Grain-size-dependent geochemical characteristics of Middle and Upper Pleistocene loess sequences from the Junggar Basin: Implications for the provenance of Chinese eolian deposits

Journal Pre-proof Grain-size-dependent geochemical characteristics of Middle and Upper Pleistocene loess sequences from the Junggar Basin: Implications for the provenance of Chinese eolian deposits

Xiaojing Li, Jinbo Zan, Rongsheng Yang, Xiaomin Fang, Shengli Yang PII:

S0031-0182(19)30739-4

DOI:

https://doi.org/10.1016/j.palaeo.2019.109458

Reference:

PALAEO 109458

To appear in:

Palaeogeography, Palaeoclimatology, Palaeoecology

Received date:

5 August 2019

Revised date:

6 November 2019

Accepted date:

11 November 2019

Please cite this article as: X. Li, J. Zan, R. Yang, et al., Grain-size-dependent geochemical characteristics of Middle and Upper Pleistocene loess sequences from the Junggar Basin: Implications for the provenance of Chinese eolian deposits, Palaeogeography, Palaeoclimatology, Palaeoecology (2018), https://doi.org/10.1016/j.palaeo.2019.109458

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© 2018 Published by Elsevier.

Journal Pre-proof

Grain-size-dependent geochemical characteristics of Middle and Upper Pleistocene loess sequences from the Junggar Basin: Implications for the provenance of Chinese eolian deposits Xiaojing Li 1

1, 2

, Jinbo Zan

1, 2

, Rongsheng Yang

1, 2

, Xiaomin Fang

1, 2

, and Shengli Yang

3

CAS Center for Excellence in Tibetan Plateau Earth Sciences and Key Laboratory of Continental Collision and Plateau Uplift, Institute

of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China, 2 University of Chinese Academy of Sciences, Beijing 3

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100049, China, College of Earth Environmental Sciences and Key Laboratory of Western China's Environmental Systems ( MOE), Lanzhou University, Lanzhou, 730000, China.

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Corresponding author: Jinbo Zan, Tel.: +86 10 8409 7172; Fax: +86 10 8409 7079; E-mail address: [email protected]

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Abstract: Thick loess-paleosol sequences are widely distributed in the arid inland basins of northwestern China. The geochemical composition of these sediments

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grain-size-dependent

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provides important constraints on the provenance of Chinese eolian deposits. geochemical

characteristics

of

Early-Middle

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Pleistocene loess deposits from these arid inland basins have not been investigated.

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The paucity of such studies hinders a full understanding of provenance changes of

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loess deposits from the Chinese Loess Plateau (CLP). In the present study, we performed a detailed major, trace and rare earth element analysis of multiple grain-size fractions of Middle Pleistocene loess deposits from the Dongwan section (DW) in the Junggar Basin. We found that the major, trace and rare earth element concentrations of the DW loess are grain-size-dependent, with higher concentrations in the fine fraction. In addition, the <5 μm and 5-20 μm fractions are more homogeneous in composition than the coarse fractions, indicating that they were extremely well mixed prior to deposition. Compared to contemporaneous loess deposits from the CLP and surface loess samples from Tajikistan and Ili Basin in Xinjiang, the loess deposits of the DW section have higher major element ratios of TiO2 /Al2 O3 and K2 O/Al2 O3 , and REE ratios of LaN/YbN, GdN/YbN and LaN/SmN, indicating that the loess deposits in the Junggar Basin have distinctive local or

Journal Pre-proof regional geochemical characteristics. These results support the view that the Junggar Basin is not a potential source region of the loess in the CLP, at least since the Middle Pleistocene. Furthermore, temporal variations in the major element and REE ratios of the coarse fractions (20-75 μm and >75 μm) from the DW section changed significantly at ~0.5 Ma, which may have arisen from intensified physical erosion or increased wind speeds in the northern Tianshan area. Keywords: Major elements; Trace elements; Rare earth elements; Chinese Loess

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Plateau; Tianshan; Central Asia. Highlights:

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● Grain-size-dependent geochemical characteristics of Central Asian loess were

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determined.

● Loess deposits in the Junggar Basin and the CLP have different geoc hemical

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characteristics.

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● Geochemical characteristics of coarse loess fractions in the Junggar Basin changed

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at ~0.5 Ma.

