Aeolian Ripples Research Articles

An Acoustic Sensor System to Measure Aeolian Ripple Morphology and Migration Rates.

Acoustic distance sensors have a long history of use to detect subaqueous bedforms. There have been few comparable applications for aeolian bedforms such as ripples. To address this, we developed a simple and reliable apparatus comprising a pair of distance sensors, a bracket upon which they are mounted, and a base upon which the bracket can slide. Our system relies on two Senix Corporation (Hinesburg, VT, USA), ToughSonic® model 14-TSPC-30S1-232 acoustic distance sensors: one to measure surface elevation changes (in this case, ripple morphology) and a second to measure horizontal location. The ToughSonic® vertical resolution was 0.22 mm and the horizontal scan distance was about 0.60 m with a locational accuracy of 0.22 mm. The measurement rate was 20 Hz, but we over-sampled at 1 KHz. Signal processing involves converting volts to meters, detrending the data, and removing noise. Analysis produces ripple morphologies and migration rates that conform with independent measurements. The advantages of this system relative to terrestrial laser scanning or structure from motion are described.

Probing Iceland's dust-emitting sediments: particle size distribution, mineralogy, cohesion, Fe mode of occurrence, and reflectance spectra signatures

Abstract. Characterising the physico-chemical properties of dust-emitting sediments in arid regions is fundamental to understanding the effects of dust on climate and ecosystems. However, knowledge regarding high-latitude dust (HLD) remains limited. This study focuses on analysing the particle size distribution (PSD), mineralogy, cohesion, iron (Fe) mode of occurrence, and visible–near infrared (VNIR) reflectance spectra of dust-emitting sediments from dust hotspots in Iceland (HLD region). Extensive analysis was conducted on samples of top sediments, sediments, and aeolian ripples collected from seven dust sources, with particular emphasis on the Jökulsá basin, encompassing the desert of Dyngjunsandur. Both fully and minimally dispersed PSDs and their respective mass median particle diameters revealed remarkable similarities (56 ± 69 and 55 ± 62 µm, respectively). Mineralogical analyses indicated the prevalence of amorphous phases (68 ± 26 %), feldspars (17 ± 13 %), and pyroxenes (9.3 ± 7.2 %), consistent with thorough analyses of VNIR reflectance spectra. The Fe content reached 9.5 ± 0.40 wt %, predominantly within silicate structures (80 ± 6.3 %), complemented by magnetite (16 ± 5.5 %), hematite/goethite (4.5 ± 2.7 %), and readily exchangeable Fe ions or Fe nano-oxides (1.6 ± 0.63 %). Icelandic top sediments exhibited coarser PSDs compared to the high dust-emitting crusts from mid-latitude arid regions, distinctive mineralogy, and a 3-fold bulk Fe content, with a significant presence of magnetite. The congruence between fully and minimally dispersed PSDs underscores reduced particle aggregation and cohesion of Icelandic top sediments, suggesting that aerodynamic entrainment of dust could also play a role upon emission in this region, alongside saltation bombardment. The extensive analysis in Dyngjusandur enabled the development of a conceptual model to encapsulate Iceland's rapidly evolving high dust-emitting environments.

Coastal aeolian sediment transport in an active bed surface layer: Tracer study and conceptual model

AbstractCoastal aeolian sediment transport is influenced by supply‐limiting factors caused by sediment sorting by grain size. Sorting processes can lead to coarsening of the bed surface and influence the formation of aeolian ripples. However, the influence sorting processes and bedforms might have on the magnitude of the transport is not fully understood. This study explores sorting processes and their influence on the magnitude and mode of aeolian transport by using sediment tracers. Sand was painted in different colors according to particle size and placed on a supratidal beach in Noordwijk, the Netherlands. Several experiments were conducted with varying wind speeds. Surface sampling and cameras tracked the sand color movement on the bed surface, and wind velocity was measured. The tracer experiments showed that ripples developed in moderate wind conditions. Once the ripples had formed, the supply of finer tracer grains in the downwind direction decreased over time, while the supply of coarser grains remained constant. A linear relationship between ripple migration speed and wind speed was found. For higher wind speeds, no ripples or differences in transport of grain size fractions were observed. Instead, alternating phases of erosion and deposition of the bed surface were observed, which could not be related to local variations in wind velocity. Based on these results and literature, a conceptual model was developed for an active bed surface layer with two transport regimes corresponding to moderate (I) and high (II) wind speeds. The conceptual model is intended to guide the selection of aeolian sediment transport models as a function of wind speed, bed characteristics, and upwind sediment supply. For Regime I, transport could be modeled using a linear relationship between sediment transport and wind speed and for Regime II using a third power relationship in combination with a process‐based model accounting for supply limitations.

