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David Thomas - One of the best experts on this subject based on the ideXlab platform.
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late quaternary lunette dune sedimentation in the southwestern Kalahari Desert south africa luminescence based chronologies of aeolian activity
Quaternary Science Reviews, 2002Co-Authors: M P Lawson, David ThomasAbstract:Abstract Through the application of luminescence dating techniques, great advances have recently been made in the understanding of the late Quaternary histories of many dryland regions, including southern Africa. Here, this work has largely applied optical dating to sediments from the extensive linear dune systems which dominate the Kalahari region of the central southern African interior. Relatively little attention has been given to localised aeolian accumulations such as lunette dunes, despite their wide distribution over many parts of the region, in particular the arid southwest Kalahari. To redress this, we consider the palaeoenvironmental significance of pan margin lunettes, and investigate the timing of lunette dune development in the southwest Kalahari through the application of optical dating to sediments from lunette dunes associated with four closed basins (pans). Ages show that localised aeolian activity occurred frequently throughout the past 18 ka, indicating that the factors controlling lunette sedimentation are markedly different from those determining regional linear dune development. It is proposed that the mechanisms controlling sediment supply to lunette dunes are controlled at both the regional and local (pan-specific) scales by factors such as groundwater levels. Where pans support more than one discrete lunette, the time since any individual lunette crest received significant accumulations of sediment increases with distance away from the pan margin. Outer and inner lunette sequences may be chronologically disparate features. Inner lunette formation may arise in response to modifications to the local atmospheric boundary conditions by the presence of large, downwind outer lunette features, or by shifts in sediment supply.
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new chronological evidence for the nature and timing of linear dune development in the southwest Kalahari Desert
Geomorphology, 1997Co-Authors: Stephen Stokes, David Thomas, Paul A ShawAbstract:Abstract The age of the linear dunes in the southwest Kalahari Desert has not previously been directly established. By applying the optical dating method to samples from the dunefield a number of questions relating to the timing and nature of linear dune development are addressed. The last major period of simple linear dune development occurred 17,000–10,000 years ago, probably from the reworking of earlier aeolian sediments. Dune emplacement occurred under more arid, probably windier, conditions than those prevailing today, which are only responsible for episodic surficial dune sand movement. As well as simple linear dunes, the dunefield also contains more complex linear dune patterns. Samples from one area in the eastern dunefield demonstrate that multi-phase development is more probable for complex dune patterns as opposed to single-phase evolution under a complex wind regime.
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holocene aeolian activity in the southwestern Kalahari Desert southern africa significance and relationships to late pleistocene dune building events
The Holocene, 1997Co-Authors: David Thomas, Stephen Stokes, Paul A ShawAbstract:Little is known about the timing of dune-building episodes in the southwestern Kalahari Desert. Previously, arid phases when dune activity occurred have been inferred from gaps in chronologies derived from information that attests to former humid periods. To redress this, optically stimulated luminescence (OSL) dating was applied to sediment samples taken from a range of aeolian landforms in the SW Kalahari. Four periods of dune building are identified in the late Pleistocene and Holocene. Holocene aeolian activity at 6 ka and 1-2 ka led to the construction of localised dune forms, in contrast to late Pleistocene times when the extensive field of linear dunes that dominates the region was emplaced. Late-Holocene aridity was however sufficient to allow rapid emplacement of the lunette dune at Witpan. The record of aeolian activity is evaluated in the context of humid chronologies and an ocean-atmosphere model of Holocene climate change, with good agreement obtained. It is concluded that the application of ...
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dunefield activity and interactions with climatic variability in the southwest Kalahari Desert
Earth Surface Processes and Landforms, 1997Co-Authors: Joanna E. Bullard, Ian Livingstone, David Thomas, G S F WiggsAbstract:An analysis is undertaken of the temporal variability of climatic parameters that influence dunefield aeolian activity. Data from seven meteorological stations in the southwestern Kalahari Desert are used, spanning the period 1960–1992. Erosivity is considered through analysis of wind data, and erodibility through analysis of precipitation and potential evapotranspiration, which together influence dune surface plant growth. The data are integrated using Lancaster's ‘mobility’ index which provides a measure of potential dune surface sand transport. This is renamed ‘potential dune surface activity index’, to reflect the actual characteristic that is measured. The subsequent analysis indicates that dunefield activity is episodic and temporally variable, that both erosivity and erodibility vary through time, and that present levels of activity cannot be characterized by a single simple state. © 1997 by John Wiley & Sons, Ltd.
