The Xiashu loess deposit is widespread in the lower reaches of the Yangtze River (LRYR). This area is now situated in a warm-humid north subtropics monsoon climatic zone, with a mean annual precipitation of 1,000–1,200 mm, and a mean annual temperature of 15–17°C (Yang et al., 2017). In most cases, the Xiashu loess covers on the terraces along the Yangtze River and its tributaries.

The Xiashu loess deposit accounts for 35–50% in the LRYR (Figure 1), especially prevalent on the hills, highlands, terraces, and other geomorphic units along the Yangtze River and its tributaries. In addition, loess also occurs on the Yangtze River Delta, the continental shelf and islands of the East China Sea. The elevation of the Xiashu loess exposures varies significantly, from over 60 m above sea level to 20 m below sea level. The thickness of the Xiashu loess reaches a maximum of over 50 m in Qingshan, Dagang and Zhenjiang. In Nanjing, the Xiashu loess deposit is distributed as platforms on the river terraces of the Yangtze River, with surface elevations of 30–40 m and thicknesses of approximately 20 m. The outcropping landforms of the Xiashu loess are mostly characterized by highlands and river terraces, and the underlying strata are mainly composed of gravel layers or Cretaceous sandstones. The loess strata are generally horizontal, slightly inclined, inconsistent with the bedrock inclination.

The Xingang (XG) profile (32° 08′ 45.5″N, 118° 50′ 51.6″E) (Figure 1) is located in the Xingang Development Zone on south bank of the Yangtze River, approximately 20 km northeast of Nanjing City, Jiangsu Province. The XG profile is a typical Xiashu loess profile exposed by engineering excavation. The profile is about 15.7 m in thickness, with interbedded loess and paleosol (Table 1), and has three steps dividing the profile into three distinct sections: from bottom to top, these are XG-1, XG-2 and XG-3, respectively. The complete profile has four paleosol layers and three loess layers (Table 1). A weathering crust and Cretaceous sandstone are visible in the lower part. A total of 81 samples were collected from bottom to top at equal intervals of 20 cm for grain-size analysis and quartz grain surface microtextural and morphological observations.

The grain-size analysis results show that the Xiashu loess in XG, Nanjing mainly consists of fine components <50 μm. No particles >250 μm were found in any samples. The content of sand grains (>50 μm) is low (range 5.5–8%, average 6.84%) (Table 5). The silt (50–5 μm) content varies from 62.41 to 75.02% (mean 68.82%). The average content of coarse silt (50–10 μm) is 53.57%; this is the dominant grain size both in XG profile and for the loess on the CLP, and is regarded as a characteristic grain size in eolian loess deposit (Liu, 1985; Guo et al., 2002; Qiao et al., 2003). The average fine silt (10–5 μm) content is 15.25%. Finally, clay (＜5 μm) is the sub-dominant grain size (average 24.34%), second only to the coarse silt in XG profile (Table 5).

The average grain size (Mz) of samples from XG profile varied from 6.23φ to 6.68φ (mean 6.47φ) (Table 5). The sorting coefficient (Sd) ranges from 1.52 to 1.79, with a mean value of 1.65, indicating a poor sorting; skewness (Sk) varies from 0.54 to 0.84 (mean 0.67), indicating positive skewness; kurtosis (Kg) ranges from 2.53 to 3.13 (mean 2.80), showing that most samples are sharply peaked.

