Ergebn. der Schwed. Sudpolar Exped., 1901-3, Bd. 3, 1911.
Hedley, C. “The palæographical relations of Antarctica.” London, Proc. Linnæan Soc., 124, 1911-2, p. 80.
CHAPTER IV
As the land began to rise, the first effect was an increased snowfall on the higher summits, and increased rainfall on the rising coast lands. The rivers had an increasing fall towards the sea, and rapidly carved out deep narrow valleys, which were later developed by the ice into the great fiords of Norway and other heavily glaciated regions. But on the whole the first beginnings of the Ice Age occurring towards the close of the Pliocene period are obscure, and are likely always to remain so, for the simple reason that the advancing and retreating ice-sheets have wiped out most of the evidence of the conditions which immediately preceded their advent. The deteriorations of the climate had begun long before the geographical changes outlined at the close of the last chapter were complete, for mollusca of the Pliocene beds in East Anglia indicate progressive refrigeration of the North Sea at the same time as it became increasingly shallow. At the close we have great shell-banks with northern species which must have been piled up by powerful easterly winds; these easterly winds show that the storm-tracks had been driven south of their present course and suggest that the glacial anticyclone already existed over Scandinavia. At the present day similar shell-banks are forming on the coast of Holland under the influence of the prevailing westerly winds. The next series of deposits in this region are fresh-water “forest beds,” attributed to a greatly extended Rhine, and belong to the period when the North Sea had become a plain.
It is no part of the plan of this work to go over the geological ground of the Quaternary Ice Age, which has already been so frequently and so efficiently covered. All I can hope to do is to give a
brief general account of the succession of climatic changes involved, necessarily incomplete since so much of the world is at present insufficiently explored for glacial traces. But a certain amount of explanatory introduction is necessary.
In Europe and North America there are distinct traces of several separate glaciations with “interglacial” periods when the climate approached or became even warmer than the present. The timerelations of these glaciations are not yet fully worked out, but there seems little doubt that they were contemporaneous in the two continents. The correlation is not perfect, however, since the United States geologists recognized altogether five glaciations. The explanation appears to be that the equivalent of the Rissian glaciation in America is double; two stages, the Illinoian and Iowan, being recognizable, separated by a retreat of the ice. The series is as follows:
Alps. Northern Europe. North America. Date. B.C.
I Gunz ?
Jerseyan or ?
Nebraskan
40,000- 18,000
The correlation is based on the amount of weathering and erosion which the various deposits have undergone. The time which has elapsed since the glaciers of the last or Wurm stage were in active retreat has been estimated by comparing the growth of peat-bogs, river-deltas, etc., during historical times with that since the last retreat of the ice. But the most conclusive method is due to the Swedish geologist Baron de Geer, who has actually counted the years since the ice in its final retreat left any particular point between Ragunda and the south of Sweden. The work is based on the idea that the lamination observed in certain marine and lacustrine clays in Sweden is seasonal, the thick coarse layers being due to the floods produced by the rapid melting of the ice in summer, and the thinner
and finer layers being due to the partial cessation of melting in winter. By correlating one section with another it is possible to date any particular layer with great exactness, and further to prove the existence of several great climatic fluctuations during the retreat. The topmost limit of the section is given by the surface of the old floor of Lake Ragunda in Jemtland, which received its waters from one of the permanent glaciers and was accidentally drained in 1796. De Geer finds that the edge of the last great ice-sheets lay over southern Scania about 12,000 years ago, and further estimates 8000 years for the retreat across the Baltic. These results are in general agreement with those obtained by other methods, and we may accordingly, with some confidence, put the date when the ice-sheet of the Wurm glaciation finally left the coast of Germany at about 18000 . .
