Dunes on the skeleton coast,Namibia (South West Africa): Geomorphology and grain size relationships

Simple, and locally compound, transverse and barchanoid dunes dominate the 2000 km² Skeleton Coast dunefield in northwestern Namibia/South West Africa. Dune height and spacing are closely correlated (r = 0-89) and decrease across the dunefield from southwest to northeast, with an accompanying change from transverse to barchanoid ridges and ultimately barchans. The dunes are aligned transverse to the dominant strong south and south southwest onshore winds. Alignment patterns indicate that surface roughness changes between coastal plain and dunes cause dune-forming winds to swing to the right over the dunes, but resume their original direction beyond. Grain size and sorting vary at three scales: the dune, the dune landscape and through the dunefield. Overall the sands, derived from three localities by deflation from beaches supplied by vigorous longshore drift, become progressively finer and better sorted across the dunefield paralleling changes in dune height and spacing. A statistically significant relationship (r = ?0?65) was established between dune spacing and the phi grain size of the coarser fraction of the dune sands, demonstrating the importance of the protective effects of coarse grains, and suggesting that the morphometry of simple transverse dunes may be controlled by the scale of turbulence associated with the threshold wind speed required to move the coarsest fraction of the dune sand.     

Investigating parabolic and nebkha dune formation using a cellular automaton modelling approach

Vegetation plays an important role in shaping the morphology of aeolian dune landscapes in coastal and semi‐arid environments, where ecogeomorphic interactions are complex and not well quantified. We present a Discrete ECogeomorphic Aeolian Landscape model (DECAL) capable of simulating realistic looking vegetated dune forms, permitting exploration of relationships between ecological and morphological processes at different temporal and spatial scales. The cellular automaton algorithm applies three simple rules that lead to self‐organization of complex dune environments, including nebkhas with distinctive deposition tails that form in association with mesquite‐type shrubs, and hairpin (long‐walled) parabolic dunes with trailing ridges that evolve from blowouts in association with vegetation succession. Changing the conditions of simulations produces differing landscapes that conform qualitatively to observations of real‐world dunes. The model mimics the response of the morphology to changes in sediment supply, vegetation distribution, density and growth characteristics, as well as initial disturbances. The introduction of vegetation into the model links spatial and temporal scales, previously dimensionless in bare‐sand cellular automata. Grid resolutions coarser than the representative size of the modelled vegetation elements yield similar morphologies, but when cell size is reduced to much smaller dimensions, the resultant landscape evolution is dramatically different. The model furthermore demonstrates that the relative response characteristics of the multiple vegetation types and their mutual feedback with geomorphological processes impart a significant influence on landscape equilibria, suggesting that vegetation induces a characteristic length scale in aeolian environments. This simple vegetated dune model illustrates the power and versatility of a cellular automaton approach for exploring the effects of interactions between ecology and geomorphology in complex earth surface systems. Copyright © 2007 John Wiley & Sons, Ltd.     

Residual dune ridges: Sedimentary architecture,genesis, and implications for palaeo-climate reconstructions

Sedimentary architecture and genesis of residual dune ridges in a temperate climate are presented and implications for their use as archive of changes in long-term precipitation and wind climate are discussed. Residual dunes are common features of wet aeolian systems, where they form sets of shallow ridges, oriented perpendicular to the prevailing wind direction. Residual dune ridges of the study area are vegetated and typically elevate 0.6 to 2.5 m above the surrounding interdune flats. They develop on the lower stoss side of active transgressive dunes, triggered by periods of elevated groundwater table and hence colonization of the foot of the dune by rapid growing pioneer vegetation. Stabilized by plants, the growing ridge detaches from the active transgressive dune and gets abandoned within years in the course of the downwind-migration of the transgressive dune. Grain-size data suggest a main sediment supply from the transgressive dune and only minor input from other sources. Ground-penetrating radar reveals that the residual dune ridges are composed of windward-dipping as well as leeward-dipping sedimentary beds. Leeward-dipping strata reflect sediment supply from the parental dune, whereas windward-dipping beds are seen to result from sediment redistribution along the ridge and sediment supply from the adjacent swales during the ridge growth period. Multi-annual to multi-decadal variability in precipitation leads to the development of sequences composed of tens of ridges, spanning time periods of several centuries. Spacing of individual ridges in these sequences is controlled not by long-term variability in precipitation alone, but probably also reflects variable wind intensity which affects the migration rate of the parental dune. The important role of vegetation in ridge construction makes these landforms a demonstrative example of landscape development by geo-biosphere interacting processes.     

