Size-differentiated REE characteristics and environmental significance of aeolian sediments in the Ili Basin of Xinjiang, NW China

Journal of Asian Earth Sciences 143 (2017) 30–38

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Size-differentiated REE characteristics and environmental significance of aeolian sediments in the Ili Basin of Xinjiang, NW China

MARK

⁎

Xiuling Chena,c, Yougui Songb, , Jinchan Lia,c, Hong Fanga, Zhizhong Lia,c, Xiuming Liua,c, Yue Lib, Rustam Orozbaevd a

Institute of Geography, Fujian Normal University, Fuzhou 350007, China State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China c State Key Laboratory of Subtropical Mountain Ecology (Funded by Ministry of Science and Technology and Fujian Province), College of Geographical Sciences, Fujian Normal University, Fuzhou 350007, China d Research Center for Ecology and Environment of Central Asia (at Bishkek), Chinese Academy of Sciences, Bishkek 720040, Kyrgyzstan b

A R T I C L E I N F O

A B S T R A C T

Keywords: Loess REE Provenance Size fraction The Ili Basin

Aeolian loess in the Ili Basin is an important geological archive for studying the changes in paleoclimate and sources of dust particles. Size-differentiated rare earth elements (REE) may help to distinguish potential dust sources. This study investigates the size-differentiated REE characteristics from three sites including the Zhaosu loess and the Kekdala desert sediments from the Ili Basin, and the Chaona loess from the Chinese Loess Plateau (CLP). Our results show that the patterns of variation of the REE characteristics in different size fractions can act as improved source tracers for aeolian sediments. Moreover, the REE characteristics of the < 2 μm particles are sensitive indicators for distinguishing dust particles transported over long distances in the semi-arid areas with limited pedogenesis such as the Ili Basin. However, it should be interpreted cautiously in the CLP due to the postdepositional chemical weathering. The REE characteristics of coarse fractions are effective tracers for tracking changes in proximal dust sources and regional boundary level circulations. Our study has implications for identifying the exact source(s) of the Ili loess, which is helpful to understand paleoclimate changes and westerly circulation patterns in Central Asia.

1. Introduction The unique location of Central Asia between the well-studied European loess sequences to the west and the extensive CLP region to the east enables researchers to carry out interregional paleoclimatic investigations along a west-east transect across the entire Eurasian loess belt of the Northern Hemisphere (Machalett et al., 2008; Song et al., 2010). Therefore, Central Asian loess contains valuable archives about the aridification of inland Asia, Northern Hemisphere dust sources, past atmospheric circulation, and the evolution of climate change (Dodonov, 1991; Ding et al., 2002; Fang et al., 2002; Machalett et al., 2008; Song et al., 2010, 2014a; Li et al., 2016a, 2016b). The Ili Basin in the Xinjiang Province in northwest China is located in the Central Asian hinterland (Fig. 1). A large number of loess studies have been carried out in the region in recent years (Ye et al., 2000; Shi, 2002, 2005; Jia et al., 2010; Song et al., 2010, 2012, 2014a, 2015; E et al., 2012; Yang et al., 2014). The results of those studies show that the loess deposits in the basin are mainly distributed on river terraces, low uplands, the

piedmonts, and desert margins (Song et al., 2014a). Geochronological studies suggest that although there are some disparities in the precise age of the deposits determined using different dating methods, most loess sections exposed in the basin have developed since the last interglacial period (Ye et al., 2000; Shi, 2005; Song et al., 2010, 2012, 2014a; E et al., 2012; Yang et al., 2014; Li et al., 2016a, 2016b). However, the paleoclimatic significance of climatic proxies of the loess deposits such as the magnetic properties, grain size and geochemical characteristics remains ambiguous (Ye et al., 2000; Shi et al., 2007; Song et al., 2010, 2014a; Xia et al., 2010; Zhang et al., 2013), which make it difficult to reconstruct the climate history accurately. For example, the relationship between magnetic susceptibility and pedogenic intensity is much more complicated in the Ili Basin than that in CLP (Ye, 2000; Shi et al., 2007; Jia et al., 2010; Song et al., 2010, 2014a; Li and Song, 2011; Chen et al., 2012). The susceptibility enhancement mechanism in this area contains both the Alaska wind model and the pedogenesis model (Song et al., 2010). Therefore, multiple influence factors including source mineral components should

⁎ Corresponding author at: State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, No. 97, Yanxiang Road, Xi'an 710061, China. E-mail addresses: [email protected], [email protected] (Y. Song).

http://dx.doi.org/10.1016/j.jseaes.2017.03.030 Received 19 October 2016; Received in revised form 14 January 2017; Accepted 25 March 2017 Available online 27 March 2017 1367-9120/ © 2017 Elsevier Ltd. All rights reserved.

