A total of 117 beachrock samples were collected from the Yaeyama Islands, southwestern Japan, in order to determine their formative period. They consist of 46 fossil shells, 36 calcarenite samples and 35 fossil corals, respectively. They were radiocarbon dated and their isotope fractionations (δ
13C) were also analyzed. Then calibrated ages were calculated by computer program. The results are summarized as follows;
1. Beachrocks of the Yaeyama Islands have formed during the following seven stages:
i.e. 5,560~5,260 cal BP, 5,020 cal BP, 4,700~4,660 cal BP, 4,360~4,050 cal BP, 3,810~3,710 cal BP, 3,530~500 cal BP, and recent time, respectively.
2. Figures of the maximum, minimum and the average δ
13C values of 117 beachrock samples indicated 9.4‰, -4.95‰ and 2.97‰, respectively, however 80 samples were out of the range of marine organisms (δ
13C:2‰~-2‰).
3. Lagoon ecosystem of coral reef have been influenced by meteoric water that contained soluble calcium carbonate and carbon hydrate originated in Quaternary Ryukyu Limestone bed consisted of the islands.
4. Therefore calcium carbonate that cemented beach sediments had not only occupied from sea-water, but also it had derived from meteoric water. This process may be well understood by considering the post-depositional diagenesis.
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