Chemical studies on venting fluids from submarine plate boundaries (KAIKO-TOKAI Project)

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Plate boundaries on the deep seafloor, such as mid-oceanic ridges and plate subduction zones, can be regarded as openings or windows through which much amount of chemical components comes up from or goes down to the earth interior chiefly by seawater convection. Since the venting fluids have quite different characteristiecs from ambient seawater, it is suggested that the fluids should bear significant parts of chemical flux between the ocean and the lithosphere. This study has paid attention to dissolved gas components, such as methane, helium, carbon dioxide etc. in venting fluids, because their concentrations and stable isotope ratios give us much useful information on the sources of the gases associated with pore fluid behaviors. As for the cold seepage at subduction zones, this study forms a part of Japan-France KAIKO-TOKAI Project which had been successfully conducted in the eastern Nankai Trough subduction zone between 1993 and 1997. We took many pore fluid samples and bottom seawaters for their chemical characterization. We have also promoted chemical investigations for submarine hydrothermal vents at various locations of mid-oceanic ridges (East Pacific Rise, Mid-Atlantic Ridge, Indian Ridge), back-arc basins (Okinawa Trough, Mariana Trough, Manus Basin), and arc volcanoes (Myojin Knoll). Accumulated data including time-series data have elucidated complicated chatacteristics of spatial and temporal variations of hydrothermal activity in the world. We have published 29 original papers on this project so far. These results of this study will play important role as preliminary databases for more detailed researches in the future.