Abstract
Molluscan assemblages containing warm water species now not living in Hokkaido, such as Trapejium liratum, Meretrix lusoria, Mactra veneriformis, Macoma contabulata, etc., are found in several horizons of Holocene shallow marine sediments and shell mounds in Hokkaido. Based on the detailed datings of the warm water molluscan assemblages in the Okhotsk Sea coast area of Hokkaido, four stages of warming of the Okhotsk Sea coast are recognized. These are 7,200-5,000 years BP, 4,200-3,200 years BP, 2,500-2,300 years BP, and 1,000-900 years BP. Correlative warm stages are also recognized in the offshore sediments of the Japan Sea and the Okhotsk Sea, based on the planktonic diatoms in the core samples. These warm stages indicate the activation of the Tsushima warm current, when mollusks invaded synchronously into the Okhotsk area.
