The succession of diatom assemblages and Holocene sea-level changes during the last 6,000 years at Sado Island, central Japan: the holocoene development of Lake Kamo-ko I.

Abstract

The diatoms in core sediments from Loc. 3 were analyzed to clarify the paleoenvironmental changes of Lake Kamo-ko during the last 6, 000 years. Diatoms were grouped into three ecological categories; marine, brackish and freshwater. Based on the ecological spectra of the diatoms, the sediments were divided into five diatom zones. The results are discussed with reference to Holocene sea-level changes as follows : Marine Diatom Zone 1 (MD-1) is characterized by marine species Diploneis novae seelandiae indicating that this site might have had a wide channel with the sea at the time of Holocene transgression. Transitional Zone 2 (Tr-2) is dominated by marine diatoms and freshwater ones alternately. Freshwater Diatom Zone 2 (FD-2) is characterized by freshwater planktonic species Melosira italica suggesting that sedimentation took place under standing-water conditions such as a freshwater lake. This may reflect the small regression at c.a. 4, 500 yBP. Marine Diatom Zone 2 (MD-2), first reported here, is dominated by marine species Diploneis suborbicularis and Nitzschia granulata, and may reflect the small transgression at c.a. 3, 000 yBP. Transitional Zone 4 (Tr-4) is considered to reflect the process of emergence at this site.