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 SO₄^2-, 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 SO₄^2-, 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.