Driving forces of aeolian desertification in the source region of the Yellow River: 1975–2005

The source region of the Yellow River, located in the northeastern portion of the Qinghai–Tibet Plateau, plays a critical role in water conservation, biodiversity protection, and wetland conservation. Aeolian desertification of this area is an important concern. Remote sensing and GIS technology were employed to assess the trends in aeolian desertification from 1975 to 2005. The monitoring results showed that, aeolian desert land increased from 15,112 to 17,214 km² during 1975–2005. In addition, it was found that the area of aeolian desertification increased rapidly from 1975 to 1990, was stable from 1990 to 2000, and slightly decreased from 2000 to 2005. Increasing temperature, overgrazing, and drainage of wetlands have been key driving factors of aeolian desertification. Thus, to control the expansion of aeolian desert lands in the source region of the Yellow River and to rehabilitate existing desert areas, the priority should be given to altering human behavior in these areas.     

五十万年来毛乌素沙漠的变迁

对沙漠-黄土边界带的古风成砂-黄土-古土壤序列的研究表明，毛乌素沙漠至少在0.5MaB.P.就已出现，但也并非从其出现之初就持续至今。沙漠-黄土边界带的风成沉积所记录的沙丘活化和固定的多次转变是第四纪时期毛乌素沙漠对气候振荡响应的结果。五十万年来，沙漠-黄土边界带的石峁剖面记录了13层古风成砂，代表了13次沙漠大规模的南侵。沙漠的南侵不仅可以发生在冰期鼎盛之时，同样也可以发生在间冰期中的寒冷气候幕出现之时。此外，鉴于沙漠-黄土边界带具有气候敏感性的特点，石峁剖面的地层记录还表明了第四纪时期不仅有冷期与暖期的多次旋回，而且冷期和暖期发生时还有次一级的气候波动。     

Wind regimes and aeolian geomorphology in the western and southwestern Tengger Desert,NW China

Wind is the primary control on the formation of aeolian geomorphology. In this study, we combined wind regime data from automated weather stations in the western and southwestern Tengger Desert of the Inner Mongolia region in China with remote‐sensing data to analyse the relationship between the wind energy environment and aeolian geomorphology. Tengger Desert is one of the main dust storm sources in northwestern China. Therefore, efforts aimed at controlling desertification and dust storm require a deeper understanding of the processes that govern the formation and subsequent evolution of dunes in this area. Wind speed was largest in the northwest (3.3 m/s in the Xiqu station) and smallest in the southeast (1.2 m/s in the Haizitan station). Potential sand transport was also largest in the northwest (195 in the Jiahe station) and smallest in the southeast (33 in the Tumen station). The sand‐driving wind (5.92 m/s) directions were from the NW and SE quadrant across the study area, at >76% of all sand‐driving wind, reaching 99% in the Tumen station. The sand‐driving wind in the NW quadrant reached >48%, and in the SE quadrant, >12% of all sand‐driving wind in all stations. In the study area, sand dunes included crescent, dune networks, transverse, and coppice dunes. Dune crest directions had similar trends from upwind to downwind, at 133° in the middle region, and 124° in the southwestern region. Mean dune spacing changed with dune patterns; the maximum spacing for crescent dunes was 147 m, for dune networks 118 m, and for transverse dunes it was 77 m. The mean crest length was 124 m (maximum) for crescent dunes in the northwest, 121 m for transverse dunes, and 84 m for dune networks. However, because of gullies in the southern region, the mean crest length was only 58 m (least) for the crescent dunes in that area. The defect density ranged from 0.007 to 0.014. The spatial differences in dune patterns reflected the evolution of the dune field, where older dunes had been formed upwind and younger downwind. Copyright © 2014 John Wiley & Sons, Ltd.     