1. Introduction

The widespread, thick Quaternary eolian loess-paleosol sequences in northern China are a valuable archive of paleoenvironmental information, including about regional aridification and the evolution of atmospheric circulation in Asia (Heller and Liu, 1982; Liu, 1985; Sun et al., 1998; An et al., 2001; Ding et al., 2002; Guo et al., 2002; Chen et al., 2014; Lü et al., 2011; Nie et al., 2015; Zhang et al., 2016). Tracing the provenance of Chinese eolian deposits is important for understanding the formation mechanisms, transport pathways and sedimentary processes of eolian dust in the mid- latitude areas of Asia. Thus, this topic has attracted much recent research attention in paleoclimatology (Liu, 1985; Sun et al., 2006; Chen and Li, 2011; Kapp et al., 2011; Stevens et al., 2013; Nie et al., 2015). Furthermore, understanding the

Journal Pre-proof provenance of Chinese eolian deposits can provide additional information on climate change and tectonic activity, due to the close relationship of these processes with dust formation (Ding et al., 2002; Guo et al., 2002). It is widely accepted that the wind field near the ground surface is the main agent of dust transport to the Chinese Loess Plateau (CLP). The source areas of Chinese eolian deposits are generally considered to be the vast regions of the northern edge of the Tibetan Plateau and the Central Asian Orogenic Belt (Li et al., 2009; Chen and Li, 2011; Pullen et al., 2011; Sun et al., 2002, 2018; Stevens et al., 2013; Xiao et al., 2012; Nie et al., 2015, 2018), where

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sediment production is closely related to high altitude surface processes such as glacial grinding, freeze-thaw weathering and structural denudation.

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However, the sources of Chinese eolian deposits remain controversial, in

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particular it is unclear whether they are derived from a single, uniform source region, or whether they are derived from mixed and recycled material from multiple distant

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sources. One of the main reasons for this uncertainty is that most previous studies have focused primarily on a comparison of physicochemical properties between the

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Neogene loess deposits in the CLP and modern eolian materials in the potential source

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areas. However, the existing evidence suggests that tectonic activity on the northern

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edge of the Tibetan Plateau is relatively strong (Lease et al., 2007; Li et al., 2014; Yang et al., 2019), and the atmospheric circulation patterns in the potential dust source areas (e.g., Gobi Desert) have changed substantially since the late Cenozoic (Fang et al., 2002; Zan et al., 2010; Cheng et al., 2019). This could have resulted in the input of fresh detritus or coarse clastic material from new source areas, causing temporal changes in the physicochemical properties of eolian materials in the potential source regions of Chinese eolian deposits. In addition, most previous geochemical studies have focused primarily on the <20 or <38 μm fractions of upper Pleistocene loess deposits in the arid inland basins of northwestern China (Maher et al., 2009). However, the grain-size-dependent geochemical characteristics of Early-Middle Pleistocene loess deposits from potential source regions of Chinese eolian deposits have not been investigated. Consequently,

Journal Pre-proof inferences based on the comparison of the geochemical characteristics of the <20 μm fraction between the loess deposits of the CLP and the potential source regions may lead to incomplete information regarding changes in the provenance of Chinese eolian deposits. In order to resolve these issues, detailed geochemical analysis of multiple grain-size fractions from older loess deposits in the potential source regions of Asian dust is required. In the northern margin of the Tibetan Plateau and the arid Gobi

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region of Central Asia, thick Pleistocene eolian loess sequences are widely distributed

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in the downwind regions of several large deserts (Fang et al., 2002). Since these loess deposits are closely associated with the deposits of the upwind deserts, their

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geochemical properties and mineral composition can provide new insights into

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temporal changes in the physicochemical properties of eolian materials in the potential source regions of Chinese eolian deposits. In the present study, we

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performed detailed major, trace and rare earth element (REE) analysis of multiple grain-size fractions from Middle Pleistocene loess deposits at Dongwan (DW), in the

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Junggar Basin. Our aims are to provide a detailed comparison of grain-size-dependent

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geochemical characteristics of contemporaneous loess deposits of the CLP and the

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Junggar Basin, and then to use the results to determine possible long-term variations in the geochemical properties of eolian materials in the potential source region of Asian dust.

2. Geological and geographical setting Loess deposits are extensively distributed along the southern margin of the Junggar Basin. The Dongwan (DW) loess section (44° 04' N, 85° 47' E; ~840 m a.s.l.) lies on the northern slope of the Tianshan, at the southern edge of the Gurbantunggut Desert (Fig. 1). The area has a temperate continental climate which is influenced by the westerlies and the Mongolian High; the mean annual temperature is ~7 ℃, and the mean annual precipitation is ~190 mm. Under the modern climatic regime, the spring rainfall accounts for about 40% of the year total (Fig. 1c). At the southwestern edges of the Gurbantunggut Desert, the number of sand–dust storm days is about 19

Journal Pre-proof days per year. The dust storm occurs frequently in spring and early summer (Qian et al., 2005). It has been argued that loess in the northern Tianshan is derived mainly from the Gurbantunggut Desert transported by northwesterly winds (Sun, 2002). The DW section has a thickness of 71 m, with the Matuyama-Brunhes boundary (0.78 Ma) located at the depth of 69 m (Fig. 2), as determined by detailed paleomagnetic analysis (Fang et al., 2002). The basal age of the section is ~0.8 Ma, assuming a mean sedimentation rate of 8.9 cm/kyr. Paleosols in the DW section are generally very

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weakly developed because of the low precipitation in the region.

