Investigation on the source of sandstorm in northern China and discussion on its material source

Wang Yong Chi Zhenqing Yao Liu Xun min

(Institute of Geology, Chinese Academy of Geological Sciences, Beijing, 100037)

Based on the study of geochemistry and isotopic composition of dust samples in Beijing-He Lin-Ejina Banner, the causes and weather conditions of dust storms in Ejina Banner, one of the dust storms in northern China, are analyzed, and the material composition and sources of dust samples in Beijing-Ejina Banner are discussed. The characteristics of Sm/Nd isotopes and oxides show that the surface dust in Ejina Banner-Beijing area mainly comes from in-situ weathering and erosion of bedrock or soil, and the contribution of foreign components is small. The occurrence of sandstorms in northern China is mainly caused by the deterioration of local climatic conditions and ecological environment.

Keywords sandstorm; Geochemical composition; source

1 overview of sandstorm

Dust weather is divided into four categories: floating dust, sand blowing, sandstorm and strong sandstorm. Sandstorm refers to the weather phenomenon that strong winds blow up a lot of dust on the ground, making the air very turbid and the horizontal visibility less than 1km; There are at least three conditions for the formation of sandstorms, one is the weather situation favorable to strong winds or strong winds, the other is the distribution of dust sources, and the third is unstable air conditions. Strong wind is the driving force of sandstorm, and the dust source is the material basis of sandstorm formation. Unstable air state is an important local thermal condition, which is conducive to the increase of wind power and the development of strong convection, thus carrying more dust.

There are many researches on sandstorms at home and abroad, but most of them are descriptions of phenomena and disasters, which are limited to case analysis or weather situation characteristics analysis. So far, it is not clear how the dust from the wind moves, because people can't track the whole movement process of the dust. The images provided by the satellite can only see the scene of dust flying over the Pacific Ocean, but can't observe the movement on land. Due to the lack of various data, it is impossible to establish a calculation model of sandstorm at present. How far dust can float depends on the floating height of its grains in the air. So far, only a special laser measuring instrument can be used to observe the floating height of dust. The defect of the laser measuring instrument is that the measuring range is small, and it can only detect a small area in the air and cannot judge the direction of dust.

Sandstorm is a major disastrous weather in northern China. It is sudden and destructive, and it is difficult to predict and defend. The average annual days of sandstorms in northwest China, most of North China, Qinghai-Tibet Plateau and Northeast Plain are generally above 1 day (Qian Zhengan et al., 1997), which is the main influence area of sandstorms (Gao Shangyu, 2000; Jinming Hu, 1999), the average annual sandstorm days in most areas south of Tianshan Mountain are above 10, which is a sandstorm-prone area; The Tarim Basin and its surrounding areas, Alashan and the northeast of Hexi Corridor are the high frequency areas of sandstorms. The annual average number of sandstorms is more than 20 days, and some are close to or more than 30 days, such as 36 days in Minfeng, Xinjiang, 365,438+0 days in Keping and 30 days in Minqin, Gansu (Xu et al., 65,438+0997).

At present, China is in the rising period of non-frequent sandstorm weather. In recent years, the intensity of sandstorms in China has increased and the number of serious sandstorms has increased. According to statistics, there were 8 sandstorms in China in 1960s, including 13 times in 1970s, 14 times in 1980s and more than 20 times in 1990s, and the scope of sandstorms became wider and wider, resulting in heavier losses.

Based on the geology and geochemistry of surface sediments in a sandstorm source and passing area in the north, this paper analyzes the variation characteristics of surface sediments in different areas, studies the contribution of source materials to local sandstorms during the occurrence and movement of sandstorms, and discusses the source and propagation path of sandstorms. The samples in this work are taken from the surface wind deposits along the route from Beijing to Ejina Banner (Figure 1).

