Early Pleistocene glacio-eustatic sea-level fluctuations as deduced from periodic changes in cold- and warm-water molluscan associations in the Shimokita Peninsula, Northeast Japan
Abstract Sea-level oscillations of apparently glacio-eustatic origin are recognized in the Lower Pleistocene Hamada Formation, based on changes of molluscan associations. There are two cold-water molluscan associations, the Turritella saishuensis-Macoma-Clinocardmium and the Acila-Macoma Associations, and one warm-water Limopsis crenata-Limatula-Nuculana Association. Their distribution in time and space indicates that the study area was dominated by cold water, and was invaded periodically by warm water at least three times in the Early Pleistocene between 1.10 Ma and 0.89 Ma. Judging from their species composition, the paleodepth was greater during the times of warm-water intrusion than during the times dominated by cold water. The phenomena are reasonably explained by a combination of climatic and sea-level changes.