This period fixed, we have a datum for estimating the duration of the interglacial periods. The moraines of the Wurm glaciation present everywhere a very fresh appearance, and the chemical change which the boulders they contain have undergone is slight, while weathering extends to a depth of scarcely a foot. The moraines of the Riss glaciation are weathered somewhat more deeply, and those of the Mindel glaciation very much more. Assuming that chemical weathering has proceeded uniformly during the interglacial periods and ceased during the glaciations, Penck and Brückner, who have studied exhaustively the glaciation of the Alps, find that the RissWurm interglacial lasted about three times as long as the interval between the Wurm glaciation and the present day, or 60,000 years, and the Mindel-Riss interglacial about twelve times as long, or 240,000 years. No data are available for the Gunz-Mindel interglacial, but it is provisionally made equal to the Riss-Wurm, another 60,000 years.
No possibility of such direct measurement of the duration of the glacial periods themselves has yet been found. Penck and Brückner assume that the duration equalled that of the Riss-Wurm interglacial, or 60,000 years in each case. This seems unnecessarily long. The Yoldia Sea, the deepest part of which coincided with the centre of the Scandinavian glaciation, appears to have reached its greatest
depth not more than 6000 years after the maximum of glaciation, indicating a lag of this period. The subsidence of the land due to the weight of the ice-sheet may have commenced some time before the maximum of glaciation, but the duration of the subsidence can hardly have been more than 10,000 years, and this is the limit for the second half of the Wurm glacial period. Further, we know that during the growth of the ice-sheets there was comparatively little melting, for the rivers then had little power of carrying debris. Recent measurements in Greenland give the rate of ice-growth on the surface of the ice-sheet as 40 cm., or 15 inches a year; let us say a foot, and assume a marginal loss equivalent to half this amount over the whole ice-sheet. This gives us an average increase of six inches a year, or 10,000 years for growth to a maximum thickness of 5000 feet. On these grounds the estimated duration of the Rissian glacial period has been reduced to 30,000 years, and that of the Wurm period to 22,000 years. Only in the case of the long and complicated Mindelian period, which, as will be seen later, was virtually a series of overlapping glaciations from various centres, has the figure of 60,000 years been accepted. In the present state of our knowledge no estimate of the duration of the Gunz-Mindel interglacial can have any value, and the dates are accordingly carried back only to the Mindelian. In this way we obtain the time-scale given on page 48.
The fourfold glaciation has been recognized with certainty only in Europe and North America, and even in these countries there is considerable doubt whether the northern ice-sheets shrank back as far as their present narrow limits during the interglacial periods. The long Mindel-Riss interglacial, which was probably the Chellean stage of flint industry,[3] was characterized by a very warmth-loving fauna, and it is possible, even probable, that during this period the glaciers melted completely away, except on the very highest summits. Of the climate of the Gunz-Mindel interglacial (termed by J. Geikie the “Norfolkian,” from the Cromer Forest Bed), we have comparatively little evidence. If the suggestion put forward in the following chapter is correct, the Gunz-Mindel interglacial was merely a local incident in the glaciation of the Alps, and not a true interglacial at all. Even the Cromer Forest Bed itself is not conclusive, since it is a river deposit
largely composed of material drifted from lower latitudes. The RissWurm interglacial (J. Geikie’s “Neudeckian”) nowhere gives us evidence of a climate as warm as the present, and as regards the Scandinavian and Canadian ice-sheets may have been merely an extensive and prolonged oscillation of the ice-edge.
As regards the glaciation of Norway, the question has been investigated recently by H. W. son Ahlmann, who has published a long and detailed paper in English in volumes 1 and 2 of the Swedish Geographiska Annalen. He concludes that the morphology of Norway, without reference to stratigraphical or biological data, gives conclusive evidence of two glaciations. The first of these was the greater, and between that time and the second smaller glaciations there occurred an interglacial period of such considerable length that the greater part of the present gorges was then formed by fluvial erosion.
