Geochemical and magnetic characteristics of fine-grained surface sediments in potential dust source areas: Implications for tracing the provenance of aeolian deposits and associated palaeoclimatic change in East Asia

Palaeogeography, Palaeoclimatology, Palaeoecology 323-325 (2012) 123–132

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Geochemical and magnetic characteristics of fine-grained surface sediments in potential dust source areas: Implications for tracing the provenance of aeolian deposits and associated palaeoclimatic change in East Asia Xunming Wang a, b,⁎, Dunsheng Xia c, Caixia Zhang a, Lili Lang a, Ting Hua a, Shuang Zhao c a b c

Key Laboratory of Desert and Desertification, Cold & Arid Regions Environmental & Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, China College of Resources Science and Technology, Beijing Normal University, Beijing 100875, China CAEP, MOE Key Laboratory of Western China's Environmental Systems, Lanzhou University, 730000, China

a r t i c l e

i n f o

Article history: Received 27 July 2011 Received in revised form 22 January 2012 Accepted 3 February 2012 Available online 13 February 2012 Keywords: Gobi desert Dust source area Surface sediment Rock magnetism Geochemistry

a b s t r a c t We investigated the geochemical and magnetic characteristics of fine-grained surface sediments (fractions b 63 μm) of 182 samples collected at 20 sites in potential dust source areas of arid China (along the southern border of Mongolia). We found coefficients of variation ranging from 2.8 to 35.2% for the four magnetic parameters (χlf, ARM, SIRM, and the S-ratio) in these samples, and this variation may have great significance for interpreting the magnetic characteristics of aeolian deposits far from their source regions. In addition, the results also show that chemical weathering in the potential dust source areas did not play a major role in pedogenesis and evolution of the characteristics of the fine-grained surface sediments because there were variations of only 5% in the values of the chemical index of alteration, which is usually considered to indicate the intensity of chemical weathering, versus variations of 14% in the Rb/Sr ratio. In addition, there was no significant positive correlation between the magnetic susceptibility and the Rb/Sr ratio, which shows that in the source areas, weathering caused little magnetic enhancement in the fine-grained surface sediments. Therefore, in addition to pedogenesis effects on the windblown sediments deposited far from their source regions, the geochemical and magnetic characteristics of fine-grained surface sediments in source areas exhibited complicated patterns. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The Gobi deserts of China and southern Mongolia, sometimes described as “desert pavements” or “stony deserts”, developed from alluvial fans that formed from the Upper Pleistocene to the Holocene (Vassallo et al., 2005) and from wadis (Wang et al., 2010); other Gobi landscapes formed as a result of aeolian–fluvial interactions (McFadden et al., 1987) that have occurred for at least the past 40 000 yr (Feng et al., 1998; Feng, 2001) and possibly for as long as 420 000 yr (Lü et al., 2010). These gobis have been described as “wide, shallow basins of which the smooth rocky bottom is filled with sand, silt or clay, pebbles or, more often, with gravel” (Cable and French, 1943; Cooke, 1970). Among the gobis in this region, the Ala Shan Gobi and the adjacent Southern Mongolia Gobi are believed to be the dominant sources of dust emissions in central Asia (Natsagdorj et al., 2003; Wang et al., 2006, 2008). Throughout the Quaternary, large amounts of aeolian sediments from these gobi and ⁎ Corresponding author at: Cold & Arid Regions Environmental & Engineering Research Institute, Chinese Academy of Sciences, No. 320 West Donggang Road, Lanzhou 730000, Gansu Province, China. Tel.: + 86 931 496 7491; fax: + 86 931 827 3894. E-mail address: [email protected] (X. Wang). 0031-0182/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2012.02.005

sandy deserts accumulated in the Chinese Loess Plateau (Liu, 1985; Sun et al., 2000; Sun, 2002a,b), provided a source of aeolian iron ions that regulate phytoplankton growth in some areas of the ocean (e.g., Bishop et al., 2002; Tsuda et al., 2003), and were deposited in ice cores (e.g., Bory et al., 2002). After accounting for other processes such as pedogenesis, the geochemical characteristics of these sediments that have been deposited far from their source regions have been used as proxies for past climate changes (e.g., Kukla et al., 1988; Maher, 1988; Maher and Thompson, 1991; Sun and Liu, 2000; Hao and Guo, 2005; Bloemendal et al., 2008; Sun et al., 2008). In addition, the isotope and element characteristics of the deposits and of paleosols within the Chinese Loess Plateau have been used to infer changes in the provenance of the loess and as proxies for variations of the Asian monsoons (e.g., J. Chen et al., 1999, 2006, 2007; T. Chen et al., 2005; Sun, 2005; Chavagnac et al., 2008; Rao et al., 2009; Sun and Zhu, 2010). This is especially true for the magnetic properties of the loess sediments, which were reviewed in detail by Tang et al. (2003) and Liu et al. (2007). Most of the aeolian sediments deposited far from their source regions are the fine fractions. The particle sizes of these aeolian sediments have been discussed in detail. For instance, Pye and Tsoar (1990) reported that the long-distance transport mainly comprised

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particles less than 16 μm in diameter, but Pye (1987) had previously argued that coarse quartz grains with diameters >30 μm (ranging up to 50 μm) can potentially travel 1000 km through the upper air. Zhang et al. (1999) noted that there were few coarse particles (i.e., >100 μm) in aerosol samples, and Liu (1985) suggested that the dominant materials in the loess sediments have diameters b63 μm (>4 φ units). However, the heterogeneity of the surface sediments (e.g., Hülle et al., 2010) and of the provenance epochs of the Gobi deserts (e.g., Owen et al., 1997) in this region are still poorly understood, except for a few studies of the characteristics of the surface sediments in southern Mongolia (e.g., Maher et al., 2009). In addition, although this whole area is considered to be a region with high dust emissions, the dust records maintained by the China Meteorological Administration reveal occasional shifts in the location of the main dust sources within this region. For instance, during different heavy duststorm events, the sources were sometimes located in the western regions and sometimes in the eastern regions of the gobi areas (e.g., Zhou and Zhang, 2003). Therefore, a combination of heterogeneity of the surface sediments and shifts in the dust source region can alter the particle size, mineralogy, and other chemical characteristics of aeolian sediments that are transported far from their source areas, leading to discrepancies in interpretations of the significance of deposits of aeolian sediments as proxies of past climate change. Our poor understanding of these surface sediments has hindered our ability to identify the sediments transported from different source areas, and has decreased our confidence in using these sediments as proxies of past climate changes. To deepen our understanding of the source areas, we used field surveys to obtain sediment samples from the Ala Shan Gobi, which lies in northern China along the border of southern Mongolia, and performed magnetic, element or oxide, and mineralogy analyses of the samples. We then used geographical information system (GIS) software and statistical analysis to