Figure 1 Sampling Route of Surface Dust Samples in Beijing-Ejina Banner

2. Discuss the material composition characteristics and sources of sandstorms in northern China.

Ejina Banner in Inner Mongolia is located on the edge of Badain Jaran Desert and in the north of Hexi Corridor. The area is close to the desert and rich in sand source conditions. Hexi Corridor is a channel for cold air activities and one of the main sources of sandstorm weather in China. According to statistics, from 1952 to 1994, among the 48 strong and extra-strong sandstorms recorded in China, there were only 6 in Ejina Banner. It not only caused serious losses to agricultural production, but also caused human and animal casualties. Investigating and analyzing the characteristics of sandstorm occurrence in this area and discussing its weather conditions can provide basis for the forecast and prevention of sandstorm weather.

2. 1 Sm and Nd isotopic characteristics of dust samples

2. 1. 1 samarium neodymium isotope principle

Rare earth element Nd has two isotopes in nature: 143Nd and 144Nd. 144Nd decays from radioactive element 147Sm, so it is also called radioactive Nd. 143Nd is mostly concentrated in acidic aluminosilicate, which is usually called terrigenous Nd. The isotopic composition of neodymium is expressed by the ratio of 143Nd/ 144Nd. It is generally believed that in many geological processes, the differentiation of w(Sm)/w(Nd) is very small, and the Sm-Nd system can be kept closed.

Using Nd isotopes to study provenance evolution and paleoenvironment (Liu Jihua,1998; Meng Xianwei, 200 1) started with the study of marine sediments. In recent years, some people have compared the nd isotopic composition of fine sediment (clay), dust and aerosol on the ocean surface with that of ancient shields and deserts to determine the source of aeolian materials (Grousset,1988; Donald, 1988). Many studies have proved that fine clastic sediments can represent the average composition of continental crust in a considerable area, and the nd model age of clastic sedimentary rocks is considered to reflect the average crustal age of the sediment source area because of the almost negligible Sm/Nd differentiation in the deposition process. Detrital sediments themselves can also reflect the chemical composition and isotopic composition characteristics of provenance.

It is a new attempt to identify the provenance by using the Nd isotopic composition of continental sediments, and it is still in the exploration stage. Fine clastic sedimentary rocks are the product of mechanical destruction of original rocks. During the weathering, crushing, transportation and deposition of rocks, the SM/ND ratio remains unchanged, and the formed sedimentary rocks maintain the Sm/Nd ratio and isotopic characteristics of the original rocks, and the model age is the same as that of the original rocks, so the source area of sediments can be identified.

2. 1.2 experimental analysis method

Sm-Nd isotope determination was carried out in the isotope laboratory of Institute of Geology, Chinese Academy of Geological Sciences, using MAT-26 1 solid isotope mass spectrometer. Re band is used for ionization band, Ta band is used for evaporation band, and M+ ions are received by adjustable Dofara cylindrical receiver. The mass fractionation was corrected with143 nd/144 nd = 0.7219, and the standard determination result was j.m. nd2o3143 nd/1444 nd = 0.5164 nd. The blank of Sm, Nd ND process is 5×10-11g. The decay constant is 6.54×10-12a-1.

2. 1.3 test results

See table 1 for the test results.

Table 1 Sm-Nd Isotope Test Results of Surface Dust Samples in He Lin-Ejina Banner

The model ages of all samples fall in the middle-late Proterozoic. The provenance of these surface samples may mainly be the ancient crust of Middle-Late Proterozoic. These clastic sediments may represent the products of mixing two end-member components in different proportions in the middle and late Proterozoic. Therefore, the surface dust sample may be the crust differentiated from the mantle in Mesoproterozoic. The Nd model age (TDM) of clastic sediments is 0.9 ~ 65438±0.75 ga, which supports this conclusion. The younger age of the model is obviously caused by the addition of a large number of young materials. As can be seen from the table 1, the Nd values of 147Sm/ 143Nd of most samples are between 0.1~ 0.13, iNdicating that there is no obvious difference in Sm/Nd during the deposition process, so these are based on. The high Nd isotope in surface dust samples indicates that the old crust (Mesoproterozoic) may be the main source region.

From the Sm-Nd relationship, the Sm/Nd ratio of all dust samples has little change, but 143Nd/ 144Nd has a certain change, and the model age also changes greatly (Figure 2). Features of the same source region are not shown.

Fig. 2 143Nd/ 144Nd and the model age of dust samples.

