Quantifying the flux of dissolved inorganic carbon in surface sediments of the Haima cold seep area, northern South China Sea

Quantifying the contribution of different carbon sources to dissolved inorganic carbon (DIC) flux in cold seep environments is critical for understanding the global carbon cycle. Pore water geochemical compositions provide insights into the biogeochemical processes of different DIC sources at cold seeps. Here, δ¹³C_DIC values as well as SO₄^2–, DIC, Ca²⁺, Mg²⁺, and PO₄^3– concentrations of three push cores were analyzed with a reactive transport model to distinguish the DIC sources and calculate the DIC budget in the shallow sediments of the Haima cold seeps. The shallow depths of the sulfate methane transition zone (SMTZ) indicate significant methane flux and anaerobic oxidation of methane (AOM). The model results confirm that AOM is the primary biogeochemical process consuming sulfate at three sites, accounting for 99.5%, 91.5%, and 52.1%, respectively. The Sr/Ca vs. Mg/Ca ratio shows that high-Mg calcite precipitation occurred at ROV4 and ROV5 sites, while the carbonated phase precipitating at ROV01 site was 73.8% aragonite accompanied by 26.2% of high-Mg calcite. Moreover, extremely low δ¹³C_DIC values indicate the presence of deep-sourced biogenic methane at the three sites. Based on the δ¹³C mass balance, the contribution of methane to DIC by AOM and methanogenesis is 99.6%, 95.4%, and 62.1%, respectively. Thus, methanogenesis is another primary source of DIC at the Haima cold seeps. Our study documents the influence of deep-sourced methane and methanogenesis on DIC flux at seeps and demonstrates that the DIC budget of seep sediments is a major contributor to the marine carbon pool and the marine carbon cycle.