Geochemical Characteristics And Environmental Significance Of Surface Sediments From Different Types Of Sand Dunes In The Dinggye Area,Tibet

Aeolian deposits are an important part of the Earth’s terrestrial landscape system,mainly found in arid,semi-arid and partially semi-humid areas,and their formation and evolution have a great impact on local climate,air quality and people’s lives.Elemental geochemical signatures contain a wealth of information on surface processes and climate change,and are important for the analysis of physical sources and depositional environments.In this paper,we use the Dinggye area of Tibet as the study area,and analyze the geochemical characteristics of whole rock samples and fractionated samples of surface sediments from different types of sand dunes（mobile dunes,climbing sand sheets and nebkhas）,and then reveal the chemical weathering intensity and physical sources of different types of sand dunes.Principal conclusions are as follows:（1）The elemental composition of the surface sediments of different types of dunes in the Dinggye area is characterized by:the major elements are rich in SiO₂,Al₂O₃ and CaO,and the trace elements are dominated by V,Sr,Ba,Zr and Rb.Each type of dune has different elemental contents,coefficients of variation and migration characteristics,and the chemical composition of the major and trace elements of the same type of dune has a good consistency.In mobile dunes,the variation coefficients of TiO₂,La,V,Cr,Co,As and Ce are>20%and their contents vary widely;compared to the upper continental crust（UCC）,SiO₂,V,Co,Ni,As,Pb,Rb and Ga are relatively enriched.Climbing sand sheets have coefficients of variation>20%for CaO,MgO,La,Cr,Co,Ni and Ce,which are highly variable,with Rb remaining consistent with the UCC and SiO₂,CaO,V,Co,As and Pb being relatively enriched.The nebkhas have a good agreement with little variation in content of the elements except Co,Pb,Y,Zr,Rb and Ce,with SiO₂ close to UCC and CaO,As,Pb,Ni,V,Co,Y,Zr,Rb and Ga relatively enriched.（2）Particle size effects on the geochemical characteristics of different types of dune surface sediments in the Dinggye area:there are distinctly different elemental compositions and transport characteristics between the coarse and fine particles of the various types of dunes.The fine particles of mobile dunes are enriched in most major and trace elements,with coefficients of variation<20%for SiO₂,Al₂O₃,Na₂O,MgO,K₂O,As,Sr,Pb,Rb and Ga.CaO,TiO₂,Y,Zr and La are relatively enriched in the fine particles,while CaO has a large variation in migration characteristics in the coarse particles and TiO₂,Y,Zr and La are relatively deficient in the coarse particles.Climbing sand sheets have higher contents of SiO₂,K₂O,Cr,Co,Cu,Pb,Rb and Ga in the coarse particles,most major and trace elements have coefficients of variation<20%in the fine particles,elemental content changes are more stable,CaO,Ba,Y and Zr are relatively enriched in the fine particles and relatively deficient in the coarse particles.The SiO₂,MgO,K₂O,Na₂O,Co,Ni,Sr,Zr and Ga contents of the nebkhas are relatively high in the coarse particles,and SiO₂,Al₂O₃,Fe₂O₃,K₂O,La,Zn,As,Rb,V,Ni,Sr,Pb and Nb have smaller coefficients of variation in the fine particles,relative to the UCC,with CaO,V,Cr,Y and Zr being relatively relatively enriched in the fine particles and relatively deficient in the coarse particles.（3）The weathering intensity of different types of dunes in the Dinggye area:The analysis based on chemical alteration index（CIA）,Na/K,Rb/Sr,residual accumulation coefficient and leaching coefficient shows that the flowing dunes in the Dinggye area are in a low and moderate chemical weathering stage,the climbing sand sheets are in a low chemical weathering stage and the nebkhas are in a moderate chemical weathering stage.A-CN-K and A-CNK-FM triangles reflect that the plagioclase of the surface sediments of different types of dunes in the Dinggye area is the first to decompose and is currently in a weaker,moderate chemical weathering stage of decalcification of Ca and Na,while the mobile dunes and nebkhas are in a stronger chemical weathering stage,alkali metal element content varies considerably between sand dune types.Material sources of coarse and fine particles in different types of sand dunes in the Dinggye area:Based on scatter plots of tracer elements,confirmed by principal component analysis（PCA）and multidimensional scaling analysis（MDS）,it is shown that the material source of coarse and fine particles of mobile dunes is mainly nearby riverine sediments,lacustrine sediments are an important source of sand for coarse and fine particles of climbing sand sheets,riverine sediments are an important source of sand for coarse particles of scrub sand mounds,while the source of sand for fine particles is mainly paleosand refurbishment.