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.     

Investigation of the age and migration of reversing dunes in Antarctica using GPR and OSL,with implications for GPR on Mars

GPR provides high resolution images of aeolian strata in frozen sand in the McMurdo Dry Valleys of Antarctica. The results have positive implications for potential GPR surveys of aeolian strata on Mars. Within the Lower Victoria Valley, seasonal changes in climate and a topographically-constrained wind regime result in significant wind reversals. As a consequence, dunes show reversing crest-lines and flattened dune crests. Ground-penetrating radar (GPR) surveys of the dunes reveal sets of cross-strata and low-angle bounding surfaces produced by reversing winds. Summer sand transport appears to be dominant and this is attributed to the seasonal increase in solar radiation. Solar radiation which heats the valley floor melts ice cements making sand available for transport. At the same time, solar heating of the valley floor generates easterly winds that transport the sand, contributing to the resultant westward dune migration. The location of the dune field along the northern edge of the Lower Victoria Valley provides some shelter from the powerful föehn and katabatic winds that sweep down the valley. Topographic steering of the winds along the valley and drag against the valley wall has probably aided the formation, migration and preservation of the dune field. Optically-stimulated luminescence (OSL) ages from dune deposits range from 0 to 1.3 kyr showing that the dune field has been present for at least 1000 yr. The OSL ages are used to calculate end-point migration rates of 0.05 to 1.3 m/yr, which are lower than migration rates reported from recent surveys of the Packard dunes and lower than similar-sized dunes in low-latitude deserts. The relatively low rates of migration are attributed to a combination of dune crest reversal under a bimodal wind regime and ice cement that reduces dune deflation and restricts sand entrainment.     

Holocene Dune Activity in the Thar Desert,India

The Thar Desert dune system in north-west India and eastern Pakistan provides a rich archive of past environmental, geomorphological and climatic change. Much of the knowledge about the timing of dune accumulation in the Thar stems from scattered and sporadic records, based on older luminescence dating protocols. If the Thar dune record is to be incorporated within a growing multiproxy framework of past climate and environmental dynamics, it is necessary to generate a systematic record of the timing of dunefield accumulation. From this, relationships to climate and other drivers of dune activity may then be better established. To this end, an intensive programme of field sampling and optically stimulated luminescence (OSL) dating was carried out from a dunefield in the east-central Thar Desert. This study presents the first detailed Holocene dune accumulation history from the region, and sheds light on the development of the multi-generational parabolic dune systems. In contrast to previously published work, we identify the importance of the Holocene and the last millennium as periods with a number of preserved accumulation phases. OSL ages suggest that accumulation was persistent during the early and mid-Holocene (within 11.7-5.5 ka), late Holocene (2-1 ka), as well as two major phases in the last millennium (600 – 200 a and within the last 70 a). Potential drivers of dune mobility in the last century include a strong anthropogenic dimension. Rapid net accumulation is recorded in the last 70 years, with rates varying between 2 and 5 m/year, in an environment where agricultural pressures have increased dramatically with the advent of irrigation schemes expanding into dunefield areas. © 2019 John Wiley & Sons, Ltd.     

The relative role of constructive and destructive processes in dune evolution on Cape Lookout National Seashore,North Carolina,USA

