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The movement of active dunes is tightly linked to climatic conditions (e.g., wind regime, temperature and precipitation) as well as human influence (e.g., grazing, dune fixation and greening). Dune migration rates can be studied to draw conclusions of changing wind conditions over time. The Gonghe Basin (GB), located on the north‐eastern Tibetan Plateau (TP), offers a good testing ground for these assumptions. The intramontane basin is highly influenced by two major wind regimes: the mid‐latitude Westerlies and the East Asian summer monsoon. To investigate environmental changes, this study combines optical remote sensing techniques with climatic datasets. High‐resolution satellite images of the last five decades, such as CORONA KH‐4B, are used to map dunes and calculate their respective migration rates. Further, height information was extracted as well. Climatic changes from the ERA‐5 reanalysis dataset and normalized difference vegetation index (NDVI) values were processed alongside. Relating the dunes' surface processes to climate model data shows an accordance between slowing migration, expanding vegetation and a decrease in sand drift potential. From 1968 to present time, an average dune migration rate of 7.3 m a−1 was extracted from the satellite images, with an overall reduction of −1.81 m a−1. The resultant drift potential (RDP) values for the GB are calculated to be below 10 m3 s−3 with a spatial decrease, following a direction from the NW to the SE, fitting well with a corresponding decrease in the migration rates. Our results indicate a good agreement between the development of aeolian landforms and the ERA‐5 climate reanalysis model data, even in a high‐altitude setting with complex topography, which is known to influence such datasets. [ABSTRACT FROM AUTHOR]

Seasonal changes in wind regime have driven the formation and emergence of reversing dunes and crest reversal in the inland arid and coastal areas of Asia, but due to the strong prevailing winds, the reversing dunes or reversing crest can be flipped. Therefore, the transient reversing dunes or crest reversal will be ignored and unobserved. To investigate dune morphology and sedimentology concerning seasonal alternation of the wind regime, we reconstructed dune topographies using aerial drone photos and analyzed the grain‐size parameters and internal sedimentary structures of dunes. Morphological results show that wind‐blown sands from the lee side are transported and deposited on the upper stoss side because of the reversing winds. Then, the dune crestal area is flattened, surface sand compositions were reorganized from fining to coarsening at the dune crest. Combining these field surveys with numerical simulation results, we found that the internal sedimentary structures are composed of high‐angle cross‐strata and low‐angle bounding surfaces. The dip angles of the bounding surfaces gradually decrease from the bottom to the top because of the reversing wind erosion on the lee side. The increase in sand flux on the lee side plays a critical role in shaping the dip angle of the bounding surfaces due to the speed‐up effect. Plain Language Summary: Under the influence of the East Asian monsoon system, near‐surface wind strength and direction are characterized by strong northwest winds in spring and winter, and weak southeast winds in summer. The seasonal changes in wind regimes reshape the dune forms and reorganize the grain‐size characteristics and internal sedimentary structures of the dunes. After the influence of reversing wind, dune crestal areas are flattened, then dune crests are flipped, oriented in the opposite direction. The sands on the downwind side were transported and deposited on the upwind side by the reversing wind, causing the grain fining characteristics on the dune crest to change into coarsening characteristics. Moreover, field surveys show that internal layering structures in dune shift from a southeast dip in the lower part to a northwest dip in the upper part. It reveals that sand deposition on the downwind side caused by NW winds becomes sand deposition on the upwind side induced by SE winds. Numerical simulation results quantitatively illustrate that the internal sedimentary structures are composed of high‐angle cross‐strata and low‐angle bounding surfaces. The dip angles of the bounding surfaces gradually decrease from the bottom to the top because of the reversing wind erosion on the lee side. Key Points: Reversing winds produce reversing slip faces and rework sands on dune crestal areasHigh‐angle cross‐strata are truncated by low‐angle bounding surfaces in asymmetrical bidirectional wind regimesThe bounding surface angle decreases with height, which is caused by the reversing wind erosion due to the speed‐up effect [ABSTRACT FROM AUTHOR]

