Dune Orientation Research Articles

Simulations of dune morphology under tri-directional wind regimes and application to dunes on Mars

Dune morphology was simulated using coupled models of wind flow and sand transport for 4728 tri-directional wind regimes and bed conditions. The dominant control of dune morphology is sand coverage on the bed. Dunes on a fully sand-covered bed tend to form a periodic pattern of long crests with a relatively uniform spacing. In contrast, dunes on a starved bed have greater diversity of crest orientations and shapes, including complex shapes that have not been simulated or observed in bidirectional wind regimes. These specific dune shapes resulting from the tri-directional wind regime persist regardless of whether the transport capacity of the weakest wind is comparable to or only 1/10th that of the dominant wind.On sand-covered beds, dunes generally have only a single modal orientation (approximately that with maximum gross bedform-normal transport). The exceptions are where two strong winds diverge by 90° (two dune orientations arise), where three winds have triradial symmetry (three dune orientations), or winds have modest deviations from triradial symmetry (two dune orientations).On a starved bed, increasing the divergence angle between two strong winds produces a highly generalized sequence of: barchan dunes (divergence angle ∼30° between the two dominant winds), squat barchans or domes (divergence angle of ∼60°), dunes with two or three crest orientations (divergence angles ∼90° or 120°), to slug-shaped or boomerang-shaped dunes (divergence angle 180°, i.e., reversing winds). The simulated morphologies include a wide variety of Martian dune shapes, which allows their formative wind regimes to be inferred.

An open-source GIS approach to understanding dunefield morphologic variability at Kati Thanda (Lake Eyre), Central Australia

Future climate projections indicate an expansion of the world’s drylands, and with that a commensurate increase in the mobilization of unconsolidated desert sediments such as sand and dust. It is therefore increasingly important to investigate the large-scale formation of dryland landscapes such as dunefields in order to better understand the processes responsible for their genesis, evolution, and thresholds for mobilization. Assessing dunefield morphologic variability, including analysis of the morphologic relationship between aeolian bedforms and other landforms such as fluvial channels and bedrock uplands, underpins such investigations. So far, however, meaningful investigations of erg-scale geomorphic patterns have been limited. This is in part due to the technological limitations of geographic information system (GIS) tools, particularly in the case of open-source datasets and software, which has effectively hindered investigations by colleagues in drylands of the global south where many of the world’s dunefields are located. Recent years have overseen the increasing availability of open-source remote sensing datasets, as well as the development of freely available software which can undertake geographic object-based image analysis (GEOBIA). These new tools facilitate cartography and statistical analysis of dunefields at large scales. In this study we make use of open-source GIS to characterise a morphologically diverse linear dunefield southwest of Kati Thanda (Lake Eyre) in central Australia. We focus on three parameters; dune orientation, spacing and Y-junctions using semi-automated GEOBIA, and investigate these in the context of local fluvial channels, depressions (pans) and uplands. Our results suggest a possible correlation between dune orientation, wind regime and the role of uplands as deflective barriers to longitudinal dune migration; dune spacing and sediment supply, likely relating to the location of both ephemeral and abandoned fluvial channels; and Y-junction frequency with underlying topography. Our study provides a framework for understanding process-based interactions between dunes and other landforms, as well as the first completely open-source approach which can be applied to linear dunefields worldwide.

Parabolic dune distribution, morphology and activity during the last 20 000 years: A global overview

AbstractParabolic dunes form and migrate in almost every climate and geographic zones of the world, ranging from the tropical coastlines to the arid continental deserts. Despite their extensive distribution and their importance within the aeolian sediment landscape, the understanding of their morphological development, activity and link with climates remains somewhat limited to local or regional scales. A good understanding of the present climate conditions under which parabolic dunes are formed and/or reactivated would be significantly helpful to constrain past climate models. Similarly, an improved knowledge of parabolic dunes behaviour during past climatic episodes would provide some valuable long‐term data to better predict their future activity. This review first aims at providing a non‐exhaustive global database on parabolic dune morphology and the present wind regimes with which they are associated. To do so, the morphology of 750 dunes distributed worldwide was first analysed using a high‐resolution global digital elevation model, suggesting an intrinsic relationship between the different measured morphoparameters. The analysis of the associated local wind regimes shows that parabolic dunes develop under strong unidirectional winds, which are more conspicuous in coastal than continental environments. Dunes of different ages are globally aligned with present prevailing winds, which suggests a prevalent control of long‐term global atmospheric circulation on dune orientations. Finally, this study explores the link between parabolic dune activity and climates over the past 20 000 years by reviewing ages from the literature and combining them with the ones compiled in the INQUA Dunes Atlas Chronologic Database. Overall, it appears that changes towards drier conditions have triggered dunes migration during both warm and cold periods of the Last Glacial Maximum, Holocene Climate Optimum, Roman Climate Optimum, Medieval Climate Optimum and Little Ice Age. The present day aeolian activity is predominantly linked with deteriorating environmental conditions caused by human disturbances.

