Nicholas Lancaster

Geomorphology of desert dunes

1. Introduction 2. Sand Transport by the Wind 3. Dune Morphology and Morphometry 4. Dune Sediments 5. Dune Dynamics 6. Controls of Dune Morphology 7. Sand Seas 8. Palaeoenvironments and Dunes 9. Review and Prospect

Influence of vegetation cover on sand transport by wind: field studies at Owens Lake, California.

Field studies conducted at Owens Lake, California, provide direct measurements of sand flux on sand sheets with zero to 20 per cent cover of salt grass. Results from 12 different sand transport events show that aerodynamic roughness length and threshold wind shear velocity increase with vegetation cover as measured by vertically projected cover and roughness density (λ). This results in a negative exponential decrease in sediment flux with increasing vegetation cover such that sand transport is effectively eliminated when the vertically projected cover of salt grass is greater than 15 per cent. A general empirical model for the relation between sand flux and vegetation cover has been derived and can be used to predict the amount of vegetation required to stabilize sand dune areas.

Late Pleistocene and Holocene dune activity and wind regimes in the western Sahara Desert of Mauritania

The western Sahara Desert in Mauritania is dominated by extensive sand seas consisting largely of linear dunes. Analyses of Landsat images, geomorphic and stratigraphic studies, and optically stimulated luminescence dating of dunes in the Azefal, Agneitir, and Akchar sand seas provide evidence that three main generations of dunes were formed during the periods 25–15 ka (centered around the Last Glacial Maximum), 10–13 ka (spanning the Younger Dryas event), and after 5 ka. The wind regimes that occurred during each of these periods were significantly different, leading to the formation of dunes on three distinct superimposed trends—northeast, north-northeast, and north—and the development of the sand seas as composite geomorphic features.

Age and dynamics of linear dunes in the Namib Desert

Bristow, C. S., Duller, G. A. T., Lancaster, N. (2007). Age and dynamics of linear dunes in the Namib Desert. Geology, 35(6), 555-558. Sponsorship: American Chemical Society

Sediment flux and airflow on the stoss slope of a barchan dune

Abstract Measurements of sediment flux on the windward slope of an isolated barchan using an array of 30 sand traps provide new data that can constrain models of dune dynamics. The data show that at low wind incident speeds, flux increases up the dune exponentially, whereas at higher wind speeds the increase with distance approaches linearity. Wind profile measurements, conducted at the same time as the flux measurements, indicate that, although wind speed at a given height increases by 1.2 times from dune toe to brinkline, wind shear velocity derived from the profile data decreases up the dune and is in many cases below transport threshold values. This demonstrates that conventional wind profiles, derived from anemometry on dunes, do not measure the part of the boundary layer that is significant for sediment transport.

The Namib Sand Sea: Dune Forms, Processes and Sediments

Lancaster (geology, Arizona State U.) reports on investigations of dune forms, processes and sediments in the Namib Sand Sea of southwestern Africa, which are used to develop models for the formation of dunes and sand seas and their rock equivalents. Annotation copyright Book News, Inc. Portland, Oregon, USA. Contents: Regional physiographic & climatic setting; Dune morphology & morphometry: Dune sediments; Dune processes; Controls of dune morphology; Accumulation of the sand sea. References; Appendices.

Combining ground penetrating radar surveys and optical dating to determine dune migration in Namibia

Ground penetrating radar (GPR) profiles across a complex linear dune in the Namib Sand Sea have been used to image sets of cross-stratification and their bounding surfaces. A combination of radar facies analysis and radar stratigraphy has been used to interpret the radar profiles and define a relative chronology. Thick sets of cross-stratification indicate when the dune was most active, whereas thin sets of cross-stratification are interpreted to indicate the increased prevalence of wind reversals and lower rates of dune migration, with bounding surfaces formed during periods of stabilization, non-deposition or erosion. A drilling and dating campaign was designed on the basis of the dune stratigraphy as defined by the GPR survey. Sampling was targeted at large sets of cross-stratification formed when the dunes were most active, and avoiding bounding surfaces formed when the dune was stable or even eroded. The results from optical dating give ages between 0.34 ± 0.02 ka and 1.57 ± 0.07 ka, indicating a time-averaged dune migration rate of 0.12 m a−1 over the past 1600 years.

Flux of Eolian Sediment in the McMurdo Dry Valleys, Antarctica: A Preliminary Assessment

Data from passive sediment traps installed at locations throughout the McMurdo Dry Valleys of Antarctica provide for an initial assessment of rates of transport of dust (sediment transported in suspension by wind) in these areas. The flux of eolian sediment in these areas is divided into material of sand and silt and clay size. Sites in close proximity to the valley floors tend to be dominated by movement of sand, whereas sites on ice-covered lakes and glaciers are dominated by transport of silt- and clay-sized sediment. Material transported by wind is likely derived from distal fluvial sediments. Rates of dust flux are generally much lower than in other desert environments. Controls on the flux of eolian material in this environment appear to be sediment supply and, to a lesser extent, wind speed.

Identification of sand sources and transport pathways at the Kelso Dunes, California, using thermal infrared remote sensing

The Kelso dune field is located in the eastern Mojave Desert, California, at the terminus of a sand-transport pathway, which has its primary source at the Mojave River Wash 50 km to the west. Initial examination of 1984 airborne thermal infrared multispectral scanner (TIMS) data showed significant spectral variations that indicate potential mineralogic heterogeneities within the active dunes. This result prompted the collection of a suite of 48 sand samples in 1990, and the acquiring of new TIMS data in 1995. This new data set was used to test a newly developed linear spectral retrieval algorithm in conjunction with a spectral library of end-member minerals. Results of this analysis produced images of end-member minerals that showed marked variations within the dunes. In addition, standard petrographic techniques revealed that the dunes contain mineralogic variations and were much less quartz rich (∼42%) than previously reported (∼70%–90%). Point-count results agreed with the spectral data to within an average of 5.3% for TIMS-derived and 3.1% for laboratory-derived mineralogic abundances. High concentrations of several of the end-member minerals on the surrounding alluvial fans indicate a potential nearby source for these minerals. Most evident is the presence of potassium feldspar from the fan that emanates from the Providence Mountains east of the dunes. This previously unidentified potential sand input is not visible with other remote sensing techniques and was confirmed with additional field sampling. Much of the quartz and some plagioclase feldspar, however, appear to have been transported from the Mojave River Wash source as previously reported. This study also validates the potential of using thermal remote sensing from future satellite-based instruments to globally monitor desert fringe areas susceptible to the changing conditions of sand encroachment.

Response of eolian geomorphic systems to minor climate change: examples from the southern Californian deserts

Abstract Eolian processes and landforms are sensitive to changes in atmospheric parameters and surface conditions that affect sediment supply and mobility. The response of eolian geomorphic systems to minor climate change can be examined through process-response models based on a combination of relations between short-term changes in climatic variables and eolian activity and the geologic and geomorphic record of Holocene eolian activity. At both time scales, eolian activity in southern Californian deserts is strongly controlled by variations in precipitation. Wind energy is not a limiting factor in this region. Formation of eolian deposits is a product of climatic changes that increase sediment supply from fluvial and lacustrine sources and may, therefore, be closely tied to periods of channel cutting and geomorphic instability. During intervening periods, eolian deposits migrate away from sediment source areas and are reworked, modified, and degraded. Remobilization of existing dormant dunes is a product of reduced vegetation cover and soil moisture in periods of drier climates. The major control on these processes is decadal to annual changes in rainfall that determine vegetation cover and soil moisture content.