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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Keywords:
- the Shenhu area /
- hydrate drilling area /
- gas hydrate /
- sedimentary environment
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致谢: 本次研究样品由广州海洋地质调查局于2007年在南海北部陆坡神狐海域实施“中国海域天然气水合物钻探”(GMGS1)航次钻探获取，感谢该航次科学家们为研究样品的采集付出的辛勤劳动；感谢中国科学院广州能源研究所分析测试中心、武汉上谱分析科技有限责任公司和中国科学院广州地球化学研究所为本文研究提供的测试；感谢两位评审专家提出的宝贵意见。
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图 1 南海北部神狐海域水合物钻孔分布图
本文研究的站位为SH3钻孔[4]
Figure 1. Map showing the locations of the sediment core in this study
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图 2 SH3钻孔沉积物中TN、TC、TS、TOC含量以及TOC/TN和TOC/TS的比值随深度变化曲线
Figure 2. The contents of TN, TC, TS, TOC, TOC / TN and TOC / TS with depth of the sediment in the SH3 core
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图 3 SH3钻孔沉积物中元素U、Mo的含量、U/Mo、U/Th和V/Sr随深度变化曲线
Figure 3. The changes in U, Mo, U / Mo, the ratios of U/Th and V/Sr with depth in the SH3 core sediments
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图 4 SH3钻孔沉积物Al/Ti、La/Sm、Zr/Y、LREE/HREE、MREE/HREE比值随深度变化的曲线
Figure 4. The curve of Al/Ti, La/Sm, Zr/Y, and LREE/HREE, MREE/HREE ratios with depth in SH3 core
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图 5 SH3钻孔孔隙水中SO42-离子浓度随深度的变化图(a)及SH3钻孔水合物饱和度随深度的分布(b)
(数据来自：GMGS1: Measuring the Concentration, Nature, and Distribution of Gas Hydrate, Geotek Field Report, 12 June, 2007)
Figure 5. Variation in SO42- concentration with depth in SH3 pore water (a), and hydrate saturation with depth distribution in the SH3 Drilling core (b)
(Data from GMGS1: Measuring the Concentration, Nature, and Distribution of Gas Hydrate, Geotek Field Report, 12 June, 2007)
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图 6 SH3钻孔所有样品在Cr/V-Y/Ni和Co/Th-La-Sc相图的分布
Figure 6. Distribution of Cr/V-Y/Ni and Co/Th-La/Sc phase diagrams for all samples in SH3 core
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图 7 SH3钻孔沉积物稀土元素UCC (Upper Continent Crust)标准化配分曲线
Figure 7. Comparison of the UCC (Upper Continent Crust) normalized distribution curve of the rare earth elements in the SH3 core sediments
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图 8 SH3钻孔中LREEN/HREEN和MREEN/HREEN混合模式图
(LREEN: LaN+ PrN+NdN; MREEN: SmN+ EuN+ TbN; HREEN: HoN+ YbN+ LuN)
Figure 8. Mixed diagram of LREEN/HREENand MREEN/HREEN
(LREEN: LaN+ PrN+NdN; MREEN: SmN+ EuN+ TbN; HREEN: HoN+ YbN+ LuN)
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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