We may, accordingly, consider the Ice Age as fourfold or double, according to the point of view from which we regard it. In the Alps and other mountain ranges on the borders of the great northern icesheets, which respond very readily to small changes, it was fourfold. In the peripheral regions of the northern ice-sheets themselves it has an appearance of being threefold or fourfold. In the more central regions of these great ice-sheets, where response to climatic change is very slow, there is no evidence of more than two glaciations; but in these regions, where the destructive effect of the ice reached its maximum, it is only by the merest chance that evidence of interglacial periods is preserved at all. And finally, in all other parts of the world we have evidence of only two glaciations at most.
There is one deposit which is of considerable importance in the study of interglacial climates, and that is the loess. Loess is an exceedingly fine-grained homogeneous deposit resulting from the gradual accumulation of wind-blown dust on a surface sparsely covered with vegetation. It is to be seen accumulating at the present day in parts of south-east Russia and central Asia. Its formation, except in closed basins, needs a climate of the steppe character, with not much rainfall, and especially with a long dry season. Now loess was very extensively developed in Europe during the
Quaternary Its occurrence is peculiar, since it is found most widely developed resting on the deposits of the Rissian glaciation, and is never found resting on the moraines of the Wurm glaciation. A little loess is found below the Riss moraines, and it has also been found between the Riss and Wurm moraines. In the pre-Rissian loess an implement of Acheulian age was discovered in 1910 at Achenheim (Alsace), by R. R. Schmidt and P. Wernert, indicating that the deposit was formed towards the close of the Chellean industry, when the climate was already cold and dry. In the same section the younger loess seems to fill completely the Riss-Wurm interglacial, since Mousterian implements were found at the base and Aurignacian implements in the middle. The younger loess contains remains of the jerboa and other rodents at present inhabiting the Siberian steppes. It is therefore reasonable to conclude that steppe conditions prevailed in central Europe through practically the whole of the RissWurm interglacial, and the same probably applies to the corresponding pre-Wisconsin interglacial in America. But if a steppe climate prevailed in central Germany there must have been very severe conditions in Scandinavia, and probably the ice-sheet maintained a quite considerable area there throughout the whole period, though without encroaching on the Baltic basin. In North America the loess was deposited by westerly winds, indicating that the ice-development was not sufficient to impose anticyclonic conditions in place of the present prevailing westerly winds, and the same appears to be true of Europe. Similar climatic conditions were developed for a short time at the close of the Wurm glaciation, but without any appreciable development of loess. (See Chapter XIII.)
BIBLIOGRAPHY
Wright, W. B. “The Quaternary Ice-age.” London, Macmillan, 1914.
Brooks, C. E. P. “The correlation of the Quaternary deposits of the British Isles with those of the continent of Europe.” Ann. Rep. Smithsonian Inst., 1917, pp. 277-375
de Geer, G. “A thermographical record of the late Quaternary climate.” Ber. Internat. Geologenkongr., Stockholm, 1910. “Die Veränderungen des
Klimas, ” p. 303.
—— “A geochronology of the last 12,000 years ” Ber Internat Geologenkongr , Stockholm, 1910, Vol. 1, p. 241.
Penck, A., and Brückner, E. “Die Alpen in Eiszeitalter.” Leipzig, 3 Vols., 1901-9.
Ahlmann, H. W., son. “Geomorphological studies in Norway.” Stockholm, Geografiska Annaler, 1, 1919, pp. 1-157, 193-252.
Richthofen, F “On the mode of origin of the loess ” Geol Mag , 1882, p 293
CHAPTER V
The literature of the glacial period in Europe is stupendous and is, further, of a highly contradictory nature. Space does not permit of any summary of the great conflict between the monoglacialists and the polyglacialists; it is sufficient to say that the latter often went to extremes and so laid themselves open to defeat, but the twofold nature of the glaciation is now widely accepted. It must be understood, however, that the following summary represents the views of a certain section of geologists only, views which are not universally held. In the British Isles especially, where the remains of the maximum glaciation completely dominate those of all the others, the theory of a single glaciation still largely prevails.