Coastal dune fields at the São Francisco River strandplain,northeastern Brazil: morphology and environmental controls

Extensive coastal dune ?elds occur on the Quaternary strandplain associated with the São Francisco River mouth. Two different generations of dunes are identi?ed. One is inactive, already ?xed by vegetation, comprising parabolic dunes. The other generation is active, bordering the present‐day shoreline and transgressing over the inactive dune ?eld. Three morphological provinces in the active coastal dune ?elds are recognized. On the updrift side of the São Francisco River mouth, they are: (a) sand‐sheet with shrub coppice and shadow dunes; (b) isolated dunes of the barchan‐transversal type up to 5 m high, and interdune areas; and (c) a 23 m high compound dune, with superimposed small dunes. The same provinces are recognized on the downdrift side of the river mouth, with two important exceptions: the barchan‐transversal and compound dunes are replaced, respectively, by (i) zibar‐type dunes up to 5 m high, and (ii) a 19 m high precipitation dune, which is associated with numerous blowouts. The prevailing eastern winds from August to January favour the development of the aeolian bedforms and the migration of dunes. The shoreline orientation almost transversal to the winds and the great supply of ?ne‐grained sediments contribute to the formation of barchan‐transversal types and compound dunes in the updrift side. On the other hand, in the downdrift side the shoreline orientation is almost parallel to the prevailing winds. This fact, in association with a coarser grain size in the beachface, favours the formation of zibar‐type and precipitation dunes with numerous blowouts. The rate of migration of individual dunes is about 20 to 24 m per year. This study suggests that the aeolian sedimentation is a relatively recent phenomenon at the Quaternary strandplain of the São Francisco River. The ?rst generation of dune ?elds initiated some time after 3000 years BP and the second generation originated some centuries ago. Copyright © 2004 John Wiley & Sons, Ltd.     

Changes in vegetation cover on the Younghusband Peninsula transgressive dunefields (Australia) 1949–2017

Studies have shown that the impact of climate change, human and animal actions on coastal vegetation can turn stabilized dunes into active mobile dunes and vice versa. Yet, the driving factors that trigger vegetation changes in coastal dunes are still not fully understood. In the transgressive dunefields of the Younghusband Peninsula (south-east coast of South Australia) historical aerial photographs show an increase in vegetation cover over the last ~70 years. This study attempts to identify the causes of the changes in vegetation cover (1949 to 2017) observed in a typical section of the coastal dune systems of the Peninsula. Vegetation cover was first estimated for various years using the available historical aerial photography (long-term changes – 1949 to 2017) and recent satellite imagery (short-term annual changes – 2010 to 2017) for the area, and then results were discussed against the observed changes in climatic variables and rabbit density, factors that could have played a role in this transformation. Results of long-term changes show that the vegetation cover has increased significantly from 1949 to 2017, from less than 7% vegetation cover to almost 40%, increasing dune stabilization and forming parabolic dune systems. Periods with the largest growth in vegetation cover (1952-1956 and 2009-2013) coincide with a significant decline in rabbit numbers. Rabbit density was found to be the primary factor linked to the rapid vegetation growth and stabilization of the dunefield, for both decadal long-term (last 68 years) and annual short-term changes (last 8 years). Other factors such as changes in rainfall, aeolian sediment transport, land use practices, and the introduction of invasive plants have apparently played a limited to negligible role in this stabilization process. © 2018 John Wiley & Sons, Ltd.     

The effects of interdune vegetation changes on eolian dune field evolution: a numerical‐modeling case study at Jockey's Ridge,North Carolina,USA

