The study of Geochemical Processes in the trench Environment using trace constituent and isotopic techniques.

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The purpose of this study is to understand geochemical processes in super-deep trench environment, such as the Japan and Izu-Ogasawara trenches.Trenches exhibit characteristic topographic features of V shaped valley exceeding 7000 m depth, and are important zone for subduction of sedimentary materials. Similarly to isolated basins, the oceanographic environment inside trenches is considered to be different from that of typical ocean floor. However, systematic investigations of oceanography and geochemistry of the trench environment are few to date and largely remains "untouched". We have investigated by collecting samples of seawater, sediments and particulate matter from the deepest parts of the Japan and Izu-Ogasawara trenches for various analysis. For comparison, we have also carried out geochemical study in the Indian Ocean including the Sulu Sea, South China Sea and Andaman Sea. This year, we have further extended our research to the Japan Trench, Sea of Okhotsk and the Japan Sea and published a final report.Summary of the outcomes are ;1) We obtained the data on the vertical distributions of natural and man-made radionuclides and some heavy metals throughout the water column of the Japan and Izu-Ogasawara trenches for the first time, and published a paper in Oceanologica Acta,2) We published the results and their implications of rare earth elements in particulate matter collected by sediment trap in the Japan Trench in Earth and Planetary Science Letters,3) We continued to work on dissolved rare earth elements and In using ICP mass spectrometry and published the results and their implications in two papers in Geochimica et Cosmochimica Acta,4) We have obtained the data on Nd isotopes in surface seawater which can be interpreted by different sources, water circulation. The results will be published shortly.We hope that these results will stimulate further systematic and comprehensive study in near future.