The grain-size frequency curve (Figure 3) of XG profile is characterized by a major peak and other three small peaks. The major peak is located near 5.5φ, with other three small peaks near 2φ, 8φ, and 10.5φ respectively. The small peak near 2φ indicates that the XG profile contains a certain amount of coarse-grained materials, namely sand content. The two small peaks near 8φ and 10.5φ suggest that the Xiashu loess contains more fine-grained materials in the two places. On the whole, the frequency curve is characterized by a striking high content of sand, fine silt and clay component, which is unusual for pure windblown loess, the small peak near 8φ probably indicate a small amount of far - distant component, and the small peak near 10.5φ may indicate the existence of a large amount of clay component in XG profile. The frequency curve of XG profile suggests that the Xiashu loess is probably a admixture composed of near - source materials and long - transport materials. On one hand, it is likely that the Xiashu loess is primarily a near - source deposit, so the sand, silt and clay were mixed together during transportation, resulting in more sand materials in XG profile, and these materials have undergone a strong weathering and pedogenesis process in the warm-humid north subtropical monsoon climate after deposition, causing a obviously high content of clay component. On the other hand, the join up of a small amount of far - distant fine materials caused a small peak near 8φ and a poor sorting in XG profile.

81 samples from XG profile were computed using the function, as shown in Table 6, all the samples from XG profile and ZKT profile are identified as ED, and all the floodplain deposit samples from NT profile are identified as FD. Therefore, the Xiashu loess in XG profile is considered as a eolian deposit.

By end-member modeling analysis, the XG Xiashu loess profile was divided into four end-members (EM) (Figures 4, 5), from EM1 to EM4, they account for 11.97, 23.06, 57.64, and 7.33% on average respectively (Figure 5). EM1 (modal size 1.0 um) should be the product of weathering and pedogenesis after deposition. According to Tsoar and Pye (1987), dust particles >20 μm are unlikely to be transported in long-term suspension across a large height range, and the >10 μm components were regarded as nearby-source materials for Xiashu loess, whereas <5 μm components were regarded as far-distant materials that was mainly derived from the arid regions of northwestern China by long-range transport (Wu et al., 2020). Therefore, we suggest that EM2 (modal size 4.5 μm, accounting for 23.06%) represent the far-source materials that were mainly originated from the arid regions of NW China. EM3 (modal size 22.4 μm, accounting for 57.64%) and EM4 (modal size 44.8 μm, accounting for 7.33%) represent nearby-source materials that were mainly derived from local sources, which account for at least 64.97% in total and were presumed probably originated from the floodplains in the lower reaches of the Yangtze River, the Huai River, and the Yellow River to the north during glacial periods.

SEM observations of quartz grains from XG profile show that 55% grains observed are characterized by subangular outlines (Figures 6A,B, 7, 8), with a certain number of angular particles (29%) and round particles (5%) (Figure 8). The XG profile quartz grains show various shapes, such as plates, strips, triangles, cubes, pointed, slightly rounded, irregular, etc (Figures 6A,B). Mechanically produced features are abundant and varied, indicating that the quartz grains have experienced a range of external forces. Characteristics typical of eolian quartz grains in XG profile are common (Figures 6C–F), e.g. dish-shaped depressions (Figures 6C–F) and crescentic marks (Figure 6F) have occurrence rates 61% (Figure 8) and 48% (Figure 8), respectively. Other eolian features like upturned plates and meandering ridges (Figure 6D) also occurred on the grains, as well as pockmarks, formed pockmarked surfaces (Figures 6D,F), indicating quartz grain surface erosion during wind transportation. These features are similar to the microtextures characteristics in quartz grains from the loess on the CLP (Figure 9), reflecting transportation process by wind power. Meanwhile, many subaqueous features are also observed on the quartz grains: for example, V-shaped cracks (Figures 6G,I), grooves (Figure 6I), conchoidal fractures (Figure 6H), parallel striations caused by scratch (Figures 7C–E), triangular cracks (Figure 6H), underwater polished surfaces (Figures 7A,B,F), as well as bulbous edges and smooth edges (Figures 7F,G) caused by abrasion in subaqueous environments. V-shaped cracks, the most representative feature produced by grain-to-grain collisions in subaqueous environments (Mahaney, 2002; Krinsley and Doornkamp, 2011), reach an occurrence rate of 63% in XG profile (Figure 8), suggesting a fluvial environment. A majority of quartz grains with high relief and abundant mechanical V-shaped cracks (Figures 6G–I), as well as parallel striations (Figures 7C–E) caused by scratch demonstrated that the quartz grains underwent a high-energy subaqueous transport. Some grains have surface microtextures that are typical of a subaqueous fluvial environment, with dish-shaped depressions or small impact pocks superimposed. This suggests the quartz grains were abraded in a fluvial environment before undergoing eolian transportation. In addition, 13% of the quartz grains (<20 μm) in XG profile have aeolian features and sharp edges on their surfaces, but without subaqueous characteristics, it is presumed that these grains were originated from far-distant materials. The grains in XG profile also exhibit silica dissolution and silica precipitation textures indicative of chemical weathering. Silica dissolution mainly comprise oriented etch pits (Figure 7H), solution pits and crevasses. Oriented etch pits are oriented and aligned, which are likely to be developed in rivers, lakes or marine environments, where the water affects the grains by dissolution (Mahaney, 2002; Krinsley and Doornkamp, 2011). 18% of the grains show oriented etch pits in XG profile, indicating a fluvial environment. Silica precipitation has produced silica globules (Figure 7I), silica pellicles and silica flowers, indicating that the quartz grains have undergone post-depositional weathering and pedogenesis.

