Aeolian Bedforms Research Articles

Surface waves in aeolian bedforms

Nature repeat the stripe pattern in aeolian bedform twice, one is sand ripples, the other is linear dunes. Being stripes, these two share many common features, e.g., the whole appearance, the processes of development from a homogeneous state, and the decorated defects. By computer simulations using coupled map lattices, the universal mechanism being at work behind the similarities is revealed. Furthermore, the pattern selection at distinct scales of ripples and linear dunes is discussed. So the results of ripple scale experiments may be used to infer dune scale property.

Comment on “Minimal size of a barchan dune”

It is now an accepted fact that the size at which dunes form from a flat sand bed as well as their "minimal size" scales on the flux saturation length. This length is by definition the relaxation length of the slowest mode toward equilibrium transport. The model presented by Parteli, Durán, and Herrmann [Phys. Rev. E 75, 011301 (2007)] predicts that the saturation length decreases to zero as the inverse of the wind shear stress far from the threshold. We first show that their model is not self-consistent: even under large wind, the relaxation rate is limited by grain inertia and thus cannot decrease to zero. A key argument presented by these authors comes from the discussion of the typical dune wavelength on Mars (650 m) on the basis of which they refute the scaling of the dune size with the drag length evidenced by Claudin and Andreotti [Earth Planet. Sci. Lett. 252, 30 (2006)]. They instead propose that Martian dunes, composed of large grains (500 microm), were formed in the past under very strong winds. We emphasize that this saltating grain size, estimated from thermal diffusion measurements, is far from straightforward. Moreover, the microscopic photographs taken by the rovers on Martian Aeolian bedforms show a grain size of 87+/-25 microm together with hematite spherules at millimeter scale. As those so-called "blueberries" cannot be entrained more frequently than a few hours per century, we conclude that the saltating grains on Mars are the small ones, which gives a second strong argument against the model of Parteli.

Overview of the Opportunity Mars Exploration Rover Mission to Meridiani Planum: Eagle Crater to Purgatory Ripple

The Mars Exploration Rover Opportunity touched down at Meridiani Planum in January 2004 and since then has been conducting observations with the Athena science payload. The rover has traversed more than 5 km, carrying out the first outcrop‐scale investigation of sedimentary rocks on Mars. The rocks of Meridiani Planum are sandstones formed by eolian and aqueous reworking of sand grains that are composed of mixed fine‐grained siliciclastics and sulfates. The siliciclastic fraction was produced by chemical alteration of a precursor basalt. The sulfates are dominantly Mg‐sulfates and also include Ca‐sulfates and jarosite. The stratigraphic section observed to date is dominated by eolian bedforms, with subaqueous current ripples exposed near the top of the section. After deposition, interaction with groundwater produced a range of diagenetic features, notably the hematite‐rich concretions known as “blueberries.” The bedrock at Meridiani is highly friable and has undergone substantial erosion by wind‐transported basaltic sand. This sand, along with concretions and concretion fragments eroded from the rock, makes up a soil cover that thinly and discontinuously buries the bedrock. The soil surface exhibits both ancient and active wind ripples that record past and present wind directions. Loose rocks on the soil surface are rare and include both impact ejecta and meteorites. While Opportunity's results show that liquid water was once present at Meridiani Planum below and occasionally at the surface, the environmental conditions recorded were dominantly arid, acidic, and oxidizing and would have posed some significant challenges to the origin of life.

Mössbauer mineralogy of rock, soil, and dust at Meridiani Planum, Mars: Opportunity's journey across sulfate‐rich outcrop, basaltic sand and dust, and hematite lag deposits

