Significance of interdune deposits and bounding surfaces in aeolian dune sands

Bounding surfaces and interdune deposits provide keys for detailed interpretations of the development, shape, type, wavelength and angle of climb of aeolian bedforms, as well as overall sand sea conditions. Current alternate interpretations of bounding surfaces require very different, but testable models for sand sea deposition. Two perpendicular traverses of Jurassic Entrada Sandstone, Utah, reveal relations among cross-strata, first-order bounding surfaces, and horizontal strata. These field relations seem explicable only as the deposits of downwind-migrating, climbing, enclosed interdune basins (horizontal strata) and dune bodies consisting of superimposed smaller crescentic dunes (cross-stratified deposits). A 1.7 km traverse parallel to the palaeowind direction provides a time-transgressive view showing continuous cosets of cross-strata, first-order bounding surfaces and interdune deposits climbing downwind at an angle of a few tenths of a degree. Changes occur in the angle of climb, cross-strata structure, and interdune deposits; these reflect changes in depositional conditions through time. A 1.5 km traverse perpendicular to the palaeowind direction provides a view at an instant in geological time showing first-order bounding surfaces and interdune deposits forming flat, laterally discontinuous lenticular bodies. The distribution of interdune sedimentary structures in this traverse is very similar to that of some modern interdune basins, such as those on Padre Island, Texas. Hierarchies of bounding surfaces in an aeolian deposit reflect the bedform development on an erg. The presence of three orders of bounding surfaces indicates dune bodies consisting of smaller, super-imposed dunes. The geometry of first-order bounding surfaces is a reflection of the shape of the inter-dune basins. Second-order bounding surfaces originate by the migration of the superimposed dunes over the larger dune body and reflect individual dune shape and type. Third-order bounding surfaces are reactivation surfaces showing stages in the advance of individual dunes. The presence of only two orders of bounding surfaces indicates simple dunes. Modern and Entrada interdune deposits show a wide variety of sediment types and structures reflecting deposition under wet, damp, and dry conditions. Interdune deposits are probably the best indicators of overall erg conditions and commonly show complex vertical sequences reflecting changes in specific depositional conditions.     

Aeolian dune‐field development in a water table‐controlled system: Skeiđarársandur,Southern Iceland

Aeolian dune fields characterized by partly vegetated bedforms undergoing active construction and with interdune depressions that lie at or close to the water table are widespread on Skei?arársandur, Southern Iceland. The largest aeolian dune complex on the sandur covers an area of 80 km² and is characterized by four distinct landform types: (i) spatially isolated aeolian dunes; (ii) extensive areas of damp and wet (flooded) interdune flat with small fluvial channels; (iii) small aeolian dune fields composed of assemblages of bedforms with simple morphologies and small, predominantly damp, interdune corridors; and (iv) larger aeolian dune fields composed of assemblages of complex bedforms floored by older aeolian dune deposits that are themselves raised above the level of the surrounding wet sandur plain. The morphology of each of these landform areas reflects a range of styles of interaction between aeolian dune, interdune and fluvial processes that operate coevally on the sandur surface. The geometry, scale, orientation and facies composition of sets of strata in the cores of the aeolian dunes, and their relationship to adjoining interdune strata, have been analysed to explain the temporal behaviour of the dunes in terms of their mode of initiation, construction, pattern of migration, style of accumulation and nature of preservation. Seasonal and longer‐term flooding‐induced changes in water table level have caused episodic expansion and contraction of the wet interdune ponds. Most of the dunes are currently undergoing active construction and migration and, although sediment availability is limited because of the high water table, substantial aeolian transport must occur, especially during winter months when the surface of the wet interdune ponds is frozen and sand can be blown across the sandur without being trapped by surface moisture. Bedforms within the larger dune fields have grown to a size whereby formerly damp interdune flats have been reduced to dry enclosed depressions and dry aeolian system accumulation via bedform climb is ongoing. Despite regional uplift of the proximal sandur surface in response to glacial retreat and unloading over the past century, sediment compaction‐induced subsidence of the distal sandur is progressively placing aeolian deposits below the water table and is enabling the accumulation of wet aeolian systems and increasing the likelihood of their long‐term preservation. Wet, dry and stabilizing aeolian system types all co‐exist on Skei?arársandur and the dunes are variously undergoing coeval construction, accumulation, bypass, stabilization and destruction as a result of interactions between localized factors.     

