The processes and mechanisms of severe sandstorm development in the eastern Hexi Corridor China, during the Last Glacial period

Journal of Asian Earth Sciences 62 (2013) 769–775

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The processes and mechanisms of severe sandstorm development in the eastern Hexi Corridor China, during the Last Glacial period Qingyu Guan ⇑, Baotian Pan ⇑, Jing Yang, Lijuan Wang, Shilei Zhao, Hongjie Gui Key Laboratory of Western China’s Environmental Systems, Ministry of Education, College of Earth and Environmental Science, Lanzhou University, Lanzhou, Gansu 730000, PR China

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Article history: Received 3 May 2012 Received in revised form 3 October 2012 Accepted 14 November 2012 Available online 23 November 2012 Keywords: Last Glacial period Hexi Corridor Severe sandstorms Loess

a b s t r a c t Dust transported by sandstorms has been an important feedback in climate change in the past, and its environmental effects are predicted to have a great impact on future global climatic change. Investigating the grain-size classes and the standard deviations of the modern sandstorm samples, and the samples in the Shagou section (situated in the eastern Hexi Corridor), lead us to suggest that the sand fraction within the range of 275.4–550 lm in this section can be used as a sensitive indicator of severe sandstorms. We selected the size range in the L1 stratum of the Shagou loess section as indicative of temporal changes in sandstorm intensity in the eastern Hexi Corridor and found that during the Last Glacial period, severe sandstorms in the eastern Hexi Corridor occurred with high frequency during these periods: I (70–54 ka B.P.), II (51–48 ka B.P.), III (45–42 ka B.P.), IV (38–33 ka B.P.), V (31–28 ka B.P.) and VI (26–12 ka B.P.) In general, the frequency and intensity of dust storms in the early (MIS 4) and late (MIS 2) periods were both high but they were reduced in the middle period (MIS 3). The primary factors controlling severe sandstorms are hydrology and wind power, followed by the expansion of the source extent. Reduced precipitation caused the source region of sandstorms to expand; in addition, wind speeds also increased at this time. These factors may have directly contributed to the abundance of severe sandstorms. Based on the grain size from a loess section (the Shagou section) in the eastern Hexi Corridor, we propose an evolutionary sequence of the severe sandstorms during the Last Glacial period. This sequence is consistent with the dust records in the Arctic, the Antarctic and low-latitude (the central equatorial Pacific) areas. Thus globally synchronous periods of high dust activity occurred in the Last Glacial period. The strong winds proposed here provide a potential explanation for the global consistency of dust flux changes during the Last Glacial period. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Dust outbreaks in Asia are among the most dramatic of meteorological phenomena in the northern middle latitudes. Recent research has found that modern dust storm outbreaks over central Asia are associated with southeast-directed cold air surges and compressed atmospheric temperature gradients in southern Siberia north of 48°N (Roe, 2009). The major part of the Asian hinterland is dominated by northwestern China, where it covering an area of 1,570,000 km2 (Fang et al., 2002) and containing significant repositories of information needed for studying climatic changes in central and eastern Asia (Yang et al., 2003). In China, sandstorms, especially severe sandstorms, are important natural phenomena in the northwestern desert and nearby areas during the spring (Liu, 1985; Sun et al., 2003; Zhou and Zhang, 2003; Roe, 2009; Guan et al., 2010). Sandstorms are storms with a wind velocity ⇑ Corresponding authors. Tel.: +86 931 8912574; fax: +86 931 8912315. E-mail addresses: [email protected] (Q. Guan), [email protected] (B. Pan). 1367-9120/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jseaes.2012.11.030

greater than 10 m s1 and horizontal visibility of less than 1 km and entrain sand and dust into the atmosphere (Zhao, 1993; Derbyshire et al., 1998; Ye et al., 2000). A severe sandstorm, often referred to as a dark storm, has a wind velocity of more than 25 m s1, and can result in horizontal visibility of less than 50 m (Derbyshire et al., 1998; Ye et al., 2000). Due to the extremely arid climate and the sparse/bare vegetation cover, most of the sand in the arid areas of northwestern China is transported, where even up to 108–109 tons of sand particles are deflated, transported, and subsequently deposited in northern China and beyond every year (Zhang et al., 1997; Wang et al., 2005). Approximately 50% of China’s inland dust is transported outside the Chinese mainland to the Pacific Ocean (Rea, 1994) and even farther to western North America (Husar et al., 2001; McKendry et al., 2001), Greenland and the Arctic (Biscaye et al., 1997; Svensson et al., 2000; Bory et al., 2003). Dust plays a critical role in the Earth’s climate system and serves as a natural source of iron and other micronutrients in remote regions of the ocean (Martin, 1990; Watson et al., 2000; Jickells et al., 2005; Winckler et al., 2008). Dust affects climate,

