Spectral and petrologic analyses of basaltic sands in Ka'u Desert (Hawaii) – implications for the dark dunes on Mars

Dark aeolian deposits on Mars are thought to consist of volcanic materials due to their mineral assemblages, which are common to basalts. However, the sediment source is still debated. Basaltic dunes on Earth are promising analogs for providing further insights into the assumed basaltic sand dunes on Mars. In our study we characterize basaltic dunes from the Ka'u Desert in Hawaii using optical microscopes, electron microprobe, and spectral analyses. We compare the spectra of terrestrial and Martian dune sands to determine possible origins of the Martian dark sediments. Our results show that the terrestrial sands consist primarily of medium to coarse sand‐sized volcanic glass and rock fragments as well as olivine, pyroxene, and plagioclase minerals. Grain shapes range from angular to subrounded. The sample composition indicates that the material was derived from phreatomagmatic eruptions partially with additional proportions of rock fragments from local lava flows. Grain shape and size indicate the materials were transported by aeolian processes rather than by fluvial processes. Spectral analyses reveal an initial hydration of all terrestrial samples. A spectral mineralogical correlation between the terrestrial and Martian aeolian sands shows a similarity consistent with an origin from volcanic ash and lava. We suggest that the Martian deposits may contain similar abundances of volcanic glass, which has not yet been distinguished in Martian spectral data. Copyright © 2011 John Wiley & Sons, Ltd.     

Advances in automated detection of sand dunes on Mars

This paper describes advances in an automatic approach for the detection of sand dunes of Mars, based on supervised learning techniques. A set of features (gradient histogram) is extracted from the remotely sensed images and two classifiers (Support Vector Machine and Random Forests) are trained from this data. The evaluation is conducted on 230 MOC‐NA images (spatial resolution between 1·45 and 6·80 m/pixel) leading to about 89% of correct detections. A detailed analysis of the detection results (dune/non‐dune) is performed by dune type or bulk shape, confirming high performances independently of the way the dataset is analysed. This demonstrates the robustness and adequacy of the automated approach to deal with the large variety of aeolian structures present on the surface of Mars. Copyright © 2012 John Wiley & Sons, Ltd.     

The influence of local topography for wind direction on Mars: two examples of dune fields in crater basins

In this work, we perform an analysis of large dark dunes within Moreux Crater and Herschel Crater on Mars using High Resolution Imaging Science Experiment (HiRISE) and Context Camera (CTX) data sets. These data allow us to conduct a detailed analysis of dune morphology and slip faces, concluding that the studied dune fields are influenced by topographically‐controlled complex wind directions. Our morphological analysis reveals that inside Moreux Crater in particular, the topographic setting dominates the wind flow direction, leading to the development of a sand transport pathway encircling the central peak of the crater. The dune fields in Herschel Crater are also affected by winds controlled by variable topography as suggested by the presence of complex dunes and dune fields. Our analysis indicate that the studied dune systems is not the result of paleo‐wind regimes. Furthermore, we perform thermal inertia measurements using thermal emission spectrometer (TES) data, which indicate that the studied dune fields consist of medium sand 250–500 µm in diameter. Copyright © 2012 John Wiley & Sons, Ltd.     

Evolution of the Tharsis region of Mars: insights from magnetic field observations

Mars Global Surveyor (MGS) observations of crustal magnetic fields over Tharsis provide new constraints on models for the thermal and magmatic evolution of this region. We analyze the distribution of magnetic field anomalies over Tharsis surface units of Noachian, Hesperian and Amazonian age. These data suggest that early Noachian crust underlies the Tharsis province, and formed contemporaneously with the existence of a martian dynamo. This crust either pre-dates the formation of Tharsis, or formed during the earlier phases of Tharsis volcanism. The preservation of strong magnetic field anomalies over some of the earliest Noachian and topographically high units, together with the observation of magnetic field anomalies over Hesperian- and Amazonian-age surface units, indicate that a large fraction of the magnetized crust has remained cool (below the blocking temperature of the magnetic carrier) throughout the construction of Tharsis. Moreover, the distributions of magnetic anomaly amplitudes over Noachian, Hesperian, and Amazonian surface units suggest that the youngest units overlie sites of prolonged intrusion and have undergone a greater extent of thermal demagnetization. The absence of magnetic anomalies around the Tharsis Montes and Olympus Mons argues for strong, localized heating, as would be expected at volcanic centers. We show that end-member models for progressive thermal demagnetization of a Noachian magnetized crustal layer are consistent with the anomaly amplitude distributions. We integrate the magnetic field observations with constraints from tectonics, gravity, and topography, and present a revised scenario for the evolution of the Tharsis region.     