When ice began to accumulate on the rising Scandinavian plateau it naturally formed at first on the Norwegian mountains near the Atlantic, which was the chief source of snowfall. These mountain glaciers spread rapidly down the steep seaward slopes to the west and more slowly down the gentler landward slopes to the east. At this stage the centre of the ice-sheet, and consequently the centre of the glacial anticyclone, as soon as the latter developed a definite existence, lay quite near the Norwegian coast. Under anticyclonic conditions the circulation of the winds round the centre is in the same direction as the motion of the hands of a watch, combined with an outward inclination at an angle of about thirty to forty-five degrees. Consequently, while the centre lay in Norway, due north of the Alps, the prevailing winds in the latter must have been from north-east, and therefore very cold. Accordingly, this stage is probably contemporaneous with the Gunz glaciation of the Alps. In
the same way, over the North Sea area the winds must have been easterly, causing the currents which piled up the great shell-banks of the East Anglian coast, already referred to as marking the end of the Tertiary and beginning of the Quaternary period.
But the ice which reached the northern North Sea broke up into icebergs not far from the coast, and floated away, while that which moved east into the north of Sweden could only be dissipated by melting and ablation, processes which we have reason to believe went on very slowly. Hence ice began to accumulate and spread over a wide area east of the main Scandinavian mountain chain. Fresh snow was deposited directly on this ice-surface, until it gradually overtopped the mountains which originally gave rise to it, and reversed the flow, so that the ice actually moved uphill across the mountain chain. As the center of the ice-sheet moved eastward the glacial anticyclone moved with it, and this new position to the eastward caused an alteration in the direction of the prevailing winds over the rest of Europe. The Alps were now south-west of the anticyclonic centre, and the winds in that district accordingly became easterly instead of north-easterly. Of course, the glacial anticyclone was now more intense, but in summer in central Europe easterly winds are naturally so much warmer than north-easterly winds that at first this increase in intensity was not enough to counterbalance the change in direction, and there was a slight improvement in the Alpine climate. In the same way, over the North Sea district the prevailing winds had now become south-easterly instead of easterly, which would make for a slight rise of temperature, as also would the occasional depressions which would be able to make their way in from the westward, bringing warm moist air from the Atlantic and occasional rainfall. By this time the process of elevation had converted the North Sea floor into an extensive plain.
From Sweden and the Gulf of Bothnia the ice spread out in all directions, extending in the east to the foot of the Ural Mountains, which formed an independent centre of glaciation; in the south-east over a large part of European Russia, where it reached as far south as latitude 40° in the Dnieper valley; in the south over almost the whole of Germany as far as the Riesengebirge and Harz Mountains;
and in the south-east over the whole of Holland and the North Sea basin. It should be noted that Holland and Denmark were glaciated, not by Norwegian ice, but by ice from the Baltic sheet which had crossed southern Sweden. The North Sea glacier extended across East Anglia as far as Cambridge, while a northern branch of it swept across Caithness and the Orkney and Shetland Islands, but most of the British Isles were glaciated from independent centres—the Scottish Highlands, the Pennines, Cumberland, Wales and northern Ireland.
With the growth of the glaciated area, and particularly with its extension south-westward across the North Sea, the Alpine climate again became very severe, and the local glaciers and Piedmont icesheets of the Alps reached their maximum development in the Mindelian. At the same time the central plateau of France developed a local plateau glacier of its own, and the Pyrenees underwent their first and greatest glaciation, no traces of the Gunzian having been found in this range.
The British Isles show an interesting outward migration of the local centres of maximum ice-development. The Scandinavian glacier which invaded East Anglia extended arctic anticyclonic conditions across the North Sea, and induced a heavy snowfall over the high lands of Great Britain. These, in consequence, developed independent glaciers, which on their eastern sides fused with the Scandinavian glacier and, partly by deflecting its flow, partly by intercepting some of its snowfall, pushed it back into the North Sea plain. The Scottish glaciers became strong enough to encroach on Ireland, partly in the north-east, and partly by way of the Irish Sea and St. George’s Channel (then a valley) on to the south-east. This further extension of the cold area enabled the Irish glaciers to develop, and these in turn pushed back the Scottish glaciers until Ireland was solely glaciated by Irish ice.
