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FIGURE 1 Gypsum casts, mud cracks, and ripples from the Barr River Formation north of Elliot Lake, Ontario, Canada.

can only be precipitated from seawater by molecular oxygen (Kirschvink et al., 2000; Kopp et al., 2005). Similarly, Neoproterozoic glacial events are associated with apparent bursts of oxygenation and may have stimulated evolutionary innovations like the Ediacara fauna and the rise of Metazoa. We argue here that Precambrian glaciations are generally followed by fluctuations in apparent redox parameters, consistent with a postulate by Liang et al. (2006) that significant quantities of peroxide-generated oxidants are formed and released through glacial processes.

LOW-LATITUDE GLACIATION AS A SNOWBALL EARTH

Despite assertions to the contrary (Lovelock, 2006), climatic regulatory mechanisms have not always maintained large open areas of water on Earth’s surface. Substantial evidence exists that large-scale continental ice sheets extended well into the tropics, yielding sea ice at the equator (Embleton and Williams, 1986; Evans et al., 1997; Sohl et al., 1999; Sumner et al., 1987). The deposition of banded iron oxide formations (BIFs) associated with glacial sediments implies both sealing off of air-sea exchange and curtailing the input of sulfate to the oceans, which otherwise would be reduced biologically to sulfide, raining out Fe as pyrite. The Snowball Earth hypothesis (Kirschvink, 1992) accounts for the peculiarities of low-latitude tillites, BIFs, abrupt and broadly synchronous glacial onset and termination, and many other features of these events (Evans, 2000; Hoffman, 2007; Hoffman and Schrag, 2002; Hoffman et al., 1998). No alternative hypothesis even attempts to explain as many diverse features of the Precambrian glacial record.

Initially, the most fundamental result driving the Snowball Earth hypothesis was a soft-sediment fold test on a varvite-like member of the ~635 Ma Marinoan-age Elatina formation in South Australia, which implied incursion of sea ice into subtropical latitudes (Figure 2) (Sumner et al., 1987). A few years later, Evans et al. (1997) demonstrated similarly robust results from the ~2.22 Ga Makganyene glaciation in South Africa, indicating that at least two intervals of geological time, separated by more than a billion years, experienced low-latitude glaciation. Comparison of less robust paleomagnetic data for all Precambrian glaciations with well-documented paleolatitudes for Phanerozoic glacial deposits yields an interesting schism. With the possible exception of the Archean Pongola event, there is a total absence of evidence for polar or subpolar glaciation throughout the Precambrian, while marine glacial sedimentation never breaches the tropics through the Phanerozoic (Evans, 2003). While the counterintuitive Precambrian polar glacial gap must be largely an artifact of the paleogeographic and



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