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1 Department of Earth Science, University of Southern California, Los Angeles, California 90089-0740
The concept of a Neoproterozoic "snowball Earth"–an ice age in the tropics (Kaufman, 1997)–has captured the imagination of the public and scientists alike. The idea of global-scale glaciation, indicated by the abundance of Neo-proterozoic glacial deposits (some with low-latitude paleomagnetic signatures), is not new (see Hoffman and Schrag [2002] and Eyles and Januszczak [2004] for developmental perspectives). This idea was given new life by Kirschvink (1992), who suggested that Earth might have been completely glaciated due to unusually high planetary albedo from an equatorial supercontinent. In this scenario, the hydrologic cycle would have been reduced, possibly leading to patchy land glaciers, but sea ice thick enough to prevent photosynthesis in the oceans. Iron concentrations would build up in the ice-covered oceans after oxygen was depleted, and ultimately the ecosystem would crash. Kirschvink termed this scenario "snowball Earth."
A snowball Earth could eventually be thawed when enough carbon dioxide had outgassed from ongoing volcanic processes to create a super-greenhouse (Kirschvink, 1992). Iron formation would precipitate as the ice melted and oxygen re-entered the marine system; iron formation does indeed occur in some Neo-proterozoic glacial deposits. Carbonate deposits (termed "cap carbonates"), more typical of warm conditions, overlie Neoproterozoic glacial deposits (Figure 1). Hoffman et al. (1998) tied the cap carbonates to the postglacial greenhouse, where a vigorously reinstated hydrologic and weathering cycle would serve to flush ions from readily available continental debris into the ocean to rapidly precipitate carbonate rocks on top of glacial deposits. The hypothesis was tidy, and had predictive power. Furthermore, it did not escape notice that the diversification of metazoan life occurred just after the "snowball" glaciations–could there be a link?
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A new study from Ruben Rieu et al. (this issue of Geology) addresses this debate in a novel way. A consequence of a snowball Earth should be the inhibition of the planet's hydrologic cycle, which might allow for some measure of physical weathering, but should stop virtually all chemical weathering of continental silicates. Through proxies for chemical weathering (the mineralogical index of alteration, MIA, and the chemical index of alteration, CIA), Rieu et al. find variation in continental silicate weathering throughout a Neoproterozoic glaciation, arguing for the waxing and waning of an active hydrologic cycle capable of both physical and chemical weathering. In fact, the CIA and MIA attain similar levels during the ice age and during the deposition of the postglacial cap carbonate. This seems to contradict the snowball Earth hypothesis: it denotes a cyclic, warm, active hydrologic and weathering cycle during the ice age, not unlike what we might predict from a typical Pleistocene glacial/interglacial succession. Such indications of warm, humid interglacials during the ice age are completely at odds with the hydrologic and weathering cycle predicted by the snowball Earth hypothesis.
The debate does not stop here. Modelers point out that the ice-albedo feedback is too simple and does not account for enough parameters to be considered realistic (e.g., Poulsen, 2003), while others counter that the models, with their roots in modern climate, are inappropriate for the non-actualistic conditions of Neoproterozoic time (Schrag and Hoffman, 2001). The case for iron formation may be somewhat overstated; where abundant, it seems to have formed in association with rift deposits (Young, 2002), and new data demonstrate the oceans may have been largely anoxic throughout much of the Neoproterozoic, not just during glaciation (Canfield et al., 2007). Many different hypotheses are available to explain the cap carbonates (e.g., Grotzinger and Knoll, 1995; Hoffman et al., 1998; Kennedy et al., 2001; Ridgwell et al., 2003; and Shields, 2005). Finally, the case for global ecosystem crash has not withstood careful scrutiny (Corsetti et al., 2003; Corsetti et al., 2006); biomarkers indicating a "healthy" photosynthesis-based ecosystem have been noted from within glacial deposits (Olcott et al., 2005), and post-glacial diversity trends appear sufficiently disconnected from the glaciations to preclude a cause and effect (e.g., Grey et al., 2003). Nevertheless, the snowball Earth hypothesis has reinvigorated interest in the Neoproterozoic and spurred on many scientific investigations in a way that only a truly provocative and innovative hypothesis can, regardless of the current lack of consensus.
Much–one might even say most–of the geochemical work surrounding snowball Earth has focused on postglacial cap carbonates. The new study by Rieu et al. reminds us that there is a lot to learn from glacial deposits themselves, which remain a relatively untapped resource with respect to novel analyses. Furthermore, preglacial rocks should provide a target to help us understand the processes that initiated the Neoproterozoic glaciations in the first place.
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REFERENCES CITED |
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Corsetti, F.A., Awramik, S.M., and Pierce, D., 2003, A complex microbiota from snowball Earth times: Microfossils from the Neoproterozoic Kingston Peak Formation, Death Valley, USA: National Academy of Sciences (USA) Proceedings, v. 100 pp. 4399-4404 doi: 10.1073/pnas.0730560100.[CrossRef]
Corsetti, F.A., Olcott, A., and Bakermans, C., 2006, The biotic response to Snowball Earth: Palaeo-3, v. 232 pp. 114-130.[CrossRef]
Eyles, N., and Januszczak, N., 2004, "Zipper-rift": A tectonic model for Neoproterozoic glaciations during the breakup of Rodinia after 750 Ma: Earth-Science Reviews, v. 65 pp. 1-73 doi: 10.1016/S0012-8252(03)00080-1.
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Kirschvink, J.L., 1992, Late Proterozoic low-latitude global glaciation; the snowball Earth: in Schopf, J.W. and Klein, C., eds., The Proterozoic biosphere; A multidisciplinary study, New York, Cambridge University Press pp. 51-52.
Olcott, A.N., Sessions, A.L., Corsetti, F.A., Kaufman, A.J., and de Oliviera, T.F., 2005, Biomarker evidence for photosynthesis during Neoproterozoic Glaciation: Science, v. 310 pp. 471-474 doi: 10.1126/science.1115769.
Poulsen, C.J., 2003, Absence of a runaway ice-albedo feedback in the Neoproterozoic: Geology, v. 31 pp. 473-476 doi: 10.1130/0091-7613(2003)031<0473:AOARIF>2.0.CO;2.
Ridgwell, A.J., Kennedy, M.J., and Caldeira, K., 2003, Carbonate deposition, climate stability, and Neoproterozoic ice ages: Science, v. 302 pp. 859-862 doi: 10.1126/science.1088342.
Schrag, D.P., and Hoffman, P.F., 2001, Life, geology and snowball Earth: Nature, v. 409 pp. 306 doi: 10.1038/35051168.[GeoRef]
Shields, G.A., 2005, Neoproterozoic cap carbonates; A critical appraisal of existing models and the plumeworld hypothesis: Terra Nova, v. 17 pp. 299-310 doi: 10.1111/j.1365-3121.2005.00638.x.[CrossRef][Web of Science][GeoRef]
Young, G., 2002, Stratigraphic and tectonic settings of Proterozoic glaciogenic rocks and banded iron-formations;Rrelevance to the snowball Earth debate: Journal of African Earth Sciences and the Middle East, v. 35 pp. 451-466 doi: 10.1016/S0899-5362(02)00158-6.[CrossRef][GeoRef]
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