Sedimentological Study of the Relation between the Decomposition and Maturation of Organic matter and Formation of Authigenic Minerals during Early Stage of Diagenesis

本研究课题包括两大主题。一是沉积物中有机质的来源及含量波动机制;通过沉积学和地球化学技术，包括沉积物结构和结构分析，以及常量、微量和稀土元素地球化学分析，探讨了天然气水合物与自生碳酸盐岩形成的成因关系。第二个主题是天然气水合物的性质、控制自生碳酸盐碳氧同位素组成变化的因素以及沉积序列中与天然气水合物相关的现象。天然气水合物的重要性长期以来被地质学家们所忽视或低估，但目前的研究强调了天然气水合物在沉积物中的潜在重要性， ...更多信息 如果生物甲烷的产率大到足以使间隙水相对于甲烷过饱和，那么在一定的压力-温度条件下，“过量甲烷”可以形成冰状甲烷水合物。天然气水合物相对于环境水的浓度大于80。180的间隙水的耗尽的总体趋势可以解释为水合物形成的结果。在进一步的埋藏过程中，甲烷水合物最终会分解，并为沉积物提供大量的甲烷和富含180的水。碎屑三价铁被还原，甲烷被“氧化”（厌氧氧化），然后间隙水的碱度预计将急剧增加。这一系列的反应导致重氧、含铁碳酸盐的沉淀，通常是菱铁矿。本研究从四国近海和美国东南部的几个海底岩芯样品和青森、山形县新近纪硅质岩中发现了这种不寻常的菱铁矿。少

This research porject includes two major thema. One is the origin of organic matter in sediments and the mechanism of fluctuation of the content. The other is the genetic relation between gas-hydrate and formation of authigenic carbonates.The mechanism of the concentration and dispersion of organic matter (organic carbon) in sediments was investigated using the sedimentological and geochemical techique including sedimentary structure- and texture-analyses and the major, minor, and rare earth element geochemistry. Results of this theme was reported last year and published in the University Journal.The second theme was focused on the nature of natural gas hydrate, factors controlling the variation of carbon and oxygen isotopic composition of authigenic carbonates, and gas-hydrate related phenomena in sedimentary sequence. The importance of gas hydrate had long been ignored or minimized by geologists, but the present study has stressed the potential importance of gas hydrate both in sedim … More entary and diagenetic processes.If the production rate of biogenic methane is large enough to make the interstitial water supersaturated with respect to methane, the "excess methane" could form ice-like methane hydrate under a certain pressure-temperature condition. Gas hydrate concentrates more 80 relative to ambient water. General trend of the depletion of 180 of the interstitial water may be interpreted as the result of hydrate formation. Upon further burial, methane hydrate would finally decompose and supply the sediments with large amounts of methane and 180-enriched water. Detrital ferric iron is reduced, and methane is "oxidized" (anaerobic oxidation), then alkalinity of the interstitial water is expected to increase dramatically. This seriese of reaction leads to the precipitation of heavyoxygen, Fe-bearing carbonated, usually siderite.The present study found such unusual siderite from several under-the sea core samples taken from the continental margin offshore Shikoku and SE America and the Neogene siliceos rocks in Aomori and Yamagata Prefectures. Less