The Mössbauer (MB) spectrometer on Opportunity measured the Fe oxidation state, identified Fe‐bearing phases, and measured relative abundances of Fe among those phases at Meridiani Planum, Mars. Eight Fe‐bearing phases were identified: jarosite (K,Na,H3O)(Fe,Al)(OH)6(SO4)2, hematite, olivine, pyroxene, magnetite, nanophase ferric oxides (npOx), an unassigned ferric phase, and metallic Fe (kamacite). Burns Formation outcrop rocks consist of hematite‐rich spherules dispersed throughout S‐rich rock that has nearly constant proportions of Fe3+ from jarosite, hematite, and npOx (29%, 36%, and 20% of total Fe). The high oxidation state of the S‐rich rock (Fe3+/FeT ∼ 0.9) implies that S is present as the sulfate anion. Jarosite is mineralogical evidence for aqueous processes under acid‐sulfate conditions because it has structural hydroxide and sulfate and it forms at low pH. Hematite‐rich spherules, eroded from the outcrop, and their fragments are concentrated as hematite‐rich soils (lag deposits) on ripple crests (up to 68% of total Fe from hematite). Olivine, pyroxene, and magnetite are primarily associated with basaltic soils and are present as thin and locally discontinuous cover over outcrop rocks, commonly forming aeolian bedforms. Basaltic soils are more reduced (Fe3+/FeT ∼ 0.2–0.4), with the fine‐grained and bright aeolian deposits being the most oxidized. Average proportions of total Fe from olivine, pyroxene, npOx, magnetite, and hematite are ∼33%, 38%, 18%, 6%, and 4%, respectively. The MB parameters of outcrop npOx and basaltic‐soil npOx are different, but it is not possible to infer mineralogical information beyond octahedrally coordinated Fe3+. Basaltic soils at Meridiani Planum and Gusev crater have similar Fe‐mineralogical compositions.

Late Quaternary dune accumulation along the western margin of South Africa: distinguishing forcing mechanisms through the analysis of migratory dune forms

The west coast of South Africa is situated at a critical location between temperate and subtropical oceanic and atmospheric circulation systems, and palaeoenvironmental proxies from this region have the potential to elucidate issues concerning variations within these systems over glacial–interglacical cycles. While semi-arid climates have hindered the preservations of organic proxies, a variety of aeolian bedforms have been analysed in an effort to improve our understanding of environmental change in the region. Optically stimulated luminescence measurements of 51 samples from 15 reticulate dune sites along the west coast have enabled the identification of phases of aeolian activity, as well as periods of relative geomorphic stability. Combined with data derived from previous studies of the region's sediment accumulating deposits and other palaeoenvironmental proxies, periods of increased windiness are identified at 16–24, 30–33, 43–49 and 63–73 ka From approximately 17–12 ka, decreasing transport capacity resulted in the stabilisation of the west coast's dune fields. During the Holocene Altithermal (∼ 4–8 ka), despite reduced wind strength, increased aridity resulting from higher temperatures and a reduced influence of moisture bearing westerly systems appears to have trigged widespread remobilisation of the region's dune fields. The combination of ages from a variety of dune forms with different development mechanisms, and comparisons with a range of proxy data sources, have allowed for an enhanced interpretation of the region's aeolian archives, moving beyond simple correlations between dune activity and “aridity.”

Coastal aeolian dune development, Sólheimasandur, southern Iceland

A coastal aeolian dune field on the distal parts of Sólheimasandur, southern Iceland is currently undergoing active construction. Ongoing sand deflation in the foreshore is demonstrated by the preservation of sand ridges in the downwind lee of obstacles, while a backshore sandsheet is characterised by actively accumulating long-wavelength wind ripples. The main region of aeolian accumulation is in the immediate backshore where sand has accumulated to a thickness of up to 8 m and is largely stabilized by vegetation. Surrounding interdune areas have undergone repeated flooding by fluvial incursions from the adjacent Jökulsá River, and are characterised by mud drapes which lap against the lower flanks of neighbouring dunes. Widespread sand adhesion structures testify to the often damp nature of the substrate. Over 1200 vegetated and stabilized dunes (nebkhas) occupy a 5 km 2 area of wind-deflated gravel sandur plain downwind from the main site of aeolian accumulation. The nebkhas show strong relationships between plan form shape and height, indicating that they have attained equilibrium morphology for the prevailing climatic conditions. Within the nebkha field, dunes demonstrate a predictable downwind decrease in height, spatial concentration and degree of sand stabilization. A time series of aerial photographs documents the temporal evolution of the dune field and demonstrates that aeolian accumulation commenced about 1940 and has been ongoing to the present day. Prior to 1940, repeated outburst floods associated with catastrophic lake drainage events from the Sólheimajökull glacier 8 km inland were of sufficient magnitude to repeatedly destroy any incipient aeolian bedforms that developed. Although strong onshore winds and high rates of sediment supply promote aeolian sediment transport, dune field development is rare in humid boreal and sub-arctic climates as encountered in southern Iceland because the availability of sediment for aeolian transport is usually severely limited by damp substrate conditions. Thus, the airflow tends to be undersaturated with respect to its potential sand transporting capacity and dune construction can only occur because of the stabilizing influence of vegetation.