Quantifying Late Pleistocene to Holocene Erosion Rates in the Hami Basin, China: Insights Into Pleistocene Dust Dynamics of an East Asian Stony Desert

AbstractWind is one of the major processes modifying the land surface in the Hami Basin, as evidenced by wind deflation gravel lags, yardangs, and gravel mantled eolian ripples. We report erosion rates for the Hami Basin using cosmogenic 10Be measurements. Bedrock erosion rates average 0.121 ± 0.0293 mm yr−1, which is similar to those of other wind‐eroded arid basins in East Asia, but is anomalously low when modern near‐surface wind speeds are considered. We posit that interglacial periods experienced lower erosion rates than glacial periods, and that the Hami Basin was likely once a larger dust producer than at present, with Pleistocene dust emissions driven by the eolian deflation of alluvial terrace deposits as well as bedrock weathering and abrasion. Wind erosion and dust production in the Hami Basin was largely regulated by the synoptically controlled spatial‐temporal distribution of precipitation and subsequent landscape evolution since at least the late Pleistocene.

Ancient Winds, Waves, and Atmosphere in Gale Crater, Mars, Inferred From Sedimentary Structures and Wave Modeling

AbstractWave modeling and analysis of sedimentary structures were used to evaluate whether four examples of symmetrical, reversing, or straight‐crested bedforms in Gale crater sandstones are preserved wave ripples; deposition by waves would demonstrate that the lake was not covered by ice at that time. Wave modeling indicates that regardless of atmospheric density, winds that exceeded the threshold of aeolian sand transport could have generated waves capable of producing nearshore wave ripples in most grain sizes of sand. Reversing 3‐m‐wavelength bedforms in the Kimberley formation are interpreted not as wave ripples but rather as large aeolian ripples that formed in an atmosphere approximately as thin as at present. These exhumed bedforms define many of the ridges at outcrops that appear striated in satellite images. At Kimberley these bedforms demonstrably underlie and therefore predate subaqueous beds, suggesting that a thin atmosphere existed at least temporarily before subaqueous deposition ceased in the crater. The other three candidate wave ripples (Square Top, Hunda, and Voe) are consistent with modeled waves, but other origins cannot be excluded. The predominance of flat‐laminated (non‐rippled) beds in the lacustrine Murray formation suggests that some aspect of the lake was not conducive to formation or preservation of recognizable wave ripples. Water depths may generally have been too deep, lakebed sediment may have been too fine‐grained, the lake may have been smaller than modeled, or the lake may have been covered by ice.

Quantifying vegetation and its effect on aeolian sediment transport: A UAS investigation on longitudinal dunes

Studying the role of vegetation in regulating aeolian sediment transport is complicated by the diversity of plant geometry and spatial distribution. Using Unmanned Aerial Systems (UAS) surveys of four partially vegetated sand dunes in the Simpson Desert, this study explored statistical associations between vegetation and the location and quantity of aeolian ripples. Employing mosaic image classifications, Digital Surface Models (DSM), and Canopy Height Models (CHM), four core independent metrics were computed: The fractional cover (fc); frontal area index (λ), mean gap length (L), and shadow casting or Shadow Area Ratio (SAR). The strongest predictor of aeolian ripple abundance was the mean scaled gap length (individually scaled by the lesser of an adjacent plant’s width or height) (Lsf-) (R2 = 0.83). Lsf- (and Lh-, which only used plant height) effectively resolved the spatial and structural distribution of vegetation, which was partially governed by the composition of functional plant types. fc was also strongly associated with ripple abundance (R2 = 0.81). Ripple cover varied continuously with fc without a clear threshold for the onset of sand transport, though the curve flattened above fc ≈ 25–30%. Moderate associations were found for SAR (R2 ≤ 0.57) and λ (R2 = 0.63). Shadow lengths (in units of plant height) of 1–3 best explained the location of ripples. The efficacy of shadow casting was affected by the signal to noise ratio in the DSMs at the scale of very small plants. UAS data nevertheless displayed strong potential for advancing the study of vegetation and aeolian activity.

