1990 年 29 巻 3 号 p. 193-208
The problem of the maximum depth to which the sea level dropped during the last glaciation is even more difficult and obscure than that of high sea levels. Some evaluations have been proposed, based on the estimations of the depth of the Holocene sediment base in the coastal plain or on the consideration of submerged sediments or geomorphological features now found on continental shelf. Many Japanese geologists estimate -100 to -140m for the last glacial low stand sea level, but some doubt is cast on their data and opinions. Our estimate of -80±5m was a conservative interpretation based on dates for shells and peat obtained from the shelf and coastal plains of the Japanese Islands.
1. Charts show that wherever sizable inland seas are separated from the ocean by narrow straits, current erosion holes (sea caldrons) exist either in the narrow straits or directly adjacent to them. The Tsushima strait is a part of the shelf, although there is a relatively deep hole (to -160m) along the east side of Tsushima Island. The depth of this deep hole attains 50m below the surrounding relict wave-cut terrace (-110m to -115m). In this case, the depth of the relict wave-cut terrace may reflect the sea level (about -100m) at the time of the Tsushima strait formation.
2. As a result of continuous seismic profiling surveys in Ishikari Bay, the buried valleys on the continental shelf are known to be deeper than 90m. The buried valleys were formed at the stage of the lowest sea level, during the last Glacial Age. However, we observed that Ishikari river may be 10m to 15m deep near its mouth. The available data indicate a relative rise of sea level since the valley was cut, amounting to about 80m.
3. The breadth to depth ratio of the straits around the Japanese Islands seems to reflect each still standing sea-level stage, such as -100±10m, -80±5m and -45±5m, after the formation of these straits.
4. For most of the Pleistocene, the Japanese Islands were connected with the Korean Peninsula, and the present major islands themselves were tied to each other. It is sure that large mammals such as elephants migrated into the Japanese Islands through land bridges. In the early Shimosueyoshi transgression (about 100, 000y.B.P.), when the sea level was about -100m, the Japan Sea was connected with the Pacific through narrow passages located in the Korean and Tsugaru straits. At the time of the last Glacial Age, when the sea level fell to about -80m, land bridges between Honshu and other lands were never formed. We now believe that it might have been 12, 000 years ago when the sea level rose to about -45m. This was the final stage of the land bridge in the Soya strait between Sakhalin and Hokkaido.