Two earth years of Mössbauer studies of the surface of Mars with MIMOS II

The element iron plays a crucial role in the study of the evolution of matter from an interstellar cloud to the formation and evolution of the planets. In the Solar System iron is the most abundant metallic element. It occurs in at least three different oxidation states: Fe(0) (metallic iron), Fe(II) and Fe(III). Fe(IY) and Fe(VI) compounds are well known on Earth, and there is a possibility for their occurrence on Mars. In January 2004 the USA space agency NASA landed two rovers on the surface of Mars, both carrying the Mainz Mossbauerspectrometer MIMOS II. They performed for the first time in-situ measurements of the mineralogy of the Martian surface, at two different places on Mars, Meridiani Planum and Gusev crater, respectively, the landing sites of the Mars-Exploration-Rovers (MER) Opportunity and Spirit. After about two Earth years or one Martian year of operation the Mossbauer (MB) spectrometers on both rovers have acquired data from more than 150 targets (and more than thousand MB spectra) at each landing site. The scientific measurement objectives of the Mossbauer investigation are to obtain for rock, soil, and dust (1) the mineralogical identification of iron-bearing phases (e.g., oxides, silicates, sulfides, sulfates, and carbonates), (2) the quantitative measurement of the distribution of iron among these ironbearing phases (e.g., the relative proportions of iron in olivine, pyroxenes, ilmenite and magnetite in a basalt), (3) the quantitative measurement of the distribution of iron among its oxidation states (e.g., Fe2+, Fe3+, and Fe6+), and (4) the characterization of the size distribution of magnetic particles. Special geologic targets of the Mossbauer investigation are dust collected by the Athena magnets and interior rock and soil surfaces exposed by the Athena Rock Abrasion Tool and by trenching with rover wheels. The Mossbauer spectrometer on Opportunity at Meridiani Planum, identified eight Fe-bearing phases: jarosite (K,Na,H30)(Fe,AI)(OH)6(S04)2, hematite, olivine, pyroxene, magnetite, nanophase ferric oxides (npOx), an unassigned ferric phase, and a metallic Fe-Ni alloy (kamacite) in a Fe-Nimeteorite. Outcrop rocks consist ofhematite-rich spherules dispersed throughout S-rich rock that has nearly constant proportions of Fe3+ from jarosite, hematite, and npOx (28%, 35%, and 19% of total Fe). Jarosite is mineralogical evidence for aqueous processes under acidsulfate conditions because it has structural hydroxide and sulfate and it forms at low pH. Hematite-rich spherules, eroded from the outcrop, and their fragments are concentrated as hematite-rich soils (lag deposits) on ripple crests (up to 68% of total Fe from hematite). Olivine, pyroxene, and magnetite are primarily associated with basaltic soils and are present as thin and locally discontinuous cover over outcrop rocks, commonly forming aeolian bedforms. Basaltic soils are more reduced (Fe3+/IFetotal ∼0.2–0.4), with the fine-grained and bright aeolian deposits being the most oxidized. Basaltic soil at Meridiani Planum and Gusev crater have similar Fe-mineralogical compositions. At Gusev crater, the Mossbauer spectrometer on the MER Spirit rover has identified 8 Fe-bearing phases. Two are Fe2+ silicates (olivine and pyroxene), one is a Fe2+ oxide (ilmenite), one is a mixed Fe2+ and Fe3+ oxide (magnetite), two are Fe3+ oxides (hematite and goethite), one is a Fe3+ sulfate (mineralogically not constrained), and one is a Fe3+ alteration product (npOx). The surface material in the plains have a olivine basaltic signature (Morris, et al., Science, 305: 833, 2004; Morris, et al., J. Geophys. Res., 111, 2006, Ming, et al., J. Geophys. Res., 111, 2006) suggesting physical rather than chemical weathering processes present in the plains. The Mossbauer signature for the Columbia Hills surface material is very different ranging from nearly unaltered material to highly altered material. Some of the rocks, in particular a rock named Clovis, contain a significant amount of the Fe oxyhydroxide goethite, α-FeOOH, which is mineralogical evidence for aqueous processes because it is formed only under aqueous conditions.

