Eustatic sea-level change at the Mid-Pleistocene climate transition: new evidence from the shallow-marine sediment record of Japan

Abstract

Abstract Based on the excellent sedimentary record, we have identified five depositional sequences in the upper part of the Omma Formation, exposed along the Japan Sea coast of central Japan. The base of this stratigraphic section is placed just below the top of the Jaramillo Subchron, and the top is located below the Brunhes-Matuyama boundary. Biostratigraphy supports the interpretation that the lower three sequences correlate with marine oxygen isotope stages (MIS) 28–26, 26–24 and 24–22, respectively. We have reconstructed a eustatic sea-level curve from analyses of sedimentary facies, fossil molluscs, basin subsidence and sediment accumulation. The resulting sea-level curve shows that mean eustatic sea-level dropped between 1.0 and 0.9 Ma, and that sea-level was lowest at MIS 22, which has the heaviest δ 18O value between MIS 28 and 22. The total fall in mean sea-level is inferred to be 20–30 m. This independent sea-level record demonstrates that ice-sheet volume rapidly increased at the Mid-Pleistocene Climate Transition (MPT) and culminated in MIS 22, as inferred from the δ 18O record. The upper two depositional sequences imply the contribution of precession to changes in ice sheet volume would have arisen in MIS 21, just after the MPT.