Changes in vegetation cover within dune fields can play a major role in how dune fields evolve. To better understand the linkage between dune field evolution and interdune vegetation changes, we modified Werner's (Geology, 23, 1995: 1107–1110) dune field evolution model to account for the stabilizing effects of vegetation. Model results indicate that changes in the density of interdune vegetation strongly influence subsequent trends in the height and area of eolian dunes. We applied the model to interpreting the recent evolution of Jockey's Ridge, North Carolina, where repeat LiDAR surveys and historical aerial photographs and maps provide an unusually detailed record of recent dune field evolution. In the absence of interdune vegetation, the model predicts that dunes at Jockey's Ridge evolve towards taller, more closely‐spaced, barchanoid dunes, with smaller dunes generally migrating faster than larger dunes. Conversely, the establishment of interdune vegetation causes dunes to evolve towards shorter, more widely‐spaced, parabolic forms. These results provide a basis for understanding the increase in dune height at Jockey's Ridge during the early part of the twentieth century, when interdune vegetation was sparse, followed by the decrease in dune height and establishment of parabolic forms from 1953‐present when interdune vegetation density increased. These results provide a conceptual model that may be applicable at other sites with increasing interdune vegetation cover, and they illustrate the power of using numerical modeling to model decadal variations in eolian dune field evolution. We also describe model results designed to test the relative efficacy of alternative strategies for mitigating dune migration and deflation. Installing sand‐trapping fences and/or promoting vegetation growth on the stoss sides of dunes are found to be the most effective strategies for limiting dune advance, but these strategies must be weighed against the desire of many park visitors to maintain the natural state of the dunes. Copyright © 2009 John Wiley & Sons, Ltd.     

WIND ENERGY VARIATIONS IN THE SOUTHWESTERN KALAHARI DESERT AND IMPLICATIONS FOR LINEAR DUNEFIELD ACTIVITY

The southwestern Kalahari linear dunefield, which displays marked morphological variability, possesses a partial but temporally and spatially variable vegetation cover and has frequently been described as a palaeodunefield. Palaeo status has been ascribed on the basis of several criteria including the presence of vegetation, but also because dunes are thought to be out of alignment with modern resultant potential sand-moving wind directions and because present-day wind energy is regarded as low. For the period 1960–1992, wind data from eight dunefield meteorological stations are analysed in detail to examine these assertions. Potential sand transport directions, including spatial and temporal variations, and potential drift directions for the windiest three month periods, are calculated and explained. It is concluded that the present-day potential sand transport environment is markedly variable from year to year and from place to place. While periods of low sand transport energy do occur, it is also noted that the 1980s possessed considerable potential for sand transport in the dunefield. Directional variability is also relatively high, perhaps exceeding that under which linear dunes can be expected to form. Because linear dune aeolian activity has a number of states, however, the present-day wind environment may allow dune surface aeolian activity to occur which does not alter the overall pattern of the dunes.     

Morphodynamics and climate controls of two aeolian blowouts on the northern Great Plains,Canada

Blowouts are the most regionally pervasive active aeolian landform on the northern Great Plains of North America. This study reports a long‐term investigation into the morphological development of two adjacent blowouts in a continental dune field. The blowouts were monitored for a decade in the Bigstick Sand Hills of southwestern Saskatchewan, Canada. Topographic changes were determined from dense arrays of erosion pins in each blowout (1 per 4 m², n = 171; and 1 per 16 m², n = 150). Pin measurements were made 16 times between May 1994 and May 2004. Over the decade both blowouts expanded and more than doubled in volume. Differences in form–flow interactions have caused the larger of the two blowouts to deposit more than a metre of sediment within the deflation basin, and the smaller blowout to erode by more than a metre. A negative feedback effect was triggered when the larger blowout reached a critical size around 1994 (60 m × 36 m × 8·1 m, length × width × height) when sediment was no longer eroded from the deflation basin. A positive feedback in the smaller blowout continues to facilitate erosion from the deflation basin. Monthly observations since 2002 indicate that aspect plays an important role in the development of these blowouts by creating a spatial asymmetry in sediment availability. Sediment is more readily available throughout the year on south‐facing slopes, which receive greater insolation than north‐facing slopes and are often drier and more frequently thawed in this cold‐climate environment. Comparisons between climate data from a remote meteorological station 45 km to the southwest and sediment transport indices developed from the erosion pin data produced very few correlations significant at the 95 per cent confidence level. Nevertheless, the signs of the correlation coefficients indicate that sediment erosion and deposition in both blowouts respond similarly to the following climate variables recorded at the remote station: (i) the amount of precipitation, (ii) the transport capacity of the wind and (iii) transporting winds from a directional wedge between 180 and 330°. Taken altogether, the results from this study highlight the importance of climate and feedback effects in blowout development that may be extended to other blowouts in continental and coastal settings. Copyright © 2006 John Wiley & Sons, Ltd.     

