MICROBIAL SULFUR CYCLING IN THE AFTERMATH OF NEOPROTEROZOIC GLACIATION

High precision measurements of ³³S and ³⁶S abundances in carbonate associated sulfate (CAS) from a Neoproterozoic cap carbonate of the Sete Lagoas Formation in south central Brazil were conducted on samples previously analyzed for δ³⁴S, δ¹³C, δ¹⁸O, and ⁸⁷Sr/⁸⁶Sr compositions. By Neoproterozoic standards, the limestone seafloor precipitates at the base of the section contain high abundances of CAS that decrease rapidly up the stratigraphic column where the sediments are predominantly composed of dolomicrite. Associated with declining sulfate concentrations is a strong positive δ³⁴S trend, which begins near +21‰ at the base and rises to as high as +47.5‰ at the top of the exposure. The extreme enrichment in ³⁴S abundances and depletion in sulfate concentrations is the expected consequence of bacterial sulfate reduction and extreme rates of carbonate accumulation in the glacial aftermath. The new measurements of ³³S and ³⁶S indicate that the post-glacial microbial system effected a small but significant degree of sulfide oxidation, likely associated with a disproportionation pathway. This result is consistent with the presence of initially oxygen stratified waters within the depositional basin; a view supported by carbon isotope results that suggest high primary productivity and production of oxygen in shallow marine environments. The linearly correlated sulfur isotopic trend between Δ³³S' and δ³⁴S' is characteristic of closed system isotopic fractionation that is likely associated with bacterial sulfate reduction operating within the sediments under an oxidized water column of rapidly decreasing sulfate abundance.