Geographic object‐based image analysis (GEOBIA) of the distribution and characteristics of aeolian sand dunes in Arctic Sweden

AbstractCurrent climate change in the Arctic is unprecedented in the instrumental record, with profound consequences for the environment and landscape. In Arctic Sweden, aeolian sand dunes have been impacted by climatic changes since their initial formation after the retreat of the last glacial ice sheet. Dune type, location and orientation can therefore be used to explore past wind patterns and landscape destabilisation in this sensitive area. However, knowledge of the full spatial extent and characteristics of these dunes is limited by their inaccessibility and dense vegetation cover. Geographic object‐based image analysis (GEOBIA) permits the semi‐automatic creation of reproducible parameter‐based objects and can be an appropriate means to systematically and spatially map these dunes remotely. Here, a digital elevation model (DEM) and its derivatives, such as slope and curvature, were segmented in a GEOBIA context, enabling the identification and mapping of aeolian sand dunes in Arctic Sweden. Analysis of the GEOBIA‐derived and expert‐accepted polygons affirms the prevalence of parabolic dune type and reveals the coexistence of simple dunes with large coalesced systems. Furthermore, mapped dune orientations and relationships to other geomorphological features were used to explore past wind directions and to identify sediment sources as well as the reasons for sand availability. The results indicate that most dune systems in Arctic Sweden were initially supplied by glaciofluvial and fluvial disturbances of sandy esker systems. Topographic control of wind direction is the dominant influence on dune orientation. Further, our approach shows that analysing the GEOBIA‐derived dune objects in their geomorphological context paves the way for successfully investigating aeolian sand dune location, type and orientation in Arctic Sweden, thereby facilitating the understanding of post‐glacial landscape (in)stability and evolution in the area.

Effect of the Mazar Tagh Mountains on Aeolian landforms in the western Taklamakan Desert, northwestern China

Topographic obstacles across the Earth and other planetary bodies are important factors affecting the morphology of aeolian landforms. Compared to general-scale obstacles (e.g., shrubs, nebkhas, residual hills), the effects of mountain-scale obstacles on aeolian landforms have rarely been investigated. Here, we studied the effect of the Mazar Tagh Mountains as a natural mountain-scale obstacle on aeolian landforms in the western Taklamakan Desert, northwestern China. A comprehensive analysis was conducted on the characteristics of aeolian landforms under the effect of the Mazar Tagh Mountains through aeolian sand parameters derived from the wind regime, grain-size parameter and the recent dune growth mechanisms. Our findings show that the Mazar Tagh Mountains change the spatial distribution of drift potential and grain-size parameters from the north to the south, resulting in distinctly different dune orientations and dune morphologies on both sides of the mountains. It was also found that mountains could block most sand transported from the north to the south. The Mazar Tagh Mountains block approximately 13.4 % of the resultant drift potential and 62.1 % of sand supply from north to south, respectively. However, aeolian landforms inside the sand transport channel or notch show high consistency since there is no obstacle. Combined with wind flow simulations, we suggest that mountains affect aeolian landforms by disrupting the wind flow and sand supply. Finally, we infer that the effects of mountain-scale obstacles on the dynamics of aeolian landform may be used to bring new constraints on the spatial distribution patterns of dune fields.

Local Wind Regime Induced by Giant Linear Dunes: Comparison of ERA5-Land Reanalysis with Surface Measurements