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Fig. 1. (a) Locations of the study sections. (b) The Ili Basin in the Tianshan Mountains (modified from Song et al., 2010).

loess (Ye, 2000, 2001; Sun, 2002a, 2004; Li et al., 2011, 2012). Some studies argue that the dust is transported from the west of the Ili Basin (Ye, 2000, 2001; Sun, 2002a, 2004; Jia et al., 2010; Li et al., 2011, 2012, 2015). However, this result is mainly inferred from geomorphological information, air circulation systems, and in part from grain size and mineralogical properties, lacking demonstration of more reliable proxies. A large number of effective diagnostic methods of dust provenance have been developed in recent years in CLP, such as Hf, Nd, Sr, and Pb isotopes in bulk and grain size fractions (Sun, 2002b; Sun and Zhu, 2010; Újvári et al., 2015), zircon U-Pb, and heavy minerals (Bird et al., 2015; Nie et al., 2013, 2015). However, those methods are expensive and time consuming compared with the rare earth element (REE) analysis. REE analysis has also been widely applied to provenance studies because of the chemical inertness of REEs (Henderson, 1984; Taylor and McLennan, 1985; Wang et al., 1989; Zhang, 1996; Yang et al., 2007; Hao et al., 2010; Ferrat et al., 2011). However, due to

be taken into account before paleoclimate reconstructions (Song et al., 2010, 2014a; Jia et al., 2010; Li and Song, 2011). Regarding grain size, some researchers argued that the clay fraction (< 2 μm) in the Ili loess might indicate the intensity of the westerlies (Ye, 2001; Li et al., 2011; E et al., 2014; Li et al., 2015, 2016a, 2016b), whereas others suggested that it might indicate the change in precipitation patterns (Shi, 2002). Mineral and geochemical characteristics were inconsistent even in different layers of the same section (Zeng and Song, 2013), suggesting that caution should be exercised in the interpretation of their paleoenvironmental significance. The physicochemical properties of aeolian sediments are closely related to the origin of mineral dust; therefore, source tracing of deposits in the Ili Basin is helpful to understand the climate implications of different proxies, and to provide crucial insights into the evolution of atmospheric circulation and the aridification history of inland Asia. Only a few studies have focused on the source of the Ili 31

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characteristics (Fig. 1).

the physical sorting and chemical weathering inherent in transportation and deposition, the mineralogical and elemental composition of sediments varies with particle size (Chang et al., 2000; Yang et al., 2007; Xiong et al., 2010). Therefore, bulk REE content and other related parameters can be significantly different for different grain size fractions, which make them insensitive source indicators, especially for multiple sources. Size-independent studies can eliminate the effects of grain size; thus, size-differentiated REE characteristics are more reliable to distinguish potential dust sources (Cullers et al., 1987; Yang et al., 2004; Chen and Li, 2011). The aim of this work is to provide reliable REE-based source tracers by optimizing the detection of compositional differences among different types of aeolian deposits from the adjacent region (sand deposits and loess sediment from the Ili Basin) and the same type of aeolian deposits with obviously distinct sources (loess deposits from the Ili Basin and CLP).

