Paleoceanographic responses to sea level variability in the East Sea since the last glacial period: Multi-proxy approach

The oceanographic conditions in the East Sea have significantly changed over glacial-interglacial cycles, but the underlying mechanisms remain poorly understood due to limited data. We present multi-proxy records from three sediment cores along the western margin of the East Sea to reconstruct past oceanic conditions since Marine Isotope Stage (MIS) 5. Our results reveal contrasting conditions between glacial and interglacial periods. First, detrital Nd isotopes provide evidence for provenance changes suggesting a reduced contribution of seawater inflow from the Tsushima Warm Current compared to the local fluvial influx from Japanese islands during the last glacial period, most likely linked to sea level drop. As a result, the East Sea likely experienced surface water freshening, particularly during MIS 2, when seawater exchange through the Korea/Tsushima Strait was (almost) entirely restricted, as evidenced by decreasing oxygen isotope ratios in planktonic foraminifers. Authigenic Nd isotopes suggest that this freshening caused strong water column stratification, limiting vertical mixing below ∼800 m. Consequently, bottom waters likely became oxygen-depleted, as indicated by decreases in total organic carbon to total sulfur contents and cerium anomalies and an increase in authigenic molybdenum concentrations. The reduced horizontal and vertical water mixing also possibly limited nutrient supply during the last glacial period, resulting in diminished primary productivity. This study emphasizes the distinctive response of the East Sea's oceanographic conditions to climate-driven sea level fluctuations and highlights its potential contribution to the global ocean carbon reservoir during glacial periods.