Fig. 1. (a) Distribution of loess deposits, major patterns of atmospheric circulation in As ia and the

Journal Pre-proof location of the Dongwan (DW) section in the Junggar Basin and the Baishui loess section in the Chinese Loess Plateau. (b) Distribution of loess deposits in the northern Tianshan (modified from Fang et al (2002). (c) The monthly variation of precipitation/temperature of meteorological

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stations from the Tacheng and Urumqi (the data are from Xinjiang statistical yearbook 2018).

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Fig. 2 . Field photograph, magnetostratigraphy and median grain size (Md) of the Dongwan (DW) loess section in the Junggar Basin. The magnetostratigraphy and median grain size are from Fang

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et al. (2002). "S" are soil layers, which are separated by loess layers (stippled pattern). The Matuyama-Brunhes (M/B) boundary is located at the depth of 69 m. Ages for geomagnetic excursions in the Brunhes and Matuyama chrons are from Laj and Channell (2007) and Roberts (2008).

3. Materials and methods A total of 20 bulk samples were collected at 3- to 4- m intervals from the DW section for geochemical analysis. Previous studies have shown that the coarse and fine components of Chinese eolian deposits may undergo different transport processes and have different sources (Hao et al., 2008; McTainsh et al., 2013; Zan et al., 2015; Wang

Journal Pre-proof et al., 2017). Bulk grain size analyses demonstrated that mass percentage of the loess samples in the DW section was dominated by 20–75 μm fraction, with the values varying from 50.0 to 66.7% (Zan et al., 2015). In contrast, the <2 μm fractions has low contribution in all of the samples. Recent work indicates that geochemical analysis of multiple grain-size fractions (<5 μm, 5-20 μm and >75 μm) of surface sand and loess samples from the major Chinese deserts and the CLP can provide new insights into the provenance change of the Chinese eolian deposits (Chen et al., 2007; Xiong et al., 2010). For this reason, the bulk loess samples from the DW section were

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divided into four grain-size fractions: <5 μm, 5-20 μm, 20-75 μm and >75 μm. The <5 and 5–20 µm fractions were separated by gravitational settling based on Stokes’ Law,

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and two coarse fractions (20–75 and >75 µm) were isolated by wet sieving.

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Prior to geochemical measurements, the grain-size fractionated samples were ground to a powder and the carbonate was then removed by treatment with 1 mol/L

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acetic acid. About 20 mg of each sample was then weighed and transferred to a Teflon container to which 1 ml of concentrated HNO 3 and 1 ml of concentrated HF were

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added (Yang et al., 2015). After heating in an ultrasonic bath for 20 min, the

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containers were oven-dried at 190 °C for 24 hr. The solutions were then dried at

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150 °C on a hot plate and HNO 3 was added before heating to a temperature of 150 °C for 24 hr. Finally, the solutions were diluted with ultrapure water to a weight of ~45 g for analysis of major, trace and rare earth elements (REE) (Yang et al., 2015). Major and trace element concentrations were respectively measured using the ICP-OES (Leeman Labs Prodigy-H) and ICP-MS (X-7, Thermo-Elemental, USA) at the Institute of Tibetan Plateau Research, Chinese Academy of Sciences. The relative standard deviations are less than 2% for major elements and 5% for trace elements. The detection limits of the major elements were less than 0.008 μg L−1 . In addition, two standard samples (GSR-1, GSR-2) and two blank samples are simultaneously analyzed.

4. Results

Journal Pre-proof 4.1 Major element characteristics Compared to the upper continental crust (UCC), the grain-size fractionated samples from the DW loess are enriched in Mg, Fe, Al, Mn and Ti and depleted in Na and Ca (Fig. 3a-d). In addition, the majority of the major elements have higher concentrations in the fine fractions (<5 μm and 5-20 μm) than in the coarse fractions (20-75 μm and >75 μm). In general, MgO, FeO, MnO and TiO 2 are enriched in the <5 and 5-20 μm fractions, whereas they are depleted in the 20-75 μm and >75 μm

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fractions. By contrast, the CaO and Na2 O contents of the <5 μm and 5-20 μm

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fractions are much lower than those of the coarse fractions (Fig. 3e).

