ARTICLE IN PRESS

Quaternary International 175 (2007) 71–85

Provenance of aeolian sediment in the Taklamakan of western , inferred from REE and major-elemental data

Xiaoping YangÃ, Bingqi Zhu, Paul D. White

Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China

Available online 24 March 2007

Abstract

To identify the provenance of aeolian deposits in the extensive field in the is of great importance for understanding the formation of this, the largest sea in China. The opinions from earlier studies are quite dipolar and are as follows: (a) local origin of the dune in different parts of the desert on the basis of various heavy mineral assemblages; and (b) strong homogenization of the sands in the entire Taklamakan on the basis of geochemical data from whole-rock samples. By separately examining the REE characteristics and major-elemental composition in coarse and fine fractions of the samples from aeolian deposits, this paper provides new data for interpreting sources of aeolian deposits in the Taklamakan. The sampling sites are distributed in four different fluvial systems, i.e. the areas of the Keriya River, Niya River and Cele River in the southern part and the northern margin near the (Fig. 1). Our results show that there are some significant differences in concentrations of trace elements and in REE features between the coarse and the fine fractions of the aeolian sediment. The major-elemental and REE data suggest that the coarser sand is different from area to area in the desert rather than a homogenization of the entire basin. The fine fractions (mainly silts) are more homogenized. The regional difference of coarse fractions in the study areas is consistent with the fluvial and wind systems in the basin. This confirms that the sands are often mixed between the northern and southern parts but there is much less mixing along the east–west direction. It should be emphasized that not only glacial and aeolian processes but also fluvial and lacustrine processes have jointly contributed to the formation of the huge sand sea. In addition, little variation is found between old and young sands in two sediment sequences in the central part of the desert, indicating consistency of sand sources in a given site during the last 40 ka. r 2007 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction aeolian dust from the Asian is a major source of pelagic sediment in the western Pacific (Rea et al., 1998) To understand the formation of a landscape it is of great and an important part of the sediment in the central North importance to investigate the sediment sources, particularly Pacific (Kyte et al., 1993). In addition, the dust significantly in sand sea environments. Determining the sources of affects the chemical composition of seawater in the western aeolian sediment in the Taklamakan Desert, western China Pacific Ocean. It is recognized that the rare earth elements is crucial not only to the understanding of the formation of (REE) patterns of western Pacific Ocean seawater in the the sand sea but also to develop a better understanding of areas near show close affinities to Chinese loess past and present global climate systems, because the (Greaves et al., 1999). In this context knowledge about the Taklamakan Desert is a large source for global dust sources of aeolian sediment in the Taklamakan has global production (Zhang et al., 2003). The mineral dust that is significance. being deposited in at recent times as well as Based on the major-elemental and mineral compositions during the Last Glacial period was recognized to be mainly and grain size distributions of the sand samples from from the Taklamakan and from the of Inner southwestern part of the desert, Honda and Shimizu (1998) Mongolia (Svensson et al., 2000; Bory et al., 2003). The reconfirmed that the sand from the Taklamakan has higher feldspar/quartz and calcite/quartz ratios, finer grain sizes ÃCorresponding author. Tel.: +86 10 62008389; fax: +86 10 62032495. and less rounding than most desert sands (Zhu et al., 1981; E-mail addresses: [email protected], [email protected] Besler, 1991; Yang, 1991). However, the opinions about (X. Yang). sources and transport histories of the sands are divided

1040-6182/$ - see front matter r 2007 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2007.03.005 ARTICLE IN PRESS 72 X. Yang et al. / Quaternary International 175 (2007) 71–85 between (a) various sources of sand in each part of the the basin (Li and Zhao, 1964). Deep incision takes place desert because of different heavy mineral assemblages (Zhu along all rivers in foreland desert margins owing to et al., 1981; Yang, 1991, 2002) and (b) homogenized sands the uplifting of the surrounding mountains. During the in the entire desert because of consistent major-elemental Quaternary, the Keriya River incised 117 m into the and isotopic features in whole-rock samples (Honda and forelands of ( Expedition Shimizu, 1998; Hattori et al., 2003). Based on the Team of Academia Sinica, 1978). Extensive sand investigation of trace elements, in particular REE, and with various forms occur in the Taklamakan, known in major-elemental compositions in two grain size fractions of local folklore as a place for journey without coming back a larger number of samples from a wide geographical area due to its vast area of active dunes. However, the sand sea of the Taklamakan, this paper intends to provide new data is divided by various rivers originating from surrounding for deciphering the sources of aeolian sediment with spatial mountains. In the central and south Taklamakan, the and temporal considerations. rivers originating from the Kunlun Mountains flow mainly Trace elements including REE and major-elemental northwards. Large oases in the desert margins and long compositions have become robust techniques for examin- green belts along the river courses crossing the desert ing the sediment sources in deserts (Muhs et al., 1995; enable millions of people to live in the basin under an Pease et al., 1998; Honda and Shimizu, 1998; Wolfe et al., extremely arid climate. Due to their sensitive nature, lakes 2000; Pease and Tchakerian, 2002, 2003; Zimbelman and and river courses have undergone large-scale changes in Williams, 2002; Hattori et al., 2003). REE are useful tools response to climate fluctuations and human activities since for investigating sediment characteristics and provenance the Late Quaternary (Yang, 1991; Yang et al., 2002, because they are among the least soluble of the trace 2006a). elements and are less mobile during weathering than many This paper deals with desert areas along the Keriya other trace elements (Taylor and McLennan, 1985; River, Niya River and Cele River as well as the Tarim Rollinson, 1993), and also because the changes of River (Fig. 1). At present these rivers with their headwaters REE characteristics are mainly controlled by provenance located in Kunlun Mountains (Keriya, Niya and Cele) dry rather than diagenesis (Taylor and McLennan, 1985). In up in the Taklamakan, whereas the Keriya flows into the the case of aeolian sands, the difference in REE character- desert much further than the Niya and Cele rivers. The istics is largely controlled by compositions in minerals Tarim River, the longest River in the , has because certain REE occur only in specific minerals. headwaters in the western Kunlun Mountains and Tian- shan Mountains and flows eastwards on the north margin 2. Regional setting of the desert along a tectonic graben. The discharge of all these rivers is largely dependant on melting glaciers and The Taklamakan Desert lies in the Tarim Basin of snow in the mountainous of upper reaches (Yang southern Xinjiang, with an area of 337,000 km2 being the et al., 2006b). largest sand sea in China (Zhu et al., 1980, 1981). Like desert research elsewhere (Goudie, 2002), the Unconsolidated sediments generally reach a thickness of classification of dune types in the Taklamakan is based 500–600 m, occasionally over 900 m in the south margin of on the surface appearance and their relationship to wind