The potential interactions between aeolian and fluvial systems are critical in understanding environmental change. Whether aeolian products originated from the river depends on the river characteristics and wind regimes. In southern Tibet, aeolian sand dunes are widespread along the Yarlung River, and thus provide an ideal example to explore the relationships between large fluvial systems and aeolian products. Although previous studies postulated that those aeolian products were originated from the Yarlung River, few empirical data actually exist to support this idea. We analysed the spatial pattern, sand transportation directions, and granulometric characteristics of the sand dunes to evaluate the potential influence of the Yarlung River on the aeolian processes. Comparison of the dune field distribution against channel width and longitudinal profile of the Yarlung River shows that sand dunes are clustered at the wide and flat segments of the river. Analyses of >4,600 sand dune orientations using Google Earth imagery show that the dunes located at the southern and northern sides of the Yarlung River demonstrate opposite sand transportation directions that generally point against the river. This suggests that the sand grains probably originated from the river. Grain size distributions from dune samples show unimodal or unimodal‐like patterns. This means that each sand dune field was dominantly fed by a simple transportation process rather than by compound processes from smaller‐level fluvial systems, which in turn implies that the Yarlung River was the most possible candidate for the provenance of these aeolian deposits. This study provides new constraints to demonstrate that the Yarlung River provides both accommodation space and sorted fluvial sand for the aeolian sand dunes along the river. [ABSTRACT FROM AUTHOR]

Numerous studies have explored paleoenvironmental conditions in China's dune fields around the mid-Holocene (MH), ∼6000 calendar years ago, often based on individual paleosol-aeolian sand depositional sequences and local-scale landscape dynamics. So far, continent-scale modeling studies have only indirectly assessed dune activation and stabilization processes via changes in simulated precipitation, net precipitation (precipitation minus evaporation), and effective moisture (precipitation divided by evaporation). Here we conducted a comprehensive study using gridded data on potential evapotranspiration, precipitation, and surface wind velocity from 13 climate models participating in the Paleoclimate Modeling Intercomparison Project phases 4 (PMIP4) and compared the model results with geological records. Our evaluation of spatial patterns of dune activity in northern China around the MH reveals that the intensity of annual MH dune activity exhibited significant differences between the eastern and western parts of Chinese deserts compared to those of the preindustrial period (PI). Dune activity was significantly weaker in the eastern region, while in the western part, it was considerably stronger. The key drivers of this disparity were the distinct spatial variations in near-surface wind speed and effective moisture between the two regions. Compared with the optically stimulated luminescence ages of the paleosol-aeolian sand sedimentary sequences derived from a compilation of geological records from 88 sites across northern China, the model results suggest that the weakening of dune activity and the process of dune stabilization in eastern China around the MH were mainly attributed to a significant increase in effective moisture and a concurrent decrease in near-surface wind speed due to the intensified East Asian summer monsoon. Additionally, the weakening of westerlies in western China resulted in reduced effective moisture, contributing to increased sand availability for dune development and subsequent desert expansion. • Mid-Holocene dune activity in Chinese deserts varied regionally, differing from preindustrial period. • Varied wind speed and moisture caused different mid-Holocene aeolian activities in northern China's western and eastern desert belt. • Numerical simulations aid in interpreting field-based geological findings comprehensively. [ABSTRACT FROM AUTHOR]

The evolution of barchan‐to‐parabolic dunes can be driven by vegetation establishment, which may be linked to climate change and/or human activity. However, little is known of the impact of changes in wind strength on vegetation development and the resulting impacts on the evolution of dune morphology and sedimentological characteristics. To address this issue, we studied the morphology and grain‐size characteristics of barchan, barchan‐to‐parabolic and parabolic dunes in the Mu Us Desert in north China, which was combined with an analysis of changes in normalized difference vegetation index (NDVI) and climatic variables during 1982–2018. The results reveal a trend of increasing growing‐season NDVI which was related to a significant decrease in drift potential (DP). Therefore, we suggest that the initiation of dune transformation was caused by the reduced wind strength which favored the establishment and development of vegetation. To reveal the response of sedimentological reorganization during the processes of dune transformation, grain‐size characteristics along the longitudinal profile of the three different types of dunes were examined. The decreasing wind strength led to the transport of fine sands on the upper part of the windward face of the dunes, resulting in a progressive coarsening of the grain‐size distribution (GSD) and a reduction in dune height at the crest area. No distinct trend in sorting and mean grain‐size was observed on the windward slope of the barchan‐to‐parabolic dune, indicating that the sand in transit had little influence on the GSD. Conversely, progressive sorting and coarsening of the sand occurred towards the crest of the parabolic dune. This indicates that vegetation development limited the transport of sand from upwind of the dune, and affected a shift in the dune source material to the underlying source deposits, or to reworked pre‐existing aeolian deposits, and resulted in the trapping of sand in the crest area. © 2020 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