Coastal dunes are dynamic features that are continuously evolving due to constructive (e.g., wind- and wave-driven sediment transport) and destructive (e.g., elevated total water levels during storm events) processes. However, the relative importance of these processes in determining dune evolution is often poorly understood. In this study, ten lidar datasets from 1997 to 2016 are used to determine the relative role of erosion and accretion processes driving foredune change on the coast of Cape Lookout National Seashore, North Carolina, USA. Beach and dune morphometrics reveal that dune toe locations have generally retreated since 1997, while dune crest heights accreted by 0.01–0.02 m/year. We develop three univariate metrics that represent (1) the potential for erosion, i.e., total water level impact hours per year, (2) accretion, i.e., dune building hours per year, and (3) the relative net effect of foredune accretion and erosion processes, i.e., constructive–destructive dune forcing (CDDF) ratio, and test the correlative power of these metrics in explaining changes in foredune morphology. The total water level impact hours per year metric explained as much as 66% and 67% of the variance in dune crest and toe elevations, respectively, across the nearly two decades of dune evolution. The greatest number of dune building hours per year and largest dunes within the study site co-occurred at locations exposed to the dominant cross-shore wind direction as a result of varying shoreline orientation. The CDDF ratio was positively correlated to changes in the dune toe elevation in approximately 70% of dunes within the study site, outperforming the impact and dune building hours per year metrics. Our results show that these three metrics can provide first-order estimates of dune morphometric change across multiple spatial and temporal scales, which may be particularly useful at sites where lidar acquisition is intermittent.     

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.     

Developing a 150 ka luminescence chronology for the barrier dunes of the southern Cape, South Africa

The southern coastline of South Africa exhibits extensive aeolianites and spectacular barrier dunes; the ages and palaeoenvironmental significance of which have, until recently, received limited attention. We present preliminary results for samples taken from a single 6.5 m section in the seaward barrier dune near to the town of Wilderness. The reliability of these ages has been investigated using a series of dose recovery and pulsed irradiation experiments, as well as through analyses of single aliquot and single grain D_e distributions. Results indicate that the SAR protocol is providing accurate and precise D_e determinations and that adequate sensitivity correction is being made. The application of pulsed irradiations does not result in significant changes in the dose–response curves or averaged D_e's. Single grain D_e distributions are broad, with significant levels of overdispersion, which may reflect beta dose heterogeneity effects.

Peaks in aeolian activity date to 157–154 and 140–125 ka, with a slower rate of deposition recorded from 115 to 85 ka. Brief phases at 40 and 21 ka are also identified. Some ages, notably those from OIS 3 and OIS 2, are surprising from a geomorphological perspective, given the likely distance of the dune from the shoreline at such times, although they are not inconsistent with evidence from the east coast of South Africa. The results reveal multiple phases of Late Quaternary barrier dune accumulation, and suggest that aeolian activity in this region has been episodic and potentially more complex than previously thought.     

Growth by extension,and reworking,of a south‐western Kalahari linear dune

Optically stimulated luminescence (OSL) dating studies of linear (longitudinal) dunes have been used extensively to elucidate late Quaternary environments and climates in arid or formerly arid regions, yet understanding of the development of such dunes is incomplete. In particular, conflicting opinions have been presented regarding the propensity of linear dunes to migrate laterally, the degree to which they rework their own sediment during accumulation and whether they form primarily by extension, as opposed to lateral sand movement from adjacent interdunes. This study focuses on this last point, although the importance of the other controversies is discussed in context. A simple linear dune in the south‐western Kalahari, which has a prominent termination on a pan (playa) surface, provides an opportunity to directly test hypotheses of dune extension. Chronostratigraphy along a ~600 m transect along the crest of the dune, constrained by 42 OSL ages, reveals that the dune grew by extension on occasions in the late Pleistocene and early Holocene, but has also been subject to reworking along its length, which has continued until recent times. Dune development by extensional growth is suggested to operate under environmental conditions more conducive to net accumulation, whereas reworking is largely independent of conditions throughout the last ~18 ka, and may represent seasonal fluctuations in the position of the dune crest. The relative significance of these two modes of development is suggested to be a key control on the efficacy of linear dunes as archives of environmental and climatic change. Copyright © 2011 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.     

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.     

