Transverse Dunes Research Articles

Degradation of a transgressive coastal Dunefield by pines plantation and strategies for recuperation (Lagoa Do Peixe National Park, Southern Brazil)

The transgressive dunefield of the Lagoa de Peixe Natural Park was modified drastically due to intensive pine plantation and the leeward development of the associated degraded areas. The present study analyzed the disturbances in the dunefield of the Lagoa do Peixe Natural Park due to pine tree plantation and the impact of the subsequent deforestation program conducted in the region. Aerial photographs, satellite images, and DGPS topographic data collected over a 70-year-long period were retrieved, analyzed, and compared, which allowed the observation of the geomorphological evolution of the dunefield. In addition, a profile of GPR on the inlet (during a period with the channel closed) was analyzed. In 1948, the surface of the sand barrier was occupied by high transverse dunes and low barchan dunes. Pine tree plantation on the inner side fixed the transgressive dunes and, consequently, avoided the filling of the shallow lagoon, although degraded areas were generated on the lee side of the pines. Simultaneous pine plantations in the backshore avoided the aeolian sediment input to the dunefield, generating a large interdune area along with the development of a few parabolic dunes, which resulted in cannibalization of the transgressive dunes. In 2001, pine trees occupied 15.03% of the total area analyzed in the present study, while the degraded area accounted for 10.81% of the total area. Progressive deforestation was performed (ring bound technique for tree gradual death), maintaining three lines of pines in contact with the dunes, to promote autochthonous vegetation growth, thereby preventing the filling of the adjacent lagoon with aeolian sediments. By the year 2018, the pine tree plantation area reduced to 3.25%, the dunefield area was 79.03%, and the extension of the degraded areas had increased and reached 17.71% of the total area. The pine tree plantation and the deforestation for conservation purposes are the main factors explaining the degradation of the dunefield during the period between 1948 and 2018, while regional climatic oscillations contributed as the secondary factor. Although internationally controversial, the present case study demonstrates that the removal of this exotic vegetation, through dune vegetation recovery programs, is often unsuccessful and may generate more degraded areas. However, in the case presented here, it was essential to remove the forest to ensure the dune field geodynamics and, therefore, the base (biotope) of the natural system (maintenance of the lagoon and the dunefield).

Blowout Morphometrics and Mass Balances

The Gonghe Basin on the Qinghai-Tibet Plateau has a cold, arid climate and has suffered severe land degradation. Climate change as well as anthropogenic activities including overgrazing have resulted in widespread blowout development and the formation of some of Earth’s largest blowouts. The blowouts are part of an aeolian dominated landscape that passes from deflation zone to grass covered plain, and then through blowouts of increasing size and complexity to transverse barchanoid dunes that are migrating into the valley of the Yellow River. A combination of structure-from-motion (SfM) optical drone mapping, ground-penetrating radar (GPR) and soil pits are used to investigate blowout scour hollows and depositional lobes. Comparisons of the volumes of sediment removed from the scour hollows with the volumes of sediment deposited within adjacent lobes varies between sites. The lobe volume is invariably less than the volume of the scour hollow. This can, in part, be attributed to aeolian reworking of the lobe, distributing sand further downwind and uplifting of dust. However, much of the difference in volumes between the scour and lobe can be attributed to the measurement technique, particularly where GPR was employed to calculate lobe volumes. The wavelength of the GPR limits its ability to resolve thin layers of sand resulting in an underestimate of the deposited sand at the margins of a lobe where the sand thickness is equal to, or less than, the wavelength of the GPR. For thin sand layers, beneath the resolution of the GPR, soil pits suggest a closer match between the volume of sand eroded from the scour and the volume of the lobe, albeit with large measurement uncertainty. We put forth two hypotheses to explain the spatio-temporal evolution of the blowout dune field. The downwind increase in blowout dune size could either reflect a downwind propagation of aeolian instability; or it could result from an upwind propagation of the instability, which started at the highest points in the landscape and has subsequently migrated in a northwesterly direction, towards lower elevations. Recent optically stimulated luminescence dating appear to support the latter hypothesis.