2.2 Geochemical characteristics of dust samples

Geochemical methods are intuitive, economical and effective in sediment provenance identification, and it has become a new trend to qualitatively identify sediment provenance attributes by using the genetic specificity of geochemical combination. However, for terrigenous clastic sediments, due to their complex sources, other methods are needed to identify the variation characteristics of provenance more effectively.

In order to understand the geochemical characteristics of surface dust samples in Beijing-Ejina Banner and explore the possible sources of dust, the oxide content of surface dust samples in Beijing-Ejina Banner was analyzed by XRF method.

Table 2 shows the oxide content characteristics of the test samples. As can be seen from the table, the contents of SiO2 and CaO vary greatly.

The content of SiO2 in Sc24 and Sc25 samples is the lowest, and the content of CaO is the highest. The samples were collected from the northwest mountainous area of Beijing, surrounded by carbonate rocks, indicating that the surface sand samples are the products of in-situ weathering. Sc29 and Sc30 samples collected from Guyang-Wuyuan area in western Inner Mongolia have the highest content of SiO2 _ 2 and the lowest content of CaO.

Silicon, aluminum, calcium, potassium, magnesium, manganese, sodium, titanium, etc. They are MagmaElemental-loving, and their atomic structure is inert gas type, so they are stable in the process of natural migration. Iron and phosphorus are siderophiles. Figure 3 shows that from the dust source area to the source area far away from the dust source area, the content of MagmaElemental-loving and iron-loving elements in the sand sample changes greatly, indicating that the main source of dust changes with the transportation process. It is possible that local rock weathering has the greatest influence on oxide content in sand samples.

Table 2 Oxide characteristic unit of surface dust samples in Beijing-Ejina Banner:%

Fig. 3 Variation characteristics of main oxides in surface sand samples from Beijing to Ejina Banner.

Dust comes from the wind-blown dust on the surface soil in a certain area. The chemical elements of various soils have certain characteristics and differences, and specific soils have certain concentrated distribution areas. For different kinds of topsoil, the Si-Al ratio W (SiO _ 2)/W (Al _ 2O _ 3) and Si-Al-Fe ratio [W (SiO _ 2)/W (Al _ 2O _ 3+Fe _ 2O _ 3)] between elements are relatively stable, so they are often used to preliminarily and qualitatively identify the source of soil. These two ratios were compared with those of several typical topsoil in China (Table 3). As can be seen from the table, the two ratios of different topsoil types are very different. These two ratios of surface dust samples are close to those of loessial soil and brown desert soil. Compared with the national soil type distribution map, brown desert soil is mainly distributed in northern Xinjiang, and loessial soil is mainly distributed in Xinjiang, northwest Gansu, Inner Mongolia, northwest Ningxia and Shaanxi. Therefore, this area may be the main source of dust.

According to the above analysis, the Si-Al ratio W (SiO _ 2)/W (Al _ 2O _ 3) and Si-Al-Fe ratio W (SiO _ 2)/W (Al _ 2O _ 3+Fe _ 2O _ 3) of dust in sand samples are close to those in brown soil, and their sources are mainly soil dust including northwest China. Of course, dust may also include local dust mixed along the way and ground secondary dust composed of them.

Table 3 Si-Al ratio of surface dust samples and several typical soils in Beijing-Ejina Banner

Desert soil series is an important soil resource in the desert area of northwest China, including grey desert soil, grey brown desert soil, brown desert soil and cracked soil. * * * has the same characteristics as follows: porous desert crust with low humus content and strong surface aggregation, gypsum and soluble salt are accumulated at a small depth in the profile, with obvious residual adhesion and iron staining, and the whole profile is thin in thickness and high in gravel content (except cracked soil and gray desert soil). In the process of soil formation, the main manifestations are calcification (lime accumulation), gypsum, salinization and weak iron, and the aeolian effect is quite obvious.

Brown desert soil is a soil developed in temperate desert conditions with coarse bone matrix, which occupies a large area in northwest China. Compared with grey desert soil, the accumulation of humus is weak, there is almost no humus layer, and the content of organic matter in the surface layer rarely exceeds 0.5%, and the content does not change much with the increase of depth.

