Climate and sea-level variation during MIS 21 from a sediment core in Osaka Bay, Japan: a sign of termination of the Mid-Pleistocene Climate Transition

The Quaternary climate system is strongly dominated by changes in the Earth’s orbital elements. The dominant cyclicity changed from about 41 ka (obliquity cycle) to about 100 ka (eccentricity cycle) during the interval known as the Mid-Pleistocene climate transition (MPT), roughly 1.25-0.7 Ma. This study reports climate and sea-level variation for the marine oxygen isotope stage (MIS) 21, encompassing the end of the MPT, based on a pollen, diatom and sulfur record from a 50-m thick sequence in a core from Osaka Bay, western Japan. An extremely warm climate coincided with the sea-level highstand of substage 21.5, when the warm-temperate element Quercus (Cyclobalanopsis) exceeds 40% of total arboreal pollen. This was followed by a warm-temperate to temperate and humid climate that continued until the end of MIS 21. Climate was dominated by precessional cyclicity, with an inverse correlation between temperature and precipitation. The postglacial sea-level rise reached its highest peak in substage 21.5, when paleo-Osaka Bay reached its maximum extent including the Kyoto and Nara Basins. At this time pelagic diatoms were dominant in the central part of the bay. Sea level dropped below the Osaka Bay sill (about -60m at present) during substage 21.4, followed by a rise above the sill in substage 21.3, and a drop at 21.2. Sea level remained below the sill during substage 21.1. The thermal maximum and sea-level peak occurrence just after the rapid postglacial sea level rise, after which there was a gradual decline in temperature and sea-level accompanied by precession-related oscillations; these features are typical of the post-MPT interglacials dominated by 100-ka cyclicity. These features may be a sign of termination of the MPT.