The southern margin of the ice-sheet did not extend beyond the Thames valley, but at some stage the English Channel carried floating ice, which formed the deposits of ice-borne boulders, of which that at Selsey is a well-known example.
This great ice-sheet nowhere formed marked terminal moraines, but its deposits fade away in thin beds of stiff boulder-clay. This absence of moraines is probably connected with the great thickness of the ice-sheets, which did not leave any appreciable nunataks or rocky “islands” exposed in its path, so that there was nothing to give rise to detritus on the surface of the ice. All the transportation had to be carried on beneath the ice-sheets, and these, penetrating into comparatively low latitudes where the sun is powerful in summer, would suffer gradual melting and ablation for some distance from their margins. Near the actual ice-limit the motion must have been slow and the thickness of the ice small, so that conditions were against the accumulation of thick beds of detritus.
On the borders of the ice-sheet the climate cannot have been over-rigorous, for pre-Chellean man was able to live almost up to the ice-edge. The air must have been extremely cold, and there was a belt of high arctic climate round the ice, but in the south and southwest this appears to have been very narrow, and sub-arctic conditions, no worse than those in which many races live to-day, prevailed not very far from the ice. The configuration of the icesurface largely explains this. A high steeply sloping wall of ice causes intensely violent winds, carrying dense clouds of drift-snow— blizzards, in fact, similar to those now experienced in parts of Antarctica under similar circumstances, which sweep the land bare of all life for a considerable distance. But a low and gradually sloping surface, such as seems to have existed near the borders of the maximum glaciation, favours instead comparatively gentle winds without much drift snow. It is only on the north-west ice-ridge, where ice-cliffs fronted the sea and where severe storms from the Atlantic were frequent in winter, that blizzards occurred.
When the land in Scandinavia began to sink under the ice-load more rapidly than the supply of snow could build up the surface of the ice-sheet the force which pushed out the ice in all directions from the centre gradually died away, and the ice-masses over the North Sea area—now probably again below sea-level—and the low grounds of Europe were left derelict, with no resources but the snowfall on their own surfaces. Under these conditions they melted
away more or less rapidly While these derelict ice-masses were still large, the auxiliary peripheral centres in the Alps, Pyrenees and British Isles maintained an independent existence for a while, probably with fluctuations similar to those which marked the close of the last glaciation in the Alps, though the evidence of these has now been wiped away. It is even likely that the beginnings of the weakening of the central source of supply helped the British ice to divert the Scandinavian ice into the North Sea. Had there been any powerful rivers bearing great masses of detritus from the south, as there are in Siberia, some of these derelict ice-sheets might have been preserved for a time, at least, as “fossil ice,” but in western Europe conditions were not favourable for this.
With the disappearance of the ice-sheets the general climate of Europe must have passed through a series of stages of amelioration, of which traces can be found here and there, though the details are lost to us. Ultimately temperate conditions again prevailed; and for a very long time, approaching a quarter of a million years, Europe cannot have differed greatly from present climatic conditions. In Scandinavia the mammoth roamed in forests of birch, pine and spruce; further south the mammoth is absent, and we find instead more southern forms—Elephas antiquus, resembling the Indian elephant, Rhinoceros merckii, a southern form, the sabre-toothed tiger, cave-lion, cave-bear and cave-hyæna, wolf, beaver, horse and various forms of deer, while the flora included even such warmthloving trees as the fig. Obviously, during part of this interglacial period, the climate must have been even warmer than the present.
