Abstract
This paper describes the Holocene environ-mental change in the downstream part of the Iwaki-gawa Lowland based on the analyses of diatom assemblages, grain size, sulfur content, sedimen-tary facies, and AMS 14C ages of the sediment cores.
The result indicates that the study area had a formation of a brackish lake during the early stage of the Jomon transgression following the development of a high salinity water condition around 7,500-7,000calBP. Subsequently the area changed to a lake environment dominated by fresh surface water after 6,000calBP, because of the formation of sand spits protecting the lake from the Sea of Japan and a relative sea level fall. It is estimated that a stratified lake condition was dominant during the period. The water area and depth of the lake decreased due to the growth of a bay head delta after 3,000calBP. After this period, the studied area changed to a brackish lagoon condition such as in seen in the present-day Lake Jusanko. It is inferred that the shallow lake topography led to the condition under which freshwater and seawater easily mixed together. Although these environmental changes of the lake had occurred simultaneously in the study area submerged by the Jomon transgression, the period of change from a fresh water to a lagoonal environment varied among sites depending on the differences in sediment supply from each river.
