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Saimaiti, A.;  Fu, C.;  Song, Y.;  Shukurov, N. Loess in Xinjiang, China. Encyclopedia. Available online: https://encyclopedia.pub/entry/32112 (accessed on 20 June 2024).
Saimaiti A,  Fu C,  Song Y,  Shukurov N. Loess in Xinjiang, China. Encyclopedia. Available at: https://encyclopedia.pub/entry/32112. Accessed June 20, 2024.
Saimaiti, Akemu, Chaofeng Fu, Yougui Song, Nosir Shukurov. "Loess in Xinjiang, China" Encyclopedia, https://encyclopedia.pub/entry/32112 (accessed June 20, 2024).
Saimaiti, A.,  Fu, C.,  Song, Y., & Shukurov, N. (2022, October 31). Loess in Xinjiang, China. In Encyclopedia. https://encyclopedia.pub/entry/32112
Saimaiti, Akemu, et al. "Loess in Xinjiang, China." Encyclopedia. Web. 31 October, 2022.
Loess in Xinjiang, China
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The loess in the arid area of Xinjiang is located at the eastern end of the Central Asia Loess Belt, and paleoclimate research about it is of great importance for understanding the mechanism of interaction of the Eurasian Westerly monsoon system and the aridity of Central Asia. Loess sediments have been mainly distributed on the river terraces and windward piedmont of the Tianshan Mountains and the Kunlun Mountains since the late Pliocene (mainly late Pleistocene). Grain size and age data show that Xinjiang loess deposits at some locations are rapid and discontinuous or sedimentary hiatus. The Siberian High system largely controlled dust mobilization and loess accumulation in northern Xinjiang but not southern Xinjiang. In southern Central Asia, the intensity of dust activity may be determined by the Caspian Sea–Hindu Kush Index (CasHKI) and local circulation. 

Xinjiang loess provenance arid area spatial distribution

1. Introduction

Loess, a kind of silt-sized terrestrial sediment, is one of the most extensive deposits in terrestrial ecosystems and is valuable for elucidating paleoclimate changes [1]. In terms of geographical settings, loess is intermittently distributed in arid and semi-arid regions in the middle latitudes of the northern and southern hemispheres [1]. Loess deposits can be found at altitudes ranging from several meters near the coasts (such as in Argentina and New Zealand) to 5300 m north of the Kunlun Mountains of China [2]. The thickest and most continuous loess deposits in the world are located in China [3]. At present, there are three modes of understanding of loess provenance: continental glacier provenance–river transport (CR), mountain provenance–river transport (MR) and mountain provenance–river transport–desert transition (MRD) [1][4].
Central Asia (CA) extends to the Caspian Sea to the east of the Tianshan Mountains and is located at the eastern end of the dust belt [5]. The modern northern boundary of Asia’s summer monsoons can be divided into an approximate boundary of the westerly region and the monsoon region [6]. The Asian westerlies are located on the west side of mid-latitude Asia, and their extent is basically the same as that of arid regions of CA. The region is characterized by an arid climate, sparse vegetation and a fragile ecological environment. It is a unique mountain basin structure with associated vertical divisions that include ice and snow, alpine meadows, forests, grasslands, oases, and deserts. The special topography of CA is a natural barrier and source of dust materials for the deposition of loess, and the area is affected by westerly circulation and basin circulation throughout the year, which is the main driving force of dust for the formation of piedmont loess. The loess in Siberia and CA are usually 10 to <200 m thick [4]. The Xinjiang loess is located in the arid region of CA and is an important part of CA loess. Compared with other loess distribution areas, the loess in this area is thinner and has scattered accumulation characteristics [7]. Loess is one of the most important archives of paleoenvironmental changes, including dust sources, temperature, precipitation, paleoclimatic variability and atmospheric circulation in CA [8][9][10]. The loess in arid areas of the Xinjiang and Chinese Loess Plateau (CLP) together constitute a rare terrestrial sedimentary record of the evolution of Asian continental climate and its dynamic correlation. However, due to the vague interpretation of indicators by regional differences in climate and topography, the Cenozoic climate and environmental changes in the arid region and their driving mechanism in Xinjiang are still under debate [11][12]. The inter-regional atmospheric circulation interaction between the mid-latitude westerly belt and the Asian monsoon circulation is controversial [6][11]. Furthermore, spatial–temporal distribution of CA loess deposits and their specific paleoclimatic implications have not yet been as clearly defined when compared to the substantial deposits at the western and eastern extremities of the eastern Eurasian loess belt, despite increasing research in recent years [7][13][14][15][16]. Therefore, the study of Xinjiang loess is helpful to understand basic scientific issues such as when and why the semi-arid environment in Asia was formed and its phase relationship with paleoclimate change in the East Asian monsoon region [17].