3. Methods ZSP and CN samples were separated into five size fractions (< 2 μm, 2–16 μm, 16–32 μm, 32–63 μm, and > 63 μm), and TKP samples were separated into six fractions with the > 63 μm fraction further divided into two parts at the boundary of 125 μm. The coarse fractions > 32 μm were extracted by sieving, and fine fractions < 32 μm were extracted using a standard settling method based on Stoke’s Law (Lu, 2000). Different size fractions were dissolved in Teflon bombs. After adding mixed acid (MOS HCl, HNO3 and HF), the samples in the Teflon bomb were sealed and heated in Microwave Digestion System. HClO4 was added when the samples reached room temperature and the mixture was dried at 120 °C. The resultant salt was re-dissolved in HNO3 and high purity water and then diluted to ∼40 ml for analysis. REE concentrations were determined using an ICP-MS (X Series 2, ThermoFisher, USA). Replicate analyses of samples showed relative standard deviations (RSDs) of REEs to be < %, and internal standard Rh and Re (5 μg L−1) values exhibited recovery rates of 90–97%. The detection limits of the 14 elements were found to be < 0.0084 μg L−1 (n = 12), with correlation coefficients of 0.9998–0.9999 of the standard curve. A standard loess sample (GBW 07454) and parallel samples for quality control were simultaneously analyzed and the deviations were less than 10%. Experiments on the separated particle size fractions were performed at the State Key Laboratory of Loess and Quaternary Geology, Chinese Academy of Sciences. REEs were measured at the Key Laboratory for Subtropical Mountain Ecology (Ministry of Science and Technology and Fujian Province Funded), Fujian Normal University. Among the related parameters, ΣREE corresponds to the total REE concentration (excluding Y) and LREE/HREE is the ratio of light REE concentration (from La to Eu) to heavy REE concentration (from Gd to Lu), showing the degree of REE fractionation. The Eu anomaly (δEu) is calculated as δEu = EuN / Eu∗ = EuN / SmN ∗ GdN , where N represents chondrite-normalized REE (C1 chondrite: Zhang, 1997). The C1 chondrite and Upper Continental Crust (UCC: Taylor and McLennan, 1985) composition used for the normalization are shown in Table 1.

2. Geographic background and sampling The Ili Basin is semi-surrounded by the Tianshan orogenic belt. The topography of the basin is characterized by an opening to the west occupied by the Central Asian Gobi desert, and higher mountains of Tianshan to the north, east, and south (Fig. 1). The basin has a temperate semiarid continental climate and upper level westerlies prevail at high altitudes throughout the year. Moisture from the Atlantic Ocean or the Mediterranean Sea can be carried by the westerly airflow and reach the basin to form precipitation. The winter climate is controlled mostly by the Mongolian high-pressure system and is influenced predominantly by the northern branch of the main westerly airstream. The summer climate is mainly affected by the Indian lowpressure system and the southern branch of the westerly airstream that shifts northward towards the basin. Strong surface winds occur frequently from April to July and the rainfall primarily occurs in the spring and summer (Ye, 2001; Song et al., 2010, 2014a). The topography has a great influence on the mean annual temperature (MAT) and mean annual precipitation (MAP). The MAT varies from 2.6 °C to 10.4 °C depending on the local topography. The MAP in the plains ranges from 200 mm to 500 mm, with a general trend of higher MAP in the eastern part (Ye, 2001; Dai et al., 2007; Song et al., 2010, 2014a). Loess sediments are widely distributed in the Ili Basin. They are mostly found in river terraces, piedmonts, and desert margins. Considering the dust transportation pathways and geomorphological locations, we selected two sections in the Ili Basin for the REE analysis. The first is the Zhaosu Section (ZSP) (80.25°E, 42.69°N, 1875 m asl) near the China-Kazakhstan border, a loess section typical of the Ili Basin (Song et al., 2010, 2012; Li et al., 2011, 2012; Li and Song, 2011). The second is the Kekdala deposit (TKP) (80.545°E, 43.97°N, 605 m asl) in the Takelmukul Desert, a representative sand deposit in the western Ili Basin (Fig. 1) (Chen et al., 2010, 2013; Li et al., 2010). Five loess samples and one paleosol sample were collected from the ZSP section; and three sand samples and two paleosol samples were collected from TKP. In order to compare these with the loess deposits that are obviously distinct in source, we also selected five loess samples and two paleosol samples from the Chaona section (CN) (107.2°E, 35.1°N, 1495 m asl), a representative loess deposit in the central CLP (Song et al., 2000, 2014b; Chen et al., 2007; Wu et al., 2007; Nie et al., 2013; Wang et al., 2016) to carry out the comparative study of REE

4. Results 4.1. REE concentrations in different size fractions Total REE concentrations excluding Y (ΣREE) in different size fractions are presented in Table S1 and show significantly different characteristics (Table 2 and Fig. 2). The ΣREE values of CN samples are lower in 16–32 μm and 32–63 μm fractions, but higher in fine (< 16 μm) and coarse (> 63 μm) fractions (Table 2). In the 16–32 μm fraction, ΣREE values vary between 116.52 and 156.23 μg g−1, with an average value of 137.45 μg g−1. ΣREE values are the lowest in the 32–63 μm fraction, averaging 113.71 μg g−1. ΣREE values in the 2–16 μm, < 2 μm, and > 63 μm fractions are higher than those of the 16–32 μm and 32–63 μm fractions. ΣREE values and inter-sample variability reach a peak for all samples in the < 2 μm fraction, averaging 220.58 μg g−1, with a coefficient of variation of 13.66%. Paleosol samples (CN50 and CN1000, see Table S1) contribute the most to this substantial fluctuation (Fig. 2a).

