Radiocarbon Ages and Significance for Beachrock Samples Collected from the Eastern Part of Maibahbama, Southeast of Miyako Island

Abstract

 Comprising beachrock, Maibahbama lies southeast of Miyako Island. It is approximately 400 m in length and 3 to 17 m in width. Fossil shell, coral and calcarenite samples were collected from the eastern part of Maibabahbama in order to determine their ages using accelerator mass spectrometry (AMS). ¹⁴C age of a Nerita albicilla yielded an age of Modern (100.7 ± 0.3 pMC); however, Tridacna gigas yielded an age of 981 ± 22 yr BP. Coral fragments yielded an age of 1746 ± 23 yr BP, while four calcarenite samples were between 1350 ± 23 and 1567 ± 22 yr BP. The above samples were collected from two outcrops standing 1 m apart and within a 10 cm circle. Five of the seven samples coincided with the estimated ages of past tsunamis within an error range of ±3σ. Consequently, the several samples collected from beachrock at the eastern part of Maibahbama might have been washed ashore by past tsunamis before being cemented. Formation of the eastern part of the beachrock started at 980 yr BP and ended recently. Therefore, they do not coincide with the older ages of other tsunami boulders (Porites sp.) nearby, reported previously.