Emergence and growth of sand dunes results from the dynamic interaction between topography, wind flow and sediment transport. While feedbacks between these variables are well studied at the scale of a single and relatively small dune, the average effect of a periodic large-scale dune pattern on atmospheric flows remains poorly constrained, due to a pressing lack of data in major sand seas. Here, we compare local measurements of surface winds to the predictions of the ERA5-Land climate reanalysis at four locations in Namibia, both within and outside the giant linear dune field of the Namib Sand Sea. In the desert plains to the north of the sand sea, observations and predictions agree well. This is also the case in the interdune areas of the sand sea during the day. During the night, however, an additional wind component aligned with the giant dune orientation is measured, in contrast to the easterly wind predicted by the ERA5-Land reanalysis. For the given dune orientation and measured wind regime, we link the observed wind deviation (over $$50^{\circ }$$ ) to the daily cycle of the turbulent atmospheric boundary layer. During the night, a shallow boundary layer induces a flow confinement above the giant dunes, resulting in large flow deviations, especially for the slower easterly winds. During the day, the feedback of the giant dunes on the atmospheric flow is much weaker due to the thicker boundary layer and higher wind speeds. Finally, we propose that the confinement mechanism and the associated wind deflections induced by giant dunes could explain the development of smaller-scale secondary dunes, which elongate obliquely in the interdune areas of the primary dune pattern.

Coexistence of Two Dune Growth Mechanisms in a Landscape‐Scale Experiment

AbstractIn landscape‐scale experiments at the edge of the Gobi desert, we show that various dune types develop simultaneously under natural wind conditions. Using 4 years of high‐resolution topographic data, we demonstrate that, depending on sand availability, the same wind regime can lead to two different dune orientations, which reflect two independent dune growth mechanisms. As periodic oblique dunes emerge from a sand bed and develop to 2 m in height, we analyze defect dynamics that drive the non‐linear phase of pattern coarsening. Starting from conical sand heaps deposited on gravels, we observe the transition from dome to barchan and asymmetric barchan shapes. We identify a minimum size for arm elongation and evaluate the contribution of wind reversals to its longitudinal alignment. These experimental field observations support existing theoretical models of dune dynamics boosting confidence in their applicability for quantitative predictions of dune evolution under various wind regimes and bed conditions.

Demarcation of Coastal Dune Morphology and Dune Patterns using Geospatial Models: A Case Study from Manapad Coastal Stretch, Tamil Nadu, South India

Abstract Coastal sand dunes are the dynamic depositional landform which are formed near the coast, due to aeolian activity. Coastal sand dune acts as a natural barrier against coastal erosion hence its periodic monitoring is essential. Modification of this landform due to anthropogenic activities was observed in the coastal stretch towards the south of Tamil Nadu, even though these dune ecosystems were meant to be conserved and preserved in the coastal zone regulation notification. Present study, focuses on finding key techniques required to demarcate the morphological parameters such as ridge line, crest portion, toe, dune orientation, and its pattern from the open source ALOS World 3D (AW3D) DEM for periodic monitoring of coastal dune ecosystems. Topographic models such as hill shading, slope, aspect, curvature, and hybrid techniques were performed. The outcomes obtained from topographic models were validated with spatial profiling and field survey. From the study it is observed that, hybrid techniques particularly slope overlaid on hill shade, slope, gaussian stretched hill shade image, and spatial profiling are the key techniques to bring out the morphological variations and the parabolic dune patterns, which were hitherto not reported in earlier literatures. Gaussian stretched hill shade image enhanced the micro landforms which were difficult to extract from normal hill shading. The slope image of the study is found to be more informative in exhibiting the minor dunes and patterns compared to shaded relief image. The ridge portion of the dune, crest and limb portion and coastal plains were differentiated based on the slope value ranges. The hybrid image formed by overlaying slope and curvature data on hill-shaded image is more informative in providing most of the parabolic patterns formed by micro and meso scale dunes. Spatial profile could be used as an additional tool to validate the information’s derived from topographic models, wherever there is limitation in field accessibility.

Geomorphology of aeolian dunes in the western Sahara Desert

Although the western Sahara Desert is dominated by aeolian dunes, research on the geomorphology of aeolian dunes throughout this region as a unit has been limited compared with other large deserts. In the present study, comprehensive research is conducted on the geomorphology of dunes in the western Sahara Desert based on aeolian parameters and dune growth mechanisms. We identified the controlling factors on the dune morphology and formation of superimposed dunes. The results suggest that the spatial patterns of dune morphology are closely correlated with the drift potential (DP) and dune orientation. A well-defined gradient of declining DP, resultant drift potential (RDP), and directional wind variability from north to south was found, indicating a gradual weakening trend in the DP along the sand flow path. Linear and transverse dunes are distributed on the central part of the desert. Barchan dunes are distributed on the western coastal areas and desert margins. Lastly, star dunes are distributed in the upwind on the northern desert margin. We concluded that the wind regime and sand availability directly impact the dune morphology in the western Sahara Desert by considering the growth mechanisms. This study provides valuable reference data on regional aeolian activities that can be used by subsequent regional aeolian geomorphology studies in the western Sahara Desert.

