Sedimentary pyrite sulfur isotopes track the local dynamics of the Peruvian oxygen minimum zone

Sulfur cycling is ubiquitous in sedimentary environments, where it mediates organic carbon remineralization, impacting both local and global redox budgets, and leaving an imprint in pyrite sulfur isotope ratios (delta S-34(pyr)). It is unclear to what extent stratigraphic delta S-34(pyr) variations reflect local aspects of the depositional environment or microbial activity versus global sulfur-cycle variations. Here, we couple carbon-nitrogen-sulfur concentrations and stable isotopes to identify clear influences on delta S-34(pyr) of local environmental changes along the Peru margin. Stratigraphically coherent glacial-interglacial delta S-34(pyr) fluctuations (>30) were mediated by Oxygen Minimum Zone intensification/expansion and local enhancement of organic matter deposition. The higher resulting microbial sulfate reduction rates led to more effective drawdown and S-34-enrichment of residual porewater sulfate and sulfide produced from it, some of which is preserved in pyrite. We identify organic carbon loading as a major influence on delta S-34(pyr), adding to the growing body of evidence highlighting the local controls on these records. To explore the importance of local vs. global sulfur-cycle controls on variations in pyrite sulfur isotopes, the authors couple carbon-nitrogen-sulfur concentrations and stable isotopes of sediments from the Peruvian oxygen minimum zone, identifying a major role for the local organic carbon loading.

Automated summary

The study demonstrates that variations in sulfur isotope ratios (δS-34(pyr)) in Peruvian margin sediments are influenced by Oxygen Minimum Zone intensification/expansion and organic matter deposition. Higher microbial sulfate reduction rates result in more effective drawdown and S-34-enrichment of residual porewater sulfate and sulfide, which is preserved in pyrite. These findings emphasize the importance of local environmental changes in shaping sulfur isotope ratios.