Table 1 C1 chondrite and Upper Continental Crust composition used for the normalization.a

C1 UCC a

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

0.312 30

0.813 64

0.123 7.1

0.603 26

0.197 4.5

0.074 0.88

0.260 3.8

0.047 0.64

0.323 3.5

0.072 0.8

0.211 2.3

0.033 0.33

0.210 2.2

0.032 0.32

C1 chondrite: Zhang (1997); UCC: Taylor and McLennan (1985).

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Table 2 Changes of REE concentrations in different particle size fractions of CN, ZSP, and TKP samples. Size (μm)

<2

2–16

16–32

32–63

> 63 63–125

CN

ΣREE (μg g−1) AVG (μg g−1) CV (%)

191.7–274.3 220.6 13.7

182.4–214.6 203.7 5.0

116.5–156.2 137.5 10.3

83.4–131.0 113.7 14.8

131.2–200.8 162.1 16.7

ZSP

ΣREE (μg g−1) AVG (μg g−1) CV (%)

147.5–191.5 172.8 8.6

165.8–200.3 181.3 6.7

135.2–167.7 149.4 8.5

122.9–153.4 134.4 8.8

65.7–148.4 108.7 27.8

TKP

ΣREE (μg g−1) AVG (μg g−1) CV (%)

176.7–203.3 189.1 5.7

179.9–213.6 198.3 6.3

135.6–190.9 168.3 13.8

116.7–156.1 140.0 10.3

107.2–147.0 124.6 15.3

ΣREE values for ZSP and TKP samples are generally lower in coarse fractions compared to fine fractions, clearly different from the ΣREE values for the CN samples (Table 2, Fig. 2b and c). The variability of ΣREE values in different ZSP samples for the < 2 μm, 2–16 μm, 16–32 μm, and 32–63 μm fractions are not obvious, but increase sharply in the > 63 μm fraction. These characteristics are similar to the < 2 μm, 2–16 μm, and 32–63 μm fractions of the TKP samples (see Table S1). The obvious difference between ZSP and TKP samples is that large fluctuations exist not only in TKP coarse particle fractions, but also in 16–32 μm fractions.

REE concentrations normalized to chondrite are shown in Fig. 3. The chondrite-normalized REE distribution patterns for different size fractions from these three sections are uniform in shape, similar to those of Chinese loess (Guo et al., 2013; Chen et al., 2013). They present a type of fastigiate “L”, enriched in light REEs (LREE), depleted of heavy REEs (HREE), and with hardly any Ce anomaly and a notably negative Eu anomaly. The similarities in REE patterns between the CLP loess, Ili Basin loess, and desert sand sediment imply that the aeolian sediments deposited in these two areas have experienced well mixing and multiple recycling processes (Gallet et al., 1996; Jahn et al., 2001). However, it is insufficient to distinguish potential sources using identical characteristics like these, while the Upper Continental Crustnormalized (UCC: Taylor and McLennan, 1985) patterns can reveal remarkable differences among them (Fig. 4). The < 2 μm fractions of CN, ZSP, and TKP samples are characterized by similar patterns, showing flat “W-type“ convex distributions, with enrichments relative to UCC and apparent enrichments of middle REE (MREE) (Fig. 4a). The ZSP and TKP samples show a similar variability and similar ΣREE values, while the CN samples have higher

a

300

CN50 CN250 CN400 CN550 CN750 CN850 CN1000

250 200

4.3. REE parameters in different size fractions Among REE parameters, the δEu (chondrite-normalized) value is generally determined by original rock minerals. Different protoliths lead to a distinct Eu anomaly because of the different diagenetic processes (Taylor and McLennan, 1985; Wang et al., 1989). The δEu values in different fractions (see Table S2), as well as REE concentrations, show different variabilities between the CN samples of CLP and the ZSP and TKP samples of the Ili Basin (Fig. 5). δEu values for CN samples