characterize the spatial distribution of the magnetic and geochemical characteristics of the fine-grained surface sediments. Our goal was to enhance understanding of the heterogeneity of fine-grained surface sediments in the dominant source areas, thereby providing a stronger basis for assessing the effects of this heterogeneity on the magnetic and geochemical characteristics of the sediments in deposition areas and improving our ability to draw inferences about past climate based on the dust contained in ocean and lake sediments and ice cores that have been affected by Chinese and Mongolian dust. 2. Regional environment and sampling criteria Our field surveys (40°N to 43°N, 100°E to 105°E; Fig. 1A) were conducted in the Ala Shan Gobi, an extremely arid area of western Inner Mongolia that lies adjacent to southern Mongolia. This region has one of the highest dust-emission frequencies in central Asia, and is therefore the dominant aeolian dust source in this region (Zhou and Zhang, 2003). The regional environment and surface characteristics have been described by Wang et al. (2005). From 9 to 13 June 2010, we obtained gobi surface samples at 20 sites (Fig. 1B); each sample was obtained from the top 1 cm over a surface area of 20 × 20 cm. The principal sampling criteria included the presence of a smooth and intact (sealed) surface, a range of gravel cover values among the samples, no vegetation cover, the absence of biological or physical crusts, and no sign of anthropogenic impacts; in addition, we chose samples that appeared to be visually representative of the dominant surface sediments at each site. Because we expected that the elemental composition, mineralogy, and magnetic characteristics of the surface samples would vary at each site, we therefore collected 10 samples at most sites (Sites 1–7 and 11–20), but only 3 to 5 samples at sites 8 to 10, with all samples collected within 200 m of the central sampling point at each site.

Fig. 1. (A) Aeolian geomorphology in China and Mongolia and (B) a shaded-relief digital model of the Ala Shan Plateau and locations of the sampling sites (1 to 20).

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Table 1 Descriptive statistics for the magnetic characteristics (×10− 8 m3 kg− 1 for χlf and ARM, and × 10− 5 A m2 kg− 1 for SIRM) of the fine surface sediments collected from 20 sites in China's Ala Shan Gobi (n = 182). Locations of the sampling sites are shown in Fig. 1. N

χlf ARM SIRM S-ratio

182 182 182 182

Range

Minimum

490.59 239.93 3441.76 0.26

92.33 84.56 3707.07 0.80

Maximum

582.92 324.49 4148.83 1.05

Mean Value

Std. error

291.17 188.98 2140.81 0.89

7.60 3.40 48.42 0.00

3. Sample treatment and analytical methods Each of the 3 to 10 samples collected at each site was analyzed separately so that we could calculate the magnitude of the variation among the samples at each site. The results for these samples were then averaged to provide a single mean value for each of the 20 sites, and these means were then used to calculate the spatial patterns for each parameter throughout the study area (as described at the end of this section). All samples were air-dried and the finegrained surface sediments (fractions b63 μm) were extracted by sieving. Oldfield et al. (2009) found a strong link between particle size classes and magnetic response across a wide range of depositional environments, and in natural materials (e.g., lake sediments), some magnetic characteristics were particularly sensitive to the finer

Std. deviation

Variance

102.53 45.86 653.28 0.03

10512.14 2103.44 426777.11 0.00

Skewness

Kurtosis

Value

Std. error

Value

Std. error

0.30 0.07 0.29 2.47

0.18 0.18 0.18 0.18

− 0.39 0.12 0.44 17.09

0.36 0.36 0.36 0.36

magnetite fractions and some to the coarser magnetite fractions (e.g., King et al., 1982). We nonetheless chose to analyze the fractions b63 μm in diameter because these fractions account for most of the sediments deposited in the Chinese Loess Plateau and most of the fractions collected in ice cores and lake sediments (e.g., Liu, 1985). They are also the dominant dust-sized fractions transported by near-surface winds (Pye and Tsoar, 1990). All samples were stored in sealed containers until analysis in the lab. For the magnetic analysis, we packed known weights (6.2 to 7.2 g, with an average of 6.9 g) of the samples into plastic pots. We then measured the low-frequency (0.47 kHz) and high-frequency (4.7 kHz) magnetic susceptibility (χlf and χhf, respectively) using a Bartington Instruments (Witney, Oxon, U.K.) MS2 magnetic susceptibility sensor. The difference between low- and high-frequency

Table 2 Descriptive statistics for the magnetic characteristics of the fine surface sediments collected from 20 sites in China's Ala Shan Gobi. Locations of the sampling sites are shown in Fig. 1. Sample Site 1 (n = 10) Site 2 (n = 10) Site 3 (n = 10) Site 4 (n = 10) Site 5 (n = 10) Site 6 (n = 10) Site 7 (n = 10) Site 8 (n = 5) Site 9 (n = 3) Site 10 (n = 4) Site 11 (n = 10) Site 12 (n = 10) Site 13 (n = 10) Site 14 (n = 10) Site 15 (n = 10) Site 16 (n = 10) Site 17 (n = 10) Site 18 (n = 10) Site 19 (n = 10) Site 20 (n = 10)

Std. deviation Mean Std. deviation Mean Std. deviation Mean Std. deviation Mean Std. deviation Mean Std. deviation Mean Std. deviation Mean Std. deviation Mean Std. deviation Mean Std. deviation Mean Std. deviation Mean Std. deviation Mean Std. deviation Mean Std. deviation Mean Std. deviation Mean Std. deviation Mean Std. deviation Mean Std. deviation Mean Std. deviation Mean Std. deviation Mean

χlf (×10− 8 m3 kg− 1)

ARM (×10− 8 m3 kg− 1)

SIRM (×10− 5 A m2 kg− 1)

S-ratio

27.67 162.24 40.36 274.19 51.03 267.07 25.44 380.81 52.67 382.36 94.73 296.36 40.22 191.01 25.65 142.68 17.55 148.38 10.39 157.67 56.92 354.70 15.52 297.82 27.48 384.79 36.21 515.04 36.98 273.56 31.11 330.08 29.36 318.20 49.67 337.45 16.75 161.65 16.09 193.10

8.79 112.06 15.04 154.51 22.28 201.88 8.69 206.05 14.63 210.61 27.57 190.47 16.40 128.24 13.25 125.78 4.43 115.44 3.70 103.45 19.55 190.14 6.76 169.89 12.47 208.68 15.58 291.37 15.01 189.59 14.55 213.12 10.93 212.31 22.27 218.53 14.69 235.45 9.36 167.58

182.65 1114.36 241.01 1815.67 381.14 2257.40 143.98 2466.19 286.06 2487.34 518.36 2089.51 238.17 1305.05 209.61 1133.21 106.40 1086.11 51.96 1112.78 302.39 2203.65 96.32 1995.18 162.19 2586.22 242.78 3700.75 264.02 2199.09 217.63 2559.78 217.31 2446.31 357.29 2603.17 171.23 1959.41 135.24 1836.11

