Geochemical characteristics of the eolian deposits in the Zoigê basin and their implications for provenance and weathering intensity

Quaternary International xxx (xxxx) xxx–xxx

Contents lists available at ScienceDirect

Quaternary International journal homepage: www.elsevier.com/locate/quaint

Geochemical characteristics of the eolian deposits in the Zoigê basin and their implications for provenance and weathering intensity Lin Qia, Yansong Qiaoa,b,∗, Haitao Yaoa, Yan Wanga, Shasha Pengc, Shuaibin Yanga a

Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China Key Laboratory of Active Tectonics and Crustal Stability Assessment, Beijing 100081, China c Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, Guangdong, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Eolian sedimentology Geochemistry Provenance Zoigê basin Tibetan paleoclimate

Eolian deposits in the Zoigê basin at the northeastern region of the Tibetan Plateau (TP) were analyzed for their major-trace elements and SmeNd isotopic compositions to explore the provenance and weathering intensity. The results are compared with the data from the Lingtai section (LT) in the Chinese Loess Plateau (CLP). Eolian deposits from different sections in the Zoigê basin have resembling geochemical compositions, reflecting similar provenance. Their major element compositions are close to those of the UCC, and their UCC-normalized majortrace element abundances and chondrite-normalized rare earth element (REE) abundances are similar to the LT samples. However, the SiO2, Al2O3, TiO2, Zr, Hf, Sc, Co, V, Ni concentrations and SiO2/TiO2, TiO2/Al2O3, Co/Th, La/Sc, Th/Sc, εNd(0), 147Sm/144Nd ratios of the Zoigê sections are quite different from those of the LT section, which suggests different provenances for the eolian deposits in these two regions. The relatively high abundances of SiO2, Zr, Hf and low abundances of trace elements indicate that local sediments on the TP are the major contributors to the eolian deposits in the Zoigê basin. Eolian deposits from the Zoigê basin have experienced the early Na and Ca removal stage, and chemical weathering parameters suggest a moderate weathering condition in the source and deposition regions.

Declarations of interest none. 1. Introduction The Zoigê basin, located in the northeastern region of the Tibetan Plateau (TP), is a faulted basin formed by tectonic movement. Eolian deposits in this basin are mainly distributed on the broad river terraces of the Yellow River and its tributaries. This accumulation was assumed to have begun during the Last glacial stage, inferred from the time of lake disappearance and river terraces formation in this region (Li, 1991; Wang and Xue, 1997; Hu et al., 2018). Previous studies have focused on the physical characterizations. The grain-size and quartz surface morphology show that dust materials in this basin are eolian deposits. As ice wedges and few pollens have been found in the loess, eolian deposits in this region was assumed to come from the glacial deposits produced on the TP (Sheng, 2010). However, direct and specific evidence is needed to verify this supposition. Geochemical analysis, which is a useful method for paleoclimate

∗

study and dust source discrimination (Sun, 2002; Liang et al., 2009; Hao et al., 2010; Ferrat et al., 2011), has not been applied to the eolian deposits in this area so far. In this study, we perform geochemical analysis on samples from Jiaerduo (JED), Tangke (TK) and Qihama (QHM) sections in the Zoigê basin (these three sections are abbreviated as “REG”) to measure their composition of major-trace elements and SmeNd isotopes so as to provide geochemical data for future study. Eolian dust materials usually come from large arid areas without dense vegetation cover. Strong winds are necessary to carry these dust materials. According to the multi-year statistics of modern wind direction, the Zoigê basin is dominated by westerly, southwesterly, northwesterly and northeasterly winds. Taking the location of the Zoigê basin into account, the TP or the deserts in north China may be the potential source areas for the eolian deposits in the Zoigê basin. We compared the REG geochemical signatures with those of the Lingtai (LT) section, which were well-documented loess on the Chinese Loess Plateau (CLP) and mainly blown from the deserts in northern China. The main objective of this study is to reveal the geochemical similarities and differences between the REG and LT sections, so as to provide geochemical constraints on the source and reveal the weathering

Corresponding author. Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China. E-mail address: [email protected] (Y. Qiao).

https://doi.org/10.1016/j.quaint.2019.03.018 Received 4 July 2018; Received in revised form 18 March 2019; Accepted 19 March 2019 1040-6182/ © 2019 Elsevier Ltd and INQUA. All rights reserved.

Please cite this article as: Lin Qi, et al., Quaternary International, https://doi.org/10.1016/j.quaint.2019.03.018

Quaternary International xxx (xxxx) xxx–xxx

L. Qi, et al.

Fig. 1. Location of the study area and sampling sites. JED, TK and QHM represent the sampling sites in Jiaerduo, Tangke and Qihama respectively. Table 1 Depth (m) and major element concentrations (wt.%) of samples from the REG and LT sections. The data were recalculated on a volatile-free basis. The data for samples from the LT section are from Qiao et al. (2011). The UCC values are from Taylor and McLennan (1985). Depth(m)