The influence of dunes on mixing in a migrating salt‐wedge: Fraser River estuary,Canada

Dune bedforms and salt‐wedge intrusions are common features in many estuaries with sand beds, and yet little is known about the interactions between the two. Flow visualization with an echosounder and velocity measurements with an acoustic Doppler current profiler over areas of flat‐bed and sand dunes in the highly‐stratified Fraser River estuary, Canada, were used to examine the effect of dunes on interfacial mixing. As the salt‐wedge migrates upstream over the flat‐bed, mixing is restricted to the lower portion of the water column. However, as the salt‐wedge migrates into the dune field from the flat bed, there is a dramatic change in the flow, and large internal in‐phase waves develop over each of the larger dunes, with water from the salt‐wedge reaching the surface of the estuary. The friction Richardson number shows that bed friction is more important in interfacial mixing over the dunes than over the flat‐bed, and a plot of internal Froude Number versus obstacle (dune) height shows that the salt‐wedge flow over the dunes is mainly supercritical. Such bedforms can be expected to cause similar effects in interfacial mixing in other estuaries and sediment‐laden density currents, and may thus be influential in fluid mixing and sediment transport. Copyright © 2010 John Wiley & Sons, Ltd.     

Geochemical modeling of evaporation processes on Mars: Insight from the sedimentary record at Meridiani Planum

New data returned from the Mars Exploration Rover (MER) mission have revealed abundant evaporites in the sedimentary record at Meridiani Planum. A working hypothesis for Meridiani evaporite formation involves the evaporation of fluids derived from the weathering of martian basalt and subsequent diagenesis. On Earth, evaporite formation in exclusively basaltic settings is rare. However, models of the evaporation of fluids derived from experimentally weathering synthetic martian basalt provide insight into possible formation mechanisms. The thermodynamic database assembled for this investigation includes both Fe²⁺ and Fe³⁺ in Pitzer's ion interaction equations to evaluate Fe redox disequilibrium at Meridiani Planum. Modeling results suggest that evaporation of acidic fluids derived from weathering olivine-bearing basalt should produce Mg, Ca, and Fe-sulfates such as jarosite and melanterite. Calculations that model diagenesis by fluid recharge predict the eventual breakdown of jarosite to goethite as well as the preservation of much of the initial soluble evaporite component at modeled porosity values appropriate for relevant depositional environments (< 0.30). While only one of several possible formation scenarios, this simple model is consistent with much of the chemical and mineralogical data obtained on Meridiani Planum outcrop.     

Cross‐sectional profiles of sand ripples,megaripples, and dunes: a method for discriminating between formational mechanisms

Cross‐sectional profiles of sand ripples, megaripples, and sand dunes provide a useful tool for discriminating between formation by ripple and dune processes. Feature width, defined as the basal break in slope along the profile to either side of the crest, represents a good standard for comparison of profile attributes across more than three orders of magnitude. Aspect ratio (height/width) as a function of log width separates measurements into clusters representing differing mechanisms of formation. Scaling both height and distance for individual profiles by feature width facilitates comparison of profile shapes across three orders of magnitude in width. The data presented here should prove useful for evaluating possible mechanisms of origin for aeolian features observed remotely, including on planetary bodies. Copyright © 2012 John Wiley & Sons, Ltd.     

Locally modified surface winds on linear dunes as derived from directional raingauges

The study deals with local airflow patterns induced by 12–16 m high, partly vegetated dunes, under various prevailing wind conditions. Detailed empirical data were derived by means of an unconventional use of directional raingauges. The utility and accuracy of the method have been previously tested in a combined empirical and numerical study over ridges on a similar scale. Results are presented on the nature, extent and intensity of locally modified patterns of surface flow and on their dependence on wind conditions and slope inclination. Results may serve to verify or refine existing perceptions or models of the flow affecting sand movement over dunes. For example, deflected surface wind flow and even stationary vortices were found to form under winds with resultant angles of incidence of only 5–10° relative to the axes of the dunes, and on slopes inclined only 12–15°, i.e. lower angles than often assumed in previous studies of dune morphology. Various effects of the resulting surface flow on the dune ecosystem are also referred to, e.g. the distribution of settling dust and seed dispersal. Copyright © 2002 John Wiley & Sons, Ltd.     