0.02 0.87 0.00 0.90 0.01 0.90 0.00 0.89 0.00 0.89 0.00 0.89 0.01 0.88 0.02 0.87 0.02 0.85 0.02 0.86 0.00 0.89 0.00 0.89 0.00 0.89 0.01 0.89 0.00 0.90 0.00 0.90 0.07 0.94 0.00 0.89 0.00 0.90 0.00 0.87

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susceptibility (χlf − χhf) is expressed as a mass-specific term (χfd) and as a percentage of the low-frequency susceptibility (χfd%). Anhysteretic remanent magnetization (ARM) was imparted using a DTECH AF demagnetizer with a peak AF field of 100 mT and a DC bias field of 0.04 mT. Measurements were expressed as the ARM susceptibility (χARM) by dividing the remanence by the steady field. Saturation isothermal remanent magnetization (SIRM) at 1 T was determined using an MMPM10 pulse magnetizer (Magnetic Measurements Ltd., Aughton, U.K.). All remanence measurements were performed using a Minispin magnetometer (Molspin Ltd., Newcastle on Tyne, England). SIRM is expressed on a mass-specific basis. IRMs in the reverse fields (−20, − 100, and −300 mT) were expressed as percentages of the reverse saturation of the SIRM (S− 20, S− 100, and S− 300), and the Sratio (the ratio of the remanence remaining after DC demagnetization at 300 mT to the SIRM) was calculated. All magnetic parameters were measured at the Key Laboratory of Western China's Environmental Systems, Lanzhou University. Concentrations of 28 elements and oxides were determined at the Key Laboratory of Desert and Desertification, Chinese Academy of Sciences: P, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Ga, As, Rb, Sr, Y, Zr, Nb, Ba, La, Ce, Nd, Pb, SiO2, Al2O3, Fe2O3, MgO, CaO, Na2O, and K2O. To do so, we used a fully automated sequential wavelength-dispersive X-ray fluorescence (XRF) spectrometer (AXIOS, PANalytical B.V., Almelo, The Netherlands) equipped with a Super Sharp Tube for the Rh-anode, with the following settings: 4.0 kW, 60 kV, 160 mA, and a 75 μ UHT Be end window. We used version 5 of the company's SuperQ software for the XRF analyses. Samples were prepared as follows: After drying

the samples at 105 °C, 4-g samples were compressed (at 30-t pressure) into a 32-mm-diameter pellet and then stored in desiccators. After the XRF analyses were finished, the concentrations of the elements were calibrated using the Chinese National Standards for rock (GBW07103 and GBW07114 (GSR01 and GSR12), GBW07120 and GBW07122 (GSR13 and GSR15)), for soil (GBW07401 and GBW07408 (GSS01 and GSS8), GBW0743 and GBW07430 (GSS9 and GSS16)), and for water sediments (GBW07301a and GBW07318 (GSD01 and GSD14)). Following the manufacturer's instructions, we calibrated the equipment and found that the analytical uncertainties (relative standard deviations) were less than ±5% for most elements and oxides, except for Co, Cr, V, and Pb; the uncertainties of all measured elements and oxides are provided in Online Supporting Table S1. In addition to the magnetic and elemental analyses, we performed semi-quantitative mineralogical analysis of the samples (with relative errors of about 10%) using an X'Pert Pro Multi-Purpose X-ray Diffractometer (XRD; PANalytical) that scanned with Co Kα1 radiation at 45 kV and 50 mA, with scan angles ranging from 0° to 167° (2θ), a precision of 0.0025°, and a resolution of 0.037°. To reveal spatial trends in the magnetic parameters and the element or oxide concentrations in the study area, we used the ordinary kriging method (Lloyd and Atkinson, 2001) to interpolate between the mean values of each parameter at the 20 sampling points using GIS software. Because our sampling area only covered 3° of latitude and 5° of longitude, we only discuss the spatial trends within this small region rather than assuming that our results are representative of the larger region surrounding our study area. For our statistical

Fig. 2. Spatial trends for χlf, ARM (× 10− 8 m3 kg− 1), χfd%, and SIRM (×10− 5 A m2 kg− 1) in China's Ala Shan Gobi. Because our sampling area only covered 40°N to 43°N and 100°E to 105°E, we only discuss the spatial trends within this small region.

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Table 3 Descriptive statistics for the elements and oxides in the fine surface sediments of China's Ala Shan Gobi (n = 182). All values are expressed as mg/kg, except for those labeled with asterisks, which are expressed as %. Locations of the sampling sites are shown in Fig. 1. Element or oxide

Range

P Ti V Cr Mn Co Ni Cu Zn Ga As Rb Sr Y Zr Nb Ba La Ce Nd Pb *SiO2 *Al2O3 *Fe2O3 *MgO *CaO *Na2O *K2O

390.62 3638.31 75.73 232.68 333.03 13.39 27.73 22.89 81.74 6.12 17.85 28.95 604.43 38.86 1795.48 19.05 281.41 37.14 57.99 34.99 20.40 12.00 3.50 2.35 5.54 9.25 1.52 0.76

Min.

656.68 3831.01 103.69 122.45 617.49 13.07 36.34 18.88 52.21 9.93 10.27 66.91 218.60 25.79 448.80 12.66 461.64 35.73 49.36 29.37 3.81 44.81 9.71 4.83 2.71 4.86 1.28 1.75

Max.

1047.30 7469.32 179.42 355.13 950.52 26.46 64.07 41.77 133.95 16.05 28.12 95.86 823.03 64.65 2244.28 31.71 743.05 72.87 107.35 64.36 24.21 56.81 13.21 7.18 8.26 14.11 2.80 2.51

Mean Value

Std. error

896.95 5964.01 134.89 215.24 723.17 17.17 52.28 27.68 68.51 12.65 15.06 78.60 265.61 46.27 1305.77 24.24 547.68 54.02 74.36 48.60 12.77 54.38 11.29 6.10 4.10 7.39 1.85 2.07

5.49 52.27 1.36 3.81 3.84 0.13 0.41 0.28 0.71 0.09 0.14 0.40 5.53 0.57 31.38 0.27 3.72 0.59 0.84 0.59 0.31 0.14 0.05 0.03 0.06 0.08 0.02 0.01

analyses, we used version 18.0 of PASW Statistics (SPSS Inc., Chicago, IL, U.S.A.). In total, we analyzed 182 samples of fine-grained surface sediments obtained from 20 sites in the study area.

