Geomorphology 106 (2009) 180–185

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Geomorphology

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Geomorphological hierarchies for complex mega- and their implications for mega- evolution in the Badain Jaran

Zhibao Dong a,b,⁎, Guangqiang Qian b, Wanyin Luo b, Zhengcai Zhang b, Shengchun Xiao b, Aiguo Zhao b a Department of Geography, Shaanxi Normal University, b Key Laboratory of Desert and Desertification, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, China article info abstract

Article history: The evolution of mega-dunes is sometimes attributed to factors other than the wind but evidence for this is Received 3 July 2008 lacking. It is assumed that the dominance of wind in maintaining the evolution of mega-dunes should be Received in revised form 20 October 2008 characterized by regular height–spacing relationships that have been found for simple dunes or wind ripples Accepted 21 October 2008 which are dominantly formed by the wind. In this context, we studied the height–spacing relationship for Available online 5 November 2008 the complex reversing mega-dunes in the , which features the tallest mega-dunes in the world. The complex mega-dunes were divided into three hierarchical orders according to the cumulative Keywords: fi Aeolian geomorphology probability plots of dune height and spacing measurements, and the coef cients of variability of dune heights Dune patterns and spacings were in accordance with values reported for other . The relationship between dune Dune height–spacing relationship spacing and height for all the three orders of dunes could be expressed reasonably well by a uniform linear function that was also applied to the height–wavelength relationship for wind ripples in other deserts. This relationship was found to be similar to those for several other deserts and subaqueous bedforms. This implies that there should be few unique factors in maintaining the evolution of complex mega-dunes in the Badain Jaran Desert compared with the superimposed simple dunes and dunes in other deserts, dune fields and subaqueous bedforms, and that the tallest mega-dunes on the earth can be maintained by the wind. © 2008 Published by Elsevier B.V.

1. Introduction wind ripples (Bagnold, 1941; Sharp, 1963; Howard, 1977; Anderson, 1987). A correlation between dune morphology and the dynamics of Regular, ordered patterns are the most striking feature of aeolian bedforms of different orders would help researchers to understand bedforms, and attracted early interest from geomorphologists as the the dynamics of mega-dunes based on studies of small-scale dunes. patterns were considered a classic example of self-organization in a However, this correlation is poorly understood. Although it is easy to geomorphic system (Anderson, 1990; Hallet, 1990). However, how the understand how the wind forms ripples and simple dunes, the regular geomorphic patterns evolve remains poorly understood, evolution of large-scale dunes such as mega-dunes is sometimes especially for mega-dunes (Wilson, 1972; Folk, 1976; Lancaster, considered to be controlled by factors other than the wind (e.g., 1982; Brown, 1983; Wasson and Hyde, 1983; Lancaster et al., 1987; Wilson, 1972). Havholm and Kocurek (1988) monitored a transverse Lancaster, 1988, 1989). It has been found that in all sand seas and dune crescent draa in the Algodones, southern California, for a year. They fields, a hierarchical system of aeolian bedforms is superimposed on found that the draa was oriented by the long-term resultant wind, the observed patterns (Wilson, 1972; Lancaster, 1988, 1995). Similar implying the predominant action of the wind. However, they were not bedform hierarchies have also been found in sub-aqueous environ- able to provide further evidence. Recent investigations (Bristow et al., ments (Ashley, 1990). Usually, three orders of aeolian bedforms can be 2007) showed that mega-dunes, such as the large complex linear identified: wind ripples, individual simple dunes, and compound and dunes in the northern Sand Sea, might be composite forms of complex mega-dunes, which are also referred to as “draa” (Wilson, many different ages reworked by the wind. Whether the wind can 1972; Lancaster, 1995). However, dynamic studies in the field and in dominate the evolution of mega-dunes thus requires further study. laboratory simulations have mainly been conducted on small-scale The Badain Jaran Desert of China features the tallest mega-dunes bedforms, resulting in various explanations for the regular spacing of on earth (Dong et al., 2004), with a maximum dune height of about 480 m. How such large dunes form has always puzzled researchers. Several researchers have speculated about the evolution of these mega-dunes. Most early researchers thought that the mega-dune ⁎ Corresponding author. Cold and Arid Regions Environmental and Engineering patterns were controlled by the underlying humps that defined the Research Institute, Chinese Academy of Sciences, No. 320, West Donggang Road, Lanzhou, Province 730000, China. Tel.: +86 931827 1167; fax: +86 931827 7169. surface relief (Lou,1962; Sun and Sun,1964). Lou (1962) thought that E-mail address: [email protected] (Z. Dong). the large mega-dunes and their patterns formed when drifting sand

