Global sea level controlled the deep low-salinity pool evolution in the Japan Sea since the last glacial period

Understanding past changes in oceanic circulation and the corresponding heat, salt delivery variations are essential for assessing the climatic roles of ocean dynamic processes since the last glacial period. Unravelling salinity budget variation in the North Pacific and its controls is important to better understand the North Pacific Intermediate/Deep Water formation and associated climate impacts. The Tsushima Warm Current (TWC), the northernmost Kuroshio branch entering the semi-closed Japan Sea through the shallow strait, is regarded as a fundamental component for oceanographic changes in the Japan Sea.

To obtain a comprehensive history of the Japan Sea salinity budget, this study reconstructed the evolution history of the TWC inflow by compiling paleo-hydrographic records in the Japan Sea, including the radiolarian, diatom, coccolithophore, and planktonic foraminiferal assemblages. Following a persistent but weakened TWC inflow during Marine Isotope Stage 3, radiolarian assemblage data revealed that the TWC taxa disappeared since ~30 thousand years ago (ka). The synchronous onset of the low salinity anomaly event was in response to the cutoff of saline TWC inflow due to the rapid fall in global sea level at 30 ka. Extreme restriction of seawater exchange caused a persistent freshening of the glacial Japan Sea and formed a low-salinity water mass in the upper ocean. The compiled microfossil data confirmed that the Japan Sea accumulated excess freshwater during the glacial sea-level lowstands and the low-salinity pool extended downward to ~900 m depths. Coinciding with the peak of the low salinity anomaly event (minimum values of ∼20 psu), re-emerging TWC inflow after 19 ka reflected the reconnection of the open ocean to the Japan Sea. The persistent TWC inflow mainly drove the reduction in magnitude of the deep low-salinity pool during the last deglaciation. As a large and isolated freshwater sink for the glacial North Pacific, the deep low-salinity pool evolution could potentially have strong impacts on the North Pacific salinity budget and subsequent large-scale circulation.