Table 4 Principal-components analysis for the 28 elements within the fine-grained surface sediments (n = 182). The table shows the principal components (PCs) with eigenvalues > 1. High positive values (>0.6) are shown in boldface. Element or oxide

Principal components PC1

PC2

PC3

PC4

PC5

P Ti V Cr Mn Co Ni Cu Zn Ga As Rb Sr Y Zr Nb Ba La Ce Nd Pb SiO2 Al2O3 Fe2O3 MgO CaO Na2O K2O

− 0.681 − 0.916 − 0.749 − 0.766 0.616 0.578 0.360 0.744 0.735 0.862 0.426 0.775 0.447 − 0.903 − 0.878 − 0.950 − 0.315 − 0.690 − 0.555 − 0.704 0.929 − 0.562 0.762 − 0.468 0.709 0.242 0.207 0.782

− 0.139 0.182 0.612 0.516 0.465 0.582 0.742 0.626 0.327 0.029 0.723 0.422 0.207 0.327 0.403 0.210 0.370 0.509 0.671 0.595 − 0.169 − 0.324 0.427 0.815 0.527 0.449 − 0.511 0.439

0.237 0.123 0.014 − 0.125 − 0.032 0.141 − 0.029 − 0.014 − 0.013 0.314 0.060 0.352 − 0.554 0.080 − 0.038 0.096 − 0.305 0.023 − 0.024 0.023 0.058 0.634 0.406 0.119 − 0.215 − 0.549 − 0.319 0.369

0.120 − 0.162 − 0.167 − 0.027 − 0.050 0.297 0.297 0.016 − 0.073 − 0.158 − 0.012 0.010 − 0.486 − 0.121 − 0.183 − 0.107 0.693 0.027 0.067 0.004 0.106 0.293 − 0.146 − 0.140 − 0.205 0.343 0.047 0.003

− 0.254 − 0.173 − 0.002 0.204 − 0.434 − 0.122 0.380 0.071 − 0.140 − 0.004 − 0.107 0.149 0.012 − 0.037 0.048 − 0.005 0.148 0.114 0.044 − 0.058 − 0.130 0.048 0.049 − 0.017 0.284 − 0.465 0.389 0.061

Std. deviation

Variance

74.01 705.15 18.30 51.43 51.78 1.74 5.55 3.75 9.57 1.20 1.87 5.35 74.67 7.64 423.38 3.60 50.12 7.95 11.30 7.96 4.18 1.86 0.64 0.47 0.83 1.13 0.24 0.15

5477.53 497229.99 334.72 2645.24 2680.67 3.04 30.76 14.09 91.60 1.44 3.49 28.64 5575.19 58.44 179249.33 12.93 2512.12 63.21 127.61 63.31 17.48 3.47 0.41 0.22 0.68 1.28 0.06 0.02

Skewness

Kurtosis

Value

Std. error

Value

Std. error

− 0.50 − 0.49 0.15 0.22 0.88 0.81 − 0.55 0.73 2.06 0.28 1.64 0.87 5.88 − 0.33 − 0.01 − 0.48 0.88 0.01 − 0.03 − 0.22 0.09 − 2.22 0.53 − 0.12 1.66 1.43 0.70 0.69

0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18

0.21 0.36 − 0.60 − 0.77 1.90 4.39 0.96 1.32 11.41 − 0.18 12.72 0.41 39.05 − 0.45 − 0.88 0.18 1.21 − 0.74 − 0.39 − 0.81 − 0.41 6.98 − 0.07 − 0.01 4.76 7.32 1.77 − 0.10

0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36

4. Results 4.1. Rock magnetism Table 1 summarizes the values of the magnetism parameters for the 182 samples. In our study area, the coefficient of variation ranged up to 35.2% for χlf, 24.3% for ARM, 30.5% for SIRM, and 2.8% for the Sratio. At the same site, there were also large variations in the magnetism parameters measured in the different samples. For instance, the coefficient of variation ranged up to 32.0% for χlf, 14.5% for ARM, and 24.8% for SIRM. These results show that there are large differences in the magnetic characteristics of the fine-grained surface sediments even though they were all collected within a relatively limited area and were collected using the same sampling methods. The variations at the same site may have resulted from small variations in the landforms that were not observed during our field work. In addition, the mean χlf value in the fine-grained surface sediments was 291.17 × 10 − 8 m 3 kg − 1, which was far higher than those in the loess and paleosol sections of the Chinese Loess Plateau. The individual summary statistics for each of the 20 sites (Table 2) and the spatial trends in the magnetic parameters (Fig. 2) also show large differences in the sediment magnetic properties. For instance, high values of χlf and SIRM usually appeared in the central to northcentral regions of our study area, whereas high values of χfd% and ARM appeared in the eastern third and the eastern half (respectively) of our study area. In this region, the spatial trends in χlf were highly consistent with the trends in dust emission intensity because high dust emissions usually occurred in the central part of our study area (e.g., Wang et al., 2006). 4.2. Element and oxide characteristics Table 3 summarizes statistical data on the elements and oxides. In our study area, the coefficient of variation of the concentrations of the 28 elements and oxides ranged between 3.4 and 32.7%, with a mean

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Fig. 3. Spatial trends in the contents of Ti (mg/kg) and of Fe2O3, CaO, and Al2O3 (%) within the fine-grained surface sediments at 20 sites in China's Ala Shan Gobi near southern Mongolia. Because our sampling area only covered 40°N to 43°N and 100°E to 105°E, we only discuss the spatial trends within this small region.

value of 14.1%. The elements with the highest coefficient of variation were Cr (23.9%), Sr (28.1%), Zr (32.4%), and Pb (32.7%). The oxides and element with the lowest coefficient of variation were SiO2, Al2O3, and Rb, all with coefficients of variation of less than or equal to 6.8%. In addition, there were different correlations among the contents of the elements and oxides for the 182 samples from the 20 sites (Online Supporting Table S2). Our principal-components analysis showed that five principal components (PCs) with eigenvalues > 1 could be extracted from the data, and that these PCs explained 85.7% of the total variance (Table 4). If we consider only loadings with magnitudes greater than 0.60 (i.e., elements or oxides that contributed most strongly to each PC), PC1 included Mn, Cu, Zn, Ga, Rb, Pb, Al2O3, MgO, and K2O with positive values, which shows that these elements and oxides may have identical sources (i.e., they may come from the same or similar minerals). For PC2, values that met this criterion were for V, Ni, Cu, As, Ce, and Fe2O3. For PC3 and PC4, SiO2 and Ba, respectively, met this criterion. PC5 contained no loadings greater than 0.60, which indicates that the contributions of the elements and oxides to this component are very complicated. Our analysis of the spatial trends (Fig. 3) shows that concentrations of Fe2O3 and Ti were high in the central regions of our study area, whereas the concentrations of the carbonates (for which the main fractions are indicated by the CaO content) were highest in the western part of the study area. There were no clear spatial trends for Al2O3.

