Tephrochronology of Late Pleistocene Tekano Formation, Nagano Prefecture, Central Japan, with Environmental Changes Reconstructed from Pollen Spectra

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The Pleistocene Takano Formation, which is distributed in Takano, northern Nagano Prefecture, consists of lake sediments with many tephra layers. Pollen spectra from the Takano Formation are correlated to those from Lake Nojiri and Lake Suwa, using widespread tephra layers as key beds, to clarify the regional vegetation changes reflecting climatic oscillations.Tephra layers from the Takano Formation are identified by the compositions of grain and heavy minerals; the refractive indices of volcanic glass, orthopyroxene, and hornblende; and the major element composition of the glass. As a result, tephras Tt-D, Aso-3, On-Pm1, K-Tz, On-Kt, Aso-4, On-Ng, Tt-E, DKP, and AT are recognized in ascending order.Pollen analysis was done for the sediments from 2.2m below the Tt-D horizon to 2.0m above the DKP horizon, covering about 11.5m in stratigraphic thickness. Eight local pollen assemblage zones, from the bottom TKN-I to VIII, are established based on the changes of major pollen types. Zones TKN-I to VI show high occurrences of Cryptomeria and Cupressaceae-type pollen, which characterized the Early Glacial (MIS 5a-5d) pollen assemblages in central Japan. Zone TKN-I is characterized by abundant Pinaceae pollen with Cryptomeria pollen, which indicates a cool climate. TKN-II (intercalating Tt-D) is characterized by pollen of temperate deciduous broadleaved trees such as Quercus subgen. Lepidobalanus, Carpinus/Ostrya, Fagus crenata type, Ulmus/Zelkova/Hemipletea, and Cercidiphyllum, which show the warmest climate conditions in this section. TKN-III (intercalating On-Pm1, K-Tz) yields more Pinaceae pollen with less pollen of temperate deciduous broadleaved trees than TKN-II, and TKN-IV (intercalating Aso-4) is characterized by abundant Pinus subgen. Haploxylon and Betula pollen, reflecting a climate change to cold and dry. TKN-V (On-Ng is near the bottom) yields more values of deciduous tree pollen, which indicate a warm climate. TKN-VII (intercalating Tt-E and DKP) is characterized by dominant Pinaceae pollen (Picea, Abies, etc.), and Larix pollen, indicating remarkable cold and droughts in this zone. TKN-VIII is characterized by Picea and temperate broadleaved tree pollen, and was under a warm climate. These environmental changes are supported by the correlation to the pollen spectra of adjacent areas using widespread tephras as key beds. These correlations allow for a new chronological aspect of the horizon of DKP correlating to MIS4.

Tephrochronology of Late Pleistocene Tekano Formation, Nagano Prefecture, Central Japan, with Environmental Changes Reconstructed from Pollen Spectra

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