The action of wind and water in a mid‐Cretaceous subtropical erg‐margin system close to the Variscan Iberian Massif,Spain

Aeolian processes and ephemeral water influx from the Variscan Iberian Massif to the mid‐Cretaceous outer back‐erg margin system in eastern Iberia led to deposition and erosion of aeolian dunes and the formation of desert pavements. Remains of aeolian dunes encased in ephemeral fluvial deposits (aeolian pods) demonstrate intense erosion of windblown deposits by sudden water fluxes. The alternating activity of wind and water led to a variety of facies associations such as deflation lags, desert pavements, aeolian dunes, pebbles scattered throughout dune strata, aeolian sandsheets, aeolian deposits with bimodal grain‐size distributions, mud playa, ephemeral floodplain, pebble‐sand and cobble‐sand bedload stream, pebble–cobble‐sand sheet flood, sand bedload stream, debris flow and hyperconcentrated flow deposits. Sediment in this desert system underwent transport by wind and water and reworking in a variety of sub‐environments. The nearby Variscan Iberian Massif supplied quartzite pebbles as part of mass flows. Pebbles and cobbles were concentrated in deflation lags, eroded and polished by wind‐driven sands (facets and ventifacts) and incorporated by rolling into the toesets of aeolian dunes. The back‐erg depositional system comprises an outer back‐erg close to the Variscan highlands, and an inner back‐erg close to the central‐erg area. The inner back‐erg developed on a structural high and is characterized by mud playa deposits interbedded with aeolian and ephemeral channel deposits. In the inner back‐erg area ephemeral wadis, desiccated after occasional floods, were mud cracked and overrun episodically by aeolian dunes. Subsequent floods eroded the aeolian dunes and mud‐cracked surfaces, resulting in largely structureless sandstones with boulder‐size mudstone intraclasts. Floods spread over the margins of ephemeral channels and eroded surrounding aeolian dunes. The remaining dunes were colonized occasionally by plants and their roots penetrated into the flooded aeolian sands. Upon desiccation, deflation resulted in lags of coarser‐grained sediments. A renewed windblown supply led to aeolian sandsheet accumulation in topographic wadi depressions. Synsedimentary tectonics caused the outer back‐erg system to experience enhanced generation of accommodation space allowing the accumulation of aeolian dune sands. Ephemeral water flow to the outer back‐erg area supplied pebbles, eroded aeolian dunes, and produced hyperconcentrated flow deposits. Fluidization and liquefaction generated gravel pockets and recumbent folds. Dune damming after sporadic rains (the case of the Namib Desert), monsoonal water discharge (Thar Desert) and meltwater fluxes from glaciated mountains (Taklamakan Desert) are three potential, non‐exclusive analogues for the ephemeral water influx and the generation of hyperconcentrated flows in the Cretaceous desert margin system. An increase in relief driven by the Aptian anti‐clockwise rotation of Iberia, led to an altitude sufficient for the development of orographic rains and snowfall which fed (melt)water fluxes to the desert margin system. Quartzite conglomerates and sands, dominantly consisting of quartz and well‐preserved feldspar grains which are also observed in older Cretaceous strata, indicate an arid climate and the mechanical weathering of Precambrian and Palaeozoic metamorphic sediments and felsic igneous rocks. Unroofing of much of the cover of sedimentary rocks in the Variscan Iberian Massif must therefore have taken place in pre‐Cretaceous times.     

Textural characteristics and genesis of the aeolian sediments in the Kuwaiti desert

Several types of aeolian deposits have been recognized in Kuwait: (a) smooth sand sheets that resemble desert floor sand, (b) immobile sands that include rugged vegetated sand sheets and wadi fill deposits, and (c) mobile sands that form active sand sheets and sand dunes. Simple size frequency curves illustrate the genetic relationship between the various aeolian sediment types. The four size parameters, namely, mean size, sorting, skewness and kurtosis, were calculated. Scatter plot diagrams of sorting versus mean size and sorting versus kurtosis are effective in differentiating smooth sand sheet deposits from dune sands. Active sand sheet deposits can also be recognized because they are usually located between the two end members–smooth sand sheets and dune sands. Size parameters change with location regardless of their types. Coarsening and positive skewness usually increase downwind. Mineralogical and textural characteristics of the aeolian deposits in Kuwait revealed that they are mostly derived from the lower Mesopotamian muddy flood plain deposits, the sand fraction of the Al-Dibdibba gravelly deposits and the disintegrated material from calcretic and gypcretic duricrusts. Distribution of depositional and deflational areas indicates that the northern desert of Kuwait is characterized by a positive sand budget, whereas the southern desert has a negative sand budget.     