Sample

Type

SiO2

Al2O3

Fe2O3

CaO

MgO

K2O

Na2O

TiO2

MnO

P2O5

Total

LOI

CIA

REG 0.8 2.0 3.4 5.6 2.8 4.0 1.0 3.0 5.0 7.5

JED4 JED10 JED17 JED28 TK28 TK40 QHM5 QHM15 QHM25 QHM35

Loess Loess Loess Paleosol Loess Loess Loess Loess Loess Paleosol

73.08 72.87 73.00 71.98 76.78 75.91 73.60 74.21 72.58 73.80

14.05 14.11 14.02 14.92 12.51 12.98 14.09 13.48 13.99 13.83

3.71 3.89 3.71 4.56 2.96 3.05 4.04 3.65 4.25 3.49

1.15 1.21 1.31 0.70 0.97 0.98 0.72 1.08 1.04 1.12

1.92 1.92 2.09 1.89 1.32 1.45 1.72 1.75 1.94 1.78

2.94 2.85 2.87 3.19 2.53 2.58 2.90 2.71 3.03 2.84

2.19 2.18 2.01 1.74 2.10 2.17 1.95 2.12 2.18 2.16

0.71 0.73 0.73 0.79 0.62 0.65 0.74 0.74 0.73 0.72

0.08 0.08 0.08 0.13 0.06 0.06 0.10 0.09 0.10 0.08

0.17 0.17 0.17 0.12 0.15 0.17 0.13 0.17 0.15 0.19

100 100 100 100 100 100 100 100 100 100

4.14 2.78 3.53 3.58 4.34 4.70 3.63 3.41 3.34 4.02

61.29 61.40 61.41 66.28 61.07 61.40 64.75 61.63 61.45 61.46

73.78 2.04

13.80 4.85

3.73 13.27

1.03 19.07

1.78 13.26

2.84 6.89

2.08 6.87

0.72 6.78

0.09 22.18

0.16 13.98

3.75 14.86

62.21 2.86

69.97 70.17 68.96 70.17 69.65

15.87 15.30 15.94 15.79 16.15

5.26 5.22 5.74 5.46 5.67

0.72 1.21 1.03 0.79 0.64

2.17 2.10 2.25 2.00 1.97

3.24 3.14 3.34 3.17 3.56

1.69 1.76 1.63 1.59 1.31

0.83 0.79 0.82 0.79 0.82

0.14 0.12 0.14 0.13 0.14

0.13 0.19 0.16 0.13 0.09

3.97 3.59 3.90 3.68 3.85

67.59 64.26 66.08 67.85 69.20

69.78 0.73

15.81 1.98

5.47 4.29

0.88 26.75

2.10 5.57

3.29 5.18

1.60 10.96

0.81 2.36

0.13 7.40

0.14 25.74

3.80 4.16

67.00 2.82

66.00

15.20

5.00

4.20

2.20

3.40

3.90

0.68

0.07

Average CV(%) LT 25.0 45.0 65.0 50.4 70.4

LT250 LT450 LT650 LT1000 LT1200 Average CV(%)

Paleosol Loess Loess Loess Paleosol

100 100 100 100 100

UCC

With regard to the flat terrain, winding channels and sluggish streams, wetland develops extensively in this basin. The exposed rocks, which are uplifted by Cenozoic tectonism of the Himalaya, are mainly Triassic slate, phyllite, schist, sandstone, siltstone, mudstone and Neogene conglomerate (She et al., 2006; Tang et al., 2012). River-lake facies, glacial sediments and wind accumulations formed during the Late Neogene and Quaternary are present mainly in the flat areas (Sheng, 2008; Dong et al., 2010).

intensity for the eolian deposits in the Zoigê basin. This research can help to understand the atmospheric circulation pattern in the northeastern region of the TP and reveal the environmental settings of source and deposition regions. 2. Regional setting The Zoigê basin is surrounded by the Minshan Mountain to the east, the Anyemaqen Mountain and the Bayan Hara Mountain to the west, the Gahai Lake to the north, and the Qionglai Mountain to the south (Fig. 1). Officially, it belongs to the Ruoergai, Hongyuan and Aba Counties of Aba Tibetan-Qiang Autonomous Prefecture in Sichuan Province and the Maqu County of Gannan Tibetan Autonomous Prefecture in Gansu Province. Gentle hills and flat valleys are distributed in the basin and their mean altitude ranges from 3400 to 3900 m. Climate in this area is characterized by semi-humid continental monsoon with annual mean temperature ranges from 0.7 °C to 3.3 °C (Hu et al., 2015). The vegetation is dominated by subalpine meadow, subalpine shrub meadow and swamp meadow. Rivers in this area mostly belong to the Yellow River system. Two of the Yellow River tributaries, the Baihe River and the Heihe River flow from south to north across the basin.

3. Material and methods Eolian deposits in the Zoigê basin are mainly distributed on the river terraces of the Yellow River and its tributaries. Their thickness is mostly less than 10 m, while occasionally more than 20 m. The JED section (33°01′N，101°34′E) is located at the western edge of the Zoigê basin and about 20 km northwestern from the Aba County, with thickness of 7.8 m. The TK section (33°28′N，102°28′E) is located at the second terrace of the Baihe River in the middle part of the basin, and about 60 km southwestern from the Ruoergai County with thickness of 29 m. The QHM section (33°24′N，101°58′E) is 7.8 m thick and was found on the southern terrace of the Yellow River, about 85 km southwestern 2

Quaternary International xxx (xxxx) xxx–xxx

L. Qi, et al.

from the Maqu County in the western region of the Zoigê basin. In these sections, loess, paleosol and eolian sand were distinguished according to their color and structure. The loess layers have a 10 YR 8/4 Munsell color and the paleosol layers are characterized by a 2.5 YR 6/6 Munsell color. Both of them are consisted of silt. The eolian sand shows a 10 YR 8/2 Munsell color and is composed of medium and fine sand. Loess and paleosol samples of different depths from the JED, TK, QHM sections were collected for major-trace elemental and SmeNd isotopic measurements. Sampling depths and sample types are shown in Table 1. Samples for major and trace elemental measurement were immersed in 1 N acetic acid solution for 24 h at room temperature to remove the carbonates. Then, the insoluble fractions were washed by deionized water in order to remove residual acid. In the following step, the insoluble fractions were dried and grounded into 74 μm (Chen et al., 2008; Qiao et al., 2011). Major element concentrations were determined by X-ray fluorescence spectrometry using a 3080E spectrometer. Analytical precision are less than 3% except for MnO and P2O5 (up to 10%). Loess on ignition (LOI) was obtained by weighing before and after 1h of heating at 950 °C. Trace element compositions were determined using an X-series ICP-MS. Analytical precision are less than 10%. Both the major and trace element measurements were accomplished at the National Research Center for Geoanalysis, Chinese Academy of Geological Sciences, Beijing. Analytical precisions were determined by three re-analyzed samples. In order to reduce the contribution of coarse particulates from local region, only the < 20 μm fractions were measured for SmeNd isotopic analyses (Sun, 2002). Samples for isotope measurement were immersed in 2.5 N HCl solution for 20 h to remove the carbonates. The insoluble fractions were washed by deionized water in order to remove residual acid. Sm and Nd concentrations were measured on a MAT262 mass spectrometer and Nd isotopic compositions were measured on a MCICP-MS mass spectrometer. The measurements were performed at the Institute of Geology, Chinese Academy of Geological Sciences, Beijing. Analytical accuracy is less than 10%. The accuracy was monitored using multiple analyses of an international standard of JMC Nd2O3. One standard was inserted in every three samples. The JMC Nd2O3 gave a value of 143Nd/144Nd = 0.511123 ± 10(2σ). The measured Nd isotopic ratios were normalized to 146Nd/144Nd = 0.7219. Nd isotope is generally expressed as the ten thousand part deviation from the 143 Nd/144Nd ratio of modern chondrite (0.512638) (Chen and Li, 2011):

