Stratigraphy at Naka-ikemi consist of a lower sand and gravel layer and an upper humus layer which includes widespread tephras, Ata, K-Tz, Aira-Tn (AT), and K-Ah. By comparing pollen spectra of the upper humus layer below AT for five drilling cores, vegetation changes at around Naka-ikemi during the late Pleistocene are reconstructed. Overall dominance of Cryptomeria pollen below AT indicated that Cryptomeria japonica forests spread widely through the periods. Differences in pollen assemblages between drilling points showed that deciduous oak forests distributed on the northern hills, alder forests grew in the wetland, and Cryptomeria japonica forests extended along the southern valley. At about 80,000 years ago, Castanea/Castanopsis pollen increased, with an enlargement of depositional area to the south, and was taken over by an increase of deciduous broadleaf tree pollen, followed by an extention of depositional area to the north. At around 50,000 years ago, marked by a brief increase of Hydrangea type pollen, Alnus and Lysichiton pollen increased, Sciadopitys pollen decreased, and Cupressaceae type pollen increased. Simultaneously, the depositional environment of Naka-ikemi changed from lake to moor. An increase of Tsuga, Pinus, and Betula pollen just below AT showed the onset of a cold phase leading to the last glacial maximum.
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