Reconstruction on surface ocean circulation and climate changes in the North Pacific region during the past 100 kyrs based on phytoplankton fossils, calcareous nannofossils and diatoms

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The quantitative and qualitative analyses of nannofossil and diatom assemblages from the Quaternary sections distributed in the Pacific side of Japan (Choshi Area) and the eastern equatorial Pacific (EEP) were performed to determine changes in surface circulation systems in the North Pacific and implications for climatic changes from 1Ma to 0.5Ma. Stratigraphic occurrences of two characteristic species of nannofossils in the Choshi area allow us to monitor the movement of the oceanographic front around the NW Pacific. At approximately 0.85Ma, the Oyashio Current, a cool and nutrient-rich surface current in the NW Pacific, suddenly invaded into the Pacific side around the Japanese islands. The timing of this strong invasion of the Oyashio Current coincides with the initial event of the mid-Pleistocene Transition (MPT : 0.90-0.6OMa), corresponding to the transition time to high frequency ice volume fluctuation during the late Pleistocene (e.g. Raymo, 1990). This means that the oceanographic frontal system along the NW Pacific has been controlled by global climate change occurred in the high latitude during the middle Pleistocene. Following this drastic southward migration of the front, the cyclic oscillation between the warm Kuroshio and cool Oyashio Currents was suggested. At the same time, changes in diatom assemblages in cores at the EEP region strongly indicate that the Intertropical Convergence Zone (ITCZ) in the EEP had migrated during the Mid Pleistocene Transition. These results suggest that the subtropical gyre circulation in the North Pacific region had become stronger during the Mid-Pleistocene Transition than today and this remarkable event should have strong relationship to the establishment of glacial-interglacial cycles during the late Quaternary.