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Project Progress

The objective is to document and interpret the longer-term history of the Proterozoic ocean. We have made great progress in this project, having now produced sulfur isotope records for the Neoproterozoic of Australia, Namibia and the western U.S. We have found at least four major positive excursions in seawater sulfur isotopic composition, two of which are closely associated with "cap carbonates" following glacial epochs [pub: Hurtgen et al., in press, EPSL]. We have introduced a new model for the evolution of Proterozoic climate that involves enhanced methane fluxes to the atmosphere during a prolonged period of low oceanic sulfate concentration followed by decreased methane fluxes and a reduced methane-greenhouse effect in the late Proterozoic as the result of rising oceanic sulfate concentration. This ultimately led to the first Snowball Earth episode [pub: Pavlov et al., submitted, Geology]. We have also constructed a model for the marine nitrogen cycle that predicts that a major increase in nitrogen isotopic values of oceanic DIN should have occurred at the Archean/Proterozoic boundary.