Stoss-side mud-drapes: deposits of interdune pond margins

Arcuate mud-drapes have been observed in an interdune area in southern Tunisia. They are believed to have been formed by deposition from ponded water around aeolian dunes during flood events. They are preferentially preserved on the stoss-side of the dunes as a result of subsequent dune migration, and exhibit extensive evidence of animal tracks. They are useful in illustrating dune-interdune dynamics and recording the type of animals present.     

Sedimentology and paleohydrology of Late Quaternary lake deposits in the northern Namib Sand Sea, Namibia

The Namib Sand Sea is the largest active desert dunefield in southern Africa, and is comprised mainly of large north-south linear dunes. In the interdune areas of the northern Sand Sea eleven small areas of calcareous lacustrine sediment have been studied. These beds are typically less than a metre thick and are dominantly comprised of calcareous sandstones to mudstones and sandy limestones. The carbonates are mainly magnesian calcites (1–14% MgCO₃) with some protodolomite and aragonite. Calcified reed casts and fresh to brackish water gastropods, diatoms, and ostracods are present in some beds. δ¹⁸O values indicate a hot and dry climate. A number of enriched δ¹³C values may reflect high salinity, low organic populations, or carbonate recrystallization.These carbonate-rich lacustrine deposits are indicative of increased periods of moisture availability in this normally hyperarid region during the Late Quaternary. The origin of the water responsible for depositing these sediments may be: (1) ponding at the end point of the Tsondab River, which at one time extended farther west into the Sand Sea; (2) flooding into interdune corridors when water levels rose in rivers such as the Kuiseb; (3) groundwater seepage into depressions either through dunes that border rivers or from the underlying Tsondab Sandstone; and (4) increased rainfall. We do not believe that there is evidence to support a major increase in precipitation over the region. However, even a small increase in precipitation in the headwaters of valleys that drain toward the Sand Sea might: (1) generate enough additional runoff to extend the terminal point of rivers such as the Tsondab farther into the dunes; (2) cause lateral flooding from major valleys into interdune corridors; and (3) recharge aquifers. The sedimentary records at Narabeb, Ancient Tracks, and West Pan, which lie along the old course of the Tsondab River, favor a ponded river origin for them, whereas groundwater seepage is favored at other sites. The chronology of deposition, based on radiocarbon dates, suggests that ponding and recharge occurred earlier in the lower, western part of the area, and later in the east. This is in harmony with the view that the end point of the Tsondab River progressively retreated eastward between about 30 and 14 ka BP, as dunes blocked its route.     

Aeolian stratification and facies association in desert sediments, Arran basin (Permian), Scotland

ABSTRACT Permian aeolian sediments on the island of Arran are divisible into dune (including draa) and interdune deposits. Both types display a distinctive and unusually wide variation in grain size. The dominant features of the dune deposits are grainfall lamination, sandflow lamination, and inverse graded lamination associated with ripple-form lamination and normal graded lamination. The flat-lying aeolian interdune deposits are characterised by granule and sand ripples, horizontal lamination in coarse sand and granules, plane bed lamination and inverse graded lamination. Associated structures include ripple-form lamination and deflation lags. Three types of trace fossil associated with completely bioturbated horizons occur in some low-angle dune and interdune deposits.
The aeolian facies interfinger with alluvial fan deposits giving rise to three recognizable facies belts. Marginal aeolian deposits are associated with fluvial conglomerates and are dominated by interdune deposits and occasionally very thin barchan deposits (set height 3-37 cm). Intermediate aeolian deposits are characterized by interbedded crescentic dune, small draa (dune set height 5 cm-4.5 m) and interdune deposits, and rare fluvial and lake sediments. Basinal aeolian deposits are dominated by draa deposits (dune set height 0.2-28 m) associated with rare interdune sediments. Transverse dunes and draas were moved by north-eastern palaeowinds towards the foot of the alluvial fans. The aeolian sediments were deposited in a fault-bounded desert basin.     

