Temperature and salinity changes in the Oyashio-Kuroshio mixed water region over the last 30 kyr

The Subarctic North Pacific has a strong halocline that prevents deep water coming up to the surface. Sedimentary data show low algal productivity in glacial time, implying increased polar ocean stratification (Sigman et al., 2004; Jaccard et al., 2005). Stable surface ocean stratification has a potential for controlling the atmospheric carbon dioxide concentration in glacial time. However, surface ocean salinity change in the Subarctic North Pacific is not known very much. If the water mass properties, such as temperature and salinity, have been changed in the Western Subarctic North Pacific, it must influence on the eastern coast of Japanese island via the Oyashio Current. We present Mg/Ca (a proxy for sea surface temperature) and δ18O record from planktonic foraminifera Globigerina bulloides in MD01-2420 (36.04_LdegN, 141.49_LdegE; 2101m) located off central Japan over the last 30 kyr. These data provide δ18O of seawater (δ18OSW) and we used it for a proxy for past sea surface salinity. The reconstructed SST and δ18OSW fluctuated synchronously resulting in alternation between cold, low-δ18OSW and warm, high-δ18OSW conditions. These fluctuations correspond to the Northern Hemispheric climate changes such as the Younger Dryas and Bolling-Allerod. Paired reconstruction of SST and δ18OSW in MD01-2420 indicates that the surface salinity at the core site have been changed not only by the displacement of the Subarctic Front but also by the water mass properties change in the Oyashio Current. The δ18OSW increase from 23 to 19 ka was presumably due to property changes in the Oyashio Current. The amplitude of the millennial-scale fluctuations in δ18OSW was _Lsim0.5 permil. Using modern δ18OSW vs salinity relationship, the δ18OSW changes suggest _Lsim1.5 psu changes in salinity. These data imply that there is a possibility of the millennial-scale salinity changes occurred in the glacial Subarctic North Pacific.