Ice-wedge Casts Showing Climatic Change Since the Late Pleistocene in the Source Area of the Yellow River, Northeast Tibet

The source area of the Yellow River is located in the northeastern Tibetan Plateau, and is a high-elevation region with the annual mean temperature of −3.9 ℃. The ice-wedge casts discovered in this region are recognized as two types. One was found in sandy gravel beds of the second terrace of the Yellow River. This ice-wedge cast is characterized by higher ratio of breadth/depth, and are 1–1.4 m wide and about 1 m deep. The bottom border of the ice-wedge cast is round arc in section. Another discovered in the pedestal of the second terrace has lower ratio of width/depth, and is 0.3–1.0 m wide and 1–2 m deep. Its bottom border is sharp. Based on the TL dating, the former was formed at the middle Holocene (5.69 ± 0.43 ka BP and 5.43 ± 0.41 ka BP), that is, the Megathermal, and the latter was formed at the late Last Glacial Maximum (13.49 ± 1.43 ka BP). Additionally, the thawing-freezing folders discovered in the late Late Pleistocene proluvium are 39.83 ± 3.84 ka BP in age. The study on the ice-wedge casts showed that the air temperature was lowered by up to 6–7 ℃ in the source area of the Yellow River when the ice-wedge casts and thawing-freezing folds developed.