Sand movement patterns in the Western Desert of Egypt: an environmental concern

Wind action is the most dominant agent for erosion and deposition in the vast Western Desert of Egypt. Analysis of wind data from seven meteorological stations distributed along the Western Desert reveals that this desert is characterized by high-energy wind environments along the northern and southern edges and low-energy wind environments throughout the rest of the desert. Accordingly, sand drift potential follows the pattern of wind energy. Maximum sand drift potential was observed at the southern edge (571 vector units, which equals 40 m³/m width/year). Sand drift direction was observed towards the southeast except at the southern part of the desert where the trend of sand movement was towards southwest. The major dune type recognized on satellite images was the simple linear type. Linear dunes are generally associated with bimodal wind regime. Rates of sand drift potential and sand dune migration were greatest at East of Owinate region at the extreme southern part of the desert. Measurements of crescentic sand dune advance from two satellite images reveal a maximum advance rate of about 9 m/year at the southern part of the desert. Dune movement creates potential hazard to the infrastructures in this open desert.     

Aeolian sand sea development along the mid‐Cretaceous western Tethyan margin (Spain): erg sedimentology and palaeoclimate implications

The existence of a mid‐Cretaceous erg system along the western Tethyan margin (Iberian Basin, Spain) was recently demonstrated based on the occurrence of wind‐blown desert sands in coeval shallow marine deposits. Here, the first direct evidence of this mid‐Cretaceous erg in Europe is presented and the palaeoclimate and palaeoceanographic implications are discussed. The aeolian sand sea extended over an area of 4600 km². Compound crescentic dunes, linear draa and complex aeolian dunes, sand sheets, wet, dry and evaporitic interdunes, sabkha deposits and coeval extradune lagoonal deposits form the main architectural elements of this desert system that was located in a sub‐tropical arid belt along the western Tethyan margin. Sub‐critically climbing translatent strata, grain flow and grain fall deposits, pin‐stripe lamination, lee side dune wind ripples, soft‐sediment deformations, vertebrate tracks, biogenic traces, tubes and wood fragments are some of the small‐scale structures and components observed in the aeolian dune sandstones. At the boundary between the aeolian sand sea and the marine realm, intertonguing of aeolian deposits and marine facies occurs. Massive sandstone units were laid down by mass flow events that reworked aeolian dune sands during flooding events. The cyclic occurrence of soft sediment deformation is ascribed to intermittent (marine) flooding of aeolian dunes and associated rise in the water table. The aeolian erg system developed in an active extensional tectonic setting that favoured its preservation. Because of the close proximity of the marine realm, the water table was high and contributed to the preservation of the aeolian facies. A sand‐drift surface marks the onset of aeolian dune construction and accumulation, whereby aeolian deposits cover an earlier succession of coastal coal deposits formed in a more humid period. A prominent aeolian super‐surface forms an angular unconformity that divides the aeolian succession into two erg sequences. This super‐surface formed in response to a major tectonic reactivation in the basin, and also marks the change in style of aeolian sedimentation from compound climbing crescentic dunes to aeolian draas. The location of the mid‐Cretaceous palaeoerg fits well to both the global distribution of other known Cretaceous erg systems and with current palaeoclimate data that suggest a global cooling period and a sea‐level lowstand during early mid‐Cretaceous times. The occurrence of a sub‐tropical coastal erg in the mid‐Cretaceous of Spain correlates with the exposure of carbonate platforms on the Arabian platform during much of the Late Aptian to Middle Albian, and is related to this eustatic sea‐level lowstand.     

