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《Geochimica》 2017-06
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Variations in fluid sources and seepages archived in carbonate chimneys from the northern South China Sea

LIU Guan-yong;WANG Xu-dong;HUANG Hui-wen;SUN Yue-dong;LIANG Qian-yong;LI Niu;HU Yu;FENG Dong;CAS Key Laboratory of Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences;University of Chinese Academy of Sciences;MLR Key Laboratory of Marine Mineral Resources,Guangzhou Marine Geological Survey, China Geological Survey;Shanghai Engineering Research Center of Hadal Science and Technology, College of Marine Sciences, Shanghai Ocean University;Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology;  
Seep carbonates represent excellent archives of fluid sources and dynamics of seepage activities near the seafloor. Three carbonate chimneys recovered from Site 3 of the northern South China Sea were studied using mineralogical and geochemical analyses, including those of carbonate carbon and oxygen isotopes, sulfur isotopes of pyrite, and major element composition of the bulk samples. Chimney A is a pipe with an outer diameter of 16 cm and a central fluid passage diameter of 5 cm; chimney B is a cone with a bottom diameter of 8 cm; and chimney C is also a pipe with an outer diameter of 20 cm and a central fluid passage diameter of 12 cm. These chimneys all share a negative carbon isotope composition(δ~(13)CVPDB values as low as –55.7‰) and positive oxygen isotope composition(δ~(18)OVPDB values as high as 5.4‰). Methane represents the predominant carbon source of all carbonates. Incorporation of ~(18)O-enriched fluids from destabilization of gas hydrates is also suggested(chimney C). In addition, we report that the outer layer of each chimney generally bears higher δ~(13)C values, lower δ~(18)O values, and higher δ~(34)S values than the inner layers of the chimneys. These variations are presumed to reflect a gradual increase in the fluid seepage intensities during the formation of the chimneys, probably due to the limitation of the formation space from the outer layer to the inner layer of the chimneys. In summary, variations in the sources of fluids and seepage dynamics can be archived in carbonate chimneys.
【Fund】: 国家自然科学基金(41422602);; 国家水合物专项南海天然气水合物勘查与试采环境评价项目(GZH201100311);; 青岛海洋科学与技术国家实验室开放基金项目(QNLM2016ORP0204)
【CateGory Index】: P736.3
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