Géomorphologie : relief, processus, environnement, 2006, n° 4, p. 275-284

Desert research in northwestern – a brief review Recherches consacrées aux déserts du nord-ouest de la Chine – une brève revue

Xiaoping Yang*

Abstract are widely distributed in northwestern China in a wide range of geomorphological and tectonic settings, from 155 m below sea level to more than 5000 m above sea level (asl.). In this paper the author briefly reviewed some of the recent studies aiming to under- stand the formation and changes of the sand seas in northwestern China. Loess sequences in Loess Plateau indicated that deserts in western China may have existed already 22 Myr ago, but geomorphological and sedimentological evidence found in the deserts sug- gests a much younger age. The geomorphological link between the Tertiary deserts and the present-day ones is not yet understood. In many parts of the deserts in northwestern China, late Pleistocene or even Holocene lacustrine and fluvial sediments are buried under the , showing abrupt Late Quaternary environmental changes in the areas. Although the mechanisms for the formation of the megadunes in the Badain Jaran are controversially argued in various publications - landforms control vs. groundwater con- trol -, geomorphological investigations are essential in order to solve this dispute. It is envisaged that geomorphology would be really crucial to the development of the Earth system sciences, dunes representing significant information archives for understanding the Earth system. Key words: aeolian process, desert, dune, groundwater, lake, China.

Résumé Les déserts sont largement représentés dans le nord-ouest de la Chine où on les trouve à des altitudes allant de -155 m à plus de 5 000 m dans des contextes géomorphologiques et tectoniques variés. Cet article dresse une brève revue de travaux récents sur la for- mation des déserts sableux du nord-ouest de la Chine et les changements qui les ont affectés. Les séquences de loess sur le Plateau de Loess attestent l’existence de déserts dans le nord-ouest de la Chine depuis au moins 22 millions d’années, mais d’autres données géo- morphologiques et sédimentologiques suggèrent des âges beaucoup plus récents. Le lien entre les déserts du Tertiaire et les déserts actuels n’est en fait pas encore entièrement compris. Dans les déserts du nord-ouest de la Chine, des sédiments lacustres et fluviaux d’âge pléistocène, ou même holocène, sont enfouis sous les dunes en de nombreux endroits, ce qui montre que les secteurs dunaires ont connu de brusques changements environnementaux pendant le Quaternaire supérieur. D’autre part, les mécanismes à l’origine de la formation des méga-dunes du désert de Badain Jaran tels qu’ils sont présentés dans les diverses publications consacrées à ce sujet étant controversés (rôle des formes de relief par rapport à celui des eaux souterraines), des études géomorphologiques paraissent essentielles pour résoudre cette polémique. La géomorphologie pourrait constituer un apport crucial dans le développement des sciences du système Terre, les dunes des déserts sableux constituant des archives pouvant fournir des informations précieuses pour comprendre la dynamique du système terrestre. Mots clés : processus éolien, désert, dune, aquifère, lac, Chine.

Version française abrégée « Gobi » ou déserts caillouteux (fig. 1). Cette note a pour but de présenter brièvement les progrès récemment accom- En Chine, les déserts appartiennent à une large gamme de plis dans l’étude géomorphologique des dunes dans les contextes géomorphologiques et tectoniques entre 150 m au déserts du NW de la Chine. Le problème de l’initiation de la dessous et 5 000 m au-dessus du niveau de la mer. Selon la formation des « mers de sable » en Chine occidentale est terminologie chinoise (Zhu et al., 1980), les déserts sont en fait lié à plusieurs questions cruciales qui relèvent des classés en « déserts ou mers de sable », « régions ou Sciences de la Terre. On admet généralement que les dépôts terres sableuses » (qui sont surtout des dunes stabilisées) et de loess en Chine ont une origine éolienne et proviennent de

* Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O.Box 9825, Beijing 100029, China. E-mail: [email protected]; [email protected] Xiaoping Yang

ces déserts. Les études des séquences de sols dans les loess les sables n’est pas encore complètement connu. Ainsi l’ap- miocènes formant les plateaux de loess occidentaux suggè- parente controverse à propos de l’explication de la genèse rent que des déserts ont existé en Asie intérieure depuis au des mégadunes du désert de Badain Jaran confirme à nou- moins 22 Ma (Guo et al., 2002). En revanche, nos investi- veau l’importance de la géomorphologie dans la compré- gations conduites dans les déserts du NW de la Chine ont hension des processus à la surface de la Terre. Du reste, la révélé que leur genèse serait beaucoup plus récente. Zhu et géomorphologie représentait la discipline la plus importan- al. (1981) avaient déjà noté que celui du Taklamakan déri- te dans la conception que Richthofen (1886) se faisait des vait d’un désert initial dont la formation remonterait au sciences du « système Terre ». Cet article illustre aussi le Pléistocène moyen. La paléogéographie des régions du Tak- fait que l’exploration des dunes reste à entreprendre pour lamakan pendant le Quaternaire a été interprétée comme un comprendre la dynamique du système Terre. ensemble de divers lacs (Norin, 1932 ; Jäkel, 1991) ou de cônes torrentiels et fluviaux (Zhu et al., 1981). Une question Introduction clé, qui mérite des investigations futures, est celle de la rela- tion géomorphologique entre les déserts actuels et ceux dont Physically and culturally diversified desert landscapes are l’existence est déduite des sédiments éoliens plus anciens extensive in the interior of Asia (e.g. Goudie, 2002). Gener- (miocènes). ally speaking, deserts of China are distributed in a wide La plupart des interprétations antérieures présupposaient range of geomorphological and tectonic settings, from the que la sécheresse s’était continuellement amplifiée à l’inté- Turpan Depression 155 m below sea level to the intramon- rieur de l’Asie durant le Quaternaire (cf. Berg, 1907). tane basins in the western Tibetan Plateau at elevations Cependant, des études plus récentes consacrées aux ter- > 5000 m above sea level (asl.). In Chinese terminology rasses du fleuve Keriya dans le désert du Taklamakan ont (Zhu et al., 1980) the deserts are classified into sandy montré que le ruissellement était plus actif et les précipita- deserts (sand seas), sandy lands (mostly stabilized dunes) tions plus élevées durant le maximum local de la dernière and Gobi (gravel deserts, fig. 1). Sandy deserts or sand seas glaciation qu’à l’heure actuelle (Yang et al., 2002). La chro- occur mainly in three altitudinal zones in China. Here the nologie des dépôts éoliens et lacustres au centre et à la lower zone of the aeolian landforms refers to the extensive bordure méridionale du Taklamakan a été établie en utili- sandy deserts and sandy lands located in northern China at sant la méthode de datation OSL (Optically Stimulated elevations up to 2000 m asl., the second between 2800 m Luminescence) (Yang et al., 2006a). Sur la base de ces nou- and 3200 m asl. in the Chaidamu Basin of northeastern velles datations, les lacs se sont formés au centre du Tibetan Plateau, and the third on the Tibetan Plateau from Taklamakan entre 30 000 et 2 000 ans BP. Une autre pério- 3500 m to 4900 m asl. (Jäkel, 2002). The present-day mor- de correspondant à un milieu lacustre a pris place entre phology of these deserts is a residue of long and short-term 40 000 et 30 000 ans BP. Des prismes sableux ont été datés variations due to interactions between authigenic and exter- à environ 40 000 ans et 18 000 ans BP par OSL (Yang et al., nal forces within the Earth system. The water cycles 2006a), ce qui indique une diminution de la température de crossing these sandy landforms impact greatly on regional plus de 12°C par rapport à l’actuelle. water balance and on the availability of local surface and Les dunes, dont la hauteur maximum est de 460 m dans le ground waters. These deserts may directly influence the désert de Badain Jaran, ne sont pas seulement les plus éle- global climate system through sediment cycles such as dust vées des déserts chinois mais aussi dans le monde. Plusieurs emissions. Therefore, greater knowledge of the evolution of facteurs invoqués, comme la superposition de dunes succes- desert landscapes would considerably increase our under- sivement formées durant plusieurs périodes et associées à standing of the Earth system. This paper was intended to des fluctuations climatiques, la présence d’un substratum briefly review recent progresses in the study of the geomor- enfoui, etc., auraient joué un rôle décisif dans la genèse de phology of desert dunes in northwestern China. ces mégadunes (Yang, 1991; Yang et al., 2003). Une nouvel- le explication de leur genèse repose sur le rôle crucial que Ages of the sand seas in joueraient des aquifères étendus. Il a été suggéré que des ri- northwestern China vières souterraines s’écoulaient depuis les montagnes du NE du plateau tibétain en direction du désert de Badain The initial age of the sand seas in western China is relat- Jaran. On a avancé l’hypothèse selon laquelle les eaux ed to answers to several crucial questions in Earth Sciences. s’écoulaient jusqu’au niveau du sommet des dunes et au- Earlier numerical modeling showed that the high pressure raient ainsi autorisé la stabilité des sables sur celles-ci over Mongolia-Siberia (causing winter monsoon in northern (Chen et al., 2004). Le point faible de cette hypothèse rési- Asia) was formed due to the uplifting of the Tibetan Plateau de dans le fait que les eaux se seraient forcément écoulées (e.g. Manabe and Terpstra, 1974). Consequently, the initial vers l’aval avant d’atteindre le sommet des dunes, car l’eau age of the sand seas was supposed to be same as the age of présente dans ces dernières est stockée dans un système ou- a high Tibetan Plateau. Loess deposits in China are widely vert. L’explication ayant trait à la morphologie du soubas- accepted to be of aeolian origin and from the deserts. As the sement enfoui n’est pas convaincante non plus car, d’une chronology of loess has improved, the assumption of the ini- part, la plupart des dunes sont complètement couvertes par tial age of the deserts is also revised. The recently influential des sables éoliens (fig. 3) et, d’autre part, le matériau sous studies of the Miocene loess-soil sequences in the western

