The Most Severe Glaciations in Earth History: Presaged by Greenhouse Conditions?

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9630928 Hoffman The most extreme glaciations in Earth history occurred near the end of the Proterozoic eon, when ice margins extended at sea level to within 10 o of the equator. Glaciogenic sediments in many regions are sandwiched between typical warm-water carbonates, implying very abrupt climatic oscillations. Carbonates preceding the glacial deposits are extraordinarily enriched in 13C 13C-4% PDB). The highly positive 13C values coupled with low seawater 87 Sr/ 86 Sr suggests that a drawdown of atmospheric CO2 (plausibly related to high organic productivity and enhanced storage of carbon on a deep anoxic ocean) was the cause of glaciation. In contrast to the state of continental emergence (and hence high weathering rates) that presaged Cenozoic and late Paleozoic glacial epochs, stratigraphic and radiogenic isotopic evidence suggest that Neoproterozoic glaciations occurred in response to continental inundation. Accordingly, the Neoproterozoic glaciations may have had more in common with the short-lived Ashgill (late Ordovician) glacial event. These ancient occurrences illustrate the importance of considering the entire geological record, which reveals events and processes not predicted by current models, nor expressed in the 1.4% of Earth history represented by the Cenozoic. Renewed funding is requested to continue integrated stratigraphic investigations. Highly successful results from the first year of funding showed that secular variations in 13C values of carbonates in the pre-Vendian Otavi Group of Namibia are regionally consistent, and are not influenced by meteoric diagenesis, nor do they correlate with changes in primary lithofacies. Large isotopic shifts are noted across the two major sequence boundaries, but across other minor boundaries 13C values show no apparent change. The Otavi Group was initially selected because of the close association of thick carbonates and a glacial diamictite. A second pre-Vendian glacial event--newly recognized in the Otavi Group- -was first indicated by preliminary carbon isotope data. And, subsequent field work confirmed the existence of two stratigraphically distinct glacial deposits, which required wholesale revision of the regional stratigraphy. These observations were subsequently corroborated by U-Pb zircon dating of an ash layer within the Otavi carbonates. These early results demonstrate the efficacy ofthe three-pronged approach involving physical sequence stratigraphy, isotope chemostratigraphy, and U-Pb chronostratigraphy. Carbon and strontium isotopic studies will continue to be undertaken by Alan J. Kauf man at Harvard University, while single-grain U-Pb zircon analyses will be continued at the Massachusetts Institute of Technology under the direction of Samuel A. Bowring. Additional funding is requested to support the C- and Sr-isotopic zircon analyses, as well as a part-time salary for Kaufman and a laboratory technician. The cost of most field operations for Hoffman in Namibia will be borne elsewhere. ??