GEOCHEMISTRY OF THE SEDIMENTS IN SHENHU HYDRATE DRILLING AREA, NORTHERN SOUTH CHINA SEA
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摘要:
海底水合物形成分解/甲烷渗漏的甲烷以及相关的生物地球化学过程可能对海底的沉积环境产生影响，因此识别水合物的形成分解/甲烷渗漏对海洋沉积环境改造有助于了解水合物成藏特征及其形成分解过程。选取南海北部神狐海域2007年水合物钻探区的SH3钻孔沉积物为研究对象，对SH3钻孔岩心的碳硫数据、主微量元素，尤其是氧化还原敏感元素(U、Mo、U/Mo、V/Sr)进行分析测试，同时结合SH3钻孔孔隙水数据和前人对神狐水合物钻探区的研究成果等进行对比研究。结果表明南海北部神狐海域沉积物来源除河流沉积物以外，同时还有少量中国黄土以及大陆岛弧的长英质岩浆岩沉积物；通过对U、Mo、U/Mo以及碳硫数据分析，发现SH3钻孔在10~25 mbsf(meter below the seafloor)层位为硫酸盐驱动的甲烷厌氧氧化作用(Anaerobic oxidation of methane, AOM)造成的还原沉积环境，AOM作用导致了在这一层位发生了LREE/HREE、MREE/HREE的分馏；SH3钻孔沉积物在约180~215 mbsf的含水合物层位出现了浊流沉积的次氧化的沉积环境，同时其赋存的细粒沉积环境也导致了轻重稀土元素的分馏，与水合物饱和度存在一定的相关性。
Abstract:The formation and decomposition of gas hydrate as well as the methane leakage caused by associated biogeochemical processes may change the depositional environment of the seabed. To identify the decomposition of hydrate as well as the change in marine environment caused by the seeping methane will contribute to the understanding of the processes of hydrate formation and decomposition. In this paper, we selected the Core SH3 as a case from the Shenhu hydrate drilling area in the northern part of South China Sea to study the formation and deterioration of hydrate. Carbon and sulfur contents, main and trace elements, especially redox sensitive elements are measured. Data from pore water and previous researches are also studied. The study suggests that the sediments of the Shenhu area are mainly sourced from a fluvial origin with a little of loess and felsic igneous materials. From the data of U, Mo contents and U/Mo ratio, as well as carbon and sulfur contents, it is inferred that the layers at 10~25 mbsf in the core of SH3 is mainly formed in a reduced depositional environment, resulted from sulfate reduction by the anaerobic oxidation of methane. The change in redox condition of depositional environment may lead to the fractionation of LREE/HREE and MREE/HREE in the layers. In the SH3 core sediment, there is a sub-oxidized depositional environment at the level of about 180~215 mbsf. It is supposed to be caused by the deposition of turbidites. The depositional environment of fine-grained sediments, at the same time, also leads to the fractionation of light and heavy rare earth elements. To sum up, we have succeeded in revealing material source, environment change in the Shenhu area and the specific role of hydrate-sedimentary environment in this paper and the results are very useful for understanding of the origin of gas hydrate.
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Key words:
- the Shenhu area /
- hydrate drilling area /
- gas hydrate /
- sedimentary environment
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表 1 SH3钻孔主要稀土元素特征与主要构造环境对比
Table 1. Major provenance types and corresponding elemental characteristics used to define the tectonic setting of the SH3
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