Fig. 1. Map of the study area showing the location of sampling sites. The right map is a zoomed view of the square marked in the left map. ARTICLE IN PRESS X. Yang et al. / Quaternary International 175 (2007) 71–85 73 directions (Zhu et al., 1981). The types of dunes in the plate; (d) samples were finally treated with 2 ml (1:1) HNO3 Taklamakan are quite diversified, including longitudinal and at this stage no residue was detected. The solutions dunes, barchains, barchan chains, pyramid dunes, dome- were finally transferred to clean flasks and were diluted shaped dunes, etc. (Institute of Desert Research of with 1% HNO3 to 50 ml. Sample preparation was Academia Sinica, 1980; Zhu et al., 1981; Ho¨vermann and performed at the Institute of Geology and Geophysics, Ho¨vermann, 1991; Ja¨kel, 1991; Wang et al., 2002). Under Chinese Academy of Sciences (CAS). ICP-MS analyses the influence of predominantly northern winds, the final were carried out at the Chinese Ministry for Nuclear direction of dune migration is towards the southwest in Industries, and the results are listed in Table 1. This ICP- most parts of the Taklamakan. It is only in the southwest MS offers very low detection limits (e.g. the lowest corner of the desert that the dunes migrate toward the detection limit for europium is 0.04 ng/ml in solutions) southeast (Zhu et al., 1981). The highest dunes in the and its analytical uncertainties (relative standard deviation) Taklamakan, being ca. 100 m in height, are distributed in are less than 75% and under 71% for measuring REE. the area between Keriya and Niya rivers (Institute of REE plots in this study are normalized to chondritic Desert Research of Academia Sinica, 1980). meteorites, using values recommended in Boynton (1984). Major-elemental abundance of samples from dune ridges 3. Samples and analytical methods was determined using X-ray fluorescence spectrometer (Shimadzu XRF-1500) at the Institute of Geology and Sediment samples were collected from dunes and inter- Geophysics, CAS (Table 2). Fusion glasses were prepared dune areas in three different regions in the Taklamakan by mixing the sample with flux (Li2B4O7) in the proportion Desert (Fig. 1). A southern sampling is along the 1:10. Chinese national standards of rocks were used for desert road, which belongs to the reaches of the Niya River calibration. The relative precision of the measurements is (sample sites 4–10) where the runoff was significant in the 2% for Na2O and MgO and better than 2% for other past. The northern sampling region is within the area of the major elements. The chemical index of alteration (CIA) is palaeo-channels from the Tarim River (sample sites 1–3). calculated using a modification of the formula proposed by Samples were also taken from the catchment of the Keriya Nesbitt and Young (1982), i.e. CIA ¼ [Al2O3/ River (sample sites 11–21). Samples of aeolian sediment (Al2O3+CaO*+Na2O+K2O)] 100 (ratio in molecular were also collected from three formerly formed sand proportions). Here the CaO* refers to the amount of CaO wedges in the fourth fluvial system, i.e. areas of Cele River only incorporated in the silicate fraction and is calculated (sample sites 22–24). These samples of aeolian sediment using CaO* ¼ 0.35 2 (Na2O in weight %)/62 (Honda and represent features of four independent fluvial systems Shimizu, 1998). because three rivers (Niya, Keriya and Cele) have head- waters in the Kunlun Mountains and flow parallel and northwards into the desert, and the Tarim River flows 4. Results eastwards along the northern margin of the desert. Consequently, samples from these four different fluvial For comparison of elements between aeolian deposits of systems should provide insight on the characteristics and the dunes in various areas, only samples from the dune provenance of the aeolian deposits in this extensive sand ridges are considered in order to avoid differences sea. To examine the possible temporal variations in associated with different sorting dynamics on various provenance two sediment profiles (sample site 5 and 7) positions of a dune. The relative concentration of 30 were examined. Sample sites were identified using a GPS as elements in the catchments of four different river systems well as topographic maps. are shown in Fig. 2. The elements are shown as Coarse grain size fractions (40.250 mm) and fine grain enrichments normalized to the composition of the upper size fractions (o0.053 mm) were isolated by dry sieving. At continental crust (UCC, Taylor and McLennan, 1985). some locations only a course or a fine sample could be There are clear differences between the fine and coarse isolated. The concentration of trace elements was measured fractions of the samples. Relative to the upper continental on an inductively coupled plasma mass spectrometer (HR- crust, the coarse fraction in general shows a depletion of ICP-MS, Finnigan MAT, Element I). To prepare the elements, whereas the fine fractions are characterized with samples, 40 mg of sample powders were dissolved by an enrichment of most elements. The enrichment of digestion with ultra-pure acids in four steps: (a) a mixture elements in the fine fractions arises from higher concentra- of 1 ml HF and 0.5 ml (1:1) HNO3 was added to the sample tion of clay and heavy minerals. High concentrations of in capped Teflon bombs, shaken with an ultra-sonic device SiO2 occurs in both fractions because of a relatively high and then heated for 24 h in order to break down silicates percentage of quartz (Table 2). The enrichment of Ca in and other salts; (b) re-dissolved in 1 ml HF and 0.5 ml (1:1) both fractions reflects the high carbonate concentration HNO3 as well as one drop H2MnO4 in capped Teflon throughout the sand sea (Fig. 2, Table 2). There is also bombs for 7 days on a hot plate to further break down a difference in CIA values between the coarse and silicates, fluorides and zircons; (c) 2 ml (1:1) HNO3 were fine fractions (Table 2), confirming the difference in added to further break down fluorides and dried on a hot major-elemental compositions between the two-grain size ARTICLE IN PRESS 74 X. Yang et al. / Quaternary International 175 (2007) 71–85

Table 1 Concentration (ppm) of traces elements in the sand samples taken from the Taklamakan Desert

C 1 F 1 C 2 F 2 C 3 F 3 C 4 F 4 C 5-1 F 5-1 F 5-2 C 5-3 F 5-3 F 5-4

Sc 7.96 13.3 6.10 11.2 4.25 12.4 4.06 13.5 11.1 11.7 12.6 4.13 9.82 10.9 V 67.5 102 52.3 81.0 40.3 89.5 39.1 63.8 95.7 88.5 94.4 41.4 72.5 82.3 Cr 44.0 117 31.9 97.6 26.4 97.8 21.7 97.5 68.5 105 84.7 23.5 61.7 68.9 Mn 402 1056 324 868 215 935 267 1116 674 859 817 269 690 679 Co 9.18 9.78 6.57 9.65 3.92 9.89 4.51 10.4 14.3 9.23 21.2 4.14 9.77 15.0 Ni 33.6 40.4 31.0 52.5 29.9 36.1 23.0 37.6 46.8 62.2 67.6 21.8 45.4 53.5 Zn 69.2 110 41.6 87.3 29.2 93.3 26.5 61.8 85.2 89.0 96.2 27.6 70.8 78.5 Rb 180 58.9 135 61.3 121 58.9 109 57.5 129 60.4 119 124 64.3 107 Sr 235 299 321 284 257 302 276 322 281 286 245 220 289 271 Y 9.93 64.9 9.96 48.6 9.43 54.92 9.29 59.5 19.5 48.0 20.5 9.53 35.9 19.9 Zr 43.6 741 48.9 450 38.2 724 43.0 421 85.6 378 93.7 36.1 236 79.4 Cs 8.89 2.91 5.18 2.94 3.25 2.75 2.98 2.72 8.37 2.94 8.97 2.78 3.23 6.93 Ba 916 634 849 550 792 914 827 1256 549 456 521 645 462 523 La 14.8 129 14.3 109 13.0 107 15.7 138 28.1 86.2 30.6 14.9 51.1 28.2 Ce 29.1 257 28.3 218 24.6 223 31.8 274 54.8 174 62.4 29.5 102 57.5 Pr 3.26 29.1 3.13 23.8 2.78 24.1 3.49 30.8 6.18 19.3 6.89 3.28 11.4 6.37 Nd 11.9 108.4 11.3 86.8 9.6 85.5 12.5 107.8 21.7 69.7 25.3 11.7 39.9 23.0 Sm 2.52 20.4 2.39 16.8 2.08 16.9 2.45 21.4 4.39 13.6 4.94 2.62 8.66 4.73 Eu 1.02 2.63 0.953 2.28 0.879 2.48 0.925 2.97 1.14 2.14 1.20 0.862 1.61 1.14 Gd 2.13 18.8 2.22 14.8 1.86 15.5 2.14 18.4 3.89 11.9 4.32 2.01 7.53 4.20 Tb 0.423 3.23 0.409 2.59 0.353 2.55 0.388 3.07 0.787 2.13 0.833 0.411 1.46 0.810 Dy 2.05 14.2 2.04 10.8 1.87 10.7 1.94 13.4 3.88 10.0 4.17 1.91 7.28 4.07 Ho 0.392 2.59 0.387 1.96 0.369 2.11 0.375 2.33 0.730 1.82 0.794 0.360 1.38 0.771 Er 1.23 8.76 1.25 6.57 1.14 7.09 1.11 7.79 2.31 5.93 2.49 1.13 4.37 2.42 Tm 0.168 1.12 0.162 0.824 0.157 0.984 0.149 0.969 0.315 0.776 0.347 0.146 0.581 0.319 Yb 1.15 7.90 1.19 5.68 1.09 6.73 1.06 6.55 2.15 5.34 2.26 1.02 3.91 2.15 Lu 0.181 1.20 0.166 0.866 0.175 1.02 0.153 0.985 0.322 0.809 0.325 0.152 0.586 0.324 Hf 1.58 26.0 1.80 16.2 1.39 25.0 1.62 14.4 3.13 13.5 3.12 1.41 8.56 2.83 Ta 1.29 0.811 0.861 2.56 0.009 2.78 0.422 0.039 0.584 0.737 0.942 0.400 1.38 0.883 W 2.31 6.38 1.52 3.79 0.0092 2.52 0.758 0.109 1.97 2.72 2.09 0.614 2.73 0.928 Th 5.60 53.4 5.24 42.0 5.19 40.6 6.33 55.5 10.9 31.1 12.6 6.22 19.4 11.1 U 1.43 12.0 2.04 7.95 1.61 9.41 1.58 10.3 3.30 6.70 3.93 1.74 4.75 3.32 Eu* 1.34 0.41 1.27 0.44 1.37 0.47 1.24 0.46 0.85 0.52 0.79 1.15 0.61 0.79