The termination of the last major phase of aeolian sand movement,coastal dunefields,Denmark

Optically stimulated luminescence (OSL) dating of sand samples from stabilized (or inactive) coastal dunes in Denmark provides information on the age of the termination phase of the last major aeolian activity period. A total of 26 sand samples were taken from four different coastal dunefields around the North Sea, Skagerrak and Kattegat coasts of Denmark. The OSL dates indicate that the last major phase of aeolian activity terminated between ad 1860 and 1905. Most of the dunes examined in this study were active around 1820, during a period documented to have been very stormy. A dune management scheme started around 1792, and this no doubt was a major cause of dunefield stabilization, but an overall decline of storminess, particularly spring and summer storminess, around the end of the 19th century must also have contributed to the increasing inactivity of coastal dunes. The new OSL dates on aeolian sand movement agree well with historical data and data from topographic maps on dune movement. This agreement supports the observation from earlier work that OSL dating of recent aeolian sand movement is accurate over the last few decades to centuries. Copyright © 2005 John Wiley & Sons, Ltd.     

A process‐based hypothesis for the barchan–parabolic transformation and implications for dune activity modelling

The introduction of vegetation to bare barchan dunes can result in a morphological transformation to vegetated parabolic dunes. Models can mimic this planform inversion, but little is known about the specific processes and mechanisms responsible. Here we outline a minimalist, quantitative, and process‐based hypothesis to explain the barchan–parabolic transformation. The process is described in terms of variations in the stabilization of wind‐parallel cross‐sectional dune slices. We hypothesize that stabilization of individual ‘dune slices’ is the predictable result of feedbacks initiated from colonization of vegetation on the slipface, which can only occur when slipface deposition rates are less than the deposition tolerance of vegetation. Under a constant vegetation growth regime the transformation of a barchan dune into a parabolic dune is a geometric response to spanwise gradients in deposition rates. Initial vegetation colonization of barchan horns causes shear between the anchored sides and the advancing centre of the dune, which rotates the planform brinkline angle from concave‐ to convex‐downwind. This reduces slipface deposition rate and allows vegetation to expand inward from the arms to the dune centre. The planform inversion of bare barchans dunes into vegetated parabolic dunes ultimately leads to complete stabilization. Our hypothesis raises several important questions for future study: (i) are parabolic dunes transitional landforms between active and vegetation‐stabilized dune states? (ii) should stabilization modelling of parabolic dune fields be treated differently than linear dunes? and (iii) are stabilized parabolic dune fields ‘armoured’ against re‐activation? Copyright © 2012 John Wiley & Sons, Ltd.     

Sedimentological,modal analysis and geochemical studies of desert and coastal dunes,Altar Desert,NW Mexico

Sedimentological, compositional and geochemical determinations were carried out on 54 desert and coastal dune sand samples to study the provenance of desert and coastal dunes of the Altar Desert, Sonora, Mexico. Grain size distributions of the desert dune sands are influenced by the Colorado River Delta sediment supply and wind selectiveness. The desert dune sands are derived mainly from the quartz‐rich Colorado River Delta sediments and sedimentary lithics. The dune height does not exert a control over the grain size distributions of the desert dune sands. The quartz enrichment of the desert dune sands may be due to wind sorting, which concentrates more quartz grains, and to the aeolian activity, which has depleted the feldspar grains through subaerial collisions. The desert dune sands suffer from little chemical weathering and they are chemically homogeneous, with chemical alteration indices similar to those found in other deserts of the world. The desert sands have been more influenced by sedimentary and granitic sources. This is supported by the fact that Ba and Sr concentration values of the desert sands are within the range of the Ba and Sr concentration values of the Colorado River quartz‐rich sediments. The Sr values are also linked to the presence of Ca‐bearing minerals. The Zr values are linked to the sedimentary sources and heavy mineral content in the desert dunes. The Golfo de Santa Clara and Puerto Peñasco coastal dune sands are influenced by long shore drift, tidal and aeolian processes. Coarse grains are found on the flanks whereas fine grains are on the crest of the dunes. High tidal regimens, long shore drift and supply from Colorado Delta River sediments produce quartz‐rich sands on the beach that are subsequently transported into the coastal dunes. Outcrops of Quaternary sedimentary rocks and granitic sources increase the sedimentary and plutonic lithic content of the coastal dune sands. The chemical index of alteration (CIA) values for the desert and coastal dune sands indicate that both dune types are chemically homogeneous. The trace element values for the coastal dune sands are similar to those found for the desert dune sands. However, an increase in Sr content in the coastal dune sands may be due to more CaCO₃ of biogenic origin as compared to the desert dune sands. Correlations between the studied parameters show that the dune sands are controlled by sedimentary sources (e.g. Colorado River Delta sediments), since heavy minerals are present in low percentages in the dune sands, probably due to little heavy mineral content from the source sediment; grain sizes in the dune sands are coarser than those in which heavy minerals are found and/or the wind speed might not exert a potential entrainment effect on the heavy mineral fractions to be transported into the dune. A cluster analysis shows that the El Pinacate group is significantly different from the rest of the dune sands in terms of the grain‐size parameters due to longer transport of the sands and the long distance from the source sediment, whereas the Puerto Peñasco coastal dune sands are different from the rest of the groups in terms of their geochemistry, probably caused by their high CaCO₃ content and slight decrease in the CIA value. Copyright © 2006 John Wiley & Sons, Ltd.     