276 Géomorphologie : relief, processus, environnement, 2006, n° 4, p.275-284 Desert research in northwestern China - a brief review

1 : déserts ou mers de sable, 2 : régions ou terres sableuses, 3 : Gobi, 4 : loess, 5 : loess, Gobi, 4 : 3 : régions ou terres sableuses, déserts 2 : ou mers de sable, 1 : 1: sandy deserts, 2: sandy lands, 3: Gobi, 4: loess, 5: mountains; 6: rivers, 7: lakes. lakes. 7: rivers, 6: mountains; 5: loess, Gobi, 4: 3: sandy lands, sandy deserts, 2: 1: . Répartition des déserts en Chine, incluant les noms des désertsRépartition incluant mentionnés dans le texte des déserts en Chine,

Distribution of desert landscapes in China, with the names of deserts mentioned in the text. with the names of deserts of desertmentioned in the text. 1 – Distribution landscapes in China, Fig. 1 – Fig. lacs. 7 : fleuves, 6 : montagnes ;

Géomorphologie : relief, processus, environnement, 2006, n° 4, p.275-284 277 Xiaoping Yang

Loess Plateau suggested that deserts existed in the Asian the development of a strong high pressure in northern Asia, interior at latest 22 Myr ago (Guo et al., 2002). resulting in a strong influence of winter monsoon accompa- In contrast, investigations carried out in the deserts of nied by arid conditions in the interior of Asia (Ruddiman China showed a much younger age. Zhu et al. (1981) con- and Kutzbach, 1989; Guo et al., 2004). The lithostratigraph- cluded that the had been developed from ic studies of Red-earth Formation in northern China showed the initial desert that was formed in middle Pleistocene. Tak- that both the Tibetan uplifting and the ice-building lamakan, as the largest desert in China, has drawn great processes would have played important roles in the aridifi- attention from desert geomorphologists. There are four main cation in Asian interior during Late Miocene and Pliocene opinions about its palaeogeography during the Quaternary. (Guo et al., 2004). (1) A fresh water lake formed in the interior of the Takla- makan during the Quaternary (Norin, 1932): this was based Records of climatic changes on observations of lacustrine sediments along the rivers flowing into the desert and on the western margin of the Much of the earlier concepts assumed a continuous inten- basin. Norin (1932) reported Quaternary erosion forms due sification of dryness in the interior of Asia during the to waves and residues of lacustrine sediments on the south- Quaternary (e.g. Berg, 1907). However, more recent studies ern slope of the Mazhatage Mountains in the southwestern of the river terraces along the Keriya River in the Takla- part of the Tarim Basin. makan Desert showed that runoff and precipitation during (2) A great lake appeared in the Tarim Basin during the the local peak of the last glaciation were much higher than Early Pleistocene (A Russian expert named Shumef, origi- at present (Yang et al., 2002). Furthermore, the availability nally written in Russian, cited from Zhu et al., 1981): the of water was much larger than at present owing to the cold lake level was supposed to be as high as 1250 m asl. and temperature and reduced evaporation at that time (Yang et extended to the Turpan-Hami depression. The Lop Nuer was al., 2002). Clear climatic variations during the last glacia- seen as the final residue of this formerly large lake. The tion were recognized also in a sediment core from the dunes were thought to be formed along the old shorelines. Chaidamu Basin where the second altitudinal zone of aeo- (3) Alluvial and fluvial fans may exist: the sediments lian landforms is located. The proxy data from this core underlying the Taklamakan Desert were supposed to be of suggested