Beyond δ 34 S: Advances in understanding pyrite sulfur isotope systematics in modern marine sediments

Ongoing studies of sedimentary pyrite highlight pronounced sulfur isotope heterogeneity in pyrite from modern marine sediments, complicating reconstructions of geological atmospheric oxygen. Recent advances in multiple sulfur isotope analyses have substantially improved our understanding of sedimentary sulfur cycling. Here, we synthesize published datasets to evaluate how multiple sulfur isotopes record key processes in modern marine sediments and to provide a framework for interpreting geological sulfur isotope signatures. Both pyrite δ34S and Δ33S show considerable variabilities, controlled by sedimentation rate, sulfate availability, organic carbon accumulation, ocean redox fluctuations, and anaerobic oxidation of methane coupled to sulfate reduction. Compared with traditional δ34S measurements, multiple sulfur isotopes help constrain metabolic fractionation from diagenetic overprints, thereby narrowing the range of plausible interpretations. However, multiple sulfur isotope signatures are rarely diagnostic of a single sulfur-cycling process on its own. Instead, Δ33S is most useful when integrated with δ34S and independent geological evidence.