[M-IS01_29PM2] Land-ocean linkages in East Asian marginal seas

[MIS01-12] Long-term evolution of the North Pacific subtropical gyre: Implication from the late Quaternary record

*Yurika UJIIE¹ (1.Faculty of Science, Shinshu University)

Keywords:North Pacific subtropical gyre, Kuroshio, water column structure, Pleistocene

The North Pacific subtropical gyre drives a transportation of huge amount of heat from low to high latitude area to maintain warm climate in the northwestern Pacific area. This gyre system largely controls the zonal temperature gradient and west-east asymmetric climate, currently observed in the Pacific Ocean. The stepwise enhancement of these temperature gradients has partly been observed in the equatorial and east Pacific area since the late Pliocene. However, a lack of long-term observation in the west Pacific Ocean impedes a better understanding of the development of the Pacific climate.The Kuroshio Current, flowing from the Okinawa Trough to eastward off the Japan, act as a heat-transfer along the North Pacific subtropical gyre margin. The variation in this surface current would reflect to the changes of the West Pacific climate. Especially, the Okinawa region is an ideal place for paleocenographic reconstruction, as (1) the Kuroshio Current shows an oscillation with surrounding water masses and (2) the sediments are buried in high rate. Through the short-term paleoceanographic records in the Okinawa region, the planktonic foraminiferal assemblage showed the decrease of the Kuroshio indicator and increase of the coastal- and cold-water masses indicators under the modern Kuroshio path (the East Chia Sea) during MIS 2. Interestingly, the long-term record, which was the first to cover the past 200 kyrs in this region, represented different oceanic condition during MIS 6. The indicator of the upper intermediate water in the subtropical gyre increased over whole of the Okinawa region at this time. Moreover, the Mg/Ca paleo-temperatures in the surface and upper intermediate layers showed that warming in the upper intermediate layer was continuing from MIS 6 to MIS 5e, while warming in this layer was rapidly stopped at MIS 2. Both records of the paleo-temperature and planktonic foraminiferal assemblage congruently suggest the development of the intermediate water in the North Pacific subtropical gyre during MIS 6, instead of the dominance of cold water mass observed during MIS 2. The intermediate water has likely been undergone an independent process from the changes of the surface water masses at least by MIS 5. Even the 200 kyrs record successfully inferred two different glacial mechanisms of MIS 2 and 6, associating with the changes of surface water masses and deeper waters. Future study with longer record will lead a comprehensive understanding how the modern water column structure has been developed in the Pacific Ocean.