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《Marine Geology & Quaternary Geology》 2018-05
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Authigenic minerals at site GMGS2-16 of northeastern South China Sea and its implications for gas hydrate evolution

ZHAO Jie;WANG Jiasheng;CEN Yue;SU Pibo;LIN Qi;LIU Jiarui;State Key Laboratory of Biogeology and Environmental Geology,China University of Geosciences;School of Marine Science,China University of Geosciences;Guangzhou Marine Geological Survey;National Institute for South China Sea Studies;  
The sulfate-methane transition zone(SMTZ)is a pivotal biogeochemical zone within an anoxic sediment column,in which anaerobic oxidation of methane(AOM)may affect the formation of authigenic minerals,such as carbonate,pyrite,barite and gypsum.In this paper,authigenic minerals in58 marine sediment samples from site GMGS2-16 at the gas hydrate drilling area in the northeastern South China Sea were analyzed.We investigated the type,content,distribution,morphology and isotopic composition of the authigenic minerals to explore the formation mechanism of them and their implications for methane events.Carbonate,pyrite and gypsum are the three dominant authigenic minerals at site GMGS2-16,and also found is the presence of elemental sulfur particles.The distribution of the authigenic mineral contents varies greatly and shows several high concentration peaks.The authigenic carbonates are formed in irregular clumps.These carbonates have extremely negative carbon isotope(-37.3‰VPDB~-51.7‰VPDB)and heavy oxygen isotope compositions(3.13‰VPDB~4.95‰VPDB),which are typical characteristics of methane-derived carbonates,suggesting that AOM is the trigger for carbonate precipitation.Authigenic pyrites are mostly in forms of irregular masses,rod-tube and organism-filling aggregates.The sulfur isotopic values(δ~(34)S)of the authigenic pyrites range from-41.7‰VCDT to 27.1‰VCDT.Anomalously positive excursions of sulfur isotopic values probably stem from enhanced-AOM in the SMTZ because of high methane flux.The AOM-derived authigenic carbonates are basically consistent with theδ~(34)S-anomaly authigenic pyrite production,indicating the occurrence of episodic methane events,which may be related to natural gas hydrate dissociation at site GMGS2-16.Authigenic gypsums occur as hyaline prisms more or less elongated and as dull prismatic or lenticular crystals,and some of them are attached to the authigenic pyrites.We tentatively propose that the precipitation of the authigenic gypsums results from ion exclusion during the formation of gas hydrates and/or pyrite oxidation caused by the change in environmental redox conditions.Hence,carbonate-pyrite-gypsum authigenic mineral assemblage in marine sediments may have the possibility to be used as a proxy for development of natural gas hydrate.
【Fund】: 广州海洋地质调查局外协专题“天然气水合物成矿区沉积物自生矿特征研究”(GZ20110030-50603);; 国家自然科学基金项目“海洋天然气水合物成藏海区沉积物中自生矿物对古硫酸盐-甲烷转换带的指示”(41472085) “孟加拉湾大陆边缘沉积物中自生黄铁矿及其古环境指示意义”(41772091);; 国家重点研发计划全球变化及应对重点专项“海洋储碳机制及区域碳氮硫循环耦合对全球变化的响应”下属课题“海洋沉积的碳氮硫效应和储碳机制”(2016YFA0601102);; 国家水合物专项项目“天然气水合物成矿理论及分布特征”(DD20160211)
【CateGory Index】: P618.13
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