C 5-5 F 5-5 C 5-6 C 6 F 6 C 7-1 F 7-1 F 7-2 C 7-3 F 7-3 C 7-4 F 7-4 C 8 F 8

Sc 8.54 10.4 11.3 4.64 13.3 4.40 12.7 14.1 4.49 11.6 6.81 12.2 4.85 10.7 V 58.6 79.5 81.4 33.7 96.6 50.0 122 104 53.4 81.6 66.2 91.3 45.8 87.6 Cr 50.9 78.3 70.1 28.4 134.0 32.1 126 92.0 31.2 97.4 40.1 122 18.3 134 Mn 517 750 724 313 1083 317 933 888 285 839 415 977 304 783 Co 14.9 10.1 18.1 6.91 14.7 6.96 12.6 20.6 6.21 11.7 8.88 11.2 5.12 10.6 Ni 82.4 42.3 54.5 42.0 40.6 28.5 76.0 56.9 37.9 40.4 45.7 53.3 19.0 51.2 Zn 81.8 80.2 84.9 32.5 103 84.8 184 157 79.7 155 105 142 30.5 86.0 Rb 146 65.1 105 95.6 55.5 75.6 64.2 154 82.3 61.0 95.2 56.0 79.2 64.9 Sr 251 309 283 265 321 263 273 244 277 268 229 288 342 354 Y 9.26 39.4 20.0 9.02 70.9 9.53 64.4 26.8 9.75 54.3 12.1 64.5 11.9 47.4 Zr 31.3 344 79.4 39.9 936 52.1 795 129 50.0 675 73.6 927 56.6 425 Cs 9.03 3.20 7.08 3.18 2.77 3.10 3.25 12.3 2.34 3.05 3.95 2.94 1.77 3.71 Ba 587 424 508 579 1169 622 634 595 576 647 636 985 959 591 La 13.4 70.1 32.6 12.2 153 11.8 107 37.9 14.5 97.5 16.0 138 20.0 90.9 Ce 27.8 143 66.0 25.6 305 25.4 193 81.1 27.1 189 31.8 282 37.8 185 Pr 3.06 15.7 7.34 2.81 35.1 2.76 23.8 8.79 2.98 22.2 3.87 31.4 4.19 20.8 Nd 11.1 58.2 27.0 10.6 122.6 10.0 86.9 31.2 10.4 78.4 14.2 110 14.6 76.5 Sm 2.27 11.7 5.39 2.23 23.2 2.10 16.2 6.11 2.09 14.5 3.13 21.1 2.89 14.4 Eu 0.646 1.80 1.30 0.828 3.08 0.858 2.46 1.56 0.824 2.26 0.981 2.73 1.10 2.15 Gd 2.10 10.0 5.13 2.15 22.6 2.06 16.7 5.53 1.96 13.3 2.83 20.6 2.62 12.8 Tb 0.363 1.81 0.845 0.389 3.40 0.398 2.97 1.10 0.380 2.48 0.546 3.29 0.448 2.14 Dy 1.81 8.38 4.05 1.79 15.2 1.84 12.7 5.10 1.87 10.7 2.58 14.6 2.28 10.1 Ho 0.340 1.51 0.750 0.328 2.60 0.360 2.38 1.02 0.363 1.99 0.490 2.62 0.439 1.82 Er 1.11 5.00 2.37 1.09 8.88 1.11 7.98 3.22 1.12 6.50 1.57 8.78 1.39 5.76 Tm 0.140 0.647 0.317 0.135 1.12 0.149 1.03 0.425 0.155 0.813 0.199 1.10 0.177 0.749 Yb 0.958 4.43 2.16 0.990 8.22 1.03 7.43 3.03 1.03 6.06 1.50 7.98 1.30 5.18 Lu 0.143 0.666 0.304 0.140 1.20 0.158 1.076 0.456 0.141 0.912 0.216 1.19 0.197 0.765 ARTICLE IN PRESS X. Yang et al. / Quaternary International 175 (2007) 71–85 75

Table 1 (continued )

C 5-5 F 5-5 C 5-6 C 6 F 6 C 7-1 F 7-1 F 7-2 C 7-3 F 7-3 C 7-4 F 7-4 C 8 F 8

Hf 1.09 11.8 2.79 1.32 31.1 1.52 24.6 3.70 1.42 19.6 2.81 32.8 2.03 14.4 Ta 0.164 2.02 0.969 0.466 3.90 0.468 2.42 1.36 0.416 2.46 0.576 3.37 0.388 0.542 W 0.149 2.36 1.91 0.702 3.62 0.595 2.21 2.10 0.483 2.53 1.66 4.10 0.759 3.59 Th 4.85 28.8 12.1 4.93 65.8 4.31 43.5 16.0 4.60 36.8 5.72 60.8 7.51 37.0 U 1.30 6.49 3.36 1.63 12.8 1.41 9.92 4.33 1.40 8.47 2.22 12.1 1.67 8.09 Eu* 0.90 0.51 0.75 1.16 0.41 1.26 0.46 0.82 1.24 0.50 1.01 0.40 1.22 0.48