5. Discussion In response to the uplift of the Qinghai-Tibetan Plateau and the associated changes in Asian atmospheric circulation patterns (e.g., An et al., 2001; Guo et al., 2002), the fine-grained surface sediments of the Tarim Basin and of the northeastern Qinghai-Tibet Plateau (e.g., Honda et al., 2004; Stevens et al., 2010; Pullen et al., 2011) and the sediments of the gobi and sandy deserts of arid northern China and southern Mongolia have been transported to the south and east by the prevailing northwest wind (Fig. 1A). Many of these sediments have accumulated in the Chinese Loess Plateau, forming the largest aeolian deposit in Asia. Despite the significance of pedogenesis and other controls on changes in the geochemical characteristics of the deposited aeolian sediments, the heterogeneities of the fine-grained surface sediments in dust source areas play a major role in determining variations of the characteristics of the deposited sediments. For instance, the observed differences in the spatial trends of magnetic characteristics of the surface materials may have resulted from variations in the mineral assemblages, which were represented in this study by variations in the element and oxide concentrations. In our study area, three of the magnetic parameters of the fine-grained surface sediments (χlf, ARM, and SIRM) appear to be strongly positively correlated with the Ti, V, Cr, Y, Zr, Nb, La, Ce, Nd, and Fe2O3 contents, and strongly negatively correlated with the Ga, Rb, and Pb contents (Table 5). However, there were no significant correlations between

X. Wang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 323-325 (2012) 123–132 Table 5 Values of Pearson's correlation coefficient between the magnetic parameters and the element and oxide contents of the fine-grained surface sediments collected from 20 sites in China's Ala Shan Gobi (n=182). High correlations (b−0.6 and >0.6) are shown in boldface. Element or oxide

χlf

ARM

SIRM

S-ratio

P Ti V Cr Mn Co Ni Cu Zn Ga As Rb Sr Y Zr Nb Ba La Ce Nd Pb SiO2 Al2O3 Fe2O3 MgO CaO Na2O K2O

0.38 0.82 0.95 0.92 − 0.27 − 0.25 0.05 − 0.29 − 0.38 − 0.70 − 0.02 − 0.47 − 0.21 0.88 0.94 0.87 0.38 0.77 0.71 0.81 − 0.85 0.22 − 0.42 0.81 − 0.26 0.01 − 0.34 − 0.44

0.60 0.88 0.75 0.64 − 0.26 − 0.31 − 0.28 − 0.46 − 0.38 − 0.50 − 0.13 − 0.56 − 0.20 0.72 0.73 0.81 0.10 0.52 0.42 0.56 − 0.73 0.31 − 0.39 0.67 − 0.43 − 0.23 − 0.28 − 0.49

0.52 0.92 0.88 0.80 − 0.30 − 0.38 − 0.22 − 0.47 − 0.44 − 0.65 − 0.14 − 0.60 − 0.23 0.85 0.89 0.90 0.18 0.64 0.55 0.70 − 0.85 0.29 − 0.46 0.73 − 0.42 − 0.16 − 0.30 − 0.53

0.29 0.41 0.34 0.28 − 0.05 − 0.12 − 0.17 − 0.16 − 0.14 − 0.14 − 0.02 − 0.21 0.03 0.32 0.30 0.35 0.13 0.22 0.14 0.24 − 0.25 0.15 − 0.13 0.29 − 0.22 − 0.01 − 0.21 − 0.16

the element or oxide contents and the S-ratio. The magnetic susceptibility had correlations (R 2) ranging from 0.05 to 0.72 with six representative elements and oxides (Fig. 4). In addition, we found strong correlations between the Fe2O3 content and the magnetic susceptibility in a band stretching from the northwest to the center of our study area (Fig. 5), and this is consistent with the regions from which high dust emissions have been reported in this region (e.g., Zhou and Zhang, 2003; Wang et al., 2006). However, the reasons for these correlations are unclear. Rb is usually enriched in K-rich minerals such as muscovite, biotite, and potassium feldspar, and Sr may be enriched in Ca-bearing minerals and silicate fractions such as limestone, hornblende, albite, calcite, clinochlore, amphibole, orthoclase, illite, and muscovite (e.g., Nesbitt and Wilson, 1992). In our study area, the XRD analysis of the fine-grained surface sediments revealed that the contents of Cabearing minerals was 9% for albite, 12% for clinochlore, 6% for amphibole, 9% for orthoclase, 7% for illite, 6% for muscovite, and 3% for dolomite. Variations in the contents of the K-rich and Ca-bearing minerals may play major roles in determining the Rb/Sr ratio, which is used as an indicator of the degree of pedogenesis, which in turn serves as an index of the degree of chemical weathering (Retallack, 1994; Gallet et al., 1996) and as a proxy for variations in the East Asia summer monsoon (Chen et al., 1999), even though a study suggested that the pedogenesis information documented by the Rb/Sr ratio did not agree with the soil morphology (Hao and Guo, 2001). In the areas of extremely arid China and southern Mongolia that we studied, which have high dust emissions, the Rb/Sr ratios in the fine-grained surface sediments averaged 0.31, with a standard deviation of 0.04 (n = 182), and there were no obvious spatial trends in our study area (Fig. 6). In contrast, the Rb/Sr ratios in the Loess Plateau ranged between 0.4 and 1.2 (Ji et al., 2001), so the low Rb/Sr ratios in the source regions suggest that after deposition of these windblown sediments far from their source areas, pedogenesis and chemical weathering strongly affect the deposited aeolian sediments, leading to enrichment in Rb or depletion of Sr. These post-deposition changes may serve as proxies of climate change, such as variations in the East Asia summer