b

ZSP120 ZSP220 ZSP320 ZSP420 ZSP540 ZSP620

250 200

300

150

100

100

100

50 2-16

16-32

32-63

>63

Size fraction/ m

50 <2

2-16

16-32

32-63

>63

Size fraction/ m (

TKP40 TKP80 TKP100 TKP140 TKP320

200

150

<2

c

250

150

50

73.1–107.8 86.2 15.9

concentrations and a greater variability. Uniform UCC-normalized REE patterns are also observed in the 2–16 μm fraction of the CN and ZSP samples. ΣREE values and fluctuations in CN samples decrease to the levels found in ZSP samples. However, patterns of TKP samples for this fraction do not follow this trend, presenting a positive slope from LREE to HREE (Fig. 4b). In the 16–32 μm fraction, the UCC-normalized patterns of TKP samples are almost the same as those in the 2–16 μm fraction. The ZSP 16–32 μm fraction also displays flat “W-type“ patterns as those in the aforementioned two fractions. Besides, they show an apparent decrease in the total REE budget and a slight enrichment of HREE relative to LREE. The CN samples exhibit clear differences in this fraction, with further decreases in ΣREE concentrations and an apparently negative slope from LREE to HREE (Fig. 4c). TKP and ZSP 32–63 μm fractions exhibit similar patterns to those of the 16–32 μm fractions, while CN samples show obvious differences (Fig. 4d). Compared to the 16–32 μm fraction, ΣREE values for the CN 32–63 μm fraction decrease and UCC-normalized patterns display a positive LREE/MREE slope and a negative MREE/HREE slope. However, the ΣREE values of the CN samples increase suddenly in the > 63 μm fraction, with a return to flat “W-type“ convex patterns, which are remarkably different from the 32–63 μm fraction (Fig. 4e).

4.2. REE patterns in different size fractions

300

> 125

loess(sand) sample

<2

2-16

16-32

32-63

63-125

>125

Size fraction/ m

paleosol sample)

Fig. 2. Variations in REE concentrations in different particle size fractions of representative samples from (a) the Chaona loess section (CN) in the Chinese Loess Plateau (CLP) and (b) the Zhaosu loess section (ZSP) and (c) the Kekdala sand deposit (TKP) in the Ili Basin.

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Fig. 3. Chondrite-normalized REE distribution patterns in different size fractions for the CN, ZSP and TKP sections. Diagrams (a)–(e) indicate the < 2 μm, 2–16 μm, 16–32 μm, 32–63 μm, and > 63 μm fractions respectively.

decrease from < 2 μm to 2–16 μm fractions, increase from 2–16 μm to 32–62 μm fractions, and then decrease again in > 63 μm fractions (Fig. 5a). δEu values for ZSP and TKP samples show an overall increase with coarser fractions (Fig. 5b and c), especially for TKP samples. The variations in different particle sizes are also observed in LREE/HREE values. As shown in Fig. 6, LREE/HREE values for CN samples first increase and then decrease as the particle size increases, inversing the

trend of ZSP and TKP samples. The LREE/HREE values of these three sections are roughly similar to < 2 μm fractions, but different from the rest of the fractions.

5. Discussion REE contents of sediments are controlled by rock types of the source

Fig. 4. UCC-normalized REE distribution patterns in different size fractions for the CN, ZSP and TKP sections. Diagrams (a)–(e) indicate the < 2 μm, 2–16 μm, 16–32 μm, 32–63 μm, and > 63 μm fractions respectively.

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Fig. 5. δEu variations in different particle size fractions. (a) CN section (b) ZSP section (c) TKP section.