C 9 F 9 C 10 F 10 C 11 F 11 C 12 F 12 C 13 F 13 C 14-1 F 14-1 C 14-2

Sc 3.20 11.7 6.35 10.4 3.65 9.71 6.16 10.7 15.2 14.8 5.87 12.6 4.05 V 34.8 86.8 53.2 78.7 44.5 76.0 62.1 78.3 144 108 47.7 75.3 43.5 Cr 18.2 132 42.5 100 23.9 61.2 32.8 94.3 116 162 43.3 89.6 21.2 Mn 304 943 393 798 237 676 368 823 780 1161 347 940 244 Co 4.26 9.11 6.75 9.96 5.47 11.6 7.45 9.99 21.8 12.3 6.09 10.7 4.20 Ni 22.8 40.8 35.4 37.6 33.2 49.5 17.5 53.1 58.8 37.2 26.6 34.8 21.8 Zn 25.9 88.0 36.5 76.8 30.6 75.4 35.8 76.9 124 112 37.2 53.5 24.5 Rb 59.5 55.5 61.4 58.6 49.6 78.2 96.6 58.8 307 54.0 60.2 60.9 55.7 Sr 277 280 359 338 374 521 248 290 167 303 245 302 188 Y 8.18 51.7 11.8 45.2 12.0 28.7 11.1 40.0 9.90 72.8 13.4 55.5 10.4 Zr 40.0 623 37.7 413 23.1 132 24.0 287 40.4 651 55.0 586 30.4 Cs 1.62 2.64 1.69 3.03 2.08 4.45 3.54 2.57 15.3 2.59 2.08 3.14 1.85 Ba 643 662 508 552 276 463 980 421 978 625 454 723 383 La 14.9 112 20.5 92.3 14.7 41.3 22.2 63.0 17.2 130 18.2 112 12.9 Ce 33.9 223 36.9 186 28.9 82.2 42.0 129 35.4 280 36.0 230 26.0 Pr 3.02 24.8 3.90 20.5 3.45 9.72 4.42 14.9 3.56 30.9 4.32 25.1 2.98 Nd 11.1 87.6 14.0 74.2 11.5 35.0 15.5 53.5 13.2 107 15.9 92.4 11.2 Sm 2.01 16.8 2.75 14.2 2.42 7.12 2.76 10.4 2.37 20.6 3.24 17.5 2.18 Eu 0.731 2.28 0.926 1.93 0.620 1.50 1.11 1.82 0.927 2.89 0.860 2.40 0.651 Gd 1.72 14.6 2.33 11.3 2.09 6.15 3.05 10.7 2.45 19.2 2.94 16.7 1.95 Tb 0.331 2.46 0.465 1.97 0.426 1.20 0.502 1.83 0.415 3.37 0.531 2.64 0.376 Dy 1.57 11.4 2.37 9.52 2.32 5.87 2.44 8.87 2.19 16.6 2.70 12.2 1.94 Ho 0.290 1.99 0.445 1.72 0.429 1.09 0.427 1.53 0.382 2.85 0.503 2.20 0.373 Er 0.928 6.36 1.34 5.53 1.35 3.36 1.42 5.01 1.24 9.27 1.64 7.30 1.22 Tm 0.120 0.816 0.171 0.687 0.178 0.454 0.175 0.634 0.157 1.20 0.206 0.940 0.168 Yb 0.826 5.59 1.17 4.87 1.20 3.07 1.21 4.45 1.13 8.44 1.40 6.43 1.07 Lu 0.120 0.828 0.165 0.725 0.174 0.461 0.174 0.658 0.166 1.28 0.199 0.985 0.169 Hf 1.36 10.9 1.30 14.1 0.93 4.70 0.98 10.5 1.57 23.5 1.91 19.4 1.06 Ta 0.060 0.908 0.369 0.223 0.391 0.050 0.649 1.93 1.32 2.92 0.608 0.051 0.414 W 0.105 0.588 0.528 2.72 0.611 1.36 0.925 3.22 1.56 12.14 0.867 0.078 0.601 Th 6.15 45.1 5.77 37.2 4.83 15.2 7.70 21.0 5.45 49.7 6.05 41.1 4.57 U 1.34 8.46 1.45 7.07 1.26 4.07 1.73 5.08 2.14 10.6 1.73 8.88 1.30 Eu* 1.20 0.44 1.12 0.46 0.84 0.69 1.17 0.53 1.18 0.44 0.85 0.43 0.97

F 14-2 C 14-3 F 14-3 C 14-4 F 14-4 C 15-1 F 15-1 C 15-2 F 15-2 C 16 F 16 C 17 F 17

Sc 12.2 4.22 11.1 3.93 14.7 5.11 12.2 4.72 11.6 4.28 12.1 6.34 11.0 V 86.0 46.3 80.7 41.5 107 50.8 90.1 45.6 82.7 44.0 82.4 55.6 76.1 Cr 96.6 24.0 86.7 20.9 137 29.7 105 26.0 82.2 34.4 126 37.9 83.6 Mn 963 278 822 250 1151 306 918 302 877 282 947 372 751 Co 9.95 4.80 10.5 4.57 11.5 5.58 9.87 5.16 10.4 5.34 10.8 9.78 11.1 Ni 32.3 28.1 40.6 18.8 38.4 26.6 42.8 22.9 33.3 20.2 52.4 36.3 35.4 Zn 79.2 28.7 82.6 23.8 108 34.8 103 25.3 81.3 29.1 79.7 49.6 75.8 Rb 56.7 85.8 61.4 84.0 52.5 75.8 58.8 84.9 60.1 66.2 56.5 104 66.6 Sr 294 304 290 311 289 266 292 291 303 281 284 338 270 Y 48.2 9.63 44.1 8.53 78.6 10.4 52.1 11.1 46.6 9.18 70.5 9.86 37.4 Zr 410 39.8 332 54.0 1014 33.3 416 36.8 366 41.1 766 38.1 269 Cs 2.85 2.25 3.30 1.99 2.68 2.33 2.86 2.06 3.11 2.21 2.88 5.51 4.66 Ba 672 738 579 730 647 611 703 758 578 664 772 515 352 La 108 16.2 90.8 11.4 144 16.9 95.0 20.1 89.5 12.4 127 13.8 65.8 Ce 213 31.1 183 22.6 295 31.8 206 37.4 179 24.8 248 26.0 131 Pr 23.4 3.46 20.8 2.52 32.8 3.52 24.2 4.03 20.1 3.02 28.6 3.14 14.9 Nd 87.7 12.4 73.8 9.5 121 13.1 87.1 14.9 75.8 11.0 108 11.5 57.2 Sm 16.4 2.56 15.3 1.95 23.0 2.79 17.1 2.74 14.3 2.09 20.0 2.26 11.2 Eu 2.39 0.976 2.35 0.891 2.85 0.903 2.73 0.918 2.07 0.928 2.74 0.783 1.90 ARTICLE IN PRESS 76 X. Yang et al. / Quaternary International 175 (2007) 71–85

Table 1 (continued )

F 14-2 C 14-3 F 14-3 C 14-4 F 14-4 C 15-1 F 15-1 C 15-2 F 15-2 C 16 F 16 C 17 F 17

Gd 15.2 2.30 13.8 1.79 20.2 2.51 19.7 2.68 13.2 2.68 20.6 2.13 10.6 Tb 2.48 0.418 2.28 0.319 3.54 0.463 3.08 0.459 2.18 0.454 3.43 0.384 1.76 Dy 11.1 2.06 10.0 1.66 16.5 2.51 13.3 2.30 10.1 2.24 16.5 1.99 8.35 Ho 1.92 0.390 1.75 0.318 3.08 0.429 2.15 0.424 1.79 0.370 2.83 0.374 1.48 Er 6.57 1.26 5.74 1.02 10.3 1.31 7.45 1.40 5.84 1.25 9.37 1.15 4.80 Tm 0.810 0.165 0.716 0.136 1.35 0.208 0.904 0.179 0.764 0.145 1.21 0.154 0.616 Yb 5.69 1.14 5.05 0.930 9.48 1.10 6.51 1.18 5.17 1.08 8.56 1.00 4.19 Lu 0.836 0.185 0.769 0.133 1.413 0.221 0.945 0.178 0.799 0.161 1.28 0.156 0.615 Hf 14.1 1.54 12.5 1.88 34.7 1.28 15.0 1.29 12.6 1.63 27.0 1.28 9.53 Ta 0.070 0.378 1.97 0.348 3.51 0.422 2.50 0.478 2.51 0.421 0.055 0.778 0.304 W 0.082 0.698 3.67 0.574 3.14 0.914 2.72 0.645 3.38 1.06 0.113 1.45 1.66 Th 40.9 6.96 35.4 4.21 58.3 5.30 31.1 6.93 32.8 4.18 50.3 4.36 23.4 U 8.44 1.73 7.32 1.47 12.8 1.39 7.90 1.54 7.17 1.28 11.4 2.90 5.36 Eu* 0.46 1.23 0.49 1.46 0.40 1.04 0.46 1.03 0.46 1.20 0.41 1.09 0.53