Journal Pre-proof Fig. 3. UCC-normalized major-element composition of four grain-size fractions and the average composition of each fraction for typical loess/paleosol samples from the Dongwan (DW) section in the Junggar Basin. UCC data are from Taylor and McLennan (1985). UCC: upper continental crust. Note that the y-axis is on a log scale.

4.2 Trace element characteristics The UCC-normalized trace element content for each grain-size fraction are shown in Figure 4a-e. Compared with the coarse fractions (20-75 and >75 μm), most

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of the trace elements have higher concentration values in the <5 and 5-20 μm fractions, except for Be, Sr and Sn (Fig. 4a-d). The trace element concentrations are

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more homogeneous in the fine fractions, whereas the 20-75 μm and >75 μm fractions generally exhibit large variations in trace element concentrations, with most of the

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trace elements being depleted overall (Fig. 4c-e).

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Fig. 4. UCC-normalized trace element patterns of four grain-size fractions and the average composition of each fraction for typical loess/paleosol samples from the Dongwan (DW) section in the Junggar Bas in. UCC data are from Taylor and McLennan (1985, 1995). UCC: upper continental crust.

4.3 Rare earth element characteristics REEs are widely used to identify the sources of eolian sediments (Taylor and McLennan, 1985; Chen and Li., 2011). The fractions are characterized by steep light-REE and relatively flat heavy-REE characteristics, with negative Eu anomalies

Journal Pre-proof (Fig. 5a-d). In addition, the grain-size fractionated samples generally have similar REE distribution patterns to the UCC (Taylor and McLennan, 1985), especially the 20-75 μm and >75 μm fractions (Fig. 5c, d). Notably, the REE concentrations of the <5 μm and 5-20 μm fractions are generally much higher than those of the coarse

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fractions, which is attributed to the lower quartz concentration (Hu and Yang., 2016).

Fig. 5. Chondrite-normalized REE patterns of four grain-size fractions and the average composition of each fraction for typical loess/paleosol samples from the Dongwan (DW) section in the Junggar Basin. Chondrite data are collected from Taylor and McLennan (1985).

4.4 Element ratios of the major, trace and rare earth elements There is a substantial grain-size dependency of the inter-element ratios of the trace and rare earth elements of the DW loess deposits (Fig. 6a,b). Compared to the

Journal Pre-proof 20-75 μm and >75 μm fractions, the <5 μm and 5-20 μm fractions have lower Zr/Hf and Y/Nb ratios, and higher Ce/Yb ratios. The Zr/Hf ratios of the <5 μm and 5-20 μm fractions generally range from 36-38 and from 38-40, respectively (Fig. 6a). By contrast, in the 20-75 μm and >75 μm fractions, the Zr/Hf ratios range from 39-43 and from 38-42, respectively (Fig. 6a). A similar pattern of variation is observed in the Y/Nb and Eu/Yb ratios, which respectively exhibit minimum values in the <5 μm and 5-20 μm fractions, and maximum values in the 20-75 μm and >75 μm fractions. Unlike the Zr/Hf and Y/Nb ratios, the Ce/Yb ratios generally exhibit maximum values

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in the <5 μm and 5-20 μm fractions and minimum values in the 20-75 μm and >75 μm fractions, with respective ranges of 20-25 and 14-22 (Fig. 6b). In addition, the

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element ratios of the trace and rare earth elements typically have a larger variability in the 20-75 μm and >75 μm fractions compared with the <5 μm and 5-20 μm fractions.

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The inter-element ratios of the clay fraction (<5 μm) are especially uniform and they

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are tightly clustered in the scatter plots (Fig. 6a,b).

Fig. 6. Scatter plots of Zr/Hf vs. Y/Nb, and Eu/Yb vs. Ce/Yb for different grain-size fractions of typical loess/paleosol samples from the Dongwan (DW) section in the Junggar Basin.

There are notable differences between the element ratios of TiO 2 /Al2 O3 and K2O/Al2 O3 between the grain-size- fractionated samples of the loess deposits of the

Journal Pre-proof Junggar Basin compared with those of the CLP (Fig. 7a-c). For each grain-size fraction, the DW samples have much higher TiO 2 /Al2 O 3 and K 2 O/Al2 O 3 ratios than the Baishui loess samples. Specifically, the TiO 2 /Al2 O3 ratios of the clay fraction (<5 μm) in the Baishui and DW sections range from 0.03-0.05 and 0.06-0.08, and the K2O/Al2 O3 ratios range from 0.16-0.20 and 0.21-0.23, respectively (Fig. 7a). With increasing grain size, the TiO 2 /Al2 O3 and K 2 O/Al2 O 3 ratios of the 20-63 μm fraction at Baishui increase to 0.06-0.07 and to 0.18-0.21, respectively. For the DW section, the TiO 2 /Al2 O3 and K 2 O/Al2 O3 ratios of the 20-75 μm fraction increase to 0.06-0.08

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and to ~0.22, respectively (Fig. 7c).

