Palaeoceanography of the Japan Sea Across the Mid-Pleistocene Transition: Insights From IODP Exp. 346, Site U1427

Large-scale atmospheric circulation patterns, such as the East Asian monsoon, have been proposed as possible feedbacks of the mid-Pleistocene transition (MPT). Marine sediments of the Japan Sea (JS) record variations in the East Asian monsoon over long timescales and may be crucial for understanding of the MPT. To interpret these sediments correctly an understanding of the JS palaeoceanography is necessary. So far, the JS palaeoceanography has been extrapolated across the MPT from studies of the most recent glacialinterglacial cycles. These suggest a good connection and unrestricted water-mass exchange with the open ocean during interglacial sea-level highstands, while during glacial sea-level lowstands the JS is nearly isolated. Glacial isolation often results in poor carbonate preservation and unusually low oxygen isotope (delta O-18) ratios from low-saline/low-delta O-18 waters accumulating in the basin. Using the sediments of Integrated Ocean Drilling Program (IODP) Site U1427, a shallow-water site in the southern JS, we present a continuous foraminiferal delta O-18 record encompassing the MPT. This record shows the JS-typical low glacial delta O-18 values in the late phase of the MPT, across Marine Isotope Stages (MIS) 24-17, while earlier MPT glacials, across MIS 39-25, are characterized by high delta O-18 values. We propose that high glacial delta O-18 values are the result of an improved connection between the shallow, southern JS and adjacent ocean during early MPT glacials. The impact of this palaeoceanographic mode, if continued to deep-water sites, would make the interpretation of dark/light sediment layers as glacial/interglacial deposits uncertain.

Automated summary

This study highlights the importance of understanding the palaeoceanography of the Japan Sea (JS) in analyzing the mid-Pleistocene transition (MPT). Through the analysis of sediments from IODP Site U1427, the study reveals that the δO-18 values in the JS vary during different stages of the MPT, indicating changes in the connection between the JS and adjacent ocean.