Can sand dunes be used to study historic storm events?

Knowing the long‐term frequency of high magnitude storm events that cause coastal inundation is critical for present coastal management, especially in the context of rising sea levels and potentially increasing frequency and severity of storm events. Coastal sand dunes may provide a sedimentary archive of past storm events from which long‐term frequencies of large storms can be reconstructed. This study uses novel portable optically stimulated luminescence (POSL) profiles from coastal dunes to reconstruct the sedimentary archive of storm and surge activity for Norfolk, UK. Application of POSL profiling with supporting luminescence ages and particle size analysis to coastal dunes provides not only information of dunefield evolution but also on past coastal storms. In this study, seven storm events, two major, were identified from the dune archive spanning the last 140 years. These appear to correspond to historical reports of major storm surges. Dunes appear to be only recording (at least at the sampling resolution used here) the highest storm levels that were associated with significant flooding. As such the approach seems to hold promise to obtain a better understanding of the frequency of large storms by extending the dune archive records further back to times when documentation of storm surges was sparse. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.     

Formation mechanism and development pattern of aeolian sand landform in Yarlung Zangbo River valley

Aeolian sand landforms in the Yarlung Zangbo River valley can be divided into 4 classes and 21 types. The river valley has favourable environment conditions for the development of aeolian sand landforms. Simulation of MM4 mid-scale climate model showed that the near-surface flow field and wind vector field during the winter half year in the river valley are generally favourable for the aeolian sand deposition and as a whole they also affect the distribution mneu and sites of aeolian sand landforms. Sand dunes and sand dune grouup in the river valley developed mainly in three ways, namely windward retarding deposition, leeward back flow deposition and bend circumfluence deposition. Through alternating positive-reverse processes of sand dune formation under wind actions and sand dune vanishing under water actions, sand dunes developed fmm primary zone thmugh main-body zone then to vanishing zone where climbing dunes and falling dunes are declining and are even disappearing. Project supported by the National Natural Science Foundation of China (Grant No. 49471009) and Xi’an State Key Laboratory of Loess and Quaternary Geology (Grant No. 9401)     

Morphology of the Itapeva to Tramandai transgressive dunefield barrier system and mid‐ to late Holocene sea level change

The surface morphology of the transgressive dunefield barrier extending from Itapeva to Tramandai along the northern littoral of Rio Grande do Sul, is examined and an attempt is presented to link morphological changes across the barrier to Holocene sea level changes. The 4·5 km wide Holocene barrier displays two typical morphologies: an inner part dominated by large‐scale, continuous alongshore, overlapping dunefield phases comprising sand sheets, dunes, deflation plains and precipitation ridges; and an outer part dominated by discontinuous, medium‐ to small‐scale, triangular to lobate transgressive dunefield phases, cut by both active and relict (palaeo‐) creeks or washouts. Holocene sea level in the region rose to c. +1 to +3 m above present reaching a maximum around 5100 years bp and then progressively fell to the present level. We argue that the effect on barrier development was to suppress the development of a drainage system during the rising and maximum stages, and encourage the development of an organized drainage system in the form of regularly spaced washouts during the falling period, and that this change in sea level from rising to falling therefore produced the large‐scale differences in barrier morphology. Copyright © 2006 John Wiley & Sons, Ltd.     

