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其含量波动的机制。二是天然气水合物与自生碳酸盐形成的成因关系。运用沉积学和地球化学技术，包括沉积构造和结构分析，以及主、次、稀土元素地球化学，研究了有机质(有机碳)在沉积物中的聚集和分散机制。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一直被地质学家忽视或低估，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。如果生物甲烷的产生率足够大，使间隙水相对于甲烷达到过饱和，那么在一定的压力-温度条件下，过剩的甲烷可以形成冰状的甲烷水合物。与周围水相比，天然气水合物的浓缩度超过80%。180%的间隙水耗竭的总趋势可以解释为水合物形成的结果。在进一步埋藏后，甲烷水合物最终会分解，并为沉积物提供大量甲烷和富含180的水。碎屑铁被还原，甲烷被“氧化”(厌氧氧化)，那么间隙水的碱度预计会急剧增加。这一系列的反应导致了重氧、含铁碳酸盐，通常是菱铁矿的沉淀。本研究从四国和美国东南部大陆边缘的几个海底岩心样品以及青森县和山形县的新近系硅质岩中发现了这种不寻常的菱铁矿。较少

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