The XG profile and other reported Xiashu loess profiles present typical characteristics of eolian origin, including a uniform loose and porous texture, massive structure, significant vertical joints, and disseminated iron-manganese films. The Xiashu loess covers hills, highlands, terraces, and other geomorphic units at varying elevations, and shows a range of origins and characteristics. In addition, the Xiashu loess strata are composed of alternating dark-colored (paleosol) and light-colored (loess) layers, resembling the typical loess-paleosol sequence on the CLP of northern China (Liu, 1985). Furthermore, although in some cases, the underlying strata of the Xiashu loess are composed of gravel layers, both of reported results and this study show that there are no sand layers or gravel layers in Xiashu loess profiles (Hao et al., 2010; Li et al., 2018b; Jiang et al., 2020), These depositional features distinguish aeolian deposits from fluvial deposits.

The grain size composition of XG profile shows obvious aeolian characteristics, most notably the fine grain size and high clayey silt fraction. Silt is the dominant component in all layers, with an average content of 68.82%, followed by clay and sand. The components with grain size＜50 µm in XG profile account for 93.16% on average. This clear dominance of the fine fraction (<50 µm) is considered to be a typical feature for eolian loess deposit (Liu, 1985; An et al. 1990; Guo et al. 2002). Coarse silt (50–10 µm), which is the primary grain size group (average content 53.57%) (Table 5) in XG profile, is regarded as a key characteristic of aeolian loess on the CLP (Liu, 1985). The grain size compositions and grain size parameters of the loess are highly uniform between different layers of XG profile, suggesting that the Xiashu loess was likely to have been transported by a single external force. In addition, the grain size characteristics of loess in XG profile, Zhenjiang profile and Nanjing Zhoujiashan profile are strongly consistent, indicating a shared provenance (Li et al., 1997, Li et al., 2001b, Qiao et al., 2003, Hao et al., 2010; Jiang et al., 2020).

