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Science Journals — AAAS SCIENCE ADVANCES | RESEARCH ARTICLE CLIMATOLOGY Copyright © 2017 The Authors, some Snowball Earth climate dynamics and Cryogenian rights reserved; exclusive licensee geology-geobiology American Association for the Advancement 1,2 3 4 5 6 of Science. No claim to Paul F. Hoffman, * Dorian S. Abbot, Yosef Ashkenazy, Douglas I. Benn, Jochen J. Brocks, original U.S. Government 7 8,9 10 11,12 Phoebe A. Cohen, Grant M. Cox, Jessica R. Creveling, Yannick Donnadieu, Works. Distributed 13,14 15 16 17 18 Douglas H. Erwin, Ian J. Fairchild, David Ferreira, Jason C. Goodman, Galen P. Halverson, under a Creative Malte F. Jansen,3 Guillaume Le Hir,19 Gordon D. Love,20 Francis A. Macdonald,1 Adam C. Maloof,21 Commons Attribution Camille A. Partin,22 Gilles Ramstein,11 Brian E. J. Rose,23 Catherine V. Rose,24† Peter M. Sadler,20 NonCommercial License 4.0 (CC BY-NC). Eli Tziperman,1 Aiko Voigt,25,26 Stephen G. Warren27 Geological evidence indicates that grounded ice sheets reached sea level at all latitudes during two long-lived Cryogenian (58 and ≥5 My) glaciations. Combined uranium-lead and rhenium-osmium dating suggests that the older (Sturtian) glacial onset and both terminations were globally synchronous. Geochemical data imply that 2 CO2 was 10 PAL (present atmospheric level) at the younger termination, consistent with a global ice cover. Sturtian glaciation followed breakup of a tropical supercontinent, and its onset coincided with the equatorial Downloaded from emplacement of a large igneous province. Modeling shows that the small thermal inertia of a globally frozen surface reverses the annual mean tropical atmospheric circulation, producing an equatorial desert and net snow and frost accumulation elsewhere. Oceanic ice thickens, forming a sea glacier that flows gravitationally toward the equator, sustained by the hydrologic cycle and by basal freezing and melting. Tropical ice sheets flow faster as CO2 rises but lose mass and become sensitive to orbital changes. Equatorial dust accumulation engenders supraglacial oligotrophic meltwater ecosystems, favorable for cyanobacteria and certain eukaryotes. Meltwater http://advances.sciencemag.org/ flushing through cracks enables organic burial and submarine deposition of airborne volcanic ash. The sub- glacial ocean is turbulent and well mixed, in response to geothermal heating and heat loss through the ice cover, increasing with latitude. Terminal carbonate deposits, unique to Cryogenian glaciations, are products of intense weathering and ocean stratification. Whole-ocean warming and collapsing peripheral bulges allow marine coastal flooding to continue long after ice-sheet disappearance. The evolutionary legacy of Snowball Earth is per- ceptible in fossils and living organisms. INTRODUCTION The Cryogenian period (50) encompasses the paired Neoproterozoic For 50 years, climate models of increasing complexity have hinted that Snowball Earths and the brief nonglacial interlude (Fig. 2A). The Earth is potentially vulnerable to global glaciation through ice-albedo term “cryochron” (27) was proposed for the panglacial epochs, on feedback (Fig. 1) (1–23). Independent geologicalevidencepointstocon- the assumption that their onsets and terminations were sharply de- on February 2, 2018 secutive “Snowball Earth” (24) episodes in the Neoproterozoic era (24–34) fined in time, which now appears to be the case (Table 1). The older and at least one such episode in the early Paleoproterozoic era (Fig. 2B) cryochron has come to be known as “Sturtian” and the younger one (35–40). Strangely, virtually no ice sheets are known to have existed has come to be known as “Marinoan,” after Sturt Gorge and Marino during the intervening 1.5 billion years (Gy) of the Proterozoic glacial Rocks near Adelaide, South Australia, where they were recognized gap (Fig. 2B). The oldest Snowball Earth (35–40) was broadly coeval and mapped over 100 years ago (51). [As originally defined (52), these with the Great Oxidation Event (Fig. 2B), the first rise of molecular regional terms did not refer exclusively to the glacial epochs, but the oxygen (41–45), whereas the younger tandem (Fig. 2A) was associated original terminology has been superseded by the formal periods of the with the emergence of multicellularity in animals (Fig. 3) (46–49). International Time Scale (Fig. 2A) (50, 53). We find it convenient to 1Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA. 2School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia V8P 5C2, Canada. 3Department of Geophysical Sciences, University of Chicago, Chicago, IL 60637, USA. 4Department of Solar Energy and Environmental Physics, Ben-Gurion University of the Negev, Midreshet Ben-Gurion, 84990, Israel. 