Migration of barchan dunes in the western Quruq Desert,northwestern China

Barchan dunes are common on Earth, Mars and Titan. Previous studies have shown that their formation, migration and evolution are influenced by the wind regime and other factors, but details vary among regions. Understanding barchan morphology and migration will both improve our understanding of dune geomorphology and provide a basis for describing the environmental conditions that affect the formation and development of these dunes on Earth and other planets. Here, we provide detailed measurements of barchan dune migration in China's Quruq Desert, in the lower reaches of the Tarim River. We monitored their migration direction and rate, and their morphological changes during migration, by comparing Google Earth images acquired in 2003 and 2014. The dunes migrated west-southwest, close to the local resultant drift direction. The migration rate averaged 8.9 to 32.1 m year⁻¹, with obvious spatial variation. In addition to the wind regime, the migration rate depended on dune morphology, density and vegetation cover; the rate was negatively related to dune height, density and vegetation cover, but positively linearly related to the length/width ratio (L_U/W) and to the decrease in this ratio from 2003 to 2014. We found correlations among the dune morphometric parameters, but the relationships were weaker than in previous research. Due to the complexity of the factors that affect the processes that underlie sand dune development and migration, the morphological changes during dune migration were also complex. Our measurements suggest that the aeolian environment played a dominant role in dune migration and its spatial variation in the Quruq Desert. These results will support efforts to control dune migration in the western Quruq Desert and improve our understanding of dune morphodynamics. © 2019 John Wiley & Sons, Ltd.     

Morphodynamic interactions of continental shelf,beach and dunes: the Cabopino dune system in southern Mediterranean Spain

The complex morphodynamic interactions between nearshore, shoreface and dune systems are usually simplified by studying these zones and their associated processes in isolation. However, the established relationships between each of them suggests that an integrated approach is required to examine the genesis, evolution and adaptation of the entire morphodynamic system. The Cabopino dune system in the southern Spanish Mediterranean Sea provides a clear example of a linked morphodynamic system in which a relatively large dune system has been generated and grown through the supply of sediments from an adjacent littoral supply environment. Here, we present a conceptual model of how the nearshore has provided suitable conditions for beach and dune development. We purport that synchronization of sediment activation in the marine and aeolian sections of the system have played a major role in this microtidal setting in which temporal aspects are not only tied to storm action, but to large sedimentary features moving alongshore. © 2019 John Wiley & Sons, Ltd.     

Embryo dune development drivers: beach morphology,growing season precipitation,and storms

For development of embryo dunes on the highly dynamic land–sea boundary, summer growth and the absence of winter erosion are essential. Other than that, however, we know little about the specific conditions that favour embryo dune development. This study explores the boundary conditions for early dune development to enable better predictions of natural dune expansion. Using a 30 year time series of aerial photographs of 33 sites along the Dutch coast, we assessed the influence of beach morphology (beach width and tidal range), meteorological conditions (storm characteristics, wind speed, growing season precipitation, and temperature), and sand nourishment on early dune development. We examined the presence and area of embryo dunes in relation to beach width and tidal range, and compared changes in embryo dune area to meteorological conditions and whether sand nourishment had been applied. We found that the presence and area of embryo dunes increased with increasing beach width. Over time, embryo dune area was negatively correlated with storm intensity and frequency. Embryo dune area was positively correlated with precipitation in the growing season and sand nourishment. Embryo dune area increased in periods of low storm frequency and in wet summers, and decreased in periods of high storm frequency or intensity. We conclude that beach morphology is highly influential in determining the potential for new dune development, and wide beaches enable development of larger embryo dune fields. Sand nourishment stimulates dune development by increasing beach width. Finally, weather conditions and non‐interrupted sequences of years without high‐intensity storms determine whether progressive dune development will take place. Copyright © 2017 John Wiley & Sons, Ltd.     

