The formation of cold-seep carbonates is closely related to cold-seep activity, making them important archives for recording cold seep activity throughout geological history. Constraining the carbon chemistry of the fluid during carbonate precipitation (e.g., pH, dissolved inorganic carbon content) provides crucial insights into the carbon cycle of cold seep systems. The boron proxy system (including B/Ca ratios and boron isotopes) has been widely applied to reconstruct fluid carbon chemistry. However, due to the compositional complexity of cold-seep carbonates, research in this area remains limited. This study investigates cold-seep carbonates from the South China Sea through detailed mineralogical and geochemical characterization, accounting for the influences of detrital material, mineral phases, and boron sources on the boron proxy system to obtain reliable records of the carbon chemistry of fluids precipitating cold-seep carbonates. The reconstructed precipitating fluids exhibit characteristics of low pH (6.7 – 7.5) and high DIC concentrations (17 – 85 mmol/kg). The low pH is primarily attributed to the addition of CO₂ from methanogenesis and aerobic oxidation of methane (AeOM), while the high DIC concentration results from bicarbonate (HCO₃⁻) generated during anaerobic oxidation of methane (AOM). By integrating U-series dating of cold-seep carbonates, this study further constrains the evolution of the carbon chemistry of fluids precipitating cold-seep carbonates in the South China Sea since the Last Glacial Period, explores its relationship with environmental changes, and reveals the response mechanism of cold-seep carbonate precipitation to climate variability.

Carbon chemistry,Cold-seep carbonates,Boron isotopes,Uranium series dating