| 摘要: |
| 长江三峡地区的巫山黄土位于润湿地区，其来源及成因具有重要的古气候意义。黄土的粒度常用于指示其成因及源区，目前关于巫山黄土成因和来源还是存在许多争议，并且过去对巫山黄土粒度的研究主要集中于单一剖面，缺乏空间上的系统对比研究。基于此，对长江三峡巫山地区三个海拔梯度变化明显的黄土剖面进行系统的粒度组成分析，结果表明：顶部望天坪、中部圣泉和底部江东嘴剖面的粒度频率曲线大多表现为不对称的多峰特征。圣泉剖面以粉砂级粒径含量为主的特点说明它的风成成因，物源主要来自近缘河谷；江东嘴剖面呈现出两种完全不一样的峰型，可能由两种不同成因混合而成；位于望天坪剖面沉积物粒径要细得多，推测为远源风成物质。 |
| 关键词: 长江三峡地区 巫山黄土 粒度 |
| DOI：10.7515/JEE192023 |
| CSTR：32259.14.JEE192023 |
| 分类号: |
| 基金项目:国家自然科学基金项目（41991321，41690111） |
| 英文基金项目:National Natural Science Foundation of China (41991321, 41690111) |
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| Grain size characteristics and source enlightenment of the Wushan loess in the Yangtze Three Gorges area |
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ZHU Xiaoyu, LIU Lianwen, MENG Xianqiang
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1. Ministry of Education Key Laboratory of Surficial Geochemistry, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China
2. State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, China
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| 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 Wangtianping, middle Shengquan and bottom Jiangdongzui sections is 9.9 μm, 30.8 μm, 73.9 μm, respectively. 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 completely 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 middle Shengquan section and the bottom Jiangdongzui 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 Jiangdongzui 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. |
| Key words: Three Gorges area of the Yangtze River Wushan loess grain size |