2. Loess Distribution in Xinjiang

Xinjiang (75–90° E, 35–45° N) is located in the center of Eurasia and covers an area of more than 1.6 square kilometers. Xinjiang is surrounded by the Tianshan Mountains and the Kunlun Mountains [18]. From north to south, it consists of the Altay Mountains, Junggar Basin, Tianshan Mountains (including North Tianshan Mountains, South Tianshan Mountains and Ili Basin), Tarim Basin and Kunlun Mountains. The Taklamakan Desert (330,000 km2) and the Gurbantonggut Desert (48,800 km2) are located in the center of the Tarim Basin and the Junggar Basin. CA is not only the main global dust source area, but it is also a loess accumulation area [19]. Regarding the climate system of Xinjiang, with the Tianshan Mountains as the boundary, most of the areas and the mountains of the northern Tianshan Mountains are mainly controlled by westerly circulation. However, the eastern Tianshan Mountains, eastern Junggar Basin and southern Tianshan Mountains are affected by different circulation systems with seasonal changes [19]. The distribution characteristics of the loess in Xinjiang are quite different from those of the loess in the CLP, namely the so-called ‘piedmont loess’, which is mainly controlled by terrain and wind circulation [3]. The loess in Xinjiang is attached to the bedrock of the windward slope of the Tianshan Mountains and the Kunlun Mountains. Loess strata develop mainly in the foothills of the desert edge of Tarim and the Junggar Basin and cover the northern and southern piedmont of the Tianshan Mountains, the western edge of the Junggar Basin and the northern Kunlun Mountains [1][17][19].
From the perspective of a large region, Xinjiang loess is mainly distributed in the Tacheng area in the west of the Junggar Basin, the northern slope of the Tianshan Mountain, the Ili Valley region and the northern Kunlun Altun Mountain at the southern edge of the Tarim Basin [3][19].
Recently, based on the composition of the loess, climate differences and topographic effects, the loess was divided into three subregions in the CA regions: Western (subregion I), Northern (sub-region II) and Eastern (subregion III) [19]. According to this division scheme, the loess at the edge of the Tarim Basin belongs to subregion III, and the Ili Basin, Tacheng, Bole and the loess at the northern foot of the Tianshan Mountains are uniformly divided into subregion II. The division scheme is slightly different from the early Xinjiang loess division scheme proposed by Liu Dongsheng [20]. Combined with the above division, researchers found that the loess in Xinjiang is mainly deposited in river terraces, higher mountain terraces and open river valley regions [17].
The loess is widely developed in the Ili Basin. Loess research in the Yili Basin began earlier than in other areas in Xinjiang, and rich research achievements have been achieved in this region [21][22][23][24][25]. In the Ili Basin, loess covers the river terraces of the upper and middle reaches of the Ili River and its tributaries (Kunes, Tekes and Kashi Rivers) and extends across the edge of the deserts [26]. Paleosols are characterized mainly as light brown or taupe and are densely and weakly developed with white mycelium patches. Some loess–paleosol strata, especially at the bottom, contain gravel and sand, showing the characteristics of multiple genesis processes [10]. In the Ili Basin, loess is widely distributed on the river terraces from the southern slope of North Tianshan to the northern slope of the south Tianshan Mountains. The distinction between loess and paleosol in the western Ili Basin, with an arid climate and low altitude, is generally inconspicuous. On the second terrace of the Kashi River, the thickness of the loess is greater than 20 m at elevations of 1250–1700 m [15]. Previous survey data show that along the Ili Basin from west to east, the loess is a lens, and the grain size is gradually fine [3][23]. Horizontally, the distribution of the loess from the plain to the windward slope of the mountain front is the most concentrated and thickest, and few loess are distributed above the forest line [3]. The thickest loess sediment reported in the Ili Basin is approximately 202 m [3]. Field observations in recent years have shown that loess in the north forms a belt zone along the southern foothills of the Tarbagatai Mountains. The loess in the south is mainly distributed in the northern pediments of the Barluk Mountains at an elevation of 800–1200 m [27]. Loess sediments are also distributed between the Irtysh River and the southern slopes of the Altai Mountains [28]. The loess area of the Zhaosu basin is several meters to dozens of meters thick, covers the Tex River Terrace and the foothills of the Tianshan Mountains and ranges from 1300 m to 2100 m above sea level [29].
Along the southern edge of the Junggar Basin and the northern slope of the Tianshan Mountains, the loess is distributed in different geomorphic units (such as river terraces and piedmonts) from 700–2400 m above sea level. With increasing altitude, the loess shows the characteristics of lens distribution, and the loess thickens from the Junggar Basin to the south [3][19]. Although there is extensive loess deposition in the north of the Tianshan Mountains, the distribution is not continuous, and the thickness is not consistent [18]. To the west of Kuitun, the loess rapidly thins or even disappears, while the loess around Bole gradually thickens from west to east and is approximately 2 to 20 m thick on the Bortala River platform of the Bortala River [30]. The thickness of the loess sediment between Tacheng and Yumin in the western Junggar Basin is several meters to 30 m [31]. The loess sediments north of the Tianshan Mountains have a maximum elevation of about 2400 m, and most are between 1000 m and 1400 m [18]. Loess sediments spread from Kashi in the west to Ruoqiang in the east and constitute a loess belt to the northern slopes of the Kunlun Mountains between 1500 m and 4500 m [3][32]. Thin loess sediments occur sporadically between the Tarim River and the southern slopes of the Tianshan Mountains. Loess in this subregion is characterized by gray-to-light-grayish–yellow, homogeneous, coarse silt and fine sand. Pedogenesis is generally very weak; it is difficult to recognize paleosol layers in the field, but they can be distinguished with careful examination and by indicators [13]. In the broad and flat watershed of the Hotan–Kriya River, drilling core data show that the CA thickness reaches 670 m; above 3400 m, the loess suddenly thins, and the loess in the Kashgar and Pishan area is only 8–10 m thick [3]. New datasets on grain size and geochemical studies suggest that the Kunlun Mountains are the main source of the sand fraction, and both Kunlun and Tianshan Mountains are the main sources of the dust fraction [28][33].

