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

通过对日本太平洋一侧（铫子区）和赤道东太平洋（EEP）第四纪剖面超微化石和硅藻组合的定量和定性分析，探讨了1 Ma ~ 0.5Ma北太平洋表层环流系统的变化及其对气候变化的指示意义。在铫子地区的两个特征种的超微化石的地层发生，使我们能够监测运动的海洋学锋周围的西北太平洋。在0.85Ma左右，西北太平洋冷而富营养的表层流--亲潮流突然侵入日本列岛附近的太平洋一侧。这一次Oyashio海流强烈入侵的时间与中更新世过渡的初始事件（MPT：0.90-0.6OMa）相吻合，对应于晚更新世期间高频冰量波动的过渡时间（例如Raymo，1990）。这说明中更新世以来，沿着西北太平洋的海洋锋系统受高纬度全球气候变化的控制。随着锋面的南移，黑潮暖流和亲潮暖流之间存在着周期性的振荡。同时，硅藻组合的变化在EEP地区的核心强烈表明，热带辐合带（ITCZ）在EEP迁移在中更新世过渡。这些结果表明，中更新世之交北太平洋地区的副热带环流比今天增强，这一显著事件与晚第四纪冰期-间冰期旋回的建立有密切关系。

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.