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monsoon. However, the scatterplots of magnetic susceptibility as a function of the Rb/Sr ratios in our study area showed no significant positive correlations (Fig. 7). These results suggest that although pedogenesis may have increased magnetic susceptibility in depositional areas (e.g., Maher and Thompson, 1991; Banerjee and Hunt, 1993; Vidic et al., 2004), weathering is not a primary control on the magnetic and geochemical characteristics of the fine-grained surface sediments in the source regions, unlike in the depositional areas. After fine-grained surface sediments have been transported by winds far from their source regions, significant changes occur in the magnetic characteristics of these deposits within timescales of a few hundred years, during the process of soil formation (e.g., Maher et al., 2003). These changes are controlled by variations in the regional temperature and rainfall regimes, and by the contributions of magnetotactic bacteria (e.g., Bloemendal et al., 1988). For instance, in our study area (the proposed source area for the sediments deposited in the Chinese Loess Plateau), the coefficient of variation ranged up to 35.2% for magnetic susceptibility (χlf) and 2.8% for the S-ratio. In contrast, the deposits in the Loess Plateau have coefficients of variation of up to 57.7% (Song et al., 2007), 60.8 to 68.2% (Guo et al., 2009), 22.4 to 35.9% (Song et al., 2010), and 25.5 to 30.5% (Sun et al., 2010) for the magnetic susceptibility. For the S-ratio, the coefficient of variation ranged up to 12.0% (Kukla and An, 1989) and up to 2.2% (Bloemendal and Liu, 2005). These results suggest that the heterogeneities of the fine-grained surface sediments in dust source areas may affect the degree of variation in the aeolian deposits far from the source regions, and these variations will have significant consequences when using these deposits to infer post-depositional processes when the deposits are used as proxies of past environmental changes. In addition, the degree of chemical weathering can be quantified using the chemical index of alteration (CIA), which equals the Al2O3 content divided by the sum of Al2O3 + CaO* + Na2O + K2O (Nesbitt and Young, 1982) or the CaO + MgO + Na2O content divided by the TiO2 content (Ding and Ding, 2003; Yang et al., 2006). High values of CIA indicate strong chemical alteration. Here, we used the CIA index of Nesbitt and Young (1982) to quantify the intensity of the chemical weathering in our study area, with CaO* values replaced with the total concentration in the fine-grained surface sediments. There was no significant correlation between the Rb/Sr ratio and the CIA. Our results show that although there were some exceptions, most CIA values ranged between 45 and 55, and there was an obvious pattern of increasing CIA moving from west to east (and to a lesser extent, from north to south) in this region (Fig. 8). The results also indicate that in extremely arid areas with high dust emissions, chemical weathering may not play a major role in the formation and evolution of the fine-grained surface sediments, and that physical weathering (freezing and thawing, thermal weathering, and salt weathering) and other processes such as aeolian abrasion (Wang et al., 2011) may be the dominant contributors to the evolution of the finegrained fractions in this region. The variations of CIA and similar indices in the Loess Plateau and in paleosols far from the dust source regions may therefore provide clues to regional environmental changes. 6. Conclusions Despite magnetic susceptibility enhancements as a result of pedogenesis where the aeolian sediments are deposited, there was high variation in the magnetic parameters of the source sediments (coefficients of variation of 35.2% for χlf, 24.3% for ARM, 30.5% for SIRM, and 2.8% for the S-ratio) in the Ala Shan Gobi, an area with high dust emissions. In this region, the heterogeneities of the magnetic characteristics in the fine-grained surface sediments were closely related to the parent materials present in these fractions, and reflected the assemblages of elements and minerals in the parent materials. Areas with high magnetic susceptibility usually appeared toward the center of our study area,

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Fig. 4. Scatterplots of magnetic susceptibility as a function of the concentration of selected elements or oxides in the fine-grained surface sediments in China's Ala Shan Gobi. Due to significant differences in the range of concentrations of the elements and oxides, different axis scales were used.

whereas high ARM values usually appeared in the western regions. Although these regions are all areas with high dust emission, variations in atmospheric circulation during different duststorm events may result in changes in the main source area for dust emission, and this would have complicated the observed patterns of geochemical and magnetic characteristics in the fine-grained surface sediments that are deposited far from their source regions. Our results show that in extremely arid regions of China and Mongolia with high dust emission, chemical weathering may not play a major role in pedogenesis and the evolution of fine-grained surface

sediments because the variation in the value of CIA, which is usually considered to be an index of the intensity of chemical weathering, only equaled 5%, versus 14% for the Rb/Sr ratio. In addition, our results show that in our study area, there were no significant positive correlations between the magnetic susceptibility and the Rb/Sr ratio. Therefore, in addition to the effects of pedogenesis on the geochemical and magnetic characteristics of the windblown sediments after they are deposited far from their source regions, these characteristics exhibited complicated patterns in the source region that should not be ignored when interpreting the significance of aeolian sediments

X. Wang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 323-325 (2012) 123–132

Fig. 5. Spatial trends in Pearson's correlation coefficient between the magnetic susceptibility and the Fe2O3 content. Because our sampling area only covered 40°N to 43°N and 100°E to 105°E, we only discuss the spatial trends within this small region.

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Fig. 8. Spatial trends in Nesbitt and Young's (1982) chemical index of alteration (CIA) in China's Ala Shan Gobi.

as proxies of past climate changes. Finally, in a wide range of depositional environments, there are strong links between particle size classes and magnetic responses of the sediments (e.g., Oldfield et al., 2009; Maher, 2011). Because the finest fractions (i.e., b16 μm in diameter) are transported for longer distances than coarser fractions, variations in the relative proportions of fine and coarse particle classes will create variations in the magnetic responses and other characteristics of the deposited sediments. It will therefore be necessary to better understand these variations in future studies. Acknowledgments This work was supported by a grant from the National Natural Science Foundation of China (40871012). Special thanks are given to the three referees and the editor for their critical comments on an early draft of this manuscript. We thank Mr. Geoff Hart for his detailed edits of the manuscript. Appendix A. Supplementary data Fig. 6. Spatial trends in the Rb/Sr ratios of the fine-grained surface sediments in China's Ala Shan Gobi. Because our sampling area only covered 40°N to 43°N and 100°E to 105°E, we only discuss the spatial trends within this small region.

Supplementary data to this article can be found online at doi:10. 1016/j.palaeo.2012.02.005. References

Fig. 7. Scatterplots of magnetic susceptibility as a function of the Rb/Sr ratio in samples of the fine-grained surface sediments in China's Ala Shan Gobi.

An, Z.S., Kutzbach, J.E., Prell, W.L., Porter, S.C., 2001. Evolution of Asian monsoons and phased uplift of the Himalaya–Tibetan plateau since Late Miocene times. Nature 411, 62–66. Banerjee, S.K., Hunt, C.P., 1993. Separation of local signals from the regional paleomonsoon record of the Chinese Loess Plateau: a rockmagnetic approach. Geophysical Research Letters 20, 843–846. Bishop, J.K.B., Davis, R.E., Sherman, J.T., 2002. Robotic observations of dust storm enhancement of carbon biomass in the North Pacific. Science 298, 817–821. Bloemendal, J., Liu, X., 2005. Rock magnetism and geochemistry of two plio-pleistocene Chinese loess-palaeosol sequences—implications for quantitative palaeoprecipitation reconstruction. Palaeogeography, Palaeoclimatology, Palaeoecology 226, 149–166. Bloemendal, J., Lamb, B., King, J., 1988. Paleoenvironmental implications of rockmagnetic properties of Late Quaternary sediment cores from the eastern equatorial Atlantic. Palaeoceanography 3, 61–87. Bloemendal, J., Liu, X., Sun, Y., Li, N., 2008. An assessment of magnetic and geochemical indicators of weathering and pedogenesis at two contrasting sites on the Chinese Loess plateau. Palaeogeography, Palaeoclimatology, Palaeoecology 257, 152–168. Bory, A.J.-M., Biscaye, P.E., Svensson, A., Grousset, F.E., 2002. Seasonal variability in the origin of recent atmospheric mineral dust at NorthGRIP Greenland. Earth and Planetary Science Letters 196, 123–134. Cable, M., French, F., 1943. The Gobi Desert. Hodder and Stoughton, London. Chavagnac, V., Lair, M., Milton, J.A., Lloyd, A., Croudace, I.W., Palmer, M.R., Green, D.R.H., Cherkashev, G.A., 2008. Tracing dust input to the Mid-Atlantic Ridge between 14°45′N and 36°14′N: geochemical and Sr isotope study. Marine Geology 247, 208–225.

