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

这项研究旨在重建日本海晚中新世的古环境，日本海是一个半孤立的边缘海，今天与周围海域的水交换极其有限。我们分析了从日本海深海钻探岩心、三陆附近海域和本州岛日本海一侧的陆上中新世剖面获得的放射虫组合。除了我们最早的目的之外，我们不仅积累了中新世晚期的组装数据，而且还积累了直到第四纪的年轻时期的组装数据。放射虫组合和估算的古海洋学历史概述如下。1)在过去的1200万年里，日本海放射虫的多样性一直低于三陆附近的开阔海域。这表明，正如今天的情况一样，这些海域之间的海洋条件也有很大的差异，在中新世至上新世期间，特征水团占据了表层以下的日本海。2)中新世晚期，边缘海与开阔洋之间发生了短期增暖事件。这可能反映了研究区域周围大范围的古气候变化。我们处理了1200万~700万年前上中新世地层的许多样品，但本研究中使用的深海岩心在上新世时期的回收率不高，因此我们在未来的工作中需要从上新世地层中获得更好的资料。基于新泻县、长野县、千叶县和北海道等县的陆上地质剖面，我们获得了大量的放射虫数据，估算了中新世和上新世地层的年龄。陆上新近系剖面年代学格架的建立是开展古环境研究的基础，应在不久的将来进一步推进。

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.