Seasonal changes and migration of longitudinal dunes in the northeastern Rub'al Khali Desert

The morphology of small longitudinal dunes responded clearly to wind fields in the northeastern Rub'al Khali Desert. Based on local meteorological data and high-resolution satellite images, we analyzed the wind regime, drift potential (DP), dune morphology changes, and migration rates. The wind roses revealed that two opposing winds (from the northwest and southeast), both at an oblique angle to the dune crest, dominated the structure of the wind regime, with periodic changes between these directions. The annual DP from 2017 to 2019 ranged from 615 to 688 vector units (VU), and the resultant drift potential (RDP) ranged from 149 to 311 VU, indicating a high-energy wind environment. The most erosive winds (DP > 60 VU) occurred from February to June. Controlled by the wind field, dune morphology remained longitudinal from September to May. However, small longitudinal dunes (height below 5 m) transformed into transverse dunes from June to August, and formed a dune network with the large longitudinal dunes. On flat land, a series of active longitudinal dunes migrated at 7.8 to 15.1 m yr−1, with a mean of 11.9 m yr−1. Their dominant migration mode was parallel to the resultant drift direction (RDD), with the dunes extending downwind via accretion at the downwind ends. These dunes also migrate laterally by 0.4 to 0.8 m yr−1 while maintaining a symmetrical longitudinal dune morphology. These results expand our knowledge of dune migration and evolution, and provide guidance for protecting areas such as oases threatened by migrating dunes.

Field measurements of crestal flow direction and turbulent flow over two transverse dune ridges

Turbulent airflow is a fundamental factor that controls the geomorphology of sand dune systems, especially dune expansion and sandy desertification. However, little is known about turbulent air flow in dune systems, and this limits efforts to control dune expansion and sandy desertification. To obtain some of the missing information, we calculated the turbulent airflow characteristics over a transverse dune in the Tengger Desert in southeastern China. We found that the angle of airflow at the dune crest controls flow angles at middle of the dune's stoss slope and at the toe of the leeward slope, and also controls the turbulent flow characteristics over the dune surface. The critical wind direction was from the NW (wind directions ranged from 355° to 45° to the crest perpendicular line [CPL]) and SE (wind directions ranged from 65° to 165° with respect to the CPL), and about 20° and25° with respect to the CPL for northwesterly winds and southeasterly winds, respectively. The kinematic Reynolds' stress (RSk), streamwise turbulence intensity (CVu), and total kinetic energy (TKE) were largest close to the critical wind direction for winds from the NW, indicating that dune movement was mainly controlled by the critical wind direction. However, TKE, RSk, and CVu differed between southeast and northwest winds; for southeasterly winds, they were largest for wind directions from 65° to 115° with respect to the CPL and similar to the values for wind directions from 115° to 165° with respect to the CPL, indicating asymmetrical sand movement. We conclude that, for closely spaced transverse dunes, sand transport is controlled by the critical wind directions; this indicates that efficient control of sand movement and sandy desertification will benefit from sand-control structures that block airflow from the critical wind directions.

The distribution and nature of star dunes: A global analysis

Abstract Star dunes, which occur on Mars and Titan and are also evident in Earth’s stratigraphic record, are widespread in the world’s drylands and 25 provinces have been identified and characterised based on an analysis of Google Earth images. They occur in East Asia, Western Asia, the Americas, southern Africa and northern Africa. Star dunes are absent in inland Australian, Kalahari and Indian sand seas and in the southern Sahara. Morphometric measurements across a wide range of sites reveal star dune densities of 7 to 81 per 100 km2 (mean 30 per km2), dune widths of 125–3071 m (mean 895 m) and heights of 4–291 m (mean 75 m). Star dunes have a great diversity of forms, which include simple pyramids, complex features, checkerboard patterns, symmetrical and asymmetrical forms, stars along linear dunes and stars along transverse dunes. In some areas star dunes show an association with mega-domes. In this paper they are classified into 10 types, though it is clear that some dunes may show characteristics of more than one type. Star dunes develop in areas with low rainfall, variable wind directions and in association with topographic barriers. They are a more widespread and varied phenomenon than has previously been recognised.

