Daito Islands, ca. 150 km east of the Ryukyu Trench, are located on the northwestern part of the subducting Philippine Sea Plate, and are composed of two main islands, Kita (north) -Daito and Minami (sourth) -Daito. Both islands are well-known raised atolls and tectonic movements are expected to be different from the Ryukyu Islands on the Eurasian Plate.
We investigated raised coastal morphology and geology at eight localities of these islands with special reference to the occurrence of fossil corals, and detailed maps and cross sections are drawn using Electric Distance Meter. Thirty three autochthonous corals are dated by ²³⁰Th/²³⁴U method and some corals are dated by ESR and non-destructive ²²⁶Ra/²³⁸U methods. Some allochthonous corals are also dated. ²³⁰Th/²³⁴U dates on autochthonous corals cluster in 113±6 to 133±6 ka in Kita-Daito and in 113±6 to 159±10 ka in Minami-Daito. Therefore, the dated autochthonous corals, except for one relatively older date, represent the sea level of the last interglacial maximum, corresponding to isotope stage 5e. The upper limit of occurrence of autochthonous corals is 8.1 m in Kita-Daito and 11 m in Minami-Daito. Elevation of the last interglacial shoreline judged by emerged coastal morphology is 10 m in Kita-Daito and 12 m in Minami Daito. No autochthonous corals younger than the last interglacial maximum exist in these islands. However, the occurrence of allochthonous corals dated at ca. 80-100 ka suggests the possible presence of corals corresponding to isotope stage 5c and 5a, below the present sea level.
Average uplift rate in both islands is nearly the same, that is 0.03 m/ka at Kita Daito and 0.05 m/ka at Minami-Daito, however, it is significantly lower than 1.8 m/ka in Kikai Island and even 0.3 m/ka in Hateruma Island, both are located on the Eurasian Plate, being subducted by the Philippine Sea Plate. Sequence of vertical tectonic movement of Daito Islands can be estimated as follows : Daito Islands had been subsided resulting in the formation of atolls and lagoons underlain by more than 400 m thick coralline limestone (Sugiyama, 1934 and 1936), and then tectonic uplift, time of which is unknown because of dolomitization of calcareous sediments, has started in association of the northwestward migration of the Philippine Sea Plate and has resulted in the formation of upbulge which is represented by the emerged atolls, fringed by a series of fringing reefs. Uplift rate has been very slow at least since the last interglacial maximum, possibly implying that these islands are tending towards the new tectonic phase (stable to subsidence).

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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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The ²²⁶Ra/²³⁸U method based on non-destructive gamma-ray spectrometry is discussed for dating 18 specimens of Pleistocene autochthonous corals in Minami-and Kita-Daito Islands. The ²³⁸U and ²²⁶Ra contents are indirectly determined by their daughter nuclides of ²³⁴Th and ²¹⁴Pb, respectively, on account of the assumption that the radioactive equilibrium between ²⁸⁸Th and ²³⁴Th, and between ²²⁰Ra and ²¹⁴Pb has been maintained. All specimens, initially aragonitic minerals, were powdered, compressed to a disc shape of 50 mm in diameter and made airtight. These specimens have been measured by the Hyper Pure Germanium Detector for 2 to 3 days.
The ²²⁶Ra/²³⁸U dates of 15 specimens in Minami-and 3 specimens in Kita-Daito Island are obtained. The highest frequency of the dates is 120 to 130 ka, and the mean value of those is 128±27 ka, although these dates including counting error (1σ) range from 80 to 190 ka. The mean value of the ²²⁶Ra/²³⁸U dates is concordant with the weighted average of the ²³⁰Th/²³⁴U dates of the autochthonous corals, i.e. 123±1 ka, obtained from the same locations of the islands by the method of alpha-spectrometry, which is correlated with the oxygen isotope stage 5 e. In addition, the ²²⁶Ra/²³⁸U dates of 12 in 18 specimens of the islands correspond to the ²³⁰Th/²³⁴U dates of the same specimens. This implies that the ²²⁶Ra/²³⁸U dates by the present study are, as a whole, correlated with the 5 e.
The non-destructive ²²⁶Ra/²³⁸U dating by the gamma spectrometry has advantages that the elaborate and time-consuming chemical procedures are unnecessary and no change in the chemical state occurs in the sample analyzed, hence, the same sample can be used for further investigation after the non-destructive measurement. If smaller counting errors (1 σ) can be made than those by the present study on condition that a detector of lower background is prepared, the detector is heavily shielded, and sample is measured about one week, the non-destructive gamma-ray method will be more valid for dating middle to late Pleistocene corals.

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