Fossil ostracode assemblages in the Middle Pleistocene upper part of the Sawane Formation on Sado Island, Niigata Prefecture, central Japan and surface water freshening of the Sea of Japan

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Evidence of surface water freshening during the late Quaternary glacial period has been recognized in deep-sea cores from the Sea of Japan. However, no evidence of seawater freshening in shallow-water environments during the glacial period has been found in the shallow marine strata in the Sea of Japan and along its coasts. In this study, changes in fossil ostracod taxa from the upper Sawane Formation in Niigata Prefecture, central Japan were investigated to clarify environmental changes during the Middle Pleistocene. Based on the fossil ostracod taxa present and the proportions of each taxon, we estimate that relative sea level changed between the lower sublittoral and upper bathyal zones at water depth of 150-200 m. Taxa that favored colder temperatures and lower salinities were abundant during low-stand period; in contrast, despite the high sea level only one warm-water ostracod was found in the study sections. This suggests that the upper Sawane Formation underwent environmental changes between the interstadial and glacial periods. Low-salinity ostracod taxa were scarce or absent in Pliocene and lower Pleistocene strata along the coasts of the Sea of Japan, and were absent during the high-stand period in the study sections. The ostracod taxa from low-salinity environments in the study sections differ from those dominant in regional low-salinity environments, such as estuaries and river mouths, in the modern Sea of Japan. These low-salinity taxa and other ostracod species that survived the surface water freshening event in the Sea of Japan during the late Quaternary now inhabit similar temperature and salinity environments. These observations provide evidence for a desalination of surface waters in the Sea of Japan in the Middle Pleistocene, in contrast to the restriction of low-salinity water to regional shallow environments that occurred during the glacial period.

Fossil ostracode assemblages in the Middle Pleistocene upper part of the Sawane Formation on Sado Island, Niigata Prefecture, central Japan and surface water freshening of the Sea of Japan

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