日本海沿岸の更新世以降の古環境の変遷

抄録

Changes in the palaeoenvironment along the Japan Sea since the early Pleistocene are reviewed with reference to the level of sea water and to the connection between islands and the continent and the nature of ocean currents. In addition, the sea level during the Murodo and Tachikawa stages is discussed. The nine palaeoenvironment stages in ascending order are as follows; 1. Omma, 2. Utatsuyama, 3. Riss II, 4. Hiradoko, 5. Murodo, 6. Tachikawa, 7. Nyuzen, 8. Jomon transgression, and 9. Yayoi regression.
Early Pleistocene
1. Omma stage; older than ca 0.8Ma B.P.
Environmental analysis of the Omma-Manganji fossil molluscs shows that the main cold currents flow from the north and the occasional warm currents from the south.
Middle Pleistocene
2. Utatsuyama stage; ca 0.5-0.3Ma B.P.
This stage is represented by the Utatsuyama, Hanyu and Kurehayama formations in the Hokuriku region and the upper part of the Uonuma group in the Niigata area; these formations consist mainly of terrestrial materials. An embayment molluscan fauna such as Batillaria or Anadara is sometimes found in these formations, which shows a marine enviroment near the depositional basin of these formations.
3. Riss II stadial; ca 0.16Ma B.P.
Although there is no direct evidence of this stage, the climate of the stadial is proved by the distribution of morains and other evidence which suggests a climate colder than those of the Würm I, II, and III stadials. At the Riss II stadial the sea level would have been 140m lower than the present, if sea leval change corresponds to the air temperature. Therefore it is reasonable to think that the Japan Sea at that time was enclosed and that land bridges connected the islands to the Asian continent.
Late Pleistocene
4. Hiradoko stage; ca 120ka B.P.
There are several marine terraces which contain marine shells along the Japan Sea: Hiradoko in Noto, Katanishi in Oga and Kurobe in the Tango area. Ecological analysis of molluscs in these marine terrace deposits shows that the sea water temperature at this stage was the same as, or a little warmer than, that at present. The sea level may have been 5m higher than at present.
5. Murodo stage (stadial); ca 60-40ka B.P.
These are two glacial stadials in the Würm age in Japan. The earlier one is the Murodo, and the later one is the Tateyama. To build a glacial cirque requires a lot of snow, which suggests warm currents flowing into the Japan Sea. The sea level at this stage must have been about 100m deeper than at present.
6. Tachikawa stage (Tateyama stadial); ca 20ka B.P.
The coldness of the Tateyama stadial was less severe than that of the Murodo stage. So the sea level was not deeper than that of the Murodo stage, presumably 80-100m deep. But warm currents still flowed into Japan Sea before the beginning of this stage, and these warm currents opened the Tsushima and Korea straits.
Holocene
7. Nyuzen stage; ca. 10ka B.P.
A Nyuzen submerged forest is found 40m under the sea on the continental shelf off the Kurobegawa alluvial fan, Hokuriku region. The climate of this stage was 2-3°C colder than at present.
8. Jomon transgression stage; ca. 6ka B.P.
Warm currents flowed into Japan Sea. The sea level was 3-6m higher than at present. This transgression invaded most of the alluvial plains around the Japanese Islands.
9. Yayoi regression stage: 2ka B.P.
An Uozu submerged forest has been found 1-2m under the sea; such a phenomena have been found in several coasts along the Japanese islands.