Paleovegetation and paleoclimate changes since the late Early Pleistocene in Japan, a middle latitudinal East Asian country, were studied. Two long terrestrial pollen records, GS-K1 and GS-SB-1 cores, from the Osaka and Kanto sedimentary basins, respectively, have been studied palynologically. These cores consisted mainly of fluvial and shallow marine sediments. Moreover, an indirect relationship between cyclic sedimentary processes, i.e. alternating marine nonmarine sedimentation of the upper Osaka Group and the glacio-eustatic sea-level changes has been suggested.
Pollen assemblages were mainly dominated by temperate deciduous broad-leaved and evergreen coniferous tree taxa along with warm-temperate evergreen and deciduous broad-leaved tree taxa. An abundant horizon of the warm-temperate evergreen broad-leaved taxon Quercus (subgen. Cyclobalanopsis) was common in the middle Middle Pleistocene of both the core sites. This horizon corresponds to marine oxygen isotope stage 11 as per previous integrated chronological studies.
Although there were regional differences, pollen assemblages of the GS-K1 and GS-SB-1 cores showed a drastic change during the depositional age of the early Middle Pleistocene (transition from marine oxygen isotope stage 16 to stage 15). In the lower Middle Pleistocene, pollen assemblage was mainly dominated by temperate deciduous broad-leaved tree taxa such as Fagus and Quercus (subgen. Lepidobalanus) along with temperate evergreen coniferous tree taxa such as Cryptomeria and Cupressaceae, warm-temperate deciduous and evergreen broad-leaved tree taxa such as Cyclocarya and Quercus (subgen. Cyclobalanopsis). This indicates that paleovegetation was warm-temperate deciduous broad-leaved forest along with temperate evergreen conifer forest. Paleoclimate was warm and high precipitation was seen throughout the year. Quercus (subgen.
Lepidobalanus) pollen occurrence decreased in the middle Middle Pleistocene, whereas the number of temperate evergreen conifers (e.g. Cryptomeria and Tsuga) increased. This indicates the change of paleovegetation to temperate evergreen conifers alternating with temperate deciduous broad-leaved trees. Paleotemperature was warm throughout the year, although precipitation fluctuated seasonally, i.e. it was high in summer and low in winter. These records indicate that the polar front moved southward due to the changes in air mass composition during early Middle Pleistocene.