Possible formation of 'Japan Sea Proper Water' during late Miocene time

本研究旨在重建晚中新世的日本海，这是一个半孤立的边缘海，是非常有限的水交换与周围的海洋今天的古环境。我们分析了从日本海和三陆区的深海钻孔岩心和本州岛日本海一侧地区的陆上中新世剖面中获得的放射虫组合。除了我们最早的目的，我们不仅积累了晚中新世的组合数据，而且还为第四纪的年轻时期。放射虫组合和古海洋学历史的估计总结如下。1)在过去的1200万年里，日本海的放射虫多样性一直低于三陆区外海。这表明，这些海洋之间的海洋学条件有很大的差异，今天的情况也是如此，在中新世至上新世期间，日本海的表层以下有特征性的水团。2)在晚中新世期间，在边缘海和开阔洋之间发生了短期变暖事件。这可能反映了古气候的变化，在一个广泛的地区，包括研究区域。我们处理了许多1200万至700万年前的中新世上部地层样品，但本次研究中使用的上新世时间段深海岩心的回收率不高，因此在今后的工作中需要更好的上新世地层材料。根据新泻、长野、千叶和北海道的陆上地质剖面，我们获得了相当数量的放射虫数据，估算中新世和上新世地层的年代学年龄。建立陆上新近系剖面的年代学格架是进行古环境研究的基础，应进一步加强。

This study aimed at reconstructing Late Miocene paleoenvironments of the Japan Sea, which is a semi-isolated marginal sea that is extremely limited in water exchange with the surrounding seas today. We analyzed radiolarian assemblages obtained from deep-sea drilled cores in the Japan Sea and off Sanriku district and onshore Miocene sections in the Japan Sea side region of Honshu Island. Beyond our earliest purpose, we have accumulated assemblage data not only for the Late Miocene but also for younger periods up to the Quaternary. The radiolarian assemblages and the estimated paleoceanographic history are summarized as follows. 1) The diversity of radiolarians was always lower in the Japan Sea than in the open ocean off Sanriku district throughout the past 12 million years. This suggests that there have been considerable differences in oceanographic conditions between these seas as is also the today's case and that characteristic water masses occupied the Japan Sea below the surface layer during Miocene to Pliocene time. 2) Short term warming events occurred during the late Miocene, coinciding between the marginal sea and the open ocean. This probably reflected paleoclimatic changes in a wide area encompassing the studied region. We treated many samples from the upper Miocene beds of 12 to 7 million years ago, but recovery of the deep sea cores used in this study for the Pliocene time interval was not good, so we will need better material from the Pliocene beds in the future work.Based on onshore geologic sections of Niigata, Nagano, Chiba and Hokkaido Prefectures, we obtained a fair amount of radiolarian data, estimating the geochronologic ages for the Miocene and Pliocene beds. Establishment of the geochronologic framework for the onland Neogene sections is very fundamental for progressive paleoenvironmental study, and, therefore, should be more promoted in the near future.