5School of Geography and Sustainable Development, University of St Andrews, St Andrews, Fife KY16 8YA, UK. 6Research School of Earth Sciences, Australian National University, Canberra, Australian Capital Territory 2601, Australia. 7Geosciences, Williams College, Williamstown, MA 01267, USA. 8Centre for Tectonics, Resources and Exploration (TRaX), Department of Earth Sciences, University of Adelaide, Adelaide, South Australia 5005, Australia. 9Department of Applied Geology, Curtin University, Bentley, Western Australia 6845, Australia. 10College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331–5503, USA. 11Laboratoire des Sciences du Climat et de l’Environnement (LSCE), Institut Pierre Simon Laplace (IPSL), CEA-CNRS-UVSQ, Université Paris-Saclay, 91191 Gif-sur-Yvette, France. 12Aix-Marseille Université, CNRS, L’Institut de recherche pour le développement (IRD), Centre Européen de Recherche et D’enseignement de Géosciences de L’environnement (CEREGE), 13545 Aix-en-Provence, France. 13Department of Paleobiology, Smithsonian Institution, P.O. Box 37012, MRC 121, Washington, DC 20013–7012, USA. 14Santa Fe Institute, 1399 Hyde Park Road, Santa Fe, NM 87501, USA. 15School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK. 16Department of Meteorology, University of Reading, Reading, RG6 6BB, UK. 17Department of Environmental Science, Wheaton College, Norton, MA 02766, USA. 18Department of Earth and Planetary Sciences, McGill University, Montréal, Québec H3A 0E8, Canada. 19Institut de Physique du Globe de Paris, 1, rue Jussieu, 75005 Paris, France. 20Department of Earth Sciences, University of California, Riverside, Riverside, CA 92521, USA. 21Department of Geosciences, Princeton University, Princeton, NJ 08544, USA. 22Department of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2, Canada. 23Department of Atmospheric and Environmental Sciences, University at Albany, Albany, NY 12222, USA. 24Department of Geology, Trinity College Dublin, Dublin 2, Ireland. 25Institute of Meteorology and Climate Research, Department of Troposphere Research, Karlsruhe Institute of Technology, Karlsruhe, Baden-Württemberg, Germany. 26Lamont-Doherty Earth Observatory, Columbia University, P.O. Box 1000, Palisades, NY 10964–1000, USA. 27Department of Atmospheric Sciences, University of Washington, Seattle, WA 98195–1640, USA. *Corresponding author. Email: [email protected] †Present address: School of Earth and Environmental Sciences, University of St Andrews, Irvine Building, St Andrews, Fyfe KY16 9AL, UK. Hoffman et al., Sci. Adv. 2017; 3 :e1600983 8 November 2017 1of43 SCIENCE ADVANCES | RESEARCH ARTICLE Solar f lux (wrt present) follow the current informal international usage, wherein “Sturtian” 0.9 1.01.1 1.2 1.3 and “Marinoan” identify the cryochron and its immediate aftermath, including the respective postglacial cap-carbonate sequences.] The nonglacial interlude separating the cryochrons was 9 to 19 million years e <10 My d 90° (My) in duration (Fig. 2A). The worldwide distribution of late Precambrian glacial deposits f Present (Fig. 4) became evident in the 1930s (25, 54, 55),butonlyrecentlyhas 60° their synchroneity been demonstrated radiometrically (32, 56–65). a ~2 ky Previously, geologists were divided whether the distribution of gla- 30° cial deposits represents extraordinary climates (29, 66)ordiachronous products of continental drift (67, 68). The recognition that a panglacial state might be self-terminating Ice line latitudeIce Eq 69 b c ( ), due to feedback in the geochemical carbon cycle, meant that its >10 My occurrence in the geological past could not be ruled out on grounds of irreversibility. “If a global glaciation were to occur, the rate of silicate 0.1 1 10 100 1000 weathering should fall very nearly to zero (due to the cessation of nor- PCO2 (wrt present) mal processes of precipitation, erosion, and runoff), and carbon dioxide should accumulate in the atmosphere at whatever rate it is released Downloaded from Fig. 1. Generic bifurcation diagram illustrating the Snowball Earth hysteresis. from volcanoes. Even the present rate of release would yield 1 bar of Ice-line latitude as a function of solar or CO2 radiative forcing in a one-dimensional carbon dioxide in only 20 million years. The resultant large greenhouse (1D) (meridional) energy-balance model of the Budyko-Sellers type (3, 4), showing effect should melt the ice cover in a geologically short period of time” three stable branches (red, green, and blue solid lines) and the unstable regime 69 (dashed line). Yellow dots are stable climates possible with present-day forcing. Black [( ), p. 9781]. Because Snowball Earth surface temperatures are
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