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wind energy variations in the southwestern Kalahari Desert and implications for linear dunefield activity
Earth Surface Processes and Landforms, 1996Co-Authors: Joanna E. Bullard, Ian Livingstone, David Thomas, Giles F.s. WiggsAbstract:The southwestern Kalahari linear dunefield, which displays marked morphological variability, possesses a partial but temporally and spatially variable vegetation cover and has frequently been described as a palaeodunefield. Palaeo status has been ascribed on the basis of several criteria including the presence of vegetation, but also because dunes are thought to be out of alignment with modern resultant potential sand-moving wind directions and because present-day wind energy is regarded as low. For the period 1960–1992, wind data from eight dunefield meteorological stations are analysed in detail to examine these assertions. Potential sand transport directions, including spatial and temporal variations, and potential drift directions for the windiest three month periods, are calculated and explained. It is concluded that the present-day potential sand transport environment is markedly variable from year to year and from place to place. While periods of low sand transport energy do occur, it is also noted that the 1980s possessed considerable potential for sand transport in the dunefield. Directional variability is also relatively high, perhaps exceeding that under which linear dunes can be expected to form. Because linear dune aeolian activity has a number of states, however, the present-day wind environment may allow dune surface aeolian activity to occur which does not alter the overall pattern of the dunes.
Giles F.s. Wiggs - One of the best experts on this subject based on the ideXlab platform.
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modelled responses of the Kalahari Desert to 21st century climate and land use change
Scientific Reports, 2017Co-Authors: Jerome R Mayaud, Richard M Bailey, Giles F.s. WiggsAbstract:Drylands are home to over 2 billion people globally, many of whom use the land for agricultural and pastoral activities. These vulnerable livelihoods could be disrupted if Desert dunefields become more active in response to climate and land use change. Despite increasing knowledge about the role that wind, moisture availability and vegetation cover play in shaping dryland landscapes, relatively little is known about how drylands might respond to climatic and population pressures over the 21st century. Here we use a newly developed numerical model, which fully couples vegetation and sediment-transport dynamics, to simulate potential landscape evolution at three locations in the Kalahari Desert, under two future emissions scenarios: stabilising (RCP 4.5) and high (RCP 8.5). Our simulations suggest that whilst our study sites will experience some climatically-induced landscape change, the impacts of climate change alone on vegetation cover and sediment mobility may be relatively small. However, human activity could strongly exacerbate certain landscape trajectories. Fire frequency has a primary impact on vegetation cover, and, together with grazing pressure, plays a significant role in modulating shrub encroachment and ensuing land degradation processes. Appropriate land management strategies must be implemented across the Kalahari Desert to avoid severe environmental and socio-economic consequences over the coming decades.
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wind energy variations in the southwestern Kalahari Desert and implications for linear dunefield activity
Earth Surface Processes and Landforms, 1996Co-Authors: Joanna E. Bullard, Ian Livingstone, David Thomas, Giles F.s. WiggsAbstract:The southwestern Kalahari linear dunefield, which displays marked morphological variability, possesses a partial but temporally and spatially variable vegetation cover and has frequently been described as a palaeodunefield. Palaeo status has been ascribed on the basis of several criteria including the presence of vegetation, but also because dunes are thought to be out of alignment with modern resultant potential sand-moving wind directions and because present-day wind energy is regarded as low. For the period 1960–1992, wind data from eight dunefield meteorological stations are analysed in detail to examine these assertions. Potential sand transport directions, including spatial and temporal variations, and potential drift directions for the windiest three month periods, are calculated and explained. It is concluded that the present-day potential sand transport environment is markedly variable from year to year and from place to place. While periods of low sand transport energy do occur, it is also noted that the 1980s possessed considerable potential for sand transport in the dunefield. Directional variability is also relatively high, perhaps exceeding that under which linear dunes can be expected to form. Because linear dune aeolian activity has a number of states, however, the present-day wind environment may allow dune surface aeolian activity to occur which does not alter the overall pattern of the dunes.
