Post-Miocene cold-water molluscan faunas from Hokkaido, Northern Japan

Abstract

Pliocene to Recent molluscs from Hokkaido are divided mainly into three faunas, from oldest to youngest: the Takikawa-Honbetsu, the Setana and the Oyashio faunas. They correspond to the fundamental division of Japanese cold-water faunas in post-Miocene time. Biostratigraphic and radiometric data show that the Takikawa-Honbetsu stage is assigned to ca. 6.0−2.0 Ma, the Setana stage to ca. 2.0−0.6 Ma, and the Oyashio stage to 0.6 Ma-Present. The Takikawa-Honbetsu fauna, characterized by Fortipecten takahashii, flourished under cool-temperate to subarctic climatic conditions. The Setana fauna can be subdivided into two, lower and upper, by stratigraphic horizon and species composition. The lower Setana fauna, including Yabepecten tokunagai, existed under subarctic climatic conditions. The onset of the Setana stage is marked in particular by cryophilic taxa such as Chlamys islandicus. This cooling event appears to have resulted from an intensification of glaciation during the early Pleistocene. The upper Setana fauna is, in turn, characterized by Mizuhopecten tokyoensis (s.s.) and lived under cool-temperate climatic conditions. The Oyashio fauna is composed of extant cold-water species such as Mizuhopecten yessoensis that have been affected by the Oyashio current. However, a molluscan fauna characterized by warm-water elements lived in Hokkaido for a short time in the middle Pleistocene (ca. 0.4 Ma), during a warm interglacial period. The faunal succession mentioned above is presumably controlled by environmental changes, such as glacio-eustacy, climatic changes, tectonic movements, etc., which may have occurred in the northwestern Pacific during the late Cenozoic time. The appearance of these faunas in Hokkaido probably resulted from the immigration of boreal elements from the northern regions.