The Discovery of Sand Wedges of the Last Glaciation in the Hexi Corridor and Its Paleoclimatic Significance
WANG Nai ang 1, 2 , WANG Tao 1 , SHI Zheng tao 1 ,HU Gang 2 , GAO Shun wei 2 (1.Cold and Arid Regions Environmental and Engineering Research Institute, CAS, Lanzhou Gansu 730000, China; 2. Resources and Environment College of Lanz
Sand wedge is formed under cold climate, and is an important basis for reconstructing paleoenvironment. It has been reported constantly over the last 20 years in the Tibetan Plateau, North China and Northeast China. Fossil sand wedge groups at the end of the Late Pleistocene recently are found in Jiuquan, Anxi, Dunhuang and other areas in the Hexi Corridor now. The fossil sand wedges, found by the authors, were mainly formed in alluvial gravel stratum of bajada and piedmont alluvial plain. Based on the field incomplete statistics, the top-width of them is usually 12～30 cm, with a depth of 17～70 cm and a mean ratio of width to depth of 1∶1.61. Their morphometrics appear bottom-cuspate wedge, deep and narrow. Most of wedge bottom is single cusp. Eolian sand is filled in all wedge body. It is found from the granulometric composition of two sample groups that fine sand ranging from 0.25 to 0.125 mm is the maximum; followed by very fine sand ranging from 0.125 to 0.063 mm, and the physical clay, less than 0.002 mm, is less than 3%. A thin layer of grail overlies the top of the sand wedge groups. According to the morphological characteristics of profile, sand wedges have nothing to do with tectonic movement, fluvial erosion and drying crack of ground surface, but have something to do with disturbance caused by seasonal process of permafrost. 14 C dating of dripping carbonate within the sand wedges from the Anxi County shows that they were formed at about (22 500±190) a BP and (19 100±125) a BP, during the Last Ice Age. The study suggests that the Hexi Corridor was the southern periphery of the continuous permafrost zone with a climate rather cold and relatively moist at the Last Ice Age, because the region was situated in a transitional zone under the interaction of monsoon westerly. According to the temperature suggested by Romanovskiy for the formation of sand wedge within gravel, it is estimated that the annual mean air temperature during the Last Glaciation might be 11～15 ℃ colder than that at present. This estimation is coincident with the predication from mirabilite sedimentary in the Hexi Corridor and also with the new research results on the temperature-falling amplitude of middle and high latitudes in the Northern Hemisphere during glacial period. The study also points out that the annual precipitation for the formation of sand wedge might be about 100～200 mm, so the disparity of precipitation between the Last Glaciation and the present was about 100 mm. The Hexi Corridor is situated in the Northwestern part of China, belongs to the inland drainage basin at 1 400～2 500 m a. s. l., and is located in the transition zone of monsoon westerly. So the Hexi Corridor is an important position for studying the past global change owing to its ecological fragility and climatic sensitivity. The first discovery of the sand wedges in this region provides an immediate evidence of permafrost distribution in North China in the Late Pleistocene, and presents a significant basic data for reconstructing paleoclimate in the transition zone of monsoon westerly.