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airflow and roughness characteristics over partially vegetated linear dunes in the southwest Kalahari Desert
Earth Surface Processes and Landforms, 1996Co-Authors: Giles F.s. Wiggs, Ian Livingstone, David Thomas, Joanna E. BullardAbstract:There is little understanding of the flow-field surrounding semi-vegetated linear dunes, and predictions of dune mobility are hampered by a lack of empirical data concerning windflow. In an attempt to characterize the near-surface airflow upwind of and over partially vegetated linear dunes in the southwest Kalahari Desert, this study presents measurements of vertical and horizontal wind velocity profiles across cross-sectional transects of seven partially vegetated linear dunes. Vegetation surveys combined with velocity measurements from vertical arrays of cup-anemometers, placed up to 2·3 m above the ground surface, were used to gain information concerning the modification of airflow structure caused by the intrusion of the dunes into the atmospheric boundary layer and to predict the variability of aerodynamic roughness (z0) from interdune to crest. The results suggest an acceleration of flow up the windward slopes of the dunes and, as such, the data correspond to classical theory concerning flow over low hills (essentially Jackson and Hunt (1975) principles). Where the theory is incapable of explaining the airflow structure and acceleration characteristics, this is explained, in part, by the presence of a spatially variable vegetation cover over the dunes. The vegetation is important both in terms of the varying aerodynamic roughness (z0) and problems concerning the definition of a zero-plane displacement (d). It is considered that any attempts to characterize surface shear stress over the Kalahari linear dunes, in order to predict sand transport and dune mobility, will be hampered by two problems. These are the progressively non-log-linear nature of the velocity profiles over the dunes caused by flow acceleration, and the production of thin near-surface boundary layers caused by areally variable aerodynamic roughness as a result of the partially vegetated nature of the dunes.
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Dune mobility and vegetation cover in the Southwest Kalahari Desert
Earth Surface Processes and Landforms, 1995Co-Authors: Giles F.s. Wiggs, David S.g. Thomas, Joanna E. Bullard, Ian LivingstoneAbstract:As part of a wider project investigating the palaeoenvironmental significance of partially vegetated linear dunes in the southwest Kalahari, data collected in the latter part of 1992 concerning dune movement and vegetation cover suggest that sediment transport is occurring on some dune surfaces, and that the majority of surface activity occurs on the crests and upper slopes of the dunes. The data suggest that the limiting variables on surface sediment movement vary on different parts of a dune. On interdunes and lower dune slopes the primary limiting variable is available wind energy, while on dune crests and upper slopes it is vegetation cover. Ground cover by litter has much greater importance in protecting the surface sediment from erosion than rooted vegetation. From individual data points, no evidence is found to support a threshold vegetation cover below which sediment movement occurs. Rather, a gradient of activity is suggested whereby a reduction in vegetation cover increases the potential for sediment movement and surface change. However, dunes with differing amounts of mean vegetation cover display differing degrees of surface activity, and at this scale, a vegetation cover threshold in the region of 14 per cent may be recognized.
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analysis of linear sand dune morphological variability southwestern Kalahari Desert
Geomorphology, 1995Co-Authors: Joanna E. Bullard, Ian Livingstone, David Thomas, Giles F.s. WiggsAbstract:Abstract Linear dunes are the most common Desert dune form, usually occurring in extensive dunefields rather than as isolated individuals. As part of a wider project investigating the dynamics and environmental significance of linear dunes, the extensive linear dunefield of the southwestern Kalahari Desert, southern Africa, was investigated for planimetric pattern variability. Considerable intradunefield variability was identified through aerial photograph analysis of a 4000 km 2 area, leading to the development of a five-class classification scheme. This scheme was validated statistically utilising data for key planimetric pattern variables: Y-junctions, termini, orientation range, and wavelength. The application of the classification scheme thoughout the dunefield permits the identification of trends in planimetric patterns. This provides a basis for first attempts to explain aspects of planimetric variability in terms of the behaviour of linear dunes and their responses to key environmental variables.
