Multiple sulfur isotopes discriminate organoclastic and methane-based sulfate reduction by sub-seafloor pyrite formation

The marine sulfate inventory represents the largest standing pool of electron acceptors, which, via microbial reduction, is responsible for roughly half organic matter mineralization globally in sediments. In addition to oxidation buried matter, reduction can be coupled methane migrating upward. Multiple sulfur isotope ratios were measured porewater sulfate, sulfide, elemental and pyrite from core samples collected continental shelf (19–96 m water depth) Baltic Sea slope (1098 Andaman Sea, order test if their systematics used trace two different processes. For shallow sediments (<43 cm below seafloor), both ?34S ?33S values increased with increasing depth as a result organoclastic (OSR). However, sulfide decreased downcore at 43–73 where was anaerobic (AOM). Pyrite yielded positive values, reflecting produced by OSR. contrast, negative low ?0.15‰, mixing sulfides derived OSR AOM. This consistent slowly sulfate–methane transition (SMT) inferred iron speciation, carbon carbonates, magnetic susceptibility data. A stable SMT, focusing AOM same over past few millennia, further provides favorable conditions prolonged formation development cubic crystals, while difference between bulk macroscopic reflects significance throughout sediment column. We conclude that minor isotopes provide unique proxy applied differentiate origin diverse sedimentary rocks. contribution highlights strong local diagenetic controls on its multiple isotopic composition various settings.