Overview of the Microscopic Imager Investigation during Spirit's first 450 sols in Gusev crater

The Microscopic Imager (MI) on the Mars Exploration Rover Spirit has returned images of Mars with higher resolution than any previous camera system, allowing detailed petrographic and sedimentological studies of the rocks and soils at the Gusev landing site. Designed to simulate a geologist's hand lens, the MI is mounted on Spirit's instrument arm and can resolve objects 0.1 mm in size or larger. This paper provides an overview of MI operations, data calibration, processing, and analysis of MI data returned during the first 450 sols (Mars days) of the Spirit landed mission. The primary goal of this paper is to facilitate further analyses of MI data by summarizing the methods used to acquire and process the data, the radiometric and geometric accuracy of MI data products, and the availability of archival products. In addition, scientific results of the MI investigation are summarized. MI observations show that poorly sorted soils are common in Gusev crater, although aeolian bedforms have well‐sorted coarse sand grains on their surfaces. Abraded surfaces of plains rocks show igneous textures, light‐toned veins or fracture‐filling minerals, and discrete coatings. The rocks in the Columbia Hills have a wide variety of granular textures, consistent with volcaniclastic or impact origins. Case hardening and submillimeter veins observed in the rocks as well as soil crusts and cemented clods imply episodic subsurface aqueous fluid movement, which has altered multiple geologic units in the Columbia Hills. The MI also monitored Spirit's solar panels and the magnets on the rover's deck.

Geology of the Gusev cratered plains from the Spirit rover transverse

The cratered plains of Gusev traversed by Spirit are generally low‐relief rocky plains dominated by impact and eolian processes. Ubiquitous shallow, soil‐filled, circular depressions, called hollows, are modified impact craters. Rocks are dark, fine‐grained basalts, and the upper 10 m of the cratered plains appears to be an impact‐generated regolith developed over intact basalt flows. Systematic field observations across the cratered plains identified vesicular clasts and rare scoria similar to original lava flow tops, consistent with an upper inflated surface of lava flows with adjacent collapse depressions. Crater and hollow morphometry are consistent with most being secondaries. The size‐frequency distribution of rocks >0.1 m diameter generally follows exponential functions similar to other landing sites for total rock abundances of 5–35%. Systematic clast counts show that areas with higher rock abundance and more large rocks have higher thermal inertia. Plains with lower thermal inertia have fewer rocks and substantially more pebbles that are well sorted and evenly spaced, similar to a desert pavement or lag. Eolian bed forms (ripples and wind tails) have coarse surface lags, and many are dust covered and thus likely inactive. Deflation of the surface ∼5–25 cm likely exposed two‐toned rocks and elevated ventifacts and transported fines into craters creating the hollows. This observed redistribution yields extremely slow average erosion rates of ∼0.03 nm/yr and argues for very little long‐term net change of the surface and a dry and desiccating environment similar to today's since the Hesperian (or ∼3 Ga).

Analysis of large aeolian (wind-blown) bedforms using the Shuttle Radar Topography Mission (SRTM) digital elevation data

Dry land areas cover large parts of the land masses of Earth. Of these a large portion is mantled by aeolian (wind-blown) deposits and subjected to aeolian process, i.e., the transport of sand or dust by wind. One of the most prominent morphologies created by wind-driven deposits are dunes and in their larger form draas or mega dunes. These mega dunes tend to be widely spaced; very large; often forming a base on which smaller dunes form, producing a compound dune pattern. In February 2000 the Shuttle Radar Topography Mission, onboard the space shuttle Endeavour flew an interferometric radar system whose objective was to create a DEM of nearly all Earth land masses. In this paper DEM data from the C-band 90 m data set and the X-band 30 m data set were used to map and characterize the height and spacing of the large sand seas (Ergs) on Earth. Only the larger forms of dunes could be mapped reliably. The dune spacing and heights extracted from the SRTM DEMs tended to be in overall agreement with those reported in the literature. In a comparison between the X-band and the C-band data it was found that the X-band data are more sensitive to the smaller scale undulations on the compound dunes and better revealed the full height of the dunes.

A mathematical model for aeolian megaripples on Mars

The recent mathematical model for megaripples formation proposed by Yizhaq [Physica A 338 (2004) 211] is used to explain the huge megaripples formation observed on Mars. The model also predicts a few linearly unstable modes which correspond to megaripples with different wavelengths at the same wind condition as seen in some places on Mars. Studying these aeolian bedforms can provide information about the past and present climatic conditions on Mars.

