南・北大東島産後期更新世サンゴ化石の230Th/234U年代 (南・北大東島のサンゴ礁段丘に関する総合研究<特集>)

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Alpha-spectrometric 230Th/234U dating was applied to fifty Pleistocene hermatypic corals from Kita-and Minami-Daito Islands (KDI and MDI), both of which have been well-known to be one of noteworthy representatives of raised atoll since the study of Aoki (1934). 230Th/234U dates range from 113±6 to 133±6 ka (ay. 123±1 ka) for autochthonous corals from KDI, and 111±5 to 159±10 (av. 123±1 ka) from MDI, intimate that the fringing reef has been developed during the high sea level stand of the last interglacial maximum. These dates are correlative to the oxygen isotope stage 5e (OIS-5e Martinson et al., 1987). The upper limit of occurrence of dated autochthonous corals was 8.1 m on KDI, and 11 m on MDI. Besides them, somewhat younger dates corresponding to the OIS-5 a or-5 c, were picked up from some allochthonous corals in a dental limestone unit on KDI. Such a detrital limestone unit cannot be geochronologically defined, because of the allochthonous mode of occurrence of dated samples. It is, however, free of doubt that hermatypic corals were alive forming a small-scaled reef in shallow sea around KDI. The detrital limestone unit including such young corals is very likely to have been formed as a storm deposit and have been preserved in some depressions on the island. The former shoreline was proved by the presence of raised surf bench at some localities (one on KDI and three on MDI), where dated autochthonous corals were collected. The height of it was measured to be ca. 10 m on KDI and 12.2 to 12.7 m on MDI. The amount and the rate of uplift is calculated to be from ca. 4 m for KDI to 6.7 m for MDI, and approximately from 0.03 m/ka for KDI to 0.05 m/ka for MDI, respectively, assuming the sea level of +6 m during the last interglacial maximum and the constant rate of tectonic upheaval since then. Compared with the amount and the rate of uplift estimated by the same way for the last interglacial limestone on Kikai, Hateruma and Yonaguni Islands arranged along the Ryukyu Arc, the vertical displacement of both KDI and MDI is considered to have been extremely small and slow during the last 130 ka.
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南・北大東島産後期更新世サンゴ化石の230Th/234U年代 (南・北大東島のサンゴ礁段丘に関する総合研究<特集>)

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