0169-555X/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.geomorph.2008.10.015 Z. Dong et al. / Geomorphology 106 (2009) 180–185 181 was blocked by and accumulated on rock mounds or other eroded reversing mega-dunes developed from compound barchanoid mega- undulations. SunandSun(1964)proposed that the patterns of mega- dunes because they have reversed crest zones and superimposed dunes in the Badain Jaran Desert were determined by the underlying barchanoid dunes on their stoss slope. No dunes are superimposed fold undulations formed by tectonic movements. Chen et al. (2004) on the lee sides of the mega-dunes. The annual sand drift potential have suggested that the mega-dune landscapes of the Badain Jaran (DP) estimated by the method proposed by Fryberger (1979) at the Desert are maintained by the presence of ground water because nearest meteorological stations (Sharitai, southeast of the Badain the high mega-dunes are closely associated with lakes. They Jaran Desert) is 88 vector units (VU), indicating a primarily low- discovered that the mega-dunes were relatively rich in moisture wind-energy environment (see Fryberger (1979) for more details of that was likely to be acting as a cohesion agent and providing the the calculation method of drift potential, vector units and classifica- dunes with resistance against wind erosion and transportation. Other tion of wind environments). The resultant drift direction is towards researchers (Tan, 1964; Wang, 1990; Dong et al., 2004) have instead the southeast (115°), with a directional variability of 0.5. The angle recognized the dominant role of wind in maintaining the evolution of between the alignments of the mega-dune crests and the resultant mega-dunes in the Badian Jaran Desert. Tan (1964) proposed that the drift direction is 15°. contemporary mega-dunes overlapped on the old calcareous- In this study, we selected a sample site between latitudes 39°42′ cemented dunes, but he recognized that the patterns of mega- 36.5″N to 40°05′00.7″N and between longitudes 101°44′14″E and dunes were determined by the wind. Aerial photographs and 102°37′21.41″E in the mega-dune–lake area of the Badain Jaran Desert. satellite images have revealed that the mega-dunes in the Badain Here, the relationship between dune height and spacing for different Jaran Desert have orderly spacing, and Wang (1990) noted that it was hierarchical orders of dune was investigated. hard to imagine how this orderly spacing could be controlled by We first measured the height and spacing of the mega-dunes and the underlying relief. In a previous paper (Dong et al., 2004), we those of the simple dunes superimposed on the mega-dunes to define tentatively proposed that wind was the most important factor in their hierarchical orders based on the cumulative probability plots of mega-dune development in the Badain Jaran Desert according to spacing and height as introduced by Ewing et al. (2006). It is assumed their alignment. In the present study, we attempt to provide further that the dominance of wind in maintaining the evolution of mega- evidence to explain the evolution of these mega-dunes by comparing dunes should be characterized by regular height–spacing relation- bedform patterns, especially the height–spacing relationship of ships that have been found for simple dunes or wind ripples which are different hierarchical orders, and to answer a basic question: Can dominantly formed by the wind. So we then chose the height–spacing wind maintain the tallest mega-dunes on earth? relationship as the parameter to characterize the patterns of dunes of different orders. The height and spacing of the mega-dunes and of the 2. Physiographic settings and research methods simple dunes generally over 30 m high superimposed on the mega- dunes were measured on aerial photographs. To obtain these The Badain Jaran Desert lies in the northwest of the Alashan measurements, we scanned and corrected aerial photographs at a Highland in the western of China, between 39°20'N scale of 1:40,000, and measured the height and spacing of the dunes and 42°N, 99°48'E and 104°14'E, covering an area of 49,000 km2 using ArcGIS 9.0 software (http://www.esri.com/). The spacing was (Dong et al., 2004). The tallest mega-dunes in the Badain Jaran measured directly, but the height (H) was derived using the relation- Desert are found in the southeastern part of the area, which has a ship H=L sinα , where L is the length of the slipface, and α is the angle unique landscape in which mega-dunes alternate with lakes (Fig. 1). of repose of the slipface (assumed to be 31° based on measurements of These mega-dunes with their crests running from northeast (35°) to a sample of dunes in the study area). Heights and spacings of the southwest (215°), used to be considered as compound barchanoid simple dunes generally less than 30 m high superimposed on the mega-dunes (Dong et al., 2004). Our recent investigation found that mega-dunes were measured in the field because these dunes were too the tall mega-dunes should be more correctly classified as complex small to measure on the aerial photographs. The average height of two

