Miocene paleoceanography around the North Pacific based on the activities of past primary producers and benthic organisms

通过对日本太平洋侧新近纪和第四纪微化石、微量化石及相关深海岩心沉积物的定性分析，探讨了北太平洋表层环流系统的变化、深海环境的变化及其对气候变化的影响。在长石地区，纳米化石和硅藻这两个具有特征的分类群的地层出现，使我们能够监测西北太平洋周围海洋锋的运动。植物的变化可能与北太平洋周围的环流变化密切相关。例如，大约在0.85 Ma时，西北太平洋的一股凉爽且营养丰富的表层洋流——漂潮流，突然在日本岛屿周围侵入太平洋一侧。这次强流入侵的时间与中更新世过渡（MPT;0.90 ~ 0.60Ma）的初始事件相吻合，对应了晚更新世向高频冰体积波动的过渡时间（例如raymo，1990）。这意味着西北太平洋沿岸的海洋锋系统受到中更新世发生在高纬度的全球气候变化的控制。与此同时，底栖生物的活动和钙质固结层的发育变得更强，从而表明北太平洋地区副热带环流发生了变化，深海环流也发生了变化。中新世期间也发现了相同的现象，因此初级生产者的通量变化影响了海洋气候变化。

The qualitative analyses of microfossils and trace fossils from the Neogene and the Quaternary sections distributed in the Pacific side of Japan and related deep sea core sediments were performed to determine changes in surface circulation systems in the North Pacific and changes of deep sea environments, and implications for climatic changes. Stratigraphic occurrences of two characteristic taxa of nannofossils and diatoms in the Choshi area allow us to monitor the movement of the oceanographic front around the NW Pacific. Floral changes can be strongly related to the circulation changes around the North Pacific Ocean. For example, at approximately 0.85 Ma, 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;0.90〜0.60Ma), corresponding to the transition time to high frequnecy 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. At the same time, activities of benthic organisms and developments of calcareous concretion layers had become stronger and thus, these results suggest that the subtropical gyre circulation in the North Pacific region had changed and deep sea circulations also altered. During the Miocene, same phenomena were also found and thus, flux changes of primary producers affected ocean climatic changes.