Methane seepage intensities traced by sulfur isotopes of pyrite and gypsum in sediment from the Shenhu area,South China Sea

查看全文 作　　者：ZHANG [1]Mei;LU [2]Hongfeng;GUAN [1,3]Hongxiang;LIU [1]Lihua;WU [1]Daidai;WU [3,4]Nengyou

机构地区：[1]Key Laboratory of Gas Hydrate,Guangzhou Institute of Energy Conversion,Chinese Academy of Sciences,Guangzhou 510640,China;[2]Guangzhou Marine Geological Survey,Guangzhou 510760,China;[3]Laboratory for Marine Mineral Resources,Qingdao National Laboratory for Marine Science and Technology,Qingdao 266061,China;[4]Key Laboratory of Gas Hydrate,Qingdao Institute of Marine Geology,Ministry of Land and Resources,Qingdao 266071,China

出　　处：《Acta Oceanologica Sinica》索引2018年第37卷第7期,共8页

基　　金：The Qingdao National Laboratory for Marine Science and Technology under contract No.QNLM2016ORP0210;the National Natural Science Foundation of China under contract Nos 41306061,41473080 and 41376076;the Scientific Cooperative Project by China National Petroleum Corporation and Chinese Academic of Sciences under contract No.2015A-4813 摘　　要：The northern slope of the South China Sea is a gas-hydrate-bearing region related to a high deposition rate of organic-rich sediments co-occurring with intense methanogenesis in subseafloor environments.Anaerobic oxidation of methane(AOM) coupled with bacterial sulfate reduction results in the precipitation of solid phase minerals in seepage sediment,including pyrite and gypsum.Abundant aggregates of pyrites and gypsums are observed between the depth of 667 and 850 cm below the seafloor(cmbsf) in the entire core sediment of HS328 from the northern South China Sea.Most pyrites are tubes consisting of framboidal cores and outer crusts.Gypsum aggregates occur as rosettes and spheroids consisting of plates.Some of them grow over pyrite,indicating that gypsum precipitation postdates pyrite formation.The sulfur isotopic values(δ^(34) S) of pyrite vary greatly(from –46.6‰ to –12.3‰ V-CDT) and increase with depth.Thus,the pyrite in the shallow sediments resulted from organoclastic sulfate reduction(OSR) and is influenced by AOM with depth.The relative high abundance and δ^(34) S values of pyrite in sediments at depths from 580 to 810 cmbsf indicate that this interval is the location of a paleo-sulfate methane transition zone(SMTZ).The sulfur isotopic composition of gypsum(from–25‰ to –20.7‰) is much lower than that of the seawater sulfate,indicating the existence of a 34 S-depletion source of sulfur species that most likely are products of the oxidation of pyrites formed in OSR.Pyrite oxidation is controlled by ambient electron acceptors such as MnO_2,iron(Ⅲ) and oxygen driven by the SMTZ location shift to great depths.The δ^(34) S values of gypsum at greater depth are lower than those of the associated pyrite,revealing downward diffusion of 34 S-depleted sulfate from the mixture of oxidation of pyrite derived by OSR and the seawater sulfate.These sulfates also lead to an increase of calcium ions from the dissolution of calcium carbonate mineral,which will be favor to the formation of gypsum.Overall,the mineralogy and sulfur isotopic composition of the pyrite and gypsum suggest variable redox conditions caused by reduced seepage intensities,and the pyrite and gypsum can be a recorder of the intensity evolution of methane seepage. 关键词：黄铁矿渗出物华南海硫同位素沉积甲烷石膏跟踪

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Methane seepage intensities traced by sulfur isotopes of pyrite and gypsum in sediment from the Shenhu area,South China Sea

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