Fig. 1. A typical landscape in the Badain Jaran Desert with compound barchanoid mega-dunes (the photograph was taken while facing south). 182 Z. Dong et al. / Geomorphology 106 (2009) 180–185

Fig. 2. Cumulative probability plots of dune height and spacing measurements of the complex mega-dunes in the Badian Jaran Desert.

neighboring dunes, (H1 +H2)/2, was used to define the relationship points. Dune populations can be roughly recognized, although their between dune height and spacing (Fig. 1). accurate division points are hard to define. The positions of inflection points have wide ranges because both ends of each segment show 3. Results and interpretation some degree of transition to other segments. The three inflection points occur at dune heights of 2 to 4 m, 10 to 15 m and 40 to 200 m, 3.1. Hierarchical orders and dune spacings of 20 to 30 m, around 50 m and 500 to 2500 m. Inflection point c in Fig. 2 can be obviously recognized on both plots First the hierarchical orders of the complex mega-dunes in the (Fig. 2a and b) because it distinguishes complex mega-dunes from the Badain Jaran Desert are identified on a statistical basis according to the superimposed individual simple dunes. However, inflection points a measurements of dune height and spacing. Ewing et al. (2006) and b on the cumulative probability plots of spacing are rather introduced frequency plots of dune spacing and crest length to identify obscured because they are among the superimposed individual simple the statistical populations in undertaking pattern analysis of dune-field dunes. Combining Fig. 2a and b and considering their spatial parameters. They recognized discrete dune populations as line segments distribution, we divided the four populations in Fig. 2 into three separated by inflection points on the cumulative plots of dune spacing orders that constitute the hierarchical system of the complex mega- and crest length measurements. They found that single populations dunes in the study area with order I being the highest order. The characterized simple dune fields whereas multiple populations char- height of order I, II and III dunes ranges from 1.3 to 16 m, 12 to 107 m acterized compound and complex dunes or dune fields. We did not and 167 to 482 m respectively. The spacing of order I, II and III ranges measure crest length but instead measured dune height. Therefore, in from 12.5 to 75 m, 67 to 707 m and 2064 to 4582 m according to the the present study, we attempt to identify dune populations on the measurements. There are some overlaps between order I and order II. cumulative probability plots of dune spacing and heights (Fig. 2). This suggests that there may be no strict dynamic division between Fig. 2 shows that both the cumulative probability plots of dune order I and order II dunes because they are all individual simple height and spacing show four segments separated by three inflection dunes. It must be emphasized that the three hierarchical orders we

Fig. 3. Probability distribution for dune height and spacing for the three hierarchical orders of dunes. a. Dune height, b. Dune spacing. Z. Dong et al. / Geomorphology 106 (2009) 180–185 183

Table 1 Statistical parameters for the spacing of dunes of different orders.