that the local climate was much drier during the alluvial and fluvial origins. Alluvial fans and fluvial deltas period between 24 ka and 15 ka than between 32 ka and were aggregated both in the south and in the north of the 24 ka (Hövermann and Süssenberger, 1986). Taklamakan Desert during the Quaternary. The lake was The grain size changes of loess sections on desert margins restricted only in the area of Lop Nuer (Zhu et al., 1981). were shown to be very useful for deducing variations in (4) The entire eastern Taklamakan Desert was assumed to desert extent north of the Loess Plateau (Ding et al., 1999). be inundated by the large palaeo-lake Lop Nuer during the The dunes of sandy lands in semi-arid regions of China were Pleistocene and its highest lake level would be ± 1000 m asl. active during the colder epochs such as marine oxygen iso- (Jäkel, 1991). tope stages two and four, and they should be stabilized by Obviously, quite young ages mostly on the basis of field vegetation at warm times like today (Zhu and Liu, 1981). investigations in the desert are not consistent with the results Little variations could be expected in the sand seas of west- inferred from aeolian sequences in the Loess Plateau. This ern China while the arid climate was attributed to the reconfirms the complexities and challenges a desert geo- uplifting of the Tibetan Plateau. However, studies in the morphologist has to face. A key aspect would be the dune fields and luminescene chronology revealed that both geomorphological link between the present-day deserts and the stability of dunes and the extension of dune fields have the deserts inferred from early aeolian sediments. Have the undergone distinct variations in the Late Quaternary. deserts occurring 22 Myr ago continuously existed until Strongly cemented, laterite-coloured dunes were observed today? The answer could be no if these dunes were once on the eastern margin of the Badain Jaran and they were buried by lacustrine or fluvial sediments to a large extent. dated to ca 57 – 51 ka by thermoluminescene dating (Yang, Detailed examinations of corings from the Taklamakan 2004). Similar aeolian sands underlying sandy loess on the would provide clues for such a question. As the Quaternary eastern margin of the Badain Jaran were dated to ca 121 ka, sediments reach a thickness of over 1000 m in the Tarim showing that the extent of the dune fields were larger at Basin, it would be an expensive effort to obtain sediment those times than at present (Yang, 2004). cores with high resolution. Based on geomorphological and With regard to desert geomorphology, distinguished work sedimentological evidence and radiocarbon chronology, has been done in the Taklamakan Desert. Great attention Jäkel and Zhu (1991) reported that dunes in many places of was given both to the landforms and to personal experiences the Taklamakan were even younger than 20 kyr. in the books written by earlier explorers (e.g. Hedin, 1899; Although it has been widely accepted that the formation 1904). Based on air photographs, topographical maps and of deserts in western China was due to uplifting of the fieldwork, Zhu et al. (1981) described the types of dunes Tibetan Plateau, other factors may have been decisive as and discussed the potential parameters controlling the char- well. It appeared that the formation of arid climate in central acters of dunes in the Taklamakan. The publication of the Asia may also be related to the ice volume in arctic regions, relatively precise map of aeolian landforms in the Takla- because a large extension of ice in the Arctic would cause makan (Zhu et al., 1980) represents some of the key

