青森県十三湖における完新世の湖水成層化と地形環境

摘要

Lake Jusanko is a shallow lagoon that is located at the northern edge of Honshu Island in Japan. The purpose of this study is to reconstruct changes in the geomorphic environment and salinity in Lake Jusanko, by analyses of lithofacies, diatoms, sulfur content and grain size, and by performing accelerator mass spectrometry (AMS)~~（14）C dating. The results of this study indicate that the study area was submerged because of postglacial transgression in the early Holocene. After the transgression, a sandspit that divided Lake Jusanko from the Sea of Japan began forming in 7,000calBP. Subsequently, the lake was buried by delta deposits transported by the Iwaki River. Diatom analysis indicates that Lake Jusanko was a freshwater lake during 5,0002,000calBP and alagoonal brackish lake since 1,000calBP. However, the deposits during this period show a high sulfur content, which implies that the sediments were deposited under marine conditions. To clarify the cause of these contradictory results of the lake environment, the authors attempted to reconstruct the depth of the Lake Jusanko since the middle Holocene. The lake depth in 6,500calBP was more than18 m. It is considered that fresh water was identified as the major contributor to the large lake voKime in the middle Holocene. These facts reveal that the surface of the lake was composed mainly of fresh water, while salt water sank to the bottom of the lake. Consequently it is estimated that a stratified lake condition was dominant during the middle Holocene.%青森県北西部に位置する十三湖の沿岸付近においてボーリング試料を採取し，堆積物の層相観察.珪藻分析，イオウ分析，粒度分析.~(14)C年代測定を実施することにより.槻文海進以降の十三湖の地形環境および湖水環境の変遷について検討した.最終氷期以降に陸域であった調査地域は.約9,000calBP以降に禅文海進がおよんで水域となり.7,000calBP前後には塩分の高い海域となった.十三湖は，7,000calBP以降の砂州の発達により日本海と隔てられることで形成され，約6,000calBP以降には淡水の彩響が強くなり，とくに5,000~2.000calBPには淡水の影響の極めて強い湖沼であった.この時期の十三湖は現在よりも水深が大きく，流入した海水が湖底に滞留し，表層に淡水が存在する成層状態となつていたことが推定された.その後，水深が小さくなったため風波による湖水の攪乱が生じやすくなり，遅くとも約1.000calBPには汽水環境が成立していた.