Mars Exploration Rover candidate landing sites as viewed by THEMIS

The analysis of six landing sites that were candidates for the two NASA Mars Exploration Rovers (MER) benefited from recently available image data from the Thermal Emission Imaging Spectrometer (THEMIS) onboard the 2001 Mars Odyssey spacecraft. The combination of daytime and nighttime thermal infrared images from THEMIS supplemented by additional data sets has lead to new or expanded insights into the nature of each landing site. In Meridiani Planum, a layer of lighter-toned, higher thermal inertia material is observable just below the hematite-bearing layer. Gusev Crater displays a more complex stratigraphy than previously observed, including an upper layer with lobate margins. The highest inertia unit of southern Isidis Planitia is confined to topographic lows in the rim/basin margin and does not appear to be due to highland material transported onto the basin floor. The enigmatic, ovoid, blocky terrain on the floor of Melas Chasma displays higher thermal inertia than its surroundings, an indication that it contains coarser or more indurated material than the adjacent aeolian bedforms. The myriad channels of Athabasca Valles display distinctive thermal signatures despite the presence of a bright layer of dust covering the region. The presence of alluvial fans produced from spur-and-gulley erosion of the walls of Eos Chasma demonstrates that mass movements have occurred following the canyon scouring floods.

Stratigraphic evolution of an aeolian erg margin system: the Permian Cedar Mesa Sandstone, SE Utah, USA

AbstractThe Permian Cedar Mesa Sandstone of south‐east Utah is a predominantly aeolian succession that exhibits a complex spatial variation in sedimentary architecture which, in terms of palaeogeographic setting, reflects a transition from a dry erg centre, through a water table‐controlled aeolian‐dominated erg margin, to an outer erg margin subject to periodic fluvial incursion. The erg margin succession represents a wet aeolian system, accumulation of which was controlled by progressive water table rise coupled with ongoing dune migration and associated changes in the supply and availability of sediment for aeolian transport. Variation in the level of the water table relative to the depositional surface determined the nature of interdune sedimentary processes, and a range of dry, damp and wet (flooded) interdune elements is recognized. Variations in the geometry of these units reflect the original morphology and the migratory behaviour of spatially isolated dry interdune hollows in the erg centre, locally interconnected damp and/or wet interdune ponds in the aeolian‐dominated erg margin and fully interconnected, fluvially flooded interdune corridors in the outer erg margin. Relationships between aeolian dune and interdune units indicate that dry, damp and wet interdune sedimentation occurred synchronously with aeolian bedform migration. Temporal variation in the rates of water‐table rise and bedform migration determined the angle of climb of the erg margin succession, such that accumulation rates increased during periods of rapidly rising water table, whereas sediment bypassing (zero angle of climb) occurred in the aftermath of flood events in response to periods of elevated but temporarily static water table. During these periods in the outer erg margin, the expansion of fluvially flooded interdunes in front of non‐climbing but migrating dunes resulted in the amalgamation of laterally adjacent interdunes and the generation of regionally extensive bypass (flood) supersurfaces. A spectrum of genetic depositional models is envisaged that accounts for the complex spatial and temporal evolution of the Cedar Mesa erg margin succession.

Coastal dune fields at the São Francisco River strandplain, northeastern Brazil: morphology and environmental controls

AbstractExtensive coastal dune fields occur on the Quaternary strandplain associated with the São Francisco River mouth. Two different generations of dunes are identified. One is inactive, already fixed by vegetation, comprising parabolic dunes. The other generation is active, bordering the present‐day shoreline and transgressing over the inactive dune field. Three morphological provinces in the active coastal dune fields are recognized. On the updrift side of the São Francisco River mouth, they are: (a) sand‐sheet with shrub coppice and shadow dunes; (b) isolated dunes of the barchan‐transversal type up to 5 m high, and interdune areas; and (c) a 23 m high compound dune, with superimposed small dunes. The same provinces are recognized on the downdrift side of the river mouth, with two important exceptions: the barchan‐transversal and compound dunes are replaced, respectively, by (i) zibar‐type dunes up to 5 m high, and (ii) a 19 m high precipitation dune, which is associated with numerous blowouts. The prevailing eastern winds from August to January favour the development of the aeolian bedforms and the migration of dunes. The shoreline orientation almost transversal to the winds and the great supply of fine‐grained sediments contribute to the formation of barchan‐transversal types and compound dunes in the updrift side. On the other hand, in the downdrift side the shoreline orientation is almost parallel to the prevailing winds. This fact, in association with a coarser grain size in the beachface, favours the formation of zibar‐type and precipitation dunes with numerous blowouts. The rate of migration of individual dunes is about 20 to 24 m per year. This study suggests that the aeolian sedimentation is a relatively recent phenomenon at the Quaternary strandplain of the São Francisco River. The first generation of dune fields initiated some time after 3000 years BP and the second generation originated some centuries ago. Copyright © 2004 John Wiley & Sons, Ltd.