εΝd

143

Nd

144

Nd Sample

( ) (0) =

−

143

Nd

144

Nd Chondrite

( )

143

Nd

144

Nd Chondrite

( )

Fig. 2. UCC-normalized abundances of major element for samples from the REG and LT sections. (a) Data for all studied samples; (b) average data for samples from each section.

sections are dominated by SiO2, Al2O3 and Fe2O3, same as the samples from the LT section. All the loess and paleosol samples from investigated sections in the Zoigê basin exhibit similar UCC-normalized major element abundances and show obviously lower CaO and Na2O contents compared to the average UCC, which is similar to the LT samples. However, the REG samples show slightly higher SiO2, CaO, Na2O and lower Al2O3, Fe2O3, MgO, K2O, TiO2 contents than the LT samples. The coefficient of variation (CV) represents the ratio of the standard deviation to the mean. It is a useful statistic for comparing the degree of variation from one data series to another, even if the means are drastically different from one another. According to the CV of major element concentration, the REG samples have more fluctuant concentrations for most major elements except for CaO and Na2O.

× 10000.

The major, trace and SmeNd isotope data used for comparison are from the reported eolian deposits (34°59′N, 107°45′E) (Qiao et al., 2011; Tan et al., 2013; Qi and Qiao, 2014; Yang et al., 2017) located at Lingtai County in Gansu Province in the CLP. Sample pretreatment, test facilities and methods are the same as applied in the REG sections.

4.2. Trace element characteristics Trace element concentrations for the samples studied are given in Tables 2 and 3. The UCC-normalized abundances (Taylor and McLennan, 1985) for the samples from the REG and LT sections are shown in Fig. 3. The REG and LT sections have basically similar UCCnormalized abundance patterns with higher concentrations of Li, Ga, Y, Zr, Nb, Cs, Ta, Pb, Bi, Th, all REEs, lower concentrations of Be, Sr, Ba, Tl, U and similar concentrations of Sc, V, Co, Ni, Rb, Hf compared with the UCC. There are also differences between the LT and REG samples. Compared with the LT samples, the REG samples have lower contents for most of the trace elements except for Sr, Zr and Hf. The Sc, V, Co, Ni concentrations are especially low in the samples from the REG sections. Besides, the trace element concentrations for the REG sections are relatively unstable according to the higher CV. The REE can be studied as a group because of their similar chemical characteristics in nature. The chondrite-normalized REE patterns (Boynton, 1984) of the REG samples are similar to those of the LT samples (Fig. 4), with a profile of enriched light REEs, relatively flat heavy REEs and a negative Eu anomaly. The distribution patterns are

4. Results and discussion 4.1. Major element characteristics Table 1 presents the major element concentrations (recalculated on a volatile-free basis) for the samples from the JED, TK and QHM sections in the Zoigê basin. For comparison, already published geochemical data of typical eolian deposits (Qiao et al., 2011; Tan et al., 2013; Qi and Qiao, 2014; Yang et al., 2017) from the LT section are also listed. The upper continental crust (UCC) normalized abundances (Taylor and McLennan, 1985) for all the samples from different sections are shown in Fig. 2. Due to low concentrations and high analytical uncertainties, variations of MnO and P2O5 contents are not discussed in this paper. Major element compositions of the eolian deposits from the REG 3