Re‐Evaluation of Large Martian Ripples in Gale Crater: Granulometric Evidence for an Impact Mechanism and Terrestrial Analogues

AbstractMars hosts three scales of superimposed aeolian bedforms: small decimeter‐scale impact ripples, large meter‐scale ripples, and dunes. The formation mechanism for large meter‐scale ripples is enigmatic and debated. This debate is largely informed by two questions: (a) Are large ripples similar to any features on Earth? and (b) Do they have the same grain size distribution as dunes and impact ripples? We address these questions using a novel digitizing approach to produce the largest known data set of Mars ripple sand grain size to date. We find a distinction between the grain size distributions of small ripples and large ripples and suggest that analogous bedforms are found on Earth. Despite an inability to perform rigorous sedimentology on Mars, results help resolve outstanding questions in bedform physics and support a hypothesis that aeolian ripples on Mars develop by a terrestrially analogous impact mechanism.

An Idealized 3D Model of Interfacial Instability of Aeolian Bedform

An idealized morphodynamic model is constructed for formation of the aeolian sand ripples from small bottom perturbations of a two-dimensional sand bed. The main goal of the analysis is to evaluate the influence of the gravity flow (including “impact-induced gravity flow” in the reptation flux and “topography-induced gravity flow” in the creep flux) on the formation of the aeolian sand ripples and to clarify the relative contribution of various factors to the bed instability. A 3D linear stability analysis reveals that gravity flow appreciably affects the dynamics behaviors of aeolian sand ripples, which decreases the growth rate of sand ripples, tends to stabilize the sand bed, and leads to longer wavelength. We found that the competition between the destabilizing effect of reptation flow and the stabilizing effects of gravity flow leads to pattern selection. The along-crest diffusion of topography driven by impact and gravity is beneficial to the transverse stability of sand ripples, producing sand ripples with straighter and more continuous crests. For moderate values of D, the most unstable mode has zero value of the transverse wavenumber (ky = 0), thus corresponding to aeolian ripples with crests perpendicular to the wind. Moreover, when the impact angle is 9–16°, it has little effect on the characteristics of sand ripples for the initial stage of ripple development. For every increase of the impact angle by 1°, the initial wavelength only increases by about 1.5%. In conclusion, the influence of the gravity flow on the dynamics of sand ripples formation stage cannot be neglected.

Turbulent Shear Flow Over Large Martian Ripples

AbstractOn Mars, large aeolian ripples with wavelengths typically 1–3 m but lacking very coarse sand at crests have been encountered by rovers and observed from orbit. These bedforms have no terrestrial counterpart and several hypotheses for origins have been proposed. This work reports results of Computational Fluid Dynamics (CFD) experiments with ANSYS Fluent under terrestrial and Martian boundary layer conditions, using the k − ω SST turbulence model to evaluate shear stress along a topographic profile of large Martian ripples at different boundary layer wind speeds. Results indicate that, compared with Earth conditions: (1) boundary‐layer flow along large ripples under Martian conditions is less turbulent due to higher kinematic viscosity; (2) reverse‐flow vortex regions from crests at ripple lee flanks are larger; and (3) shear stresses at crests of large ripples are relatively low, so ripple flattening is less likely at high wind speeds. These results indicate Martian ripples formed by the saltation impact splash mechanism should be less constrained by shear stress effects limiting growth of exposed ripple crests, because the low‐density Martian atmosphere applies relatively low wind‐related shear stress to ripple surfaces. Other origins for the large Martian ripples are not excluded, however. On Earth, very large ripples with crests unprotected by very coarse grains do not develop due to higher wind‐related shear stresses.

