Environ Geol (2008) 56:359–365 DOI 10.1007/s00254-007-1171-2
ORIGINAL ARTICLE
Nebkha development and its relationship to environmental change in the Alaxa Plateau, China Xunming Wang Æ Honglang Xiao Æ Jinchang Li Æ Mingrui Qiang Æ Zhizu Su
Received: 19 October 2007 / Accepted: 18 December 2007 / Published online: 5 January 2008 Ó Springer-Verlag 2008
Abstract Extensive nebkha areas develop mainly under the control of aeolian processes, and their sediments record information on regional environmental changes during different periods. Such areas have developed on the dry riverbeds and deserted arable lands of China’s Alaxa Plateau, Taklimakan, and Kumutage deserts. In this paper, we studied nebkhas that had developed in the Heicheng–Juyan region to determine their CaCO3 contents, particle size distributions, and creation dates. Extensive human activities have occurred in this region since at least in the late Tang Dynasty (618–907 AD). Although historical records show that most of the region’s rivers dried up around 1372, surface water persisted in some areas until the early Qing Dynasty (1644–1911 AD). After the 1600s, extensive nebkhas began to develop due to drying of the region’s rivers. The early stages of nebkha development were controlled by both the sediment supply and the regional wind regime, whereas late stages were controlled primarily by variations in wind activity. In the Alaxa Plateau, it took about
X. Wang (&) H. Xiao J. Li Key Laboratory of Desert and Desertification, Cold & Arid Regions Environmental & Engineering Research Institute, Chinese Academy of Sciences, No. 260, West Donggang Road, 730000 Lanzhou, Gansu Province, China e-mail:
[email protected] X. Wang M. Qiang CAEP, MOE Key Laboratory of West China’s Environmental Systems, Lanzhou University, 730000 Lanzhou, Gansu Province, China e-mail:
[email protected] Z. Su Research Institute of Loess Plateau, Shanxi University, 030006 Taiyuan, Shanxi Province, China e-mail:
[email protected]
100 years for arable lands and riverbeds to evolve into gobi deserts, and during this time, several phases occurred with different levels of wind activity. The land degradation processes in this region are mainly controlled by surface water resources, and the impact of human activities such as reclamation on land degradation appear to have been overestimated in previous studies. Keywords Nebkha Environmental change Land degradation Alaxa Plateau
Introduction Nebkha dunes, which are also referred to as nabkhas, coppice dunes, or vegetated dunes, form as a result of basic aeolian processes (Tengberg 1995), and are found extensively throughout the world’s arid, semiarid, and subhumid regions (e.g., Nickling and Wolfe 1994; Parsons et al. 2003). Previous studies revealed the dynamic processes responsible for their formation (e.g., Hesp 1981; Hesp and McLachlan 2000) and suggested that the sediments that form these dunes come from adjacent rather than distant regions (e.g., Khalaf et al. 1995; Langford 2000; Marston 1986). In China and other regions of the world, the initial and subsequent development of nebkhas are usually regarded as good indicators of land degradation and the condition of the environment (e.g., Gile 1975; Zhu et al. 1981; Tengberg and Chen 1998; Wang et al. 2006a). Extensive nebkhas that have formed around arid-land shrubs, and especially Chinese tamarisk (Tamarix chinensis Lour.), have developed in the Alaxa Plateau. This area is located in northwestern China, and is a hyper-arid region with a mean annual precipitation of about 36 mm (1954–2003), a potential evapotranspiration of 3,500 mm
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(1954–2001), mean annual wind speeds exceeding 3.3 m/s, and 26 windy days per year. The Alaxa Plateau is an attractive study area for earth and environmental scientists due to its high wind activity, with the annual drift potential exceeding 200 vector units (VU) and dust storms occurring on more than 67 days per year. As a result, the area is regarded as a typical region with high dust emissions that has undergone significant environmental changes (Zhang et al. 2003; Wang et al. 2006b). Although most ground surfaces in this region are currently covered by gobi deserts, the reclamation history of this region can be traced back to at least the Western Han Dynasty (206 BC to 24 AD) of ancient China due to the formerly abundant water resources. Many ancient cities, such as Juyan and Heicheng, were founded in this region (Fig. 1). However, in the Heicheng–Juyan region, there are no records of human activity after 1372 due to the collapse of the Yuan Dynasty (1271–1368 AD) and occupation of the area by the Ming Dynasty (1368–1644 AD) in the region. Climate change plays a major role in the origins and development of human civilization (e.g., Weiss and Bradley 2001; Hsu 1998), but whether environmental change is caused primarily by human activities or climate change (particularly variations in water resources) has been debated, and the debate has not been resolved for the Heicheng– Juyan region due to the lack of historical records (Li 1991). For instance, Zhu et al. (1983) proposed that over-
Fig. 1 Location and geomorphologic characteristics of the Heicheng–Juyan region, modified after Zhu et al. (1983)
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reclamation in the region was the key cause of land degradation, whereas Zhao (1987) suggested that climate change had controlled the environmental evolution of the region. Fortunately, the nebkhas that have developed in this region provide a proxy with a relatively high resolution that can provide important insights into environmental change and land degradation. Therefore, in the study described in this paper, the origins and subsequent development of nebkhas in the Alaxa Plateau were investigated and their significance for revealing the progress of environmental change and land degradation were discussed in this region.