Order Number of Range of Mean Standard Coefficient of samples spacing (m) spacing (m) deviation (m) variability I 19 12.5 to 75 41 23 0.56 II 181 67 to 707 223 112 0.50 III 94 2064 to 4582 3317 526 0.16

recognized for the complex mega-dunes in the Badain Jaran Desert are not contradictory to the traditionally identified three orders (Wilson, 1972; Lancaster, 1988). In the present study, the order II (individual Fig. 4. The three hierarchical orders of the compound mega-dune system. simple) dunes in the traditional hierarchical system are divided into two sub-orders (order I and order II). It can be concluded from the dunes and the crest zone, and form in response to variations in the following discussion the order I and order II dunes in the Badain Jaran average wind regime over longer periods than order I dunes. They Desert are developed in different sub-environments so that their move much more slowly than order I dunes and are generally aligned difference is reflected in Fig. 2. in the same direction as the mega-dunes, except where they lie Fig. 3 shows the distribution of height and spacing for dunes of all between the horns of two neighboring mega-dunes. In the latter case, three orders. The corresponding statistical parameters are presented the orientation is more random, and is affected by modifications in the in Tables 1 and 2. Dune size differed greatly among the three orders, local airflow by the mega-dunes. although there was some overlap for orders I and II. The mean What causes the difference between order I and order II dunes is spacings between dunes of orders I, II, and III were 41, 223, and the sub-environments in which they are developed. Order I and II 3317 m, respectively, and the corresponding mean dune heights were dunes are superimposed on the windward slope of the mega-dunes. 7.1, 30.5, and 296 m. The distributions of both parameters showed a Fig.1 shows that there is usually a lake or dry lake that supplies limited high degree of scattering, with higher hierarchical orders showing a sediment to the nearby dunes (order I) at the toe of the windward greater degree of scattering. We quantified this scatter using the slope, but the northwesterly wind transports sediments from the toe coefficient of variability, defined as the standard deviation divided by to the crest of mega-dunes so that dunes (order II) on the upper (respectively) the mean spacing and the mean height. Wasson and windward slope receive relatively more sand supply. Order II dunes Hyde (1983) reported results similar to those presented here. They experience stronger wind than order I dunes owing to the speed-up found a coefficient of variability of height ranging from 0.22 to 0.65 effect on the windward slope of the mega-dunes. compared with 0.22 to 0.54 in the present study, and spacing coefficient of variability of 0.21 to 0.75 versus 0.16 to 0.56 in the 3.2. Height–spacing relationships present study, in their field study in Australian deserts, although they did not report the trend we observed for the coefficient of variability Height–spacing relationship is used here to define the significance to increase with increasing dune order. The wider relative variations in of wind in maintaining the evolution of mega-dunes in the Badain height and spacing of the higher order dunes reflect the greater Jaran Desert based on the assumption made in Section 2. Data from variability in wind intensity and direction at shorter temporal scales. studies of dune morphometry and development processes in sand Fig. 4 shows the typical hierarchical orders of the complex mega- seas, dune fields and sub-aqueous bedforms around the world have dunes in the Badain Jaran Desert. Population PD in Fig. 2 constitutes shown a good correlation between dune height and spacing, although the mega-dunes (order III). They show typical features of reversing the relationship varies between dune types and locations (Ashley, dunes, with typical slipfaces and horns (Fig. 1). The top 40 to 60 m of 1990; Lancaster, 1995). Lancaster (1988) suggested that the relation- the mega-dunes is the crest zone (reversing zone), which is the most ship could be represented by a power function, with exponents active part of the dune, and migrates back and forth in response to ranging between 0.52 and 1.72. Remote-sensing images showed variations in wind direction and forms reversed slipface. Super- that patterns for dunes of different order in the Badain Jaran Desert imposed on the windward slope of these mega-dunes are simple are regular. Fig. 5 shows the height–spacing relationships for dunes individual dunes whose size increases from toe to crest of the mega- of different orders in the study area. Data of the height–spacing dunes. The superimposed dunes found below the crest zone on the relationship for barchanoid dunes from other deserts (Lancaster, 1982, windward slope can be divided into two orders (order II and I) 1989; Hasi, 1999; Blumberg, 2006) are also presented. Previous according to dune populations identified in Fig. 2. Populations PA and studies (Lancaster, 1988; Ashley, 1990) showed that dunes and ripples PB constitute order I dunes that are distributed on the lower quarter of (wind ripples and current ripples) formed dominantly by wind had the windward slope of the mega-dunes. They form in response to similar height–spacing or height–wavelength relationships though seasonal winds and the average wind regime over a period of several they belonged to different genetic populations. So Fig. 5 also presents years because their height is less than 15 m. They move quickly and are the height–wavelength relationship for wind ripples reported by aligned in different directions at different positions in response to how Sharp (1963), Walker (1981) and Lancaster (1995) for a more extensive the local air flow is modified by the mega-dunes. Population PC comparison. The height–spacing relationships in the Badain Jaran constitutes order II dunes. They are distributed between the order I Desert and other deserts and sub-aqueous bedforms are similar (Fig. 5), and for all three orders of dunes in the Badain Jaran Desert,