132

X. Wang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 323-325 (2012) 123–132

Chen, J., An, Z.S., Head, J., 1999. Variation of Rb/Sr ratios in the loess-paleosol sequences of central China during the last 130,000 years and their implications for monsoon paleoclimatology. Quaternary Research 51, 215–219. Chen, T., Xu, H., Xie, Q., Chen, J., Ji, J., Lu, H., 2005. Characteristics and genesis of maghemite in Chinese loess and paleosols: mechanism for magnetic susceptibility enhancement in paleosols. Earth and Planetary Science Letters 240, 790–802. Chen, J., Chen, Y., Liu, L.W., Ji, J.F., Balsam, W., Sun, Y.B., Lu, H.Y., 2006. Zr/Rb ratio in the Chinese loess sequences and its implication for changes in the East Asian winter monsoon strength. Geochimica et Cosmochimica Acta 70, 1471–1482. Chen, J., Li, G., Yang, J., Rao, W., Lu, H., Balsam, W., Sun, Y., Ji, J., 2007. Nd and Sr isotopic characteristics of Chinese deserts: implications for the provenances of Asian dust. Geochimica et Cosmochimica Acta 71, 3904–3914. Cooke, R.U., 1970. Stone pavement in deserts. Annals of the Association of American Geographers 60, 560–577. Ding, F., Ding, Z., 2003. Chemical weathering history of Southern Tajikistan loess and paleoclimate implications. Science in China (Series D) 33, 505–512. Feng, Z.D., 2001. Gobi dynamics in the Northern Mongolian Plateau during the past 20,000 yr: preliminary results. Quaternary International 76 (77), 77–83. Feng, Z.D., Chen, F.H., Tang, L.Y., Kang, J.C., 1998. East Asian monsoon climates and Gobi dynamics in marine isotope stages 4 and 3. Catena 33, 29–46. Gallet, S., Jahn, B.M., Torii, M., 1996. Geochemical characterization of the Luochuan loess-paleosol sequence, China, and paleoclimatic implications. Chemical Geology 133, 67–88. Guo, Z.T., Ruddiman, W.F., Hao, Q.Z., Wu, H.B., Qiao, Y.S., Zhu, R.X., Peng, S.Z., Wei, J.J., Yuan, B.Y., Liu, T.S., 2002. Onset of Asian desertification by 22 Myr ago inferred from loess deposits in China. Nature 416, 159–163. Guo, Z.T., Berger, A., Yin, Q.Z., Qin, L., 2009. Strong asymmetry of hemispheric climates during MIS-13 inferred from correlating China loess and Antarctica ice records. Climate of the Past 5, 21–31. Hao, Q.Z., Guo, Z.T., 2001. Quantitative measurements on the paleoweathering intensity of the loess-soil sequences and implication on paleomonsoon. Science in China (Series D) 44 (6), 565–576. Hao, Q.Z., Guo, Z.T., 2005. Spatial variations of magnetic susceptibility of Chinese loess for the last 600 kyr: implications for monsoon evolution. Journal of Geophysical Research (Solid Earth) 110, B12101. Honda, M., Yabuki, S., Shimizu, H., 2004. Geochemical and isotopic studies of aeolian sediments in China. Sedimentology 51, 211–230. Hülle, D., Hilgers, A., Radtke, U., Stolz, C., Hempelmann, N., Grunert, J., Felauer, T., Lehmkuhl, F., 2010. OSL dating of sediments from the Gobi Desert, Southern Mongolia. Quaternary Geochronology 5, 107–113. Ji, J.F., Balsam, W., Chen, J., 2001. Mineralogic and climatic interpretations of the Luochuan loess section (China) based on diffuse reflectance spectrophotometry. Quaternary Research 56, 23–30. King, J., Banerjee, S.K., Marvin, J., Özdemir, Ö., 1982. A comparison of different magnetic methods for determining the relative grain size of magnetite in natural materials: some results from lake sediments. Earth and Planetary Science Letters 59, 404–419. Kukla, G., An, Z.S., 1989. Loess stratigraphy in central China. Palaeogeography, Palaeoclimatology, Palaeoecology 72, 203–225. Kukla, G., Heller, F., Liu, X.M., Xu, T.C., Liu, T.S., An, Z.S., 1988. Pleistocene climates in China dated by magnetic susceptibility. Geology 16, 811–814. Liu, T., 1985. Loess and Environments. China Ocean Press, Beijing, p. 251. Liu, Q.S., Deng, C.L., Torrent, J., Zhu, R.X., 2007. Review of recent developments in mineral magnetism of the Chinese loess. Quaternary Science Reviews 26, 368–385. Lloyd, C.D., Atkinson, P.M., 2001. Assessing uncertainty in estimates with ordinary and indicator kriging. Computers & Geosciences 27, 929–937. Lü, Y.W., Gu, Z.Y., Aldahan, A., Zhang, H., Göran, P., Lei, G., 2010. 10Be in quartz gravel from the Gobi Desert and evolutionary history of alluvial sedimentation in the Ejina Basin, Inner Mongolia, China. Chinese Science Bulletin 55 (33), 3802–3809. Maher, B.A., 1988. Magnetic properties of some synthetic sub-micron magnetites. Journal of Geophysical Research 94, 83–96. Maher, B.A., 2011. The magnetic properties of Quaternary aeolian dusts and sediments, and their palaeoclimatic significance. Aeolian Research 3, 87–144. Maher, B.A., Thompson, R., 1991. Mineral magnetic record of the Chinese loess and paleosols. Geology 19, 3–6. Maher, B.A., Alekseev, A., Alekseeva, T., 2003. Magnetic mineralogy of soils across the Russian Steppe: climatic dependence of pedogenic magnetite formation. Palaeogeography, Palaeoclimatology, Palaeoecology 201, 321–341. Maher, B.A., Mutch, T.J., Cunningham, D., 2009. Implications for provenance of the Chinese Loess Plateau. Magnetic and geochemical characteristics of Gobi Desert surface sediments. Geology 37, 279–282. McFadden, L.D., Wells, S.G., Jercinovich, M.J., 1987. Influences of eolian and pedogenic processes on the origin and evolution of desert pavements. Geology 15, 504–508. Natsagdorj, L., Jugder, D., Chung, Y.S., 2003. Analysis of dust storms observed in Mongolia during 1937–1999. Atmospheric Environment 37, 1401–1411. Nesbitt, H.W., Wilson, R.E., 1992. Recent chemical weathering of basalts. American Journal of Science 29, 774–777. Nesbitt, H.W., Young, G.M., 1982. Early Proterozoic climates and plate motions inferred from major element chemistry of lutites. Nature 299, 715–717. Oldfield, F., Hao, Q., Bloemendal, J., Gibbs-Eggar, Z., Patil, S., Guo, Z., 2009. Links between bulk sediment particle size and magnetic grain-size: general observations and implications for Chinese loess studies. Sedimentology 56, 2091–2106.

