Abstract
We present pollen assemblages of the early Last Glacial Biraotri Formation at the right bank of the Rakko River, Hiroo, in eastern Hokkaido. Nineteen tephra layers were recognized in the study section, and two of them are correlated to the widespread tephra Aso-4 and K-M, which have been dated to the early Last Glacial period. Eight local fossil pollen assemblage zones HRO-I to VIII are established based on the changes in the major arboreal pollen. The patterns of pollen assemblage change below and above the K-M pumice are similar and corresponding to two sedimentary cycles from gravel to peat. In each sequence, the characteristic pollen types change from Alnus, through Larix, Picea, to Larix from lower to upper. This pattern reflects succession linked to the bog development and increasingly cold climate. The pollen assemblage in the upper sequence indicates climatic fluctuation with a larger amplitude. The lower sequence suggests the smaller climatic fluctuation in oxygen isotope stage 5, and the upper one does larger change from the stage 5 to 4.
