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

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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.