Paleoenvironmental Analysis of Siliceous Sediments in Relation to Opening of the Japan Sea

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(1) We established analytical method to determine total sulfur content in marine sediments using a LECO-HF20 equipment. Measurements are seriously affected by a little amount of salts, particularly NaCl. It is, therefore, necessary to desalt the marine sediments before analyzing.(2) Total sulfur content (S) and total organic carbon content (C) in Neogene sediments of the Japan Sea, collected during ODP Leg 127, were analzsed. The C/S ratio of sediments reflects oxygen level in bottom water and it is about 3 in normal seawater sediments. The Leg 12 Neogene sediments show a wide range of the C/S ratio from 0.10 to 20. Late Miocene dark gray mudrocks have the minimum value of the C/S ratio, which indicates an anoxic environment due to isolation of the Japan Sea.(3) Quaternary sedimentary columns at sites 794, 795 and 797 is characterized by cyclic repetition of lighter-colored and darker-colored muddy sediments. The lighter-colored muds have larger values of the C/S ratio indicating oxyic environments. In contrast, the darker-colored muds show smaller values suggesting anoxyic conditions. This cyclicity accords with the cyclicity of Quaternary climatic change.(4) Evolution of Tertiary sedimentary basins in the circum-Japan Sea region with reference to opening of the Japan Sea is clarified on the basis of paleogeographical and paleobathymetrical maps of each interval from Eocene to latest Pliocene in age. The phenomena of the above items (2) and (3) can be interpreted by the evolution of the basins.