Identification of tsunami deposits using a combination of radiometric dating and oxygen-isotope profiles of articulated bivalves

Abstract

Four tsunami deposits (deposits I–IV) have been identified on Ishigaki Island, southwest Japan. The youngest tsunami deposit (deposit I) was caused by the Meiwa tsunami, which occurred on 24 April 1771 CE, as described in reliable historical documents. Two well-preserved specimens of articulated marine bivalve were collected from the youngest tsunami deposit (deposit I) and an additional two from the second-youngest tsunami deposit (deposit II; 920–620 cal. yr BP). The shells were tightly closed and empty inside. No encrusting epifauna or evidence of erosion was observed on the inner or outer shell surfaces. In each tsunami deposit, the 14C ages of the shells are nearly identical. The mode of occurrence and coincidence of ages mean that these shells were transported and buried alive by tsunamis. We analyzed the oxygen-isotope (δ18O) profiles of these bivalves to determine the seasons of their death, which provides clues to the seasonal timing of tsunamis. Tsunami deposits I and II were formed during spring and fall, respectively. The former supports the proposal that tsunami deposit I was caused by the 1771 Meiwa tsunami and provides regional radiocarbon reservoir age for the late 1700s; the latter provides a chronological constraint on the identification of tsunami deposit II. Thus, a combination of radiometric dating and δ18O profiles of articulated bivalves derived from tsunami deposits provides important chronologic constraints for examining paleo-tsunami events.