南海北部东沙海域水合物区浅表层沉积物的地球化学特征

对采自南海北部陆坡东沙海域水合物区HD319、HD196A和GC10站位的浅表层沉积物进行了地球化学特征研究。X射线衍射分析和扫描电镜观察表明,沉积物中存在具有天然气水合物和甲烷渗漏指示意义的自生碳酸盐、硫酸盐和黄铁矿。沉积物孔隙水化学组分分析显示,随着埋藏深度加深,SO42-、Ca2+、Mg2+和Sr2+浓度明显降低,CH4、H2S浓度的增加,以及Mg2+/Ca2+和Sr2+/Ca2+比值急剧增加,与世界上水合物区浅表层沉积物孔隙水中离子浓度异常特征吻合。沉积物顶空气游离烃分析结果和孔隙水化学组分变化,特别是SO42-、H2S和甲烷含量的急剧变化,说明研究区有丰富的气源,赋存水合物的可能性非常大,同时指示了研究区硫酸盐-甲烷界面(SMI)较浅,位于海底之下8 m左右。

GEOCHEMICAL CHARACTERISTICS OF SHALLOW SEDIMENTS IN THE GAS HYDRATE DISTRIBUTION AREA OF DONGSHA,THE NORTHERN SOUTH CHINA SEA

Geochemical characteristics of the shallow sediments from sites HD319, HD196A and GC10 in Dongsha gas hydrate distribution area, the northern slope of South China Sea was discribed in the present paper. Complicated authigenic minerals were identified by XRD and SEM, such as miscellaneous carbonates, sulphates and pyrite, which consistently indicate the existence of gas hydrate and methane seep in the research area. Chemical analysis of pore water from the sediments showed that the concentrations of SO42-, Ca2+, Mg2+ and Sr2+ decrease clearly,whereas CH4 and H2S as well as the ratios of Mg2+/Ca2+ and Sr2+/Ca2+ increase sharply as the depth increases. These geochemical characteristics are similar to the chemical compositions abnormalities in pore water of the shallow sediments,where gas hydrate forms in the other parts of the world. There should be a great amount of gas sources in the research area,based on the occurrence of free hydrocarbon in the headspace gas and the distribution pattern of chemical compositions in the sediments, specially the concentrations of SO42-, H2S and CH4 which changed sharply with depth. Furthermore, the results strongly suggest that there should be gas hydrates underneath, and the sulphate-methane interface (SMI) should be shallow, about 8m below the seafloor in the research area.