Drastic change of the subtropical circulation in the super El Niño like condition during late Holocene; evidence from the fluctuation of the Kuroshio Current

In the northwestern Pacific Ocean, the Kuroshio Current is a main component of subtropical circulation and heat transfer under the monsoon and ENSO system. The Ryukyu Arc region, especially the Okinawa Trough, is a key area for studying past variability of the Kuroshio since it is the area where the current becomes strong after diverging from the North Equatorial Current. In this area, Ujiie et al. (2003) reconstructed time-space changes in the surface water masses after 20 ka based on planktonic foraminiferal assemblage of 15 piston core samples. The fluctuations of the most characteristic species of the Kuroshio Water, which is the Pulleniatina group, represent that the effect of the Kuroshio Current diminished twice during the LGM and late Holocene, from 4.5 to 3 ka, respectively. Especially, the latter event (Pulleniatina minimum event; PME) has no cooling signal, although other water masses extended over the Ryukyu Arc region. This study focuses on the vertical fluctuations of the Kuroshio Current in the water column, comparing the fluctuations of δ ¹⁸O and δ ¹³C between surface and sub-surface habitual species in 5 piston core samples. During the PME, the δ ¹⁸O curves of Pulleniatina obliquiloculata and Neogloboquadrina dutertrei, which are living in sub-surface water near the thermocline, indicate heavy values, although sea surface temperature estimated by δ ¹⁸O of Globigerinoides sacculifer and planktonic foraminiferal assemblage has no change. At the same period, the differences of δ ¹³C between P. obliquiloculata and G. sacculifer becomes small. According to the decrease of the abundance of the Kuroshio Water species during the PME, the transport of the Kuroshio Current tends to decrease and then the thermocline is shallower in the Okinawa Trough. At present, the transport of the Kuroshio Current decreases, when the North Equatorial Current bifurcate is at northernmost position prior to the El Niño. Impacts of the PME have been recognized outside of the Ryukyu Arc region including the South China Sea and probably within the equatorial Pacific Ocean. Therefore, the PME is likely a response to a broad scale oceanographic-climatic process of the subtropical and tropical circulation.