Paleoenvironmental changes during the Pleistocene in the northern part of the Niitsu Hills, Niigata Prefecture based on fossil ostracod assemblages

Abstract

The paleoceanography of the Japan Sea has revealed environmental shifts during glacial and interglacial periods of the early Pleistocene; however, detailed paleoenvironments in the shallow seas have not been fully reconstructed. In this study, detailed environmental changes were investigated by studying fossil ostracods in the Pleistocene Kakuma and Minagawa formations distributed in the Niitsu Hills, Niigata Prefecture, Central Japan. At least 211 species belonging to 83 fossil ostracod genera were recognized in 57 samples from the two formations. Four biofacies were identified based on cluster analysis: lower sublittoral, upper sublittoral, bay, and cold water lower sublittoral. Lower sublittoral biofacies are dominant in the Kakuma Formation, whereas upper sublittoral biofacies are dominant in the Minagawa Formation. This shallowing upward trend over a few million years was caused by uplift related to compressive tectonics within the northeastern Japanese archipelago. It is evident that at 1.4 Ma during the Pleistocene, cold water covered the lower sublittoral and upper bathyal environments in the Japan Sea.