Chronologies for Late Quaternary barchan dune reactivation in the southeastern Arabian Peninsula

The Liwa region of the United Arab Emirates contains some of the largest and most areally extensive megabarchanoid sand dunes on a global scale. Here we present optical dating results on samples of aeolian sediment from deep drill cores extracted from the largest dune field of the Liwa area. Optical dating of these core sediments using the single aliquot regeneration protocol indicates Mid–Late Holocene phases of rapid dune deposition, the most recent period of reactivation began at ca 2.8 ka. This event was preceded by a period of deposition at ca 5 ka. These results suggest that the dune systems of the southeastern Arabian Peninsula are closely linked to changes in Late Quaternary global climate, particularly linked to the intensity and spatial extent of palaeomonsoon rainfall. Since the last precessional maxima at ca 9 ka, at which time a peak in monsoonal rainfall has been recognised, a significant environmental transition to widespread desert conditions occurred in an apparently abrupt fashion. During the initial period of aridification, large quantities of sand were transported and deposited in the form of large and very large (up to 160 m high) scale aeolian bedforms. Following the initial phase of aeolian accumulation, the system appears to have remained in stasis.

Predicting distribution of remobilized aeolian facies using sub-surface data: the Weissliegend of the UK Southern North Sea

Aeolian sandstones of the Permian Rotliegend Group comprise the principal gas reservoirs of the Southern North Sea Basin. The upper portion of the reservoir interval comprises a unit informally called the ‘Weissliegend’. This unit comprises mass-flow and in-situ soft-sediment deformed deposits formed when the Zechstein Sea catastrophically flooded the Rotliegend sand sea. The Weissliegend typically exhibits poorer reservoir properties than the underlying Rotliegend reservoirs and its thickness and distribution are notoriously difficult to map. Hanging vertical sections on a regionally extensive, intra-Rotliegend super-surface has led to the recognition of a preserved topography at the top of the Rotliegend. The thickness differences within the unit between the super-surface and the top Rotliegend (termed here the Upper Aeolian Unit, UAU) represent the original dune topography modified by flood-related processes, and provide an insight into the nature and scale of the aeolian bedforms that existed within the basin prior flooding. The relationship between known Weissliegend distribution and this topography provides a predictive tool for understanding Weissliegend distribution. Mapping the preserved topography reveals preserved bedforms up to 85 m high, with a bedform spacing of 8 to 10 km. The dunes deposited ‘transverse type’ strata but the scale and spacing of the bedforms are too big compared with modern transverse dunse. Instead a more complex model is proposed. The Weissliegend has highly variable thickness (0–26 m) and is commonly thicker within the interdune hollows. The model for a preserved topography at the top of the Rotliegend is further supported by thickness trends in the overlying Zechstein units which increase into the proposed interdune lows.

Stratigraphic evolution and preservation of aeolian dune and damp/wet interdune strata: an example from the Triassic Helsby Sandstone Formation, Cheshire Basin, UK

