Holocene successions of diatom fossil assemblages in alluvium, and those relations to paleogeographical changes.

Abstract

Diatoms are single-cell algae, which live in marine, brackish and fresh water. As the flustules of diatom are often preserved in sediments, diatom fossil can be an useful indicator of paleoenvironments. The purpose of this paper is to reconstruct the paleogeographical changes of alluvial plains and coastal plains in Japan, using the data on the successions of diatom fossil assemblages in alluvial deposits. At first, the author collected the lake bottom sediments and living diatom samples in five brackish lakes in Japan, in order to consider the distribution-patterns of living and dead diatoms. Based on this contribution, species of diatom can be classified into 12 groups by ecological conditions, which are defined in the paper. The author observed diatom fossil assemblages in alluvial deposits, from the 25 localities of alluvial plains and coastal plains in Japan. 'Marine transgression' in early Holocene and 'Marine regression' in middle to late Holocene, both of which are represented by diatom fossil assemblages, are observed in every locality. Those phenomena are influenced not only at sea level change but also by development of beach ridge, movement of river mouth, change of depositional ratio of river and so on.-from English summary