can be expressed well by a uniform linear function (Eq. (1)) on a log10– log10 graph. This uniform function can be extrapolated to express the Table 2 height–wavelength relationships for wind ripples that were reported Statistical parameters for the heights of the dunes of different orders. by Sharp (1963), Walker (1981), and Lancaster (1995). Order Number of Range of Mean Standard Coefficient of samples height (m) height (m) deviation (m) variability − : ; : 2 : ð Þ log10 H = 0 93 + log10 Sp or H =012Sp R =098 1 I 35 21.3 to 16 7.1 3.8 0.54 II 362 12 to 107 30.5 12.3 0.40 where H is the dune or wind ripple height, and Sp is the dune spacing III 190 167 to 48 296 64 0.22 or wind ripple wavelength. 184 Z. Dong et al. / Geomorphology 106 (2009) 180–185

regular patterns of the mega-dunes are controlled by pre-existing surface relief. Similar empirical functions (Table 3) with good correlation coefficients could be obtained if we made integrated reanalysis combining the data of height–wavelength for wind ripples and height–spacing data for dunes of other deserts, respectively. This suggests that the height–wavelength relationship is in good agree- ment with the dune height–spacing relationship reported in several dune fields. It is revealed that the H–Sp relationships for the Baidain Jaran Desert, the Skeleton Coast dune field of Namibia and the of China are close to the height–wavelength relationship for wind ripples, where the dune height increases almost linearly with an increase in dune spacing, or dune height increases at a constant rate relative to dune spacing. The H–Sp relationships for the Namib Sand Sea, Gran Desierto Sand Sea of Mexico and the barchanoid dunes estimated from SRTM by Blumberg (2006) over the world are close, where the dune height increases at a rate decreasing with increasing dune spacing relative to dune spacing. What determines the change rate of dune height with dune spacing requires further study. The height–spacing relationship we obtained (Eq. (1)) for the complex mega-dunes in the Badain Jaran Desert is also similar to that obtained by Flemming (1988, quoted by Ashley (1990)) for subaqueous bedforms (Table 3). In summary, it can be concluded according to the height–spacing relationship that there should be few unique Fig. 5. The relationship between dune height and spacing for the three hierarchical orders of dunes in the present study and in comparable studies from other areas. Fitted factors in maintaining the evolution of complex mega-dunes in the lines: Black line for wind ripples (Sharp,1963; Walker,1981; Lancaster,1995)+complex Badain Jaran Desert compared with the superimposed simple dunes mega-dunes in the Badain Jaran Desert (present study); green line for wind ripples and dunes in other deserts, dune fields and subaqueous bedforms. (Sharp, 1963; Walker, 1981; Lancaster, 1995)+barchanoid dunes in Namib Sand Sea (Lancaster,1989); gray line for wind ripples(Sharp,1963; Walker,1981; Lancaster,1995)+ barchanoid dunes of the Skeleton Coast dune-filed, Namibia (Lancaster,1982); blue line for 4. Conclusions wind ripples (Sharp,1963; Walker, 1981; Lancaster,1995)+barchanoid dunes of the Gran Desierto Sand Sea, Mexico; dark yellow line for wind ripples (Sharp, 1963; Walker, 1981; Three hierarchical orders have been identified for the complex mega- Lancaster, 1995)+barchanoid dunes estimated from SRTM over the world (Blumberg, dunes in the Badain Jaran Desert, according to the cumulative 2006); red line for wind ripples (Sharp,1963; Walker,1981; Lancaster,1995)+barchanoid probability plots proposed by Ewing et al. (2006) and the spatial dunes in the Tengger Desert (Hasi, 1999). distribution of dunes. Both the probability distributions of dune height and spacing were widely scattered. The coefficient of variability Eq. (1) indicates that dune height increases at a constant rate increased as the dune size decreased, suggesting that dunes of higher relative to dune spacing in the Badain Jaran Desert. Lancaster (1988) orders (smaller sizes) are shaped by a more variable wind regime over suggested that the relationship between dune height and spacing shorter periods. The relationship between dune spacing and height for appeared to reflect the availability of sand for dune building and the all three orders of dunes could be expressed by a uniform linear function. wind regime characteristics. However, how this relationship relates to This linear function is also applicable to the height–wavelength these two factors is not yet clear. At least, the uniform relationship that relationship for wind ripples that has been observed in other deserts. covered the height–wavelength relationship for wind ripples and the The consistency of the height–spacing relationship for the mega-dunes height–spacing relationship for all three orders of dunes in the Badain with the superimposed simple dunes and wind ripples in other dune Jaran Desert provides significant information for mega-dune evolu- fields, which are dominantly formed by the wind, suggests that the tion. Wind ripples can be considered of purely aerodynamic origin tallest mega-dunes on the earth appear to be maintained by the wind (Lancaster, 1995). Individual simple dunes of orders I and II super- regime rather than by pre-existing surface relief. imposed on the windward slopes of the mega-dunes (order III) are We have tested a tentative method for ascribing the nature of unlikely to be controlled by the underlying relief since the mega- dune evolution based on dune morphometry. The conclusions we dunes conceal that relief. Therefore, the uniform function for the reached are based on an assumption that the dominance of wind in relationship between dune height and spacing for dunes of the three maintaining the evolution of mega-dunes should be characterized by orders in the Badain Jaran Desert and its applicability to the wind regular height–spacing relationships that have been found for simple ripples in other deserts imply that the tallest mega-dunes are dunes or wind ripples which are dominantly formed by the wind. This maintained primarily by the wind, and it is improbable that the assumption sounds reasonable but knowledge about the mechanism