Let us glance for a moment at the probable conditions. One of the dominant features in the present weather of Europe is the accumulation of floating ice in the Arctic basin. This keeps the temperature low and the pressure high—forms in fact during the spring and summer months a temporary glacial anticyclone similar in kind to, though of less intensity than, that which has been described as covering the Scandinavian ice-sheet. This anticyclone maintains on its southern edges a belt of easterly winds, and these winds enter into the general circulation of the earth. Their effect is to push southward the permanent storm-centres normally situated near
Iceland and the Aleutian Islands, and it is these storm-centres which play a large part in causing the rainy weather of northern and central Europe. But occasionally—as in the remarkable spring and summer of 1921—these conditions break down. The Arctic Ocean becomes unusually ice-free and warm, the pressure falls, and in consequence the storm-centres move northward. Europe comes under the influence of the permanent anticyclones of sub-tropical latitudes, rain-bearing storms pass far to the northward, and we have a dry warm summer of the Mediterranean type.
This is presumably what happened during the long warm MindelRiss interglacial. For some reason, possibly connected with a temporary widening and deepening of the Bering Strait, the waters of the Arctic Ocean became warmer and the amount of floating ice less. Pressure became lower in the polar basin and therefore higher over the Atlantic and Europe, and fine warm conditions prevailed in Europe as the normal climate instead of only as an occasional event.
This warm interval was finally brought to a close by the renewed elevation of Scandinavia, and the ice-sheets began to develop again, heralded by a period of dry steppe climate. This time, however, the conditions were different; the elevation was not so great, and was more local. Hence the resulting glaciation was less intense; it filled the Baltic basin and extended some distance on to the North German plain and into Holland. It failed to reach the coast of Britain, but that it extended some way across the North Sea plain is indicated by the peculiar distribution of the Newer Drift of Britain, to be referred to later. In the north of Norway the slope of the ice towards the sea was very steep, so that many of the coastal hills extended above it as nunataks. The ice extended into the channel between the mainland and the Lofoten Islands (then a peninsula), but according to Ahlmann these islands were an independent centre of local glaciation, as the British Isles had been during the preceding period, and the local ice met the main ice-sheet in the fiords. On the coast of Nordland sufficient land lay bare to harbour a small Arctic flora, and Vaero, the southernmost island of Lofoten, had only small hanging snow-banks.
The interpretation of the British glacial deposits is still very much under discussion, but it seems probable that the Scottish highlands formed a subsidiary centre which glaciated the whole of Scotland and north-east England, sending a stream south-eastward, which was prevented from spreading across the North Sea plain by the presence of Scandinavian ice to the east and impinged on the coast of Yorkshire and Lincolnshire, just reaching the northern extremity of Norfolk. Many British geologists regard this development as the concluding phase of a single glaciation of Britain, but the differences in the amount of weathering undergone are against such an interpretation. At the same time there were local glaciers in Cumberland, Wales and Ireland.
In England limits of this glaciation are characterized by a wellmarked series of end-moraines, which indicate that the ice carried much surface detritus, and probably ended in a steep cliff. In Scandinavia, on the other hand, the centre of glaciation again lay over the low ground well to the east of the mountains, and the ice which reached Germany and Denmark was still largely free of surface detritus, and so did not form marked end-moraines. There was a difference, however. On this occasion, owing to the local nature of the elevation in Scandinavia, the ice-sheet did not extend its borders so far to the eastward, and the glaciation of Asia, as described in Chapter VII, was slight. Europe came more under the influence of cold north-easterly and northerly winds, and life on the ice-borders was not so easy as during the preceding glaciation. Man could still live near the ice, but he took to making his home in caves, and to clothing himself in skins for warmth.