Aeolian dunes morphodynamics and wind regime reconstruction in mid-latitudes of the Gondwana during Early Permian, Aracaré Formation, Sergipe-Alagoas Basin, Brazil

Aeolian systems play an important role in palaeoclimatic studies, allowing the inference of arid regions in different geological periods, in addition to being the best records for acquiring palaeowind data. Morphodynamic reconstruction of ancient aeolian dunes is extremely important for understanding the local wind regime. This study aims to contribute to the morphodynamic reconstruction of aeolian deposits of the Aracaré Formation in response to Gondwana's palaeoclimatic variations and to indicate the prevailing wind regime. Permian Aracaré Formation is located in the Sergipe-Alagoas Basin, northeast region of Brazil, and exhibits sand accumulation of successive sets of aeolian cross-strata which are interpreted as dry aeolian system. Aracaré Formation is characterized by smaller cross-bedded sets of barchanoid dunes at the base that are overlaid by larger cross-bedded sets of transverse dunes. Aeolian cross-strata show a repetitive alternation of grainflow and wind-ripple packages, separated by reactivation surfaces indicating seasonal variations in wind direction. Cross-strata mean vectors dip consistently towards ESE when corrected for the palaeogeographic position of the Lower Permian. Cyclic stratification pattern suggests an alternation of westerly transverse winds responsible for dune migration, followed by reverse winds that caused dune reworking and led to the formation of reactivation surfaces and the related deposition of wind-ripple. Sometimes, reverse winds are more intense (storms?), strongly eroding the lee face of the aeolian dunes, generating master reactivation surfaces. The comparison between the wind patterns of Aracaré Formation and Sakmarian climate models showed good correlation with disposition of westerlies wind regime in mid-latitude in Gondwana landmass.

Extra-large grains in Late Glacial – Early Holocene aeolian inland dune deposits of cold climate, European Sand Belt, Poland: An evidence of hurricane-speed frontal winds

Only exceptionally, grains larger than 2–3 mm are involved in dune-forming processes. In 16 Late Glacial – Early Holocene inland dunes deposits from three regions of the northern and central Poland, beds, layers, laminae and lenses containing extra-large grains, up to 20.5 × 12.5 × 11 mm in the longest axes, are present. Occurrences of the extra-large grains are documented in the dunes dispersed in three distinct physiographic regions, Biebrza Basin, Central Poland Lowlands, Przedbórz Upland and the adjacent part of the Włoszczowa - Nida Basin. An aeolian genesis of these deposits is proven by sedimentary features and morphology of the landforms, including simple forms of parabolic, longitudinal and transverse dunes, as well as large, complex depositional forms. The extra-large grains occur at various depths in the sandpit sections, ranging from a few tens of centimetres just the above contact of dune deposits with the substrate up to a few tens of centimetres below the top of the dune section. Outcrops with the extra-large grains are located on both lee and stoss sides in the dune landforms. Deposits bearing extra-large grains show nearly horizontal and low angle lamination, which is typical of the dune stoss sides, to high angle foresets, which are typical of lee sides of dune landforms. Several sites display up to seven layers of the extra-large grains, indicating a few episodes of extremely strong and relatively steady wind during the dune-forming period. Estimation of the wind velocity threshold for such coarse material shows values between 23 and 82 m s−1, for speeds 1 m above the surface. On the basis of measured dimensions and volumes of particular pebbles, a principal component analysis demonstrates that the distribution of grain dimensions is clearly heterogeneous between the three physiographic regions of northern and central Poland. The concentration of values from the Central Poland Lowlands dataset is significantly higher than in the other two regions. This might have been caused by the lack of wind-flow terrain constraints in the mostly flat area of the Central Poland Lowlands. In contrast, the relatively high variance of grain dimension parameters could have been caused by the inconstancy of wind velocity and direction over the undulated morphology of the other two regions. The prevailing western direction of transportation points to frontal, episodically hurricane-speed winds (>20–40 m s−1), which were able to transport the extra-large grains during Late Glacial – Early Holocene times, in the area south of the Scandinavian ice sheet. The relatively rare occurrence of layers and laminae with extra-large grains in the investigated dune deposits suggests specific processes leading to their formation. However, reports of 16 outcrops with the extra-large grains imply that aeolian transport of very coarse material is not only a scientific curiosity, but a process occurring on a broader scale.