Mars Global Digital Dune Database (MGD3): north polar region (MC‐1) distribution,applications, and volume estimates

The Mars Global Digital Dune Database (MGD³) now extends from 90°N to 65°S. The recently released north polar portion (MC‐1) of MGD³ adds ~844 000 km² of moderate‐ to large‐size dark dunes to the previously released equatorial portion (MC‐2 to MC‐29) of the database. The database, available in GIS‐ and tabular‐format in USGS Open‐File Reports, makes it possible to examine global dune distribution patterns and to compare dunes with other global data sets (e.g. atmospheric models). MGD³ can also be used by researchers to identify areas suitable for more focused studies. The utility of MGD³ is demonstrated through three example applications. First, the uneven geographic distribution of the dunes is discussed and described. Second, dune‐derived wind direction and its role as ground truth for atmospheric models is reviewed. Comparisons between dune‐derived winds and global and mesoscale atmospheric models suggest that local topography may have an important influence on dune‐forming winds. Third, the methods used here to estimate north polar dune volume are presented and these methods and estimates (1130 km³ to 3250 km³) are compared with those of previous researchers (1158 km³ to 15 000 km³). In the near future, MGD³ will be extended to include the south polar region. Published in 2011. This article is a US Government work and is in the public domain in the USA.     

Bomb‐fallout 137Cs as a marker of geomorphic stability in dune sands and soils,Pinery Provincial Park,Ontario, Canada

The anthropogenic radionuclide ¹³⁷Cs has been extensively utilized as a tracer of geomorphic processes in the northern hemisphere since its deposition during atmospheric testing of nuclear devices in the 1950s and 1960s. The distribution of bomb‐fallout ¹³⁷Cs was measured on a sequence of coastal dune sands and soils at Pinery Provincial Park, on the coast of Lake Huron in southern Ontario, Canada. The depth distribution within the stabilized, developed soils inland reflected the relationship between clay content and the adsorption and immobilization of the radionuclide. However, the influence of soil organic matter, silt‐sized particles and vegetation cycling on the profile distribution could not be discounted. Within the geomorphically dynamic dune sands near the coast, there was a significant activity of ¹³⁷Cs even though the sands were lacking in clay‐sized particles. Within a buried soil on the inland side of a large active dune blowout, the distribution of ¹³⁷Cs with depth was useful as a stratigraphic marker of the rates of accumulation of sands at that position. Therefore ¹³⁷Cs may be a useful alternative to erosion pins, sequential air photos and sediment traps in the monitoring of dune destabilization in coastal environments. Copyright © 2001 John Wiley & Sons, Ltd.     

Sedimentary rocks at Meridiani Planum: Origin, diagenesis, and implications for life on Mars

The MER rover Opportunity has carried out the first outcrop-scale investigation of ancient sedimentary rocks on Mars. The rocks, exposed in craters and along fissures in Meridiani Planum, are sandstones formed via the erosion and re-deposition of fine grained siliciclastics and evaporites derived from the chemical weathering of olivine basalts by acidic waters. A stratigraphic section more than seven meters thick measured in Endurance crater is dominated by eolian dune and sand sheet facies; the uppermost half meter, however, exhibits festoon cross lamination at a length scale that indicates subaqueous deposition, likely in a playa-like interdune setting. Silicates and sulfate minerals dominate outcrop geochemistry, but hematite and Fe3D3 (another ferric iron phase) make up as much as 11% of the rocks by weight. Jarosite in the outcrop matrix indicates precipitation at low pH. Cements, hematitic concretions, and crystal molds attest to a complex history of early diagenesis, mediated by ambient ground waters. The depositional and early diagenetic paleoenvironment at Meridiani was arid, acidic, and oxidizing, a characterization that places strong constraints on astrobiologial inference.     

Tephra deposition on glaciers and ice sheets on Mars: Influence on ice survival, debris content and flow behavior

We examine the consequences of pyroclastic deposits being emplaced onto ice layers on Mars, both those in the polar caps and those forming glaciers on the flanks of some of the large shield volcanoes. We show that layers of pyroclasts greater than a few meters in thickness, whether emplaced cold (as fall deposits) or hot (as pyroclastic density current deposits) act almost exclusively to protect ice layers beneath them from sublimation, irrespective of whether they are emplaced at high or low elevations or high or low latitudes. Layers less than about 2 m thick, on the other hand, can cause significant ice loss by raising the surface temperature due to their low albedo and then transmitting that increased temperature to the underlying ice, especially on a diurnal time scale. This can have a significant bearing on the emplacement history of polar water ice and on the survival time of glacial ice on shield volcano flanks. A key factor in the latter case is the timing of the episodic volcanic activity relative to the cycles of climate change driven by Mars' obliquity and eccentricity variations.     

