Land connections between Eurasian Continent and Japanese Islands - Related to human migration
Prof. Masaaki Kimura
Recent submarine, geological, and geophysical investigations including diving surveys reveal the geo-history of the Japanese islands with special reference to Ryukyu Islands and the East China Sea. Three stages are fundamentally distinguished for formation process of the Ryukyu Arc and connections from the Japanese islands to the Eurasian continent. Around 7.0-5.0 Ma, the southwestern part of Japanese Islands (Ryukyu Arc) was a part of Eurasian continent. This is the first stage of land connection between the continent and Japanese islands. Then, sea areas invaded the continent to make a separated Ryukyu Arc. In the second land connection stage between 2.0 - 1.3 Ma the East China Sea area, including most of the Okinawa Trough, may have been subaerial. At that time, the Ryukyu Arc region may have been a part of the Eurasian continent. At 0.2-0.015Ma the third stage occurred and the Ryukyu Arc probably was connected to the Chinese continent, through Taiwan as a land bridge. The paleo-land probably was submerged step by step since 0.02 Ma caused by both of the crustal movement and the sea level rising after the last Ice Age. Submarine stalactite caverns were discovered at 10 - 35 m deep off the Ryukyu Islands. The caverns have subsided since the Würm (Wisconsin) Ice Age. Stone tools were also recovered inside one of them. Additionally, archeological ruins similar to an ancient castle, estimated at about 10,000 years were discovered beneath the sea off Yonaguni Island. Existence of Holocene and such submarine ruins provide indicators of subsidence processes of the Ryukyu Arc during post Glacial sea level rising.