Stratigraphic evolution of an aeolian erg margin system: the Permian Cedar Mesa Sandstone, SE Utah, USA

The Permian Cedar Mesa Sandstone of south‐east Utah is a predominantly aeolian succession that exhibits a complex spatial variation in sedimentary architecture which, in terms of palaeogeographic setting, reflects a transition from a dry erg centre, through a water table‐controlled aeolian‐dominated erg margin, to an outer erg margin subject to periodic fluvial incursion. The erg margin succession represents a wet aeolian system, accumulation of which was controlled by progressive water table rise coupled with ongoing dune migration and associated changes in the supply and availability of sediment for aeolian transport. Variation in the level of the water table relative to the depositional surface determined the nature of interdune sedimentary processes, and a range of dry, damp and wet (flooded) interdune elements is recognized. Variations in the geometry of these units reflect the original morphology and the migratory behaviour of spatially isolated dry interdune hollows in the erg centre, locally interconnected damp and/or wet interdune ponds in the aeolian‐dominated erg margin and fully interconnected, fluvially flooded interdune corridors in the outer erg margin. Relationships between aeolian dune and interdune units indicate that dry, damp and wet interdune sedimentation occurred synchronously with aeolian bedform migration. Temporal variation in the rates of water‐table rise and bedform migration determined the angle of climb of the erg margin succession, such that accumulation rates increased during periods of rapidly rising water table, whereas sediment bypassing (zero angle of climb) occurred in the aftermath of flood events in response to periods of elevated but temporarily static water table. During these periods in the outer erg margin, the expansion of fluvially flooded interdunes in front of non‐climbing but migrating dunes resulted in the amalgamation of laterally adjacent interdunes and the generation of regionally extensive bypass (flood) supersurfaces. A spectrum of genetic depositional models is envisaged that accounts for the complex spatial and temporal evolution of the Cedar Mesa erg margin succession.     

Fluvial-aeolian interactions: Part I, modern systems

Two modern fluvial-aeolian depositional systems (Great Sand Dunes National Monument, Colorado and the Mojave River Wash, California) are remarkably similar in spite of different climates, sizes, fluvial sediment textures, and relative directions of aeolian and fluvial transport. Dune growth and migration, and deflation of blowouts create 8–10 m of local relief in unflooded aeolian landscapes. There are six prominent fluvial-aeolian interactions. (1) Fluvial flow extends into the aeolian system until it is dammed by aeolian landforms; (2) interdune areas (overbank-interdunes) upstream of aeolian dams, and alongside channels are flooded; (3) water erodes dunes alongside channels and interdunes; (4) flood waters deposit sediment in interdune areas; (5) fluvially derived groundwater floods interdunes (interdune-playas); (6) wind erodes fluvial sediment and redeposits it in the aeolian system. Unique and characteristic sediments are deposited in overbank-interdunes and in interdune-playas, reflecting alternate fluvial and aeolian processes and rapidly changing flow and salinity conditions. These fluvial-aeolian interdune deposits are characterized by irregular, concave-up bases and flat upper surfaces containing mudcracks or evaporite cement. Characteristic low-relief surfaces form in aeolian systems as an effect of flooding. Fluvial deposits are resistant to aeolian deflation. Aeolian sand is preserved when flood sediments are deposited around the bases of the dunes. Thus repetitive fluvial and aeolian aggradation tends to be ‘stepwise’ as interdune floors are suddenly raised during floods. The effects of flooding should be easy to recognize in ancient aeolianites, even beyond the area covered with overbank muds.     

Stratigraphic evolution and preservation of aeolian dune and damp/wet interdune strata: an example from the Triassic Helsby Sandstone Formation, Cheshire Basin, UK

Abstract New and previously published models of wet aeolian system evolution form a spectrum of types that may be explained in terms of aeolian dune dynamics, rate of water table rise and/or periodicity of interdune flooding. This is illustrated with an example from the Mid‐Triassic (Anisian) Helsby Sandstone Formation, Cheshire, UK. Lenses of damp and wet interdune strata exhibit an intertonguing, transitional relationship with the toe‐sets of overlying aeolian dune units. This signifies dune migration that was contemporaneous with water table‐controlled accumulation in adjacent interdunes. Downwind changes in the geometry and facies of the interdune units indicate periodic expansion and contraction of the interdunes in response to changes in the elevation of the groundwater table and episodic flooding, during which accumulation of dune strata continued relatively uninterrupted. This contrasts with other models for accumulation in wet aeolian systems where interdune flooding is associated with a cessation in aeolian bedform climbing and the formation of a bypass or erosional supersurface. Architectural panels document the detailed stratigraphy in orientations both parallel and perpendicular to aeolian transport direction, enabling a quantitative three‐dimensional reconstruction of genetically related aeolian dune and interdune elements. Sets of aeolian dune strata are composed of grainflow and translatent wind‐ripple strata and are divided by a hierarchy of bounding surfaces originating from oblique migration of superimposed dunes over slipfaceless, sinuous‐crested parent bedforms, together with lee‐slope reactivation under non‐equilibrium flow conditions. Silty‐mudstone and sandstone interdune units are characterized by wind ripple‐, wavy‐ and subaqueous wave ripple‐laminae, desiccation cracks, mud flakes, raindrop imprints, load casts, flutes, intraformational rip‐up clasts and vertebrate and invertebrate footprint impressions and trackways. These units result from accumulation on a substrate that varied from dry‐ through damp‐ to wet‐surface conditions. Interdune ponds were flooded by either fluvial incursions or rises in groundwater table and were periodically subject to gradual desiccation and reflooding. Red silty‐mudstone beds of subaqueous origin pass laterally into horizontally laminated wind‐ripple beds indicating a progressive transition from wet‐ through damp‐ to dry‐surface conditions within a single interdune.     

