Secular changes in the oxygen isotope ratios of mollusc shells during the Holocene of Central Japan

Abstract

In order to estimate changes in the relative water temperature during the Holocene, oxygen isotopes ratios were measured in mollusc shells collected from archaeological sites and natural shell beds in Central Japan. Analyses were made on the clam Meretrix lamarckii which lives on the open coast at depths of about 5–10 m. Serial samples were analyzed for each shell to obtain isotopic profiles. Daily growth-lines were examined to determine the dates or season. A living specimen was analyzed to compare with the results of the Holocene specimens. The oxygen isotope fluctuation of the specimen shows a clear correspondence with the growth rate pattern, indicating that the fluctuation reflects the annual variation of temperature. Twenty specimens of various ages ranging from 9000 to 1500 yr B.P. were analyzed. The isotopic values indicate that the water temperature was lowest at 9000 yr B.P., highest at 7000 yr B.P., and again became low during 4500-4000 yr B.P. Then it slightly increased after 4000 yr B.P. with an indistinct low temperature episode at about 2000 yr B.P. The change accords well with the climatic and sea-level changes known in central Japan.