A bed‐load transport model for rough turbulent open‐channel flows on plane beds

Data from flume studies are used to develop a model for predicting bed‐load transport rates in rough turbulent two‐dimensional open‐channel flows moving well sorted non‐cohesive sediments over plane mobile beds. The object is not to predict transport rates in natural channel flows but rather to provide a standard against which measured bed‐load transport rates influenced by factors such as bed forms, bed armouring, or limited sediment availability may be compared in order to assess the impact of these factors on bed‐load transport rates. The model is based on a revised version of Bagnold's basic energy equation i_bs_b = e_bω, where i_b is the immersed bed‐load transport rate, ω is flow power per unit area, e_b is the efficiency coefficient, and s_b is the stress coefficient defined as the ratio of the tangential bed shear stress caused by grain collisions and fluid drag to the immersed weight of the bed load. Expressions are developed for s_b and e_b in terms of G, a normalized measure of sediment transport stage, and these expressions are substituted into the revised energy equation to obtain the bed‐load transport equation i_b = ω G ^3·4. This equation applies regardless of the mode of bed‐load transport (i.e. saltation or sheet flow) and reduces to i_b = ω where G approaches 1 in the sheet‐flow regime. That i_b = ω does not mean that all the available power is dissipated in transporting the bed load. Rather, it reflects the fact that i_b is a transport rate that must be multiplied by s_b to become a work rate before it can be compared with ω. It follows that the proportion of ω that is dissipated in the transport of bed load is i_bs_b/ω, which is approximately 0·6 when i_b = ω. It is suggested that this remarkably high transport efficiency is achieved in sheet flow (1) because the ratio of grain‐to‐grain to grain‐to‐bed collisions increases with bed shear stress, and (2) because on average much more momentum is lost in a grain‐to‐bed collision than in a grain‐to‐grain one. Copyright © 2006 John Wiley & Sons, Ltd.     

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

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

Late‐Holocene buried forests on the Oregon Coast

Deposits of late‐Holocene beach sand buried conifer forests episodically emerge on beaches of the Oregon coast. Simultaneously, sand dunes buried late‐Holocene forests growing on marine terraces landward of the beaches. Dune ramps, up to 60 m in elevation, connected the beach and dune deposits. The average age of wood samples from stumps rooted on the shore platforms is 3·07 ± 1·45 ka. The average age of wood and charcoal samples embedded in forest soil on the marine terraces is 3·27 ± 1·46 ka. Between 1994 and 2006, winter storm waves exposed more than 4·5 km² of late‐Holocene forest soil on shore platforms at 19 localities. Rooted stumps without soil were uncovered at an additional 14 localities. Once exposed, wave action eroded the soil rapidly (one to two years). The intact forest soil and roots on the shore platforms must have been nearly continuously buried, protected and preserved prior to recent exposure. The late‐Holocene buried forest provides the basis for a conceptual model of coastal evolution. A three stage reversal of erosion and sand supply must have occurred: (1) wave erosion switched to seaward advancement of forests, (2) forest growth and soil development switched to burial beneath beach and dune sand and (3) burial and preservation switched to wave erosion, truncation of dune ramps and landward retreat of sea cliffs. Copyright © 2006 John Wiley & Sons, Ltd.     

A method for semi‐automated objective quantification of linear bedforms from multi‐scale digital elevation models

