Seasonal Sea Surface Temperature Variations at the Northern Limit of the Kuroshio Current in Marine Isotope Stage 20–18

Abstract This study presents a high time‐resolution alkenone unsaturation ratios (U K' 37 ) in the Chiba composite section, a marine sedimentary sequence formed in the Kazusa forearc basin in the northwestern Pacific under the influence of the Kuroshio and Oyashio currents. The U K' 37 reflects summer sea surface temperature (SST) and records glacial‐interglacial to sub‐millennial‐scale variations spanning the marine isotope stage (MIS) 19, an analog of our Holocene interglacial. At the glacial‐interglacial scale, summer U K' 37 ‐SST and previously published winter SST fluctuations are in tandem with East Asian Summer and Winter monsoon variabilities, respectively. Millennial‐scale U K' 37 ‐SST variations correspond to meltwater events in the North Atlantic. Summer temperature amplitudes maybe half those in winter, suggesting the stable dominance of the Kuroshio Current during the summer and significant winter cooling caused by the Oyashio Current intrusion. Our study reveals the unique seasonally different atmosphere‐ocean coupling system during MIS 19 at the subarctic front in the northwestern Pacific.