Abstract
In northern Hokkaido, sediments of the early half of the last glacial include several tephra layers from Mt. Rishiri. Those tephra layers are important keys for the geological correlation and the study of environmental changes during this period. We made a palynological study of peaty sediments including Rishiri-Acharu (Rs-Ac) at Toyotomi to determine the timing of Rs-Ac and to clarify the environmental changes. This tephra has been tephrochronologically positioned between Toya and Aso-4. Fossil pollen assemblages from the peaty sediments changed with the fall of Rs-Ac1, and assemblages rich in deciduous broadleaf tree pollen changed to those dominated by spruce pollen, suggesting environmental change to a colder and drier climate. After the fall of Rs-Ac2, Larix pollen increased, which indicated further environmental change to a cold and dry climate. Correlating these pollen spectra at Toyotomi to those at Haboro and Tomamae which intercalate Toya and Aso-4, Rs-Ac1 and Rs-Ac2 were repectively estimated to have fallen at the beginning of the marine isotope stage (MIS) 5b and just before the peak of MIS 5b.