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both directly by altering the radiation budget of the atmosphere (Tegen et al., 1996; Cox et al., 2008) and indirectly by influencing the biological uptake of CO2 by the oceans (Martin, 1990; Ridgwell, 2003) and the exchange of radiatively active gases with the atmosphere (Jickells et al., 2005). Thus, the dust transported by sandstorms may have been an important feedback in climate change in the past, and its environmental effects are predicted to have a great impact on future global climatic change (Harvey, 1988; Andreae, 1995; Shine and Forster, 1999). Vast swaths of the continent are blanketed every year by thick clouds of windblown dust generated in the desert regions of Asia that accumulate on land as loess (Derbyshire et al., 1998; Roe, 2009). Over geologic time, dust storms, occurring frequently in northwestern China, play an important role in the accumulation of loess deposits on the Chinese Loess Plateau (Derbyshire et al., 1998; Goudie and Middleton, 2006). Aeolian-derived Chinese loess deposits are an important climate archive over the past 2.6 Ma and debatably, even back until 22 Ma (Liu, 1985; Guo et al., 2002). Considerable work has been done on dust transport processes (Derbyshire et al., 1998; Sun et al., 2001, 2003; Zhou and Zhang, 2003; Roe, 2009; Qiang et al., 2010), sources of dust (Gallet et al., 1996; Derbyshire et al., 1998; Jahn et al., 2001; Sun, 2002; Chen et al., 2007; Guan et al., 2008; Stevens et al., 2010; Pullen et al., 2011), loess grain-size (Xiao et al., 1995; Sun et al., 2002; Sun, 2004) and loess accumulation rate (Kohfeld and Harrison, 2003; Lai et al., 2007; Prins et al., 2007) in Quaternary loess. The westerly and northwesterly winds and dust storms over the Hexi Corridor and their surrounding regions northwest of the Loess Plateau, point toward these areas as the potential source areas of the loess deposits (Derbyshire et al., 1998; Sun, 2002; Chen et al., 2007; Guan et al., 2008; Stevens et al., 2010). The area of the eastern Hexi Corridor (Fig. 1) experiences the most frequent sandstorm activity in China and across the Asian continent (Derbyshire et al., 1998; Sun et al., 2001; Wang et al., 2004, 2008). The processes and the mechanisms of sandstorm development in this area, as well as the relationship between sandstorms and global climate change and the evolution of monsoon patterns, play an important role in Quaternary. Understanding the properties of the severe sandstorms that occurred during the Last Glacial period has great relevance to the prevention and control of present-day sandstorms.

2. Materials and methods The Hexi Corridor is in a different loess distribution area than the Loess Plateau, and is much closer to the likely source of loess. The frequent sandstorm activity provides a great deal of material for the loess deposition in this area. Modern climate in this area is much drier than that in the Loess Plateau. The mean annual temperature and precipitation are about 5 °C and 300 mm, respectively. The low temperature and precipitation limit weathering processes, bioturbation and leaching are also limited, so unambiguous information regarding the evolution of sandstorms should be found in the loess stratum of those regions. Therefore, the records such as in the Shagou loess section (Fig. 1) can be used to demonstrate the evolutionary history of sandstorms in the region. The Shagou section (37°330 N, 102°490 E) is situated on the fifth terrace of the Shagou River to the north of the Qilian Mountains (Pan et al., 2001), and in the east part of the Hexi Corridor (Fig. 1). It is close to the Tengger Desert (less than 120 km from the southern edge of the modern desert) (Fig. 1). The Quaternary eolian deposits in the Shagou section have reached a thickness of approximately 230 m, which is the thickest eolian loess section discovered in the Hexi Corridor (Pan et al., 2001, 2003; Guan et al., 2007). The stratum of L1 in the section is 27.8 m thick, in which samples were collected along a 1.5–3-m-deep trench. The sampling interval was 2.5 cm in the upper part of the L1 (0–3.525 m) and 5 cm toward the base of the L1 (3.525–27.8 m). A total of 626 samples were collected. All bulk samples were airdried, and grain size and carbonate content analyses were carried out in the Key Laboratory of Western China’s Environmental System, Ministry of Education, Lanzhou University. Grain size was analyzed with a Mastersizer 2000 (Malvern Instruments Ltd., UK), with a measurement range of 0.02–2000 lm. Following the chemical pre-treatment procedure described by Konert and Vandenberghe (1997), samples were first treated with 30% H2O2 to remove organic materials. Carbonate in samples is subsequently dissolved with 10% HCl. Sample solutions were then dispersed with 0.5 N Na(PO3)6 on an ultrasonic vibrator before grain-size measurement. Carbonate content was assessed by exposing 0.5–2-g sub-samples to 3 N HCl and measuring the volume of CO2 evolved using the Bascomb (1961) calcimeter. The procedure and formula of Bascomb (1974) are: CaCO3 equivalent (%) = V  P  K/(W  T), where V = volume of CO2 evolved (cm3), P = atmospheric pressure