内蒙巴丹吉林沙漠成因的粒度分析和热发光测年新证据

巴丹吉林沙漠位于中国内蒙古自治区的西部,是中国第三大沙漠。过去通常认为该沙漠中沙丘的可能沙源来自附近的各种沉积物和岩石,包括弱水河扇的沉积物,弱固结的二叠纪页岩、白垩纪砂岩和砾岩以及戈壁阿尔泰的古老岩石。根据对风成砂的粒度观测,有证据表明在沙漠内部的风成砂沉积朝东南方向具有较弱的但又确实的变细趋势。这与沙丘横脊线的SW-NE向排列以及坡面的南东指向是一致的。因此,可以认为西北部的弱水河冲积扇是巴丹吉林沙漠风成砂的最可能的沙源。在西北部风成床沙覆盖了较老弱水河扇起源的冲积物。这种沉积物的热发光测年范围在190～100 ka BP之间。由于测量中对沉积物漂白可能不完全以及对其中平均古水含量估算均存在不确定性,同时由于对沙漠该地区风成作用开始之前堆积的沉积物进行了测年,这一数据范围代表了最大的年龄。另外三个风成砂测年结果分别接近133.66和22 ka,代表有关巴丹吉林沙漠西北部风成作用开始的最小年龄。沙漠的沙丘高度平均为200～300m,但在东南部偶尔可达450m。一些学者曾提出风成床沙覆盖了一个陡峭岛山突起的假说,来解释这些异常的空间。本次研究发现,在研究区之下是一个产状水平的白垩纪扇砾岩和砂岩的台地,可以断定在沙漠东南部呈现台地地貌,但进一步向北该台地明显延伸到这些沙丘之下。因此现在可以认为巴丹吉林沙漠中高的沙丘是不同区域气候和地貌因素相互作用的结果,而不是覆盖一个陡峭的岛山突起。对弱水河冲积扇作为巴丹吉林沙漠的主要源区的证实强调区域环境的重要性。在全新世,沿河流的绿洲植被在某种程度上阻碍了冲积扇提供沙源。现在,河西走廊的农业用水量极大地危及沿河森林,因此沿着作为天然拦沙阱的弱水河,维持足够的河水流量来保护区域性密集的沙丘植被,一定会避免具有重大威胁的沙的活动性增加。     

Sedimentology, stratigraphy and landscape evolution of a Holocene coastal dune system, Lodbjerg, NW Jutland, Denmark

The stratigraphy and landscape evolution of the Lodbjerg coastal dune system record the interplay of environmental and cultural changes since the Late Neolithic. The modern dunefield forms part of a 40 km long belt of dunes and aeolian sand‐plains that stretches along the west coast of Thy, NW Jutland. The dunefield, which is now stabilized, forms the upper part of a 15–30 m thick aeolian succession. The aeolian deposits drape a glacial landscape or Middle Holocene lake sediments. The aeolian deposits were studied in coastal cliff exposures and their large‐scale stratigraphy was examined by ground‐penetrating radar mapping. The contact between the aeolian and underlying sediments is a well‐developed peaty palaeosol, the top of which yields dates between 2300 BC and 600 BC . Four main aeolian units are distinguished, but there is some lateral stratigraphic variation in relation to underlying topography. The three lower aeolian units are separated by peaty palaeosols and primarily developed as 1–4 m thick sand‐plain deposits; these are interpreted as trailing edge deposits of parabolic dunes that moved inland episodically. Local occurrence of large‐scale cross‐stratification may record the head section of a migrating parabolic dune. The upper unit is dominated by large‐scale cross‐stratification of various types and records cliff‐top dune deposition. The nature of the aeolian succession indicates that the aeolian landscape was characterized by alternating phases of activity and stabilization. Most sand transported inland was apparently preserved. Combined evidence from luminescence dating of aeolian sand and radiocarbon dating of palaeosols indicates that phases of aeolian sand movement were initiated at about 2200 BC , 700 BC and AD 1100. Episodes of inland sand movement were apparently initiated during marked climate shifts towards cooler, wetter and more stormy conditions; these episodes are thought to record increased coastal erosion and strong‐wind reworking of beach and foredune sediments. The intensity, duration and areal importance of these sand‐drift events increased with time, probably reflecting the increasing anthropogenic pressure on the landscape. The formation of the cliff‐top dunes after AD 1800 records the modern retreat of the coastal cliffs.     

黄河源区玛多县沙漠化成因与发展过程

通过分析黄河源区玛多县沙漠化的成因和过程,结果表明,玛多县沙漠化成因类型有三种,分别为固定沙丘/古沙丘活化、滑塌陡坎及风蚀斑块,其形成都与多年冻土退化相关.在固定沙丘或古沙丘分布区,冻土退化导致热融沉陷,形成沉陷坑,沉陷坑边缘形成拉裂缝或陡坎,使下伏松散沙露出.在斜坡上,冻土退化形成滑塌陡坎,使底层土壤从侧面暴露.在平坦的冲积平原,差异性冻胀和融沉导致草皮拉裂,形成积水坑洼,冻土退化导致土壤变干,土壤的底层暴露.下伏风成沉积物暴露后,遭受风蚀,形成侧向凹槽,致使上部土体坍塌,使更多的风成沉积物暴露,这些过程的不断重复,使风蚀坑、陡坎、风蚀斑块扩大相连,最终形成流动沙丘、风蚀劣地、戈壁等地貌.     

