Quantitative assessment of methane turnover in shallow surface sediments of hydrate-bearing areas in Shenhu area of South China Sea

Anaerobic oxidation of methane (AOM) is an important process in the carbon cycle in marine sediments, especially in hydrate-bearing areas. By quantifying the pattern of methane migration and conversion flux in surface sediments, we can assess the impact of carbon, derived from methane, onto sediment carbon pool and deep ocean carbon pool more accurately. In this research, the reaction-migration model is used to fit the SO₄²⁻, dissolved inorganic carbon (DIC), and Ca²⁺ concentration of pore water and the carbon isotope of DIC is analyzed simultaneously at SH-W19-PC and SH-W23-PC Station, which are two typical hydrate-bearing areas in the Shenhu area of South China Sea. The analyzed results reveal that, the concentrations of SO₄²⁻ and Ca²⁺ in the pore water of the two stations decrease linearly with depth, while the DIC concentration increases with depth. And the δ¹³C_DIC value of the DIC in the pore water is near -25‰, indicating methane activity in these two stations. The numerical results show that the flux of dissolved methane in the pore water of the two stations are 25.9 mmol·m⁻² a⁻¹ and 18.4 mmol·m⁻² a⁻¹, respectively. And the content of DIC produced by AOM accounts for 70.7% and 60% of the total DIC content. In addition, the DIC flux released from sediment to seawater is about 60% of the DIC sink. Therefore, methane leaking from the cold seep area and hydrate-bearing area enters the overlying seawater partly in the form of DIC. These methane-derived DIC with extremely negative carbon isotope values may have a certain impact on the local deep sea carbon pool.