Abstract
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) 14C 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 5,000-2,000calBP and a lagoonal 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 than 18 m. It is considered that fresh water was identified as the major contributor to the large lake volume 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.