4

Average CV(%) UCC

2.15 2.89

3.00

20.00

2.17 2.21 2.06 2.12 2.20

48.10 46.80 46.50 43.40 44.70

45.90 4.03

1.96 6.50

1.95 2.21 1.98 2.12 1.76 2.00 1.93 1.90 1.87 1.90

Be

42.17 12.04

44.30 42.10 45.20 51.40 34.50 37.20 47.80 39.90 39.50 39.80

REG JED4 JED10 JED17 JED28 TK28 TK40 QHM5 QHM15 QHM25 QHM35

Average CV(%) LT LT250 LT450 LT650 LT1000 LT1200

Li

Sample

13.60

14.06 2.59

14.00 13.80 13.90 14.70 13.90

11.73 12.72

12.40 13.00 13.40 13.60 9.33 11.30 10.10 10.40 11.10 12.70

Sc

107.00

114.80 3.96

121.00 110.00 112.00 113.00 118.00

92.24 10.59

74.80 98.50 104.00 106.00 89.30 80.40 92.10 87.40 95.70 94.20

V

17.00

16.30 4.34

16.70 15.30 16.50 15.90 17.10

12.59 10.06

13.00 13.40 13.50 14.70 10.40 10.90 12.70 11.90 13.00 12.40

Co

44.00

42.64 4.95

44.00 40.40 42.30 41.00 45.50

32.81 12.63

36.70 35.80 36.90 38.30 27.30 26.80 32.20 29.00 33.60 31.50

Ni

17.00

20.18 1.54

20.50 19.80 20.10 20.00 20.50

17.45 5.77

18.00 17.80 18.40 19.20 15.70 16.40 17.50 16.70 17.40 17.40

Ga

112.00

108.80 4.66

105.00 110.00 103.00 116.00 110.00

99.82 19.58

117.00 119.00 118.00 117.00 72.70 104.00 79.00 77.80 81.70 112.00

Rb

350.00

113.60 8.48

114.00 124.00 106.00 122.00 102.00

128.00 12.58

143.00 144.00 138.00 124.00 113.00 142.00 103.00 116.00 112.00 145.00

Sr

22.00

33.24 6.28

32.70 32.70 30.30 35.50 35.00

30.92 12.44

27.00 31.80 30.00 35.10 25.90 27.00 30.80 30.80 32.40 38.40

Y

190.00

225.00 2.86

233.00 215.00 226.00 225.00 226.00

247.50 12.67

199.00 212.00 253.00 257.00 213.00 262.00 280.00 251.00 249.00 299.00

Zr

12.00

16.68 3.48

17.60 16.10 16.60 16.30 16.80

15.47 7.65

16.20 15.70 16.80 16.90 13.60 13.40 15.60 15.60 15.00 15.90

Nb

4.60

9.60 8.72

9.74 8.70 9.58 9.07 10.90

8.89 20.41

8.79 9.42 9.79 11.80 5.77 6.68 10.90 7.87 9.38 8.52

Cs

550.00

477.60 3.41

469.00 492.00 455.00 478.00 494.00

447.10 11.50

486.00 498.00 501.00 501.00 399.00 425.00 374.00 396.00 406.00 485.00

Ba

5.80

5.77 5.87

5.65 5.24 5.85 6.06 6.04

6.28 15.74

4.68 5.29 6.17 6.43 4.95 6.89 7.53 6.77 6.76 7.30

Hf

1.00

1.76 7.74

1.78 1.62 1.66 1.76 1.97

1.71 10.13

1.62 1.74 1.82 1.95 1.43 1.43 1.86 1.68 1.78 1.79

Ta

0.80

0.67 2.79

0.65 0.65 0.68 0.68 0.69

0.60 7.27

0.61 0.60 0.65 0.66 0.50 0.58 0.60 0.58 0.61 0.60

Tl

17.00

24.46 4.22

23.40 23.50 24.50 25.10 25.80

22.59 10.45

23.60 23.50 23.90 26.50 18.00 20.70 23.80 21.10 23.60 21.20

Pb

0.10

0.37 7.93

0.36 0.34 0.38 0.37 0.42

0.32 17.02

0.32 0.34 0.34 0.39 0.23 0.24 0.38 0.28 0.34 0.30

Bi

10.70

16.62 4.95

16.50 15.70 16.80 16.20 17.90

15.67 11.84

16.80 17.60 17.40 17.80 11.90 14.30 15.00 14.90 14.70 16.30

Th

Table 2 Trace element concentrations (ppm) of samples from the REG and LT sections. The data for samples from the LT section are from Qiao et al. (2011). The UCC values are from Taylor and McLennan (1985).

2.80

2.56 3.38

2.64 2.56 2.54 2.64 2.43

2.45 12.93

2.72 2.57 2.90 2.78 1.87 2.18 2.38 2.33 2.20 2.56

U

L. Qi, et al.

Quaternary International xxx (xxxx) xxx–xxx

Quaternary International xxx (xxxx) xxx–xxx

L. Qi, et al.

Table 3 Rare earth element concentrations (ppm) of samples from the REG and LT sections. The data for samples from the LT section are from Qiao et al. (2011). The UCC values are from Taylor and McLennan (1985). The chondrite values are from Boynton (1984). Sample

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

REG JED4 JED10 JED17 JED28 TK28 TK40 QHM5 QHM15 QHM25 QHM35

38.90 42.00 43.80 46.10 31.60 38.50 39.60 42.00 36.90 42.80

80.30 93.10 88.90 92.30 63.30 85.30 81.30 86.30 78.80 86.80

8.68 9.63 9.64 10.60 7.26 9.52 9.92 10.30 9.44 10.10

31.70 35.40 34.50 39.30 26.80 34.40 36.60 37.40 35.40 37.80

6.31 6.96 6.85 7.95 5.45 6.48 6.97 7.29 7.14 7.76

1.14 1.22 1.25 1.42 1.08 1.18 1.26 1.33 1.33 1.47

5.06 5.47 5.56 6.25 4.58 5.25 5.43 5.86 5.74 6.14

0.79 0.85 0.86 0.98 0.71 0.84 0.90 0.97 0.94 0.98

4.74 5.42 5.32 6.06 4.50 5.05 5.65 5.69 6.00 6.31

0.94 1.09 1.06 1.23 0.87 0.97 1.09 1.07 1.15 1.31

2.83 3.28 3.22 3.65 2.67 2.85 3.28 3.17 3.42 3.93

0.43 0.48 0.49 0.53 0.40 0.42 0.47 0.47 0.49 0.57

2.68 3.04 3.05 3.33 2.53 2.74 3.03 3.14 3.20 3.59

0.37 0.44 0.43 0.50 0.35 0.41 0.45 0.47 0.46 0.53

Average CV(%) LT LT250 LT450 LT650 LT1000 LT1200

40.22 10.18

83.64 10.28

9.51 9.99

34.93 10.17

6.92 10.43

1.27 9.61

5.53 9.05

0.88 10.14

5.47 10.70

1.08 12.24

3.23 11.90

0.48 10.70

3.03 10.44

0.44 12.44

44.80 42.20 41.80 45.20 45.30

95.60 92.20 94.80 99.30 87.30

10.10 9.58 9.92 11.10 11.10

37.40 34.00 33.70 40.50 41.20

7.63 6.52 6.47 7.64 8.22

1.41 1.15 1.19 1.45 1.48

6.21 5.34 5.14 6.49 6.48

0.94 0.89 0.91 1.04 1.04

5.53 5.32 5.68 6.47 6.45

1.08 1.03 1.08 1.20 1.27

3.30 3.15 3.28 3.55 3.69

0.51 0.47 0.50 0.54 0.52

3.17 3.14 3.23 3.52 3.27

0.43 0.45 0.48 0.52 0.49

Average CV(%) UCC

43.86 3.91

93.84 4.74

10.36 6.77

37.36 9.40

7.30 10.55

1.34 11.54

5.93 10.88

0.96 7.43

5.89 9.10

1.13 8.78

3.39 6.48

0.51 5.10

3.27 4.62

0.47 7.40

30.00

64.00

7.10

26.00

4.50

0.88

3.80

0.64

3.50

0.80

2.30

0.33

2.20

0.32

0.310

0.808

0.122

0.600

0.165

0.074

0.259

0.047

0.322

0.072

0.210

0.032

0.209

0.034

Chondrite

Fig. 4. Chondrite-normalized abundances of REE for samples from the REG and LT sections. (a) Data for all studied samples; (b) average data for samples from each section.

Fig. 3. UCC-normalized abundances of trace element for samples from the REG and LT sections. (a) Data for all studied samples; (b) average data for samples from each section.

