NO COLD CONDITION IN THE KUROSHIO REGION, NORTHWESTERN PACIFIC OCEAN DURING MIS 6

The northwestern Pacific area, particularly the tropical and sub-tropical regions, plays an important role in the monsoon and ENSO systems. Especially, the Kuroshio Current flows as a main component of heat transfer in the northwestern Pacific Ocean and strongly affects climate changes in this region. The Okinawa Trough is a key area for studying past variability of the Kuroshio since it is the source area where the current becomes stronger after diverging from the North Equatorial Current. This work reports the first good record for long term and high-resolution paleoceanography in this region. During IMAGES IV and VII cruises of the the R/V Marion Dufresne, two long piston cores MD98-2196 and MD01-2398 were obtained from the northern Okinawa Trough and southern Ryukyu Trench, respectively. These cores provide the first continuous record covering the last 210 ka (after MIS 7) in this area, and permit the reconstruction of the fluctuations of surface water masses, based on the planktonic foraminiferal assemblage. We recognized distinct influences of the Kuroshio Current during interglacial periods (MIS 1, 5 and 7) in core MD98-2196 and warm water conditions in core MD01-2398. On the other hand, cold water covered this area during the glacial periods (MIS 2 and 4) instead of the warm water. However, MIS 6, which is also a glacial period, is separated into two units at ca. 160 ka. The lower unit (from ca. 190 to 160 ka) represents cool conditions in the whole of the Ryukyu Arc region, and the central water masses of the north Pacific Ocean extended over the middle of the Okinawa Trough. In the upper unit (from ca. 160 to 130 ka), Globigerina bulloides drastically increased, particularly in core MD98-2196. Considering the ecology of G. bulloides, we conclude that an upwelling system developed and strengthened close to the marginal area during this glacial period (MIS 6), differing from the last glacial period (MIS 2) in the northwestern Pacific Ocean.