Fig. 1. (a) Sketch map of the East Asian monsoon. (b) Location of the Shagou and Luochuan loess section.

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(mmHg), K = constant (0.1604), W = weight of oven-dried sample used, and T = room temperature (K). Four thermoluminescence (TL) samples and one 14C sample were also collected (details about both the TL and 14C dates were described by Guan et al., 2007). The Last Glacial stratum of the Shagou section can be divided into three parts, according to the chronological dating results and the observation of the section in field. A weak paleosol complex (L1SS1, corresponding to MIS 3) in the middle of the Malan loess subdivides an upper loess unit (L1LL1, corresponding to MIS 2) from a lower one (L1LL2, corresponding to MIS 4) (Guan et al., 2007). Due to the likely inaccuracies and poor precision of the TL dates in the Shagou loess section, it is difficult to apply them directly to the reconstruction of millennial-scale climatic oscillations. However, the Last Glacial stratigraphic sequences in the Shagou section and the classic Luochuan section of the Chinese Loess Plateau (Fig. 1), show distinct similarities, both of them were loess deposits, thus the likely ages for the boundaries of L1LL1/S0 (MIS 2/MIS 1), L1SS1/L1LL1 (MIS 3/MIS 2), L1LL2/L1SS1 (MIS 4/ MIS 3) and S1/L1LL2 (MIS 5/MIS 4) in the Luochuan loess section were be used as control points (Xiao et al., 1995). Ages for the Shagou section were then interpolated between these control points. Nevertheless, the interpolated ages used in this provisional chronology should be considered as only approximate. There is an aeolian sand stratum with a thickness of 15 cm on the top of the Shagou section (Guan et al., 2010). The grain size distribution pattern of this unit is in excellent agreement with that of the modern sandstorm samples near the Shagou section, and both have a larger grain-size peak (275.4–550 lm) (Guan et al., 2010). After investigating the severe sandstorm that happened in Harbin on March 20, 2002, Xie et al. (2005) discovered that the grain size component of 420.5–500 lm was detected in this sandstorm. The sand fraction (>63 lm) is dominant in all modern sandstorm samples of the Qaidam Basin and accounts for 66.9 vol.% on average and all samples show distinct peaks around the modal grain size of 99.2–146.6 lm, with a mean value of 127 lm (Qiang et al., 2010). However, it should be pointed out that the sampling vessel was placed on a roof 3.5 m above the ground surface (Qiang et al., 2010). Hence, we infer that there was some larger grain-size particles certainly existed in the samples near the ground during the sandstorm periods. In arid areas, strong winds can displace gravels with diameters on the order of centimeters (for example the Songorine doors in the west of the Songorine Basin in northern Xinjiang). The largest wind speed in this area is over 40 m/s, which can lift gravel with diameter of 2–3 cm on the shore of Ebinur Lake to pile a gravel bank to a height of 30 cm (Wu, 1982). The Hexi Corridor is also a windy area. The greatest wind speed recorded in the Minqin region from 1961 to 1970 was 28 m/s, and the wind speed reached 38 m/s when a severe sandstorm occurred on May, 5, 1993. Because the Shagou section is adjacent to the Tengger Desert on its leeward side (Fig. 1a), the sand stratum is likely the direct result of sandstorm processes in the eastern Hexi Corridor, especially in the Tengger Desert. The stratum in the Shagou section contains materials that were carried by sandstorms in the eastern Hexi Corridor. Sandstorm and non-sandstorm aeolian processes are two important factors for loess deposition. The non-sandstorm process just provide finer particles for loess deposition, but in addition to these fine particles, there are also some coarse particles which can only be provided by sandstorm process (Liu, 1985; Lu and An, 1998; Sun et al., 2003; Sun, 2004; Guan et al., 2010; Qiang et al., 2010). Therefore, the coarse particles in loess stratum may be used as an indicator of the sandstorm events (Lu and An, 1998; Guan et al., 2010). We suggest that the grain-size of 275.4– 550 lm range in the Shagou section can probably be used as a sensitive indicator for severe sandstorms in the eastern Hexi Corridor (Guan et al., 2010). In this study, we selected this size range in the

