Temperature Controls on Carbon‐Sulfur Diagenesis in Deep Hydrothermal Subseafloor Sediments

Carbon mobilization and sulfur transformation play a significant role in deep carbon and sulfur cycling. However, sulfur biogeochemistry and its coupling with carbon and iron cycling remain poorly constrained in hydrothermal sediments. We investigated the effect of temperature on carbon‐sulfur‐iron diagenesis in subsurface sediments (≤370 m depth) of the Guaymas Basin, Gulf of California. Sediments have a low average carbon‐to‐sulfur ratio (∼1.6), especially at hydrothermal active sites (1.3–0.67). These values are well below the typical value of 2.8 for marine sediment and could reflect carbon release and relative sulfur enrichment. Elevated temperatures accelerate the thermal breakdown of sedimentary organic matter, leading to hydrocarbon expulsion, increased dissolved organic carbon release, and ultimately carbon loss. Elemental sulfur content correlates positively with reactive iron, indicating iron oxides facilitate elemental sulfur accumulation. These results highlight the temperature effect on carbon storage and iron redox chemistry control on sulfur dynamics in hydrothermal systems.