Dune Orientation Research Articles

Dune-forming winds on Titan and the influence of topography

Numerous extended dunes on Saturn's moon Titan detected by the Cassini RADAR constrain the long-term pattern of surface winds. We analyse the statistics of surface wind speed and direction and their spatial and temporal variability predicted by a general circulation model (GCM) in order to constrain surface wind predictions of this GCM by dune observations. The model shows that modern winds are sufficient for saltation and dune formation at low latitudes, in agreement with the presence of dunes there. The best condition for the dune-forming wind occurs with a threshold friction speed of 0.02 m s −1 or slightly less. The equatorial region is conducive to longitudinal dunes because of a combined effect of a high sand drift potential and obtuse bimodal wind pattern oblique to the equator caused by the seasonal reversal of the Hadley circulation. The cross-equatorial wind is steady, and is characterised by a high Weibull shape parameter ( k ∼ 4 ). The wind pattern at higher latitudes is more complex and gusty, and neither longitudinal nor transversal dunes would be able to form. Putative large-scale topography is found to have a profound influence on the near-surface wind pattern. Generally mountains cause a convergence and speeding up on the flank, while basins weaken the wind and cause a divergent flow. Longitudinal dunes can be deflected on the foot of mountains by up to 90°. If Xanadu is a hypothetical large mountain, a wind pattern converging in Xanadu that entirely disagrees with the dune observations is predicted. If instead Xanadu is a large basin, the wind arcs clockwise north of Xanadu and anti-clockwise west and southwest of Xanadu, in agreement with the dune orientations in the vicinity of Xanadu. The albedo pattern has comparatively little influence on the wind field. Isolated mountain chains cause only local-scale change in the wind pattern. However, the persistent surface easterlies in Belet, which are in conflict with the dune orientations, do not disappear by any combination of large-scale topography.

Mars Global Digital Dune Database and initial science results

A new Mars Global Digital Dune Database (MGD3) constructed using Thermal Emission Imaging System (THEMIS) infrared (IR) images provides a comprehensive and quantitative view of the geographic distribution of moderate‐ to large‐size dune fields (area >1 km2) that will help researchers to understand global climatic and sedimentary processes that have shaped the surface of Mars. MGD3 extends from 65°N to 65°S latitude and includes ∼550 dune fields, covering ∼70,000 km2, with an estimated total volume of ∼3,600 km3. This area, when combined with polar dune estimates, suggests moderate‐ to large‐size dune field coverage on Mars may total ∼800,000 km2, ∼6 times less than the total areal estimate of ∼5,000,000 km2 for terrestrial dunes. Where availability and quality of THEMIS visible (VIS) or Mars Orbiter Camera narrow‐angle (MOC NA) images allow, we classify dunes and include dune slipface measurements, which are derived from gross dune morphology and represent the prevailing wind direction at the last time of significant dune modification. For dunes located within craters, the azimuth from crater centroid to dune field centroid (referred to as dune centroid azimuth) is calculated and can provide an accurate method for tracking dune migration within smooth‐floored craters. These indicators of wind direction are compared to output from a general circulation model (GCM). Dune centroid azimuth values generally correlate to regional wind patterns. Slipface orientations are less well correlated, suggesting that local topographic effects may play a larger role in dune orientation than regional winds.

The morphology of barchan-shaped sand banks from western Torres Strait, northern Australia

Large barchan-shaped sand deposits have been observed in the north west of Torres Strait. These deposits share characteristics of both subaerial barchan dunes and subaqueous sand banks. A study of satellite imagery indicate that the deposits migrate in the direction indicated by their horns (10–15 m west per year), and that sediment is shed from their horns, features that are characteristic of barchan dunes. However the orientations of sand dunes superimposed upon the sand banks indicate the presence of mutually-evasive channels and circulation of sediment around the sand bank, a characteristic of subaqueous sand banks. The presence of mutually-evasive channels is the criteria used to categorise the deposits as sand banks. Barchan forms are known to exist in regions with limited sediment supply and unidirectional current or wind regimes. In the Torres Strait both these criteria are met. Previous work has demonstrated the presence of a net westward current through the Torres Strait that is driven by the southeast trade winds. The relatively high displacement of the wind-driven currents during the trade wind season relative to the monsoon appears to provide the necessary ‘unidirectional’ regime to form barchans. The low, and typically eastwards, displacement of the residual monsoon season current appears to have a negligible affect on the barchan form. While seasonal wind-driven currents appear to maintain the barchan shape of the sand banks, tidal currents actively maintain mutually-evasive channels observed by variations in dune orientation on the sand banks. A sediment starved environment combined with bedload transport attributed to both wind driven and tidal currents is concluded to create a unique hydrodynamic environment where sand banks can attain a barchan form.