Compared with the geochemical characteristics of soil in the northwest desert area, the surface dust samples obviously have a higher ratio of W (SiO _ 2)/W (Al _ 2O _ 3). The reason may be that dust samples have higher maturity after longer weathering and wear.

Discussion on material source and propagation path of sandstorm in northern China

The formation of sandstorms must meet the dynamic conditions of loose fine particles on the surface and wind transport, and at the same time have unstable air conditions (Qiu Xinfa, 2001; Zhang, 2002). When the dynamic conditions are satisfied, the occurrence and development of sandstorms depend on different surface conditions. The surface and cultivated land with vegetation protection have strong wind erosion resistance and are not easy to become the source of sandstorms, while the surface without vegetation coverage or with low vegetation coverage may be the potential source of sandstorms. Due to the humid climate, good vegetation coverage and small soil wind erosion, several mountain systems in northwest China are non-sandstorm source areas, while other areas may be sandstorm source areas because of the dry climate and sparse vegetation, and there are a large number of sandstorm material sources. At the same time, due to the improvement of industrialization level and the expansion of city scale, there is obvious temperature difference between cities in some areas and surrounding areas, which increases the local geothermal gradient, forms an unstable air thermal state, and promotes the occurrence of sandstorms.

The basis for judging the source of aeolian sand is that bare rocks, water bodies and construction land do not have the conditions of fine-grained materials in land resource types, so they cannot form the source of aeolian sand. Because of the high vegetation coverage, the forest land and grassland with medium and high coverage are not easy to be eroded by the wind under the protection of vegetation, and they do not have the conditions to constitute the source of sandstorms. Due to the change of vegetation coverage in a year, cultivated land and agricultural activities have a high consistency. Whether cultivated land is the source of sandstorm is determined by the characteristics of planting system and agricultural activities. The fallow period is from June 165438+ 10 to March of the following year, with little precipitation. Cultivated soil destroys surface vegetation and soil cohesion, and has weak wind erosion resistance, which has become the source of strong sandstorms. From the beginning of April to the end of May, dryland crops are sown and germinated. Although the vegetation coverage is low, irrigated soil is the source of weak sandstorm because of its high water content and strong wind erosion resistance. From the beginning of May to the end of 10, crops grew vigorously, with abundant precipitation and high surface coverage. During this period, cultivated land was a non-sandstorm material source. Grassland with low coverage in arid areas is the source of weak sandstorm because of sparse vegetation, which is difficult to effectively protect the surface soil and prevent soil fine substances from being eroded by wind. Gobi is also the material source of weak sandstorm, because fine sand is sandwiched between gravel.

Therefore, the sources of sandstorms are mainly composed of six types in winter and spring (June of the previous year 165438+ 10 to April this year) and five types in summer and autumn (May to June 10), including sandy land, saline-alkali land, gravel, bare soil and low coverage grassland.

According to the isotopic composition and geochemical characteristics of surface dust samples, the main source of surface dust in Beijing-Ejina Banner is the product of in-situ wind erosion in sandy land with high maturity, with less foreign components.

Due to the different sources and moving paths of dust, the influence range of dust will be very different. Because of the long transportation path, the concentration of dust will be significantly reduced due to settlement and removal, and generally it will evolve into floating dust, so its contribution to surface sand samples is obviously small.

4 conclusion

The main source of surface dust in Beijing-Ejina Banner is the product of local wind erosion, with high maturity and less foreign components. Because of the long transportation path, the concentration of dust will be significantly reduced due to settlement and removal, and generally it will evolve into floating dust, so its contribution to surface sand samples is obviously small.

By analyzing the sandstorms recorded in the climatic data of Ejina Banner, Lanzhou and Beijing, it is found that the sandstorms in the first two places may increase the floating dust over Beijing, but more sandstorms are not directly affected by the first two places, and the regional climate is the main cause of sandstorms in Beijing.

Sandstorms in northern China are mainly caused by wind erosion of local surface sediments. With the improvement of industrialization level, the expansion of city scale, the increase of population and the heat island effect, the temperature difference between this area and its surrounding areas is increasing, and the geothermal gradient is increasing, which promotes the occurrence of sandstorms.

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