中海・宍道湖の地史と環境変化  [in Japanese] Natural History and Environmental Changes of Lakes Nakaumi and Shinji  [in Japanese]

中海・宍道湖の自然史を, 8葉の古地理図として示した。完新統堆積前には西の大社湾に注ぐ古宍道川と東の美保湾に注ぐ二つの水系が存在した。縄文海進はこれらの二つの水系にそってすすみ, 古宍道湾と古中海湾が形成された。縄文海進高潮期には古宍道湾の中央部が埋め立てられ, 東の水域は古宍道湖となり, やがて西の中海湾へと排水するようになり, 現在の中海・宍道湖の原型ができあがった。宍道湖宅はA.D. 1600年頃を境としてそれまでの汽水環境から淡水環境へと変わった。中海では環境変化が複雑だが, 米子湾でみるとA.D. 1600年頃までは出雲国風土記にも示されている夜見島の南に美保湾に通じる水道が断続的に存在したが, その後は閉鎖的環境が急速に進んだ。これらの環境変化は中国山地の人為的な荒廃による土砂の大量流出によって起こったが, 中世の温暖期をへて, A.D. 1600年頃を中心とする寒冷期にいたる地球規模の環境変化が背景となっているものと考えられる。^<210>Pb, ^<137>Cs年代測定および過去25年間の地形変化からそれぞれ求められた宍道湖での埋積量は約0.1/gr/cm^2程度であり, 中海ではその1/3と見積もられる。

Lakes Nakaumi and Shinji are maritime coastal lakes along the Sea of Japan. Their origins and development are intimately related to post Glacial sea-level changes and they have been affected extensively by human activities since the 17th century. We have surveyed the entire areas of the lakes by various methods of echo-sounding. Drilling data in and around the lakes and more than twenty gravity cores were analysed sedimentologically and micropaleontologically. Organic remains of mollusc, pollen, diatom, ostracod, foraminifer, etc. preserved in sediments are very useful not only for correlating the strata but also for recognizing the environmental changes of the lakes. Radiometric (^<14>C, ^<210>Pb, ^<137>Cs) age data are summarized and discussed. These data have made it possible to illustrate with precise paleogeographic maps eight stages of lake development from 20,000 y.B.P. to 100 y.B.P. The exploitation of iron ores in the Chugoku Mountains strongly affected the flow regime of the Hii River and the environment of Shinji Lake. In the middle of the 17th century the Hii river was directed towards the east, flowing into Shinji Lake, and as a result the lake became dominated by a freshwater environment. This change in lake environment may have been related to global cooling and the accompanying fall of sea level during the little Ice Age. The growing fan-delta of the Hii River prograded rapidly eastwards, forming the eastern part of the Izumo Plain. During the last 100 years many engineering structures have been made in and around the lake which have changed it again to a brackish lake.