Sedimentary and redox controls on pyrite sulfur isotopes in oxic–suboxic continental shelf sediments

Abstract

Pyrite links sedimentary processes with the biogeochemical sulfur cycle and is widely used for reconstructing past redox and depositional environments. However, the controls on pyrite formation and its sulfur isotopic composition (δ³⁴S_pyr) in dynamic continental shelf settings, particularly under fluctuating oxic–suboxic conditions, remain poorly constrained. Here we investigate a late Pleistocene–Holocene sedimentary succession from the mud wedge off the Shandong Peninsula (Yellow Sea), which provides a high-resolution archive of glacial–interglacial environmental change, to resolve the relationship between δ³⁴S_pyr and shelf sedimentary evolution. We combine sulfur isotope measurements with redox-sensitive trace metals (U and Mo) and previously published sedimentological and geochemical data. Bulk δ³⁴S_pyr shows systematic variability associated with several depositional and diagenetic factors. First, δ³⁴S_pyr covaries with sedimentation rate, total organic carbon content, and water depth, indicating an important influence of depositional dynamics on pyrite formation and apparent isotopic fractionation. Second, more oxidizing conditions during the Holocene sea-level highstand coincide with generally higher δ³⁴S_pyr values, consistent with redox-related changes in sulfur burial. Third, localized sulfur cycling possibly related to anaerobic oxidation of methane may have contributed to elevated δ³⁴S_pyr in discrete intervals through isotopic mixing. These results indicate that bulk pyrite sulfur isotopes in oxic–suboxic continental shelf sediments primarily record sedimentary evolution and local diagenetic sulfur cycling, while redox-related variations in sulfur burial may further influence their stratigraphic pattern. This study provides a well-constrained example of δ³⁴S_pyr behavior in a non-euxinic shelf environment and emphasizes that depositional and early diagenetic processes must be explicitly considered when applying pyrite sulfur isotopes to paleoenvironmental reconstructions.