Formation and dynamics of sandy dunes in the inland areas of the Hexi Corridor

Abstract. Dynamic changes of aeolian landforms and desertification under global warming in a middle-latitude desert belt, the Hexi Corridor in China, considered to be one of the source and engine area of sandstorms in China and Northern Hemisphere (NH), is a typical problem of climate change and landscape response, which need a comprehensive understanding of the history and forcing mechanisms of recent landform and environmental changes in the region. Based on the existing high-resolution satellite image interpretations, field investigations and observations, comprehensive evidences from geomorphological, aeolian-physical, granulometrical and geochemical analysis, this study discussed the formation of dune landforms, the mechanism of desertification and their environmental implications in the Hexi Corridor. The analytical results show that 80 % of the sand particles flow within a height of 20–30 cm near the surface, and about half of the sand particles flow within a height of 0.3–0.5 cm near the surface in the Hexi Corridor. The average height of the typical crescent-shaped dunes is about 6.75 m, and the minimum and maximum values are between 2.6 and 11.2 m. On the inter-annual and multi-year time scales, only the crescent-shaped dunes and chains of barchan dunes are moving or wigwagging in the study area, while the parabolic and longitudinal dunes did not move. Under the influence of wind speed, strong wind days and other factors, the dunes at the edge of the Minqin Oasis move the fastest, with a moving speed of about 6.2 m/a. Affected by the main wind direction and other factors, the dunes at the edge of the Dunhuang Oasis move the slowest, with a moving speed of about 0.8 m/a. The main factors affecting the dynamic changes of sandy dunes in the Hexi Corridor are the annual precipitation, the annual average wind speed and the number of annual strong wind days, of which the annual precipitation contributes the largest, indicating that the climate factors have a most important impact on the dynamic change of sand dunes. The cumulative curve of particle size frequency of dune sediments in the Hexi Corridor basically presents a three-segment model, indicating a saltation mode dominated under the action of wind, but superimposed with a small amount of coarser and finer particles dominated by the creeping and suspension models, which is obviously different from that of the Gobi sediments with a dominant two-segment mode. The palaeo-geographical, sedimentological and geochemical evidences indicate that dune sediments in the Hexi Corridor are mainly derived from locally or in-situ raised sandy sediments, which are mainly come from alluvial plains and ancient fluvial sediments, as well as ancient lake plains and lacustrine deposits, aeolian deposits in the piedmont denudation zones of the north and south mountains and modern fluvial sediments in the corridor. In geochemical compositions of major and trace elements, the dunes in the Hexi Corridor have certain similarities and differences to other sandy dunes in the northwest and northern deserts of China or aeolian loess in the Loess Plateau. Sandy dunes in the Hexi Corridor are relatively rich in iron and Co. Considering the proportion of fine particles on the surface, the coverage rate of surface salt crust, and the potential migration of erodible sandy materials, it can be concluded that the Gobi area in the west Hexi Corridor is not the main source area of sandstorms in the middle and east of the corridor, but the north probably is. In the past half century, the warming and humidification of local climate is the main cause of the reduction of sandstorms in the study area, and the Hexi Corridor has a potential trend of anti-desertification, which is mainly controlled by climate change but not human activities. For the oasis areas of the corridor, however, the effective measures to restrict desertification depend on human activities. Restriction of the decline of groundwater is the key to preventing desertification in oases, rather than water transfer from outer river basins.