278 Géomorphologie : relief, processus, environnement, 2006, n° 4, p.275-284 Desert research in northwestern China - a brief review achievements in desert research in China. Landforms, sedi- 37 ka and 32 ka, confirming a quite large geographical mentological features such as grain sizes and heavy mineral extension of wetter climate at that time. assemblages and microstructures on the quartz grains of The centre of the Taklamakan is probably one of the most sands in the Taklamakan have been investigated by various suitable places to demonstrate the strong interactions be- authors (e.g. Zhu et al., 1981; Besler, 1991; Coque and Gen- tween aeolian, fluvial and lacustrine processes on the Earth telle, 1991; Yang, 1991). The geomorphological and surface. Dome-shaped dunes are distributed in the delta of sedimentological records of climatic changes have become the Keriya River. The dunes in this delta reach a height of important objectives of more recent studies in deserts. The 30 m and are surrounded by old river courses (fig. 2). Sand alluvial landforms in the southern of the Tarim Basin were samples taken from windward sides are dated to only a few associated with water flows during different stages of the hundreds years, indicating a relatively young age of the last glaciation (Hövermann and Hövermann, 1991). A dunes (Yang et al., 2006a). The grain size characters also chronology of aeolian and lacustrine deposits from the cen- show that the Taklamakan Desert is still in an early stage of tre and southern margin of the Taklamakan was established development (Besler, 1991). using optically stimulated luminescence (OSL) dating meth- The abovementioned studies should have made it clear ods (Yang et al., 2006a). According to these new datings, that not only the aeolian processes but also the fluvial pro- lakes were formed in the centre of the Taklamakan at around cesses have changed considerably in the Taklamakan during 2000 and 30,000 years ago. Another period of lacustrine the Late Quaternary. The hydrological systems have also environment occurred between 40,000 and 30,000 years undergone abrupt changes in the Taklamakan accordingly. ago. It was suggested that the climate was much wetter than At present the Niya River is quite short and becomes extinct at present while lakes occupied the central part of the desert in the south of the Taklamakan. Based on satellite imageries, (Yang et al., 2006a). In contrast, the development of large it was found that the Niya River flowed into the Keriya migrating dune fields was the predominant geomorphologi- River in the past (Yang et al., 2006b). Fluvial forms and sed- cal process during periods of more arid conditions. Sand iments indicate that the Keriya River was flowing into the wedges at the southern margin of the Taklamakan were first- Tarim River on the northern margin of the desert during the ly reported by Hövermann and Hövermann (1991) and were late last glacial, and at about 2000 years ago as well as dur- later dated to ca 40,000 and ca 18,000 years ago using OSL ing the Little Ice Age (Yang et al., 2002). Historical maps (Yang et al., 2006a), indicating a temperature decrease of and literature show that the Lop Nuer was a large lake in the more than 12°C when compared to present conditions. Han Dynasty (206 BC to AD 220) and became a much According to lacustrine records (Rhodes et al., 1996), wet- smaller one in the Qing Dynasty (AD 1644 – 1911). Archae- ter conditions also prevailed in the Zhungar Basin between ological excavations confirmed that irrigated agriculture was

Fig. 2 – Dunes surrounded by river courses in the centre of the Taklamakan desert. Fig. 2 – Dunes entourées de cours d’eau dans le centre du désert du Taklamakan.

Géomorphologie : relief, processus, environnement, 2006, n° 4, p.275-284 279 Xiaoping Yang

Fig. 3 – A schematic cross-profile of the landscape around the lakes in the SE of the Badain Jaran Desert. 1: present-day shifting sands; 2: palaeo-dunes; 3: outcrop of bedrock granite; 4: layer of calcareous cementations; I: zone of active and semi-active dunes; II: consolidated and semi-consolidated dunes; III: sand heaps covered by shrubs; IV: salty grasses; V: swamp grasses; VI: water surface. Fig. 3 – Coupe schématique du paysage autour des lacs dans le SE du désert de Badain Jaran. 1 : sables migrant actuellement ; 2 : paléo-dunes ; 3 : substratum granitique affleurant ; 4 : couche d’encroûtements calcaires ; I : zone de dunes actives ou partiellement actives ; II : dunes consolidées ou partiellement consolidées ; III : monticules de sable couverts par des buissons ; IV : Végétation halophile ; V : végétation de marécage ; VI : eau en surface.