Aeolian sand dunes alongside the Yarlung River in southern Tibet: A provenance perspective

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

Morphological characteristics and dynamic changes of seif dunes in the eastern margin of the Kumtagh Desert, China

The seif dune field over the gravel desert surface in the eastern margin of the Kumtagh Desert is a valuable experimental site for the observation of dune formation and dynamics. We used high-resolution remote sensing and station observation approaches, combined with wind and grain size data, to study the characteristics of the aeolian environment and the morphologies of and dynamic changes in seif dunes. We observed the ratio of the resultant drift potential (RDP) to the drift potential (DP), which was 0.37, associated with an obtuse bimodal wind regime. The drift potentials in the west-northwest (WNW) and east-northeast (ENE) directions were dominant, and the angle between the two primary DP directions was 135.00°. The dune orientations ranged from 168.75°–213.75°, which were parallel to the resultant drift direction (186.15°). The dune lengths ranged from 51.68 to 1932.11 m with a mean value of 344.91 m. The spacings of the dunes ranged from 32.34 to 319.77 m with a mean value of 93.39 m. The mean grain size of the sediments became finer, and the sorting became better from upwind tail to downwind tip, which indicated that the sediment of the seif dunes in the study region may be transported from northward to southward. The rate of increase in the length, the mean longitudinal migration rate of the dune tail, and the mean longitudinal extension rate of the dune tip (also called elongation rate) were 4.93, 4.63, and 9.55 m/a, respectively. The mean lateral migration vector of the seif dunes was approximately 0.11 m/a towards the west (–0.11 m/a), while the mean amplitude of lateral migration was 0.53 m/a, ignoring the direction of lateral migration. We found that the seif dune field formed first beside seasonal rivers, which can provide sediment, and then expanded downwind.

Etched linear dunefields of the Nullarbor Plain; A record of Pliocene-Pleistocene wind patterns across southern Australia

The southern Nullarbor Plain is covered by low amplitude (generally 1–2 m) parallel ridges and swales with variable wavelength (300–900 m). The ridges are stony; the swales hold thin deposits of unconsolidated silt and clay. The ridges and swales have previously been attributed to joint control but are not parallel to joints exposed in cave roofs. Instead, they have a striking similarity to central Australian linear sand dunes: they are parallel, regularly spaced and most importantly, fork in a consistent direction. They are here explained as the etched footprints of an extensive linear dune system that once covered much of the southern Nullarbor. Rainwater collecting in the troughs between the sand dunes dissolved the underlying limestone to produce the swales. Recognition of this dune field fills a major gap in the distribution of linear dunes across Australia. The dunes have subsequently disappeared, apart from red aeolian sand in dolines in the southeastern part of the Nullarbor; most of the sand was probably blown offshore.The ridge and swales are oriented predominantly north-south in the west, and northeast-southwest and northwest-southeast in the centre and east, due to deposition by northerly and westerly winds respectively, as shown by directions of forking. The north-south Nullarbor dunes are probably older and may have been deposited in the arid climate of the late Neogene; this would make them the oldest linear dunes in Australia, remnants of an early phase of linear dune activity that has been removed or overprinted elsewhere. The change in wind direction shown by the dune orientation could represent northward migration of sub-tropical high-pressure systems during the late Neogene.

Pattern analysis and dating for the Badain Jaran dune field, northwestern China

Pattern analysis can estimate dune formation ages, judge wind conditions and extract environmental signals from the common pattern parameters of active dune field. For the extraterrestrial dune fields, this method is a great advantage while many in situ and laboratory techniques cannot do anything. An ideal location for the large-scale pattern analysis test is the Badain Jaran desert where the tallest terrestrial aeolian dune occurs. In this study, the function forms of pattern parameters varying with the dune construction time are established, according to the natural evolution of a dune field in the duration of over 10 years. It is revealed that the relations between pattern parameters and time display power functions, in which the unknown coefficients in different conditions of wind regimes and sand availabilities can be estimated from the absolute ages of local deposits. From the northwest to southeast of the Badain Jaran desert, the dune crest length and mean spacing increase, and the defect density decreases. The pattern ages change correspondingly. Dune orientation mostly ranges between $$10^{\circ }$$ and $$60^{\circ }$$ . The crest length ages indicate that the modern active dune field began to form at about 20–75 ka BP, the dunes in the southeast and northeast are older than those in the western. This work confirms that pattern analysis offers an effective dating method for hard-to-reach or large-scale dune fields.