Due to the wind-sorting effect, prevailing northwesterly wind direction, and the decreasing carrying capacity with increasing distance from the source area, the grain size of loess on the CLP decreases from coarse to fine from northwest to southeast. Here, we choose two classical loess profiles on the CLP, one is Luochuan profile in the middle of the CLP, the other is Qinling profile in the southernmost part of the CLP. They are two representative profiles on the CLP. For example, the average grain size (Mz) of the loess layers in Luochuan profile is 6.01φ, the mean value of that in other six loess profiles in the interior of the CLP is 6.00φ (Lei, 1998; Lei, 1999), indicating that the Luochuan profile is very representative. Here, we take Luochuan profile and Qinling profile in northern China as examples, the respective contents of sand (>50 μm) and clay (<5 μm) in Luochuan profile are 6.8 and 27.4% on average, the average grain size (Mz) of Luochuan profile is 6.01φ (Liu, 1985; Lei, 1998; Lei, 1999). The respective values for the Qinling loess are 4.3%, 30.5%, and 6.59φ respectively, demonstrating the NW-SE trend toward finer grain sizes. The Xiashu loess is located far from northwest China; In addition, the warm-humid climate in the LRYR is highly conducive to post-depositional weathering and pedogenesis. Therefore, the sand content in the Xiashu loess should be lower than that of the loess on the CLP in northern China, while the contents of silt and clay materials should be higher. However, results show that sand (>50 μm) contents of the Xiashu loess in the LRYR, including XG (6.84%), Zhoujiashan of Nanjing (8.6%) and Zhenjiang (10.1%) are higher than those of the Luochuan profile (6.8%) and the Qinling profile (4.3%), while clay content in XG (24.3%), Zhoujiashan of Nanjing (21.3%) and Zhenjiang (19.1%) are lower than those in Luochuan (27.4%) and the Qinling loess (30.5%), so is the LGS Xiashu loess profile (24.9%) in East China Sea (Table 7). The average grain size of the loess layers in XG (6.42φ, Paleosol layers not included) is coarser than that in Qinling loess (6.58φ) (Liu, 1985; Lei, 1998, Lei, 1999; Xu et al., 2016). All the characteristics above suggest that the grain size of the Xiashu loess in the LRYR does not follow the NW-SE trend of decreasing grain size. On the other hand, grains larger than 20 μm are unlikely to be transported for long distances by wind. The components larger than 20 μm in XG profile account for 38.92%, and sand content (>50 μm) accounts for 6.84%, especially the >10 μm components, which were considered as nearby-source materials for the Xiashu loess (Wu et al., 2020), account for 60.51% in XG profile. In addition, end-member modeling analysis show that EM3 (modal size 22.4 um, accounting for 57.64%) and EM4 (modal size 44.8 μm, accounting for 7.33%) represent nearby-source materials that were mainly derived from local sources, account for at least 64.97% in total. The relatively high content of grains >10 μm and >20 μm in XG profile is hard to explain using the distant source theory. In other words, nearby-source materials dominate the accumulation of the Xiashu loess. Finally, the poor sorting of XG and other reported Xiashu loess profiles in the LRYR is unusual (Li et al., 1997, Li et al., 2001b, Li et al., 2018b; Qiao et al., 2003; Xu et al., 2016), if the Xiashu loess was originated from the remote Gobi Desert of northwestern China or originated from the adjacent floodplains, it should have been well sorted. Therefore, multiple characteristics suggest that the Gobi Desert in inland NW China were not the primary source materials for the Xiashu loess, the Xiashu loess is probably a admixture composed of nearby-source materials and long-transport materials. Previous studies show that dust from inland NW China can be transported to the sea of Japan (Xiao et al., 1999), and the grain-size frequency curve of XG profile has also proved this point, so we suggest that a small amount of dust from the remote Gobi Desert in areas of northwestern China was likely to be transported to the LRYR during glacial periods, and the poor sorting of XG profile and other reported Xiashu loess profiles is not difficult to understand.