Discussion of "Geochemical evidence for the origin of late Quaternary loess in central Alaska"

In a recent paper, Muhs and Budahn (2006) aim to test regional paleowind hypotheses put forth in the literature for the provenance of Quaternary loess deposits in central Alaska with geochemical evidence. The experimental design of their study consists of a comparison and multilinear regression analysis (MRA) of major element and rare-earth element (REE) geochemistry between possible source sediment material along the Yukon, Tanana, and Nenana rivers and the youngest loess units of three deposits near Fairbanks, Alaska (see their fig. 2). Such a study has been long overdue and we applaud their initiative. However, through this discussion letter, we wish to address two points. The first concerns the “opposing” hypotheses to be tested, while the second concerns aspects of their experimental design. Muhs and Budahn (2006, pp. 324–325) set out to “test the opposing hypotheses of northerly versus southerly paleowinds for the origin of Quaternary loess in central Alaska”. Literature accounts of paleowind reconstructions are either based on sand dune orientations and sand sheet locations or loess deposits. For both types of archives, sand and loess, the inferred paleowinds are presented by the authors as inconsistent and contradictory. We argue that the platform on which this paper is set up (i.e., north versus south) is built from a misinterpretation and inaccurate representations of previous work referenced as supporting evidence. Furthermore, temporal variability, an important issue, appears to be cast aside. Concerning aeolian sand deposits in the inner parts of central Alaska, Hopkins (1982) and Lea and Waythomas (1990) conclude winds came from a northeasterly direction during the last glacial maximum. Muhs and Budahn (2006) hypothesize (p. 324) from the occurrence of aeolian sands of early Holocene age north of9the Yukon River near the confluence of the Porcupine River (see Froese et al. 2005 and their fig. 1 for location) that winds must …

Observations of scatterometer asymmetry over sand seas and derivation of wind ripple orientation

Over sand seas the ERS scatterometer has observed asymmetries similar to those found over open water. In this paper this asymmetric backscatter response is investigated using C-band VV data from the ERS scatterometer and the data are analysed using a simple empirical model combined with quick-look Synthetic Aperture Radar (SAR) data to provide information on dune type and orientation. The results suggest that the azimuthal response is sensitive to the orientation of small-scale (millimetres to metres) wind-induced sand ripples, rather than the larger scale dune orientation. Additionally, the study investigates the feasibility of using such observations to determine ripple orientation over areas of active sand.

A 160,000-year record of dune development and atmospheric circulation in Southern Arabia.

Aeolian deposits in the Wahiba Sands, Sultanate of Oman, reveal patterns of atmospheric circulation over the past 160,000 years. Luminescence dating indicates a correlation of dune activity with periods of low global sea level and decreased monsoon intensity. Evidence from dune orientation and sedimentary structures shows exclusively northbound transport of sand during times of high- latitude glaciation. These results are in contrast to the current paleocirculation model that assumes an increase of northwesterly winds because of a southward shift of the Intertropical Convergence Zone. Our results indicate that the circulation pattern during glacial times was comparable to that of the present.

The European sand belt in eastern Europe ‐ and comparison of Late Glacial dune orientation with GCM simulation results

A compilation is presented of the continuation of the European sand belt, east of Poland. Dune fields encompass most of the aeolian formations in eastern Europe. Supposed sand provenance, dune orientation, and the few available datings suggest initial aeolian activity during cold stages of the Upper‐Pleni (= full)glacial and Late Glacial, similar to northwest and central Europe. Dune fomiation was primarily supply controlled. Comparison of dune orientation with Late Glacial surface wind directions simulated by various GCMs permits reconstruction of dune activation with the positioning of (winter) westerlies over glacial deposits during the Oldest Dryas. Final widespread aeolian activity occurred during the Younger Dryas. As compared to the smaller fields toward the centre of deglaciation, aeolian sand deposits of the intermediate and periglacial zone benefited from distinctly longer intervals of accumulation. In the zone of deglaciation, aeolian activity was restricted to relatively isolated basins.

Early views of the martian surface from the Mars Orbiter Camera of Mars Global Surveyor.

High-resolution images of the martian surface at scales of a few meters show ubiquitous erosional and depositional eolian landforms. Dunes, sandsheets, and drifts are prevalent and exhibit a range of morphology, composition (inferred from albedo), and age (as seen in occurrences of different dune orientations at the same location). Steep walls of topographic depressions such as canyons, valleys, and impact craters show the martian crust to be stratified at scales of a few tens of meters. The south polar layered terrain and superposed permanent ice cap display diverse surface textures that may reflect the complex interplay of volatile and non-volatile components. Low resolution regional views of the planet provide synoptic observations of polar cap retreat, condensate clouds, and the lifecycle of local and regional dust storms.