3. Loess Material Composition of Xinjiang

3.1. Mineral Composition

As the main body of loess, minerals are undoubtedly important archives for paleoenvironment changes. However, the mineralogical study of the Quaternary sediments in Xinjiang is relatively weak compared to that of other proxies. Mineral composition analysis of loess in Xinjiang is mainly conducted using XRD. The results of analysis show that there are mainly detrital minerals, followed by carbonate and clay minerals, and a small amount of heavy minerals (generally less than 5%) [34]. The detrital minerals are quartz, feldspar and mica. The carbonate minerals are calcite and dolomite, and the clay minerals are mainly illite and chlorite. Some heavy minerals (such as pyroxene, amphibole, etc.) have also been found [34]. The content of unstable minerals (such as plagioclase) in the Xinjiang loess is higher than in the Luochuan loess. Magnetite in opaque minerals is one of the important factors affecting the change in magnetic susceptibility, implying that the magnetic susceptibility of the Xinjiang loess is lower than that of the CLP. Therefore, the loess in this area may be formed under weak pedogenesis. Feldspar minerals have high hardness and light color and are resistant physical weathering, but they are more chemically weathered than quartz and are unstable minerals. The content of feldspar in Xinjiang loess is significantly higher than that of CLP, which indicates that the chemical weathering of Xinjiang loess is weaker than that of CLP. Analysis of heavy minerals shows that the combination of heavy minerals in Xinjiang soil is similar to that of loess in the CLP [2][23][35]. The clay minerals in the loess of the CLP are mainly illite and contain a certain amount of chlorite and kaolinite, and the content of montmorillonite is less [20]. Clay minerals in Xinjiang loess are mainly chlorite, followed by illite [34]. Previous studies found that the amphibole/epidote ratio might serve as a proxy for wind intensity [13]. These mineral ratios provide new approaches to reconstruct paleoenvironmental changes. The main mineral content of the loess in various regions of Xinjiang is similar, but the differences in the mineral types are quite obvious, which may be due to the different sources of materials.