Sampling location and analytical methods Extensive nebkhas, with dune heights varying between 3 and 10 m and diameters ranging from 6 to 10 m have formed around Chinese tamarisk in many areas of the Heicheng–Juyan region (Fig. 2). The sediments that underlie the nebkhas in the study area are riverbeds and formerly arable land, which dried out and were deserted because the landscape lying near extensive areas of gobi desert was subjected to high levels of wind activity that exacerbated the loss of water caused by the historical river diversion. In addition to the tamarisk that form the nucleus of many of the nebkhas, other shrubs and trees grow in the study area.
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Fig. 2 Photograph of a typical landscape in the study area and of the nebkha that was selected for intensive analysis
It was hypothesized that the sediment stratification in the nebkhas has been largely uninterrupted because the vegetation cover exceeds 60%, compared with a vegetation cover of\14% where dune surfaces are active (Wiggs et al. 1995). Therefore, the size of these nebkhas has increased due to more or less continuous sediment deposition from their early stages to the present, with hardly any wind erosion of their surfaces occurring. On this basis, the sediments in different strata within the nebkhas contain important information on environmental changes in the region. The nebkha, selected for sampling, was located in a dry riverbed, 6.10 m tall and about 8 m in diameter, and had a roughly round shape. There were no signs of degradation of this dune because the vegetation cover exceeded 80%. Field methodology included sediment collection and morphological analysis. Field investigations also included surveys of the regional environment to determine the geomorphological type, vegetation cover, development of drainage patterns, and land use. The nebkha selected for intensive analysis was excavated from its crest to its base, and sediments were sampled at 5-cm intervals throughout the profile. In addition, sediments to a depth of 20 cm below the nebkha were obtained as a sample of the region’s undisturbed sediments. Laboratory analyses included measurements of the particle size distribution in each layer, determination of the CaCO3 content in the sediments, and analyses of remote-sensing images to distinguish the geomorphological characteristics of the region. The CaCO3 contents in the sediments were analyzed in accordance with
the national standard of the People’s Republic of China (NY/ T86-1988), and grain size analysis was performed using a Mastersizer 900 with a resolution of 0.02–900 lm (Malvern Instruments Ltd., Malvern, UK). In addition, the 14C ages of the strata of the nebkha were determined using the litter collected in the sediments; measurements were obtained by means of accelerator mass spectrometry (Model 1.5SDH-1; National Electrostatics Corporation, Madison, WI, USA).