A Comparison of General Circulation Model Predictions to Sand Drift and Dune Orientations

The growing concern over climate change and decertification stresses the importance of aeolian process prediction. In this paper the use of a general circulation model to predict current aeolian features is examined. A GCM developed at NASA/Goddard Space Flight Center was used in conjunction with White`s aeolian sand flux model to produce a global potential aeolian transport map. Surface wind shear stress predictions were used from the output of a GCM simulation that was performed as part of the Atmospheric Model Intercomparison Project on 1979 climate conditions. The spatial resolution of this study (as driven by the GCM) is 4{degrees} X 5{degrees}; instantaneous 6-hourly wind stress data were saved by the GCM and used in this report. A global map showing potential sand transport was compared to drift potential directions as inferred from Landsat images from the 1980s for several sand seas and a coastal dune field. Generally, results show a good correlation between the simulated sand drift direction and the drift direction inferred for dune forms. Discrepancies between the drift potential and the drift inferred from images were found in the North American deserts and the Arabian peninsula. An attempt to predict the type of dune that would bemore » formed in specific regions was not successful. The model could probably be further improved by incorporating soil moisture, surface roughness, and vegetation information for a better assessment of sand threshold conditions. The correlation may permit use of a GCM to analyze {open_quotes}fossil{close_quotes} dunes or to forecast aeolian processes. 48 refs., 8 figs.« less

Sand-wedge and ventifact palaeoenvironmental indicators in North-West Saskatchewan, Canada, 11 ka to 9.9 kaBP

Exhumed sand wedges and ventifacts in the upper Churchill watershed of north-western Saskatchewan indicate that aeolian and periglacial processes were dominant from c. 11 to c 10.5 ka BP. Nineteen sand wedges were identified that display vertical foliation, form polygonal nets, and penetrate either till, glaciolacustrine sediment or glaciofluvial diapiric sand. The sand wedges, and sand lenses within the till and glaciolacustrine sediments, indicate permafrost aggradation after glaciation and lake drainage. Ventifacts that overlie the sand wedges have preferred keel orientations that correlate with palaeo-katabatic wind directions interpreted from stabilized parabolic dune orientations. Rat-tail lineaments on faceted surfaces of coarse-grained, granitic ventifacts also indicate palaeo-wind flow direction. Strong and weakly developed lineaments on the same einkanter are explained by opposing flow from prevailing winds, katabatic winds and anticyclonic air flow from the adjacent Laurentide Ice Sheet, or by flow separation at the ventifact keel under unidirectional air flow. Two geomorphic surfaces are identified: (1) an older surface with sand wedges and weakly developed ventifacts indicative of permafrost and brief aeolian activity; and (2) a lower and younger segment of the former Lake Agassiz basin, without permafrost indicators but with well-developed ventifacts. The upper surface, developed in association with the Beaver River Moraine and glacial Lake Meadow-McMurray, is 11–10.5 ka BP in age, synchronous with the Younger Dryas cold period. The younger, lower surface was last modified beginning at 9.9 ka BP, when the north-west outlet of Lake Agassiz opened, was susceptible to greater aeolian reworking than the upper surface, and postdates the Younger Dryas. © 1996 John Wiley & Sons, Ltd.

Large-scale stabilized dunes on the High Plains of Colorado: Understanding the landscape response to Holocene climates with the aid of images from space

Principal-component analysis of Landsat Thematic Mapper images from eastern Colorado reveals stabilized dune fields that are composed of single and compound parabolic dunes, some longer than 10 km. These dunes have been reactivated at least four times in the past 10,000 yr, at ca. 9500 to 5500 yr B.P., 5500 to {gt}4800 yr B.P., 4800 to {gt}1000 yr B.P., and {lt}1000 yr B.P., during droughts that probably exceeded the dry conditions during the 1930s. Dune orientations indicate that these landforms were molded by winds from the northwest, which now dominate during the winter and spring with the passage of Pacific air masses. This study underscores the sensitivity of the landscape of the High Plains to drought conditions, which may become more prevalent because of the anticipated effects of greenhouse warming.