Figure 7d demonstrates that all of the grain-size- fractionated samples from the

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DW section have much higher TiO 2 /Al2 O3 and K2 O/Al2 O3 ratios than those of the

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bulk loess samples from Tajikistan (Li et al., 2016). For the bulk loess samples from Tajikistan, the TiO 2 /Al2 O3 and K 2 O/Al2 O3 ratios range from 0.05-0.06 and from

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0.16-0.20, respectively; whereas in the Junggar Basin, the TiO 2 /Al2 O 3 and K 2 O/Al2 O3 ratios of the four grain-size fractions of typical loess/paleosol samples generally range

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from 0.05-0.08 and 0.20-0.25, respectively. These observations suggest that the loess

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deposits from the Junggar Basin have the different provenance as the Tajikistan loess.

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The inter-element ratios of REE for the grain-size- fractionated samples of loess deposits from the Junggar Basin and Ili Basin also exhibit clear contrasts in geochemical composition (Fig. 7e, f). For example, the LaN/YbN, LaN/SmN and GdN/YbN ratios of corresponding grain-size fractions are generally much higher in the Zhaosu section (Ili Basin) than in the DW section (Fig. 7e, f).

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Fig. 7. (a-c) Scatter plots of TiO2 /Al2 O3 vs. K2 O/Al2 O3 for the Mid-Pleistocene loess deposits of the DW section in Junggar Basin and the Baishui section in the CLP, and (d) surface loess samples from Tajikistan. (e-f) Scatter plots of LaN /YbN vs. LaN /SmN, and GdN /YbN vs. LaN /SmN for the loess deposits from the DW section in Junggar Basin and the Zhaosu section in Ili Basin. The major element data of the loess deposits from the Baishui section and Tajikistan are from Xiong et

Journal Pre-proof al. (2010) and Li et al. (2016), respectively. The REE data of late Pleistocene loess deposits from the Zhaosu section are from Jia et al. (2014). The base of the red clay-loess sequences in the Baishui section is about 6.2 Myr old (Xiong et al., 2010). In the present study, the major element data of loess deposits since ~0.8 Ma from the Baishui section were collected for further comparison.

4.5 Temporal variations in the geochemical composition of the DW loess The CaO/Al2 O3 and MgO/Al2 O3 ratios of the fine fractions (<5 μm and 5-20 μm)

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of loess samples from the DW section show little variability since 0.8 Ma (Fig. 8). By contrast, a substantial shift in CaO/Al2 O3 and MgO/Al2 O3 ratios is evident in the

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coarse fractions (20-75 μm and >75 μm) at around 0.6-0.5 Ma. At ~0.5 Ma the CaO/Al2 O3 ratios of the 20-75 μm and >75 μm fractions increase from 0.12 to 0.16

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and from 0.08 to 0.12, respectively; whereas the MgO/Al2 O3 ratios decrease from

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0.15 to 0.12 and from 0.17 to 0.12, respectively.

Fig. 8. Temporal variations in the CaO/Al2 O3 and MgO/Al2 O3 ratios of different grain-size fractions of Middle Pleistocene loess deposits from the DW section in the Junggar Basin. The chronology for the loess samples in the DW section is based on a sedimentation rate of ~8.9

Journal Pre-proof cm/kyr in the Brunhes chron (Fang et al., 2002).

For the grain-size- fractionated samples from the DW section, the long-term variations of the Zr/Nb and Zr/Al ratios have similar patterns to the major element ratios of CaO/Al2 O3 and MgO/Al2 O3 (Fig. 9); that is, the Zr/Nb and Zr/Al ratios in the fine fractions (<5 μm and 5-20 μm) vary little since 0.8 Ma, but the values of the

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20-75 μm and >75 μm fraction increase abruptly at around 0.6-0.5 Ma.

Fig. 9. Temporal variations of the Zr/Nb and Zr/Al ratios of different grain-size fractions of Middle Pleistocene loess deposits from the DW section in the Junggar Basin.

5. Discussion

Journal Pre-proof 5.1 Grain-size-dependent geochemical characteristics of the DW loess Our results demonstrate that the major, trace and rare earth element concentrations of the DW loess are grain-size-dependent, with higher concentrations in the fine fractions (<5 μm and 5-20 μm) (Figs. 3 and 4). These characteristics are consistent with previous geochemical investigations of Quaternary loess deposits of the CLP, which were attributed to the varying content of clays and heavy minerals in different grain-size fractions of loess and paleosol samples (Yang et al., 2007). It has

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been suggested that in natural environments most major and trace elements are

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usually enriched in sediments with higher contents of clay minerals, because of their adsorption to clay particles (Liang et al., 2009). However, the concentrations of the

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trace element Sr, and of major elements including Ca and Na, are relatively low in the

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fine fractions (Figs. 3 and 4), which may be the result of the strong chemical weathering of the clay component which results in the loss of the trace and major

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elements (Cox et al., 1995).

