Sea surface and subsurface temperature changes in the Okhotsk Sea and adjacent North Pacific during the Last Glacial Maximum and deglaciation

The mid- to high-latitude region of the western-central North Pacific—including the Kuroshio-Oyashio transition area and marginal seas such as the Okhotsk, Japan, and Bering Seas—is a key area for understanding climate variability of the eastern Asian continent. Despite the review by the Multiproxy Approach for the Reconstruction of the Glacial Ocean Surface (MARGO) project, the variations in sea surface and subsurface temperatures in the mid- to high-latitude region of the western-central North Pacific Ocean—including the Kuroshio-Oyashio transition area and marginal seas except for the Japan Sea—during the glacial-interglacial cycles have been insufficiently studied. Therefore, the purpose of this study was to clarify the interaction between atmospheric circulation, the sea-surface environment, and circulation in the intermediate-deep ocean in the western-central North Pacific and its marginal seas in response to global climate changes at the millennial time-scale during the last glacial-deglacial period. We determined sea surface and subsurface temperatures in the Okhotsk Sea during the Last Glacial Maximum (LGM) and the last deglaciation from measurements of alkenones and the Tetra Ether indeX of tetraethers consisting of 86 carbon atoms (TEX86) in piston core sediments, which reveal the climate response of this region to global climate changes. The TEX86-derived temperatures are different from the alkenone-derived temperatures from the same and nearby sediment samples in the Okhotsk Sea. This suggests that the different proxies reflect different aspects of thermal structure changes during the LGM and the last deglaciation. During the LGM, alkenone-derived temperatures in the Okhotsk Sea were relatively warm. In addition, during the last deglaciation, alkenone-derived temperatures changed in response to the millennial-scale climate change; from 19-10 kyr BP the main feature was higher temperatures during Heinrich Event 1 and Younger Dryas and lower during the Bølling-Allerød. The apparent warmer alkenone-derived temperatures during the cold events (LGM, H1 and YD) have also been found at many other sites in the western North Pacific and may reflect the shift in the season and depth of biomarker production from early summer and autumn to midsummer because of an expansion of the season of sea-ice cover. Empirical Orthogonal Function (EOF) analysis also indicated a shift in the alkenone production season as the first principal component. The EOF analysis further implied that the alkenone-derived temperature traced the precessional cycle of fall insolation at 45°N and millennial time-scale variability in the North Atlantic.