9510339 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 degrees of the equator. The glaciogenic sediments in many regions are intimately intercalated with typical warm-water carbonates, implying very abrupt climatic oscillations. Carbonates preceding the glacial deposits are extraordinarily enriched in 13 C (13C>+6% PDB), signifying extreme rates of proportional organic carbon burial, whereas those directly following the glacial deposits are high depleted ( 13C<-4% PDB). This implicates a drawdown of atmospheric CO2 as the cause of glaciation and a corollary increase in pO2 as enabling the initial radiation of metazoans. In contrast to the state of continental emergence that presaged Cenozoic and late Paleozoic glacial epochs, stratigraphic and radiogenic isotope 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. Funding is requested to investigate two outstanding questions central to Neoproterozoic global change: (1) Do the glacial deposits represent globally synchronous events of diachronous products or rapid continental drift? (2) Do the observed shifts in 13Ccarbonate reflect purely secular variations in ocean surface waters or in part the spatial effects of differing water masses? The proposed study will be piggy-backed on ongoing field projects by the P.I. and by Anthony R. Prave (City University of New York) on the sequence stratigraphy, structural geology and regional tectonics of the Damara orogen in Namibia, considered a keysto ne for the Neoproterozoic tectonic reorganization of Gondwanaland. The ongoing projects have identified volcanic horizons potentially suitable for U-Pb zircon chronology within or bracketing three separate glaciogenic formations. Single-grain U-Pb zircon analyses will be done by thermal ionization mass spectrometry at the Massachusetts Institute of Technology under the direction of Samuel A. Bowring, whose recent chronostratigraphic work has dramatically refined the early Cambrian time scale. The ongoing projects have already gone far towards establishing the sequence stratigraphic framework and regional facies variations for carbonates above and below the glacial deposits in northern Namibia. Sampling and interpretation of carbon isotopic variations will be grounded on this work and on follow-up diagentic studies. The isotopic and diagenetic studies will be undertaken by Alan J. Kaufman at Harvard University, who has published extensively on Neoproterozoic isotopic chemostratigraphy , including reconnaissance studies in Namibia. Funding is requested to support the U-Pb and C isotopic analyses, and salary and travel to the field for Kaufman. The cost of field operations in Namibia and support for other personnel will be borne elsewhere.