Each of them has 7 isotopes. The 147Sm decays to 143Nd by alpha decay with a half-life of about 106 Ga. The rest of isotopes can be treated as stable nuclides because of their long half-life. The 143Nd/144Nd and 147 Sm/144Nd values is distinct in rocks with different origin and age, they are often used to determine the provenance of various sediments

similar to those of the UCC (Taylor and McLennan, 1985). 4.3. SmeNd isotopic compositions Both Sm and Nd are light REE with analogous chemical properties. 5

Quaternary International xxx (xxxx) xxx–xxx

L. Qi, et al.

Table 4 SmeNd data of samples from the REG and LT sections. The data for samples from the LT section are from Qiao et al. (2011). Sample

REG JED4 JED10 JED17 JED28 TK28 TK40 QHM5 QHM15 QHM25 QHM35 LT LT250 LT450 LT650 LT1000 LT1200

143 Nd/144Nd ( ± 2σ*10−6)

εNd(0)

0.1004 0.1028 0.1014 0.1000 0.1021 0.1007 0.1001 0.1021 0.1024 0.1022

0.512084 ± 7 0.512086 ± 8 0.512089 ± 5 0.512091 ± 5 0.512095 ± 5 0.512094 ± 8 0.51208 ± 6 0.512082 ± 14 0.512087 ± 10 0.51209 ± 7

−10.80685 −10.76783 −10.70931 −10.67030 −10.59227 −10.61178 −10.88487 −10.84586 −10.74833 −10.68980

0.1075 0.1078 0.1103 0.1059 0.1103

0.512065 0.512077 0.512077 0.512076 0.512079

−11.17748 −10.94339 −10.94339 −10.96290 −10.90438

Sm (μg/g)

Nd (μg/g)

147

4.185 3.843 4.480 4.317 4.328 4.511 4.368 4.386 4.443 4.469

25.217 22.614 26.734 26.112 25.631 27.088 26.400 25.989 26.250 26.444

3.592 4.425 3.511 3.982 3.509

20.218 24.828 19.263 22.733 19.245

Sm/144Nd

± ± ± ± ±

8 9 12 12 7

Fig. 5. SiO2/TiO2 versus TiO2/Al2O3 diagram for samples from the REG and LT sections.

samples provide the first evidence for the different sources. Some trace elements, and particularly their ratios, are useful indicators of provenance as they are least affected by weathering, transporting and sorting processes (Ferrat et al., 2011). Zr and Hf are almost exclusively in a heavy mineral zircon which is normally enriched relative to upper crustal abundances during the silt production, transportation and deposition (Taylor et al., 1983). However, Zr/Hf could vary in zircons derived from different source rocks, depending on their magmatic history (McLennan, 1989). Therefore, Zr/Hf can be an effective indicator for tracing the provenance (Sun, 2002; Hao et al., 2010; Qiao et al., 2011). The commonly immobile elements, such as Th, Sc, Co, REEs and their ratios, are also useful indicators for source determination (Taylor and McLennan, 1985; Gallet et al., 1998; Yang et al., 2007). These elements, even if mobilized, have short residence time in water and hence are almost quantitatively transferred into the sediments (Yang et al., 2003). Therefore, they are commonly accepted as conservative and reliable indices of sediment provenance (Condie, 1989). All the studied samples are plotted in a TheHfeCo triangular diagram (Fig. 6). The eolian deposits from REG sections can be easily discriminated from the LT samples in the plot (Fig. 6). The REG samples have lower Co/Th and higher La/Sc, Th/Sc ratios than the LT samples (Fig. 7), indicating a different provenance for the eolian deposits in the Zoigê basin. In the diagram of 147Sm/144Nd versus εNd(0) (Fig. 8), the eolian

(Goldstein et al., 1984; Revel et al., 1996; Biscaye et al., 1997; Grousset and Biscaye, 2005). SmeNd data from the REG and LT sections are shown in Table 4. The Sm and Nd contents, 143Nd/144Nd and εNd(0) values in the REG are obviously higher than those of the LT samples. In addition, the 143Nd/144Nd values in the REG have a greater variation range. These may indicate that the eolian deposits from REG and LT sections have different sources. 4.4. Implications for provenance Although there are certain differences in the element concentrations of the samples from JED, TK and QHM sections, the similar geochemical characteristics reflect similar provenance for the eolian deposits in the Zoigê basin. The REG samples have similar major element compositions as UCC, and their UCC-normalized major-trace element abundances and chondrite-normalized REE element abundances are similar to the LT samples, indicating that the dust materials come from multiple sources and have been mixed prior to deposition in the Zoigê basin. According to the CV of major element concentration listed in Table 1, the contents of SiO2, Al2O3 and TiO2 in different depths keep relatively stable in both REG and LT sections. The TiO2/Al2O3 ratio is particularly useful as a provenance indicator, because Ti contents may be quite variable among different types of rocks, even though Al contents are relatively constant (Young and Nesbitt, 1998; Li, 2000; Sheldon and Tabor, 2009). Besides, both Ti and Al have low solubility in natural water (Broecker and Peng, 1982; Sugitani et al., 1996). Previous studies on the eolian deposits from the CLP indicate that the TiO2/Al2O3 is independent of sedimentary differentiation and can serve as an indicator to trace the provenance of silt-sized sediments (Gu et al., 1999). Compared with LT loess and paleosol, eolian deposits in the Zoigê basin have slightly higher TiO2/Al2O3 ratios (Fig. 5) indicating different source regions. Earlier studies suggest that Si mainly resides in silicates and Ti is primarily enriched in stable heavy minerals such as anatase, pyromelane and rutile (Li, 2000). With regard to their strong weathering-resistance, sometimes, the SiO2/TiO2 ratio can also be an indicator for provenance discrimination (Qiao et al., 2011; Yang et al., 2017). However, the value of SiO2/TiO2 may be affected by sedimentary sorting during transport and deposition because quartz tends to be enriched in the coarser fractions (Peng and Guo, 2001). The REG samples have higher SiO2/TiO2 ratios than the LT samples (Fig. 5). There are two explanations: 1. eolian deposits in these two regions have different sources and originate from different types of rocks; 2. the REG samples have share of coarser fractions, indicating a closer location to the source area. Anyway, the higher values of TiO2/Al2O3 for the REG

Fig. 6. TheHfeCo triangular diagram for samples from the REG and LT sections. 6

Quaternary International xxx (xxxx) xxx–xxx

L. Qi, et al.

Fig. 7. Zr/Hf versus Co/Th and Th/Sc versus La/Sc diagrams for samples from the REG and LT sections.