Wind sedimentation in the Jafurah sand sea, Saudi Arabia

The Jafurah sand sea of the Eastern Province of Saudi Arabia extends along the Arabian Gulf coastline from Kuwait in the north to the Rub Al Khali in the south, a distance of about 800 km. Sand drifts southward to south-eastward from regions of high wind energy in the north to low wind energy in the south. The aeolian landscape is zoned, with areas of deflation, transport and deposition from north to south. Drift rates in the zone of transport, near Abqaiq, range from 2 m³ m-w^-1 yr^-1 on sabkhas, to 29 m³ m-w^-1 yr^-1 on the crests of dunes. Average drift rates of approximately 18 m³ m-w^-1 yr^-1 observed during the study can cause about 1 m of accumulation per 5500 yr in a 100 km zone of deposition downwind, not including the bulk transport represented by the forward advance of dunes. Dune advance ranged from 23 m (2.9 m high dune) to 3 m (23 m high dune) during April-October 1980. The study area consists of dune, interdune, sand sheet and siliciclastic sabkha terrains, each of which is characterized by differing drift rates, and differing rates of erosion or deposition. Sedimentation occurs by lateral movement of dunes and interdunes, and vertical accretion by sand sheets and sabkhas.     

石英光释光测年揭示的晚第四纪毛乌素沙地演化

本研究利用石英光释光测年的单片再生法(Single­aliquot Regenerative­dose Protocol,简称SAR)对毛乌素沙地内部西北－东南方向5个风成砂－砂质古土壤剖面进行了年代测定,结合年代框架和剖面沉积相、磁化率及粒度特征探讨了晚第四纪以来毛乌素沙地演化和气候变化。研究表明毛乌素沙地在晚第四纪以来经历了多次沙地固定与活化的交替演化： 距今91.0ka,71.0ka,48.0ka,22.0ka,11.6ka,5.0ka,1.1ka,1.0ka和0.4ka前后风成砂沉积,沙地活化,指示气候干旱,植被覆盖度低; 在距今65ka和全新世适宜期(8.5～5.0ka),沙地固定成壤,砂质古土壤发育,指示气候湿润。另外,剖面中风成砂层数变多、厚度增加、粒径变粗指示了晚第四纪以来毛乌素沙地干旱化趋势加强。     

Geomorphology of desert sand dunes: A review of recent progress

Through the 1980s and 1990s studies of the geomorphology of desert sand dunes were dominated by field studies of wind flow and sand flow over individual dunes. Alongside these there were some attempts numerically to model dune development as well as some wind tunnel studies that investigated wind flow over dunes. As developments with equipment allowed, field measurements became more sophisticated. However, by the mid-1990s it was clear that even these more complex measurements were still unable to explain the mechanisms by which sand is entrained and transported. Most importantly, the attempt to measure the stresses imposed by the wind on the sand surface proved impossible, and the use of shear (or friction) velocity as a surrogate for shear stress also failed to deliver. At the same time it has become apparent that turbulent structures in the flow may be as or more important in explaining sand flux. In a development paralleled in fluvial geomorphology, aeolian geomorphologists have attempted to measure and model turbulent structures over dunes. Progress has recently been made through the use of more complex numerical models based on computational fluid dynamics (CFD). Some of the modelling work has also suggested that notions of dune ‘equilibrium’ form may not be particularly helpful. This range of recent developments has not meant that field studies are now redundant. For linear dunes careful observations of individual dunes have provided important data about how the dunes develop but in this particular field some progress has been made through ground-penetrating radar images of the internal structure of the dunes.

The paradigm for studies of desert dune geomorphology for several decades has been that good quality empirical data about wind flow and sand flux will enable us to understand how dunes are created and maintain their form. At least some of the difficulty in the past arose from the plethora of undirected data generated by largely inductive field studies. More recently, attention has shifted–although not completely–to modelling approaches, and very considerable progress has been made in developing models of dune development. It is clear, however, that the models will continue to require accurate field observations in order for us to be able to develop a clear understanding of desert sand dune geomorphology.     