Abstract New and previously published models of wet aeolian system evolution form a spectrum of types that may be explained in terms of aeolian dune dynamics, rate of water table rise and/or periodicity of interdune flooding. This is illustrated with an example from the Mid‐Triassic (Anisian) Helsby Sandstone Formation, Cheshire, UK. Lenses of damp and wet interdune strata exhibit an intertonguing, transitional relationship with the toe‐sets of overlying aeolian dune units. This signifies dune migration that was contemporaneous with water table‐controlled accumulation in adjacent interdunes. Downwind changes in the geometry and facies of the interdune units indicate periodic expansion and contraction of the interdunes in response to changes in the elevation of the groundwater table and episodic flooding, during which accumulation of dune strata continued relatively uninterrupted. This contrasts with other models for accumulation in wet aeolian systems where interdune flooding is associated with a cessation in aeolian bedform climbing and the formation of a bypass or erosional supersurface. Architectural panels document the detailed stratigraphy in orientations both parallel and perpendicular to aeolian transport direction, enabling a quantitative three‐dimensional reconstruction of genetically related aeolian dune and interdune elements. Sets of aeolian dune strata are composed of grainflow and translatent wind‐ripple strata and are divided by a hierarchy of bounding surfaces originating from oblique migration of superimposed dunes over slipfaceless, sinuous‐crested parent bedforms, together with lee‐slope reactivation under non‐equilibrium flow conditions. Silty‐mudstone and sandstone interdune units are characterized by wind ripple‐, wavy‐ and subaqueous wave ripple‐laminae, desiccation cracks, mud flakes, raindrop imprints, load casts, flutes, intraformational rip‐up clasts and vertebrate and invertebrate footprint impressions and trackways. These units result from accumulation on a substrate that varied from dry‐ through damp‐ to wet‐surface conditions. Interdune ponds were flooded by either fluvial incursions or rises in groundwater table and were periodically subject to gradual desiccation and reflooding. Red silty‐mudstone beds of subaqueous origin pass laterally into horizontally laminated wind‐ripple beds indicating a progressive transition from wet‐ through damp‐ to dry‐surface conditions within a single interdune.

Low‐albedo surfaces and eolian sediment: Mars Orbiter Camera views of western Arabia Terra craters and wind streaks

High spatial resolution (1.5 to 6 m/pixel) Mars Global Surveyor Mars Orbiter Camera images obtained September 1997 to June 2001 show that each of the large, dark wind streaks of western Arabia Terra originate at a barchan dune field on a crater floor. The streaks consist of a relatively thin (<1 m) coating of sediment deflated from the dune fields and their vicinity. In most cases, this sediment drapes over a previous mantle that more thickly covers nearly all of western Arabia Terra. No concurrent eolian bedforms are found within the dark streaks, nor do any dunes climb up crater walls to deliver sand via saltation to the streaks. The relations between dunes, wind streak, and subjacent terrain imply that dark‐toned grains finer than those that comprise the dunes are lifted into suspension and carried out of the craters to be deposited on the adjacent terrain. Previous eolian physics and thermal inertia studies suggest that, under modern Martian conditions, such grains likely include silt (3.9–62.5 μm), very fine sand (62.5–125 μm), and possibly fine sand (to ∼210 μm). The streaks change in terms of extent, relative albedo, and surface pattern over periods measured in years; however, through June 2001, very little evidence for recent eolian activity (dust plumes, storms, dune movement) was observed.

Sedimentology and petrography of dredged carbonate sands from Stocking Island (Bahamas). Implications for meteoric diagenesis and aeolianite formation

Piles of carbonate sand dredged onto Stocking Island (Exuma Chain, Bahamas) during World War II are now relatively well-lithified down to a depth of about 80 cm and may contain up to 8% of interstitial calcite cement, a volume comparable to that measured from 1000-yr-old beach ridges elsewhere in the Bahamas. Rates of meteoric diagenetic changes are thus more rapid than previously assessed in this area (∼600 cm 3 of calcite cement produced by m 3 of rock per year). Preservation of hydrodynamic sedimentary structures, footprints, and the lack of aeolian bedforms (e.g. dunes, ripples) on the surface of these sediments further show they have not been reworked by the wind at all. Therefore, the carbonate aeolianites of Holocene and Pleistocene ages forming the bulk of the islands of Bermuda and the Bahamas were likely deposited during relatively dry periods when the effects of meteoric diagenesis were limited. Reduced moisture, along with sediment availability and wind energy, appears as a key player in carbonate aeolianite genesis.

Computer simulation of aeolian sand ripples and dunes

A discrete computer method, which considers the principal characteristics of the motion of discrete particles caused by the airflow over the aeolian bedforms using a set of simple rules, is suggested to simulate the initiation and evolution of wind blown sand ripples and dunes. The results indicate that, if the grain-bed impacting and surface creep are taken into account, the model is capable of reproducing sand ripples vividly, and describing the reparation of any destroy to rippled surface, a phenomenon observed in field. If the saltation driven by the main flow and the secondary flow, the surface creep due to gravity, and the slide occurring when the slope of sand surface is larger than the angle of repose are considered, the model can simulate the formation and development of sand dunes whose shape and arrangement in space are in accordance with the landscape of typical deserts. After a further investigation into the simulation process, we draw the conclusion that the aeolian bedforms is a system of self-organization and hierarchy with a fractal character.