basin. According to the geochemical research on the eolian deposits from the Ganzi section (Qi and Qiao, 2014), the higher concentrations of SiO2, Zr and Hf may be related to the higher background values of rocks in the TP. In addition, geochemical characteristics show that local loose materials in the Zoigê basin on the TP have an important contribution to the eolian deposits. Tectonic movements of the TP and its associated climate changes have great influences on the environmental conditions in the basin. Disaggregation of rocks, shrink of the great lakes, shift or draught-up of the Yellow Rivers and its tributaries, and retreat of the glaciers have produced a lot of thick and loose sediments in the basin (Sheng, 2007; Dong et al., 2010; Hu et al., 2018). According to the 1:200 000 regional surface geochemical exploration data, the contents of most trace elements, especially Co, V and Ni, in different sediments in this basin are lower than those from other place in Western Sichuan (Tang, 2001; Sheng et al., 2007). These loose sediments would be easily deflated by wind and become the dust providers. Therefore, the deficiency of most trace elements in the eolian deposits in the Zoigê basin suggests that local loose materials in the basin may be an important contributor to the eolian deposits. Eolian deposits in the Zoigê basin have more fluctuant concentrations for most major and trace elements according to the CV (Tables 1–3). The relatively unstable geochemical concentrations indicate that the source for the eolian deposits in the Zoigê basin is relatively unstable. Uplift of the TP and its associated climate changes has great influences on the rock denudation rate and environmental conditions of the earth surface, such as expanding or retreating of glaciers, shrinking or expanding of lakes, shifting or drying up of rivers and variations in vegetation coverage, leading to the changeability of dust producing regions. Tectonic activities of the TP can also change the strength and direction of the plateau monsoon, which is the major force for dust transportation, resulting in the variations of potential dust source regions. Thus, variable earth surface conditions and plateau monsoon lead to the geochemical differences within the eolian deposits in the Zoigê basin.

Fig. 8. 147Sm/144Nd versus εNd(0) diagram for samples from the REG and LT sections.

deposits in the REG sections can be clearly separated from the LT samples. The REG samples have an obviously higher εNd(0) and lower 147 Sm/144Nd, also illustrating a different dust source. Geochemical evidence listed above indicates that the eolian deposits in the Zoigê basin have different sources from the loess in the CLP. Due to the influence of geographical location, the TP is likely to be the potential source and the plateau monsoon may be the transportation force for the eolian deposits in the Zoigê basin. Eolian deposits in the Zoigê basin have higher SiO2, Zr, Hf contents and SiO2/TiO2, TiO2/Al2O3 ratios than the LT eolian deposits, just as the eolian deposits from the Ganzi and Caotan sections which located in the eastern and northeastern margin of the TP respectively (Qi and Qiao, 2014; Yang et al., 2017). Previous studies on sedimentary and geochemical characteristic of the Ganzi section proved that the Ganzi loess originates mainly from local glacial and other Quaternary detrital sediments in the TP. The main driving force is the plateau monsoon (Fang, 1994; Qi and Qiao, 2014; Wen et al., 2017). Investigations of the magnetic susceptibility anisotropy, quartz grains surface features and geochemical characteristics of the Caotan section proved that fluvioglacial sediments in the TP are possible sources for the eolian deposits at the northeastern margin of the TP. The dust components from the TP increase obviously since 300 ka B.P. leading to the higher contents of SiO2, Zr, Hf and SiO2/TiO2, TiO2/Al2O3 ratios (Peng et al., 2015; Yang et al., 2017). Similar geochemical characteristics are also found in the samples of the REG sections, indicating that sediments in the TP are the major contributors for the eolian deposits in the Zoigê

4.5. Implications for weathering intensity The major and trace element concentrations (Tables 1 and 2) and their UCC-normalized abundances (Figs. 2 and 3) indicate that Ca, Na, K, Mg, Fe and Sr are the typical mobile elements rinsed during the weathering process in both REG and LT sections. Ca, Na and Sr are mainly contained in plagioclase. Besides, Ca and Sr are primarily enriched in carbonates. As carbonates were leached before chemical analysis, the concentrations of CaO, Na2O and Sr can reflect the weathering intensity of the plagioclases. The REG samples have higher CaO, Na2O and Sr concentrations than the LT samples indicating a weaker decomposition of plagioclase. 7

Quaternary International xxx (xxxx) xxx–xxx

L. Qi, et al.

CIA and high CaO, Na2O, Sr, Na2O/K2O in the eolian deposits in the Zoigê basin. Although the MgO and K2O concentrations in eolian deposits could be changed by the weathering, the AeCNeK triangular diagram shows that the REG eolian deposits stay at the early CaeNa removal stage and have not reach K removal stage yet. Thus, the lower MgO and K2O contents are independent of the weathering and may reflect the primitive elemental components of the dust providers. 5. Conclusions Comprehensive geochemical analysis of the eolian deposits leads to the following conclusions. Firstly, eolian deposits from different sections in the Zoigê basin have same geochemical compositions, reflecting their similar provenance. Secondly, eolian deposits in the Zoigê basin have relatively low abundances for most of the major and trace element, and their immobile element abundances of SiO2, Al2O3, TiO2, Zr, Hf, Ni, V, Co, Sc and immobile element ratios of SiO2/TiO2, TiO2/Al2O3, Co/Th, La/Sc, Th/Sc, εNd(0), 147Sm/144Nd are quite different from those of the CLP loess, illustrating different provenances. The relatively high abundances of SiO2, Zr, Hf and low abundances of trace elements prove that the loose sediments on the TP are the major contributors for the eolian deposits in the Zoigê basin. Thirdly, the immobile elemental concentrations in the REG samples have larger coefficient variations, which reflect a relatively unstable source. Finally, eolian deposits from the Zoigê basin have experienced the early Na and Ca removal stage, and chemical weathering parameters suggest a moderate weathering condition in the source and deposition regions.

Fig. 9. AeCNeK triangular diagram for samples from the REG and LT sections.

Acknowledgments This study is supported by the National Natural Science Foundation of China (grant no. 41772383) and the Mineral Resources Investigation and Appraisal Project of the Ministry of Land and Resources (grant nos. 41212011087118 and 1212010914041). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.quaint.2019.03.018.

Fig. 10. CIA versus Na2O/K2O diagram for samples from the REG and LT sections.