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REE patterns are important indices for identifying the provenance of eolian deposits, because these elements are immobile and insoluble during transport and

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weathering (Taylor and McLennan, 1985, 1995; Rollinson, 2014). The REE

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distribution patterns of the fine fractions (<5 μm and 5-20 μm) of the DW deposits are very similar (Fig. 5). These characteristics are quite different from those of the coarse fractions (20-75 μm and >75 μm), in which the REE distribution patterns varies substantially. These observations suggest that, compared to the 20-75 μm and >75 μm fractions, the <5 μm and 5-20 μm fractions are more homogeneous in their geochemical composition. Previous analysis of dust transport dynamics indicated that dust particles coarser than 20 μm were usually transported in short-term suspension at low levels in the atmosphere (Tsoar and Pye, 1987). By contrast, fine particles, with diameters <20 μm, can be transported in long-term suspension over a larger vertical range and for long distances. These findings support the view that the fine components of the DW loess were extremely well mixed and sorted prior to deposition, in the high wind-energy environment of the Junggar Basin. The

Journal Pre-proof homogeneous major and trace element composition of the clay fraction (<5 μm) of the DW loess provides further evidence for this inference (Figs. 3, 4 and 6). Similar geochemical characteristics are also observed in the eolian sand samples from the Taklamakan Desert (Yang et al., 2007). This research demonstrates that the coarse sand component of eolian deposits in various locations of the Taklamakan Desert exhibits contrasting REE characteristics and major element composition (Yang et al., 2007). These observations suggest that coarse particles are not readily

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wind-transported for long distances, which reduces the potential for thorough mixing.

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By contrast, the geochemical composition of the fine fractions (mainly silts) are relatively homogeneous along the predominant wind direction, indicating a high

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degree of mixing.

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To date, most previous geochemical studies of sediments from potential source

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regions of Chinese eolian deposits have focused primarily on the <20 μm fraction of loess deposits, and the grain-size-dependent geochemical characteristics have not

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been studied systematically. Our new results from the Junggar Basin provide a useful basis for comparing the geochemical characteristics of multiple grain-size fractions

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between the loess deposits of the CLP and Central Asia.

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5.2 Regional contrasts in the geochemical characteristics of loess deposits from the Junggar Basin, Tajikistan, the Ili Basin and the CLP Early studies generally argued that the Junggar Basin, the Tarim Basin and the Qaidam Basin were the important source areas of Quaternary loess deposits in the CLP, given that the three inland basins are located upwind of the CLP (Liu, 1985). However, Sun (2002) investigated the geochemical and mineralogical composition of the <20 μm fractions of upper Pleistocene loess deposits from the Junggar Basin, Tarim Basin and Qaidam Basin and challenged the conventional view. Combined with the analysis of dust transport paths and modern meteorological data, they concluded that the loess deposits of the CLP were not sourced from the three northwestern inland basins, but instead were mainly derived from the Gobi Desert in southern Mongolia

Journal Pre-proof and the adjoining arid/semi-arid areas in north China (Sun, 2002). In addition, Maher et al. (2009) emphasized that the source areas of the loess in the CLP might encompass a much larger area than any one proximal desert region via a potential “dogleg” transport path for dust from the arid basins in northwestern China. Recently, detrital zircon analyses have provided new insights into the provenance of loess deposits in the CLP, supporting the view that the eolian materials may come from vast areas of the northern edge of the Tibetan Plateau and the Central Asian Orogenic Belt, including the Qilian Mountains, the Gobi Altai and Hangay Mountains, and the

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sediments of the Yellow River (Che and Li, 2013; Nie et al., 2015; Stevens et al., 2013; Sun et al., 2018). It should be noted that most of the previous studies focused the comparison of the physicochemical properties of

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primarily on

late

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Pliocene-Pleistocene loess deposits from the CLP and those of modern eolian materials in the potential source areas of Asian dust. However, the problem of the age

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inconsistency of the sediments analyzed may lead to incomplete and imprecise information regarding the provenance of Chinese eolian deposits. In the present study,

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detailed comparison of grain-size-dependent geochemical characteristics of Middle

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Pleistocene loess deposits between the CLP and the Junggar Basin demonstrates that the grain-size fractionated samples from the DW section usually have higher major

