Fossil sand wedges in the northern shore of Qinghai Lake:discovery and paleoclimatic implications

Qinghai Lake in the northeast of the Tibetan Plateau is the biggest inland lake and largest saltwater lake of China. The lake is at an elevation of more than 3 000 m with an annual mean temperature of 1. 7 ℃.Around the lake there is no permafrost now,but ice-wedge casts and fossil sand wedges are discovered. Investigation in this area has found that there are many groups of fossil sand wedges in the northern shore of Qinghai Lake. One is located in the east of Gangcha County( 37°15'43. 7″ N,100°00'28. 4″ E),the another in the west of Quanji Township( 37°14'22. 2″ N,99°50'13. 0″ E). The fossil sand wedges found in the east of Gangcha County are named as GS1,GS2 and GS3,respectively. GS1 is 0. 9 m in breadth and 1. 9 m in depth,based on ESR dating,which formed during the Kunlun Glaciation( 774 ± 70) ka; GS2 is 1. 4 m in breadth and 1. 7 m in depth,which formed in the same layer of the profile in the east of Gangcha County,and based on the ESR dating,which also formed during the Kunlun Glaciation( 773 ± 70) ka; GS3 is 0. 5 m in breadth,1. 0 m in depth,based on ESR dating,which formed during the Penultimate Glaciation( 229 ± 20) ka. The fossil sand wedges found in the west of Quanji Township is named as QS1,which is 2. 3 m in breadth and 1. 5 m in depth.The characteristics of QS1 are quite different from GS1,GS2 and GS3. QS1 is characterized by higher ratio of breath to depth and formed at( 177 ± 17) ka and( 197 ± 18) ka based on ESR dating,thus,it is believed that QS1 formed during penultimate glaciation. Research results of fossil sand wedges have environmental significances. It is estimated that the annual mean temperature in the northern shore of Qinghai Lake was probably- 7. 5 ~- 10 ℃ in the Kunlun Glaciation and about- 9. 5 ~- 10 ℃ in Penultimate Glaciation. Temperature reduction in glaciations were conspicuous. It is the first time to find that fossil sand wedges,which formed during the Kunlun Glaciation,in this region. The discovery has significance in Quaternary paleoclimate study and the study of permafrost distribution in the northeast of the Tibetan Plateau.