LREE/HREE

11

the climate is warm and humid. The strong weathering and leaching would render REEs in a hydroxyl or heteronuclear multiple complex ion form linked with Fe and Al, which are easily preserved in fine particles (Liu, 2000; Shi et al., 2003; Hao et al., 2010). Therefore, as shown in Figs. 2 and 4, the < 2 μm fraction of the CN samples presents higher concentrations and variability in ΣREE values (Fig.2) and greater amplification of the distribution patterns (Fig. 4a) relative to those of the ZSP and TKP samples, and paleosol samples make the major contribution. This suggests that the climate in CLP is warmer and wetter than that in the Ili Basin, and climate changes in different periods contribute to the variability of REE values. Thus, some caution is needed in interpreting the clay fractions as distal dust in CLP. For the Ili Basin, REE values show that there are hardly any Ce anomalies in the ZSP and TKP samples (Fig. 3b and c). Ce3+ is easily oxidized to Ce4+ in an oxidizing environment, and it preferentially shows positive Ce anomalies with increasing intensity of pedogenesis (Yang, 2000). Therefore, hardly any Ce anomalies reflect an arid climate leading to weak pedogenesis, and fine particles of the ZSP and TKP samples mainly record distant Central Asian dust sources. A considerable amount of 2–16 μm fractions of modern dust storms likely originate from proximal source areas (Qiang et al., 2010). These particles would vary greatly in different strata because of the frequency and intensity of dust storms. Therefore, similar REE concentrations and patterns in the CN and ZSP 2–16 μm fractions and the stability in different strata (Fig. 4b) suggest that the loess dust of this fraction, either in CLP or the Ili Basin, is mainly transported from a distant source. However, the apparently different patterns found in the TKP samples (Fig. 4b) and the low LREE/HREE values (Fig. 6) in the 2–16 μm fraction indicate that, for the desert sediment, there exist specific proximal sources containing mineral dust rich in HREEs (such as zircon and garnet) (Yang, 2000) and that the dust proportion from distant sources decreases. The analysis of major elements in the 2–16 μm size fraction of the TKP samples showed great variability through the profile, also indicating multiple material sources of this size fraction (Li et al., 2014). The intensity and pathway shifts in upper level westerlies and boundary level flows can cause the changes in the dust proportion from distant and proximal sources.

CN ZSP TKP

10 9 8 7 6

<2

2-16

16-32

32-63

>63*

>125

Fig. 6. Scatter diagrams of the LREE/HREE variations in different particle size fractions of the CN, ZSP and TKP sections (> 63 μm∗: > 63 μm for CN and ZSP, 63–125 μm for TKP).

area, mineral partitioning with grain size, and the intensity of chemical weathering (Taylor and McLennan, 1985; Yang et al., 2004; Dou et al., 2015). The remarkable differences in REE-based geochemical characteristics between CN, ZSP, and TKP sediments demonstrate a significant diversity in material sources and environmental change of CLP and the Ili Basin. 5.1. The long distances implications of REE of fine fractions As mentioned above, < 2 μm fractions of TKP and ZSP exhibit strong similarity in ΣREE values (Fig. 2) and UCC-normalized patterns (Fig. 4a), while CN samples present similar patterns but higher concentrations and greater variability (Fig. 2). Fine-grained particles (< 16 μm), especially the ones in the < 4 μm fraction, are mainly transported by upper level flow and can be dispersed over wide vertical and horizontal ranges (Pye, 1987; Zdanowicz et al., 2006; Sun et al., 2007). Although fine particles in modern dust storms can be transported as aggregates from local areas, this only accounts for 2.2–3.1% of the < 2 μm fraction (Qiang et al., 2010). Thus, the similarity in patterns suggests that the clay fractions in CLP and Ili Basin are wellmixed and multiply recycling after long distance transportation. They may both be transported by upper level westerlies. This is also supported by the similarity in LREE/HREE values of the < 2 μm fractions (Fig. 6) and weak pedogenesis. REE characteristics for different period have also determined a distant source for fine particles in the Zhaosu loess (Jia et al., 2014). The loess’ fine grain component in CLP was probably transported by a high-altitude westerly airflow (Sun et al., 2004; Sun, 2004). Although mainly controlled by source rock types, some of the REEbearing minerals would dissolve during the weathering process when

5.2. Coarse fractions tracking changes in regional material sources and boundary level circulation The uniform UCC-normalized patterns and LREE/HREE ratios in the coarse > 16 μm fractions of the TKP samples (Fig. 4c–e) imply a relatively stable source. Owing to mineral particle sorting and dilution of quartz, carbonate, and other rock-forming minerals, ΣREE values decrease gradually with increasing grain size. Feldspars can be important hosts for Eu and tend to concentrate in the coarser fractions (Taylor and McLennan, 1985; Pye, 1987; Yang, 2000). As the bedrock of the Ili Basin is mainly composed of feldspar-enriched calcium 35