3.2. Granular Composition

Grain size is the most common physical parameter used as a proxy for environmental change, especially for wind strength in aeolian sediments. Many achievements have been made in loess grain size research in Xinjiang. Various parameters deduced from grain size, such as median, mean, clay, sand, silt and ratios of different fractions, are employed to conceptually and practically reflect wind dynamics or distances from source-to-sink of the arid Xinjiang interior [9][10][31][36][37][38][39][40]. Particle size composition analysis shows that most of the Xinjiang loess is dominated by coarse silt particles, with an average particle size greater than 25 μm, while the CLP loess is dominated by fine silt. This implies that the loess dust comes from inside the basin. The average grain size of the Xinjiang loess has good comparability within the region. The loess on the northern slope of the Kunlun Mountains is compared with the loess in the Yili Basin and the Junggar Basin. The former is composed of sand and silt with an average particle size greater than 30 μm, and the latter is mainly composed of silt and clay. This may be due to the composition of the granularity being affected by the regional paleoclimate, local geomorphology or near origin [31][40][41]. However, the main sources of aeolian sediments and the genetic relations among mountains, deserts and rivers in Xinjiang are still unclear.
The Ili Basin is dominated by silt that contains a certain amount of fine sand, and the particle size is coarser than that of CLP. The sorting property is worse than that of the CLP; the content of sand is less, but the content of clay is slightly higher than that of the CLP. The loess of Ili Basin is mainly composed of silty sand and contains a certain amount of fine sand. The grain content of the sand is low, and the loess content is slightly higher than that of the CLP [23][42]. The components of the end-membrane of different grain sizes in the Ili Basin indicate different meanings. For example, the component of the end-member grain size of 21.22 μm represents a relatively stable background value in atmospheric dust, and changes in content are related to the intensity of the upper-altitude westerly circulation [38]. The suspension transport component of CA material can be considered a relatively sensitive paleoclimate indicator, and 47.5 μm is also a suspension transport component of close range [38]. The grain size of the loess and paleosol in the CLP can be used as a better indicator of the intensity of winter monsoons. However, the generally weak pedogenesis in the Ili Basin has little contribution to the production of fine-grained components, which are sensitive indicators of long-distance transport of dust particles. Therefore, it is considered to be a reliable indicator of the strength of the westerly [10][43][44]. The grain size composition of the loess on the northern slope of the Kunlun Mountains is relatively uniform, and the fine particle content is relatively small. The change in the grain size of the loess in this area is considered an indicator of the degree of drought [41][45][46][47].
The grain size composition of the loess in the north Tianshan Mountains is mainly silt, which is a typical aeolian sediment. The environmentally sensitive grain sizes are <31.7 μm and 31.7–282.5 μm, which may represent the accumulation of dust after storms and wind transport close to storms. For sediment composition in this area, the trend of finer grain size in the longitudinal direction is related to the classification of sediment particles caused by topography. In the southern margin of Tarim Basin, a higher coarse size fraction (>30 μm) in loess sediments was regarded as indicating stronger westerlies and increased aridity, while fine size fractions (<20 μm) are mostly transported by the upper-level westerly jet to more remote regions [9][31][39]. Therefore, when the implications of grain size proxies will be discussed, attention should be paid to the effects of local landform, regional and global atmospheric circulation on grain size proxies.