A Broad Continuum of Aeolian Impact Ripple Morphologies on Mars is Enabled by Low Wind Dynamic Pressures

AbstractAeolian ripples are common in sandy environments on Earth and Mars. On Earth, ripples in sorted dune sands typically are <1 cm high and are erased in high winds. On Mars in similar sands, ripple wavelengths commonly exceed 2 m, with much smaller ripples superimposed. Large Martian ripple sizes and juxtaposition of multiple wavelengths have raised questions about origins and the applicability of terrestrial aeolian physics to different planetary environments. Here, two hypotheses are evaluated for large Martian ripples: (1) fluid/wind drag, analogous to ripples formed under water on Earth, as proposed previously for Martian large ripples; and (2) saltation impact splash, the mechanism creating aeolian ripples of much smaller size on Earth. This study evaluates these hypotheses with numerical experiments and Mars rover observations, and concludes that large Martian ripples develop through the saltation impact splash mechanism. The low‐density Martian atmosphere enables aeolian impact ripples to grow much higher into the boundary layer before reaching maximum heights constrained by wind dynamic pressure effects at crests. In this concept, boundary layer conditions influence mature ripple heights more directly than wavelengths. On Mars, low wind dynamic pressures, combined with the impact splash mechanism, also help to explain other distinctively Martian aeolian bedforms, including large longitudinal ripples observed by rovers and orbiters, and transverse aeolian ridges (TARs) distributed widely across the Martian surface. Compared with Earth, low wind dynamic pressures on Mars permit a wider range of ripple sizes, relative ages, morphologies, and orientations in close proximity, as displayed in rover observations.

Design and application of gas cluster accelerator for surface smoothing and nanostructures formation

A custom-built gas cluster ion source with energy up to 50 keV is constructed, and Ar, CO<sub>2</sub>, N<sub>2</sub>, and O<sub>2</sub> are used as the working gases. The clusters are formed by a metal supersonic conical nozzle with critical diameter in a range of 65–135 μm and a cone angle of 14°. The nozzle is powered in the pulsed mode, which improves the pumping conditions, and also makes it possible to increase the gas pressure in the stagnation zone to 15 atm and thereby obtain larger clusters. Based on the principle of ultrasonic expansion, gas cluster ions with an average size of 3000 atoms are obtained. The cluster beam current of 50 μA is obtained. The Ar cluster beam, which is less reactive, is used for treating surface, namely, surface smoothing and formation of self-assembled nanostructures. The Ar cluster bombardment perpendicular to the surface of the substrate is used to demonstrate the smoothing of the surface of Si wafers, Ti coating, and Au film. For the initial Si wafer, its root-mean-square (RMS) roughness of 1.92 nm decreases down to 0.5 nm after cluster beam treatment. The cleaning effect of the cluster beam is also observed very well. The one-dimensional (1D) isotropic power spectral density of the Si surface topography before and after smoothing are also discussed. The off-normal irradiation Ar cluster beam is also used to form self-assembled surface nanoripple arrays on the surface of flat ZnO single crystal substrates. The ripple formation is observed when the incident angle of the cluster beam is in a range of 30°–60°. The process of nanoripple fabrication is significantly governed by the cluster beam incident angle, energy and dose. The nano-ripples formed on the flat substrates remain eolian sand ripples and their formation starts at the incident angle of 30°. The most developed nanoripples are observed at the incident angle within a range of 45°–60°. The surface morphology and characteristic distribution of the nano-structures on the flat ZnO substrate are also analyzed by the two-dimensional (2D) power spectral density function. Next, Ar cluster beam is used for irradiating the ZnO nanorod arrays grown on the Si substrate. Due to various angles between the nanorod’s axis and the substrate normal, the conditions of the ripple formation on the nanorod facets are also realized. The dependence of wavelength on the accelerating voltage of the cluster ions and the dose are studied. Similar dependence of wavelength on accelerating voltage and dose are found for nanorods. Comparing with the flat ZnO surface, nanoripples on the ZnO nanorod faces at high irradiation doses demonstrate an ordering effect, and morphology of the ripples resembles that of more parallel steps rather than eolian ripples.

