ENHANCED EL NIÑO LIKE CONDITION IN THE NORTHWESTERN PACIFIC OCEAN DURING LATE HOLOCENE; EVIDENCE FROM DRASTIC CHANGE OF THE SURFACE CURRENT, KUROSHIO

The Kuroshio Current is a main component of subtropical circulation in the northwestern Pacific Ocean and effects climatic changes in the region. The Ryukyu Arc region is a key area for studying past variability of the Kuroshio since it is the area where the current strengthens after diverging from the North Equatorial Current. In this area, Ujiie et al. (in press) used 15 piston cores to reconstruct time-space changes in surface water masses based on planktonic foraminiferal assemblages younger than 20 ka. Of the four modern planktonic foraminiferal groups characterizing this region, the Pulleniatina group is most characteristic of Kuroshio Water. However, this species diminished in abundance twice during the LGM and late Holocene, from ca. 4.5 to 3 ka, respectively. During the latter event (the “Pulleniatina minimum event" or PME), the effect of the Kuroshio Water was greatly diminished without any cooling signal. The same event has been recognized in the South China Sea and the western equatorial Pacific Ocean. In this study, we propose a new hypothesis for the PME by considering the delicate ecological differences between Pulleniatina obliquiloculata and Neogloboquadrina dutertrei. Although both species inhabit the thermocline in Kuroshio Waters at present, their relative abundance was reversed during PME. Using 3 cores obtained from the Ryukyu Arc region, we measured d¹⁸O of P. obliquiloculata, N. dutertrei and Globigerinoides sacculifer. d¹⁸O curves of P. obliquiloculata and N. dutertrei represent heavy values during the PME, although G. sacculifer’s values do not change. Therefore, the PME indicates an environmental change in sub-surface waters. P. obliquiloculata inhabits the thermocline of the entire equatorial Pacific. However, sub-surface water is relatively cooler when the thermocline is shallower, as occurs during El Niño events. Moreover, the velocity and strength of the Kuroshio Current tends to decrease when the North Equatorial Current bifurcates in more northerly latitudes prior to El Niño. Therefore, we suggest that PME indicates the onset of the modern ENSO system around the low-latitude northwestern Pacific Ocean.