Dune deformation in a multi‐directional wind regime: White Sands Dune Field,New Mexico

As with most dune fields, the White Sands Dune Field in New Mexico forms in a wind regime that is not unimodal. In this study, crescentic dune shape change (deformation) with migration at White Sands was explored in a time series of five LiDAR‐derived digital elevation models (DEMs) and compared to a record of wind direction and speed during the same period. For the study period of June 2007 to June 2010, 244 sand‐transporting wind events occurred and define a dominant wind mode from the SW and lesser modes from the NNW and SSE. Based upon difference maps and tracing of dune brinklines, overall dune behavior consists of crest‐normal migration to the NE, but also along‐crest migration of dune sinuosity and stoss superimposed dunes to the SE. The SW winds are transverse to dune orientations and cause most forward migration. The NNW winds cause along‐crest migration of dune sinuosity and stoss bedforms, as well as SE migration of NE‐trending dune terminations. The SSE winds cause ephemeral dune deformation, especially crestal slipface reversals. The dunes deform with migration because of differences in dune‐segment size, and differences in the lee‐face deposition rate as a function of the incidence angle between the wind direction and the local brinkline orientation. Each wind event deforms dune shape, this new shape then serves as the boundary condition for the next wind event. Shared incidence‐angle control on dune deformation and lee‐face stratification types allows for an idealized model for White Sands dunes. Copyright © 2015 John Wiley & Sons, Ltd.     

Early Holocene dune field development in Dalarna,central Sweden: A geomorphological and geophysical case study

Bonäsheden, Sweden's largest continuous dune field, situated in the county of Dalarna, central Sweden, has been investigated using LiDAR (light detection and ranging) remote sensing, ground penetrating radar as well as by field observations and luminescence dating. The use of LiDAR in conjunction with geographic information system (GIS) software proved to be efficient in mapping the inactive dune field and classifying the dune morphology, especially when slope raster images were used. The dunes have formed mostly by winds from the northwest (NW) and are of a transverse type. Still other dune types, such as parabolic dunes, and transverse dunes with a deviating orientation are present. Also, there seems to be different generations of dunes, suggesting a complex palaeowind environment with a change from predominantly north‐westerly winds to more westerly winds. Luminescence dating finally allows us to have an absolute chronology of the development of the Bonäsheden dune field, revealing formation of the dune field closely following the de‐glaciation of this part of Sweden (c. 10.5 ka). The well preserved transverse shape of the majority of the dunes suggests rapid stabilization by vegetation, although sand drift still seems to have been active on a noticeable scale for at least 1500 years and also, occasionally and patchy, as coversand deposition during the Late Holocene. A simple model is proposed for the dune field development of Bonäsheden based on our findings. This model is a useful addition since the majority of present day dune field models focus on the formation of parabolic dunes or large unvegetated dune fields. Our results suggest that most models cannot adequately simulate the formation of such small dune fields as that of Bonäsheden, with apparently rapidly fixated transverse dunes in a previously glaciated, now vegetated area. Copyright © 2017 John Wiley & Sons, Ltd.     

Monitoring considerations for a dynamic dune restoration project: Pacific Rim National Park Reserve,British Columbia,Canada.

Historically, management of coastal dune systems has often involved artificial stabilization of active sand surfaces in order for coastal areas to be more easily controlled and modified for human benefit. In North America, the introduction of invasive grasses, namely European and American beach (marram) grasses (Ammophila spp.) has been one of the most successful strategies used for stabilizing active coastal dune sands. Recent research has demonstrated, however, that stabilization of coastal dunes often leads to reduced landform complexity and resilience, as well as declines in species diversity. More ‘dynamic’ restoration efforts have emerged over the past 20 years that encourage dune mobility and aeolian activity in order to provide a more resilient biogeomorphic system. In North America, there is generally little research relating restoration methods and outcomes to geomorphic responses despite the fundamental importance of sedimentary processes and dune morphodynamics in broader ecosystem function. This paper aims to better situate dynamic dune restoration within current geomorphic understanding. A brief review of key terms and concepts used in the emerging field of dynamic dune restoration is provided and expanded upon with respect to geomorphologic considerations. A case study of a recent dynamic restoration effort in Pacific Rim National Park Reserve, British Columbia, Canada is provide to demonstrate how these concepts are applied. Introduction of European marram at this site, coupled with a warming climate and increased precipitation in recent decades at this site, is thought to be associated with a rapid decline in aeolian activity, system stabilization and accelerated ecological succession. Preliminary results on the response of the dune system to mechanical removal of Ammophila are presented to provide the foundation for a research framework to guide the broader restoration project. Recommendations for improving treatment methodologies and monitoring protocols are provided to aid future restoration projects of this nature. Copyright © 2013 John Wiley & Sons, Ltd.     

