| Badain Jaran Sand Sea has the unique and tallest mega-dunes in the world, the research of mega-dunes is also a hot topic for the sand sea and the regional aeolian geomorphology. Mega-dunes formed by the wind conditions, sediment, vegetation and surrounding terrain, arround its research there had a lot of achievements, but further studies are still needed. Based on two influencing factors for mega-dunes including vegetation and sediment, this paper analyzed the vegetation and sediment characteristics and their relation with mega-dunes on the east and west side of Yihejigede Lake in the Badain Jaran Sand Sea, and aimed at enriching the research content of regional aeolian geomorphology and provided some reference for regional desertification prevention and control. Through the field investigation of vegetation and indoor experiment of sediment and statistical calculation of related parameters for the study area, the main conclusions in this paper included:The aspect of vegetation:(1) the study area had less vegetation types, mainly composed by xeromorphic species. The vertical distribution and differences of vegetation communities was obvious. There existed6families and9genera and9species, from the top to the bottom of mega-dunes vegetation communities changed in turn form herb community to shrub community to herb or subshrub community. Agriophyllum squarrosum distributed in the leeward slope, but Corispermum patelliforme Iljin and Artemisia blepharolepis Bunge in windward slope.(2) The diversity of vegetation communities was obviously different in different position of mega-dunes. In the aspect of overall community diversity measurement, the diversity index was high in middle of the two slopes of mega-dunes, the evenness index was high in the upper side of leeward slope and the middle side of windward slope. In the aspect of hierarchical community diversity measurement, for the single herb species, there only had a little difference of community diversity index and evenness index difference, shrub community diversity was high in the middle of windward slope.(3) The conditon of vegetation diversity in the southeastern sand sea was better than that in the northwest, this may be caused by the regional difference of precipitation form summer monsoon that brought a small amount of precipitation. The uniformity of temperature inside the mega-dunes may reduce moisture evaporation, and it may provide favorable conditions for vegetation growing.(4) The characteristic of vegetation communites was affected by the whole morphology of mega-dunes, there existed a well corresponding relation between the vegetation characteristic including distribution and diversity in the windward slope and the physiognomy morphology with Order I and Order II of mega-dunes. Besides the consolidation by vegetation on the surface of mega-dunes in warm period, in the broader development period of sand sea, the sparse vegetation may play an important role in the aspect of maintaining the height and lessening the sand erosion of mega-dunes, through the way of influencing sand transport rate and then intercepting the sand.The aspect of grain size for sediments:(1) The grain size distribution of surface and deep sediments was obviously different in different position of mega-dunes, it reflected the difference of transporting capacity for sediment by wind along the slope of mega-dunes. There was medium sand in the upper windward slope of the mega-dunes, and fine sand in the middle windward slope, the amount of coarse sand and very fine sand became more in the lower-middle and lower windward slope. The higher content of medium sand in the upper windward slope may play an important role in maintaining the height and lessening the sand erosion of mega-dunes. The surface and deep sediments in the leeward slope of the mega-dunes mainly composed by fine sand, the amount of medium and very fine and coare sand became more toward the bottom. The grain size distribution in the leeward was formed by the law of the smaller and larger sediments.(2) The grain size parameters of surface and deep sediments was obviously different in different position of mega-dunes. The mean grain size was ranged from medium and fine sand, sorting coefficient was ranged from very fine to worse, skewness was ranged from positive to negative, but mainly was nearly symmetry, kurtosis was ranged from broad to medium, but mainly was medium. It was easy to distinguish the sediment type in the windward slope of mega-dunes, through analyzing the relation between mean grain size with sorting coefficient and skewness and kurtosis, this may cause by different sedimentary environment, in the same sedimentary environment, the scatter diagram of sediment tended to concentrate.(3) The sedimentary environment of surface and deep sediments was obviously different in different position of mega-dunes. Thers existed lacustrine and alluvial and aeolian facies in the bottom and lower-middle windward slope of mega-dunes, and formed alternation of beds, but mainly were aeolian face in the middle and upper slope. It meaned that there was a well development of different facies in the early period of mega-dunes during its formation, and established the basement of mega-dunes, but in the later period the formation of mega-dunes mainly influenced by the wind condition.The aspect of sediment geochemical elements:(1) In the composition aspect of geochemical elements, the mainly major elements were SiO2and Al2O3and Na2O, and the mainly trace elements were Co and Sr and Ba and Zr, after the geochemical elements standardization of UCC for sediments, it showed that the major elements were leached and migrated except Si, the trace elements of Co and As and Cr and Ni were enriched, but others were leached or closed to the UCC. The law of enrichment or migration for geochemical elements of sediments may be formed in intense physical weathering and windy transportation process.(3) The chemical alteration index of sediments for the mega-dunes indicated that the sediment experienced lower chemical weathering process, it was the product of dry and cold climate. The diagram of A-CN-K and A-CNK-FM also suggested that the stage of weathering was close to the the upper continental crust for a early stage of chemical weathering, it further illustrated that intense physical weathering and windy transportation process palyed an major role in the formation of mega-dune. |
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