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

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.