Abraded pyroclastic linear paleodunes in Syria and Daedalia Plana, Mars

In the aeolian-dominated conditions of modern Mars, wind-driven sediment deposition and erosion produce a wide range of enigmatic landforms on the planet's surface. In this work, we explore a widespread occurrence of paleodunes on the surface of Mars. These fossilized bedforms preserve morphologies consistent with transverse dunes, implying an approximately unimodal ancient sediment transport azimuth and abundant sediment supply. These bedforms have previously been interpreted as layered dust deposits, and their bland, dust-like reflectance spectra are consistent with volcanic ash. Additionally, these paleodunes have undergone induration and have been significantly eroded by recent winds, causing a superimposed yardang-like erosional texture on the older dune morphology. We propose that these unusual bedforms most likely occurred as wind-mobilized pyroclastics that welded or were cemented in place after deposition and have since eroded into a field of hybrid paleobedform/proto-yardangs.

Morphological Profiles of Sand Dunes from ICESat-2 Geolocated Photons

Aeolian process leads to the transportation and accumulation of sand particles that result in sand dune landforms. The structure and shape of the sand dunes are driven by the parameters of interacting wind force and the material composition of sand within. Cross-section profiles over the sand dunes will essay the geomorphological parameters through which the steady state and rate of sand transport can be computed. National Aeronautics and Space Administration’s novel satellite namely Ice, Cloud and land Elevation Satellite-2 (ICESat-2) hosts a solo sensor namely Advanced Topographic Laser Altimeter System (ATLAS) which is a photon counting instrument that measures the round-trip time of the light pulse being emitted and reflected back from the surface determines the true height of the topographic feature on the Earth. In this article, cross-section profiles generated from the beams of ICESat-2 ground-tracks acquired over sand dunes of the Thar Desert region were analysed for detecting the geomorphological parameters. Observations from the cross-section profiles have resulted in giving unprecedented details about the shapes and morphological settings of various types of sand dunes like barchanoids, parabolic, longitudinal, and transverse dunes. Morphological parameters of sand dunes like the length of the stoss slope, crest height, slip face details, inter-arms spacing, height of the trailing arms, length of the depositional lobes, and sinuosity of the recurring crest lines were retrieved with ease from the Level-2A data product namely ATL03 of ICESat-2/ATLAS.

Inverted dune swales, Hunder, Ladakh, India

ABSTRACT The Hunder Dunes occur in the Shyok valley, Ladakh region, north-western India. These barchanoid and transverse dunes consist of sand reworked from the seasonally exposed beds of the river. Wind direction is strongly uni-modal from the northwest and is controlled by valley orientation. We observed small (10–20 m long, 5–10 m wide, and up to 1.5 m high) ellipsoidal mesas in the swales between dunes, composed of sand deflated from the Shyok River system. The mesas have a tiered, wedding cake appearance with concave upper surfaces. We interpret these mesas as inverted swale deposits consisting of stacked cycles of sediment. The cycles range from 40 to 50 cm in thickness and are composed of a from bottom to top, deposits of prior dunes, high energy fluvial deposits with climbing ripples, pool sediments with symmetrical wave ripples, and desiccation-cracked mud drapes over the ripples indicating final drying out of the pool. Up to four cycles of deposition were present in the mesas. The inverted swales are being both exhumed from beneath and buried underneath migrating dunes. Inverted swale deposits provide a record of historical depositional processes and biological communities active at the site.

Morphology, accumulation and preservation of draa systems in a Precambrian erg (Galho do Miguel Formation, SE Brazil)

