Sulfur Isotope Biogeochemistry Controlled by Sulfate Reduction Activity in Cold Seep Sediments

Abstract Cold seeps are unique deep‐sea ecosystems that play an important role in marine biogeochemical cycles. However, the dynamics and regulation of sulfur isotope biogeochemistry in seep sediments remain unconstrained. We investigated the geochemical characteristics of active seep sites, including the content and stable isotope composition of sulfur species and sulfate reduction (SR) rates. The S‐shaped downcore distribution of sulfate suggested non‐steady state diagenesis induced by lateral advection of venting fluids. 34 S‐enriched pyrite (−7.3‰) was found in surface sediment, suggesting the low sulfur isotope fractionation, calculated between 30 and 35‰. High estimates of cell‐specific sulfate reduction rates, 18–69 fmol cell −1 day −1 , together with data collected from previous studies, confirmed that elevated SR activity at seeps contributed to smaller sulfur isotope fractionation (11.5–35‰) than found typically in shelf sediments (44.5–66.5‰), leading to 34 S‐enriched pyrite (∼–10‰). These results provided insights into sulfur isotope biogeochemistry in cold seep sediments.