References Weathering processes are sequentially characterized by the early Na and Ca removal stage, the intermediate K removal stage and the more advanced Si removal stage. These trends can be determined using the Al2O3eCaO∗+Na2OeK2O triangular diagram (AeCNeK) (Nesbitt et al., 1980). The AeCNeK plot (Fig. 9) shows that in all samples from the REG and LT sections Na and Ca are in the early removal stage. Their weathering trend is subparallel to the A-CN line, suggesting the decomposition of plagioclase. However, a stronger Na and Ca depletion is observed for the LT samples. The Na2O/K2O is used here to reflect the decomposition of plagioclase as Na is removed and K is retained during the weathering (Nesbitt et al., 1980; Chen et al., 2001). Chemical index of alteration (CIA) is also widely used to evaluate the chemical weathering of terrestrial sediments (Nesbitt and Young, 1982). It is defined as CIA=(Al2O3/ (Al2O3+CaO∗+Na2O + K2O)) × 100, calculated in molar proportions and the CaO∗ refers to the amount of CaO in the silicate minerals. Eolian deposits from REG sections have higher Na2O/K2O and lower CIA values when compared with the loess from the CLP (Fig. 10) indicating a weaker decomposition of plagioclase and a moderate weathering condition. As the highest plateau in the world, the TP has low temperature and great temperature differences. As the source and deposition area for the REG eolian deposits, the TP are dominated by physical weathering presented as rock disintegration. The chemical weathering, which is manifested as element migration, is weaker, leading to relatively low

Biscaye, P.E., Grousset, F.E., Revel, M., Van der Gaast, S., Zielinski, G.A., Vaars, A., Kukla, G., 1997. Asian provenance of glacial dust (stage 2) in the Greenland ice Sheet Project 2 ice Core, Summit, Greenland. J. Geophys. Res. 102 (C12), 26765–26781. Boynton, W.V., 1984. Cosmochemistry of the rare earth elements: Meteorite studies. In: Henderson, P. (Ed.), Rare Earth Element Geochemistry. Elsevier, Amsterdam, pp. 63–114. Broecker, W.S., Peng, T.H., 1982. Tracers in the Sea. Eldigio Press, New York, pp. 26–31. Chen, J., An, Z.S., Liu, L.W., Ji, J.F., Yang, J.D., Chen, Y., 2001. Variations in chemical compositions of the eolian dust in Chinese Loess Plateau over the past 2.5 Ma and chemical weathering in the Asian inland. Science in China (Series D) 44 (5), 403–413. Chen, J., Li, G.J., 2011. Geochemical studies on the source region of Asian dust. Science China Earth Science 54 (9), 1279–1301. Chen, Y.Y., Li, X.S., Han, Z.Y., Yang, S.Y., Wang, Y.B., Yang, D.Y., 2008. Chemical weathering intensity and element migration features of the Xiashu loess profile in Zhenjiang, Jiangsu Province. J. Geogr. Sci. 18, 341–352. Condie, K.C., 1989. Plate Tectonics and Crustal Evolution. Pergamon Press, New York. Dong, Z.B., Hu, G.Y., Yan, C.Z., Wang, W.L., Lu, J.F., 2010. Aeolian desertification and its causes in the Zoige plateau of China's Qinghai-Tibetan plateau. Environmental Earth Sciences 59, 1731–1740. Fang, X.M., 1994. Origin and provenance of the Malan loess in the eastern margin of the Qinghai-Xizang (Tibetan) Plateau and its adjacent area. Science in China (Series B) 24 (5), 539–546 (in Chinese). Ferrat, M., Weiss, D.J., Strekopytov, S., Dong, S., Chen, H., Najorka, J., Sun, Y., Gupta, S., Tada, R., Sinha, R., 2011. Improved provenance tracing of Asian dust sources using rare earth elements and selected trace elements for palaeomonsoon studies on the eastern Tibetan Plateau. Geochem. Cosmochim. Acta 75, 6374–6399. Gallet, S., Jahn, B.M., Lanoe, B.V., Dia, A., Rossello, E., 1998. Loess geochemistry and its implications for particle origin and composition of the upper continental crust. Earth Planet. Sci. Lett. 156, 157–172. Goldstein, S.L., O'Nions, R.K., Hamilton, P.J., 1984. A Sm-Nd isotopic study of atmospheric dusts and particulates from major river systems. Earth Planet. Sci. Lett. 70,