This paper presents the environmental history and its responses to palaeoclimatic changes since the start of the Holocene in the eastern portion of the desert belt (sand seas and sandy lands) in northern China by comparing the aeolian sandpalaeosol sequences and their palaeoclimatic proxies. The optically stimulated luminescence (OSL) ages of the aeolian sandpalaeosol sedimentary sequences and a series of palaeoenvironmental proxies show that: (1) The large-scale dune landscape currently in the Kubuqi Sand Sea was formed during the Holocene in general; and the palaeosol was generally developed during the period of 4–2 ka, indicating conditions favorable for vegetation growth, soil development, and organic carbon accumulation due to increased precipitation or effective moisture and weakened aeolian activities; the large-scale expansion of dunes in the recent 2 ka is closely linked to human activities. The variable discharge of the Yellow River with diversions for irrigation may have resulted in a more consistent supply of aeolian particles for dune field expansion. (2) The dune landscape of the Hunshandake Sandy Land was likely formed around 12 ka, and before this, the western part of the Hunshandake Sandy Land would have been covered by a single large lake; it was obviously wetter than today in the sandy land during the period of 9.6–3 ka and the palaeosols were developed at the same time. But the aeolian activities have not been completely dormant in this long-lasting wetter epoch; because the Holocene wetter period was likely time-transgressive across the region. (3) The palaeosol of the Hulunbuir Sandy Land began to develop as early as 14.5 ka, probably continuing until the last 2 ka. The palaeosol development of various dune fields in the eastern portion of the desert belt (sand seas and sandy lands) in northern China is spatially heterogeneous, and even the palaeosol development time in different locations within each sandy land is inconsistent. During the middle Holocene (especially the 7.5–3.5 ka), all the sandy lands were stabilized in general and the intensity of aeolian activities was significantly weakened. The number of palaeoenvironmental records in the eastern portion of the desert belt (sand seas and sandy lands) in northern China has increased rapidly in the past decade, but the amount of published data still does not match the vast extent of the dune fields. It does require much more in-depth palaeoenvironmental studies for a full understanding of the relationship between aeolian activities and climate change in northern China. [ABSTRACT FROM AUTHOR]

The development and transformation of barchan-parabolic dune can be led by vegetationestablishment, which may be induced by climate change (precipitation, temperature, andwind strength) and/or human intervention. The impact of wind strength change-inducedvegetation restoration on dunes morphology and theirs surface sedimentary features isneglected in the Maowusu Sandy Land. In this study, we combined datasets of NormalizedDifference Vegetation Index (NDVI) and climatic factors to analyze temporal changes invegetation growth and their response to changes in temperature, precipitation and windstrength in the northern margin of Maowusu Sandy Land from 1982 to 2015. Growing seasonNDVI is related to the significant decrease in drift potential (DP) during the entire studyperiod. Further temporal analyses indicate that the growing season NDVI trends are largelycontributed by wind strength and temperature changes during the earlier study period1982-2006. And then, the increasing NDVI is consistent with the slightly increasing trend ofprecipitation during the recent study period 2006-2015. Thus, the transformation ofthe transitional dune cannot be explained by the increasing trend of wind strengthchange-induced vegetation alone. During the recent period, once dunes are covered byvegetation, the change in wind strength cannot mobilize the stable dunes becausethe vegetation provides an effective buffer preventing sand erosion. To reveal theresponse of sedimentological reorganization during the dune transformation processes,grain-size characteristics along the longitudinal profile of three different types of duneswere examined. From barchan dune through transition dune to parabolic dune, withthe increase of the fine sands (125-250μm) proportions, the mean grain size MZgradually decreases; in addition, as the very fine sands (<125μm) class increases, thesorting σ1 of sand dunes gradually became worse. The decreasing wind strengthlead to partly transport of fine sands on the upper part of dune windward slope,resulting in the progress coarsening and the reduction of dune height at the crestarea. No distinct trend in sorting and mean grain size on the windward slope oftransition dune, indicating that sand in transport had little influence on the particle sizedistributions. Conversely, the progressive of sorting and coarsening of sands towards thecrest of parabolic dune, implying that vegetation limited the sands from upwind ofdune, changed the effective source of dune into the underlying source deposits orreworking pre-existing aeolian deposits and trapped the sands on the crest area. [ABSTRACT FROM AUTHOR]