3.3. Major Element Geochemical Composition

The composition of the major elements of loess in different regions of Xinjiang is similar, and difference in geochemical composition between the loess and paleosol layers are not apparent, whereas they are obviously different that that of CLP [9][21][48][49]. Study of geochemical data shows that the content of Si, Al and Ca in loess in the dry and arid regions of Xinjiang are higher. From SiO2, Al2O3, CaO, Fe2O3, MgO, K2O to Na2O, the oxide content of major geochemical elements in loess decreases sequentially, which is similar to the average percentage of UCC (Upper Continental Crust, (UCC)) average content percentages. Compared to Shaanxi loess, the content of MgO, Na2O and K2O in Xinjiang loess is slightly higher, and the Fe2O3 content is lower. The Xinjiang loess was also found to contain a small amount of S, but the Shaanxi loess has little or no content [34]. The composition of other major elements is similar to the Shaanxi loess. Compared to the average value of UCC, the content of Al in Xinjiang loess is similar to UCC, while Ca, K and Mg elements in some sections are relatively enriched, and Si, Na, and Fe in some sections are slightly depleted. Among them, the enrichment of Mg and Na in the Ili loess section is mainly due to the relatively dry climate of the basin, and the active chemical elements are not easily leached. Generally speaking, higher K content in the stratum reflects a humid climate and more precipitation. By contrast, this indicates that precipitation is reduced and the climate is dry. The K element in the loess section of northern Xinjiang is more enriched than in the loess section of the west Kunlun Mountains in southern Xinjiang. This shows that the climate of northern Xinjiang is relatively humid compared to that of southern Xinjiang. The content of SiO2, Al2O3, Fe2O3 and Na2O in the loess section of the West Kunlun Mountains is relatively lower, while the content of CaO and MgO is higher. This means that the loess in this area was formed in an arid climate. In a word, the loess deposits in Xinjiang have distinctive local or regional geochemical characteristics.