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element ratios (e.g. TiO 2 /Al2 O3 and K 2 O/Al2 O3 ) than those from the Baishui section in the CLP (Fig. 7a-c). These observations support the view that the Junggar Basin is not a potential source region of the loess in the CLP, at least since the Middle Pleistocene. This inference is further supported by the topographical and meteorological characteristics of the Junggar Basin, where the windblown dusts could hardly be transported out of the basin and to the CLP due to the physical barrier of the surrounding mountains (Sun, 2002). Our results also demonstrate that, compared to the bulk or fractionated samples of surface loess deposits in the Ili Basin and Tajikistan, the DW samples are characterized by higher major element ratios (TiO 2 /Al2 O3 , K2 O/Al2 O3 ) and REE ratios (GdN/YbN, LaN/SmN) (Fig. 7d-f). These characteristics indicate that the loess deposits

Journal Pre-proof from the Junggar Basin have the different provenance as those of Tajikistan and Ili Basin. We argue that the high mountains surrounding the sedimentary basins on the western and northern margins of the Junggar Basin (Fig. 1) act as barriers for dust transport. This may explain why the loess deposits from the DW section exhibit local or regional geochemical characteristics. The Gurbantunggut Desert in the Junggar Basin serves as a holding area for dust and silt (Sun, 2002), and loess-sized silt production is closely related to high altitude weathering processes. Thus, the clastic loess-sized materials in the Junggar Basin are probably derived mainly from the

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northern Tianshan and Altai Mountains, although more detailed research is needed to confirm this.

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Recently, trace-element characteristics and analysis of atmospheric dynamics

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demonstrate that eolian dusts entrained from several large deserts in the Asian interior (e.g. Karakum) have a minor contribution to the Tajikistan loess (Li et al., 2019). In

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addition, Li et al. (2018) revealed that loess deposits in the Ili Basin exhibited local geochemical characteristics, with only a small proportion of the eolian materials from

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the distal Central Asian deserts. All these observations suggest that the contribution of

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eolian dusts from several large deserts in the Asian interior to the Central Asian loess

2017).

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deposits was overestimated previously (Song et al., 2014; Li et al., 2016; Chen et al.,

5.3 Temporal changes in the geochemical properties of different grain-size fractions of Middle Pleistocene loess deposits in the Junggar Basin Under natural conditions, the major elements Ti and Al and the trace elements Zr and Nb remain unchanged during post-depositional chemical weathering (McLennan et al., 1993; Taylor and McLennan, 1985). Thus, the respective inter-element ratios can be used to identify provenance changes of eolian deposits. For the grain-size- fractionated samples from the DW section, the long-term variations of the Zr/Nb and Zr/Al ratios exhibit similar patterns to those of the major element ratios of CaO/Al2 O3 and MgO/Al2 O3 (Figs. 8, 9); that is, the ratios of the 20-75 μm and >75 μm fractions change abruptly at around 0.6-0.5 Ma, whereas the ratios of the fine

Journal Pre-proof fractions (<5 μm and 5-20 μm) exhibit little variability since ~0.8 Ma. The homogeneity of the major and trace element composition of the <5 μm and 5-20 μm fractions of the DW loess is consistent with the fact that the fine-grained components of eolian dusts are usually well mixed and sorted prior to deposition (Sun, 2002). However, by contrast, the coarse loess-sized materials are usually poorly mixed under natural conditions, due to their relatively large volume and weight, and consequently, the geochemical characteristics of the coarse loess fractions are more

f

susceptible to the influence of the influx of weakly-weathered detritus, which is

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closely related to changes in atmospheric circulation patterns, or to the occurrence of erosional and tectonic processes in the surrounding mountains. Major and trace

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element compositions of the coarse sand component of eolian deposits across the

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Taklamakan Desert are highly variable (Yang et al., 2007), providing further support for this inference. Thus, changes in the geochemical composition of the 20-75 μm

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and >75 μm fractions of the DW loess may indicate the increasing supply of fresh, unweathered clastic sediments in areas that were not previously loess source regions.

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We hypothesize that the addition of fresh Ca-silicate minerals may account for the

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increase of CaO/Al2 O3 ratio at about 0.5 Ma. The MgO/Al2 O3 ratio mainly reflects the

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change of chlorite, and its decrease at ~0.5 Ma can be attributed to the dilution effect of the continuous addition of other minerals. Howe ver, it is important to note that intensified weathering and pedogenesis under warm and humid environments may also exert an influence on the physicochemical properties of the fine fractions of the loess deposits. However, paleosols are very weakly developed in the Junggar Basin due to low precipitation (Zan et al., 2012). Moreover, hysteresis, thermomagnetic and low-temperature magnetic experiments on particle-size fractioned samples from the DW loess section demonstrate that the concentration of pedogenically produced superparamagnetic (SP) magnetic particles in the clay fraction (<4 μm) is very low (Zan et al., 2015, 2018), supporting that the degree of pedogenesis is very weak. Thus, all these observations suggest that the homogeneous major and trace element composition of the clay fraction of the DW loess are not induced by intensified