After the ice had reached its Rissian maximum of glaciation the climate improved somewhat. The ice-edge retreated, leaving Denmark and the German coast, and vacating the Baltic basin, but not disappearing altogether from Scandinavia. At Rixdorf, near Berlin, there is a bed of gravel deposited in this “interglacial,” containing numerous and well-preserved bones of the mammoth, woolly rhinoceros, aurochs, bison, horse, reindeer, red deer and other species of Cervus, musk ox and wolf—a cold temperate to sub-arctic fauna. In south Germany fresh-water mollusca indicate
that the summers in that district were almost as warm as at present, but the winters were probably severe. As described in the preceding chapter this “interglacial” was the time of loess formation par excellence, with a continental climate and steppe conditions over much of central Europe.
Investigations at Skærumhede in Denmark show that this recession of the ice was accompanied by, and presumably due to, a fall in the level of the land relatively to that of the sea, for at the beginning of the oscillation the land lay about 350 feet above its present level, sinking gradually to only 30 feet above present. Even at its best during this interglacial the climate was almost sub-arctic in Denmark. In northern Finland, on the eastern edge of the ice-sheet, there was also an “interglacial,” with a slight improvement in the climate accompanying a temporary submergence. But in Scandinavia there are no traces of any interglacial deposits of this period, and considering the cold climates which prevailed in Denmark and North Germany, it seems probable that Scandinavia continued to be glaciated during the whole period.
The mode of life among Mousterian men, who lived during this “interglacial,” also points to a severe climate. For at this time man did not live in the open, but in caves and rock-shelters, and the practice of wearing the fur skins of animals as a protection against the cold, begun in the preceding Rissian glacial period, was not discontinued.
After the temporary subsidence had ceased, elevation again set in, causing a readvance of the ice-sheets and glaciers. The limits fell short of those of the preceding maximum, and the climate was not so severe, but in its general character it resembled that of the preceding maximum, but was much stormier, and there were probably frequent blizzards of the Antarctic type, carrying drift-snow. The new ice-sheet carried more surface detritus than its predecessors, presumably because all the high ground was not covered, and it formed high terminal moraines. The close association of cold ice and irregular masses of bare sand and stones, strongly heated by the summer sun, set up a belt of powerful convection very favourable for the development of blizzards; possibly there was something in the nature of an ice-cliff down which the cold winds could blow with great
strength. At any rate, man found the near neighbourhood of the ice unpleasant, and went, so that there are no contemporaneous human implements near the moraines. The limits of the Scandinavian icesheet ran from the Norwegian coast across Denmark from north to south, through North Germany and northern Russia, and included Finland. The ice probably did not extend far across the North Sea plain, and in the British Isles there was no ice-sheet, but the high mountains of Scotland, Ireland, Wales and Cumberland bore small local glaciers, which were long enough to reach the sea in the Scottish highlands. The Alps bore considerable glaciers, indicating a depression of the snow-line of about 3500 feet, corresponding to a temperature 11° F. lower than the present.
After this ice-development had reached its maximum limits and remained there for perhaps a thousand years, retreat set in, and the Scandinavian ice once more withdrew from Germany and Denmark to the Baltic basin. But its edge was never far from the German coast, and occasionally readvanced across it, for numerous fossiliferous deposits are intercalated in boulder-clay. The fauna and flora, which are well known, point to an arctic climate. At its best the mean temperature of July rose to about 50° F., and there was a vegetation period of three or four months with an average temperature of about 40° F., but these relatively mild conditions lasted at most for a few decades or perhaps a century at a time, and the winters were severe throughout. The duration of the whole of this “Baltic Interstadial” was from one to two thousand years.
Next followed the final readvance of the ice forming the great “Baltic” moraines which fringe the Baltic coast of Germany, turning northward in the west into Denmark and in the east into Finland. There was a corresponding re-development of glaciers in the Alps (Bühl stage) and in the mountains of Ireland and Scotland, though these probably failed to reach the sea even in Scotland. This period gave us a repetition of the climate of the preceding maxima. In this case we have definite evidence of the presence of a belt of easterly winds on the southern side of the ice-sheet, in a series of “barkans” or fossil dunes in Holland, Germany and Galicia. These dunes were formed of fine ice-deposited material, and they are crescent-shaped,