Proterozoic aeolian successions tend to exhibit a low complexity in the architectural organisation, generally related to simple dunes with low morphological diversity. Although most palaeoenvironmental reconstructions of ancient ergs confirm this scenario, the general conditions in the Precambrian landmass should have allowed the construction of larger and more complex sedimentary aeolian systems than those recorded in the Phanerozoic successions. To address this paradigm, the present work proposes to discuss the development and morphodynamics of ancient draa-scale bedforms and to understand which controlling factors produced the apparent absence of draa architecture in the Precambrian sedimentary record. For this purpose, this study examines an interval of c. 80 m of the Galho do Miguel Formation, which is a Mesoproterozoic aeolian succession composed of metasandstones showing well-preserved primary sedimentary structures and geometries with excellent vertical and horizontal exposures. A detailed sedimentological analysis allowed the recognition of six main architectural elements: (i) simple transverse dune, (ii) simple linear dune; (iii) compound transverse draa; (iv) complex linear draa, (v) dry interdune and (vi) sand sheet. The lateral and vertical relationships between these architectural elements support the evidence of coeval spatial coexistence of distinct aeolian morphologies (transverse and linear bedforms), probably controlled by lateral variations in sand saturation and in circulation pattern of the palaeowind. The stratigraphic organisation indicates different intervals of draa construction and destruction, which appear to be associated with different sedimentary pulses. The intervals of draa construction represent the vertical succession between simple and compound dunes of similar morphologies, representing temporal variation in sedimentary availability and changes in angle of dune climbing. The erosional and irregular surface at the bottom of compound sets of cross-strata can represent periods of irregular accumulation, associated with partial draa destruction (dune cannibalisation), probably due to seasonal variations in sedimentary availability throughout construction stages. Evidence of a near-surface water table level, as small wave ripples, indicates that the position of the water table was directly linked to the sedimentation pattern throughout these intervals. This relationship implies that ground water was the main controlling factor on sand availability and wind saturation during aeolian morphodynamics, as well as on accumulation and preservation processes. Moreover, in conditions of high rates of subsidence, the thickness accumulated and preserved via water table fluctuation is incorporated in the sedimentary record; otherwise, these deposits would have eroded due to aeolian deflation or fluvial reworking. Apparently, the absence of large and complex aeolian systems in the Precambrian record can be related to the absence of continuous near-surface water table during the draa development, directly decreasing their preservation potential.

Early Pennsylvanian fluvial - Aeolian interplay in the Amazonas Basin (central-western Gondwana) and its relation to marine transgression

The Monte Alegre Formation corresponds to the main reservoir rock in the Amazonas Basin petroleum system in Brazil and is correlated with the Pennsylvanian rocks of the Parnaíba and Solimões basins. Previous outcrop and well-based studies are concentrated on the southern edge of the basin and show that the Monte Alegre Formation basically represents an aeolian, deltaic and transitional marine succession. However, there is no published research that demonstrates the depositional system in the northern edge of Amazonas Basin. In this sense, outcrop-based facies and stratigraphic analysis is carried-out to understand the distal influence of marine incursion into the basin and its relationship with paleogeography in the Upper Carboniferous. The data show that the aeolian Monte Alegre system is represented by a braided fluvial system formed by five architectural elements: (i) sandy bedform, (ii) downstream-accretion macroform, (iii) gravel bars and bedforms, (iv) channel and (v) laminated sand sheets, pointing to a braided depositional system. The aeolian succession overlies the fluvial system and is represented by medium-scale trough cross-bedding and large-scale planar cross-bedded sandstones resulting from the accumulation and migration of transverse dunes toward the northwest that grade upward to dry interdunes and aeolian sand sheet deposits that are interbedded, consisting basically of translatent ripple cross-lamination and low-angle cross-stratified lamination sandstones related to water table raises. The stratigraphic record defines a wetting-upward stacking pattern of Monte Alegre Formation resulting from local sea level rise as a result the advance of the marine incursion that settled in the central-western region of Gondwana during the Late Carboniferous. Thus, in addition to corroborating the paleo wind directions predicted for the region during the Late Carboniferous, these deposits help to understand the paleoenvironmental relations between the northern and southern edges of the Amazonas Basin.

Seasonal control on coastal dune morphostratigraphy under a monsoon climate, Mui Ne dunefield, SE Vietnam