Greedy parabolics: Wind flow direction within the deflation basin of parabolic dunes is governed by deflation basin width and depth

Parabolic dunes are ‘U’ or ‘V’-shaped aeolian landforms that form on pre-existing sand deposits. Their morphology consists of an upwind deflation basin, bordered by often vegetated trailing arms and a downwind depositional lobe. The orientation of parabolic dunes is commonly attributed to the prevailing or resultant wind direction. Consequently, the orientation of parabolic dunes stabilised by vegetation growth has been used as a proxy for wind direction during past climates in several studies. However, the ability or extent of parabolic dune morphology to steer incident wind flow parallel to the orientation of the landform, and thus migrate in the direction of the current landform orientation rather than prevailing wind direction, is unknown. By numerically modelling wind flow within the deflation basin of eight parabolic dunes, we demonstrate for the first time that wind flow direction within the deflation basin of a parabolic dune is highly controlled by the depth and width of the deflation basin. The greater the depth–width ratio of the landform (i.e. the deeper and narrower the deflation basin), the greater the degree of flow steering relative to the axis orientation of the landform. These results demonstrate that future studies must exercise caution when using parabolic dune orientation as a direct proxy for prevailing wind direction, especially where parabolic dunes have a relatively high deflation basin depth–width ratio, as the deflation basin of these landforms may continue to migrate in an antecedent wind direction.

Intersections between wind regimes, topography and sediment supply: Perspectives from aeolian landforms in Central Asia

The widespread aeolian deposits of the Central Asian steppes and piedmonts offer potential to better understand the dynamics of the major Eurasian climate subsystems over Quaternary timescales. However, current assumptions linking climate processes with aeolian activity remain poorly substantiated and potentially problematic: emerging datasets suggest that loess accumulation rates, and the timing of peak dust flux along the Tien Shan piedmont, are highly variable between sites; the degree to which desert dunes reflect wind regimes and therefore provide records of climate circulation is unknown; and the presumed genetic link between the region's desert dunes and piedmont loess deposits remains hypothetical. These uncertainties raise questions as to the nature of the relationship between aeolian flux, deposition and (palaeo)climate.Here we investigate the relationship between wind regimes and the distribution, morphology and timing of two aeolian landform types (dunes and loess) in the topographically diverse Ili Basin of southeast Kazakhstan and western China. We establish late Holocene ages for three dune sites and one loess profile and infer that the location and morphology of these landforms can be used to test their genetic link with recent wind regimes. We assess the morphologic characteristics of linear dunes at four sites using a semi-automated mapping approach, and analyse the variability in dune orientations with respect to sand-shifting wind trajectories identified from meteorological reanalysis data for each site. The most frequently occurring wind direction appears to be responsible for sediment supply, while the strongest winds drive dune orientation. Wind orientations are strongly influenced by topography. Back trajectories incorporating dust transport and deposition potential to the loess site suggest both westerly and northerly delivery of fine-grained material to the piedmont, and that the most common source areas lie in regions where surface sediment contains a mixture of coarse and fine-grained material to facilitate entrainment into suspension.

Topographic Effects on Titan’s Dune-forming Winds

The Cassini mission made an unexpected discovery when it found evidence of linear dune fields on Titan’s surface. The orientation of the dunes and their interaction with topography allow scientists to estimate the dominant wind direction on the surface of Titan. There is some consensus in the community that the dune-forming winds must be net westerly, however, there is an active debate about the dune-forming wind regime. This debate has been guided by several studies of Earth dune fields considered analogous to the Titan dunes including those in Namibia, the Sahara, the Serengeti, and China. Complicating this active debate about the surface wind regime is the fact that global circulation models (GCMs) have historically not been able to reproduce westerly surface winds in the tropics. Here we use the Titan Community Atmosphere Model (CAM) to quantify the impact of topography and an added torque on Titan’s dune-forming winds. Dunes tend to form at higher elevations on Titan, and adding topography to the model alters the near-surface wind directions, making them more westerly and consistent with the dune orientations. The addition of topography and added torque create a wind regime that is consistent with linear dunes in areas of stabilized sediment.