Mass‐wasting of ancient aeolian dunes and sand fluidization during a period of global warming and inferred brief high precipitation: the Hopeman Sandstone (late Permian), Scotland

Large‐scale deformation structures in late Permian aeolian dune sands are associated with sand fluidization and injection. Exceptional precipitation and flooding of the desert margin are believed to have caused mass‐wasting by gravitational collapse and sliding of water‐saturated dunes, which loaded down‐dip strata, thus generating overpressure and triggering sand injection. This short‐lived but heavy precipitation seems to have been associated with a climatic change from arid Rotliegend dune deposition to widespread Zechstein marine conditions within the greater North Sea area, probably just before or coinciding with deposition of the rapidly expanding marine Kupferschiefer.     

Conditions favourable for the formation of warm-climate aeolian sand sheets

Aeolian sand sheets are areas of aeolian sand where dunes with slipfaces are generally absent. Sand sheets are ubiquitous to modern, warm-climate sand seas, generally occurring marginal to dune fields, although they may exist within the interior of a sand sea or independent of a dune field. Sand-sheet deposits are recognized in ancient aeolian sequences, where they may account for significant accumulations of low-angle aeolian stratification. We suggest that the occurrence of sand sheets instead of dunes indicates that conditions are outside the range within which dunes form or that one or more factors interfere with dune development while also favouring the accumulation of sand sheets. A study of six modern sand sheets in North America (located at Great Sand Dunes, Gran Desierto, Dumont, Algodones, Padre Island, and Colorado River delta) indicates that the factors favourable for sand-sheet development are: (1) a high water table, (2) surface cementation or binding, (3) periodic flooding, (4) a significant coarse-grained sediment population, and (5) vegetation. These factors are reflected in the nature of stratification and the accessory features of sand-sheet accumulations within the areas of modern sand sheets as well as in their ancient counterparts in the Triassic Dolores and Pennsylvanian-Permian Rico formations.     

基于地学规律的科尔沁沙地土地整治与生态修复规划方法

内蒙古科尔沁沙地历史上曾是科尔沁大草原,现在是农牧交错区,该区的土地利用关系到生态安全、国家粮食安全和农民生计。该区域“坨甸相间”地貌特点造成不同地形部位土壤含水量存在差异,降水量年际变化大。通过卫星影像数据解译出土地利用/覆被类型图与土壤图进行叠加,获得了显性沙地和隐性沙地数量与分布信息。首先保留耕地以外的其他地类,再利用卫星影像数据反映不同降水年型耕地的归一化植被指数(NDVI),将偏欠水年NDVI低于平均值的耕地退耕还草,消除这些耕地的冬春沙化风险,以进行土地利用布局调整。这种直接利用遥感数据获取土地利用/覆被类型和耕地的NDVI进行土地利用空间布局调整的方法,十分简单实用。本文还根据地貌和沙地降水入渗快的特点,提出了与高标准农田建设模式不同但与区域地理环境相吻合的农田整治模式,即利用自然沙丘作为区域风沙防护工程,土地开发不做大平整,不修建灌溉系统,发展雨养农业。以上根据区域地理环境条件进行土地利用布局和确定农田开发建设模式,可将农业生产与荒漠化防治和谐结合在一起,实现土地可持续利用。     

Age,origin and climatic controls on vegetated linear dunes in the northwestern Negev Desert (Israel)

The stabilized northwestern (NW) Negev vegetated linear dunes (VLD) of Israel extend over 1300 km² and form the eastern end of the Northern Sinai – NW Negev Erg. This study aimed at identifying primary and subsequent dune incursions and episodes of dune elongation by investigating dune geomorphology, stratigraphy and optically stimulated luminescence (OSL) dating. Thirty-five dune and interdune exposed and drilled section were studied and sampled for sedimentological analyses and OSL dating, enabling spatial and temporal elucidation of the NW Negev dunefield evolution.In a global perspective the NW Negev dunefield is relatively young. Though sporadic sand deposition has occurred during the past 100 ka, dunes began to accumulate over large portions of the dunefield area only at ～23 ka. Three main chronostratigraphic units, corresponding to three (OSL) age clusters, were found throughout most of the dunefield, indicating three main dune mobilizations: late to post last glacial maximum (LGM) at 18–11.5 ka, late Holocene (2–0.8 ka), and modern (150–8 years). The post-LGM phase is the most extensive and it defined the current dunefield boundaries. It involved several episodes of dune incursions and damming of drainage systems. Dune advancement often occurred in rapid pulses and the orientation of VLD long axes indicates similar long-term wind directions. The late Holocene episode included partial incursion of new sand, reworking of Late Pleistocene dunes as well as limited redeposition. The modern sand movement only reactivated older dunes and did not lengthen VLDs.This aeolian record fits well with other regional aeolian sections. We suggest that sand supply and storage in Sinai was initiated by the Late Pleistocene exposure of the Nile Delta sands. Late Pleistocene winds, substantially stronger than those usually prevailing since the onset of the Holocene, are suggested to have transported the dune sands across Sinai and into the northwestern Negev.Our results demonstrate the sensitivity of vegetated linear dunes located along the (northern) fringe of the sub-tropical desert belt to climate change (i.e. wind) and sediment supply.     