Results Age of the nebkha and its accumulation rates during different periods Dating of a wood artifact 10 cm in diameter and extending to a height 30 cm above ground level embedded in the protective walls surrounding the ruins of the ancient town of Heicheng (Fig. 3) revealed a 14C age of 1,170 ± 40 a BP, dating to around 829 AD This result shows that Heicheng was in existence by at least the late Tang Dynasty (618–907 A.D.). There are extensive signs of human settlement around this sample site. However, dating of relict leaves of Chinese tamarisk collected from the bottom of the nebkha show that its age is only 250 ± 40 a BP, which suggests that it originated around 300 years ago. On this basis, the rate of sediment accumulation averaged around 2 cm/a. In addition, based on 14C dates for litter collected within the strata of the nebkha, it took about 85 years for this nebkha to reach a
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Contents of CaCO3 and of fine and coarse particles in the sediments
Fig. 3 Ruins of the ancient city of Heicheng and the location of the sample used for 14C dating
height of 1.75 m (an accumulation rate of about 1.8 cm/a), then 120 years to increase from this height to a height of 1.95 m (an accumulation rate of only 0.16 cm/a). Over about the past century, the accumulation rate averaged 3.3 cm/a and the nebkha height increased from 1.95 to 6.10 m. The significant variations in the growth rate of the nebkha during different periods show that there were changes in wind activity and the supply of sediments in this region during the past ca. 300 years: during periods with a relatively high wind activity and with an abundant supply of sediment, the accumulation rates of nebkhas increased. However, once the region’s sediments reached equilibrium late in the period of nebkha development, the growth rate of the nebkha was controlled primarily by wind activity in the region.
The CaCO3 contents in the sediments that formed the base of the nebkha, the walls of Heicheng, the lake deposits, and the gobi desert surfaces averaged 17.7, 11.1, 16.5, and 9.6%, respectively. In the nebkha, this value averaged only 8.5% (Fig. 4), which is most similar to the value obtained in the gobi deserts. In this region, annual precipitation is only 36 mm, so leaching processes would be very weak. This belief was supported by microscopic examination of the surfaces of the grains that made up the nebkha, which showed no signs of prolonged exposure to water. Therefore, the CaCO3 component of the early nebkha sediments appear to have originated mainly from the surface of the region’s gobi deserts and were transported to the dune’s current site by wind instead of resulting from soil-forming processes. In addition, the major sources of CaCO3 during the early stages of nebkha development were mainly deserted arable lands and dry riverbeds that contained high levels of CaCO3, and the periods with higher levels of CaCO3 in the sediments of the nebkha continued until the fine materials in the surface soils of the deserted arable lands and dry riverbeds were lost to the wind, leaving primarily coarse materials behind. Grain size analyses of the sediments in different layers of the nebkha showed that during the early stages of nebkha development, the coarse fractions were relatively high. This may be because the dry riverbeds during this period provided more coarse fractions capable of being transported by the winds to the nebkha, or perhaps due to an early period of high wind activity in this region. However,
Fig. 4 Contents of CaCO3, fine particles (\16 lm), and coarse particles ([200 lm) in the different layers of the nebkha
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as wind erosion continued over long periods in the study area, the fine fractions from arable lands and riverbeds were transported to the dune, and the nebkhas developed as a result of accumulation of these sediments, whereas the gobi deserts developed as a result of the loss of these sediments and the resulting deflation. After the dune attained a height of 1.5 m, there were no significant variations in the fine and coarse fractions in different layers of sediments, and the nebkha development processes mainly reflect variations in wind activity in the region.
Discussion As is the case for mobile dunes, the development of nebkhas is closely related to vegetation cover, groundwater levels, and the sediment supply. As a result, the origin and subsequent development of nebkhas provide explicit signals of the factors responsible for regional land degradation (Wang et al. 2006a; Zhu et al. 1981). However, nebkhas only develop in regions with high levels of wind activity and with groundwater maintained within a certain range. In the Alaxa Plateau, field investigations show that nebkha development can be divided into several stages (Fig. 5). The nebkhas that have developed in the Heicheng–Juyan region are currently stable, and the main reasons why they have not degraded are that groundwater levels in this region have not decreased significantly. The amplitude of the fluctuation in the depth of water has not exceeded 0.5 m over the past 200 years (Sun et al. 2006), and this is not sufficient to damage the regional vegetation enough to