Table 3 Comparison of the H–Sp relationship obtained by regressive analysis combining the height–wavelength data for wind ripples and height–spacing data for barchanoid dunes in several deserts over the world and subaqueous bedforms respectively.

Location H–Sp relationship r2 Wind ripples (Sharp, 1963; Walker, 1981; Lancaster, 1995) H=0.067Sp1.02 0.94 Wind ripples (Sharp, 1963; Walker, 1981; Lancaster, 1995)+complex mega-dunes in the Badain Jaran Desert (present study) H=0.12Sp 0.98 Wind ripples (Sharp, 1963; Walker, 1981; Lancaster, 1995)+barchanoid dunes in Namib Sand Sea (Lancaster, 1989) H=0.059Sp0.91 0.99 Wind ripples (Sharp, 1963; Walker, 1981; Lancaster, 1995)+barchanoid dunes of the Skeleton Coast dune-filed, Namibia (Lancaster, 1982) 0.066Sp1.01 0.99 Wind ripples (Sharp, 1963; Walker, 1981; Lancaster, 1995)+barchanoid dunes of the Gran Desierto Sand Sea, Mexico H=0.055Sp0.85 0.98 Wind ripples (Sharp, 1963; Walker, 1981; Lancaster, 1995)+barchanoid dunes estimated from SRTM over the world (Blumberg, 2006) H=0.058Sp0.90 0.99 Wind ripples(Sharp, 1963; Walker, 1981; Lancaster, 1995)+barchanoid dunes in the Tengger Desert (Hasi, 1999) H=0.066Sp1.08 0.95 Subaqueous bedforms including current ripples and dunes (Ashley, 1990) H=0.068Sp0.81 0.96 Z. 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