C 18 F 18 C 19 F 19 F 20 C 21 F 21 C 22 F 22 C 23 F 23 C 24 F 24

Sc 6.06 13.7 7.59 14.2 10.5 4.42 8.72 3.34 9.17 3.74 10.4 4.76 10.2 V 59.3 92.7 44.1 103 92.2 46.4 66.8 30.5 71.1 33.2 78.1 44.2 76.0 Cr 32.4 94.9 37.2 118 73.9 35.4 63.0 21.3 64.5 16.4 83.0 358 71.7 Mn 353 995 385 1110 673 337 605 219 634 245 747 319 721 Co 7.19 12.4 8.38 11.4 15.2 8.52 9.61 4.18 10.4 5.05 11.4 8.76 10.5 Ni 20.7 45.8 20.5 39.7 46.6 36.9 53.6 27.4 42.3 24.7 34.1 58.1 36.5 Zn 34.1 110 39.4 109 77.3 39.2 67.7 28.3 69.9 28.2 80.7 36.8 76.9 Rb 108 60.6 168 53.4 104 44.8 74.1 41.4 78.0 75.6 70.0 53.2 70.7 Sr 277 292 223 295 301 1323 319 640 853 449 1162 600 1075 Y 9.91 61.3 7.30 64.9 18.4 12.2 27.1 7.67 26.4 10.8 41.2 12.5 36.4 Zr 31.1 476 13.5 688 70.2 49.1 138 30.7 128 36.7 309 36.4 214 Cs 4.48 3.08 8.42 2.65 7.56 3.01 4.36 1.92 4.70 1.72 4.00 2.03 4.02 Ba 879 634 863 765 477 235 446 232 441 389 547 348 545 La 14.7 93.8 12.6 134 26.5 16.6 40.2 14.1 42.4 20.3 72.1 16.7 56.3 Ce 27.9 194 24.2 268 52.3 36.1 79.2 28.4 84.8 40.7 145 33.0 115 Pr 3.21 23.3 2.80 29.6 5.87 3.90 9.43 3.20 9.71 4.57 16.8 3.82 13.5 Nd 12.4 87.0 10.8 114 21.7 14.5 34.2 11.8 36.2 17.1 61.2 13.6 49.4 Sm 2.61 16.2 2.10 21.3 4.45 3.01 6.97 2.24 7.26 3.31 12.0 2.64 9.90 Eu 0.959 2.67 0.943 2.80 1.03 0.718 1.52 0.530 1.44 0.810 1.93 0.712 1.79 Gd 2.44 18.0 2.01 21.0 3.77 2.70 6.56 2.07 6.83 2.95 10.6 2.39 8.83 Tb 0.430 2.93 0.322 3.22 0.755 0.498 1.19 0.345 1.13 0.493 1.87 0.464 1.64 Dy 2.26 14.4 1.59 14.5 3.66 2.44 5.59 1.60 5.64 2.35 8.82 2.44 7.82 Ho 0.417 2.62 0.279 2.55 0.700 0.463 1.06 0.278 1.01 0.411 1.59 0.475 1.43 Er 1.31 8.34 0.889 8.63 2.20 1.48 3.34 0.911 3.28 1.32 5.23 1.49 4.53 Tm 0.237 1.14 0.110 1.08 0.279 0.203 0.431 0.106 0.424 0.169 0.668 0.199 0.606 Yb 1.09 7.48 0.736 7.56 1.83 1.35 2.96 0.786 2.82 1.11 4.58 1.33 3.97 Lu 0.206 1.06 0.106 1.153 0.286 0.220 0.435 0.110 0.420 0.156 0.684 0.197 0.593 Hf 1.21 18.6 0.53 24.2 2.42 1.79 4.98 1.04 4.74 1.28 10.8 1.32 7.81 Ta 0.557 2.78 0.025 3.09 0.819 0.466 1.14 0.103 0.296 0.436 1.628 0.421 0.527 W 0.976 2.87 0.027 6.86 1.69 0.960 1.72 0.639 1.76 0.605 2.89 0.681 2.25 Th 4.53 34.2 5.49 50.0 10.4 6.14 14.6 5.03 14.7 11.4 29.1 4.74 25.6 U 1.70 8.30 1.27 10.5 3.20 3.63 3.57 1.07 4.01 2.08 6.29 1.33 5.85 Eu* 1.16 0.48 1.40 0.40 0.77 0.77 0.69 0.75 0.62 0.79 0.53 0.87 0.58

1/2 C—coarse fraction, F—fine fraction; see Fig. 1 for locations Eu* ¼ EuN/[(SmN )(GdN)] (Taylor and McLennan, 1985)

fractions. With one exception (sample 19), the CIA is other areas whereas the fine fraction of all sand samples is higher in the fine fraction (silt) than in the coarse fraction. more homogeneous (Fig. 3). In the coarse fraction the Regional variations of the sediment can be recognized in sands have different Fe2O3+MgO and Na2O contents. In the ternary plots of various compositions. Fig. 3 shows the the fine faction the difference is mainly in SiO2 content relationship between SiO2,Fe2O3 and CaO, and Fig. 4 (Fig. 4). examines the relationship between SiO2,Fe2O3+MgO, Eu/Eu* vs. LaN/YbN diagram and ternary plots of and Na2O. In the coarse fractions the sand from sand Lanthanum (La), scandium (Sc), and thorium (Th) (Fig. 5) wedges in Cele is unique compared to dune sands from also show the geochemical differences between the Keriya ARTICLE IN PRESS X. Yang et al. / Quaternary International 175 (2007) 71–85 77

Table 2 Abundance (%) of major-elements in the sand samples taken from the Taklamakan Desert

SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2OK2OP2O5 LOI Total CIA(%)

C 1 62.9 0.297 10.2 2.20 0.043 1.87 8.76 2.46 2.27 0.075 8.54 99.5 52.1 F 1 58.5 0.470 9.55 2.99 0.065 2.37 10.9 2.21 1.84 0.113 10.4 99.4 53.9 C 3 69.1 0.240 10.9 1.78 0.034 1.45 5.44 2.75 2.55 0.061 5.50 99.7 51.0 F 3 59.8 0.446 9.65 2.78 0.064 2.47 10.3 2.17 1.78 0.100 9.98 99.5 54.7 C 4 72.6 0.239 11.0 1.80 0.035 1.14 3.68 2.88 2.68 0.061 3.93 100 50.0 F 4 62.8 0.360 9.63 2.30 0.054 1.96 9.05 2.36 1.98 0.086 8.96 99.6 52.4 C 6 72.1 0.272 10.8 2.06 0.037 1.33 3.48 3.60 2.62 0.066 3.75 100.1 45.6 F 6 62.3 0.410 9.56 2.52 0.060 2.01 9.21 2.49 1.96 0.095 8.83 99.5 51.3 C 10 62.7 0.573 11.1 3.98 0.082 2.56 7.66 2.92 1.56 0.096 6.51 99.8 53.0 F 10 58.7 0.852 10.0 5.18 0.101 2.74 9.19 2.29 1.75 0.115 8.45 99.4 54.7 C 9 70.1 0.326 10.9 2.51 0.049 1.47 4.63 2.85 2.02 0.078 4.63 99.6 51.7 F 9 65.7 0.470 10.0 2.98 0.063 1.85 7.01 2.39 2.01 0.088 6.85 99.4 53.1 C 12 61.0 0.525 12.7 3.87 0.079 2.49 7.89 3.06 2.25 0.097 5.96 99.9 53.5 F 12 51.7 1.05 11.2 6.76 0.126 3.56 12.1 2.24 1.58 0.180 8.91 99.4 58.3 C 13 65.8 0.439 13.0 3.78 0.059 2.23 4.19 2.66 3.26 0.065 4.73 100 54.2 F 13 63.5 0.422 10.0 2.81 0.061 2.18 8.37 2.22 1.98 0.104 8.18 99.9 54.6 C 16 70.2 0.361 11.1 2.68 0.051 1.76 4.50 2.64 2.02 0.078 4.36 99.7 53.6 F 16 64.8 0.470 10.3 2.95 0.061 2.12 7.36 2.35 1.95 0.096 7.01 99.4 54.2 C 19 63.8 0.552 13.2 4.30 0.069 2.62 4.73 2.83 2.85 0.082 4.45 99.5 54.5 F 19 63.1 0.413 9.93 2.64 0.056 2.20 8.39 2.48 1.97 0.105 8.18 99.5 52.3 C 23 46.0 0.375 11.2 2.75 0.051 2.10 11.6 2.78 2.56 0.102 10.3 89.8 51.6 F 23 60.0 0.568 10.1 3.34 0.070 3.11 8.51 2.31 1.88 0.142 9.39 99.4 54.3 C 24 42.0 0.289 8.49 2.10 0.045 1.85 14.0 2.67 1.53 0.076 11.6 84.6 48.2 F 24 60.1 0.575 9.94 3.35 0.069 3.29 8.24 2.41 1.74 0.151 9.48 99.4 53.5

C—coarse fraction, F—fine fraction; see Fig. 1 for locations. Fe2O3 as total iron; LOI—loss on ignition at 1000 1C.

