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2006:Maquette Geomorpho Géomorphologie : relief, processus, environnement, 2006, n° 4, p. 275-284 Desert research in northwestern China – a brief review Recherches consacrées aux déserts du nord-ouest de la Chine – une brève revue Xiaoping Yang* Abstract Deserts 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 dunes, showing abrupt Late Quaternary environmental changes in the dune areas. Although the mechanisms for the formation of the megadunes in the Badain Jaran Desert 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.
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