Characteristics and OSL minimum ages of relict fluvial deposits near Sossus Vlei,Tsauchab River,Namibia, and a regional climate record for the last 30 ka

The present end‐point of the Tsauchab River is at Sossus Vlei, 30 km into the Namib Sand Sea. Interdune deposits in three depressions west and southwest of the vlei include channel and interdune lithofacies associations but no deposits typical of river end‐points or of groundwater seepage into interdune areas. The two lithofacies associations show that the Tsauchab River extended further into the sand sea in the past. It had a well‐developed channel and a higher flow than today that caused flooding of adjacent interdune areas. OSL 4‐mm aliquot minimum ages indicate that the Tsauchab River reached 2–3 km beyond its present end‐point at ca. 25 ka and ca. 9–7 ka, and that the river was more active from 0.9–0.3 ka. The eastward migration of the river end‐point since ca. 7 ka suggests a reduction in flood magnitude accompanied by the gradual invasion of the Sossus Vlei area by dunes. The regional data indicate an additional wet interval at ca. 15 ka that is so far not recorded in the Sossus area. Copyright © 2006 John Wiley & Sons, Ltd.     

Late Quaternary history of the coastal Wahiba Sands,Sultanate of Oman

Continental sediments and geomorphological features of the coastal Wahiba Sands, Sultanate of Oman, reflect environmental variability in southeastern Arabia during the late Quaternary. Weakly cemented dune sands, interdune deposits and coastal sediments were dated by luminescence methods to establish an absolute chronology of changes in sedimentary dynamics. The dating results confirm previous assumptions that during times of low global sea level sand was transported by southerly winds from the exposed shelf onto the Arabian Peninsula. Two prominent phases of sand accumulation in the coastal area took place just before and after the last glacial maximum (LGM). A final significant period of dune consolidation is recognised during the early Holocene. However, no major consolidation of dunes appears to have occurred during the LGM and the Younger Dryas. In the northern part of the Wahiba Sands, these two periods are characterised by substantial sand deposition. This discrepancy is explained by the lack of conservation potential for dunes in the coastal area, probably caused by a low groundwater table due to low sea level and decreased precipitation. While the times of aeolian activity reflect arid to hyper‐arid conditions, lacustrine and pedogenically altered interdune deposits indicate wetter conditions than today caused by increased monsoonal circulation during the Holocene climatic optimum. Copyright © 2005 John Wiley & Sons, Ltd.     

Antecedent aeolian dune topographic control on carbonate and evaporite facies: Middle Jurassic Todilto Member,Wanakah Formation,Ghost Ranch,New Mexico,USA

