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

Abstract

High-resolution seismic reflection profiles delineated the distribution of moundshaped 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 230 Th/ 234 U dating coupled with calibrated accelerator mass spectrometric 14 C 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.

Reconstruction of the East China Sea palaeoenvironment at 16 ka by comparison of fossil and modern Faviidae corals from the Ryukyus, southwestern Japan

“…The 230 Th/ 234 U age of MYK90 is 16.17 AE 0.52 ka, which is comparable to the age obtained by conventional 14 C dating (Sasaki et al, 2006). Also, the diagenesis checks on the fossil coral were performed by Sasaki et al (2006) and no evidence of diagenesis was found. For comparison with the fossil coral MYK90, a modern Faviidae coral specimen also was analysed as part of this study.…”

“…However, it was impossible to identify MYK90 as to genus because no definitive correspondence in skeletal structure with modern Faviidae corals was identified. Sasaki et al (2006) investigated the mineralogy and determined the age of this coral specimen with the method that is precisely described by Omura et al (2004). The 230 Th/ 234 U age of MYK90 is 16.17 AE 0.52 ka, which is comparable to the age obtained by conventional 14 C dating (Sasaki et al, 2006).…”

“…Sasaki et al (2006) investigated the mineralogy and determined the age of this coral specimen with the method that is precisely described by Omura et al (2004). The 230 Th/ 234 U age of MYK90 is 16.17 AE 0.52 ka, which is comparable to the age obtained by conventional 14 C dating (Sasaki et al, 2006). Also, the diagenesis checks on the fossil coral were performed by Sasaki et al (2006) and no evidence of diagenesis was found.…”

“…The Kuroshio Current transport also varies on interannual timescales, with less transport during El Niñ o years and more during La Niñ a years, but in the modern ocean seasonal variations are greater than those that occur on interannual timescales (Xiang et al, 2008). The fossil coral sample MYK90 was collected as part of the research project 'Techniques for reconstruction and interpretation of reservoir development' conducted by the Technology Research Center (TRC), Japan National Oil Corporation (JNOC; currently the Japan Oil, Gas and Metals National Corporation) (Honda et al, 1993;Sasaki et al, 2006). The aim of this programme, which started in 1986, is to understand the relationship between three-dimensional distribution of sediments and the factors controlling their distribution, and to develop a carbonate depositional model from the insular shelf to the slope around Miyako Island.…”

“…MYK90 is columnar shaped and 73 mm long and is classified in the family Faviidae. All Faviidae species have septal structures, and the columellae are a simple tangle of elongate septal teeth (MYK90 was presented as CR14-720 in Sasaki et al, 2006). Walls are composed of thickened septa and cross-linkages.…”