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climate changes. The REE characteristics of the < 2 μm particles are sensitive to the dust transported over long distances in semi-arid areas with limited pedogenesis such as the Ili Basin. As for the semi-arid areas such as CLP, the REE characteristics of this size fraction are also influenced by climate change, and caution is needed in interpreting them as long distant provenance indicators. Multiple material sources of dust from distant and proximal areas can be recorded by REE characteristics of the 2–16 μm size fractions in different strata (in different periods). Coarse fraction (especially > 63 μm) REE characteristics are effective tracers of proximal dust sources and regional boundary level circulation. Thus, the variation patterns of the REEbased geochemical parameters of different size fractions (including REE concentrations, LREE/HREE values, δEu values, and UCC-normalized patterns) can serve as more effective source tracers for aeolian sediments. Future comparative studies of size-differentiated REE characteristics of deposited dust and its potential source(s) can identify the exact source(s), and allow for a better understanding of atmospheric circulation patterns and paleoclimate changes.

alkaline volcanic rocks and alkaline volcanic rocks (Zhu et al., 2012), the rapid increase in δEu values in the > 16 μm fraction (Fig. 5c) implies an increase in the proportion of mineral dust derived from neighboring regional sources. The analogous distribution patterns and δEu values of the ZSP samples in the > 16 μm fraction imply a major stable dust source. As coarse particles with abundant HREE originated from the adjacent Ili diluvial-alluvial plain and Kazakhstan Gobi deserts join in, the LREE/ HREE ratios increase gradually while ΣREE values decrease. The mineralogical analyses of the Ili loess also showed the existence of heavy minerals enriched in HREE, such as garnet and zircon, in the coarse fractions (Yang, 2000; Ye, 2001). Nonetheless, compared with the TKP samples, the δEu values of the ZSP samples for the > 16 μm fraction show only a slight increase (Fig. 5b); this disparity indicates a difference in the detrital minerals, protoliths, and airflow patterns between the ZSP and TKP areas. Moreover, high fluctuations in ΣREE values occur in different stratigraphic units of ZSP > 63 μm fractions (Fig. 2b). Since particles of size > 63 μm can only originate from a proximate region, the apparent variability in ΣREE values in different stratigraphic units suggests the changes in the intensity and pathway of the boundary level flows in different climatic cycles, causing the shift of the material source. Thus, the REE characteristics of coarse fractions (> 16 μm) in aeolian deposits, especially in the > 63 μm fraction, are effective tracers for tracking the changes in regional material sources and boundary level circulation. The CN samples from CLP show significantly different REE-based characteristics in their coarse fractions. UCC-normalized patterns for CN samples in the 16–32 μm and 32–63 μm fractions are clearly different from those of the other two sections (Fig. 4c and d). As the short-range suspension near ground surface is the main transport route of silt fractions with average particle sizes of 20–70 μm (Pye, 1987), these differences suggest that the mineral dust found in these CLP and Ili Basin fractions were transported from different regional sources. Mixing with dusts supplied by other sources altered the geochemical composition of the dust deposited in the CN section, leading to differences in the normalized patterns from the finer fractions. Recently, the zircon U-Pb age studies have also shown multiple source regions of the quaternary loess in the central CLP (Nie et al., 2014, 2015). Fig. 2a shows that the gradual decrease in REE concentrations for the 16–32 μm and 32–63 μm fractions of the CN samples, in addition to the noticeable increase in the > 63 μm fractions. REEs are generally enriched in clay and silt fractions, but depleted in sand fractions. Nevertheless, heavy minerals such as zircon, apatite, monazite, garnet, allanite, and sphene, despite their low content in mostly sandy sediments, may constitute a significant proportion of REEs (Taylor and McLennan, 1985; Dou et al., 2015). The increase in REE concentrations in the > 63 μm fraction of the CN samples indicates that a high proportion of material sourced from the neighboring regions (enriched in the aforementioned minerals found in CLP deposits). Moreover, the decrease in δEu values in the > 63 μm fraction (Fig. 5a) also explains the existence of adjacent sources; coarse particles originated from such sources are low in feldspars. This is consistent with the results of zircon U-Pb and heavy mineral provenance studies, which also showed that substantial amounts of NE Tibet denuded material were deposited on the Loess Plateau (Nie et al., 2015). Therefore, coarse fraction REE characteristics, especially in > 63 μm fractions, are effective tracers of proximal dust sources, and the variation patterns of REE characteristics in different size fractions can serve as effective provenance tracers.