The Middle Jurassic Todilto Member of the Wanakah Formation is a carbonate and gypsum unit inset into the underlying aeolian Entrada Sandstone in the San Juan Basin. Field and thin section study of the uppermost Entrada and Todilto at Ghost Ranch, New Mexico, identified Todilto facies and their relationship to remnant Entrada dune topography. Results support the previous interpretation that the Entrada dunes, housed in a basin below sea level, were rapidly flooded by marine waters. Mass wasting of the dunes gave rise to sediment‐gravity flows that largely buried remnant dune topography, leaving ca 12 m of relief that defined the antecedent condition for Todilto deposition. Previously interpreted as seasonal varves deposited in a stratified water body, the Todilto is reinterpreted as a microbial biolaminite. Most diagnostic are organic‐rich laminae with structures characteristic of filamentous microbes and containing trapped aeolian silt, and clotted‐texture laminae with a fabric associated with calcification of extracellular polymeric substances. The spatial arrangement of Todilto facies is controlled by the dune palaeotopography. A continuous basal laminated mudstone thickens over the dune crest, reflecting the optimum conditions for microbial mat development, and is interpreted to have been deposited when marine waters submerged the topography. Subsequent drying caused emergence of the crestal area, and formation of tepee structures and a dissolution breccia. Gypsiferous mudflats and periodic ponds occupied the dune flanks and interdune area, with gypsum concentrated within the interdune area. Entrada sands remained unstable during Todilto deposition with common injection structures into the Todilto, and a remnant slope caused the downslope movement and folding of Todilto strata on the upper lee face. Although some expansion of the gypsum occurred in the subsurface, facies architecture fostered development of a dissolution front adjacent to the interdune gypsum body with section collapse of gypsiferous limestone on the dune flanks.     

Pollen diagram from the Nebraska Sandhills and the age of the dunes

Radiocarbon dates of organic alluvium beneath as much as 40 m of dune sand along the Dismal River have led to the suggestion that the Nebraska Sandhills date from the Holocene rather than the last glacial period. On the other hand, the basal layers of lake and marsh deposits in interdune depressions at three localities date in the range of 9000 to 12,000 yr B.P., implying a pre-Holocene age for the sand dunes. A pollen diagram for one of these sites, Swan Lake, indicates prairie vegetation throughout the last 9000 yr, with no suggestion that the landscape was barren enough to permit the shaping of the massive dunes characterizing the area. Sand was not transported across the site during the Holocene, either during the marsh phase, which lasted until 3700 yr B.P., or during the subsequent lake phase. The sand that buries the alluvium along the Dismal River may represent only local eolian activity, or it may indicate that the younger of the two main dune series identified by H. T. U. Smith (1965, Journal of Geology73, 557–578) is Holocene in age, and the older one Late Wisconsin in age.     

Spatial heterogeneity of cyanobacteria-inoculated sand dunes significantly influences artificial biological soil crusts in the Hopq Desert (China)

Artificial biological soil crusts (ABSCs), formed by inoculating Microcoleus vaginatus Gom. and Scytonema javanicum Born. et Flah. onto the topsoil of desert dunes, proved to be effective tools for the stabilization of moving dunes and promotion of soil fertility. As dominant driving forces in arid habitats, the abiotic environmental conditions of undulating dunes produce a gradient of abiotic stresses on cyanobacteria. Cyanobacteria are considered pioneering phototrophs in early soil crust communities in deserts. In this study, the development of ABSCs under various environmental site conditions was investigated using 16S rRNA-based polymerase chain reaction, denaturing gradient gel electrophoresis (DGGE), and soil property measurements. After inoculation in 2002 and long-term development, patchy moss crusts were observed on the dunes. All of the available sequenced bands in the DGGE gels belonged to Oscillatoriales and Nostocales. The dominant Nostocales genus in the ABSCs was still Scytonema; however, more Oscillatoriales genera were identified, which belonged to Microcoleus and Phormidium. The cyanobacterial compositions of different slope types were significantly distinct (p < 0.05), particularly those from windward slopes. The crusts of the top-dune slopes were more heterogeneous. In addition, the soil physicochemical properties and richness indices of the windward slopes were significantly lower than those of the leeward and interdune slopes (p < 0.05). Compared with uninoculated control dunes, all of the inoculated dunes had far higher biodiversities.     

Feature: The sedimentary signature of deserts and their response to environmental change

Desert sedimentary systems comprise a variety of related sub-environments including aeolian dunes, intervening interdunes, sandsheets, salt flats, playa lakes, ephemeral fluvial systems and alluvial fans. These are highly sensitive, and undergo subtle but systematic morphological and sedimentary adjustments in response to externally-imposed environmental change. This article presents a dynamic model explaining how desert successions – particularly aeolian dune and interdune environments – are determined both by intrinsic sedimentary behaviour, such as dune migration, and by the imposition of externally-forced changes such as climate change.     