Having developed in the later Quaternary Period, the Mui Ne dunefield, representing the upper-most portion of a major coastal barrier complex in SE Vietnam, formed by wind regimes associated with the Asian monsoon climate. The barrier complex is 100 km long and higher than 150 m in elevation but despite its large dimensions, research on its geomorphological and geological characterization is limited. Several active dune fields extend 2–10 km landwards from the coast while the rest of the barrier surface is stabilized. We applied ground-penetrating radar (GPR) and quartz optically-stimulated luminescence (OSL) dating to the transverse dunes of the Mui Ne dunefield to characterize the morphostratigraphy and recent aeolian sedimentation history. Currently the dunes have a relief of typically 5–10 m and a wavelength of 50–100 m. They are driven by the northeasterly winter monsoon winds and reversed with minor relief in response to the southwesterly summer monsoon. GPR profiles and OSL ages define three morphostratigraphic units: the modern, Holocene, and Pleistocene dune units. The GPR data of the modern dune unit is dominated by a series of steep reflections that dip exclusively towards the west reflecting the winter monsoon and is punctuated by reactivation surfaces. Evidence for the wind reversal is not preserved in the sediment successions but is represented by reactivation surfaces. Thus a subunit bounded by the reactivation surfaces defines annual deposition. The preservation potential of a subunit varies with the migration rate of the transverse dune, and in the fastest migrating dunes, approximately one subunit was preserved every year (1.1 ± 0.6 subunit/yr). The modern dunes are generally younger than 100 years and have migrated WSW at net rates of 0.6–2.5 m/yr. The Holocene dune unit is as thick as the modern unit although it spans a period, ten times longer, reflecting the lower preservation potential of the transverse dunes as sedimentary records, due to erosion associated with their migration. The rate of dune migration suggests that an aeolian contribution of coastal sands has occurred following the sea level highstand of the Middle Holocene, accounting for the extent of the active dunefields. This mode of the barrier accumulation is considered as an analogue for earlier highstand events. However, the dunefields only cover a fraction of the entire barrier. It is thus inferred that barrier formation occurred over a long period, probably encompassing several glacial-interglacial cycles. Sand accumulation during the lowstand, either by transport of sand from the subaerially exposed shelf and/or reworking of sand emplaced during the prior highstand, may contribute significantly to the barrier development.

Microbial influence on the accumulation of Precambrian aeolian deposits (Neoproterozoic, Venkatpur Sandstone Formation, Southern India)

In many presently active aeolian systems, processes of sediment erosion, transport and deposition are markedly influenced by vegetation, which acts as an important sediment stabilising agent. Current sedimentary models for hyperarid continental settings devoid of vegetation and with adequate sand supply and wind force envisage intense aeolian activity, resulting in the construction of extensive aeolian sand-seas (ergs). During the Proterozoic, the Earth’s surface was devoid of vegetation, yet ergs are not so common in Proterozoic continental successions. High water-table levels, and erosional reworking by fluvial or marine processes are recognised as possible factors that restricted accumulation and preservation of extensive Proterozoic aeolian systems. By contrast, Proterozoic biotic communities have traditionally been considered largely incapable of markedly influencing clastic sedimentary processes in continental settings. However, since it is now known that bacteria have colonised parts of the continental Earth's surface since the Palaeoarchean, a question arises as to whether such organisms might have exercised some control on aeolian processes via substrate stabilisation.The Neoproterozoic Venkatpur Sandstone Formation (maximum age 709 Ma) is an aeolian depositional unit comprising small, isolated barchanoid and transverse dunes, and dry and damp sand sheet palaeoenvironments. Water-table-influenced aeolian sand-sheet deposits composed of thin sand layers that alternate with a suite of microbially induced sedimentary structures (MISS) provide evidence for fossilised bacteria. These strata record the interaction between microbial mats and aeolian depositional processes, which enabled the construction and accumulation of sand deposits. As microbial mats stabilised and provided plasticity to the depositional accumulation surface, they protected underlying (i.e. accumulated) aeolian deposits from possible wind erosion, reducing the possibility that these stored deposits could be recycled as lagged input to a downwind aeolian system.The stabilising influence of the microbial mats enabled the accumulation of the aeolian deposits, resulted in a marked decrease in the availability of sand for aeolian transport, and hindered construction and accumulation of large aeolian bedforms. This depositional model highlights the significance of the microbial mats for controlling the depositional processes in Precambrian aeolian-dominated environment. Probably, this model may be applied to other pre-vegetated Earth continental environments.

