230Th/234U and 14C dating of a lowstand coral reef beneath the insular shelf off Irabu Island, Ryukyus, southwestern Japan |
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| Authors: | Keiichi Sasaki Akio Omura Tetsuo Miwa Yoshihiro Tsuji Hiroki Matsuda Toru Nakamori Yasufumi Iryu Tsutomu Yamada Yuri Sato and Hiroshi Nakagawa |
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| Institution: | Department of Cultural Properties and Heritage, Kanazawa Gakuin University, 10 Sue-machi, Kanazawa 920-1392, Japan (email: ),;Department of Earth Sciences, Kanazawa University, Kanazawa 920-1192, Japan,;Technology and Research Center, Japan Oil, Gas and Metals National Corporation, 1-2-2 Hamada, Mihama-ku, Chiba 261-0025, Japan,;Department of Earth Sciences, Kumamoto University, Kumamoto 860-8555, Japan,;Institute of Geology and Paleontology, Tohoku University, Sendai 980-8578, Japan,;Geowindow Co. Ltd., 5-7-4 Tamagawa, Chofu, Tokyo 182-0025, Japan |
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| Abstract: | Abstract High-resolution seismic reflection profiles delineated the distribution of mound-shaped reflections, which were interpreted as reefs, beneath the insular shelf western off Irabu Island, Ryukyus, southwestern Japan. A sediment core through one of the mounded structures was recovered from the sea floor at a depth of ?118.2 m by offshore drilling and was dated by radiometric methods. The lithology and coral fauna of the core indicate that the mounded structure was composed of coral–algal boundstone suggesting a small-scaled coral reef. High-precision α-spectrometric 230Th/234U dating coupled with calibrated accelerator mass spectrometric 14C ages of corals obtained reliable ages of this reef ranging from 22.18 ± 0.63 to 30.47 ± 0.98 ka. This proves that such a submerged reef was formed during the lowstand stage of marine oxygen isotope stages 3–2. The existence of low-Mg calcite in the aragonitic coral skeleton of 22.18 ± 0.63 ka provides evidence that the reef had once been exposed by lowering of the relative sealevel to at least ?126 m during the last glacial maximum in the study area. There is no room for doubt that a coral reef grew during the last glacial period on the shelf off Irabu Island of Ryukyus in the subtropical region of western Pacific. |
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| Keywords: | coral reef last glacial maximum Ryukyu Islands sealevel U-series dating |
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