| 研究実績の概要 |
Five boreholes (10-27 m in length) were drilled into late Middle Pleistocene limestone (Minatogawa Formation) along a transect perpendicular to shore in southern Okinawa Island in December 2022 and January 2022. The total cored length is 90 m. After shipping the cores to the Kochi Core Center for safe storage, each core was X-ray CT-scanned. The cores were then split in half, photographed, and their lithology, sedimentary structures, and major fossil biotic components, particularly corals, coralline algae, and foraminifers, were carefully described in September 2022. Seven additional boreholes (8-23 m in length) were drilled in the same geological formation in August and September 2022. The total cored length is 98 m. Five of these boreholes were drilled along a second transect perpendicular to shore situated 800 m to the south-west of the first transect. One borehole was drilled between the first and second transect, and another borehole was drilled along the first transect. The seven new cores were shipped to the Kochi Core Center for storage in January 2023. Sampling of the cores of the first transect was conducted at the Kochi Core Center in February 2023 for sedimentological analysis, radiometric dating, taxonomic identification of corals, crustose coralline algae, and foraminifers, as well as elemental and stable isotope geochemistry. In addition, the corals observed in the cores of the first transect were identified at the lower taxonomic level possible and their status (in situ vs. not in situ) duly noted. The thickness of coralline algal crust was also recorded.
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| 今後の研究の推進方策 |
During FY2023, cores of the seven new boreholes which are stored at the Kochi Core Center will be X-ray CT scanned. Like for the other cores, the XCT scan images will provide information about their lithology, sedimentary structures, and fossil content. The cores will then be split in half. One half will be photographed and the other half will be sampled at a later date. A visual description of the cores will be conducted at the Kochi Core Center, with particular attention given to describing the different lithologies, sedimentary structures (e.g., discontinuities, paleosols), fossil content, particularly corals, coralline algae, and foraminifers. After the visual description of the cores is completed, a sampling party will be organized. Samples will be collected for sedimentological analysis, radiometric dating, taxonomic identification of corals, crustose coralline algae, and foraminifers, as well as elemental and stable isotope geochemistry. Coralline algae and foraminifers will be identified by Prof. Yasufumi Iryu (Tohoku University) and Prof. Kazuhiko Fujita (University of the Ryukyus), respectively. Data on lithologies, sedimentary structures, and fossil content of all the cores will be integrated to produce a model of facies architecture. In parallel, samples of fossil corals will be screened to identify well-preserved specimens that can be radiometrically dated. The geochemical analyzes of the carbonate samples will be conducted by a graduate student of Nagoya University.
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