Joanna E. Bullard - One of the best experts on this subject based on the ideXlab platform.
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dunefield activity and interactions with climatic variability in the southwest Kalahari Desert
Earth Surface Processes and Landforms, 1997Co-Authors: Joanna E. Bullard, Ian Livingstone, David Thomas, G S F WiggsAbstract:An analysis is undertaken of the temporal variability of climatic parameters that influence dunefield aeolian activity. Data from seven meteorological stations in the southwestern Kalahari Desert are used, spanning the period 1960–1992. Erosivity is considered through analysis of wind data, and erodibility through analysis of precipitation and potential evapotranspiration, which together influence dune surface plant growth. The data are integrated using Lancaster's ‘mobility’ index which provides a measure of potential dune surface sand transport. This is renamed ‘potential dune surface activity index’, to reflect the actual characteristic that is measured. The subsequent analysis indicates that dunefield activity is episodic and temporally variable, that both erosivity and erodibility vary through time, and that present levels of activity cannot be characterized by a single simple state. © 1997 by John Wiley & Sons, Ltd.
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wind energy variations in the southwestern Kalahari Desert and implications for linear dunefield activity
Earth Surface Processes and Landforms, 1996Co-Authors: Joanna E. Bullard, Ian Livingstone, David Thomas, Giles F.s. WiggsAbstract:The southwestern Kalahari linear dunefield, which displays marked morphological variability, possesses a partial but temporally and spatially variable vegetation cover and has frequently been described as a palaeodunefield. Palaeo status has been ascribed on the basis of several criteria including the presence of vegetation, but also because dunes are thought to be out of alignment with modern resultant potential sand-moving wind directions and because present-day wind energy is regarded as low. For the period 1960–1992, wind data from eight dunefield meteorological stations are analysed in detail to examine these assertions. Potential sand transport directions, including spatial and temporal variations, and potential drift directions for the windiest three month periods, are calculated and explained. It is concluded that the present-day potential sand transport environment is markedly variable from year to year and from place to place. While periods of low sand transport energy do occur, it is also noted that the 1980s possessed considerable potential for sand transport in the dunefield. Directional variability is also relatively high, perhaps exceeding that under which linear dunes can be expected to form. Because linear dune aeolian activity has a number of states, however, the present-day wind environment may allow dune surface aeolian activity to occur which does not alter the overall pattern of the dunes.
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airflow and roughness characteristics over partially vegetated linear dunes in the southwest Kalahari Desert
Earth Surface Processes and Landforms, 1996Co-Authors: Giles F.s. Wiggs, Ian Livingstone, David Thomas, Joanna E. BullardAbstract:There is little understanding of the flow-field surrounding semi-vegetated linear dunes, and predictions of dune mobility are hampered by a lack of empirical data concerning windflow. In an attempt to characterize the near-surface airflow upwind of and over partially vegetated linear dunes in the southwest Kalahari Desert, this study presents measurements of vertical and horizontal wind velocity profiles across cross-sectional transects of seven partially vegetated linear dunes. Vegetation surveys combined with velocity measurements from vertical arrays of cup-anemometers, placed up to 2·3 m above the ground surface, were used to gain information concerning the modification of airflow structure caused by the intrusion of the dunes into the atmospheric boundary layer and to predict the variability of aerodynamic roughness (z0) from interdune to crest. The results suggest an acceleration of flow up the windward slopes of the dunes and, as such, the data correspond to classical theory concerning flow over low hills (essentially Jackson and Hunt (1975) principles). Where the theory is incapable of explaining the airflow structure and acceleration characteristics, this is explained, in part, by the presence of a spatially variable vegetation cover over the dunes. The vegetation is important both in terms of the varying aerodynamic roughness (z0) and problems concerning the definition of a zero-plane displacement (d). It is considered that any attempts to characterize surface shear stress over the Kalahari linear dunes, in order to predict sand transport and dune mobility, will be hampered by two problems. These are the progressively non-log-linear nature of the velocity profiles over the dunes caused by flow acceleration, and the production of thin near-surface boundary layers caused by areally variable aerodynamic roughness as a result of the partially vegetated nature of the dunes.