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L1 stratum of the Shagou loess section as indicative of temporal changes in sandstorm intensity in the eastern Hexi Corridor during the last glaciation. 3. Results From the grain-size results, it can be found that during the last glaciation, the severest sandstorms in the east Hexi Corridor occurred mainly during six periods (Fig. 2): I (70–54 ka), II (51–48 ka), III (45–42 ka), IV (38–33 ka), V (31–28 ka) and VI (26– 12 ka). The duration of period I was the longest (16 kyr), followed by period VI (14 kyr) and then by periods IV. The duration of period II, III and V are almost identical, only 3 kyr. During the six periods, the frequencies of occurrence of severe sandstorms were varied (Fig. 2): Interval I and VI had the most sandstorms, and then are intervals II, III and IV, and interval V had the least sandstorms. The average intensity of the severe sandstorms, as indicated by their coarsest grain sizes, increased during periods I, V and VI, with essentially equally high intensity, although the intensity during period I was slightly greater. Sandstorms during periods II, III and IV were less intense and were of relatively equal intensity. Generally speaking, the frequency and intensity of the severe sandstorms in the early glacial period (i.e., period I, corresponding to broadly MIS 4) and later glacial period (i.e., period VI, broadly corresponding to MIS 2) in the eastern Hexi Corridor were both greater (Fig. 2). Relative to these two periods, the intensity of sandstorms in the middle periods (i.e., periods II–IV, corresponding to MIS 3) was less. 4. Discussion Three factors have been identified to explain the higher dust fluxes occuring during glacial periods than that in interglaciations (Harrison et al., 2001): a less vigorous hydrological cycle, resulting in reduced washout and thus preserving more material for potential removal by wind; increased wind intensities, leading to increased dust entrainment; and expanded dust source areas during glacial periods. Many investigations also suggest that the increased vigor of the dust cycle is a result of the colder, drier and windier climate and the consequent reduction in vegetation cover and the expansion of dust source areas (Lancaster, 1989; Tegen and Fung, 1994; Middleton, 1997; Mahowald et al., 1999; Reader et al., 1999, 2000; Grunert et al., 2000; Wang, 2000; Wang et al., 2004, 2008; Laurent et al., 2005; Roe, 2009). Laurent et al. (2005) attempted to look at the various factors and tentatively concluded that the frequency of wind gusts is the dominant control in the modern climate. Xiao et al. (1995) suggested that the quartz component of loess is largely unaffected by weathering processes and therefore constitutes a more reliable proxy index of wind strength. They further proposed that the maximum grain size of quartz (Qmax) in the Luochuan section can serve as a proxy for maximum wind strength (Fig. 2). The periods of the frequent occurrence of the severest sandstorms indicated by the Shagou secton (except III) generally correspond closely with the peak values on the Qmax curves of the Luochuan section (Fig. 2), suggesting that the wind strength at Luochuan was more powerful during the severest sand storm periods at Shagou. Actually, only strong wind is able to move the particles with grain-size >275.4 lm. During the early part of the Last Glacial period (MIS 4), the fluctuating range of the CaCO3(%) curve was the least, indicating the least precipitation (Fig. 2). During the later part of the Last Glacial period (MIS 2), although the fluctuating range of the CaCO3(%) curve was somewhat increased compared with that of the early period, but was still smaller, indicating relatively increased precipitation with a limited range