Fig. 2. Elemental enrichment plots for four catchments in the Taklamakan Desert. Data of each catchment are the mean value of three to four samples taken from ca 30 m high dune ridges, but samples representing Cele are aeolian sediment from sand wedges (a—coarse fraction, b—fine fraction). ARTICLE IN PRESS 78 X. Yang et al. / Quaternary International 175 (2007) 71–85

Fig. 3. Ternary plots of SiO2, CaO and Fe2O3 contents (in moles) showing compositional variability as well as similarities among the four areas (a—coarse fraction, b—fine fraction).

Fig. 4. Ternary plots of SiO2,Na2O and Fe2O3+MgO contents (in moles) showing compositional differences as well as similarities (a—coarse fraction, b—fine fraction).

River area and other parts of the desert. The differences shows the REE patterns of the sands from northern part of here are not absolute separation between the samples but the Taklamakan, i.e. from the catchment of the Tarim the samples from the areas of Keriya River and Niya River River and indicates a high homogeneity. Fig. 7 examines show a much larger variability than those from other parts the REE characteristics of sand from two dunes in lower in the Eu/Eu* vs. LaN/YbN diagram. The concentrations of reaches of the Keriya River. It shows a stronger homo- La, Sc and Th in the coarse sands of Taklamakan are very geneity in the fine fractions than in the coarse fractions. similar to UCC, but quite different from the fine fraction Sample 15-2 and 14-4 were both from ridges of dunes with (Fig. 5). a height of about 20 m. The dunes are isolated from each Generally speaking, the REE features of the aeolian other by channels of episodic streams. The higher Eu sediment from the Taklamakan are rather complex. On the anomaly in coarse fraction of sample 14-4 suggests that it is basis of REE abundance data listed in Table 1, chondrite- different from other samples. normalized patterns are drawn in Figs. 6–11. Both grain Some of the different features of the sand can be easily size fractions of sediment show light REE-enriched observed in the field. Sample 12 was taken from an isolated patterns, but have different Eu anomalies. Their REE 60 m high dune on the south margin of the desert (Fig. 1), patterns indicate that the fine fraction is generally surrounded by the vegetation. The colour of the sand is characterized by a distinctly negative Eu anomaly and grey and is different from the light-yellow sands occurring the coarser fraction by a slightly positive Eu anomaly. The in most other parts of the Taklamakan. Sample 21 is from concentration of REE in the fine fractions is generally a 5 m high, semi-stable dune located west of the Keriya much higher than in coarse fractions (Table 1). Fig. 6 River (Fig. 1) and samples 13 and 16 are from dunes along ARTICLE IN PRESS X. Yang et al. / Quaternary International 175 (2007) 71–85 79

Fig. 5. Upper: Eu/Eu* vs. LaCH/YbCH diagrams. Lower: Lanthanum–scandium–thorium plots (in moles). The sand samples in the areas of Keriya River show larger ranges of variation in all plots. In the Eu/Eu* vs. LaCH/YbCH diagrams larger ranges of variations occur in samples from areas of the Niya and Keriya rivers, and samples from Cele are separated from others (here samples from dune ridges are included, but samples from Cele are aeolian sediment buried in sand wedges, a—coarse fraction, b—fine fraction). The star represents the average composition of the upper continental crust (UCC, Taylor and McLennan, 1985). the Keriya River (Fig. 1). Sample 18 is taken from an inter- from the sand wedges that formed during the Last Glacial dune area. The REE characteristics confirm that samples Maximum is different from the dune sands in other areas 13, 16 and 18 are quite homogeneous in fine and coarse of this study and slightly different from the sand deposited fractions, but the samples 12 and 21 are different from the in older sand wedges. The REE patterns confirm the Keriya sands in terms of REE concentrations and in curved different features inferred from the major-element data shape. In the coarse fraction the sand from this semi-stable (Figs. 3 and 4). dune is characterized by a slightly negative Eu anomaly. In Trace element analysis was carried out in two sedimen- the fine fractions the difference is a lower concentration of tary sequences in the interior of the desert. Only fine REE (Fig. 8). fractions are shown in Figs. 10 and 11 because the Samples 23 and 24 were taken from two sand wedges lacustrine sediments in these two sequences are very fine OSL dated to ca 18 ka (Yang et al., 2006a). And sample 22 grained and the sand percentage is minor. The difference was taken from a sand wedge OSL dated to ca 40 ka (Yang between the aeolian fine fraction and lacustrine sediments et al., 2006a). Although sample 22 contains a lower is reflected in REE abundances and patterns (Figs. 10 and concentration of REE, the REE patterns of these three 11). Although both types of sediments are depleted in samples are very homogenous and are characterized by a europium, the scale of depletion is much larger in aeolian slightly negative Eu anomaly in both fine and coarse fine fraction than in lacustrine sediments. The REE fractions (Fig. 9). The REE pattern indicates that the sand concentrations in the aeolian fine fraction are much higher ARTICLE IN PRESS 80 X. Yang et al. / Quaternary International 175 (2007) 71–85

Fig. 6. Chondrite normalized REE plots for the samples located near the Fig. 7. Chondrite normalized REE plots for the samples from two ca 20 m Tarim River, showing the difference between coarse and fine fractions and high dunes in the lower reaches of the Keriya River, samples 15-2 and 14-4 homogeneity among the samples (a—coarse fraction, b—fine fraction). are from ridges and 15-1 and 14-1 are from bases, respectively, showing that sample 14-4 is different from other samples including samples from than in lacustrine sediment. The lacustrine sediment from the same dune in both fractions (a—coarse fraction, b—fine fraction). three different periods is very similar in REE character- istics, whereas slight differences in REE concentrations exist among the aeolian fine fraction at different locations. in the interior of the desert (sampling site 5 on Fig. 1), The lacustrine sedimentation occurred prior to ca 25 ka in plotted in stratigraphic order. There are basic differences the sediment sequences of sampling site 5 (Fig. 10a), but between aeolian (samples 5-1, 5-3 and 5-5) and lacustrine (5- the lacustrine sediment in the sequence of sampling site 7 2, 5-4 and 5-6) sediment, except for the Cr/Ba ratio (Fig. 11a) was OSL dated to be younger than 27907240 a (Fig. 10a). The lacustrine sediment shows elevated Rb and (Yang et al., 2006a). The difference between aeolian and Ba levels relative to the dune sand. Fig. 11a shows variations lacustrine sediments suggests that the underlying lacustrine in elemental ratios in sampling site 7 (Fig. 1). The lacustrine deposits were not the direct source of the silt and clay in the sediment (sample 7-2) has an elevated Rb level but reduced dunes. However, the similarity both in elemental ratios and Th, La and Cr levels relative to the modern dune sands (7-3 in REE characteristics in the sands from different layers and 7-4) and the old aeolian sand beneath (7-1). through time (Figs. 10 and 11) suggests that modern sand is sourced at least in part from the old sands. With regard to 5. Discussion these elemental ratios no clear difference exists between modern and old sands in these two profiles (Figs. 10 and 5.1. Comparison between coarse and fine fractions 11). From Figs. 10 and 11 it is clear that the REE in lacustrine sediment of sampling site 5 is less abundant than The coarse grain size fraction studied here is mainly that of sampling site 7 (see Table 1 also). medium-sized sand as the content of coarse sand is very For determining aeolian sediment sources trace elements low in the Taklamakan (Zhu et al., 1981; Besler, 1991; ratios have been proven to be useful because they omit the Yang et al. 2002). The fine fraction of the samples contains problems associated with dilution by quartz or carbonate mainly silt because clay is relatively rare in the dunes of the (Muhs et al., 1995, 1996; Pease and Tchakerian, 2003). Taklamakan (Zhu et al., 1981; Besler, 1991; Yang et al., Fig. 10a shows six elemental ratios for the stratigraphic units 2002). The sediments of the Taklamakan are transported ARTICLE IN PRESS X. Yang et al. / Quaternary International 175 (2007) 71–85 81

