LOESS SEDIMENTS IN THE NORTH SLOPE OF TIANSHAN MOUNTAINS AND ITS INDICATION OF DESERTIFICATION SINCE MIDDLE PLEISTOCENE

Loess sediments in Xinjiang in the interior of Asia possess important environment information such as the formation and development of deserts,air-circulation,and the uplift of Tibetan Plateau. In this study,detailed climatic proxies of a 71 m-thick eolian loess section at Dongwanzhen in Shawan County,located at the north slope of Tianshan,are measured and analyzed. Environment history of North Xinjiang area since middle Pleistocene has been reconstructed. The environment history with a gradual drying trend can be divided into three periods. The first period from 0.80 to 0.60 Ma is a relative wet stage,the second period from 0.60 to 0.25 Ma is an accelerated drying stage,and the third period from 0.25 Ma to now is the formation of modern dry climate stage. Two significant climatic events that happened around 0.60 and 0.25 Ma promoted the process of drying. These evidences indicate the deserts existed in North Xinjiang area as early as 0.8 Ma,and the modern dry climatic pattern formed during 0.25 Ma.