Landscape change and sandy desertification in arid areas: a case study in the Zhangye Region of Gansu Province, China

The Zhangye Region of Gansu Province is an important agricultural base in arid northwestern China. During the twentieth century, especially in the last five decades, the region has experienced sandy desertification. To document the status and causes of this deterioration, satellite images, meteorological and socioeconomic data to assess landscape change from 1993 to 2002 were interpreted and analyzed. The results show that during the intervening 9-year period the area of sandy lands has increased by 642.2 km², which consist of aeolian sand dune (357.1 km²) and potential sandy land (216.3 km²). Although the development and reversion of sandy desertification co-exist, the sandy desertification in this area seems serious and is attributable to the irrational use of water and land.     

Erg deposits in the Lower Jurassic Wingate Sandstone, northeastern Arizona: oblique dune sedimentation

The Lower Jurassic erg (aeolian sand sea) deposits of the Wingate Sandstone on the Colorado Plateau are beautifully exposed near Many Farms, Arizona. These 3-D outcrops allow a detailed study of structures and sequenses in the erg body. The erg sequence comprises chiefly oblique dune deposits. The dune facies are in most cases characterized by a well-developed tripartite upbuilding. Each dune coset contains unusually thick and intricate bottomsets, medial low-angle dipping toesets, and upper steeply dipping foresets. The foresets reveal significant across-crest transport of sand and dip within a narrow range of directions towards the ESE. The bottomset beds are composed of compound cross-bedding that documents strong along-crest transport towards the SSW, whereas the toeset beds reveal upslope, downslope, and along-crest transport of sand. The ancient dunes apparently formed in a directionally varying wind flow with prevailing winds (early summer) from the NW and periodic strong winds (late summer) from the SW. The dunes were oblique not only to seasonal transport directions, but also to the resultant annual transport direction and dune migration direction. This was caused by the interaction of the dune system with the primary winds which resulted in secondary airflow and significant along-crest transport of sand. The erg deposits at Many Farms are separated by a number of super bounding surfaces suggesting several episodes of erg formation and destruction. The initial erg system was dominated by transverse dunes, but overlying ergs only contained oblique dunes. All erg systems were bounded to the SW by wide regions of erg margin environments in which aeolian sand sheet, fluvial, and lacustrine facies were deposited. Even though fluvial deposits are absent from the main part of the sequence at the study area, the effects of this system are reflected within the erg deposits at Many Farms.     

Abrasion-derived sediments under intensified winds at the latest Pleistocene leading edge of the advancing Sinai-Negev erg

Quaternary desert loess and sandstone-loessite relationships in the geological record raise questions regarding causes and mechanisms of silt formation and accretion. In the northern Sinai-Negev desert carbonate terrain, only sand abrasion in active erg could have produced the large quantities of quartzo-feldspathic silts constituting the late Quaternary northwestern Negev loess. In the continuum of source (medium to fine sand of dunes) to sink (silts in loess) the very fine sand is unaccounted for in the record. This weakens the sand abrasion model of silt formation as a global process. Here, we demonstrate that, as predicted by experiments, abrasion by advancing dunes generated large quantities of very fine sand (60-110 μm) deposited within the dune field and in close proximity downwind. This very fine sand was generated 13-11 ka, possibly synchronous with the Younger Dryas under gusty sand/dust storms in the southeastern Mediterranean and specifically in the northern Sinai-Negev erg. These very fine sands were washed down slope and filled small basins blocked by the advancing dunes; outside these sampling basins it is difficult to identify these sands as a distinct product. We conclude that ergs are mega-grinders of sand into very fine sand and silt under windy Quaternary and ancient aeolian desert environments.