8

Quaternary International xxx (xxxx) xxx–xxx

L. Qi, et al.

Sheng, H.Y., 2007. Late Neogene Geology and Environmental Evolution in the Zoigê Basin in the Northeastern Margin of the Tibetan Plateau. Ph.D. Thesis. Chengdu University of Technology, Chengdu (in Chinese with English abstract). Sheng, H.Y., 2010. Zoigê basin loess origin in the northeast Tibet Plateau. Earth Sci. J. China Univ. Geosci. 35 (1), 62–74 (in Chinese with English abstract). Sheng, H.Y., 2008. Lithostratigaphy of late Neogene deposits in the Zoige basin. northeastern margin of the Qinghai-Tibet Plateau 43 (3), 445–472 (in Chinese with English abstract). Sheng, H.Y., Yang, X.J., Bai, X.Z., Zheng, X.W., Xiong, C.L., 2007. Geochemical element spread order in Ruoergai basin. Earth Environ. 35 (1), 79–84 (in Chinese with English abstract). Sugitani, K., Horiuchi, Y., Adachi, M., Sugisakj, R., 1996. Anomalously low Al2O3/TiO2 values for Archean cherts from the Pilbara Block, Western Australia-possible evidence for extensive chemical weathering on the early earth. Precambrian Res. 80, 49–76. Sun, J.M., 2002. Provenance of loess material and formation of loess deposits on the Chinese Loess Plateau. Earth Planet. Sci. Lett. 203 (2002), 845–859. Tan, Y.L., Qiao, Y.S., Zhao, Z.Z., Wang, Y., Qi, L., Fu, J.L., Liu, Z.X., Yao, H.T., Wang, S.B., Jiang, F.C., 2013. Geochemical characteristics of eolian deposits in the Chengdu Plain of Sichuan province and the implications for provenance. Acta Geol. Sin. 87 (6), 1712–1723. Tang, W.C., 2001. The environmental geochemical characters and studies of Ruoergai pasture. Comput. Tech. Geophys. Geochem. Explor. 23 (4), 352–356 (in Chinese). Tang, Y., Sang, L.K., Yuan, Y.M., Zhang, Y.P., Yang, Y.L., 2012. Geochemistry of Late Triassic politic rocks in the NE part of Songpan-Ganzi Basin, western China: implications for source weathering, provenance and tectonic setting. Geosci. Front. 3 (5), 647–660. Taylor, S.R., McLennan, S.M., 1985. The Continental Crust: its Composition and Evolution. Blackwell, London, pp. 57–72. Taylor, S.R., McLennan, S.M., Mcculloch, M.T., 1983. Geochemistry of loess, continental crustal composition and crustal model ages. Geochem. Cosmochim. Acta 47, 1897–1905. Wang, S.M., Xue, B., 1997. Environmental evolution of Zoigê basin since 900 ka B.P. And comparison study with Loess Plateau. Science in China (Series D) 40 (3), 329–336. Wen, X.Y., Chen, M.L., Feng, W.L., Huang, C.M., 2017. Mid-late Holocene climatic changes recorded by loess deposits in the eastern margin of the Tibetan Plateau: Implication for human migrations. Quat. Int. 441, 77–88. Yang, S.B., Qiao, Y.S., Peng, S.S., Li, C.Z., Qi, L., Han, C., Tan, Y.L., Cheng, Y., Liu, Z.X., 2017. Geochemical characteristics of eolian deposits in the northeastern margin of the Tibetan Plateau and implications for provenance and weathering intensity. Quat. Sci. 37 (1), 1–13 (in Chinese with English abstract). Yang, S.Y., Jung, H.S., Lim, D.I., Li, C.X., 2003. A review on the provenance discrimination of the Yellow Sea sediments. Earth Sci. Rev. 63 (1–2), 93–120. Yang, X.P., Liu, Y.S., Li, C.Z., Song, Y.L., Zhu, H.P., Jin, X.D., 2007. Rare earth elements of aeolian deposits in Northern China and their implications for determining the provenance of dust storms in Beijing. Geomorphology 87, 365–377. Young, G.M., Nesbitt, H.W., 1998. Processes controlling the distribution of Ti and Al in weathering profiles, siliciclastic sediments and sedimentary rocks. J. Sediment. Res. 68 (3), 448–455.

221–236. Grousset, F.E., Biscaye, P.E., 2005. Tracing dust sources and transport patterns using Sr, Nd and Pb isotopes. Chem. Geol. 222, 149–167. Gu, Z.Y., Ding, Z.L., Xiong, S.F., Liu, T.S., 1999. A seven million geochemical record from Chinese red-clay and loess-plaeosol sequence: weathering and erosion in northwestern China. Quat. Sci. 4, 357–369 (in Chinese with English abstract). Hao, Q.Z., Guo, Z.T., Qiao, Y.S., Xu, B., Oldfield, F., 2010. Geochemical evidence for the provenance of middle Pleistocene loess deposits in southern China. Quat. Sci. Rev. 29, 3317–3326. Hu, G.Y., Dong, Z.B., Lu, J.F., Yan, C.Z., 2015. The developmental trend and influencing factors of Aeolian desertification in the Zoige Basin, eastern Qinghai-Tibet Plateau. Aeolian Research 19, 275–281. Hu, G.Y., Yu, L.P., Dong, Z.B., Lu, J.F., Li, J.Y., Wang, Y.X., Lai, Z.P., 2018. Holocene aeolian activity in the Zoige basin, northeastern Tibetan plateau, China. Catena 160, 321–328. Li, J.J., 1991. The environmental effects of the uplift of the Qinghai-Xizang Plateau. Quat. Sci. Rev. 10, 479–483. Li, Y.H., 2000. A Compendium of Geochemistry—From Solar Nebula to the Human Brain. Princeton University Press, Princeton, pp. 14–17. Liang, M.Y., Guo, Z.T., Kahmann, A.J., Oldfield, F., 2009. Geochemical characteristics of the Miocene eolian deposits in China: their provenance and climate implications. Geochem. Geophys. Geosyst. 10, Q04004. https://doi.org/10.1029/2008GC002331. McLennan, S.M., 1989. Rare earth elements in sedimentary rocks: influence of provenance and sedimentary processes. Rev. Mineral. Geochem. 21 (1), 169–200. Nesbitt, H.W., Markovices, G., Price, R.C., 1980. Chemical processes affecting alkalis and alkaline earths during continental weathering. Geochem. Cosmochim. Acta 44, 1659–1666. Nesbitt, H.W., Young, G.M., 1982. Early Proterozoic climates and plate motions inferred from major element chemistry of lutites. Nature 299, 715–717. Peng, S.S., Ge, J.Y., Li, C.Z., Liu, Z.X., Qi, L., Tan, Y.L., Cheng, Y., Deng, C.L., Qiao, Y.S., 2015. Pronounced changes in atmospheric circulation and dust source area during the mid-Pleistocene as indicated by the Caotan loess-soil sequence in North China. Quat. Int. 372, 97–107. Peng, S.Z., Guo, Z.T., 2001. Geochemical indicator of original eolian grain size and implications on winter monsoon evolution. Science in China (Series D) 44 (Suppl. p.), 261–266. Qi, L., Qiao, Y.S., 2014. Geochemical characteristics of eolian deposits on the eastern margin of the Tibetan Plateau and implications for provenance. Acta Geol. Sin. 88 (3), 963–973. Qiao, Y.S., Hao, Q.Z., Peng, S.S., Wang, Y., Li, J.W., Liu, Z.X., 2011. Geochemical characteristics of the eolian deposits in southern China, and their implications for provenance and weathering intensity. Palaeogeogr. Palaeoclimatol. Palaeoecol. 308, 513–523. Revel, M., Sinko, J.A., Grousset, F.E., 1996. Sr and Nd isotopes as tracers of North Atlantic lithic particles: paleoclimatic implications. Paleoceanography 11, 95–113. She, Z.B., Ma, C.Q., Mason, R., Li, J.W., Wang, G.C., Lei, Y.H., 2006. Provenance of the Triassic Songpan-Ganzi flysch, west China. Chem. Geol. 231, 159–175. Sheldon, N.D., Tabor, N.J., 2009. Quantitative paleoenvironmental and paleoclimatic reconstruction using paleosols. Earth Sci. Rev. 95 (1–2), 1–52.

9