The effects of glacio-eustatic sea-level change on Pleistocene cold-water ostracod assemblages from the Japan Sea

The Pleistocene palaeo-habitats and the extinction history of cold-water ostracods in the Japan Sea are recorded in its coastal strata. The presence of now-extinct species in the Omma Formation of central Japan (1.5 Ma) indicates that such species predominated in a shallow, open sea environment. The tolerance ranges of these species as to water temperature and salinity are inferred to have been narrower than those of most extant cold-water species in the same families that live in shallow open water, as well as in brackish inner bays. The horizons in the Omma Formation with peak relative abundances of now-extinct species differ from the horizons with peak abundances of living Japan Sea species. We therefore suggest that the now-extinct species were adapted to optimum temperature and salinity conditions that differ from those in the modern Japan Sea. The observation that few now-extinct species survived after 0.4 Ma may be explained by the hypothesis that physicochemical properties of the water masses changed. We argue that, during the glacial periods with increased amplitude (0.9-0.4 Ma), the salinity decreased as the result of glacio-eustatic changes and the closure of shallow straits owing to low sea levels, causing the extinctions.