Neogene paleogeography and marine climate of the Japanese Islands based on shallow-marine molluscs

Abstract Based on a large amount of data on Neogene shallow-marine molluscs of the Japanese Islands, paleogeography and marine climate are reconstructed at 19−17 Ma, 16 Ma, 13 Ma, 10−6 Ma, 4−3 Ma and 2−1 Ma. The definition of marine climates such as warm-temperate, mild-temperate, cool-temperate, and so on basically follows that by Nishimura (1981) which is characterized by subdivisions of the transitional zone between the subtropical and subarctic regions in the modern sea around the Japanese Islands. The distribution patterns of shallow-marine molluscan genera/species along the Japanese Islands in both modern and Neogene seas are considered to reflect accurately the surface thermal conditions. The middle Miocene climatic optimum at about 16 Ma is well displayed by the northward extension of mangrove swamp and Vicaryella -bearing faunas within the Yatsuo-Kadonosawa faunal province in northeastern Honshu and southern Hokkaido. After this warming, a gradual cooling commenced at about 14−13 Ma, as indicated by the disappearance of tropical molluscs and the occurrence of survival elements such as the genera Nanaochlamys, Kotorapecten, Sinum , and so on, which are descendants of earlier genera. However, this cooling is considered to be a minor climatic deterioration, from tropical to subtropical in southern Japan, and from subtropical to warm-temperate in central to northeastern Honshu. The late Miocene molluscan faunas of Japan are composed mainly of such endemic elements as Miyagipecten matsumoriensis, Dosinia (Kaneharaia) kaneharai, Mercenaria chitaniana and so on. This assemblage is interpreted as having flourished under mild-temperate conditions, on the basis of the thermal tolerances of these genera. Middle and late Miocene shallow-marine molluscs of the northern Pacific, including those from Hokkaido, Sakhalin and Kamchatka, are considered to have occupied a single temperate zoogeographic province. Early Pliocene and Plio-Pleistocene molluscan faunas of Japan and the Northeast Pacific display a remarkable convergence of subtropical and subarctic/cool-temperate realms.