The loess on the CLP were originated from the remote Gobi Desert in inland northwestern China. The quartz grains in the loess are mostly subangular and angular, and with characteristic shapes (e.g. water drops, spears, diamonds, spindles or spirals) (Wang et al, 1985; Liu, 1985). These forms are mostly streamlined with a long primary axis, especially the twisted spiral shape representing the typical eolian loess on the CLP (Figure 9). This is because the quartz grains were carried by airflow for a long distance, and the grains themselves also rotated around their long axis, gradually being shaped by wind friction (Wang et al, 1985; Sun and An, 2000). Previous studies show that the quartz grains from the loess on the CLP are dominated by eolian features, subaqueous environmental characteristics are rare or absent (Liu, 1985; An et al., 1990; Ding et al., 1995; Guo et al., 2000). Our results show that most of the quartz grains of XG profile not only have typical features of wind transportation (such as dish-shaped depressions, crescentic marks and pockmarked surfaces), but also display a range of subaqueous environment characteristics (e.g. V-shaped cracks, subaqueous polished surfaces, grooves, etc) with high frequencies indicating that the grains have undergone subaqueous transport. Some grains with surface features of fluvial environments have aeolian characteristics superimposed on them, suggesting that the quartz grains were firstly abraded in a fluvial environment, and then transported and deposited by wind action. These features are distinctly different from those of the quartz grains on the CLP, and are also significantly different from those of the quartz grains derive from underlying parent rocks that are almost without any transportation and typically irregularly shaped with sharp edges and extremely irregular shapes. In addition, if the Xiashu loess were originated from the remote Gobi Desert in inland of NW China, its quartz grains would have developed the same streamlined or spiral shapes as those of the loess on the CLP. However, SEM observations show that the quartz grains of XG profile present various shapes (such as plates, strips, triangles, cubes, points, and slightly rounded; and no screw - like grain occurs in the quartz grains observed), which were clearly distinct from the quartz grains on the CLP. In addition, subaqueous environmental characteristics such as conchoidal fractures and V-shaped depressions are rare in typical eolian quartz grains, only can be seen in quartz grains that have just entered the eolian environment. The subaqueous environmental features in XG profile are abundant and clearly visible, indicating that the grains have been changed little by the eolian environment and have not been transported by wind for a long distance. Quartz grains >20 µm are unlikely to be transported by wind over long distance and therefore are regarded as regional materials, SEM observations show that >20 µm quartz grain (including sand grains) in XG profile are with obvious eolian features and subaqueous characteristics, indicating a near-source deposition. 13% of the quartz grains (<20 µm) observed have eolian features on their surfaces, but without subaqueous characteristics, it is presumed that these grains were originated from far-distant materials. These characteristics suggest that the Xiashu loess should be eolian deposit that was mainly originated from near-source materials. Previous studies show that dust from inland of NW China can be transported to the sea of Japan (Xiao et al., 1999), the grain-size frequency curve and the quartz grain microtextures, as well as end-member modeling analysis indicate that the Xiashu loess may contain a certain amount of far-distant materials, so we suggest that nearby-source materials dominate the accumulation of the Xiashu loess, and the dust from the Gobi Desert in inland NW China have played a small role in the accumulation of the Xiashu loess. The surface microtextures and morphologies of the quartz grains provide a evidence for identifying the provenance of the Xiashu loess.

The accumulation of the Xiashu loess in the LRYR of southern China have significant environmental implications. In terms of sedimentary chronology, new advances in magnetostratigraphy and optically stimulated luminescence (OSL) dating have constrained the age of the lower boundary of the Xiashu loess to approximately 0.9 Ma, and the uppermost loess layer formed in the last glacial period (Qiao et al., 2003; Hao et al., 2010; Li et al., 2018b). The development of aeolian loess deposit in Asian subtropical regions since 0.9 Ma against a backdrop of global climate change. In the late Cenozoic, the overall climate trend was characterized by a global cooling, as evidenced by the expansion of continental ice sheets. This was accompanied by continental stepwise aridification, which led to the formation and development of the eolian loess deposit on the CLP (Liu, 1985; Guo et al. 1998; Sun and An, 2000; Guo 2010; Hao et al., 2010; Li et al., 2018b; Westerhold et al., 2020).