Dune phases in the Otaki‐Te Horo area (New Zealand): a geomorphic history

Optical dating, sedimentological analysis and soil profile development have been used to develop a chronology for, and an understanding of, the geomorphic evolution of the Holocene coastal plain between Otaki and Te Horo, North Island of New Zealand. The coastal plain has prograded 0·48 m a^?1 since sea‐levels reached their post‐glacial maximum 6500 years ago. Dune development on the plain, which is dependent on the supply of sediment suitable for dune building, has been episodic. Three periods of dune activity have been identified – the Foxton, Motuiti and Waitarere phases – the last two of which are believed to have resulted from anthropogenic activities. The dunes north of the Otaki River and south of Mangaone Stream are typical of a coastal dune system that extends from Patea in the north to Paekakariki in the south. However, this system is disrupted by the Otaki River and the gravels it delivers to the coastal zone. Immediately south of the river mouth the dunes are significantly smaller, coarser, and contain significantly more magnetic material. The character of the landforms is the result of: the reworking of the last glacial deposits; ongoing coastal progradation; variation in the input of sediment suitable for dune formation; the change in beach character because of gravel input; and the position of the Otaki River mouth. Copyright © 2006 John Wiley & Sons, Ltd.     

Dune formation on the humid tropical sector of the North Queensland Coast,Australia

Several previous attempts have been made to explain the apparent poor development of coastal dunes in the humid tropics in terms of lack of wind energy, failure of sand supply to the shoreline, excessive climatic wetness, salt crust formation on beaches, and the character of tropical back-beach vegetation. However, recent published reports indicate that coastal dune occurrences are more common in the humid tropics than was formerly thought, throwing suspicion on the idea that environmental conditions militate against dune formation in these areas as a whole. Evidence from the humid tropical sector of the North Queensland coast suggests that the poor development of dunes in this area primarily reflects poor sediment sorting in the beach and nearshore zone and low wind energy at the shoreline due to the nature of the coastal orientation and physiography in relation to the prevailing southeasterly winds. These limiting factors are not unique to humid tropical climates.     

Aeolian landform processes since the last deglaciation revealed by OSL chronology and stratigraphy in the Hulunbuir dune field in NE China

The Hulunbuir dune field (HLB) is situated near the northern limit of the East Asian summer monsoon (EASM), and vulnerable to climate change. The aeolian sand–paleosol sequences of this region are crucial for understanding the past landform processes in response to climate change, but not yet understood well due to chronological controversies. Here, we presented 20 optically stimulated luminescence (OSL) ages from five aeolian sand–paleosol profiles in the HLB, and reconstructed the aeolian landform processes since 18 ka. The findings of this study suggested that the HLB was probably dominated by mobile dunes before 18 ka, as 10 out of 11 aeolian samples were dated to 18–12 ka. Two strong sandy paleosol layers were found and dated to ∼9 ka and 5–0.5 ka, indicating that strong in situ pedogenic process on the accumulative sand could occur during the Holocene. The OSL ages of samples near the top of three profiles were >9.5 ka, indicating two possible surface processes. First, the land surface was stable since 9.5 ka after stabilization, with no accumulation or erosion. Alternatively, the surface could have been erosive with the eroded sediments feeding downwind active dunes. The latter explanation is consistent with the current local landforms, which has widespread blowout pits, indicators of strong wind erosion. We emphasized that the OSL age of a sand layer sample in fossil dunes implied the onset of mobile dune stabilization, not the age of dune activity, as previously stated.     

Sand dunes as potential sources of dust in northern China

While saltation bombardment of sand grains on a fine substrate can produce considerable dust, the well-sorted nature of sand dunes tends to preclude them from consideration as major dust sources. Recent research, however, has revealed that sand dunes can, in some cases, be large sources of dust. We used the PI-SWERL(Portable In-Situ Wind Erosion Laboratory) to measure in the field the potential of sand dunes and other desert landforms to emit particulate matter 10 μm(PM-10) dust in the Tengger, Ulan Buh, and Mu Us deserts of northern China. Combined with high resolution particle size measurements of the dune sand, an assessment of sand dunes as a dust source can be made. Large active transverse dunes tend to contain little to no stored PM-10, yet they produce a low dust flux. Coppice dunes stabilized by vegetation contain appreciable PM-10 and have very high dust emission potential. There is a positive correlation between the amount of PM-10 stored in a dune and its potential dust flux. Saltation liberates loose fines stored in dunes, making them very efficient dust emitters compared to landforms such as dry lake beds and washes where dust particles are unavailable for aeolian transport due to protective crusts or sediment cohesion. In cases where large dunes do not store PM-10 yet emit dust when active, two hypotheses can be considered:(1) iron-oxide grain coatings are removed during saltation, creating dust, and(2) sand grains collide during saltation, abrading grains to create dust. Observations reveal that iron oxide coatings are present on some dune sands. PI-SWERL data suggests that low dust fluxes from dunes containing no stored dust may represent an estimate for the amount of PM-10 dust produced by removal of iron oxide coatings. These results are similar to results from dunes in the United States. In addition, PI-SWERL results suggest that dust-bearing coppice dunes, which cover vast areas of China's sandy deserts, may become major sources of dust in the future if overgrazing, depletion of groundwater, or drought destabilizes the vegetation that now partially covers these dunes.     