Fig. 8. Chondrite normalized REE plots for the samples from the reaches Fig. 9. Chondrite normalized REE plot for the samples taken from sand of Keriya River, showing the uniqueness of samples 12 and 21. Sample 12 wedges near Cele, showing strong similarities among these samples (a— is from an isolated, 60 m high dune on the desert margin, and sample 21 is coarse fraction, b—fine fraction; see Fig. 1 for locations). from a 5 m high, stabilized dune (a—coarse fraction, b—fine fraction; see Fig. 1 for locations). assemblages. The positive anomalies of sand fraction should be caused by a relatively higher concentration of from the mountains into the basins by the rivers and by alkali feldspar and plagioclase and a lower concentration sheet flows. The rivers with headwaters in the Kunlun of heavy minerals, such as hornblende, muscovite and Mountains, such as Niya, Keriya, Cele, transport sediment garnet, because alkali feldspar and plagioclase have a from south to north. The northerly winds blow the distinctly high concentration of Eu but relatively lower materials back toward the south. Consequently, a complete REE content, in particular the heavy REE (Taylor and transport cycle of sediment between the sand sea in the McLennan, 1985). Hornblende, muscovite and garnet that north and the mountains in the south exists in the Tarim occur often in the heavy mineral assemblages of aeolian Basin (Yang, 1991). The aeolian sediments of Taklamakan deposits in the Taklamakan (Yang, 1991) are characterized are reported to be from alluvial and fluvial deposits (Zhu by a negative Eu anomalies (Taylor and McLennan, 1985). et al., 1981) and were exposed to glacial processes in the As quartz contains little or no REE (Taylor and mountains (Ho¨vermann and Ho¨vermann, 1991; Yang, McLennan, 1985), the Eu anomalies in the sand fractions 1991). Therefore, the silt and the sand probably have should be dependant on the ratios between feldspars and same origins, resulting from frost weathering under heavy minerals. Negative Eu anomalies should be related glacial environments. The difference in the CIA bet- to a relatively higher concentration of heavy minerals. ween the fine and coarse fractions (Table 2) should be The highest level of homogeneity was found in three caused by mineralogical differentiations. The changes in samples of aeolian sediment from the northern part of the CIA should reflect changes in the proportion of feldspar study area, i.e., the middle reaches of Tarim River. and the clay minerals (Nesbitt and Young, 1982). There- Although the REE concentrations and patterns are clearly fore, the higher CIA value of the fine fractions is probably different between the coarse and fine fractions in each related to a potentially high content of clay minerals sample, the concurrence between these three samples and feldspars. becomes obvious if the fraction of same grain size is used The changes of Eu anomalies in the samples are for comparison (Fig. 6). These sediment samples (Fig. 6) interpreted to arise directly from variations in mineral were primarily transported by the Tarim River, and ARTICLE IN PRESS 82 X. Yang et al. / Quaternary International 175 (2007) 71–85

Fig. 10. Six elemental ratios and chondrite normalized REE plots for the Fig. 11. Six elemental ratios and chondrite normalized REE plots for the samples from each stratigraphic unit in the central Taklamakan (for samples from each stratigraphic unit in the southern Taklamakan (for locations see Fig. 1, sampling site 5. a—variation of elemental ratios from location see Fig. 1, sampling site 7. a—variation of elemental ratios, b— bottom to top, b—chondrite normalized REE plot). Samples 5-1, 5-3 and chondrite normalized REE plot). A sand sample beneath the lacustrine 5-5 are aeolian, 5-2, 5-4 and 5-6 are lacustrine and their bases were OSL layer (7-2) was OSL dated to 27907240 (Yang et al., 2006a). dated to be between ca 30 and ca 40 ka (Yang et al., 2006a). reworked by wind. Fluvial sediment should become more (Figs. 4 and 5). REE contents and patterns (Fig. 7) show homogeneous as the river reaches its middle and lower differences between various locations in one dune. A reaches because the input of more sediments is reduced La–Sc–Th plot has been shown to be useful for identifying downstream due to the absence of tributaries. The sands derived from rocks of different tectonic origin in the geomorphological framework indicates that the sand on Wahiba Sand Sea of (Pease and Tchakerian, 2002). the northern margin could be from the Tianshan Moun- The La–Sc–Th plot and Eu/Eu* vs. LaCH/YbCH diagram tains because sediment would be deposited when the river show that the coarse fractions of sand from areas of Keriya course reaches the flat foreland after leaving the steep River are not homogeneous (Fig. 5). The plots for fine mountains. fractions (Fig. 5) are similar to the coarse fraction plots Although some geochemical studies (Honda and Shimi- and support this idea. This might arise from the input of zu, 1998; Chang et al., 2000) showed homogeneity of the locally derived grains of heavy minerals or clay minerals in sands in the entire Taklamakan, the results reported here small quantities. Their heterogeneous signal may result reveal some differences between sands from various sites in from flood events that have transported sediment from the desert. Our examinations show that there are funda- different source areas to different sites in this part of the mental differences in the geochemical compositions be- sand sea due to changes in the river channels. Although the tween the fine and coarse fractions in each sand sample. sediment at the lower reaches of a river should be The coarse fractions are less homogeneous than the fine homogeneous, but special events like a flood, could fractions. transport new exotic materials as the headwater area is petrologically diversified (Ma, 2002). In the present-day 5.2. Regional variations delta of the Keriya River, flood waters come during the summer season. Some of the fluvial sediments are blown to In the small area of the lower reaches of Keriya River the dunes nearby, and thus increasing the heterogeneity of both the coarse and fine fractions are not homogenous the aeolian sediments. ARTICLE IN PRESS X. Yang et al. / Quaternary International 175 (2007) 71–85 83

