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要旨:
We estimated sea surface temperatures (SSTs) in the western Okhotsk Sea during the last glacial maximum (LGM) and the last glacial termination from measurements of the alkenone unsaturation index (UK'37), the Tetra Ether indeX of tetraethers consisting of 86 carbon atoms (TEX86), and TEX86L in piston core sediments, which reveal the climate response of this region to different types of forcings. TEX86L is a modification of TEX86 proposed for high-latitude ocean regions. The TEX86L-derived temperatures were not different from the alkenone-derived temperatures from the same and nearby sediment samples. During the LGM, alkenone-derived temperatures in the Okhotsk Sea were relatively warm. These and similarly warm alkenone-derived temperatures found at many other sites in the western North Pacific may reflect a shift in the season of biomarker production from early summer and autumn to mid-summer during the LGM as a result of the expansion of the season of sea-ice cover. Empirical orthogonal function (EOF) analysis of alkenone-derived temperatures at 14 sites from the North Pacific suggested substantial cooling between 20 and 14 cal kyr BP during the deglaciation at 11 of the sites (including 4 sites in the Okhotsk Sea). This dominant EOF mode suggests either an overall seasonal bias in alkenone-derived temperatures or potential shifts in the Haptophyseae production season. The second EOF mode of variability indicated a deglacial warming trend punctuated by millennial-scale cold events, associated with reorganizations of the Atlantic Meridional Overturning Circulation. The SST pattern corresponding to this EOF mode is indicative of millennial-scale meridional shifts of the axis of the westerly jet, which may have affected heat transport across the Kuroshio. Reconstructed temperature changes, in particular in the Okhotsk Sea, provide insight into the conditions that modulated the ventilation history of intermediate waters in the North Pacific.