widely practised from ca 200 BC to ca AD 500 in the lower plenty of small calcareous tubes (length: 2–20 cm, diameter: reaches of the Keriya and Niya rivers and in many other 0.5–3 cm) having formed due to accumulation of carbonate parts of the interior of the Taklamakan (Yang et al., 2006b), around the plant roots. Such roots are indicative of an consistent with geomorphological records (Yang et al., increase in precipitation and higher plant coverage. The sur- 2002). Both the human activity and natural environmental face of the dunes could be more or less stabilized due to changes have played a role in the variations of the extension calcareous consolidation and an increase in plant coverage. of the oases during historical times (e.g. Zhu and Liu, 1981; The calcareous cementations are nowadays only preserved Wang, 1998; Yang, 1998; Mu and Liu, 2000). occasionally because the cement of calcium carbonate is vulnerable to dissolution. The outcrops of these cementa- Interpretation of megadunes tions appear sometimes like small cliffs (fig. 3) on the slopes of dunes while the underlying sands are eroded by winds. The dunes with a maximum height of ca 460 m in the The ages of the calcareous tubes could be consistent with Badain Jaran Desert are not only the highest in all Chinese the age of an old, relatively stable, dune surface formed deserts but also globally. They are higher than all dunes so under higher humidity. The radiocarbon ages of these tubes far discovered in other planets (e.g. Zimbelman, 2000; suggest a roughly 10 ka-scale periodic appearance of a more Lorenz et al., 2006). Based on field observations, two fac- humid climate in the Badain Jaran Desert (Yang, 2001). tors were suggested to be crucial to the formation of these However, dating of such calcareous tubes is quite challeng- megadunes: a) overlapping of dunes formed during various ing because the results using different methods are often far periods associated with climate fluctuations; b) underlying apart. The analysis of uranium – thorium isotopes showed bedrock landforms (Yang, 1991). The argument of the influ- that the oldest age of the tubes was 207,000 ± 10,000 years ence from bedrock landforms is from the observations done (Yang, 1991). However, the oldest radiocarbon age of the in the dune areas. On the slope of a 280 m high dune in the cemented tubes from the same area is only 31,750 southeastern Badain Jaran an outcrop of granite is visible at ± 485 years (Yang, 2001). In fact, the inorganic carbon in ca 50 m above the basis of the dune, evidencing the impor- the tubes might come from two different sources. One is tant contribution of the bedrock to the height of this dune from the solution of old carbonate in sediments, the other is (fig. 3). The different colors of the sand on the windward from the atmosphere through carbon sequestration. Little side of a megadune, accompanied by various densities of organic carbon is preserved in the dune stratigraphy owing plants, indicate that a megadune is somehow an aggregation to the processes of dissolutions. Therefore the lack of organ- of various dunes having formed at different epochs (Yang, ic materials in the dune stratigraphy is not contradictory to 1991). The layers of calcareous cementations on the dunes the idea that the dunes have been more or less stabilized by in the Badain Jaran Desert (fig. 3) were probably of pedo- vegetation during wetter periods in the past. genic origins and formed under higher soil moisture (Yang Another peculiar character of the Badain Jaran Desert is et al., 2003). On the surface of older dunes there are often the occurrence of many permanent lakes in the inter-dune

