Paleo-SST and Seawater δ 18O Reconstruction of the Kuroshio-Front Region since the Last Glacial Maximum

In the northwest Pacific, the Oyashio Current (low-temperature, low-salinity) and the Kuroshio Current (high-temperature, high-salinity) meet each other today at the east coast of Japan, and the Kuroshio-Front is located offshore Kashima in central Japan. Latitudinal gradient of SST is extremely steep in this region, and variations of SST and hydrological structure offshore Kashima are closely related to the north-south migration of the Kuroshio-Front. δ ¹⁸O_calcite (δ ¹⁸O_c) and Mg/Ca ratios of Globigerina bulloides were analyzed from piston core MD012420 (36°04.05'N, 141°48.92'E, 2101 m), recovered during the IMAGES-WEPAMA cruise to reconstruct water mass structures offshore Kashima. Both δ ¹⁸O_c and Mg/Ca ratios were measured from the same samples, then Mg/Ca-based SSTs were combined with δ ¹⁸O_c to estimate past seawater δ ¹⁸O (δ ¹⁸O_w) using a δ ¹⁸O-temperature calibration. Estimated SSTs from the core-top and Last Glacial Maximum (LGM) are 15.8° C and 6.4° C, respectively. The calculated δ ¹⁸O are AŸ_0.10‰ for the Holocene average and 0.66‰ for the LGM average. The difference in calculated δ ¹⁸O_w between LGM and Holocene, 0.76‰ , includes both global continental ice volume change and local δ ¹⁸O_w (salinity) changes. Assuming the amplitude of δ ¹⁸O_w variation due to glacial ice volume was 1.0‰ , then the residual, -0.24‰ , must be due to local δ ¹⁸O_w (salinity) change. We suggest that the Kuroshio-Front was located further south and the influence of the Oyashio Current, constituting low salinity water, was dominant offshore Kashima during the LGM.