Organic geochemical study on the degradation processes of organic matter during early diagenesis: A case study in the sediments from the continental shelf and trough off Niigata

摘要

Various organic constituents were analyzed within Holocene sediments collected from 3 sites on the continental shelf off Niigata and the Mogami trough. The accumulation and degradation process of organic matter is discussed based on the distribution of the organic constituents.The organic carbon. nitrogen. amino acid-carbon and carbohydrate-carbon contents in the core 188 sediments (Mogami trough) were two to six times higher than those in the cores 126 and KI-3 sediments (continental shelf off Niigata). The C N ratios show a decreasing trend with distance from the land (corel26 > core KI-3 > core 188). The ratios of amino acid-carbon plus carbohydrate-carbon to total organic carbon are larger in core 188 compared to core KI-3. Unstable chlorophyll a and b were detected in core 188. The relatively stable pheo compounds were detected in core KI-3.Whithin the Mogami trough, the sedimentary conditions were probably suitable for the supply and preservation of organic matter at low temperatures. In contrast. in the continental shelf off Niigata. the organic matter may have been diluted by the addition of detrital matter supplied from rivers. with a large fraction decomposed due to the relatively high temperature of the bottom water and the high porosity of the sediments. Organic matter in core 188 is mostly marine in origin whereas the organic matter in core KI-3 is derived mainly from terrestrial higher plants.The decomposition rate constants for organic matter are as follows; total organic carbons < total carbohydrate-carbons < total amino acid-carbons in core 188 and KI-3. The decomposition rate constants for organic matter in core KI-3 are two to three times larger than those in core 188. This indicates that the decomposition of organic matter is dependent on the depositional environment between the trough and continental shelf.