4. The Provenance of Loess Deposits in Xinjiang

Tracing the dust sources of Xinjiang loess helps to better understand atmospheric circulation patterns, transport paths and deposition processes of aeolian dust in CA. Abundant achievements have been made on provenance of loess in Xinjiang [5][13][27][38]. With the application of new technologies and indicators in provenance research, provenance of Xinjiang loess has also made new progress [4][5][9][16][38][50]. Some early researchers emphasized that main sources of dust in the Xinjiang loess are adjacent deserts [18][19][30]. However, this view is mainly inferred from geomorphology, atmospheric circulation, grain size and mineralogical properties. There was a lack of evidence from more reliable indicators. In recent years, methods of loess provenance research in Xinjiang have made great progress. In loess provenance tracking studies in the Ili Basin, North Tianshan Mountains and Tarim Basin, elemental geochemistry [9][13][31], heavy mineral assemblage [2][23][35], analysis of rare earth elements (REE) [16][25][51], and even various provenance tracking models have been used: for instance, Bayesian grain size end-member models [24][38], GLUE models [4], Monte Carlo models [10], etc. Among them, research on the provenance of loess in the Ili Basin and the North Tianshan Mountains is relatively abundant and uses various research methods, while research data on the provenance of loess in other regions are rare. Because loess in different regions of Xinjiang is affected by different atmospheric circulation and local circulation, loess sediments in the northern Tianshan Mountains have almost no similarities with those of the southern Tianshan Mountains [5]. Therefore, it is necessary to discuss loess provenance issues in different regions separately.
For provenance of loess in the Ili basin, it is inferred from topography, wind circulation and sediment particle size distribution that loess materials in the Ili area are likely to come from the CA desert in the west and from westerly transported dust to the Ili basin [3][18]. Recently, provenance tracking has been carried out by means of element geochemistry, heavy mineral assemblage, and REE analysis. Trace elements and REE have been shown to be powerful tools for studying the dust source of loess [52]. Rb is mainly concentrated in mica and potassium feldspar, while Sr is mainly found in calcium-bearing minerals such as plagioclase and carbonate minerals. Rb typically remains immobile during weathering and Sr is characterized by high mobility during genesis because these minerals differ in their resistance to weathering. Therefore, the Rb/Sr ratio can reflect the intensity of weathering and pedogenesis in the loess–paleosol sequence [52]. Trace element and REE studies in the Ili Basin found no correlation between Rb/Sr, ƩLREE/ƩHREE, LaN/SmN, LaN/YbN and GdN/YbN. Therefore, the influence of post-prepositional pedogenesis on loess geochemistry can be excluded. The sedimentary recovery of the Ili loess is not good, so loess provenance is likely to come from the interior of the basin [31]. The Zr/Hf ratio of the samples from potential source areas further proves that local sediments in the Ili Valley can be used as the proximal provenance of Ili loess. The relatively closed environment of Ili Basin greatly reduces massive input of distant dust.
Ili loess is composed of proximal and distal sources. The study of the composition of potential provenance in the basin shows that there are various dust sources in the Ili loess. The provenance area is not only the arid deserts, but alluvial, bedrock and loose material on the inner surface of the basin are also potential sources of loess [2][10][24]. Recently, semi-quantitative analysis and elemental geochemical methods have been used to study local sediments and distant materials in the basin. The materials of the Ili loess near sources are found to be dominant in all areas of the valley, and the local topsoil contributes a great amount to the Ili loess (approximately 54–90%), while the CA deserts contribute little to the deposits of Ili loess [4][10][31].
The proximal end of Ili Valley has different material contributions, among which alluvial sediments have the largest contribution, followed by modern riverbed sediments, and finally, modern surface dust and loose sediment make less of a contribution [2][9][10]
The proportion of the modern riverbed sediment contribution in the east of Ili is significantly lower than that of the Zhaosu Basin. However, the proportion of dust and topsoil contributions is the highest in the west of Ili and lowest in Zhaosu.
Regarding the provenance of the loess in the Tacheng Basin, the Northern Tianshan Mountains and the Kunlun Mountains, for the early stage, based on particle size distribution, meteorological observations and geomorphology, it is speculated that the Sary–Ikshikotrau Desert is the main provenance of the Tacheng Basin [30]. The main sources of loess in the north Tianshan and Kunlun Mountains are the Gulbantonggut Desert and the Taklamakan Desert [25]. Recently, some scholars have carried out elemental geochemical analysis of topsoil and alluvial at the margins of the Tacheng Basin, North Tianshan Mountains and Tarim Basins. The results showed that long-distance transport components contribute less to the loess at the edge of the basin, but the loose sediments in the piedmont slopes and flood plains around the basin are the main dust sources for the loess north of the Tianshan Mountains. Trace element and REE analysis data from the loess in North Tianshan show that there is no good correlation between Rb/Sr, ƩLREE/ƩHREE, LaN/SmN, LaN/YbN and GdN/YbN [13]. It also implies that the weathering and pedogenesis of the loess in North Tianshan is weaker, and it is likely that the dust comes from the interior of the basin. This view is consistent with understanding the provenance of loess in the Ili basin [13]. Thus, loess in the Ili Basin and northern Tianshan Mountains formed a relatively arid environment.
Therefore, the provenance and formation of Xinjiang loess are largely dependent on local topography and dust dynamics. The different sources of loess sediments in the northern and southern Tianshan Mountains in CA can also be attributed to different atmospheric dust dynamics related to the climate circulation system in different seasons. The atmospheric dust dynamics in different regions of Xinjiang need to be further explained, because the differences in provenance are related to the dynamics of aeolian dust [9][53].
Recently, a new understanding of dust dynamics in CA has been presented, and CA can be divided into two parts in terms of wind dust dynamics. The boundary is located in the north at the Tianshan Mountains and in the south at the Aral Sea. According to the latest research, it can be found that loess accumulation in southern CA is mainly determined by the Caspian Sea–Hindu Kush Index (CasHKI) [5]. However, understanding CasHKI is not enough, and much research is needed on the impact of the index. Little attention has been paid to the source and formation mechanism of loess dust at the piedmont of Kunlun Mountains. The current understanding of the provenance of loess at the edge of Tarim is consistent with that in the northern Tianshan Mountains and the Ili Basin. Remote materials contribute less to loess at the edge of the basin, and topsoil and piedmont materials in the basin are the main dust sources for loess formation. However, it has not yet been verified whether the distant source material comes from the CA desert or the Junggar Basin. The Tarim Basin is a relatively closed and arid region, and whether the dust dynamics of its marginal loess is controlled by CasHKI has not been fully discussed. Therefore, the focus of the next provenance research is to carry out quantitative or semi-quantitative research on the potential provenances of loess at the edge of Tarim Basin (such as alluvial deposits and loose sediment on the piedmont slopes) to determine the contribution ratio of potential provenances to piedmont loess.
Numerous indicators show that provenance of loess in Xinjiang has multiple sources, including loose surface sediment, alluvial deposits, surrounding mountains and adjacent deserts. Among them, alluvial deposits and loose sediment on piedmont slopes contributed more to piedmont loess deposition, while the adjacent deserts provided less material for loess.

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