Strong biological effects on sulfur reduction in modern pore water and their implications to pyrite sulfur isotopes in geological record

The S isotopic composition of sedimentary or diagenetic pyrite is an important geochemical proxy for the global S cycle and the redox evolution in the Earth’s past. Recent studies indicate that the pyrite S isotopes are controlled by various global or local extrinsic (environmental) factors, yet it remains unclear to what extent the pyrite S isotopes could be affected by some intrinsic (biological) factors, including the reaction rate constant of dissimilatory sulfate reduction (DSR, R_DSR) and the fractionation factor of DSR (Δ_DSR). Although these intrinsic factors could be determined in laboratory culture experiments, DSR in natural environments is also affected by complex biological and chemical environments. As such, in situ measurements of Δ_DSR in sediment porewater would provide more realistic estimates of isotopic fractionation in DSR. In this study, we apply the diffusion-advection-reaction (1D-DAR) model to simulate diagenetic pyrite formation in modern marine sediments, so as to quantify the intrinsic factors including sulfate reduction rate and Δ_DSR. By using the porewater geochemical profiles of four sediment drill-cores from Santa Barbara Basin and Bornholm Basin, we observe large ranges of variations in RDSR, Δ_DSR and reaction rate of H₂S oxidation. The model results indicate that variations of intrinsic factors can result in large pyrite S isotope excursions up to 56‰, suggesting the intrinsic factors could also cause large spatial or temporal variations of pyrite S isotopes of sediments or sedimentary rocks.