Aeolian Ripple Migration and Associated Creep Transport Rates

Wind-formed ripples are distinctive features of many sandy aeolian environments, and their development and migration are basic responses to sand transport via saltation. Using data from the literature and from original field experiments, we presented empirical models linking dimensionless migration rates, u r / g d ( u r is the ripple migration speed, g is the gravity acceleration, and d is the grain diameter) with dimensionless shear velocity, u*/u*t (u* is shear velocity and u*t is fluid threshold shear velocity). Data from previous studies provided 34 usable cases from four wind tunnel experiments and 93 cases from two field experiments. Original data comprising 68 cases were obtained from sites in Ceará, Brazil (26) and California, USA (42), using combinations of sonic anemometry, sand traps, photogrammetry, and laser distance sensors and particle counters. The results supported earlier findings of distinctively different relationships between u r / g d and u*/u*t for wind tunnel and field data. With our data, we could also estimate the contribution of creep transport associated with ripple migration to total transport rates. We calculated ripple-creep transport for 1 ≤ u*/u*t ≤ 2.5 and found that this accounted for about 3.6% (standard deviation = 2.3%) of total transport.

A first evaluation of the contribution of aeolian sand transport to lagoon island accretion in the Maldives

Aeolian sedimentation and dune development have not been reported from coral atolls at equatorial latitudes. This study presents high-frequency measurements of incident and near surface wind flow and aeolian sand transport on a lagoon sand cay (Maaodegalaa) in the Maldives. Sonic anemometers and Wenglor™ particle counters were operated at 1 Hz for 8 days during the Iruvai monsoon in February 2018. Sand traps were deployed to estimate sand flux and island topography and vegetation cover were surveyed using UAV (un-manned aerial vehicle) photogrammetry and a laser level (in 2017 and 2018). Flow over beach scarps is modelled using computational fluid dynamics. Maaodegalaa sand cay reaches just 0.9 m above the highest spring high tides. Nebkha, between 0.10 and 0.40 m high, are widespread and are associated with Scaevola taccada and Cyperus conglomeratus. Between 2017 and 2018 the eastern section of the sand cay accreted 0.3 m following Cyperus colonisation. Reptation and aeolian ripple development occurred during fieldwork when near-surface flows exceeded 6 ms−1. Saltation occurred at higher wind speeds (8 ms−1). The highest rates of sand transport occurred during north-east incident winds of 12 ms−1 (at 6 m), that were probably generated by surface-based density currents under cumulonimbus clouds. Spatially, higher rates of sand transport were recorded downwind of a beach scarp, probably forced by flow acceleration. We propose a conceptual model of lagoon island formation, with both over-wash and aeolian sedimentation contributing to island accretion. A period of aeolian sedimentation may be critical to the emergence of sand cays.

Numerical Study of Shear Stress Distribution Over Sand Ripples Under Terrestrial and Martian Conditions

AbstractRipples occur on Earth and Mars in a range of sizes. From terrestrial studies, ripple size is known to depend on grain‐size frequency, wind duration, wind strength (including stronger winds that can flatten ripples), and fundamental environmental factors that differ between the two planets. Here we use computational fluid dynamics (CFD) experiments to model boundary layer shear stresses applied to aeolian ripple surfaces, to investigate how these stresses might differ between Earth and Mars. CFD experiments used ANSYS Fluent, with inlet wind speeds of 10 and 15 m/s for both planetary environments. Ripple profiles for Earth and Mars were developed using saltation and reptation properties modeled by the numerical saltation model COMSALT (Kok & Renno, 2009, https://doi.org.10.1029/2009JDO11702) to develop ripple profiles using the numerical technique of Yizhaq et al. (2004, https://doi.org.10.1016/j.physd.2004.03.015). Although the CFD experiments using these inputs could not include the effects of a saltation cloud, results are robust enough to indicate clearly that for similar modeled wind speeds on Earth and Mars, boundary layer shear stress applied to ripple surfaces is greater on Earth under conditions for which sand transport is expected. These results indicate that ripples can grow larger on Mars than on Earth, because for typical Martian wind speeds the shear velocity at the ripple crests is below the fluid threshold.

