²³⁰Th/²³⁴U and ¹⁴C dating of a lowstand coral reef beneath the insular shelf off Irabu Island, Ryukyus, southwestern Japan

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 alpha-spectrometric Th-230/U-234 dating coupled with calibrated accelerator mass spectrometric C-14 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.