Anomalous mercury isotopic composition highlights the essential role of terrestrial erosion in forming cold seeps in the continental margin of the South China Sea

Cold seeps are often observed along continental margins and are key sources of methane emitted into the atmosphere, yet the role of terrestrial erosion in their formation remains poorly constrained. Mercury isotopes display mass-independent fractionation during aqueous Hg(II) photoreduction, producing negative Δ 199 Hg values in terrestrial soil and positive Δ 199 Hg in marine sediments. Here, we study the Hg concentration and isotopic composition in a 230-meter-long sediment core (GMGS2-16) from a cold seep site in the continental shelf of the northern South China Sea (Dongsha area). This core shows overall negative Δ 199 Hg values (−0.26‰ to 0.03‰), consistent with dominant terrestrial Hg input. The sulfate-methane transition zones (SMTZs) display Hg peaks and negative Δ 199 Hg excursions, supporting enhanced terrestrial influx during the formation of SMTZs. The SMTZs are associated with high chromium-reducible sulfur (CRS), high molybdenum enrichment factor (Mo EF ) and positive δ 98 Mo excursions. Integrating these geochemical proxies, we propose a mechanistic framework: enhanced terrestrial erosion delivers large quantities of Hg and sediment to the study area, increasing sedimentation rates and facilitating upward migration of methane-rich cold-seep fluids. These fluids fuel SD-AOM in SMTZs, reducing seawater sulfate to sulfide and establishing anoxic-euxinic conditions, which promote authigenic pyrite formation and Hg sequestration. This study demonstrates that Hg isotopes can serve as a novel tool for identification of SMTZs, and highlights the previously underappreciated role of terrestrial erosion in regulating cold seep formation.