日本列島の最後の陸橋(討論会-日本列島の第四紀にまつわる諸問題,<特集>20回記念総会「第四紀」討論会)

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During the time of the Monastirian transgression, the Japan sea may once have opened to the Pacific through narrow passages probably located in the Korea and Tsugaru straits of the present day. It is not difficult for us to present many geological evidences concerning the above statement. In the Early Wiirmian glacial stage (Altwurm, Fruhwurm W_I =Wv), sea level became lower and the maximum lowering in this age is now estimated by us to have been about -80m or so, as indicated by the level of the top of wave cut terraces deeply burried in the subsurface, which are thickly covered by volcanic products of the interglacial age between W_I and Wh_1 or the beginning of Wh_1. It might have been sometime during the ice age of W_I, when the Mammonteus fauna migrated to Hokkaido from the continent, via Saghalin, crossing over the Soya straits between Hokkaido and Saghalin. During the interglacial age of W_I/Wh_1; the sea level might have become much higher than during the preceding age, and it may have been about -40m at the maximum. Then the sea level again lowered as much as about -116m. In all probability, land bridges still existed in this stage (Wh_1) in many straits including both Korea and Tsugaru, because topographic surfaces of those days could have been much higher than the buried topography preserved under the sea of the present day. Then sea level became higher during the next interglacial age Wh_1/Wh_2. It reached about -94m or possibly -80m at its maximum. In this age, the land bridges which existed in the Tsugaru and Korea straits are considered to have sunken under the sea, for a time, but these bridges were soon reexposed by the rapid lowering of sea level in the glacial age of Wh_2, followed by Wh_1/Wh_2 interglacial stage. In this age, the sea level may have been as low as about -140 m. This maximum lowering of sea level is believed to have occurred from 20,000 to 18,000 Y.B.P. This age estimate is based on archeological evidence, namely the same type of Late Paloeolithic industry called the Early Shirataki which is found both in Hokkaido and north Honshu ; this cultural stage has been viewed to be approximately comparable to the Late Aurignacean. Hence, the age of this culture may be between 20,000 and 18,000 Y.B.P. This is the final stage during which the Japanese islands were united with the continent by land bridges in the Korea and Tsugaru straits. No doubt, the Japan sea existed in those days as a large inland sea, completely isolated from the Pacific. Under such circumstances, animals, plants and ancient dwellers seem to have been able to freely migrate north to south throughout the Japanese islands. After the maximum lowering of sea level, however, land bridges disappeared from both the Korea and Tsugaru straits by rapid and almost continuous rising of sea level in the latter half of Wh_2 and also Wh_3, time, the latest glacial stage. It is now believed by us that it might have been perhaps some 12,000 years ago when the sea level rose to about -70m. This was the final stage of the land bridge in the Soya straits between Saghalin and Hokkuido. Since then, Hokkaido has deen isolated from both the continent and Honshu. The age estimate in this case is also based upon the correlation of other archaeological evidences. Sea level seems to have been continuously rising since the time of the maximum lowering, with episodic lowering two or three times during the time of Wh_3 and pre-boreal and boreal stages, as is shown in fig. 4. It may have been during the Atlantic stage, especially the latter half, when sea level reached its present heignt.
地学団体研究会の論文
1966-09-25

日本列島の最後の陸橋(討論会-日本列島の第四紀にまつわる諸問題,<特集>20回記念総会「第四紀」討論会)

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