Optical dating of Holocene sand dune activities in the Horqin sand-fields in inner Mongolia, China, using the SAR protocol

The Horqin sand-field in northeastern Inner Mongolia, China, had been the fertile grassland in North China, but desertification and sand-dust storm have increasingly occurred in the past decades [Zhu and Wang, 1992. Theory and practice of sandy desertification in China (in Chinese with English abstract). Quaternary Sciences 2, 97]. To understand the Holocene sand dune activities in this region, five sand dune profiles were investigated, and 32 coarse grain quartz samples were dated by OSL using the single-aliquot regenerative-dose (SAR) protocol [Murray and Wintle, 2000. Luminescence dating of quartz using an improved single-aliquot regenerative-dose protocol. Radiation Measurements 32, 57–73]. For cross-checking, six organic-rich samples from the paleosols and sandy peat/mud were dated by both ¹⁴C and quartz OSL. With one exception, ¹⁴C and quartz OSL dating results show good agreements. Based on the consistent results of five sand dune profiles, a chronology of Holocene dune activity in Horqin sand-field is established as follows: (i) active sand dunes built up widely before 10 ka; (ii) dunes semi-stabilized between 10 and 7.5 ka ago; (iii) the dunes solidify and chernozem soils developed between 7.5 and 2.0 ka ago; and (iv) partially re-mobilization of dunes occurred since about 2.0 ka ago.     

Terrestrial photogrammetric techniques applied to the control of a parabolic dune in the Liencres dune system,Cantabria (Spain)

The spatial–temporal variations of a dune system can be determined by using diverse ‘geomatic’ methodologies: geodesy, global positioning system (GPS) and photogrammetry. In the case of the Liencres dune system, a study will be carried out using the ‘close‐range’ photogrammetry technique and the topography technique (total station and GPS). In order to determine the dynamic of the dune system it is necessary to repeat the process of study after a specific interval of time. For this reason, three dimensional data should be available in two different time periods, between which the displacement of the object of analysis (the front portion of the dune) will be significant enough to evaluate its magnitude. This work analyses the viability of photogrammetry for the determination of the spatial–temporal changes of a coastal parabolic dune. Two factors have been analysed: first, the comparison of the photogrammetric results with the results obtained from topographic methods (total station and GPS), and second, the quantification of the displacement of the dune system. The analysis of the correspondence between the movement of different parts of the dune and the influence of the intensity and direction of the prevailing wind in the area is also desired. The dune advanced 12·15 ± 0·06 m (an average of 8·5 m/yr), and the partial implications for the dynamic of human modified processes on the natural park have been established. Copyright © 2008 John Wiley & Sons, Ltd.     

Offshore aeolian sediment transport across a human‐modified foredune

Concepts derived from previous studies of offshore winds on natural dunes are evaluated on a dune maintained for shore protection during three offshore wind events. The potential for offshore winds to form a lee‐side eddy on the backshore or transfer sediment from the dune and berm crest to the water are evaluated, as are differences in wind speed and sediment transport on the dune crest, berm crest and a pedestrian access gap. The dune is 18–20 m wide near the base and has a crest 4.5 m above backshore elevation. Two sand‐trapping fences facilitate accretion. Data were obtained from wind vanes on the crest and lee of the dune and anemometers and sand traps placed across the dune, on the beach berm crest and in the access gap. Mean wind direction above the dune crest varied from 11 to 3 deg from shore normal. No persistent recirculation eddy occurred on the 12 deg seaward slope. Wind speed on the berm crest was 85–89% of speed at the dune crest, but rates of sediment transport were 2.27 times greater during the strongest winds, indicating that a wide beach overcomes the transport limitation of a dune barrier. Limited transport on the seaward dune ramp indicates that losses to the water are mostly from the backshore, not the dune. The seaward slope gains sand from the landward slope and dune crest. Sand fences causing accretion on the dune ramp during onshore winds lower the seaward slope and reduce the likelihood of detached flows during offshore winds. Transport rates are higher in access gaps than on the dune crest despite lower wind speeds because of flatter slopes and absence of vegetation. Transport rates across dunes and through gaps can be reduced using vegetation and raised walkover structures. Copyright © 2017 John Wiley & Sons, Ltd.     

