Constraints on eustatic sea-level changes during the Mid-Pleistocene Climate Transition: Evidence from the Japanese shallow-marine sediment record

Abstract

During the Middle Pleistocene, the nature of glacial–interglacial fluctuations changed from low-amplitude and a periodicity of 41 ky to high-amplitude and quasi-periodic of 100 ky. The origin of the Mid-Pleistocene Climate Transition (MPT) is an unsolved mystery. At present, there is a debate about whether the initiation of the MPT was a gradual or an abrupt process. This study investigated the process of initiation of the MPT from reconstructions of eustatic sea-level changes, as a proxy for global ice volume, based on a reexamination of lithofacies and fossil occurrences from shallow-marine sediments (Omma Formation) exposed on the west coast of Japan. The Omma Formation comprises 19 depositional sequences spanning marine isotope stages (MIS) 56–21.3, reflecting sedimentation under alluvial plain to offshore conditions. The data indicate that (1) sea-level was lowest during MIS 22 (∼0.9 Ma); (2) sea-level during MIS 34 (∼1.13 Ma) and MIS 26 (∼0.96 Ma) was lower than during any other glacial stage, except for MIS 22; and (3) sea-level during MIS 22 was at most 20 m lower than during MIS 34 and 26. Together, these findings suggest that the initiation of the MPT was a gradual, rather than abrupt, process.