チベット東南部ポテチュー谷における木片および埋没土壌の年代とその古環境復元上の意義

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タイトル別名
  • Radiocarbon Ages of Woods and Buried Soil Layers and their Significance in the Potechu Valley, Southeastern Tibet

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Geomorphplogical and environmental field research was conducted during a visit to the southeastern part of the Tibetan Plateau in 1989,as a part of the China-Japan Joint Glaciological Expedition to Qingzang Plateau, 1989. The study area is located on the southern flank of the Nyainqentanglha Mountains and is a typical U-shaped valley, called Potechu, in which head the Zepu Glacier occurs. A humid climate combined with the relatively low altitude of the valley causes a wide extent of dense coniferous forest. The geomorphological evidences and results of carbon dating obtained from the valley show the following chronological sequence of fluctuations of the Zepu Glacier. During the past 600yrBP the Glacier advanced 2km, and Neoglacial advances occurred prior to 1,000yrBP, and the maximum advance attained down to 5km from the present terminal position. A time-distance curve showing fluctuations of the glacier front during the last two millennia is illustrated. This indicates that the Neoglacial advances are more extensive than those of the Little Ice Age for the Zepu Glacier. Buried soil layers were observed at many localities downstream the valley. They are 10 to 30cm thick and contains humus and charcoal fragments. Radiocarbon ages of these soil layers concentrate from about 700yr.BP to 1,200yr.BP. Generally speaking, in high mountain regions buried humic soil layers often signify relatively warm period, while under relatively moderate climate of this valley, they do not always indicate warm climate. Many charcoal fragments contained in the buried soil layers are likely to imply the evidence of human activity rather than warm climate, in other words, at that time inhabitants in the area may have intensively burnt the forest in order to make fields and pastures. Deforestation by the human activity may have took place in the valley during the period.

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