南海南部浅表层柱状沉积物孔隙水地球化学特征对甲烷渗漏活动的指示

海底沉积物孔隙水地球化学特征能快速响应甲烷渗漏活动及其生物地球化学过程，从而记录甲烷渗漏活动特征。对采自南海南部北康盆地的3个重力沉积柱状沉积物孔隙水样品（BH-H75、BH-H13Y和BH-H61）进行了甲烷浓度、溶解无机碳（DIC）和碳同位素（δ13CDIC）、阴离子（SO42−、Cl−）以及主微量元素（Ca2+、Mg2+、Sr2+、Ba2+）等地球化学分析。（△DIC+△Ca2++△Mg2+）/△SO42−比率图解与δ13CDIC深度剖面特征揭示了有机质硫酸盐还原反应（OSR）和硫酸盐驱动-甲烷厌氧氧化反应（SD-AOM）在不同沉积柱中所占比例的不同，其中BH-H13Y沉积柱中OSR和SD-AOM共同存在；BH-H75沉积柱中OSR占主导；在BH-H61沉积柱中SD-AOM占主导，且其底部可能存在微生物产甲烷作用。硫酸盐浓度线性拟合关系指示BH-H13Y的硫酸盐-甲烷过渡带（SMTZ）的深度约为700 cmbsf。结合SO42−浓度、DIC浓度最大值和δ13CDIC最小值推测BH-H61的SMTZ深度约为480 cmbsf。BH-H61和BH-H13Y沉积柱中，较浅的SMTZ深度、上升的DIC浓度以及强烈负偏的δ13CDIC值指示研究区存在甲烷渗漏活动。此外，在BH-H61和BH-H13Y站位，硫酸盐浓度随深度降低的变化梯度在沉积柱下部较上部陡，指示向上迁移的甲烷通量在时间上逐渐增强。孔隙水中Ca2+、Mg2+、Sr2+浓度以及Mg/Ca、Sr/Ca比值变化特征指示研究区沉积物中可能有自生高镁方解石矿物生成；而BH-H61站位SMTZ界面以下，孔隙水中Ba2+浓度升高，指示了硫酸钡的溶解作用。

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 (δ13CDIC), 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 δ13CDIC 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 δ13CDIC value, the SMTZ depth of BH-H61 is estimated at about 480 cmbsf. Sallower SMTZ depths, increasing DIC concentrations and highly negative δ13CDIC 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.