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trigger degradation of the nebkhas. Therefore, once the nebkhas began to form in the region, their development processes and sediments have primarily recorded variations in the region’s sediment supply and wind regime. The age of the nebkha analyzed was only 300 years, even though historical records show that the region’s rivers and lakes had mostly dried out by 1372, which suggests that the period of nebkha development only covered a small portion of the last 600 years of desertification. However, only 200 years after 1372 did extensive nebkha areas begin to develop in this region. Therefore, although the rivers were diverted from the region by the dynastic wars that occurred around 1372, surface water still existed intermittently in this region for some time, and desertification did not occur immediately. The intermittent presence of surface water lasted for nearly 300 years more, until the collapse of the Ming Dynasty (1368–1644 AD) and the foundation of the Qing Dynasty (1644–1911 AD). Early in the Qing Dynasty period, with surface water having disappeared and the arable lands deserted, nebkhas began to develop on the riverbeds and in arable lands as desertification progressed. These results also show that, at least in the Heicheng–Juyan region, the collapse of agricultural civilization did not result from over-reclamation of the region’s land, but was instead due to an absence of surface water resources. The CaCO3 contents in the surface materials in arid and semiarid China are closely related to aridity, precipitation, and leaching processes (Gao et al. 1988). In the Heicheng– Juyan region, the contents of CaCO3, fine particles, and coarse particles in the sediments that form the different
Fig. 5 Periods of nebkha development in the Alaxa Plateau. a During the early stage, the amount of water in rivers decreased slowly, but groundwater levels remained high, so no nebkhas began to develop. b During the origin of the nebkhas, riverbeds began to emerge due to a decrease in the amount of surface water, and a few nebkhas began to form on the exposed riverbeds and in deserted arable land. c During the developing stage, river beds became thoroughly dry and extensive nebkha areas began to develop. d During the degradation stage, many nebkhas began to degenerate and evolved into mobile dunes, with only a few distributed in regions with relatively high groundwater levels
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layers of the nebkhas mainly indicate the effects of regional aridity, and further indicate variations in the regional sediment supply. During the early stages of nebkha development, the major contributors of sediment were deserted arable lands and dry riverbeds, which contain abundant CaCO3 and fine particles. During this stage, it is very difficult to appraise the variations in the regional wind environment from the contents of CaCO3 and of fine and coarse particles due to the complex nature of sediment supplies, but the trends in CaCO3, fine particles, and coarse particles suggest that it took roughly 100 years for the arable lands and riverbeds to lose their fine materials and become enriched in coarse particles after the rivers dried up. After this coarsening of the arable lands and riverbeds was mostly completed, the height of the analyzed nebkha increased from 1.5 to 3.2 m, and during this period, the variation in CaCO3, fine particles, and coarse particles in the sediments were controlled by variations in the regional wind regime and sediment supply. However, after the nebkha height exceeded 3.2 m, coarsening of the arable lands and riverbeds was complete, and the development of nebkhas was mainly controlled by the wind regime. The contents of CaCO3, fine particles, and coarse particles in the sediments during this period indicate that the wind environment has changed over the past century. During this recent period, the contents of CaCO3, fine particles, and coarse particles show that the region’s wind regime can be divided into five phases, which parallel the development of dune height from 3.1 to 3.6, 3.6 to 4.6, 4.6 to 5.2, 5.2 to 5.6, and 5.6 to 6.1 m. Dune height appears to have increased from 4.6 to 5.6 m during the late 1950s to the mid-1980s, which has previously been reported to be a period with high wind activity in arid and semiarid China (Wang et al. 2007).
Conclusions In the Alaxa Plateau, the origin and development of nebkhas were controlled by aeolian processes, but variations in sediment deposition during different periods indicate that environmental changes also were a significant factor. In the Heicheng–Juyan region, the nebkhas only began to form after the late Ming Dynasty period due to drying of the region’s rivers and lakes, and rapid land degradation also occurred during that time. In this region, the early stage of nebkha development is controlled by both the wind regime and the sediment supply, whereas the late stage was mainly controlled by variations in wind activity. It appears to have taken about 100 years for the arable lands and riverbeds in the region to complete their evolution into gobi deserts. Land degradation appears to have resulted mainly from the loss of surface water resources over the past 300 years,
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and human activities such as reclamation do not appear to be the principal causes of land degradation in the Alaxa Plateau. Although human activities may still play a major role in desertification under certain circumstances, it appears to be unfair to blame land degradation exclusively on human activities, even in the hyper-arid regions of China. Acknowledgments This work was supported by the NSFC 90702001. The CaCO3 and grain size analysis were carried out by Prof. Y Liu and H Jia of CAREERI, and 14C dating was carried out by Dr. Z Wang and Y Chen of Lanzhou University. Special thanks are due to our colleagues for their critical comments on an earlier version of this manuscript. We thank Mr. Baoshan He for his assistance in preparing the illustrations and Mr. Geoff Hart for his detailed edits of the manuscript.
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