Explaining the Structure of the Archean Mass-Independent Sulfur Isotope Record

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Science  27 May 2010:
DOI: 10.1126/science.1190298


Sulfur isotopes in ancient sediments provide a record of past environmental conditions. The long-timescale variability and apparent asymmetry in the magnitude of minor sulfur isotope fractionation in Archean sediments remain unexplained. Using an integrated biogeochemical model of the Archean sulfur cycle, we find that the preservation of mass-independent sulfur is influenced by a variety of extra-atmospheric mechanisms including biological activity and continental crust formation. Preservation of atmospherically produced mass-independent sulfur implies limited metabolic sulfur cycling prior to ~2500 Ma; the asymmetry in the record requires that bacterial sulfate reduction was geochemically unimportant at this time. Our results suggest that the large-scale structure of the record reflects variability in the oxidation state of volcanic sulfur volatiles.