拉日铁路风沙防护研究

结合工程实际,对拉日铁路的风沙灾害防治技术难题采用“永临结合”的平面工程防沙措施体系,近期以适宜于高原地区的工程措施为主,远期研究以适宜于高寒铁路沿线的植物防沙措施,以解决我国在高海拔地区铁路沙害防护技术难题。     

Toward a model for airflow on the lee side of aeolian dunes

The interaction between dunes and the primary wind results in a complex pattern of secondary airflow on the lee side of dunes. From 15 dunes studied during transverse flow conditions at Padre Island in Texas, White Sands in New Mexico, and the Algodones in California, distinct flow regions can generally be recognized, with the overall flow structure comparing favourably to that proposed for subaqueous bedforms. Downwind of dunes with flow separation is a back-flow eddy that extends about four dune-brink heights downwind from the brink of the dune. Beyond the separation cell, the velocity profiles can be divided into regions based upon segments separated by ‘kinks’ in the velocity profiles. The interior is an area above the dunes of relative high wind speed but low velocity gradient. Beneath the interior is the wake, which consists of two layers. The upper wake exhibits an uppermost portion where the flow decelerates while the remainder exhibits accelerating flow, so that the overall velocity gradient decreases downwind. The lower wake exhibits low velocity gradients and wind speeds that accelerate downwind at all heights, but primarily near the top of the layer, thereby causing the velocity gradient to increase downwind. At about eight dune heights downwind, the upper and lower wakes equilibrate to a single profile with the kink between them no longer apparent. The lowest recognizable region is the internal boundary layer. It is recognized by a relatively steep velocity gradient below the wake, and never exceeds a few tens of centimetres in height for our data set. Because of acceleration and increasing shear stress within this layer, interdune flats are at least potentially erosional. Overall, the wake and internal boundary layer show a downward transfer of momentum from upper regions so that the flow recovers. Where flow separation does not occur, simple flow expansion down the lee-face causes flow deceleration.     

High-resolution stratigraphy and depositional model of wind- and water-laid deposits in the ordovician Guaritas rift (Southernmost Brazil)

The upper portion of the Pedra Pintada Alloformation includes about 100 m of mostly eolian deposits. This paper emphasizes the vertical succession and lateral association of sedimentary facies, based on analysis of outcrop data and aerial photographs, as well as the hierarchy and origin of bounding surfaces. It aims to propose a high-resolution stratigraphic and depositional model that may be useful to exploitation of eolian reservoirs. The succession has been preserved due to basin subsidence, and is described in terms of four facies associations that constitute three dominantly eolian units. These units are sharply bounded by major flooding surfaces (super surfaces) that, in turn, are overlain by 1 to 2 m thick, dominantly water-laid facies (lacustrine, fluvial, deltaic and eolian). Both their internal organization and boundaries were controlled by changes in the base level rise rate. The basal Eolian Unit is composed of crescentic eolian dunes and damp interdune deposits ascribed to a wet eolian system. On the other hand, eolian units II and III, also characterized by crescentic eolian dunes (simple and compound) deposits, were related to dry eolian systems, since they comprise dry (eventually wet) interdune facies. Eolian Unit III is truncated by basinwide unconformity, which is then overlain by the ephemeral fluvial deposits (Varzinha Alloformation). This second type of super surface is related to climate-induced wind erosion (deflation) down to the water table level (regional Stokes surface) followed by fluvial incision linked to tectonic activity.     

Microbiotic crusts and their interrelations with environmental factors in the Gurbantonggut desert,western China

Located in the Junggar Basin in Xinjiang, the Gurbantonggut Desert is the second largest desert in China. Microbiotic crusts consisting of animalcule, lichen, moss, and algae species develop extensively in the region. Their formation, species composition and distribution pattern are closely related to the environmental conditions along the different parts of sand dune. Analysis of microbiotic crust distribution and relationship to environmental factors shows that average microbiotic crust thickness is 0.05–0.1 cm at the tops dunes, 0.2–1.5 cm in the upper part, 1.5–2.5 cm in middle and lower parts of dunes, and 1.5–5.0 cm in interdune areas, while areal coverage is 30.5, 48.5, 55.5, and 75.5%, respectively. Microbiotic crust differentiation along dune slopes is a result of the development stage and converse-succession resistance of the different microbiotic crusts. The numbers of species, thickness and degree of development of microbiotic crusts increase from the upper part to the middle and lower parts of dune slopes. The development and differentiation of microbiotic crusts at various dune slope positions are a reflection of the ecological expression of the comprehensive adaptability and natural selection of different microbiotic crust species to the local environmental conditions, and are closely related to such ecological conditions as the physiochemical properties of soils and topsoil textural stability.     

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.     

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.