Opal Phytolith Assemblages and Reconstructed Paleoenvironment since the Last Interglacial on the Foot of Mt. Iwate, Northern Japan.

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  • 洞爺火山灰以降の岩手火山テフラ層の植物珪酸体群集と古環境
  • トウヤ カザンバイ イコウ ノ イワテ カザン テフラソウ ノ ショクブツ ケ

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Abstract

We clarified the Late Pleistocene and Holocene vegetation and climatic changes by phytolith analysis of accumulated tephras (aeolian deposits including a eolian dust) after the fall of Toya Ash on the foot of Mt. Iwate. Five cold stages (KS-a-KS-e) were distinguished by a relatively high frequency of Pooideae phytolith (Festucoid) and coniferous phytolith. A tephra stratigraphy and chronology suggests that the KS-a to KS-d cold stages between the White Pumice (Towada Pumice Flow: ca.15ka) and the Aso-4 Ash (70-90ka) are correlated with those in the Last Glacial and that the KS-e cold stage beneath Aso-4 Ash corresponds with that in the Last Interglacial. A dominance of Pooideae in the KS-a and KS-d cold stages indicates a very cold climate. The occurrence of coniferous forests in the KS-b, KS-c and KS-d suggests a humid condition. Climate in the Ks-e cold stage is not colder than that in later cold stages because of the relatively small amount of Festucoid. Since the Last Interglacial after the fall of Toya Ash, the following three periods are established based on paleoenvironments reconstructed by vegetation history, periglacial phenomena, involution, and a eolian dust influx: 1) Toya Ash-K3S: a cool or cold period with humid climate and characterized by a low a eolian dust influx in the late Last Interglacial and marked deposition of primary and secondary tephras; 2) K3S-the top of the Shibutami Volcanic Ash: a very cold or cold period with humid or dry climate, characterized by a high a eolian dust influx, crack development in a eolian dust horizons, and intense involution corresponding to the Last Glacial age; 3) the Wakare Volcanic Ash (13ka-modern): a cold to cool period and humid climate, characterized by a low a eolian dust influx and humus accumulation.

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