Sedimentary pyrite d^(34)S differs from porewater sulfide in Santa Barbara Basin: proposed role of organic sulfur
Santa Barbara Basin sediments host a complex network of abiotic and metabolic chemical reactions that knit together the carbon, sulfur, and iron cycles. From a 2.1 m sediment core collected in the center of the basin, we present high resolution profiles of the concentrations and isotopic compositions of all the major species in this system: sulfate, sulfide (H_2S) elemental sulfur (S^0) pyrite, extractable organic sulfur (OS) proto kerogen S, total organic and dissolved inorganic carbon, and total and reducible iron. Below 10 cm depth, the core is characterized by low apparent sulfate reduction rates <0.01 mM/yr) except near the sulfate methane transition zone. Surprisingly, pyrite forming in shallow sediments is ~30%0 more (34)S depleted than coexisting H_2S in porewater. S^0 has the same strongly (34)S depleted composition as pyrite where it forms near the sediment water interface, though not at depth. This pattern is not easily explained by conventional hypotheses in which sedimentary pyrite derives from abiotic reactions with porewater H_2S or from the products of S^0 disproportionation. Instead, we propose that pyrite formation in this environment occurs within sulfate reducing microbial aggregates or biofilms, where it reflects the isotopic composition of the immediate products of bacterial sulfate reduction. Porewater H_2S in Santa Barbara Basin may be more (34)S enriched than pyrite due to equilibration with relatively (34)S enriched OS. The difference between OS and pyrite d^(34)S values would then reflect the balance between microbial sulfide formation and the abundance of exchangeable OS. Both OS and pyrite d34S records thus have the potential to provide valuable information about biogeochemical cycles and redox structure in sedimentary paleoenvironments.