The increasing availability of large, detailed digital representations of the Earth's surface demands the application of objective and quantitative analyses. Given recent advances in the understanding of the mechanisms of formation of linear bedform features from a range of environments, objective measurement of their wavelength, orientation, crest and trough positions, height and asymmetry is highly desirable. These parameters are also of use when determining observation‐based parameters for use in many applications such as numerical modelling, surface classification and sediment transport pathway analysis. Here, we (i) adapt and extend extant techniques to provide a suite of semi‐automatic tools which calculate crest orientation, wavelength, height, asymmetry direction and asymmetry ratios of bedforms, and then (ii) undertake sensitivity tests on synthetic data, increasingly complex seabeds and a very large‐scale (39 000 km²) aeolian dune system. The automated results are compared with traditional, manually derived, measurements at each stage. This new approach successfully analyses different types of topographic data (from aeolian and marine environments) from a range of sources, with tens of millions of data points being processed in a semi‐automated and objective manner within minutes rather than hours or days. The results from these analyses show there is significant variability in all measurable parameters in what might otherwise be considered uniform bedform fields. For example, the dunes of the Rub' al Khali on the Arabian peninsula are shown to exhibit deviations in dimensions from global trends. Morphological and dune asymmetry analysis of the Rub' al Khali suggests parts of the sand sea may be adjusting to a changed wind regime from that during their formation 100 to 10 ka BP. Copyright © 2012 John Wiley & Sons, Ltd.     

Modelling the effect of time‐dependent river dune evolution on bed roughness and stage

This paper presents an approach to incorporate time‐dependent dune evolution in the determination of bed roughness coefficients applied in hydraulic models. Dune roughness is calculated by using the process‐based dune evolution model of Paarlberg et al. ( 2009 ) and the empirical dune roughness predictor of Van Rijn ( 1984 ). The approach is illustrated by applying it to a river of simple geometry in the 1‐D hydraulic model SOBEK for two different flood wave shapes. Calculated dune heights clearly show a dependency on rate of change in discharge with time: dunes grow to larger heights for a flood wave with a smaller rate of change. Bed roughness coefficients computed using the new approach can be up to 10% higher than roughness coefficients based on calibration, with the largest differences at low flows. As a result of this larger bed roughness, computed water depths can be up to 15% larger at low flow. The new approach helps to reduce uncertainties in bed roughness coefficients of flow models, especially for river systems with strong variations in discharge with time. Copyright © 2010 John Wiley & Sons, Ltd.     

The factors influencing the abrasion efficiency of saltating grains on a clay‐crusted playa

The entrainment and subsequent transport of PM₁₀ (particulate matter <10 µm) has become an important and challenging focus of research for both scientific and practical applications. Arid and semi‐arid environments are important sources for the atmospheric loading of PM₁₀, although the emission of this material is often limited by surface crusts. It has been suggested that the primary mechanisms through which PM₁₀ is released from a crusted surface are abrasion by saltating grains or disturbance by agricultural and recreational activities. To examine the importance of saltation abrasion in the emission of PM₁₀, a series of field wind tunnel tests were conducted on a clay‐crusted surface near Desert Wells, Arizona. In a previous part of this study it was found that the emission rate varies linearly with the saltation transport rate, although there can be considerable variation in this relationship. This paper more closely examines the source of the variability in the abrasion efficiency, the amount of PM₁₀ emitted by a given quantity of saltating grains. The abrasion efficiency was found to vary with the susceptibility of the surface to abrasion, the ability of the sand to abrade that surface and the availability of material with a caliper size <10 µm within the crust. Specifically, the results of the study show that the abrasion efficiency is related to the crust strength, the amount of surface disturbance and the velocity of the saltating grains. It is concluded that the spatial and temporal variability of these controls on the abrasion efficiency imposes severe contextual limitations on experimentally derived models, and can make theoretical models too complex and impractical to be of use. Copyright­© 2001 John Wiley & Sons, Ltd.     

A distribution‐free statistical method for the spatial characterization of dune granulometry: an example from the Strzelecki Desert,South Australia

Distribution‐free statistical methods of comparative data analysis have identi?ed subtle granulometric differences attributed to the evolution of barchan form at Gurra‐Gurra waterhole. Geomorphic locations on the barchan dunes display statistically signi?cant grain‐size differences that assist in the interpretation of aeolian processes. In summer, very ?ne sands mantle the dunescape and are the fraction that most affects the parameters of sorting and skewness. The sur?cial sedimentological character is one of subtle contrasts between the processes of grain winnowing and intergranular protection. The second and third moment measures are parameters that best demonstrate the spatial granulometric differences. Dune‐forming processes at Gurra‐Gurra have produced dune sands that have a very narrow range of grain size, which, in turn, re?ects textural and mineralogical maturity, and hence an extensive transport history. The statistical techniques employed in this study can also be used for the comparison of temporal (seasonal) sedimentological change, and for the granulometric analysis and association of process for dunes of different morpho‐types. Copyright © 2004 John Wiley & Sons, Ltd.     

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

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