Decadal and seasonal development of embryo dunes on an accreting macrotidal beach: North Lincolnshire,UK

Embryo dunes are often ephemeral, but can develop to become established coastal foredunes. In 2001 a patch of embryo dunes 13.11 m² appeared on a beach in north Lincolnshire, UK and had expanded to over 3600 m² by 2011. The rate of expansion is linked to storm occurrence, where expansion is slowed during years with a higher incidence of storm surges. From July 2009–October 2010 seasonal changes in dune field topography were determined using terrestrial laser scanning (TLS) data. Vegetation is important in the development of embryo dunes, but can cause errors in TLS data. Tests evaluating the impact of vegetation on the TLS data suggest the minimum elevation value from the TLS point cloud within a 0.05 m grid cell gives a good approximation of the ground surface. Digital elevation models (DEMs) of the dunes constructed using filtered data showed the embryo dunes underwent a classic seasonal cycle of erosion during the winter and accretion during the summer. For example from October 2009 to April 2010 over 375 m³ of sediment was eroded from the dunes whereas during spring and summer 2010 the dune field gained over 600 m³ of sand. The overall magnitude of change in dune height and volume from season to season exceeded the errors associated with the construction of the DEM from the TLS data and the vegetation filtering process, which suggests TLS can be useful for documenting topographic change in vegetated dunes. After 10 years, the patch of embryo dunes is still expanding but has not yet merged with more established foredunes to landward. Aeolian process measurements indicate that, at present, the embryo dunes do not prevent sand from reaching the foredunes, however the rate of foredune progradation has slowed concurrently with the expansion of the embryo dune field. Copyright © 2013 John Wiley & Sons, Ltd.     

Volcaniclastic aeolian deposits at Sunset Crater,Arizona: terrestrial analogs for Martian dune forms

Sunset Crater in north‐central Arizona (USA) is a 900‐year‐old scoria‐cone volcano. Wind action has redistributed its widespread tephra deposit into a variety of aeolian dune forms that serve as a terrestrial analog for similar landforms and aeolian processes on Mars. Fieldwork was conducted to collect essential geomorphological and sedimentological data, and to establish a baseline for the type and morphometry of dunes, physical properties, interactions with topography, and saltation pathways. Our analyses focused primarily on coppice dunes, falling dunes, wind ripples, and sand streaks. For all collected volcaniclastic aeolian sediment samples, the sand‐size fraction dominated, ranging from almost 100% sand to 74.6% sand. No sample contained more than 1.6% silt. The composition is overwhelmingly basaltic with non‐basaltic particles composing 2 to 6% of the total. Coppice (nebkha) dunes form where clumps of vegetation trap saltating particles and create small mounds or hummocks. Mean grain size for coppice dune samples is coarse sand. Measured dune height for 15 coppice dunes ranged from 0.3 to 3.3 m with a mean of 1 m. Mean length was 6.7 m and mean width was 4.8 m. Falling dunes identified in this study are poorly developed and thin, lacking a prominent ramp‐like structure. Mean wavelength for three sets of measured ripples ranged from 22 to 36 cm. Sand streaks extend downwind for more than a kilometer and are up to 200 m in width. They commonly occur on the lee side of mesas and similar landforms and are typically the downwind continuation of falling dunes. Falling dunes, wind ripples, and sand streaks have been identified on Mars, while coppice dunes are similar to Martian shadow or lee dunes in which sand accumulates in the lee of obstacles. Copyright © 2012 John Wiley & Sons, Ltd.