Incipient bedforms in a bidirectional wind regime

Most terrestrial sand seas form at ‘horse’ latitudes, where the wind direction exhibits seasonal variation. Here, we extend the two-dimensional linear stability analysis of a flat sand bed associated with a unidirectional wind to the three-dimensional case in order to account for multidirectional wind regimes. Focusing on the simplest case of bidirectional flow regimes, we show that the transition from transverse to oblique or longitudinal patterns is controlled by the transport ratio and the divergence angle between the two flows. Our predictions agree with previous results for dune orientation, and also provide a wider range of possible alignments depending on flow strength, especially when the two winds are perpendicular, at which the transition occurs. This analysis also predicts the selected pattern wavelength, which either decreases close to the transition angle for strong winds, due to a geometric effect, or increases at low winds, when the bed slope affects the transport. This theoretical analysis is complemented by analogous subaqueous experiments, where bedforms are submitted to alternate water flows. For transverse bedforms, the experimental data validate the model at strong flows, providing evidence for the predicted geometric effect, but also for the increase of the wavelength close to the transport threshold. For longitudinal bedforms, a discrepancy is observed, which we interpret as the sign of enhanced nonlinearities induced by the development of slip faces when the flow alternately blows on both sides of the dune.

Evidence for a change in wind regime during the Last Glacial Maximum from the Sydney region

This paper presents the results of a small number of dates from sand samples collected at building sites in the eastern suburbs of Sydney. OSL ages of well-sorted, quartz sand ranges from 31 to 24 ka from within well-developed podzol soil profiles within a metre of the surface. The source of this sand appears to be from the east off the inner continental shelf during a period just prior to the maximum of the Last Glacial Maximum (LGM, Marine Isotope Stage 2) when sea-level was approximately 70–90 m below present level. During a later phase of the LGM, there is evidence from other coastal sites in central NSW and Gippsland, Victoria, that the dominant wind was from the west consistent with evidence from arid and semiarid Australia. Dune orientations in northern NSW into Queensland and other evidence at this time appear to reflect winds from the southeast quadrant. This study highlights the existence of a potential pivot area in dominant wind direction during the LGM between sustained westerly flow in southern area including Victoria and northeast Tasmania and southeast flow of northern NSW into Queensland. In central NSW, the circulation switched from easterly in Stage 2 to westerly as the glacial stage intensified in the LGM sometime after ca 25 ka.

The Combined Effect of Sediment Availability and Wind Regime on the Morphology of Aeolian Sand Dunes

AbstractAeolian‐driven sand dune orientation is a key parameter that determines dune morphology. Although wind regime, surface condition, and sediment availability all control sand dune formation, researchers have mostly put emphasis on the wind regime. However, research shows that the formation of dune orientation cannot be explained solely on wind because different dune types can form under the same wind regime in a given area. In this study, we investigate the combined effects of wind regime and sediment availability on dune orientation based on numerical simulations. The model clarifies the extent to which sediment availability influences dune orientation as well as the physical mechanisms that allow for the coexistence of different dune types. We found that linear dunes and barchans can coexist when the sediment availability is less than 10% with a localized sand source, and where barchans are situated at the extremity or on the same flank of the linear dunes. When the angle between two dominant winds is greater than 90° and sediment availability is greater than 10%, oblique dunes may occur, and they can evolve from asymmetric barchans. This study offers insight into the potential relationship between dune‐forming factors related to the wind regime and the available erodible sediment within a dune system, in view of the fact that such a model would be a valuable tool in ascertaining the causes behind complex dune formation processes.

Origin of last-glacial loess in the western Yukon-Tanana Upland, central Alaska, USA

AbstractLoess is widespread over Alaska, and its accumulation has traditionally been associated with glacial periods. Surprisingly, loess deposits securely dated to the last glacial period are rare in Alaska, and paleowind reconstructions for this time period are limited to inferences from dune orientations. We report a rare occurrence of loess deposits dating to the last glacial period, ~19 ka to ~12 ka, in the Yukon-Tanana Upland. Loess in this area is very coarse grained (abundant coarse silt), with decreases in particle size moving south of the Yukon River, implying that the drainage basin of this river was the main source. Geochemical data show, however, that the Tanana River valley to the south is also a likely distal source. The occurrence of last-glacial loess with sources to both the south and north is explained by both regional, synoptic-scale winds from the northeast and opposing katabatic winds that could have developed from expanded glaciers in both the Brooks Range to the north and the Alaska Range to the south. Based on a comparison with recent climate modeling for the last glacial period, seasonality of dust transport may also have played a role in bringing about contributions from both northern and southern sources.