ミランコビッチ・サイクルに伴う環境変動に対する海生貝類の応答様式

抄録

The middle part of the early Pleistocene Omma Formation (1.5-1.0Ma) is composed of eleven depositional sequences caused by glacio-eustatic sea-level changes associated with Milankovitch cycles (41, 000-year orbital obliquity). Each depositional sequence contains inner shelf sediments of transgressive and high-stand systems tracts. Within each depositional sequence the molluscan fauna changes from cold-water associations to warm-water associations, followed again by cold-water associations. On the basis of detailed stratigraphic distributions of molluscs and planktonic foraminifers, the following events can be recognized during the warming interval from a glacial stage to an interglacial stage : 1. initiation of inflow of the warm Tsushima Current into the Japan Sea, 2. local extinction of cold-water molluscs, 3. absence of both cold-and warm-water molluscs, 4. successful migration of warm-water molluscs. The absence of both elements may have been caused by high seasonal fluctuations of water temperature associated with the unstable inflow of the Tsushima Current.