Badain Jaran Desert Of Southeastern Sand Hills Lakes Formed

The landform of Badain Jaran Desert is always a focus to geographers, because it not only has the world's largest height difference of megadune, but also has a desert and lakes were arranged alternately distribution of special landscape.The megadune and lake are closely related. It helps to deeply understand their formation and internal relationship to research them by putting together. Many factors play an important role in the formations of megadune and lake, such as the material composition, megadunes, physiognomy zonations, chemical characters of lake water and groundwater, moisture content and migration of sand layer. However, due to the natural environment condition and the difficulty of entering in desert hinterland etc., study on the problems of material composition, physiognomy zonations, development mode of megadunes, the relations between the moisture content of sand layer and the formation of megadune was less. Previously, study on the landform of Badain Jaran Desert mainly concentrated on the larger scale and wide range of dune geomorphology and characteristics of surface material composition and distribution. At present, any result of single megadune about the systematic and quantitative, the material composition and physiognomy zonations hasn't yet published. Also did not see the published result on the relations between moisture contant of sand layer and formations of megadune and lake. The paper analysed the main-formation process of the formations of megadune and lake, and provides important grounds for settlement the formations of megadune and lake.The study on the landform of megadune and water, moisture content of sand layer has great science meaning for revealing formation mechanism and dynamic variation of megadune and lake, and also has practical significance for maintaining local ecological balance, protecting biodiversity, controlling desertification and scientifically utilizing desert water resources.In order to find out the formation of megadune, supply source of lake and the moisture effect of sand layer on the formation of megadune, densely distributed central area of megadune and lake in the southeastern Badain Jaran Desert was chosen as study area. Methods of field observation, particle size analysis, determination of chemical element, water chemical analysis, Environmental scanning electron microscopy (ESEM) and energy spectrum analysis were used to obtain the data of physiognomy zonations at the windward slope, sediment size composition, major element and trace element contents at the different parts of megadune, chemical constituents of lake water and groundwater, moisture content of sand layer, the crystal habit and chemical constituents of secondary salts. According to these data, the landform zoning characteristics of megadune windward slope, the relationship between sediment size composition and wind dynamic conditions at different parts of megadune, indicative function of content variation of major elements and trace elements on landform, space variation regular pattern of water chemical composition and its indication role on the supply source of lake and groundwater, high content of film moisture and secondary gypsum s indicating effect on the water migration and supply sources of lake water and groundwater were researched. Some results were drawn as follows:(1) According to the observational data of secondary physiognomy, the windward slope of megadune was divided into four geomorphological zones from the bottom up. The first was low-lying sandy depression zones which was undergoing wind erosion and coarsening. The second was sparse simple barchans or barchanoid chains zoning at the down of the middle part of megadune. The third was dense barchans and barchanoid chains zoning at the up of the middle part of megadune. The fourth was steep main peak zoning of megadune.(2) The paper proposed the formation patterns of megadunes. That aeolian sand material was not carried to the middle and upper part of megadune one time, but carried to the upper part of megadune by numerousness wind-force transport. Firstly, medium sand and fine sand in the lower of megadune was carried to the middle part, and shaped barchan and barchanoid chains. After repeatedly transport, finally the peak of megadune was shaped.(3) The research first discovered the secondary gypsum which has important indication function to water cycle and atmospheric precipitation seeping into megadune internal in the slopes of Huhejilin-E megadune etc. The emerging of secondary gypsum and gravity water in sand layer, imply that the precipitation has infiltrated into the deep position and exposed to the surface. These indicates that atmospheric precipitation can supply the lake water and groundwater via sand layer. Moisture entrance sand layer increase the humidity of sand layer and bond between sand particles, which was beneficial to the formation of megadunes.(4) According to the data of moisture content in the depth of4m, it is first determined that there were high content of film moisture at1-2m depth of sand layer in Badain Jaran Desert, which shows that precipitation after evaporation and transpiration there were still some residual water downward migration, and at least can become one of the supply sources of lake water and groundwater. These identify the supply source of lake water and groundwater indicated by the vegetation zonal distribution, secondary gypsum and water exudation at the slope of megadune. Meanwhile, fully proves that the sand layer moisture in the region has normal cycle and positive balance characteristics. The positive balance of moisture in the sand layer played an important role in the formation of megadunes.(5) The aeolian sand material has a double-layer structure in Badain Jaran Desert. The particle size of surface is coarser than the below10cm, indicated that the aeolian sand material was undergoing wind erosion and coarsening, and directed megadunes still exist certain activity. The particle composition of sediment has obvious difference at different parts. From the bottom to top of megadune, particle composition present taper off trend, this was caused by the differences of their formation process and the dynamic effect. The coarse particle size composition at the bottom of windward slope was determined by that the fine sand was carried away by the wind erosion and only coarse component was left. The fine particle size composition in the upper part of the windward slope was the result that the slope and height was large, which hindered the wind moving up, lead to wind transport ability decrease and generate deposition.(6) In the windward slope surface of megadune, the content of Si from bottom to top decreased gradually, which was the result of the strong stability of quartz mineral and its high purity in coarse particles. The content of Al,K,Nb,Rb,Ni increased from bottom to top, this was caused by the high content of these elements in fine sediments. The content variations of Si,Al,K,Nb,Rb,Ni play an important role in indicating the landform zoning of megadune.(7) In the Badain Jaran Desert, the chemical characteristics and spatial difference of lake water were very evident. From south to north, the chemical type of lake water gradually transit from Na-(Mg)-(Ca)-Cl-(SO4)-(HCO3) type to Na-(K)-Cl-(SO4) type. The southern lakes close to the Yabulai Mountains, receive the supply of precipitation infiltration of the Yabulai Mountains more than northern lakes, which was the reason that the ions concentration of southern lakes were lower than the northern lakes. The lake surface altitude of southern lakes were generally higher than the northern lakes, so the ions of soluble salt move with the lake water from southeast to northwest, and accumulate in the north lakes, leading to a higher salinity in the northern lakes. In general, the directions of low ion concentrations represent the source direction of recharge water, and the directions of high ion concentrations mean the advance and convergence direction of flow.(8) The data of chemistry characteristics of lake water and lake surface altitude indicates that the main supply source come from local precipitation and precipitation infiltration migration of the Yabulai Mountains and Heishantou Mountains. In addition, the hierarchical cluster analysis of lake water and precipitation also indicate the lake water of southeastern and precipitation has homology. In the southeastern lakes of Badain Jaran Desert, the elevations of lake surface were higher than the groundwater level in the Hexi Corridor, which indicates that the formation of desert lakes was not necessarily associated with the water supplies from the Qilian Mountains.