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Fig. 2. The processes of severe sandstorm development in the eastern Hexi Corridor during the Last Glacial period and comparison with other records. The grain size [275.4– 550 lm (%)] and CaCO3(%) of the Shagou loess section is compared with the maximum grain size of quartz (Qmax) in the Luochuan section (Xiao et al., 1995) and the d18O (‰) of the GRIP ice core record (GRIP Project Members, 1993), the LLS (Laser Light Scattering) of the GISP2 ice core record (Ram and Koenig, 1997), the laser dust (ng/g) (Lambert et al., 2008) and CH4 (ppbv) (Loulergue et al., 2008) of the EPICA Dome C (EDC) ice core record and the PC72 232Th flux (lg cm2 kyr1) in the central equatorial Pacific record (Winckler et al., 2008).

(Fig. 2). During the middle of the Last Glacial period (MIS 3), the CaCO3(%) curve range and absolute values are greatest, suggesting increased but highly variable precipitation compared to before and after (Fig. 2). The variations in precipitation may largely dominated the changes of the severest sandstorms, resulting in a greater frequency and intensity of the severest sandstorms during the early and later part of the Last Glacial period, but reduced in frequency and intensity during the middle part of the Last Glacial period. In the arid area of northwestern China, hydrological conditions and source conditions have a close interlinked relationship: when the area of the vegetation cover and the surface water increases as a result of increased precipitation, the area of the source of dust decreases, and the frequency of severe sandstorms is reduced. If precipitation is reduced, leading to a decrease in the area of the vegetation cover and surface water (often causing them to disappear entirely), the dust and sand source area will increase. If there are strong winds during this period, then severe sandstorms can occur with greater frequency. We suggest that the primary factors controlling severe sandstorms in the eastern Hexi Corridor during the last glaciation were changes in hydrology and wind power, followed by the expansion of source areas. Many studies in the arid area of northwestern China and surrounding areas support the proposed sequence of the severe sandstorms occurring in the eastern Hexi Corridor during the last glaciations (Fig. 2). For example, numerous studies focusing on the Tengger Desert (Hofmann, 1993; Pachur et al., 1995; Zhang et al., 2002), the Badain Jaran Desert (Norin, 1980; Wünnemann and Hartmann, 2002; Yang et al., 2003), the Taklimakan Desert (Yang et al., 2002, 2006), the Tibet Plateau especially in the Qaidam Basin (Chen and Bowler, 1986; Hövermann and Süssenberger, 1986; Fang, 1991; Li et al., 1993; Hövermann, 1998), Western Mongolia (Grunert et al., 2000) and southern Siberia (Chlachula, 2003), have found that humid climate events occurred in these deserts during MIS 3. Due to differences in dating materials and

methods amongst other things, accurate comparisons cannot be made between these humid climate events and the sandstorm records here, However, the observations still suggest that some humid climate periods existed during MIS 3. In these humid climate periods, the frequency and intensity of the severest sandstorms would be smaller than during both MIS 4 and MIS 2, as discussed above. In addition to the humid climate events, some dry events and cold events (mainly concentrated during MIS 4 and MIS 2) were also be found in the arid areas of northwestern China and surrounding areas. The timing of these events roughly corresponds to the periods the severe sandstorm frequently occurred at Shagou. For instance, Zhang et al. (2002) suggested that sand-dune activation, the expansion of the desert, and/or eolian activity strengthened during the periods of 20,000–24,000 14C yr BP, 24,660–25,820 14C yr BP and 35,160 14C yr BP in the in the Tengger Desert. The large dunefield east of the Uvs Nuur (Western Mongolia) was formed during the period 20,000–13,000 yr ago and 46,000 yr ago, likely representing arid periods (Grunert et al., 2000). Yang et al. (2004) considered that there was a hyperarid period between 18,600 and 12,800 14C yr BP in the Badain Jaran Desert. Southward expansion of the Mu Us desert occurred during the periods of 75–55 ka, 48–36 ka and 17–10 ka (Sun and Ding, 1998). Paleoenvironmental conditions in the eastern part of the Chinese desert belt were marked by an extensive widening of dune formation at 21–13 ka and eolian dunes began to appear after 16.6 ± 1.5 ka in the northern Hunshandake Desert (Yang et al., 2004, 2008). Vandenberghe et al. (2004) found very cold conditions with permafrost around 25–20 ka in the Mu Us desert. The glaciers in the Tianshan Mountains advanced at 13.6–25.3 ka, 39.5–40.4 ka and 64.2–71.7 ka (Zhao et al., 2010). A marked drop in mean annual temperature of approximately 8 °C (to ca. 9/10 °C) and a decline in the mean annual precipitation by 250 mm around the Last

