Abstracts of Annual Meeting of the Geochemical Society of Japan

Our research group is acquiring detrital-zircon-age spectra from Paleo-Mesozoic sandstones of Japan and Mongolia to reveal the pre-Cenozoic structural development of East Asia. In this talk, I will introduce the following three points: (1) the temporal change of detrital-zircon-age spectra of the sandstones from major Japanese geologic belts, (2) the temporal change of detrital-zircon-age spectra detected from the sandstones of Mongolia, and (3) my model of the pre-Cenozoic structural development of East Asia.

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Cs+ has been considered as an importnat radionuclide for performance assessment of high-level radioactive waste disposal. It is expected that migration of Cs+ in rock would be retared by sorption on clay minerals. Sorption of Cs+ on clay minerals, however, might compete with other cations such as NH4+ and K+ in groundwater. Therefore, it would be quite important to esimate the reaction controlling the NH4+ concentration in groundwater for evaluation of Cs+ migration in rock. In the present study, thermodynamic calculations were conducted to estimate the reaction controlling the NH4+ concentration in the Horonobe groundwaters. As the results, the Horonobe groundwaters were plotted on the mineral stability field of muscovite similar to illite which is a dominant clay mineral in the Horonobe sedimentary rocks, and additionally an activity ratios of H+ for NH4+ and K+ for groundwaters were plotted on the line of slope 1. These results suggest that the NH4+ concentration in the Horonobe groundwaters could be controlled by an ion exchange reaction between NH4+ and K+ on illite in the sedimentary rocks.

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Hydrochemical change of groundwater was evaluated based on the observation around large-scale underground facility in crystalline and sedimentary rocks.

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Coral cores from massive corals could record marine environmental and ecological changes in their annual bands with monthly temporal resolution in the present and/or the past. We discovered large massive Porites corals living at active volcanic island of Satsuma Io-Jima, located 50 km south from Kyushu area, southern part of Japan. Satsuma Io-Jima provides a unique opportunity to observe marine organism living under extreme environments of volcanic gases emission and different types of hydrothermal activities from sea flower. We collected eleven coral cores from four different conditions around the island to test if corals could record volcanic and hydrothermal activities and how corals could survive in extreme environments such as very low pH condition with CO2 emission. Coral extension rate for the site near hydrothermal vent was significantly small (1-2mm/year) relative to that for general condition of Porites corals (ca. 10-20 mm/year), suggesting that coral growth was influenced by hydrothermal activity. We will demonstrate our preliminary results of geochemical approaches of δ18O, δ1３C, Sr/Ca, Mg/Ca, Ba/Ca, and F/Ca in coral skeletons and in surrounding seawater and discuss the possibility for reconstructing the past hydrothermal events and relationship between marine ecosystem and extreme environments at volcanic activity as the analogues for coral adaptation to future ocean acidification.

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