Diatom assemblages and Holocene sea level changes at the Tamatsu site in Kobe, western Japan1)

Diatom assemblages were analysed to clarify the changes in sedimentary environments caused by the Holocene sea level changes, and the results were cross-checked with information from the field observations of sedimentary facies. Diatoms were grouped into three ecological categories; marine, brackish and freshwater. Based on the ecological spectrum of the diatoms, the sediments were divided into five diatom zones as follows: (1) Freshwater diatom zone 1 (FD-1) is dominated by freshwater diatoms. (2) Transitional zone 1 (Tr-1), where a brackish species begins to appear and freshwater diatoms are replaced by marine ones as the dominant. This zone indicates when the sea penetrated into this site at the beginning of the Holocene transgression. (3) Marine diatom zone (MD) is dominated by marine diatoms. This zone is subdivided into three subzones on the basis of the dominant species; a) Subzone MD-a is characterized by Nitzschia granulata and Diploneis pseudovalis. b) Subzone MD-b is dominated by Melosira sulcata and Cyclotella stylorum. c) Subzone MD-c is dominated by Cocconeis scutellum, Cyclotella striata, and Nitzschia lanceolata. Typical planktonic species, Thalassionema nitzschioides and Thalassiosira spp., also occur with low frequencies. This subzone contains the Akahoya tephra intercalary at a horizon of -1.3m and is considered to coincide with the culmination of the Holocene transgression. (4) Transitional zone 2 (Tr-2) is subdivided into two subzones; the subzone Tr-2a is dominated by the brackish species Terpsinoe americana, and the subzone Tr-2b is dominated by freshwater diatoms, intermixed with two marine species, Nitzschia granulata and Cyclotella stylorum. (5) Freshwater diatom zone 2 (FD-2) is characterized by the aerophilous species. In this study, the environmental changes caused by the Holocene sea level changes were clearly reflected in diatom assemblages. The replacement of freshwater diatoms by marine ones in the zone Tr-1 was so remarkable that we could easily trace the beginning of the transgression. At this site, the horizon indicating the beginning of the transgression based on the diatom assemblages precedes that based on macrofossil records by about 2m. The upper limit of marine deposits based on the diatom assemblages is indicated by Terpsinoe americana, the dominant species of subzone Tr-2a, and agrees with information from the field observations of sedimentary facies.