Late pleistocene paleowind directions, Atlantic Coastal Plain, U.S.A

Parabolic dunes, transverse dunes and fields of irregular dunes are common along the left banks, the north or east sides, of streams of the Atlantic Coastal Plain from Georgia to Delaware and probably beyond. Similar dunes occur in the Appalachian Piedmont, but are less common. The features have been eroded, to some extent, and are normally recognizable on topographic maps only as areas of irregular topography with wet and dry depressions. The larger parabolic dunes can be recognized on aerial photographs and satellite images because the associated soils are poor and the vegetative cover relatively sparse. The dunes are of eolian origin and late Pleistocene in age. It is thought that most of the dunes are late Wisconsin in age and contemporary with the Carolina Bays. A small sample of paleowind directions based on average directions of parabolic dune axes indicates that late Wisconsin winds in Georgia were westerly and grain sizes suggest that the winds were stronger than present-day coastal winds of the area. Paleowind directions shift progressively toward the south across the Carolinas and are southwesterly in North Carolina. They change abruptly to the northwest in the Delmarva Peninsula. Following the theory that Carolina Bays are former lakes that are elongated perpendicular to the effective wind direction, the fact that paleowind directions derived from Carolina Bay orientations are virtually identical to those from parabolic dune orientations, strongly supports the wind orientation theory of Carolina Bay origin and confirms the validity of the small sample of directions obtained from parabolic dunes. By analogy with dunes of the eastern shore of Lake Michigan and the Atlantic and Pacific Coasts, the Atlantic Coast Plain dunes do not necessarily indicate arid conditions. The sediment load of some streams may have been sandier than at present and streams with sandy, braided channels may have been common.

Sedimentologic Controls on Production in Giant Eolian Reservoirs in Western U.S. Overthrust: ABSTRACT

The Jurassic Nugget Sandstone is a 1500-ft (457-m) thick eolian reservoir that produces prolifically throughout the Wyoming Overthrust belt. Nugget fields exhibit exceptional, but predictable reservoir anisotropy. Based on detailed core, log, and production analyses, this study documents Nugget reservoir behavior as a strongly layered system with preferred directional permeability. Anschutz Ranch East and Painter Reservoir fields contain numerous fieldwise vertical permeability barriers that are interpreted as amalgamated inland sabkhas (interdunes) associated with significant diastems. The inland sabkhas contain internal erosion surfaces and abundant eolian ripples, and they are commonly disturbed by burrowing or salt growth. Carbonate and silicate cements render the inland sabkha deposits tight, creating vertical permeability barriers (K/sub v/ less than or equal to 0.001 md). Strongly preferred directional permeability parallels dune strike. Dune orientations determined from dipmeter logs and oriented cores relate directly to production rates and premature breakthrough of injected fluids. Preferred permeability along the predominant dune axis can be 5-50 times greater than permeability across dune cross-bedding. Detailed well data documenting dune orientation and diastems should be used early in field development to determine appropriate spacing and the well pattern. By incorporating these concepts into a reservoir management plan, eolian reservoirs can be moremore » efficiently exploited.« less

Speculation on martian north polar wind circulation and the resultant orientations of polar sand dunes

Dunes in the Martian north polar erg show two dominant orientations. When seen at frost cap minimum, dunes north of 80°N record east winds, dunes south of 80°N record west winds. Many of the transverse dunes are considered to be reversing dunes. Dunes in two fields may have reversed at least once during the lifetime of the Viking Orbiters. Poor agreement exists among published predictive models of north polar winds and the interpretations derived from the major published map of the north polar dunes. We propose that the average polar winds are: (1) strong, off-pole northwest winds in fall; (2) moderate west winds in winter; (3) latitude-dependent weak to strong off-pole northeast winds in spring; and (4) weak west winds in summer. Viking images of near-polar clouds confirm much of the hypothesis. Images discussed in other studies can be given alternative interpretations that support this hypothesis also. Over millenia, the combination of reversing west and east winds could produce the binodal distributions of dune orientations observed at the north pole.

Quaternary deposits in southwestern Afghanistan

Geologic evidence in the closed Seistan Basin of southwestern Afghanistan and adjacent parts of Iran and Pakistan indicates that a lake as much as 65,000 sq km in size occupied this closed depression during Pleistocene time. The deposits consist mostly of lacustrine silt and clay and have a maximum observed thickness of about 250 m. A layer of alluvial gravels overlies the sequence. The deposits are probably early or middle Pleistocene in age; they are old enough to have sustained nearly 300 m of erosion over large areas but are not faulted or detectably folded in the central part of the basin although they are upwarped along the west edge of the basin. Sand dunes cover extensive areas of the basin. Dune orientation shows that the strong surface winds enter the basin blowing toward the south-southeast and then are deflected to the east, apparently as a response to mountains bordering the basin on its south side. The Gawdezereh, a large deflation depression, may be a result of an augmented excavation ability of winds that oc urs where turbulence is created along a zone of deflection.