Reversing transverse dunes: Modelling of airflow switching using 3D computational fluid dynamics

Airflow dynamics across dune surfaces are the primary agent of sediment transport and resulting dune migration movements. Using 3D computational fluid dynamic modelling, this study examined the behaviour of near surface airflow travelling over transverse (reversing) dunes on a beach system. Wind direction was modelled in two opposing directions (both perpendicular to dune crestline) to investigate surface alteration of flow on the dune topography. Surface shear stress, velocity streamlines and potential sediment flux were extracted from the modelling. The work shows that under SW winds the surface (under the configuration measured) underwent almost 10% more aeolian flux than with opposing NE winds of the same magnitude. Differences were also noted in the airflow behaviour with SW winds staying attached to the surface with less turbulence while NE winds had detached flow at dune crests with more localised turbulence. The work provides detailed insights into how 3D airflow behaviour is modified according to incident flow direction of reversing dune ridges and the resulting implications for their topographic modification. These dune types also provide interesting analogues for similarly scaled Transverse Aeolian Ridges found on Mars and the findings here provide important understanding of flow behaviour of such landforms and their potential movement.

Late Pleistocene lake level history of Lake Mungo, Australia

Lake Mungo is a currently dry lake basin in the semi-arid zone of southeastern Australia. The transverse dune system on the downwind side contains a record of human occupation of international importance. It also contains one of the most continuous records of climate change over the last glacial cycle in the Australia desert. In this paper we provide a framework for the interpretation of lake level history from before the arrival of people (>41 ka) until after the establishment of the pastoral industry in the area. We present 83 optically stimulated luminescence ages from the Lake Mungo lunette. The lake level history is reconstructed from 34 stratigraphic sections along three transects through the lunette. The dating reveals considerable lake level fluctuations through time which occur over a depth range of ∼10 m in the basin. At its height, probably at multiple times before 20 ka, the lake held more than 1 km3 of water and at its final level at ∼19 ka, contained only 0.03 km3. The inception of Lake Mungo appears to have taken place during the mid-Pleistocene between ∼256 and 369 ka. During the last glacial cycle, Lake Mungo was almost continuously wetter than present from shortly after 60 ka until ∼19 ka. The Upper Mungo, Arumpo and Zanci units represent a succession of lake filling and drying events, briefly interspersed by soils. The final Zanci unit does not represent a single high lake phase, but an initial lake filling followed by a series of short-lived lake level events within a brief period of a few thousand years. At the conclusion of this event, the lake remained dry until the present day. Four OSL ages from a linear dune upwind of Lake Mungo indicates regional aeolian activity from ∼30 ka until present. Widespread erosion that produces the characteristic topography of the lunette began after the arrival of British pastoralists and traditional aboriginal ways of life overlapped briefly with this erosion. The presence of water in Lake Mungo closely corresponds to periods when regional surface temperature was colder than present during the late Pleistocene. Our new data supports a model that decreased evaporation and increased runoff were primarily responsible for increased availability of surface water in the hydrological cycle.

Starved versus alluvial river bedforms: an experimental investigation

AbstractLaboratory experiments were conducted to investigate the formation of river bedforms under sediment supply‐limited conditions, i.e. when a motionless substratum is bared by the dynamics of the mobile sediments. Three series of experiments were organized in a laboratory flume by fixing all the hydrodynamic and morphodynamic parameters but varying the thickness of the initial layer of mobile sediments which covers the rigid bottom of the flume. At the end of all the experiments, which lasted for the same amount of time, the formation of transverse sand dunes was observed. For decreasing , the rigid bottom of the flume was bared progressively earlier during the experiment and the measurements showed a clear tendency of the bedforms to lengthen, i.e. to increase their crest‐to‐crest distance. Moreover, under strong supply limitation, the two‐dimensional transverse dunes turned into three‐dimensional barchanoid forms and into isolated barchan dunes characterized by an abrupt reduction in bedform heights. A two‐dimensional Fourier analysis of the bottom profile was performed, providing the amplitude of the main streamwise and spanwise harmonic components of the bottom morphology as a function of . © 2019 John Wiley & Sons, Ltd.

Stabilization of a straight longitudinal dune under bimodal wind with large directional variation.

