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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.
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.
Paleoflood slackwater deposits have recently been investigated in small to moderately sized rivers. However, our understanding of the characteristics of paleoflood slackwater deposits in large rivers remains limited, which poses a notable obstacle to accurately predicting the magnitude, frequency and force of extraordinary floods. To address this research gap, this study compares the characteristics of paleoflood slackwater deposits in the middle Yarlung Zangbo River (YZR) and the upper Hanjiang River in China. Paleoflood slackwater deposits and aeolian deposits (e.g. loess, paleosol S0 and aeolian sand) were investigated and sampled from these locations. For the paleoflood slackwater deposits and aeolian deposits, sedimentology (e.g. grain size distribution, magnetic susceptibility and geochemical elements) was analyzed in the laboratory. The macroscopic features of the paleoflood slackwater deposits in the middle YZR valley are very similar to those of the upper Hanjiang River valley. The paleoflood slackwater deposits were dominated by sand and silt in the middle YZR and by silt and sand in the upper Hanjiang River; this implied that the paleoflood slackwater deposits were suspended sediments of floodwater from different source regions. Compared with the paleoflood slackwater deposits in the upper Hanjiang River and other large rivers in China (e.g. the Yellow and Yangtze Rivers), the paleoflood slackwater deposits contained more coarse particles in the middle YZR; this result may be closely related to the particular hydrogeologic and geomorphic conditions (e.g. rich sandy sediment, high riverbed slope and narrow valley) in the southeastern Tibetan Plateau. Compared with the paleoflood slackwater deposits in the upper Hanjiang River, the paleoflood slackwater deposits presented considerably worse sorting in the middle YZR; this may be closely related to the short transport distance in the middle YZR. The magnetic susceptibility values of the paleoflood slackwater deposits were relatively higher in the middle YZR and the upper Hanjiang River, suggesting that these slackwater deposits with minimal pedogenesis may contain a relatively high ferromagnetic mineral content because of the formation process of slackwater deposits. The sediment provenance of the paleoflood slackwater deposits may be closely related to that of the loess in the middle YZR valley, which implies that the sediment provenance of deposits of different genetic types may be greatly affected by the particular hydrologic–climatic and geomorphic conditions in the southeastern Tibetan Plateau.
The movement of aeolian materials and dynamics of dune landforms are results of the transportation and accumulation of sandy sediments under the influence of climate, in which the processes that gave birth to sand dune formations are complex as the arid area has experienced different periods of humidity and aridity. These dynamics are still not well understood at present, especially at the inter-decadal time scale.Based on a comprehensive analysis of geomorphological, sedimentological, geochemical, and ecohydrological data, this study discussed the dynamical changes of different dune landforms during the past half-century and their formation mechanism in the Hexi Corridor, northern China. The results show that on the inter-annual and inter-decadal scales, the crescent-shaped dunes move the fastest among different dune types. Dunes in Minqin move the fastest (at 6.2 ± 0.5 m yr À1 ) and dunes in Dunhuang move the slowest (at 0.8 ± 0.2 m yr À1 ). The dune sands are mainly medium sand (21.7%-57.4%) and fine sand (23.2%-53.0%) with an average grain size of 0.07 ± 0.01 mm $0.24 ± 0.06 mm, similar to the world's average particle size of sand dunes. Annual precipitation and spring precipitation, annual average wind speed, and number of annual strong wind days are the main factors affecting the dynamic changes of dunes in the Corridor, indicating a major influence of climatic factors. STC (sufficient transport capacity) is evidenced for both the western and eastern Hexi Corridor, however, SSS (sufficient sand supply) and SA (sand availability) are the favourable factor for dune formation in the east part but is the limiting factor for the west. In the past half-century, although the vulnerability of land degradation is a function of climate change, geomorphological processes, and anthropogenic pressure in the Hexi Corridor, water resource limitations have been more important.
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