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Date: Wednesday, April 18, 2012
Speaker: Alan J. Kaufman, University of Maryland
Topic: Pick A Snowball Fight: Sulfur and Clumped Isotope Constraints on the Snowball Earth Hypothesis
Time and Location: 1:00 PM, with Q&A to follow; in a 1st floor conference room at the American Center for Physics, 1 Physics Ellipse, College Park, MD-- off River Rd., between Kenilworth Ave. and Paint Branch Parkway.
Abstract: The most critical constraint on any model of the Snowball Earth is the source of alkalinity for the enigmatic carbonates that consistently cap Neoproterozoic glacial strata worldwide. In one end-member hypothesis the materials required for the caps are derived from the intense weathering of the continents under a CO2-charged atmosphere. The distinct lack of clay, particularly kaolinite, and silica in cap lithofacies poses problems for this extreme view. Alternatively, carbonate alkalinity may have formed through 1) the dissolution of pre-existing carbonate, or 2) through bacterial sulfate reduction in sediments or the water column using methane or other low molecular weight organic compounds as terminal electron acceptors. Water column anoxia - similar to the present-day Black Sea - is supported by the observation that many of the glacial diamictites are cemented with iron oxides; high concentrations of both iron and manganese are also present in post-glacial carbonates. Sulfur isotope compositions of sulfides, barites, and structurally bound sulfate in cap carbonate lithofacies reveal extreme positive δ34S excursions. It is envisioned that during the ice age, bacterial sulfate reduction in deep anoxic oceans would have caused 34S enrichment (and 31C ) in seawater as sulfide was removed as pyrite. In the aftermath, however, rising temperatures, dilution of sulfate-depleted seawater, and wind-driven upwelling likely resulted in the primary precipitation of cap dolostone. Continued 34S enrichment in anachronistic cap lithofacies (as high as +50 permil) is modeled as a sedimentary phenomenon during extreme rates of carbonate accumulation (in excess of bacterial sulfate reduction), assuming that the glaciation lasted longer than the precipitation of the caps. The most striking result of the model calculations concerns a limit on the magnitude of the δ34S fractionation between oceanic sulfate and bacterially reduced sulfide, irrespective of the initial oceanic sulfur mass. The notable carbon isotope anomaly in the caps can also be explained through extreme rates of carbonate precipitation. In contrast, the sulfur isotope anomaly is inconsistent with a continental source of alkalinity, or with the alkalinity that may have been produced during a sudden release of methane into post-glacial seas.
Biography: Alan J. Kaufman, who has been a Professor at the University for the past 15 years, has published over 80 research articles (cited over 5000 times) primarily on Precambrian Earth history and critical transitions throughout the Phanerozoic Eon. His research focuses on the determination of changes in the isotopic composition of the oceans through time, by the analysis of stratigraphic suites of little-altered carbonate rocks. Thus far, most of these studies have centered on Neoproterozoic (ca. 1000-544 million-year-old) sedimentary successions in Svalbard/East Greenland, Namibia, arctic Canada, Alaska and Siberia, India, Brazil, and the western USA. Temporal variations in C, S, and Sr isotopes can be used as stratigraphic tools within and between basins, and through detailed correlations allow us to order key tectonic, biogeochemial, and paleoenvironmental events in Earth history.