CHARACTER AND ORIGIN OF AUTHIGENIC CARBONATES IN THE WESTERN CONTINENTAL SLOPE OF THE EAST JAPAN SEA
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摘要: 对日本海西部大陆坡沉积物柱状样中的自生碳酸盐样品进行了X射线衍射、扫描电镜、地球化学和碳氧同位素组成的系统研究。X射线衍射和扫描电镜分析结果表明,碳酸盐主要组成矿物为颗粒状自生高镁方解石微晶,放射状自生文石微晶仅在一个层位出现。结合碳酸盐的地球化学组成,认为研究区碳酸盐来自于富Ca2+、Mg2+和HCO3-流体的沉淀。中度亏损的13C (-33.85‰~-39.53‰)表明碳来自于甲烷的厌氧氧化,同时,这也是研究区海底存在甲烷冷泉的重要证据。重氧同位素比值(5.28‰~5.31‰)则指示着富18O流体来源,而该流体应源于天然气水合物的分解。综上可知,研究区碳酸盐来自于研究区甲烷冷泉上升流的沉淀,指示着海底更深处天然气水合物的存在与分解。Abstract: Authigenic carbonates collected from a sediment core on the western continental slope of East Japan Sea have been synthetically studied on X-ray diffraction (XRD), scanning electron microscope (SEM), geochemistry, and carbon and oxygen isotope. The analysis results on XRD and SEM show that carbonate minerals are mainly composed of authigenic high-Mg calcite (HMC) in grain shape, while authigenic elongate aragonite only appears in one layer. Combining the analysis on geochemical composition, we deduce that the studied carbonates come from fluids enriched in Ca2+, Mg2+, and HCO3-. The moderately depleted 13C (-33.85‰~-39.53‰) reflects that carbon should be mainly derived from the anaerobic oxidation of methane, which is also an evidence for gas venting in the local seafloor. The heavy 18O values (5.28‰~5.31‰) should be closely related with 18O-rich fluids, resulting from the decomposition of gas hydrate. Therefore, the studied carbonates should precipitate from upward cold seep fluids enriched in methane, indicating the occurrence and dissociation of underlying gas hydrate.
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Key words:
- authigenic carbonate /
- mineralogy /
- geochemistry /
- carbon and oxygen isotope /
- gas hydrate
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