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ICE-WEDGE CASTS DISCOVERED IN THE SOURCE AREA OF YELLOW RIVER, NORTHEAST TIBETAN PLATEAU AND THEIR PALEOCLIMATIC IMPLICATIONS

Cheng Jie~(①) Zhang Xujiao~(①) Tian Mingzhong~(①) Yu Wenyang~(①) Tang Dexiang~(②) Yue Jianwei~(③)(①School of the Earth Sciences and Resources,China University of Geosciences,Beijing100083;②Institute of Process Engineering,Chinese Academy of Sciences,Beijing100080;③Institute of Geographical Sciences and Natural Resources Research,Chinese Academy of Sciences,Beijing100101)  
The source area of Yellow River is located at Northeastern Tibetan Plateau,and is a high elevation region with the annual mean temperature of-3.89℃ and annual precipitation of 322mm.In this area,permafrost is widely but isolatedly developed.The permafrost layers are about 6.5~65.5m in thickness,while active layers are about 0.8~1.5m.Our field work has confirmed that frost-heaved mound,stone polygon,debris slope,and freeze-thaw terrace are common in this area,but active and in-active ice wedges are absent.Ice-wedge casts and freeze-thaw folds,however,have been discovered in this area.Two types of ice-wedge casts were recognized.One type was found in sandy gravel beds of the second terrace of Yellow River,which is characterized by higher ratio of breadth to depth,and is 0.8~1.4m in breadth and 0.5~0.9m in depth.The lower border of the ice-wedge cast is round curve on section.The other type was discovered in bedrocks of the second terrace and has lower ratio of breadth to depth,and is 0.3~1.0m in breadth and 0.7~2.0m in depth.Its lower border is sharp.Based on the TL dating,the former was formed at the Middle Holocene(5.69±0.43kaB.P.and 5.43±0.41kaB.P.),and the latter was formed during the deglaciation period,about 13.49±1.43kaB.P.,which is correlated to the oldest Dryas in age. In additiona,the freeze-thaw folds were discovered from the late Late Pleistocene proluvium,and is 39.83±3.84kaB.P.in age.These discoveries infer that the temperature during the formation of the ice-wedge casts and the thawing-freezing folds might have dropped remarkably to 6~7℃ and that there might exist a sharp temperature-drop event in the Megathermal.
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