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 delta C-13-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 delta C-13-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.

Automated summary

This study quantitatively assessed the cycling of dissolved inorganic carbon (DIC) in methane-rich marine sediments and found that methane is the main source of DIC, with varying sources of methane at different sites. The study also highlighted the influence of external thermogenic methane and the methanogenic zone on DIC cycling. Accurately assessing DIC cycling in methane-rich marine sediments is important for estimating their contribution to the oceanic carbon cycle.