珪藻遺骸群集からからみた北海道オホーツク海沿岸藻琴における 完新世中期～後期の堆積環境

抄録

Diatom assemblages in sediments were analyzed to clarify sedimentary environments during the mid- to late-Holocene and to determine the upper limit of marine facies at the Mokoto site along the Okhotsk Sea in Hokkaido.
There was a replacement of marine and brackish-water diatoms by freshwater ones at +1.80 m altitude. Marine and brackish-water diatoms occurred over 40 % at the horizons below +1.80 m altitude, where a marine diatom Cocconeis scutellum and brackish-water diatoms Bacillaria paradoxa, Navicula peregrina and Synedra tabulata occurred. This indicates that sedimentation took place under a lagoon-like environment. Peat accumulation and the dominance of freshwater epiphytic diatoms indicate peat moor environments at the horizons above +1.80 m altitude. At about 5,500 yrs BP, the Mokoto site changed from lagoon littoral to peat moor conditions. Based on the diatom assemblages in sediments, the upper limit of marine facies can be regarded to be +1.80 m altitude, indicating the mid-Holocene sea-level high stand at the Mokoto site.