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

深海海底的板块边界，如大洋中脊和板块俯冲带，可以看作是大量化学成分主要通过海水对流从地球内部升降到地球内部的开口或窗口。由于喷发流体的性质与周围海水有很大的不同，因此认为喷发流体应承担大洋和岩石圈之间的化学通量的重要部分。由于甲烷、氦、二氧化碳等气体组分的浓度和稳定的同位素比值为我们提供了许多与孔隙流体行为有关的气体来源的有用信息，因此本研究关注了排气流体中的甲烷、氦、二氧化碳等溶解气体成分。至于俯冲带的冷渗漏问题，本研究是1993-1997年在南海海槽东部俯冲带成功实施的日法开海东海项目的一部分。我们采集了大量的孔隙流体样品和海底海水进行了化学表征。我们还推动了对大洋中脊(东太平洋隆起、大西洋中脊、印度洋海脊)、弧后盆地(冲绳海槽、马里亚纳海槽、马努斯盆地)和弧形火山(Myojin Knoll)不同地点海底热液喷口的化学研究。包括时间序列数据在内的累积数据揭示了全球热液活动时空变化的复杂特征。到目前为止，我们已经发表了29篇关于这个项目的原创论文。本研究的这些结果将为今后更详细的研究提供初步的数据库。

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.