Initiation Age of a Peaty Soil Layer in the Subalpine Zone of Mount Tairappyo, the Mikuni Mountains, Central Japan

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  • SASAKI Akihiko
    Institute of Geography, Graduate School of Science, Tohoku University
  • KARIYA Yoshihiko
    Human and Earth Environment Course, Graduate School of Science and Technology, Chiba University

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  • 三国山地平標山の亜高山帯に分布する泥炭質土層の生成開始期
  • ミクニ サンチ タイラッピョウヤマ ノ アコウザンタイ ニ ブンプ スル デイタンシツドソウ ノ セイセイ カイシキ

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Abstract

A peaty soil layer is frequently observed on the alpine and subalpine slopes of snowy mountains in central and northern Honshu, Japan. The genesis of the peaty soil layer is closely related to ground surface stability, plant succession and hydrological condition on these slopes. Therefore, the peaty soil layer plays an important role in reconstructing paleoenvironment in the snowy mountains. In particular, the data on the initiation of the peaty soil layer can be regarded as the significant evidence of the landscape change since the Last Glacial age. However, the previous studies on the peaty soil layer have rarely presented the precise age when the formation of such a soil layer was initiated.<br>The following two opinions were previously presented on the initiation age of the soil layer in the subalpine slopes of Mount Tairappyo (36°48′N, 138°49′E; 1, 984m a. s. l.). One is that the peaty soil layer began to develop in the late Holocene, and the other is that initiation age of the soil layer considered to be older than the middle Holocene. The discrepancy causes large confusion in the paleoenvironmental interpretation in this area.<br>This paper aims to clarify the initiation age of the peaty soil layer in the subalpine zone of Mount Tairappyo.<br>The results are summarized as follows:<br>1) The peaty soil layer is approximately 50cm thick, and extensively covers fossil snowpatch hollows and fossil periglacial smooth slopes. 2) The peaty soil layer commonly intercalates the middle Holocene tephra layers such as the Kikai-Akahoya (7, 200cal y BP), the Myoko-Akakura (6, 600cal y BP), the Asama-Tairappyosan 3 (6, 600cal y BP) and the Asama-Tairappyosan 2 (6, 300cal y BP) in its lower part. 3) The age of the boundary between the peaty soil layer and the underlying slope deposits is tephrochronologically determined as 8, 000 to 7, 000cal y BP in most parts of the study area, even those fossil snowpatch hollows where snow disappears latest in this area. The peaty soil layer which was formed in the early to the middle Holocene is, however, lacking in a restricted central part of fossil snowpatch hollows. 4) The peaty soil layer began to accumulate in the whole area, except for the central part of fossil snow patch hollows, by 8, 000 to 7, 000cal y BP. Periglacial and nivation processes became less active by 8, 000 to 7, 000cal y BP.

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