Abstract: Background, aim, and scope The Wushan loess in the Three Gorges region of the Yangtze River is located in the wet area, and its source and genesis have important paleoclimatic significance. Previous studies have been conducted on the formation, provenance and times of Wushan loess, but the origin and provenance of Wushan loess are still controversial. Different scholars choose different locations in different regions to study Xiashu loess, which may be the main reason for the debate on its origin. The grain size composition of the sediment directly reflects the dynamic condition, sedimentary environment and transportation direction of the transport medium, which is often used as a discriminant index for the origin and origin of loess. However, in the past, studies on loess particle size of Wushan mainly focused on a single section, lacking systematic comparative studies in space, and lacking comparative studies on different sedimentary types. Based on this, this paper systematically analyzed the grain size of three loess profiles with obvious changes in elevation gradient in Wushan area of the Three Gorges of Yangtze river in China, and discussed the origin and material source of Wushan loess. Materials and methods This paper collected three loess profiles of different heights on both sides of the Yangtze river in Wushan area, they are the Jiangdongzui profile at the bottom of the east bank of the Yangtze river at an elevation of about 190.4 m, the middle Shengquan profile near Chengnanjiu wharf in Wushan county at an elevation of about 248.6 m, and the top Wangtianping peak profile at the south bank of the Yangtze river in Wushan county at an elevation of about 1413.5 m, opposite the entrance of Daning river and the Yangtze river. These sections are developed on the limestone of the Triassic, and the lithology of the sections is relatively uniform, mainly composed of brown and yellow silty sand and sandy clay, containing a small amount of calcareous nodules. Soil weathering degree is weak, no obvious ancient weathering layer. According to the data, the loess in the study area belongs to the late Pleistocene loess. The three sections have a vertical span of 1223 m and a horizontal span of only 5 km, which may represent different loess accumulation types in the Three Gorges area of the Yangtze river. Based on this, this paper analyzes the systematic grain size composition of three loess sections with obvious elevation gradients. Particle size testing was completed in particle size analysis laboratory, key laboratory of surface geochemistry, ministry of education, Nanjing university, with the Mastersizer 2000 laser particle size analyzer produced by Malvern, UK. The testing range was 0.02—2000 μm, and the error of repeated measurement was less than 2%. Results The results show that the average particle size of the top Wanganting, Zhongshengquan and Bottom Jiangdongzui sections is 9.9 μm, respectively. At 30.8 μm and 73.9 μm, the particle size frequency curves of Wushan loess mostly show asymmetric multi-peak characteristics. Discussion The characteristics of the particle size distribution on the particle size frequency curve of Shengquan section indicate the aeolian genesis. The source of the material is mainly from the near valley. The particle size frequency curve of the Jiangdongzui section shows two complete Different peak shapes may be mixed by two different causes. The first peak type is basically consistent with the Shengquan section, showing similar aeolian genesis; the second type shows that the coarse grain content is high, which may be affected by the later river reform, presumably except for the near-river valley dust and the later river sediment. The contribution of the material; the particle size of the sediment in the Wangtianping section is much finer than that of the sediments in the central Shengquan section and the bottom Jiangdongzuo section, which is presumed to be a distant source of wind. Conclusions Based on the above analysis, it can be concluded that the distant source of dust in north China may be the main source region of loess in the wangtianping profile. While the shengquan profile is mainly the near source valley eolian deposition, the jiangdong mouth profile is mainly the near source valley eolian deposition and/or fluvial facies deposition. Recommendations and perspectives In this paper, the grain size analysis of three loess profiles in Wushan area of the Three Gorges of the Yangtze River, which have obvious changes in elevation gradient and may be different in origin, is carried out systematically, to explore the origin and material source of Wushan loess, this study will help to identify the location, properties and transport routes of sediment sources of dust, and is of great significance to explain regional and global climate and environmental changes.
Keywords: Three Gorges area of the Yangtze River; Wushan loess; grain size