| Ujimqin Sandy Land is one of the five major sandy lands in Inner Mongolia and one of the main dust sources affecting Beijing,Tianjin and Hebei.The development and construction of aeolian landforms are based on sediment.It includes a wide range of data,including information on environmental evolution and climate change,which can,to some extent,depict how the desert has evolved and grown.In this thesis,the characteristics of grain size and geochemical elements of surface sediments in Ujimqin Sandy Land were studied by field sampling and indoor experiment,and the significance of environmental indication was discussed.The main conclusions are as follows:（1）The grain size composition of surface sediments in Ujimqin sandy land is mainly composed of medium sand and coarse sand,with an average content of 31.76%and 33.57%respectively.Followed by fine sand,the average content of 18.44%;the silt and very coarse sand content is less,almost no clay and very fine sand.In terms of spatial distribution,the contents of clay,silt,fine sand and very fine sand basically show a decreasing trend from the western and eastern regions to the central region,while the contents of medium sand,coarse sand and very coarse sand show the opposite trend,with the highest content in the central region.The grain size frequency distribution curves in different regions of the study area show a bimodal state,and the probability cumulative curve exhibits a two-stage distribution.（2）The average particle size of surface sediments in Ujimqin Sandy Land is 1.24Φ,and the spatial distribution shows a decreasing trend from northeast to southwest.The average sorting coefficient is 1.45Φ,and the sorting is poor.The average value of skewness is 0.36Φ,which belongs to extremely positive skewness distribution.The average kurtosis value is 1.83Φ,which is a very narrow peak.In addition to the average particle size and skewness,any two sediment particle size characteristics have a positive association with one another.The sediment’s average particle size increases,or more precisely,the particles get smaller,sorting is worse,the skewness is more negative,and the kurtosis is wider.（3）The characteristics of geochemical elements in surface sediments of different regions in Ujimqin Sandy Land are similar.The total mineral content is mainly Si O2,followed by Al2O3,both of which are relatively stable and less affected by environmental climate.Compared with UCC,only Si O2was enriched,and other elements migrated and leached.The highest content of trace elements in different regions of the study area is Ba,followed by Sr and Rb,and the remaining trace elements are below 20μg/g.Compared with UCC,the migration and leaching rules of trace elements in different regions of the study area are basically consistent.Only Ba shows slight enrichment,Rb shows slight element migration,and other elements show leaching.（4）Based on the analysis of provenance evidence based on the characteristics of geochemical elements,the Ujimqin sandy land has a tendency of increased chemical weathering from west to east and from north to south.The global sediments in the study area were formed in a cold and dry climate and experienced low chemical weathering.The content of clay minerals in its clastic sedimentary rocks is high and has undergone a process of redeposition.Surface sediments’compositional maturity and mineral stability index gradually increased from west to east,and the highest value appeared in the middle,indicating that the poor mineral stability in the west-central region was close to the provenance.（5）Based on the analysis of provenance evidence of sedimentology,it is considered that the sediments in Ujimqin sandy land are derived from the near-source sediments of’in situ sand’.The sedimentary environment is relatively simple,mostly lake sediments,and also formed by the transportation and accumulation of rivers originating from the surrounding high mountains,and then continuously transported to the surrounding areas through wind action. |
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