Aeolian sand sorting and megaripple formation

Sand is blown across beaches and deserts by turbulent winds. This seemingly chaotic process creates two dominant bedforms: decametre-scale dunes and centimetre-scale ripples, but hardly anything in between. By the very same process, grains are constantly sorted. Smaller grains advance faster, while heavier grains trail behind. Here, we argue that, under erosive conditions, sand sorting and structure formation can conspire to create distinct bedforms in the ‘forbidden wavelength gap’ between aeolian ripples and dunes. These so-called megaripples are shown to co-evolve with an unusual, predominantly bimodal grain-size distribution. Combining theory and field measurements, we develop a mechanistic understanding of their formation, shape and migration, as well as their cyclic ageing, renewal and sedimentary memory, in terms of the intermittent wind statistics. Our results demonstrate that megaripples exhibit close similarities to dunes and can indeed be mechanistically characterized as a special type of (‘reptation’) dune. Wind-mediated ripples form on a centimetre scale in sand, and in dunes on a scale spanning tens of metres, but patterns on intermediate scales are rare. A theory now fills the gap by predicting megaripples, which resemble structures seen on Mars.

Young, small-scale surface features in Meridiani Planum, Mars: A possible signature of recent transient liquid and gas emissions

Enigmatic small-scale (<1 m) depositional and erosional features found in basaltic sands partly covering bedrock exposures, imaged at several locations in the equatorial Meridiani Planum region by the Mars Exploration Rover Opportunity, may be evidence of previously unrecognized, geologically young (or even contemporary) Martian surface processes. Leveed fissures appear to have formed by venting from beneath; possible explanations include aeolian blowholes near crater margins, volcanic fumarole activity, or gas/vapour escape resulting from the decomposition of small pockets of ground ice, methane clathrates or hydrated sulphate minerals. Some leveed fissures cross-cut and are therefore younger than aeolian ripples thought to have last been active c. 50,000 years ago. Erosional gutters are sharply defined and fresh-looking, internally terraced, sometimes are deeper near one end, and in one case seem to give way to small depositional fans downslope; they have the appearance of having been formed by liquid flow rather than by wind erosion. There is evidence elsewhere that contemporary ground-ice thaw and consequent transient surface run-off may occur occasionally under present conditions at low, near-equatorial latitudes on Mars; short-lived (even for just a few minutes) meltwater emission and flow at the surface could form gutters before evaporating. Further possibilities are the decomposition of buried pockets of methane clathrates (which theoretical considerations suggest might be present and stable even in equatorial regions) giving rise to both methane gas venting and transient surface water, or the release of liquid brines by decomposition of hydrated magnesium sulphate minerals or deliquescence of perchlorates. Dry granular flow mechanisms proposed as explanations for Recurring Slope Lineae seem inadequate to explain the morphologies of leveed fissures and gutters.

Particle size characteristics of aeolian ripple crests and troughs

AbstractRipples are prevalent in aeolian landscapes. Many researchers have focused on the shape and formation of sand ripples, but few have studied the differences in the particle size of sand on crests and in troughs along bed, especially the variations caused by changes in friction velocity and the wind‐blowing duration. A particle size of 158 μm (d) was used to create aeolian ripples in a wind tunnel under four friction velocities (u*) with different wind duration times (t). Samples were collected from the surfaces of ripple crests and troughs, respectively, at seven sites, and particle sizes were measured using a Malvern Mastersizer 2000. The main results were: (i) The particle size distributions of sand in troughs are unimodal with slight variations of particle size parameters, including mean particle size, standard deviation, skewness and kurtosis, etc., under different conditions, while these particle size parameters of sand on crests change with friction velocity and deflation time. Moreover, some of the particle distribution curves for the sand on crests do not follow typical unimodal curves. (ii) With increasing friction velocity or deflation duration, the sand on the crests shows a coarsening process relative to those on the bed surface. The particle size of sand on crests at a 1 m bed increases linearly with friction velocity (d = 344·27 + 34·54 u*) at a given wind‐blowing duration. The particle sizes of sand on crests at 1 m, 2 m and 4 m beds increase with a power‐law relationship (d = a + tb, where a and b are fitting parameters) with deflation time at a given friction velocity. (iii) The probability cumulative curves of sand showed a three‐section pattern in troughs and on most of the crests but a four‐section pattern at crest locations due to increased influence by friction velocity and deflation time. The proportions of the sediment moved by suspension, saltation and creep in the three‐section pattern were within the ranges of 0·2% to 2·0%, 97·0% to 98·9%, and 0·8% to 3·0%, respectively. For the four‐section pattern, suspension accounted for 0·3% and 3·0%, and the proportion of creep increased with friction velocity and deflation time, while saltation decreased accordingly. Although these results require additional validation, they help to advance current understanding of the grain‐size characteristics of aeolian ripples.

