Quantitative Study On Material Sources Of Megadunes In Southeastern Badain Jaran Sand Sea

The Badain Jaran Sand Sea,the second largest desert in China,is located on the Alxa Plateau in the northwest of China.It is famous for its unique landscape of megadunes and lakes.At present,the research of megadunes mainly focuses on the sediment,morphology and formation process of megadunes.Sand source is the material basis for the formation and development of desert.Clarifying the sand source of Badain Jaran Sand Sea can provide a theoretical basis for the research of regional aeolian sand geomorphology.Quantitative research on the material source of aeolian sand on the surface of megadunes is of great significance for improving the research on the provenance of megadunes and enhancing the understanding of the formation mechanism of megadunes and desert.In this study,the aeolian sand on the surface of the megadunes in the southeastern desert was taken as the research object.Through field investigation,analysis of sediment grain size characteristics and analysis of geochemical elements,the provenance and source of the sand mountain were determined,the contribution rate of sand material from different sources was quantified,and the transport mechanism of sand mountain material was discussed.Preliminarily drawing the following conclusions:（1）The aeolian sand on the surface of the megadunes is mainly composed of fine sand and medium sand（89.83%）,with an average content of 50.30%and 39.53%,respectively.The average particle size ranges from 1.80 to 2.38Φ,and the average particle size is 2.12Φ（229.69μm）,belonging to the fine sand range.The separation coefficient ranges from0.41 to 0.79,and the mean separation coefficient is 0.58.The frequency curve of aeolian sand size on the surface of megadunes presents a unimodal state,with the peak size appearing in the range of 185-272μm,showing a single dynamic transport characteristic.（2）The main oxides of the aeolic sand on the surface of megadunes are SiO₂,with an average content of 82.99±1.45%,followed by Al₂O₃,with an average content of7.75±0.49%.Others contain relatively few oxides.Compared with the UCC value,all samples of megadune sands were enriched in SiO₂,and other major oxides were depleted.The average concentrations of Cr,Sr,Zr and Ba were greater than 100μg/g.For the megadunes sand,Cr and Ni were enriched relative to the UCC,and other trace elements were depleted.The results of chemical alteration index show that the aeolian sand is in the primary stage of continental chemical weathering.The results of source rock analysis show that the aeolian sand on the surface of megadunes sand is from felsic source rock.（3）The sensitive components of aeolian sand on the surface of megadunes were extracted by end-member model analysis and grain gradation-standard deviation method.The end-member model analysis method extracted three end-members of the aeolian sand on the surface of the megadunes,and the grain size frequency curves showed an obvious single-peak form.The cumulative probability curve results showed that all end-members were dominated by saltation material.The results of comparative analysis of grain size endmembers and potential provenance showed that end member 1（EM1）was provided by the northern part of Beida-Yabrai Gobi sediments,and end member 3（EM3）was provided by the fine particle composition of the Ejin Gobi sediments and the fluvial sand of the Heihe River in the northwest part.Two sensitive components were extracted by the grain size standard deviation method,and the grain size standard deviation curve showed a"double peak"shape.The difference of surface sediment size composition between the northwestern provenance area and the southern provenance area is the main reason for the"double peak"pattern of surface aeolian sand size-standard deviation curve.（4）Taking TiO₂/Al₂O₃-K₂O/Al₂O₃as the discrimination index of aeolian sand provenance for analysis,the positions of aeolian sand and other regional samples in the scatter plot can reflect their provenance correlation.The results show that the fluvial sand of the Heihe River and the Ejin Gobi sediments are the sources for the megadune sands,and that the Beida-Yabrai Gobi sediments are an additional sand source.The data set of trace elements of surface aeolian sand and potential provenance area were analyzed by multi-dimensional scaling.The results show that the fluvial sand of the Heihe River,the Ejin Gobi sediments and the Beida-Yabrai Gobi sediments are similar to the surface aeolian sand in geochemical element characteristics.These results indicate that the above area may be the main source area of aeolian sand.（5）The quantitative results of composite fingerprint method on the aeolian sediment sources of the megadunes in the southeastern Badain Jaran Sand Sea show that the the Ejin Gobi sediments is the main source of the megadunes sand in the northwestern desert,with a contribution rate of 66.51±11.13%,followed by the the fluvial sand of the Heihe River and the Beida-Yabrai Gobi sediments.The contribution rates of sand were19.64±7.08%and 13.85±7.45%,respectively.The Heihe River is the only river near the Badain Jaran Sand Sea,and it transports sediments from the Qilian Mountains to the Ejin basin.Weathered debris from the Altay Mountains could also be transported to the basin by alluvial processes associated with temporary runoff events on the Gobi surface.Then,aeolian processes sort these fluvial and alluvial sediments,entrain suitable particles,and transport them downwind to the sand sea,contributing to the development of the megadunes.In the southeastern sand sea,the Beida-Yabrai Gobi sediments could be transported to the southern edge of the sand sea by temporary alluvial processes.The sorting action of the southeast wind could further deliver fine particles to the megadune region.