Quantitative assessment of dissolved inorganic carbon cycling in marine sediments from gas hydrate-bearing areas in the South China Sea

Quantitatively assessing dissolved inorganic carbon (DIC) cycling in methane-rich marine sediments is essential for understanding the contributions of different carbon sources to the global carbon cycle. Here, the data of δ13C-DIC, SO42−, DIC, Ca2+, Mg2+, PO43−, and a reactive transport model were used to analyze the DIC and methane source and calculate the DIC budget in the shallow sediments at two methane seep sites in the Xisha Trough (CL27A) and to the west of the Haima cold seeps in the Qiongdongnan Basin (CL44). Model results show that methane contributed to 92.75% and 79.95% of DIC sources through anaerobic oxidation of methane (AOM) versus only 7.25% and 12.90% by organic matter via organoclastic sulfate reduction (OSR) and methanogenesis at both sites. However, the methane sources vary between the two sites. External thermogenic methane that is decoupled from contemporaneous organic matter deposition drives the AOM at CL27A. A combination of upwardly diffusing external biogenic methane and internal methane derived from in situ methanogenesis drives the AOM at CL44. Internal methane flux from depth reveals the deep methanogenic zone influences the DIC cycling in the shallow sediments at CL44. It is accompanied by the upward diffusive DIC flux that can be identified by the δ13C-DIC data when modeling, which contributes to the remaining 7.15% of the DIC source. Neglecting this DIC source that is set as a fixed value at the lower boundary will misunderstand the DIC cycling in the shallow sediments. This study identified and highlighted the influence of external thermogenic methane and the methanogenic zone on the DIC cycling in marine sediments, and demonstrated assessing DIC cycling accurately in methane-rich marine sediments is important to estimate its contribution to oceanic carbon cycling. •The δ13C-DIC minimum approaching the δ13C of organic matter is related to external thermogenic methane.•Neglecting upward 13C-rich DIC flux will misunderstand the DIC cycling in shallow marine sediments.•The δ13C-DIC data can identify the upward 13C-rich DIC flux in modeling.