Owen, L.A., Windley, B.F., Cunningham, W.D., Badamgarav, J., Dorjnamjaa, D., 1997. Quaternary alluvial fans in the Gobi of southern Mongolia: evidence for neotectonics and climate change. Journal of Quaternary Science 12, 239–252. Pullen, A., Kapp, P., McCallister, A.T., Chang, H., Gehrels, G.E., Garzione, C.N., Heermance, R.V., Ding, L., 2011. Qaidam Basin and northern Tibetan Plateau as dust sources for the Chinese Loess Plateau and paleoclimatic implications. Geology 39, 1031–1034. Pye, K., 1987. Aeolian Dust and Dust Deposits. Academic Press, London. Pye, K., Tsoar, H., 1990. Aeolian Sand and Sand Dunes. Unwin Hyman, London. Rao, W., Chen, J., Yang, J., Ji, J., Li, G., 2009. Sr–Nd isotopic characteristics of different particle-size fractions of eolian sands in the deserts of northern China. Geological Journal of China Universities 15 (2), 159–164 (in Chinese). Retallack, G.J., 1994. A pedotype approach to latest Cretaceous and earliest Tertiary paleosols in eastern Montana. Geological Society of America Bulletin 106, 1377–1397. Song, Y., Fang, X., Torii, M., Ishikawa, N., Li, J., An, Z., 2007. Late Neogene rock magnetic record of climatic variation from Chinese eolian sediments related to uplift of the Tibetan Plateau. Journal of Asian Earth Sciences 30, 324–332. Song, Y., Shi, Z., Fang, X., Nie, J., Ishikawa, N., Qiang, X., Wang, X., 2010. Loess magnetic properties in the Ili Basin and their correlation with the Chinese Loess Plateau. Science in China (Earth Sciences) 53, 419–431. Stevens, T., Palk, C., Carter, A., Lu, H,., Clift, P.D., 2010. Assessing the provenance of loess and desert sediments in northern China using U–Pb dating and morphology of detrital zircons. Geological Society of America Bulletin 122, 1331–1344. Sun, J.M., 2002a. Provenance of loess material and formation of loess deposits on the Chinese Loess Plateau. Earth and Planetary Science Letters 203, 845–859. Sun, J.M., 2002b. Source regions and formation of the loess sediments on the high mountain regions of Northwestern China. Quaternary Research 58, 341–351. Sun, J.M., 2005. Nd and Sr isotopic variations in Chinese eolian deposits during the past 8 Ma: implications for provenance change. Earth and Planetary Science Letters 240, 454–466. Sun, J.M., Liu, T.S., 2000. Multiple origins and interpretations of the magnetic susceptibility signal in Chinese wind-blown sediments. Earth and Planetary Science Letters 180, 287–296. Sun, J.M., Zhu, Z., 2010. Temporal variations in Pb isotopes and trace element concentrations within Chinese eolian deposits during the past 8 Ma: implications for provenance change. Earth and Planetary Science Letters 290, 438–447. Sun, J.M., Liu, T., Lei, Z., 2000. Sources of heavy dust fall in Beijing, China on April 16, 1998. Geographical Research Letters 27, 2105–2108. Sun, Y.B., Tada, R., Chen, J., Liu, Q.S., Shin, T., Tani, A., Ji, J., Yuko, I., 2008. Tracing the provenance of fine-grained dust deposited on the central Chinese Loess Plateau. Geophysical Research Letters 35, L01804. Sun, Y.B., Wang, X., Liu, Q., Clemens, S.C., 2010. Impacts of post-depositional processes on rapid monsoon signals recorded by the last glacial loess deposits of northern China. Earth and Planetary Science Letters 289, 171–179. Tang, Y.J., Jia, J.Y., Xie, X.D., 2003. Records of magnetic properties in Quaternary loess and its paleoclimatic significance: a brief review. Quaternary International 108, 33–50. Tsuda, A., Takeda, S., Saito, H., Nishioka, J., Nojiri, Y., Kudo, I., Kiyosawa, H., Shiomoto, A., Imai, K., Ono, T., Shimamoto, A., Tsumune, D., Yoshimura, T., Aono, T., Hinuma, A., Kinugasa, M., Suzuki, K., Sohrin, Y., Noiri, Y., Tani, H., Deguchi, Y., Tsurushima, N., Ogawa, H., Fukami, K., Kuma, K., Saino, T., 2003. A mesoscale iron enrichment in the western Subarctic Pacific induces a large centric diatom bloom. Science 300, 958–961. Vassallo, R., Ritz, J.F., Braucher, R., Carretier, S., 2005. Dating faulted alluvial fans with cosmogenic 10Be in the Gurvan Bogd mountain range (Gobi-Altay, Mongolia): Climatic and tectonic implications. Terra Nova 17, 278–285. Vidic, N.J., Singer, M.J., Verosub, K.L., 2004. Duration dependence of magnetic susceptibility enhancement in the Chinese loess-palaeosols of the past 620 ky. Palaeogeography, Palaeoclimatology, Palaeoecology 211, 271–288. Wang, X., Dong, Z., Yan, P., Yang, Z., Hu, Z., 2005. Surface sample collection and dust source analysis in northwestern China. Catena 59, 35–53. Wang, X., Zhou, Z., Dong, Z., 2006. Control of dust emissions by geomorphic conditions, wind environments and land use in northern China: an examination based on dust storm frequency from 1960 to 2003. Geomorphology 29, 292–308. Wang, X., Xia, D., Wang, T., Xie, X., Li, J., 2008. Dust sources in arid and semiarid China and southern Mongolia: impacts of geomorphologic setting and surface materials. Geomorphology 97, 583–600. Wang, X., Zhang, C., Zhang, J., Hua, T., Lang, L., Zhang, X., Wang, L., 2010. Nebkha formation: implications for reconstructing environmental changes over the past several centuries in the Ala Shan Plateau, China. Palaeogeography, Palaeoclimatology, Palaeoecology 297, 697–706. Wang, X., Zhang, C., Wang, H., Qian, G., Luo, W., Lu, J., Wang, L., 2011. The significance of Gobi desert surfaces for dust emissions in China: an experimental study. Environmental Earth Sciences. doi:10.1007/s12665-011-0922-2. Yang, S.L., Ding, F., Ding, Z.L., 2006. Pleistocene chemical weathering history of Asian arid and semiarid regions recorded in loess deposits of China and Tajikistan. Geochimica et Cosmochimica Acta 70, 1695–1709. Zhang, X., Arimoto, R., An, Z., 1999. Glacial and interglacial patterns for Asian dust transport. Quaternary Science Reviews 18, 811–819. Zhou, Z., Zhang, G., 2003. Heavy dust events in North China, 1954–2002. Chinese Science Bulletin 48, 1224–1228.