南海东沙海域末次冰期异常沉积事件与水合物分解

Abnormal Sedimentary Events and Gas Hydrate Dissociation in Dongsha Area of the South China Sea during Last Glacial Period

对南海东沙海域陆坡区973-3柱状样开展沉积学和年代学、有孔虫同位素和壳体B/Ca比值、碳酸钙和黄铁矿含量的分析,发现该岩芯浮游有孔虫Globigerinoides ruber和底栖有孔虫Uvigerina peregrina的δ13 C在末次冰期同时在多个层位发生负偏,最大负偏达-2.03‰;有孔虫δ13 C负偏层位环境[CO32-]浓度相对偏低;负偏层位同时出现大量黄铁矿,最高含量达17%.在负偏层位以下沉积的碳酸盐溶解作用强烈,CaCO3含量最低,沉积物颜色偏深.究其成因与水合物分解关系较大.甲烷渗漏事件发生在末次冰期,说明末次冰期的海平面下降是水合物分解的主要诱因.根据有孔虫δ13 C负偏的次数和程度推断至少发生过4次甲烷渗漏,渗漏的强度基本相当.

Core 973-3 is located at the potential gas hydrate area on the slope within Dongsha area of the South China Sea,with the water depth of 1026 m.According to the sedimentology,chronology,isotope and B/Ca ratio of the foraminiferal tests,carbonate and pyrite content,the light δ^13 C values of planktonic Globigerinoides ruber and benthic Uvigerina peregrine are found in several core layers with the most negative value of-2.03‰.In the δ^13 C distinctly depleted layers,the concentration of environmental[CO3^2-]is relatively low,and the pyrites are abundant up to 17%,which indicates abnormal seepage of methane.Below the δ^13 C depleted layers,the carbonate intensely dissolves,and its content decreases to the bottom.It suggests that it has probably been caused by the gas hydrate dissociation.The seepage and anaerobic oxidation of methane resulted from the dissociation and release of gas hydrates is the primary reason of the depleted δ^13 C values of foraminiferal tests,carbonate dissolution and the deposits of enormous pyrites.Methane release events occurred in the last glacial period,which implies that the descending sea level induced the gas hydrate dissociation.According to the δ^13 C depleted record of foraminiferas,it is concluded that there were at least four times release events of methane,and their intensities were almost at the same level.