Coarse‐sand beach ridges at Cowley Beach, north‐eastern Australia: Their formative processes and potential as records of tropical cyclone history

AbstractStorm surges generated by tropical cyclones have been considered a primary process for building coarse‐sand beach ridges along the north‐eastern Queensland coast, Australia. This interpretation has led to the development of palaeotempestology based on the beach ridges. To better identify the sedimentary processes responsible for these ridges, a high‐resolution chronostratigraphic analysis of a series of ridges was carried out at Cowley Beach, Queensland, a meso‐tidal beach system with a &gt;3 m tide range. Optically stimulated luminescence ages indicate that 10 ridges accreted seaward over the last 2500 to 2700 years. The ridge crests sit +3·5 to 5·1 m above Australian Height Datum (ca mean sea‐level). A ground‐penetrating radar profile shows two distinct radar facies, both of which are dissected by truncation surfaces. Hummocky structures in the upper facies indicate that the nucleus of the beach ridge forms as a berm at +2·5 m Australian Height Datum, equivalent to the fair‐weather swash limit during high tide. The lower facies comprises a sequence of seaward‐dipping reflections. Beach progradation thus occurs via fair‐weather‐wave accretion of sand, with erosion by storm waves resulting in a sporadic sedimentary record. The ridge deposits above the fair‐weather swash limit are primarily composed of coarse and medium sands with pumice gravels and are largely emplaced during surge events. Inundation of the ridges is more likely to occur in relation to a cyclone passing during high tide. The ridges may also include an aeolian component as cyclonic winds can transport beach sand inland, especially during low tide, and some layers above +2·5 m Australian Height Datum are finer than aeolian ripples found on the backshore. Coarse‐sand ridges at Cowley Beach are thus products of fair‐weather swash and cyclone inundation modulated by tides. Knowledge of this composite depositional process can better inform the development of robust palaeoenvironmental reconstructions from the ridges.

Dune‐like dynamic of Martian Aeolian large ripples

AbstractMartian dunes are sculpted by meter‐scale bed forms, which have been interpreted as wind ripples based on orbital data. Because aeolian ripples tend to orient and migrate transversely to the last sand‐moving wind, they have been widely used as wind vanes on Earth and Mars. In this report we show that Martian large ripples are dynamically different from Earth's ripples. By remotely monitoring their evolution within the Mars Science Laboratory landing site, we show that these bed forms evolve longitudinally with minimal lateral migration in a time‐span of ~ six terrestrial years. Our observations suggest that the large Martian ripples can record more than one wind direction and that in certain cases they are more similar to linear dunes from a dynamic point of view. Consequently, the assumption of the transverse nature of the large Martian ripples must be used with caution when using these features to derive wind directions.

Migrating meter-scale bedforms on Martian dark dunes: Are terrestrial aeolian ripples good analogues?

Abstract Using automatic bedform mapping, principal component analysis and clustering we present a multiscale morphodynamic survey of dunes and meter-scale ripples on Herschel crater, Mars. The main purpose of this study is to assess if the morphology and temporal evolution of Martian meter-scale ripples is comparable to the morphodynamic characteristics of terrestrial aeolian impact ripples. We demonstrate that the spatial variations of the mapped dune patterns are correlated with substrate topography, which also influences the spatial distribution of the height and celerity of dunes’ slipfaces. This topographic forcing is also patent on the spatial distribution of the ripples, thus proving that a multiscale coupling of active bedforms exists on Herschel under the present surface conditions. We found that Martian meter-scale ripples are morphologically distinct from terrestrial aeolian ripples, presenting a lower degree of straightness. Only ∼3% of the mapped ripples can be considered sinuous or straight bedforms. Moreover, we conclude that this two-dimensional sub-population is restricted to well define dune settings, where factors that promote the elongation of the meter-scale ripples were identified: gravity transport on higher slopes, bedform obliquity and flow convergence on the leeward side of dunes. We also report that the different sets of ripples that were mapped and segmented do not present a transverse migration. Therefore we conclude that terrestrial aeolian ripples are not good analogues for Martian meter-scale bedforms, either in terms of morphology or dynamic evolution.