Both major-elemental composition and REE patterns abundant hornblende, mica and epidote as well as metallic suggest that the aeolian sand from the sand wedges in Cele minerals in the aeolian sediments of the southern Takla- is different from other sand samples (compare Fig. 9 with makan should come from various metamorphic rocks in Figs. 6–8). The source material in the sand wedges could be the surrounding mountains around the Tarim Basin. initially transported by the Cele River from its headwater The compositions of the dunes sands in Taklamakan, area in the Kunlun Mountains. The sand has been fixed in that result from this study, are not fully consistent with the the wedges since the Last Glacial Maximum, and no opinions of Honda and Shimizu (1998) and Chang et al. exchange with other sand has taken palace since then. (2000). Honda and Shimizu (1998) concluded that low The difference in major-elemental compositions and in regional variations in mineral compositions occur in the REE characteristics is a result of different mineral sands of Taklamakan. Their analysis is based on analysis of assemblages, which arise from varying rock types in source the modal grain size fractions of 11 sand samples and were areas. According to the 1:5,000,000 geological map of examined using X-ray diffraction methods. Chang et al. Xinjiang (Ma, 2002), the rocks in the headwater areas of (2000) examined major minerals of three sand samples from Cele River are mainly Tertiary sandstones and conglom- the northern margin of the Taklamakan and reiterated the erates, and Permian olivine-rich rocks and olivine pyrox- idea about homogeneity of the sands. Light minerals (quartz enolite. Upper Proterozoic and Palaeozoic metamorphic and feldspar) are the most abundant minerals in all desert rocks and Palaeozoic granite as well as Cretaceous dunesinChina(Yang, 1991). In the areas of the Keriya sandstones and conglomerates occur in the headwater River, Taklamakan, the percentage of heavy minerals varies areas of Keriya River. In the headwater areas of Niya between 6.7% and 17.5% (Yang, 1991). The significance of River there are mainly Middle Proterozoic metamorphic a lower heavy mineral concentration would be reduced using rocks and Devonian dolomite. Regional variations of whole-rock analysis. Our present study, based on 24 petrological features do exits in the Kunlun Mountains. sampling sites and separation of samples into two different For example, there are much more Lower Proterozoic grain size fractions, indicates that regional variations of rocks in the mountainous areas west of Yutian (Fig. 1). In sands do occur in the Taklamakan, although some features the eastern part of the Kunlun Mountains facing the Tarim are quite homogenous. Basin, Carboniferous carbonates and Jurassic quartzose Our conclusions on the mineralogical maturity of the sandstones are widely distributed (Ma, 2002). sands in Taklamakan are consistent with the conclusions of Extensive investigations by the Chinese Academy of Honda and Shimizu (1998). The maturity of aeolian sands Sciences during the 1960s (Zhu et al., 1981) revealed that is positively correlated to the richness of quartz and there are regional variations in heavy mineral assemblages consequently to the concentration of SiO2, although this in the sand of the Taklamakan. In the western margin of feature may be inherited from the parent material (Muhs, the desert there is a large delta () jointly formed by 2004). Compared with many other dune fields such as the three rivers originating from the western Kun Lun of , Zallaf Sand Sea of Libya Mountains and the Pamir Plateau (Fig. 1). Owing to the (Muhs, 2004) and the Saudi (Honda and influence of the different fluvial-sourced sediments the dune Shimizu, 1998), the Taklmakan sands are characterized by sands vary distinctly even within the relatively small delta a lower SiO2 content, and a higher content of Al2O3+ region. There is a high concentration of metallic minerals Na2O+K2O. and epidote in the north, high concentrations of mica and garnet in the middle and high concentrations of hornblende 5.3. Other indicators for the heterogeneity of the sand in and pyroxene in the south, because the rivers flow parallel Taklamakan and eastwards in this area. Samples from the lower reaches of the Keriya River show high concentrations of horn- Sr and Nd isotopic compositions vary considerably in blende and epidote. Secondarily abundant heavy minerals the moraines distributed in the mountains surrounding the are ilmentite, ferroferrite and actinolite. More recent Taklamakan (Chang et al., 2000), suggesting that the studies have confirmed that hornblende and epidote are original source of the sands were not homogeneous. In the two most abundant minerals in the heavy mineral addition, glaciers in the north flank of Kunlun Mountains assemblages of the dune sands along the Keriya River were restricted to individual valleys and a west–east (Yang, 1991). In addition, hornblende is abundant in the mixture of the sediments in the initial source regions is heavy mineral assemblages of all aeolian sands distributed highly unlikely. Moreover, not all the sands were sourced north of Kunlun Mountains, although differences exist from moraines. Intensive fluvial erosion and weathering between the catchments of various rivers (Zhu et al., 1981). processes would have produced a large amount of the loose Garnet is more abundant in the aeolian coarse fraction materials increasing the heterogeneity of the source west of Yutian than in the east, and it is high in the sediment. The extensive mountainous landforms surround- southeast corner of the Taklamakan. This demonstrates ing the Taklamakan and the extensive sand sea could be a that specific characteristics of heavy mineral assemblages great laboratory for examining the feasibility of various can be recognized in the aeolian sands occurring in the methods designated for inferring the source of aeolian areas of each river (Zhu et al., 1981). The relatively sediments. ARTICLE IN PRESS 84 X. Yang et al. / Quaternary International 175 (2007) 71–85

The sedimentological features indicate that dunes in the information of heavy mineral assemblages is shown to be Taklamakan are still in an early stage of development and an useful tool for investigating sand sources. there is an over-abundance of fine sediment in the grain size distribution (Besler, 1991). The sand sea of Taklamakan is rejuvenated by the input of fluvial sediments (Besler, 1995) Acknowledgements and the fine fraction is later blown out if it is exposed to surface processes. A large fine grain fraction supports the We thank the National Natural Science Foundation for idea of heterogeneity in the Taklamakan, because the large much-appreciated supports (Grant nos.: 40425011, sediment volume reduces the potentials of mixing and the 40671020). Sincere thanks are extended to Norm Catto, effects of aeolian abrasion (Yang, 1991). The relatively low Andrew Goudie and Daniel Muhs for valuable comments, CIA of both fine and coarse fractions (Table 2)also linguistic help and for constructive suggestions. indicates a low degree of weathering, which is related to, in part, a limited exposure time. Only the fine grain fraction can be easily mixed. The coarse grains, which cannot be References easily transported by wind for long distances, remain parallel in a west–east direction. It should also be mentioned Besler, H., 1991. The Keriya Dunes: first results of sedimentological that Taklamakan belongs to a low wind-energy environment analysis. Die Erde .-H. 6, 73–88. among the sand seas on Earth (Goudie, 2002), and this Besler, H., 1995. The Keriya dunes in the Taklimakan Sand Sea: factor reduces the possibility of a homogenous sand sea. sedimentological evidence for a polygenetic evolution. Die Erde 126, 205–222. The ground wind directions are quite complex in the Bory, A., Biscaye, P., Grousset, F., 2003. Two distinct seasonal Asian Tarim Basin, but the predominant wind directions are source regions for mineral dust deposited in Greenland (NorthGRIP). north and northeast (Zhu et al., 1981). Therefore, the wind Geophysical Research Letters 30(4), 1167, doi:10.1029/2002GL016446. would mainly transport sediments from north to south. Boynton, W., 1984. Cosmochemistry of the rare earth elements: meteorite Generally speaking, there is a lack of agents, which cause a studies. In: Henderson, P. (Ed.), Rare Earth Element Geochemistry. Elsevier, Amsterdam, pp. 63–114. mixing of sediment between the east and west and thus Chang, Q., Mishima, T., Yabuki, S., Takahashi, Y., Shimizu, H., 2000. Sr reduces the possibility of homogenous sands on a large and Nd isotope ratios and REE abundances of moraines in the scale. This factor could play a large role in controlling the mountain areas surrounding the Taklimakan Desert, NW China. heterogeneity of dune sand. This can be shown by the Geochemical Journal 34, 407–427. unique features of the sediment in sample locations 12 and Goudie, A., 2002. Great Warm Deserts of the —Landscapes and Evolution. Oxford University Press, Oxford. 21 (Fig. 8). The uniqueness of sample 12 can be linked to its Greaves, M., Elderfield, H., Sholkovitz, E., 1999. Aeolian sources of rare isolated location, which reduces sand exchange with other earth elements to the Western Pacific Ocean. Marine Chemistry 68, dunes. The same could be true for sample 21 because it has 31–37. been stabilized since about 20 ka (Yang et al., 2006a). Hattori, Y., Suzuki, K., Honda, M., Shimizu, H., 2003. Re-Os systematics of the Taklimakan Desert sands, moraines and river sediments around the Taklimakan Desert, and of Tibetan soils. Geochimica et Cosmochimica Acta 67, 1195–1205. 6. Conclusions Honda, M., Shimizu, H., 1998. Geochemical, mineralogical and sedimen- tological studies on the Taklimakan Desert sands. Sedimentology 45, For aeolian sediments in the desert dunes, our study 1125–1143. shows that different grain size fractions from one site can Ho¨vermann, J., Ho¨vermann, E., 1991. Pleistocene and Holocene geomorphological features between the Kunlun Mountains and the produce very different major-elemental and REE signals. Taklimakan Desert. Die Erde Erg.-H. 6, 51–72. The coarse fractions of sand samples are characterized by a Institute of Desert Research of Academia Sinica, 1980. The map of aeolian much lower concentration of REE and a positive Eu landform in Taklimakan Desert 1:1,500,000. Cartographic Publishing anomaly, whereas the fine fraction has a high concentra- House, Beijing. tion of REE and a negative Eu anomaly in the Taklamakan Ja¨kel, D., 1991. The evolution of dune fields in the Taklimakan Desert since the Late Pleistocene—notes on the 1:2,500,000 map of dune Desert. Within the Taklamakan, regional variation is less evolution in the Taklimakan. 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