Great changes have taken place in global climate and environment in the middle of the Quaternary (Hao et al., 2010; Westerhold et al., 2020). The Mid-Pleistocene Transition (MPT) at about 1.25–0.7 Ma marked a shift in the primary climatic cyclicity from a 41 ka cycle to a 100 ka cycle, caused a significant increase in global ice volume and global cooling (Ruddiman et al., 1989; Clarket al., 2006; Hao et al., 2010; Elderfield et al., 2012; Westerhold et al., 2020). On the other hand, a significant tectonic movement (the “Kunlun-Yellow River movement”) on the Tibetan Plateau at about 1.2–0.6 Ma resulted in a period of rapid uplift. The "Kunlun-Yellow River Movement" raised the plateau and many surrounding mountains to a critical height of 3,500 m by around 0.9 Ma, causing widespread growth of glaciers which brought the Tibetan Plateau into the cryosphere and induced the first ice age in China since the Quaternary (Fang et al., 1996; Cui et al., 1998, Cui et al., 2011; Li et al., 2001a). The uplifted Tibetan Plateau serves as a cold source in winter and a heat source in summer, thereby intensifying both the winter monsoon and the summer monsoon, and also split the westerly jet into two streams. Thus resulted in the intensification and northward displacement of the Siberian high-pressure system. Global cooling and the uplifted Tibetan Plateau led to the enhancement of the Siberian high pressure, and caused a severe aridification in Asian interior and the intensification of the east Asian winter monsoon (Ruddiman et al., 1989; Kutzbach et al., 1997). Hence, the dust deposition rate in northern China accelerated during 1.2–0.9 Ma, created extensive areas of eolian loess. The development of the exceptionally thick and coarse loess layers L15 (deposited at about 1.2 Ma) and L9 (deposited at about 0.9 Ma) on the CLP is a strong evidence for the cold-dry event in this period: L15 and L9 correspond to marine oxygen isotope stages 38 and 22, representing two periods of severe global cooling (Liu, 1985; Ding et al., 2002; Guo et al., 2000; Lu et al., 2000, Lu et al., 2008; Sun and Guo, 2017). Global cooling and the uplifted Tibetan Plateau also strongly influenced the LRYR region in southern China, resulting in a severe regional cooling and aridification. In addition, retreat of the coastline of the western Pacific Ocean increased the area of the continental land mass: for example, during the LGM, the coastlines of the East China Sea and the Yellow Sea had retreated eastwards by 6–7° of longitude (about 600 km), and the sea level had dropped by 150 m (Wang, 1999, Xu et al., 2012). The area of the LRYR, the Huai River and the Yellow River to the north is low, flat, and densely covered with rivers and lakes. In glacial periods, with declining of the sea level, the reduced river and lake levels yielded large areas of desiccated rivers, floodplains and lake beds, promoted these areas as a dust source for the accumulation of the Xiashu loess. The intensified winter monsoon produced strong wind erosion capacity and provided enough power to transport these regional fine materials, the river valleys and multistage river terraces of the Yangtze River and its tributaries provide good geomorphological conditions for loess deposition. The far-distant dust from the Gobi Desert of northwestern China may have played a small role in the accumulation of the Xiashu loess as well.

Sporopollen analysis of a typical loess stratum in Nanjing shows that herbaceous pollen dominated the total pollen assemblage, the sporopollen are mainly comprised of Compositae and Gramineae (subordinately artemisia and Chenopodiaceae), while arboreal pollen was mainly composed of Pinus, indicating a relatively arid and cold climate (Hao et al., 2010). The Ganzi loess in western Sichuan and the red earth in Chengdu plain (Figure 1) in the upper reaches of the Yangtze River have been confirmed as near-source eolian deposits formed in this period (Zhao et al., 2007; Qiao et al., 2007; Wang et al., 2018). The occurrence of sustained dust deposit reflects an important climate and environment change in the source and/or deposition regions (Pye, 1995). This means that the environment of the whole Yangtze River Basin generally deteriorated at around 0.9 Ma. Therefore, we suggest that under the background of MPT, the strengthened east Asian winter monsoon and regional aridification driven by global cooling and the rapid uplift of the Tibetan Plateau are suggested as the primary drivers for the Xiashu loess. The development of the Xiashu loess was a regional response of the northern subtropical region in southern China to the MPT global cooling event.