Pore water geochemistry of shallow surface sediments in the southern South China Sea and its implications for methane seepage activities

The geochemical characteristics of pore water in seabed sediments may quickly respond to the changes in the methane seepage and related biogeochemical processes. In this paper, methane, DIC and its carbon isotope value (δ¹³C_DIC), anions (SO₄²⁻, Cl⁻), major and trace elements (Ca²⁺, Mg²⁺, Sr²⁺, Ba²⁺) are analyzed for the pore water samples (BH-H75, BH-H13Y and BH-H61) collected from the Beikang Basin in the southern SCS. The (△DIC+△Ca²⁺+△Mg²⁺)/△SO₄²⁻ ratios and δ¹³C_DIC show that organoclastic sulfate reduction (OSR) and sulfate-driven anaerobic oxidation of methane (SD-AOM) vary from different columns. For the column of BH-H13Y, OSR and SD-AOM occur together. However, OSR is dominant in column BH-H75, while SD-AOM dominates the BH-H61 column. There may be microbial methanogenesis at the deeper layer in the BH-H61 column. Based on the linear fitting sulfate concentrations, the sulfate-methane transition zone (SMTZ) of BH-H13Y is estimated to be about 700 cmbsf. According to SO₄²⁻ concentrations, the maximum DIC concentration and the minimum δ¹³C_DIC value, the SMTZ depth of BH-H61 is estimated at about 480 cmbsf. Sallower SMTZ depths, increasing DIC concentrations and highly negative δ¹³C_DIC values recorded in BH-H61 and BH-H13Y columns suggest a remarkable methane seepage in the study aera. The gradients for sulfate concentrations of lower part of BH-H61 and BH-H13Y columns are steeper than that of the upper part, indicating that the methane flux upward migration increases with time. Features of Ca²⁺, Mg²⁺ and Sr²⁺ concentrations and Mg/Ca and Sr/Ca ratios in pore water indicate the possibility of the formation of high-Mg calcite. Below the SMTZ interface at BH-H61 column, Ba²⁺ concentrations increase with depth, indicating the barium sulfate dissolution occurs.