Paleoceanographic Change of the Sanriku Area during the Last 90,000 Years Based on the Analysis of a Sediment Core (KH 94-3, LM-8)

摘要

The occurrence of warm-water planktonic foraminifera, the δ~18O of both ben- thic and planktonic foraminifera, and the total organic carbon content were meas- ured for a sediment core (KH94-3, LM-8) collected from the continental slope off Sanriku area, on the east coast of Japan.The occurrence of warm-water planktonic foraminifera increased during the interglacial periods of the oxygen isotopic sub- stage 5_1 and stage 1 and decreased during the glacial periods from the oxygen iso- topic stage 4 to stage 2. This indicates that the core site was affected by the warm-water mass derived from the Kuro-shio Current during the oxygen isotopic substage 5_1 and stage 1. The δ~18O curve of benthic foraminifera shows a very similar pattern to the standard δ~18O curve, but the δ~l8O curves of planktonic foraminifera are quite different from the standard curve. This suggests that the surface water con- dition at the core site has been changing over the last 90,000 years. The total or- ganic carbon content increased during oxygen isotopic stages 5_1, 2, and 1, sug- gesting possible increases in surface productivity during these periods. Based on comparison of the paleotemperatures calculated from the oxygen isotopic values in three piston cores which were retrieved off the east coast of Japan, it is suggested that the surface temperatures off Kashimanada and Sanriku decreased by 7.6℃ and 4.4℃ , respectively, during the last glacial maximum (LGM). Such large tem- perature drops are believed to be caused by the southward migration of the mixed water mass and the Oyashio Current into