280 Géomorphologie : relief, processus, environnement, 2006, n° 4, p.275-284 Desert research in northwestern China - a brief review basins (figs. 3 and 4). The salinity and chemistry of these of high resolution seismological techniques would be need- lakes vary considerably: some of them contain fresh water ed for trying to find answers to these questions. while others are filled with brine. However, the shallow Theoretically, environmental isotopes would provide groundwater from all lake basins is fresh. The relatively clues to the origin of waters in the desert region. But the re- high concentration of tritium in the shallow groundwater sults of measurements reported by various authors (e.g. indicates a young age of the water, mostly younger than Geyh et al., 1996; Yang and Williams, 2003; Chen et al., 100 years. The deep groundwater is probably of old age due 2004; Yang, 2006) are not fully consistent with each other, to very low content of tritium (Yang and Williams, 2003). leading to different explanations of water sources. Geyh et Based on the relatively young ages, it was concluded that al. (1996) pointed out that the groundwater in the western the shallow groundwater in the Badain Jaran Desert is from margins of Badain Jaran Desert is recharged locally because local precipitation. The higher shorelines in the lake basins its isotope data (δ18O, δ2H, 14C) are clearly distinct from the were seen as evidence of wetter epochs characterized by Heihe River whose headwater is in the Tibetan Plateau. The much more rainfalls (Yang and Williams, 2003). The fluctu- spatial difference in the ion chemistries, tritium contents and ations of local climate were suggested to be the key factor δ18O in the samples taken from various localities also sug- having caused the changes of lake level not only in the inte- gests that the shallow groundwaters in the areas around the rior of the desert (Yang and Williams, 2003) but also in lakes in the Badain Jaran Desert (Yang and Williams, 2003) desert margins in the southeast (Yang, 2006). It is, however, and in the eastern margins of this desert (Yang, 2006) are not clear whether the deep groundwater has played a role in mainly recharged by infiltration of local rainfall, consistent the recharge of the desert lakes. with the interpretation of water ages (Yang and Williams, A new and quite interesting explanation for the formation 2003). However, also based on the δ18O and δ2H composi- of the megadunes is that groundwater could maintain the tions, Chen et al. (2004) argued that the water underneath dunes. It was suggested that underground rivers were flow- the desert is sourced from the northeastern Tibetan Plateau. ing from the mountains of northeastern Tibetan Plateau to A new effort would be needed to resample all the waters in the Badain Jaran Desert. In 20 to 30 years, the water could order to reexamine the values of the environmental isotopes. flow from the mountains to the desert lakes. The water was Probably a longtime monitoring of the waters would be believed to flow even up to the ridges of the dunes and to helpful to understanding the water sources and the relation- have kept the sands stable on the dune (Chen et al., 2004). ship between landforms and hydrological features in the The weak point of this hypothesis is that the water would sand sea. flow downwards before reaching the ridges of the dunes, as the water in the dunes is stored like in an open system. But Prospects the explanation with bedrock morphology is not sufficiently convincing, either, because most of the dunes are complete- Although the basic aspects such as distribution of deserts ly covered by aeolian sands (fig. 4) and the materials and types of dunes in China are well understood (e.g. Zhu et beneath the sands are not thoroughly investigated. In fact, it al., 1980) and it is now accepted that the dune landscapes is not clear whether there are any high bedrock outcrops or undergo changes in response to climatic fluctuations, many fluvial sediments under most of the sand dunes. Application fundamental questions relating desert geomorphology in northwestern China are still waiting for an- swers. For example, the time of initial forma- tion of present-day deserts in western China is not clear. The triggering factors for the initia- tion of a dry climate are still debatable. Is this related to the uplifting of Tibetan Plateau, to an increase in ice volume in the arctic regions or to other potential factors? Variations in the sediment sequences of the dunes should re- flect the climate changes. However, are these local, regional or global signals? Much more synthesis work is therefore badly needed in order to find and to interpret geomorphologi- cal records in the deserts. The appearance of controversial explanations on the formation of the megadunes in the Badain Jaran Desert re- confirms the importance of geomorphology in Fig. 4 – An eight-meter deep lake with an area of one km2 in the Badain Jaran understanding Earth surface processes. Just as Desert, ca 300 m-high dunes in the back. Richthofen (1886) already pointed out, the Fig. 4 – Un lac de 8 m de profondeur et d’une superficie de 1 km2 dans le désert forms and features of earth surface build the de Badain Jaran. Les dunes à l’arrière plan ont une hauteur d’environ 300 m. basis for all other geographical knowledge.

Géomorphologie : relief, processus, environnement, 2006, n° 4, p.275-284 281 Xiaoping Yang

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