Early Holocene Paleolake Evolution And Otindag Sandy Land

At present, global changes have been paid widespread attention, and the response relationship between regional and global environmental evolution is extremely sensitive. Otingdag sandy land, located in an arid to semi-arid climate transition zone over middle Inner Mongolia plateau, is situated in the north of marginal Asian summer monsoon zones with a very fragile ecological environment, where exists dune sand, semi-fixed dune, fixed dune.At the same time, desert hinterland distributes a many of lakes and became the typical area of sand-land and lake evolution, where is the dream place of paleolake evolution and important place that sand-land increased with the decrease of lakes. The study on response of climatic change of marginal monsoon transition zone to change in lake area is benefit to understand environmental evolution at that place. The paper, combining with field work and experiment in laboratory, conducted grain size index analysis and OSL dating methods for profiles and constructed the time series of the lake evolution. According to the elevations of the lake shore in study area, using DEM of lake, the temporal and spatial pattern of the lake in early-middle Holocene has been inversed. The main conclusions follow as:1. Based on the systematical field work, there are still has lakeshore terraces, the altitude of high lakeshore terrace in north of Haga Nuur over Otingdag sandy land is 1313 m, illustrating that sand-land evolution has offered the geology landscape evidence to the paleolake evolution. 1) it inferred from terrace profile and altitude of research points that paleolake level was higher 38 m, 39 m, 40 m, 41 m, 43 m than present lake, the OSL dating on beach sediment was formed at 9.6±1、8.9±0.8、10.3±0.8、8.1±0.6、7.5±0.6 ka, illustrating that the first high lake level occurred in middle-early Holocene with wet climate. 2) OSL dating results of lake sedimentaryshowing that: the rising period of the lake was from 9.6 to 7.5 ka, when the lake area expanded, the lake level rose continuously, and climate was wet abnormally. The present of 7.5 ka as the change point of dry-wet climate with the 1313m’s paleolake level, was the boom period of lake development with the area of 244km2, 243km2 larger than the present. The lake extended to east area-Bayan Nuur, to north area–Uritayan Nuur, and south and west, becoming the large lake correspondingly. At the same time, the sand land surrounding the lake was submerged, forming enormous wet-land ecosystem in sand land,indicating that climate was very wet and ecological environment was extremely good. The process of lake evolution could be divided into 2 stages:the first stage is the rising period of the lake from 9.6 ka to 7.5 ka, when the climate was warm and wet in lake area, and the lake level rose continuously.The second stage is from 7.5 ka to the present, when the climate became dry gradually, with the lake shrinkage and paleolake disintegration, appearing to the present size of Haga Nuur.2. The results of grain size analysis of profile demonstrated that the surfaces of Aeolian sand and beach sand are significant different. The grain size of Aeolian sand is 0~5 mm, while beach sand about 0~4.2 mm. The surface morphology of Aeolian sand is comparing single, including dish-shaped pits, crescent shaped pits, and pockmarked pits etc. the dominant grain sand feature is rough and surface voids. But beach sand’s characteristic is complicate and changeable, conchoidal fractures, pockmarked pit, celluar silica, parallel cleavage,V-shaped pits, and triangle pit with dish-shaped pits and pockmarked pits.