Opening of the Japan Sea and Marine Biogeography during the Miocene

抄録

A sharp changeover of benthic molluscs from warm water to cold water faunas is widely recognized in the Japan Sea coast early Middle Miocene deposits dating from about 15 Ma ago and lying within the Blow's N. 9 planktonic foraminiferal zone. The change in the molluscan faunas is attributable to a regional change in paleogeography around the Japan Sea. Recent paleomagnetic work shows that the clockwise rotation of Southwest Japan took place within a very short interval, probably between 15 and 14Ma ago, at about the same time as the faunal change. The sharp change in marine faunas may be reasonably explained by the rotation and opening of the Japan Sea. As Southwest Japan rotated clockwise around the pivot located in the northern East China Sea, the seas around the Tsushima Straits, a passageway of the warm Kuroshio Current, might be closed. The Japan Sea was turned to open toward the north, and thus brought the fauna under the influence of cold, northern water.