Palaeoceanographic records related to glacio-eustatic fluctuations in the Pleistocene Japan Sea coast based on ostracods from the Omma Formation

Abstract

Six palaeo-water depth oscillations in response to glacio-eustacy during ca. 1.5–1.25Ma were recognized based on the changes of ostracod assemblages from the coastal sediments of the Omma Formation in the Noto Peninsula on the coast of the Japan Sea in central Japan. This is the first report of glacio-eustatic sea-level changes from the shallow-coastal sediments between ca. 1.5 and 1.3Ma, and the process of stepped intensification of the magnitude of the sea-level change is illustrated clearly for the first time.Shallow cold-water and relatively deeper warm-water fossil assemblages appear alternatively in the Omma Formation. This mode of occurrence indicates that sea-level rise occurred repeatedly in parallel with a marine climatic warming. These palaeo-water depth changes occurred with a periodicity of a few tens of thousands of years, and the six sea-level rises can be assigned to oxygen isotope stages 49–39, when compared with the oxygen isotope fluctuation curve from the North Atlantic deep-sea sediments records. The first three sea-level changes probably correlate with stages 49, 47 and 45, and had a magnitude of fluctuation of 10–20m. This was followed by stages 43, 41 and 39, with a magnitude of fluctuations of 50–60m for stage 43 and 70–80m for stage 39. The process of intensification of the amplitude is reflected in the increase of the thickness of warm water influx into the Japan Sea at high sea-level stand during this period, which developed from 20–30m into ca. 100m.