Late Proterozoic Snowball Earth Brian Morgan Colby College December 3, 2012

Questions What is the evidence to support Snowball Earth? How could the Earth overcome a global glaciation? How could organisms survive “icehouse” condition? What are other hypotheses?

Glacial Deposits Many Late Proterozoic ( Ma) deposits contain glacial debris Glacial deposits found on almost every continent Many of these rocks were deposited at low latitudes Breakup of supercontinent Rodinia during the Proterozoic, leaving vast oceans at the poles separated from equatorially positioned landmasses

Banded Iron Formations (BIF) Sedimentary rocks consisting of oxides, sulfides, or carbonates interlayered with iron-poor chert Form when: Oxygen deficiency in stagnant bottom waters caused by widespread ice cover Dissolved iron accumulated in the oxygen-poor oceans Mixing with oxygen waters during deglaciation caused precipitation of iron

Cap Carbonates Continuous layers of limestone and/or dolostone that sharply overlie Late Proterozoic glacial deposits Found in Namibia, NW Canada, Australia, Morocco, Western U.S. Formed during deglaciation due to rising temperatures and increased ocean surface area Increased precipitation Dissolves CO 2 in atmosphere Caused by intense carbonate and silicate weathering

Paleomagnetism Paleomagnetic reconstructions of sedimentary deposits suggest deposition at low latitudes Multiple reversals of geomagnetic field, implying hundreds of thousands of years near paleoequator Many scientists came to the same conclusion that Late Proterozoic glacial strata were deposited at low latitudes

Breaking out of Snowball Earth

Microscopic Organisms Could eukaryotic life survive millions of years of ice-covered oceans? Many organisms likely died off due to unfavorable conditions Metazoans that evolved before the glaciations could survive near volcanic islands or in areas of thin ice Small diversity of organisms may have led to rapid speciation in aftermath of glaciations because abundance of nutrients and lack of predators

Alternate Hypotheses Fairchild and Kennedy, 2007

Conclusions Many lines of evidence supporting Snowball Earth hypothesis Late Proterozoic glacial deposits found on nearly every continent BIFs suggesting anoxic bottom water Cap carbonates directly overlie glacial deposits Paleomagnetic reconstructions suggest near-equatorial position CO 2 buildup in the atmosphere became great enough to overcome global glaciation Microscopic organisms survived global glaciation

References Corsetti, F. A., A. N. Olcott, and C. Bakermans, 2006: The Biotic Response to Neoproterozoic Snowball Earth. Palaeogeography, Palaeoclimatology, Palaeoecology, v. 232, p Evans, D. A. D., 2006: Proterozoic low orbital obliquity and axial-dipolar geomagnetic field from evaporate paleolatitudes. Nature, v. 444, p Eyles, N. and N. Januszczak, 2004: ‘Zipper-rift’: a Tectonic Model for Neoproterozoic Glaciations During the Breakup of Rodinia after 750 Ma: Earth- Science Reviews, v. 65, p Fairchild, I. J., and M. J. Kennedy, 2007: Neoproterozoic glaciation in the Earth System. Journal of the Geological Society, London, v. 164, p Hoffman, P. F. and R. S. Hildebrand, 2009: Snowball Earth. Hoffman, P. F., A. J. Kaufman, G. P. Halverson, and D. P. Schrag, 1998: A Neoproterozoic Snowball Earth. Science, v. 281, p Hoffman, P. F. and D. P. Schrag, 2002: The snowball Earth hypothesis: testing the limits of global change. Terra Nova, v. 14, p Kennedy, M. J., N. Christie-Blick and A. R. Prave, 2001: Carbon isotopic composition of Neoproterozoic glacial carbonates as a test of paleoceonographic models for snowball Earth phenomena. GEOLOGY, v. 29, p Kirschvink, J. L., Late Proterozoic Low-Latitude Global Glaciation: the Snowball Earth, p in The Proterozoic Biosphere: A Multidisciplinary Study. J. W. Schopf and C. Klein, eds., New York, New York: Cambridge University Press; 1348 pp. Micheels, A. and M. Montenari, 2008: A snowball Earth versus a slushball Earth: Results from Neoproterozoic climate modeling sensitivity experiments. Geosphere, v. 4, p Rothman, D. H., J. M. Hayes, and R. E. Summons, 2003: Dynamics of the Neoproterozoic Carbon Cycle. PNAS, v. 100, p

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