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

对分布于日本太平洋一侧(长石地区)和赤道东太平洋(EEP)的第四纪剖面中的超微化石和硅藻组合进行了定量和定性分析，以确定北太平洋表层环流系统的变化及其对1~0.5 Ma气候变化的意义。中石地区两种特征超微化石的地层产状使我们能够监测西北太平洋周围海洋锋面的运动。在大约0.85 Ma时，西北太平洋上一股凉爽而富含营养物质的表层洋流--大洋流突然侵入日本岛屿周围的太平洋一侧。Oyashio洋流强烈侵入的时间恰好与中更新世转变的初始事件(MPT：0.90-0.60 Ma)相吻合，对应于晚更新世向高频冰量波动的转变时间(如Raymo，1990)。这意味着西北太平洋海洋锋面系统受全球气候变化的控制，气候变化发生在中更新世中期的高纬度地区。随着锋面的剧烈南移，提出了黑潮暖流与冷水洋流之间的周期性振荡。同时，EEP区岩芯中硅藻组合的变化强烈地表明EEP区的热带辐合带(ITCZ)在中更新世过渡期间发生了迁移。这些结果表明，北太平洋副热带环流在中更新世过渡时期比今天变得更强，这一显著事件应该与晚第四纪冰川-间冰期旋回的建立有很强的关系。

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.