Evaluating the impact of methane flux on the trace element geochemistry of sedimentary pyrite: Insights from the South China Sea

Abstract

The trace element (TE) content of pyrite has been used as an archive of ocean and sediment pore fluid chemistry, depending on the respective formation conditions and potentially providing valuable insight into past atmospheric and oceanic conditions and sedimentary environments. The formation of authigenic pyrite is predominantly governed by organoclastic sulfate reduction (OSR) and sulfate-driven anaerobic oxidation of methane (SD-AOM). While methane flux significantly influences TE enrichment of marine sediments, its impact on the TE geochemistry of pyrite remains poorly understood. This study investigates the geochemistry of pore water, sediment, and handpicked pyrite from a gravity-piston core collected at the Haima seeps on the South China Sea continental slope. The current sulfate-methane transition zone (SMTZ) is located at approximately 200 cm below the seafloor (cmbsf), as indicated by sulfate penetration depth and the extremely negative δ¹³C values of dissolved inorganic carbon. The presence of a paleo-SMTZ in shallower sediment layers (above 200 cmbsf) is inferred from consistently low δ¹³C values (< −40 ‰) of carbonate nodules, suggesting higher methane flux during past seepage episodes. Relatively higher δ³⁴S values of pyrite (−35.5 ‰ to −6.4 ‰) relative to background OSR-derived pyrite and elevated TS/TOC ratios (0.36 to 1.94) relative to typical marine sediments throughout the core suggest that pyrite formation was predominantly driven by SD-AOM. While TE enrichment (e.g., Mo, U, As) in bulk sediment is confined to the upper 20 cmbsf, reflecting seawater-derived TE input under high methane flux conditions, pyrite TE content remains largely invariant across the current and the paleo-SMTZ. Such lack of variability probably results from a dilution effect of the locally enhanced pyrite formation and/or competing TE sequestration by organic phases. The absence of a correlation between seawater TE concentrations and pyrite TE contents challenges the use of SD-AOM-derived pyrite as a seawater chemistry proxy. However, consistently elevated Mo/Cd ratios in pyrite, independent of methane flux variations, serve as a robust discriminator between SD-AOM- versus OSR-derived pyrite. Our findings improve the understanding of TE dynamics in methane-rich sediments, refining the role of pyrite geochemistry as a paleoenvironmental proxy.