Dune field development,interactions and boundary conditions for crescentic and stellate megadunes of the Al Liwa Basin,the Empty Quarter

Within the greater Ar Rub' al Khali (Empty Quarter) sand sea lies an internal depocentre, the Al Liwa Basin, which comprises a variety of mega‐scale dune types. Crescentic dunes dominant the north of the basin while megadunes of stellate or star form are a major landform of the south‐eastern reaches. Their development into dune fields is determined by the style and rate of dune–dune interactions, the boundary conditions imposed by a multi‐modal wind regime, fluctuating groundwater levels, and sediment availability under an assortment of climatic conditions throughout the Quaternary. As a result, dune field patterns are a collective response to these perturbations in space, time and environment. The R‐statistic is a collective measure of these responses, and is a metric capable of identifying the degree of pattern maturity or self‐organization of the aeolian system, and the pathways from which patterns evolve. The spatial signature of the southerly located star dunes is characterized by two definitive patterns of organization: the first, one of complete spatial randomness, the second, a low degree of spatial uniformity. In isolation, these results appear to be unrelated to those for crescentic dunes of the region in which a significantly higher degree of pattern dispersion is the norm. However, when spatial statistical measures are integrated with the theoretical understanding of dune–dune interactions and the involvement of environmental agents, the complex morphodynamic pathways and linkages between regional dune fields is better understood. In this case, both constructive (e.g. merging, lateral linking) and regenerative activity (e.g. calving) have played important roles in the development of dune size, and associated adjustments in spacing, and dune numbers, and subsequently dune field patterns. Synergetic patterns are emblematic of this vast dunescape, whereby transitional geographic, morphologic, dimensional and environmental modifications exist between the mega‐crescentic and mega‐stellate dunes of the Empty Quarter. Copyright © 2012 John Wiley & Sons, Ltd.     

Morphologic and sedimentologic patterns of active aeolian dune‐fields on the east coast of Maranhão,northeast Brazil

The eastern portion of the Maranhão coast is characterized by large active dune‐fields located in a tropical setting. This article combines the analysis of dune‐field morphologic patterns with the classical analysis of grain size and heavy minerals to study the sedimentary dynamics of the active aeolian system in this region. Based on the heavy mineral analysis, the main sedimentary suppliers feeding the system are the westward alongshore drift bringing sand from the coast east of the Parnaíba River mouth and the river itself. The absence of well‐defined variation patterns of the two morphometric parameters studied, dune spacing and crest length, reflects complex sedimentary dynamics and transport pathways, inside the aeolian system, despite the unidirectional wind. During the wet season, the interdune plains are flooded and the system is intensely reworked by intermittent drainages. During the dry season, the deposits formed by the drainages and interdune lakes become temporary internal sedimentary sources for the system. Due to this combined aeolian‐fluvial transport, the sediment source area has a planar geometry, with contributions from the beach and interdune plains, and not linear as expected in a typical case of source only in the beach. The areal limits of the dune‐fields is the main boundary condition controlling the dune‐field patterns, as dune spacing and crest length, by changing the sedimentary inflow–outflow balance and interactions between migration dunes. Copyright © 2015 John Wiley & Sons, Ltd.