It has been observed that the direction in which a sand dune extends its crest line depends on seasonal variation of wind direction; when the variation is small, the crest line develops more or less perpendicularly to the mean wind direction to form a transverse dune with some undulation. In the case of bimodal wind with a large relative angle, however, the dune extends its crest along the mean wind direction and evolves into an almost straight longitudinal dune. Motivated by these observations, we investigate the dynamical stability of isolated dunes using the crest line model, where the dune dynamics is represented by its crest line motion. First, we extend the previous linear stability analysis under the unidirectional wind to the case with nonzero slant angle between the wind direction and the normal direction of the crest line, and show that the stability diagram does not depend on the slant angle. Second, we examine how the linear stability is affected by the seasonal changes of wind direction in the case of bimodal wind with equal strength and duration. For the transverse dune, we find that the stability is virtually the same with that for the unidirectional wind as long as the dune evolution during a season is small. On the other hand, in the case of the longitudinal dune, the dispersions of the growth rates for the perturbation are drastically different from those of the unidirectional wind, and we find that the largest growth rate is always located at k=0. This is because the growth of the perturbation with k≠0 is canceled by the alternating wind from opposite sides of the crest line even though it grows during each duration period of the bimodal wind. For a realistic parameter set, the system is in the wavy unstable regime of the stability diagram for the unidirectional wind, thus the straight transverse dune is unstable to develop undulation and eventually evolves into a string of barchans when the seasonal variation of wind direction is small, but the straight longitudinal dune is stabilized under the large variation of bimodal wind direction. We also perform numerical simulations on the crest line model, and find that the results are consistent with our linear analysis and the previous reports that show that the longitudinal dunes tend to have a straight ridge elongating over time.

Meteorological application of wind speed and direction linked to remote sensing images for the modelling of sand drift potential and dune morphology

AbstractWind velocity and its direction are important erosive factors affecting sand drift potential and dune morphology in arid and semi‐arid environments. The study assesses sand drift potential using wind meteorological data acquired from eight synoptic stations of Kerman province in southeastern Iran for a statistical period of 2006–2010. Three diagrams including wind rose, storm rose and sand rose were plotted for the selected stations using WR Plot View.8 and Sand Rose Graph 3.0. The analyses were performed at seasonal and mean annual time scales. The unidirectional index and wind energy classification were compared with sand dune morphology obtained from satellite images. The seasonal analysis indicated intermediate directional variability and high energy winds, with the most erosive winds occurring in winter. The total sand flux in all directions and discharge sand flow in the resultant drift direction ranged from 53.4 to 319.1 and from 13.8 to 247.6 t·m−1·year−1, respectively. The transverse dune was the main type of sand dune detected in the region with an intermediate directional variability, implying positive sand supplies. Dune morphology coincided reasonably well with the resultant drift direction. An appropriate match was found between the wind data analyses and the morphology of dunes observed in the satellite images.

Mesoproterozoic erg and sand sheet system: Architecture and controlling factors (Galho do Miguel Formation, SE Brazil)

Aeolian systems of the Precambrian time were established in different palaeoclimatic and palaeogeographic conditions than the present. Different morphologies of dunes/draas, as linear, oblique and star forms, are widespread in the recent aeolian systems. However, the ancient aeolian deposits are rarely interpreted as these types of bedforms and usually referred to as transverse dunes. The nature and complexity of bedform morphology in Precambrian aeolian systems, as well as the factors that allow the construction, accumulation and preservation of these environments are still not well defined. Extensive exposures of Galho do Miguel Formation in southeastern Brazil represent a Mesoproterozoic Erg succession and exhibit a distinct facies architecture, which contributed to the understanding of these deposits. Three architectural elements were identified: (i) tangential cross-stratified sandstone with superimposed trough cross-bedded sandstone, (ii) planar-laminated sandstone, and (iii) extensive flat-bedded sandstone dominated by wrinkly laminae. These elements are organized into three succession groups representative of simple transverse dune, oblique compound draa and sand sheet systems. The simple to compound dune/draa system corresponds to the most central parts of the erg, associated with a higher sediment availability and lower water-table level. Conversely, the sand sheet system occurs under low sediment availability and higher water-table level in the marginal portions of the erg. Precambrian climatic oscillations may have changed the aeolian system morphology, allowing the alternation of simple and complex aeolian dunes, resulting in high-frequency changes in sand sheet system, and the remobilization of previously accumulated sediments over large geographic areas.