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Glacial Maximum (21–19 ka) (Frenzel, 1992) was followed by the most intensive loess deposition (probably occurring around 18– 16 ka), with the highest accumulation rates on the northern Altai Plain and in the Yenisei Basin (Chlachula, 2003). However, there are also some differences in the periods of these dry events and cold events between studies. These may be controlled by the dating materials and methods or may also be controlled by the local climate environment. Although it is impossible to make a oneto-one correspondence between these cold–dry events and the periods in which the severe sandstorm frequently occurred in the eastern Hexi Corridor during the Last Glacial period, these cold– dry events tend to support the severe sandstorm sequence built by us. Therefore, we suggest that the climate during the early (MIS 4) and later (MIS 2) part of the Last Glacial period was cold and dry with severe sandstorms greater in frequency and intensity. However, the climate during the middle of the Last Glacial was relatively humid, and the severe sandstorms occurrence and intensity was suppressed despite their existence during some periods. Comparing the evolutionary sequence of the severe sandstorms with the dust records in the Arctic (Ram and Koenig, 1997), the Antarctic (Lambert et al., 2008) and low-latitude areas (Winckler et al., 2008) in Fig. 2, we suggest that the evolutionary sequence of the severe sandstorms and the dust records are highly consistent, although some differences occur in the detail (especially larger during MIS 3) and are likely attributable to the local climatic or environmental influences. Similar to the record of severe sandstorms, in the three dust records, the dust deposition intensity in the early period (MIS 4) and the later period (MIS 2) of the last glaciation was the greatest, with the middle period (MIS 3) showing considerably weaker accumulation. The consistent roughly variations among different dust records suggests that global synchronicity in dust response occurred during the Last Glacial period. In Asia, the equatorial Pacific, the Arctic and Antarctica, the dominant processes regulating dust generation apparently experienced a coherent response to global climate change rather than to local factors. The six periods, in which severe sandstorms frequently occurred, all correspond roughly to stages of low global temperature (Loulergue et al., 2008) (Fig. 2). In periods I, III, IV and V of frequently occurring severe sandstorms, the extremely cold events Heinrich 6 (H6), H5, H4 and H3 occurred separately in the Northern Hemisphere high-latitude areas; during the stage VI, there were also three extremely cold events: H2, H1 and the Younger Dryas (GRIP Project Members, 1993) (Fig. 2). Some high-resolution records indicate that millennial-scale cold periods of the last glaciation and deglaciation were marked by abrupt dust flux increases (Mayewski et al., 1997; Fischer et al., 2007; Yancheva et al., 2007; Tjallingii et al., 2008). McGee et al. (2010) considered that steeper meridional temperature gradients during glacial periods would have been accompanied by enhanced gustiness in dust source areas, contributing to and plausibly driving increases in global dust emissions; thus, they suggested that the gustiness hypothesis is a robust potential explanation for the global consistency of glacial–interglacial dust flux changes. We also consider the strong winds hypothesis (McGee et al., 2010) can probably be a potential explanation for the global consistency of dust flux changes in the last glaciation, as highlighted in our data.

5. Conclusions During the last glaciation, severe sandstorms in the eastern Hexi Corridor exhibit six periods of increased frequency and/or severity. Many further studies in the arid area of northwestern China support the temporal sequence of the severe sandstorms proposed here. The frequency and intensity of these severe sandstorms in the early period (i.e., period I, corresponding to MIS 4)

