Geochemical Characteristics of Sediments at Site HQ-48PC in Qiongdongnan Area,the North of the South China Sea,and Their Implication for Gas Hydrates
WU Lu-shan1,2,YANG Sheng-xiong2,LIANG Jin-qiang2,SU Xin1,YANG Tao3,ZHANG Xin2,CHENG Si-hai2,LU Hong-feng2(1.School of Marine Geosciences,China University of Geosciences,Beijing 100083,China;2.Guangzhou Marine Geological Survey, Guangzhou,Guangdong 510760,China;3.Center for Marine Geochemistry Research,State Key Laboratory for Mineral Deposits Research, Nanjing University,Nanjing,Jiangsu 210093,China)
The Qiongdongnan area is one of the most prospective areas of the gas hydrate exploration in the northern continental slope of the South China Sea.Headspace gas and pore water samples were collected from sediment core in Site HQ-48PC in this area.Comprehensive studies on methane concentrations in headspace gas,anions,cations and trace element contents in pore water indicate that strong anaerobic methane oxidation(AMO) occurres at the sulfate-methane interface(SMI),with the calculated depth of 6.05 mbsf by Linear Regression.For example,the concentration of dissolved sulfate(SO2-4) decreases linearly to zero,while that of methane increases abruptly;the concentrations of pyrite and organic carbon in sediments reach maximum value,and that of dissolved Ba2+ behaves as a Ba front character.The concentrations of alkalinity(HCO-3) increase clearly with the increasing of the depth above the SMI;those of Ca2+,Mg2+,Sr2+ show a decreasing trend,and the ratio of Mg2+/Ca2+ increases sharply.The main product of authigenic carbonates is calcite above the SMI.The concentration of HCO-3 decreases slowly with the increasing of depth below the SMI;and concentration of Mg2+,Sr2+ show the same trend with the increasing of depth above the SMI,but the concentration of Ca2+ shows little fluctuation,which results in a decreasing ratio of Mg2+/Ca2+.The main product of authigenic carbonates is dolomite in this region.The concentrations of phosphate(PO3-4) and ammonium(NH+4) in pore water of sediment core in Site HQ-48PC are high and increase clearly with depth;the increasing trend has an enantiomorphous relation to the decreasing trend of the concentration of SO2-4.These geochemical characteristics are similar to those of the shallow sediments where the gas hydrate occurs in the world,strongly indicating that there should be gas hydrate reservoirs underneath.
【Fund】： 国家重点基础研究发展计划项目(2009CB219508);; 国土资源部公益性行业科研专项(200811014);; 中国地质调查局基础调查项目(GZH200200201)
【CateGory Index】： P736.21
【CateGory Index】： P736.21