[Display omitted] • "Yardang-controlled dune" is systematically illustrated with morphology and dynamics. • In three patterns, dunes form before, beside and behind the yardangs. • CFD simulations reveal that the yardang-induced low-velocity winds shape the dunes. • Remote sensing measurements support the CFD, which agrees with three patterns. • The aspect ratios of the streamlined yardangs control the linear dune formation. The yardang-dune coexistence and the control of yardangs on dune formation reveal the joint action of wind erosion and deposition. Yardangs exhibit distinctive characteristics of streamlining and clustering, which apply unique and complicated impacts on the wind field and sand transport. However, previous studies lack systematic investigation on how yardangs exert control over dune morphology, and the intricate wind dynamics and mechanisms involved remain unclear. In this study, the "yardang-controlled dune" is systematically illustrated, supported by thorough studies of the yardang-controlled dune morphology and dynamics. Through the monitoring of the 6722 km2 dunes in the Qaidam Basin (QB), three typical yardang-dune coexistence patterns are identified and summarized: Pattern I-windward dune, crescentic dunes form on the upwind sides of the sparsely arranged yardangs; Pattern II-corridor dune, crescentic dunes form beside the narrowly arranged yardangs in the corridors and migrate; Pattern III-leeward dune, linear dune forms on the downwind side from isolated yardang tail. Computational fluid dynamics (CFD) modeling is used to analyze the dynamics and mechanisms of the three patterns and the corresponding real yardang topographies. CFD simulations show that with the control of yardangs, the winds in the locations corresponding to the dunes have significantly low velocities, leading to the wind entrainment decrease, sand accumulation and subsequent dune formation. CFD also reveals that the aspect ratios of yardangs control the formations of wind shadows and linear dunes on the yardang tails. The simulation results agree well with the actual landforms observed in the QB of the three patterns. Additionally, the dune leeward slope width and migration velocity statistics based on remote sensing provide further support for the observations and simulations. This study sheds light on the intricate influences of yardangs on wind deposition, underscoring their role in shaping the landform of dunes. [ABSTRACT FROM AUTHOR]

Abstract: Sedimentary sequences occurring in desert dunes reflect changes in desert systems, and as such may contain signals useful for recognizing spatial and temporal changes of deserts and their response to regional or even global climate fluctuations. Geomorphological and palaeoenvironmental studies within the dune fields of the Asian middle-latitudes have provided some solid evidence for interpreting the history of these sand seas. Using the Hunshandake (Otindag) Sandy Land, a sandy area covered primarily by stabilized dunes and located in the semi-arid zone of eastern Inner Mongolia, China (Fig. 1), as an example, we studied the initiation and variation in the dune landscape in the eastern portion of the desert belt in northern China. On the basis of physical and biochemical indicators in the sediments and OSL chronology, we herein argue that this dune system in the middle latitudes of eastern Asia is much younger than previously assumed and that it has responded sensitively to climate change during the late Quaternary. Geological evidence from the Sandy Land suggests that most of the current dunes are of late Pleistocene or even Holocene age. Palaeosols intercalated in the aeolian sequences and their OSL chronology show that the climate of the Hunshandake was much wetter than today between 9.6 ka and 3 ka. This resulted in stabilization of the dunes in the eastern and central portions of the Sandy Land. Epochs of reworking or stabilization of the dunes are broadly consistent with the fluctuations in northern hemisphere solar radiation although with an obvious time lag. Because the increase rate of annual precipitation was not sufficient to fully stabilize the dunes in more arid part of the region, some active dunes persisted even during this long-lasting wetter epoch. We conclude that periods of Holocene dune stabilization due to palaeosol formation varied along the climate gradients across the various sandy lands of northern China, and in general it began earlier and lasted longer in the east than in the west. The general nature of the sandy lands and their counterparts in the western portion of the desert belt during the LGM and mid-Holocene climate optimum is discussed in comparison with their current states. [Copyright &y& Elsevier]