Journal Pre-proof weathering and pedogenic processes. A large body of evidence suggests that enhanced aridification and/or increased wind speeds may have caused the grain size increase of the loess deposits in the Junggar Basin and Tarim Basin at ~0.5 Ma (Fang et al., 2002; Zan et al., 2010). In addition, a recent study found that the lithogenic magnetic susceptibility of Chinese eolian deposits increased rapidly at ~0.6-0.5 Ma, as a result of the accelerated denudation of the NE Tibetan Plateau (Zan et al., 2019). Furthermore, the major

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element ratios of Na2 O/Al2 O 3 and MgO/Al2 O3 of the eolian deposits in the central

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CLP increased substantially after 0.6 Ma (Xiong et al., 2010), possibly because of the increasing input of fresh, unweathered silicates due to intensified tectonic activity and

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erosional processes in the northern Tibetan area.

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After ~0.6-0.5 Ma, intensified physical erosion and/or increased wind speeds in

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the vast area of the northern Tianshan (Fang et al., 2002) are closely related to Quaternary global cooling or/and tectonic uplift (Xiong et al., 2010). Field

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investigations and seismo-stratigraphic analyses of thrust–fold belts along the margins of several late Cenozoic intermontane basins in the NE Tibetan Plateau revealed that

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the growth strata and unconformities were developed around 0.6-0.5 Ma (Li et al.,

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2014), indicating a tectonically active setting since the middle Pleistocene. In addition, alkaline-enriched volcanic rocks are widely distributed along the West Kunlun range with the interval of 0.67- 0.44 Ma (Liu and Maimaiti, 1989). The intensified physical erosion in northern Tibetan Plateau would have caused an increase in the input of fresh detritus to the loess deposits of the Junggar Basin. These coarse clastic sediments were mostly derived from new source areas that had not previously served as loess source regions in the Junggar Basin; moreover, the freshly-eroded bedrock may not previously have been a source of loess material. Thus, the increased input of such materials may account for the substantial changes in the geochemical composition of the 20-75 μm and >75 μm fractions of the DW loess at ~0.6-0.5 Ma.

Journal Pre-proof 6. Conclusions Our results demonstrate that the major, trace and rare earth element concentrations of Middle Pleistocene loess deposits from the Junggar Basin are grain-size-dependent, with higher concentrations in the fine fractions (<5 and 5-20 μm). For the <5 and 5-20 μm fractions of the DW loess, long-term variations in the element ratios show little variability since ~0.8 Ma; by contrast, the inter-element ratios of the 20-75 and >75 μm fractions increased abruptly at around 0.6-0.5 Ma. The

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homogeneity of the element composition of the fine fractions suggest that the fine

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components of the DW loess were extremely well mixed and sorted prior to deposition. Increasing input of fresh clastic sediments may account for the significant

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changes in the geochemical composition of the 20-75 and >75 μm fractions of the

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DW loess at ~0.5 Ma. The disparity between the grain-size-dependent geochemical characteristics of contemporaneous loess deposits between the CLP and the Junggar

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Basin demonstrates that the latter is not a potential source region of the loess in the CLP, at least since the Middle Pleistocene. In addition, regional contrasts of the

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geochemical characteristics of loess deposits between the Junggar Basin, Tajikistan

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and Ili Basin indicate that the clastic loess-sized materials in the Junggar Basin are

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derived from the northern Tianshan and Altai Mountains. Acknowledgments

This work was co-supported by the Second Tibetan Plateau Scientific Expedition (STEP) program, the Youth Innovation Promotion Association, CAS, China (grant 2016071), the National Natural Science Foundation of China (grants 41877456, 41571198, and 41620104002), and the Strategic Priority Research Program of Chinese Academy of Sciences (grant XDA20070201). We thank Dr. Shi Zhengtao for his field assistance. Data Availability Datasets

related

to

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https://data.mendeley.com/datasets/jg7j4fjh5f/draft?a=546389ba-586c-4987-912e-233833a52b2 8

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Zhang, J., Li, J.J., Guo, B.H., Ma, Z.H., Li, X.M., Ye, X.Y., Y, H., Liu, J., Yang, C., Zhang, S.D., Song, C.H., Hui, Z.C., Peng,T.J., 2016. Magnetostratigraphic age

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Journal Pre-proof Declaration of interests

The authors declare that they have no known competing financial interests or personal

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relationships that could have appeared to influence the work reported in this paper.