Modeling sulfur isotope fractionation and differential diffusion during sulfate reduction in sediments of the Cariaco Basin

Sulfur isotope composition (δ34S) profiles in sediment pore waters often show an offset between sulfate and sulfide (Δδ34SSO4–H2S) much greater in magnitude than S isotope fractionations observed in pure cultures. A number of workers have invoked an additional reaction, microbial disproportionation of sulfur intermediates, to explain the offset between experimental and natural systems. Here, we present an alternative explanation based on modeling of pore water sulfate and sulfide concentrations and stable isotope data from the Cariaco Basin (ODP Leg 165, Site 1002B). The use of unique diffusion coefficients for 32SO42- and 34SO42-, based on their unequal molecular masses, resulted in an increase in the computed fractionation by almost 10‰, when compared to the common assumption of equal diffusion coefficients for the two species. These small differences in diffusion coefficients yield calculated isotopic offsets between coeval sediment pore water sulfate and sulfide without disproportionation (up to 53.4‰) that exceed the largest fractionations observed in experimental cultures. Furthermore, the diffusion of sulfide within sediment pore waters leads to Δδ34SSO4–H2S values that are even greater than those predicted by our model for sulfate reduction with unique diffusion coefficients. These diffusive effects on the sulfur isotope composition of pore water sulfate and sulfide can impact our interpretations of geologic records of sulfate and sulfide minerals, and should be considered in future studies.