Acknowledgments We are grateful to Ed Derbyshire, Prof. Jimin Sun and Keyan Fang for editing and polishing the manuscript. This work was supported by the National Natural Science Foundation of China (NSFC) [Nos: 41302149, 41572162, 41210002]; the Project in the Public Interests of Fujian [No: K3-296]; National Basic Research Program of China (No. 2013CB955904); International partnership Program of Chinese Academy of Sciences (CAS) (No: 132B61KYS20160002), and the Scientific and Technological Innovation Team of CAS [No: CAS Renzi (2013)47]. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jseaes.2017.03.030. References Bird, A., Stevens, T., Rittner, M., Vermeesch, P., Carter, A., Ando, S., Garzanti, E., Lu, H.Y., Nie, J.S., Zeng, L., Zhang, H.Z., Xu, Z.W., 2015. Quaternary dust source variation across the Chinese Loess Plateau. Palaeogeogr., Palaeoclimatol., Palaeoecol. 435, 254–264. http://dx.doi.org/10.1016/j.palaeo.2015.06.024. Chang, Q., Mishima, T., Yabuki, S., Takahashi, Y., Shimizu, H., 2000. Sr and Nd isotope ratios and REE abundances of moraines in the mountain areas surrounding the Taklimakan Desert, NW China. Geochem. J. 34 (6), 407–427. http://dx.doi.org/10. 2343/geochemj.34.407. Chen, J., Li, G.J., 2011. Geochemical studies on the source region of Asian dust. Sci. China (Ser. D) 54, 1279–1301. http://dx.doi.org/10.1007/s11430-011-4269-. Chen, Q., Liu, X.M., Heller, F., Hirt, A.M., Lü, B., Guo, X.L., Mao, X.G., Zhao, G.Y., Feng, H., Guo, H., 2012. Susceptibility variations of multiple origins of loess from the Ily Basin (NW China). Chin. Sci. Bull. 57 (15), 1844–1855. http://dx.doi.org/10.1007/ s11434-012-5131-1. Chen, X.L., Fang, X.M., An, Z.S., Han, W.X., Wang, X., Bai, Y., Hong, X., 2007. An 8.1 Ma calcite record of Asian summer monsoon evolution on the Chinese central Loess Plateau. Sci. China (Ser. D) 50 (3), 392–403. http://dx.doi.org/10.1007/s11430-0070016-. Chen, X.L., Li, Z.Z., Ling, Z.Y., Jin, J.H., Cao, X.D., 2010. Holocene Aeolian deposits and environmental evolution in Yili Valley, Xinjiang. Mar. Geol. Quat. Geol. 30 (6), 35–42. http://dx.doi.org/10.3724/SP.J.1140.2010.06035. (in Chinese). Chen, X.L., Li, Z.Z., Jia, L.M., Guo, L.C., 2013. Rare earth element characteristics of desert sediments in Ili valley and their environmental implication. Quat. Sci.. 33 (2), 368–375. http://dx.doi.org/10.3969/j.issn.1001-7410.2013.02.19. (in Chinese). Cullers, R.L., Barrett, T., Carlson, R., Robinson, B., 1987. Rare-earth element and mineralogic changes in Holocene soil and stream sediment: a case study in the Wet Mountains, Colorado, U.S.A. Chem. Geol. 63 (3–4), 275–297. http://dx.doi.org/10. 1016/0009-2541(87)90167-7. Dai, X.G., Li, W.J., Ma, Z.G., Wang, P., 2007. Water-vapor source shift of Xinjiang region during the recent twenty years. Prog. Nat. Sci.: Mater. Int. 17 (5), 569–575. http://dx. doi.org/10.1080/10020070708541037. Ding, Z.L., Ranov, V., Yang, S.L., Finaev, A., Han, J.M., Wang, G.A., 2002. The loess record in southern Tajikistan and correlation with Chinese loess. Earth Planet. Sci. Lett. 200 (3–4), 387–400. http://dx.doi.org/10.1016/S0012-821X(02)00637-4. Dodonov, A.E., 1991. Loess of Central Asia. GeoJournal 24 (2), 185–194. http://dx.doi. org/10.1007/BF00186015.

6. Conclusion The comparative study of aeolian sediments between the Ili Basin and CLP indicates significantly different characteristics of the UCCnormalized patterns and REE parameters of different size fractions. These differences are likely caused by the diversity in dust sources and 36

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