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

本研究项目包括两大主题。一是沉积物中有机质的来源及其含量波动机制。二是天然气水合物与自生碳酸盐形成的成因关系。采用沉积地球化学、构造分析、主、次、稀土元素地球化学等沉积学和地球化学方法研究了沉积物中有机质（有机碳）的富集和分散机理。这一主题的研究结果已于去年发表在《大学学报》上。第二个主题是天然气水合物的性质、自生碳酸盐碳氧同位素组成变化的控制因素以及沉积层序中天然气水合物的相关现象。长期以来，天然气水合物的重要性一直被地质学家忽视或低估，但本研究强调了天然气水合物在沉积、沉积和成岩过程中的潜在重要性。如果生物甲烷的生产速率足够大，使间隙水相对甲烷过饱和，那么在一定的压力-温度条件下，“多余的甲烷”可以形成冰状甲烷水合物。天然气水合物的浓度比周围的水高80%。间隙水枯竭的总趋势可以解释为水合物形成的结果。经过进一步的埋藏，甲烷水合物最终会分解，为沉积物提供大量的甲烷和富含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重氧、含铁的碳酸盐沉淀，通常是菱铁矿。本研究从四国和东南美洲大陆边缘以及青森县和山形县新近纪硅岩的几个海底岩心样本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