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and later period (i.e., period VI, corresponding to MIS 2) of the last glaciation in the eastern Hexi Corridor were increased in contrast to the middle periods (i.e., periods II–IV, corresponding to MIS 3). We propose that when precipitation in the eastern Hexi Corridor decreased and temperature dropped, changes in vegetation allowed the expansion of dust source areas. Wind speeds and gustiness also increased, inducing severe sandstorms. The proposed evolutionary sequence of severe sandstorms in the east Hexi Corridor during the Last Glacial period is consistent with dust records in the Arctic, the Antarctic and in low latitudes (Fig. 2), which indicates that global synchronicity in dust activity occurred during the last glaciation. The strong winds hypothesis is a potential explanation for the global consistency of dust flux changes over this period. Acknowledgements We thank Prof. J.L. Xiao for kindly and quickly providing the data of the Luochuan section. We thank Dr. T. Stevens for improving the English and for his constructive suggestions. We are grateful to another anonymous reviewer for providing useful comments, which helped to improve the manuscript. This work was supported by the National Natural Science Foundation of China (Grant No. 41171015, 40701016), the Program for New Century Excellent Talents in University (Grant No. NCET-11-0208), the Science and Technology project of Gansu Province (Grant No. 1107RJZA212), the Fundamental Research Funds for the Central Universities (Grant No. lzujbky-2010-96). References Andreae, M.O., 1995. Climate effects of changing atmospheric aerosol levels, World Survey of Climatology, Future Climates of the World, Henderson, A., Sellers, A. (Eds.), vol. 16, Amsterdam, Elsevier, 341392. Bascomb, C.L., 1961. A calcimeter for routine use on soil samples. Chemistry and Industry 45, 1826–1827. Bascomb, C.L., 1974. Physical and chemical analyses of <2 mm samples. In: Avery, B. W. Bascomb, C.L. (Eds.), Soil Survey Laboratory Methods, Harpenden: Soil Survey technical monograph 6, pp. 14–40. Biscaye, P.E., Grousset, F.E., Revel, M., Van der Gaast, S., ZieLinski, G.A., Vaars, A., Kukla, G., 1997. Asian source of glacial dust (stage 2) in the Greenland Ice Sheet Project 2 Ice Core, Summit, Greenland. Journal of Geophysical Research 102 (C12), 26765–26781. Bory, A., Biscaye, P., Grousset, F., 2003. Two distinct seasonal Asian source regions for mineral dust deposited in Greenland (North GRIP). Geophysical Research Letters 30 (4), 1167. http://dx.doi.org/10.1029/2002GL016446. 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. Chen, K., Bowler, J., 1986. Late Pleistocene evolution of salt lakes in the Qaidam Basin, Qinhai Province, China. Palaeogeography, Palaeoclimatology, Palaeoecology 54, 87–104. Chlachula, J., 2003. The Siberian loess record and its significance for reconstruction of Pleistocene climate change in north-central Asia. Quaternary Science Reviews 22, 1879–1906. Cox, P.M., Harris, P.P., Huntingford, C., Betts, R.A., Collins, M., Jones, C.D., Jupp, T.E., Marengo, J.A., Nobre, C.A., 2008. Increasing risk of Amazonian drought due to decreasing aerosol pollution. Nature 453, 212–215. Derbyshire, E., Meng, X.M., Kemp, R.A., 1998. Provenance, transport and characteristics of modern aeolian dust in western Gansu Province, China, and interpretation of the Quaternary loess record. Journal of Arid Environments 39, 497–516. Fang, J.Q., 1991. Lake evolution during the past 30,000 years in China, and its implication for environmental changes. Quaternary Research 36, 37–60. Fang, X.M., Shi, Z.T., Yang, S.L., Yan, M.D., Li, J.J., Jiang, P., 2002. Loess in the Tian Shan and its implications for the development of the Gurbantunggut Desert and drying of northern Xinjiang. Chinese Science Bulletin 47 (16), 1381–1387. Fischer, H., Siggaard-Andersen, M., Ruth, U., Röthlisberger, R., Wolff, E., 2007. Glacial/interglacial changes in mineral dust and sea–salt records in polar ice cores: sources, transport, and deposition. Reviews of Geophysics 45 (1). http:// dx.doi.org/10.1029/2005RG000192. Frenzel, B., 1992. Maximum cooling of the last glaciation (about 20,000– 18,000 years B.P.). In: Frenzel, B., Péczi, M., Velichko, A.A. (Eds.), Atlas of Paleoclimates and Paleo-environments of the Northern Hemisphere, Late Pleistocene–Holocene. Geographical Institute, Hungarian Academy of Sciences and Gustav Fisher Verlag, Budapest, Stuttgart, pp. 97–99.

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