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Dune mobility and vegetation cover in the Southwest Kalahari Desert
Earth Surface Processes and Landforms, 1995Co-Authors: Giles F.s. Wiggs, David S.g. Thomas, Joanna E. Bullard, Ian LivingstoneAbstract:As part of a wider project investigating the palaeoenvironmental significance of partially vegetated linear dunes in the southwest Kalahari, data collected in the latter part of 1992 concerning dune movement and vegetation cover suggest that sediment transport is occurring on some dune surfaces, and that the majority of surface activity occurs on the crests and upper slopes of the dunes. The data suggest that the limiting variables on surface sediment movement vary on different parts of a dune. On interdunes and lower dune slopes the primary limiting variable is available wind energy, while on dune crests and upper slopes it is vegetation cover. Ground cover by litter has much greater importance in protecting the surface sediment from erosion than rooted vegetation. From individual data points, no evidence is found to support a threshold vegetation cover below which sediment movement occurs. Rather, a gradient of activity is suggested whereby a reduction in vegetation cover increases the potential for sediment movement and surface change. However, dunes with differing amounts of mean vegetation cover display differing degrees of surface activity, and at this scale, a vegetation cover threshold in the region of 14 per cent may be recognized.
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analysis of linear sand dune morphological variability southwestern Kalahari Desert
Geomorphology, 1995Co-Authors: Joanna E. Bullard, Ian Livingstone, David Thomas, Giles F.s. WiggsAbstract:Abstract Linear dunes are the most common Desert dune form, usually occurring in extensive dunefields rather than as isolated individuals. As part of a wider project investigating the dynamics and environmental significance of linear dunes, the extensive linear dunefield of the southwestern Kalahari Desert, southern Africa, was investigated for planimetric pattern variability. Considerable intradunefield variability was identified through aerial photograph analysis of a 4000 km 2 area, leading to the development of a five-class classification scheme. This scheme was validated statistically utilising data for key planimetric pattern variables: Y-junctions, termini, orientation range, and wavelength. The application of the classification scheme thoughout the dunefield permits the identification of trends in planimetric patterns. This provides a basis for first attempts to explain aspects of planimetric variability in terms of the behaviour of linear dunes and their responses to key environmental variables.
David J Nash - One of the best experts on this subject based on the ideXlab platform.
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heat treatment as a universal technical solution for silcrete use a comparison between silcrete from the western cape south africa and the Kalahari botswana
PLOS ONE, 2017Co-Authors: Patrick Schmidt, David J Nash, Sheila Coulson, Matthias B Goden, Graeme AwcockAbstract:Heat treatment was one of the first transformative technologies in the southern African Middle Stone Age (MSA), with many studies in the Cape coastal zone of South Africa identifying it as an essential step in the preparation of silcrete prior to its use in stone tool manufacture. To date, however, no studies have investigated whether heat treatment is necessary for all silcrete types, and how geographically widespread heat treatment was in the subcontinent. The aim of this study is to investigate experimentally whether heat treatment continued further north into the Kalahari Desert of Botswana and northernmost South Africa, the closest area with major silcrete outcrops to the Cape. For this we analyse the thermal transformations of silcrete from both regions, proposing a comprehensive model of the chemical, crystallographic and `water'-related processes taking place upon heat treatment. For the first time, we also explore the mobility of minor and trace elements during heat treatment and introduce a previously undescribed mechanism - steam leaching - causing depletion of a limited number of elements. The results of this comparative study reveal the Cape and Kalahari silcrete to respond fundamentally differently to heat treatment. While the former can be significantly improved by heat, the latter is deteriorated in terms of knapping quality. These findings have important implications for our understanding of the role of fire as a technical solution in MSA stone tool knapping, and for the extension of its use in southern Africa. Silcrete heat treatment - at least in the form we understand it today - may have been a strictly regional phenomenon, confined to a narrow zone along the west and south coast of the Cape. On the basis of our findings, silcrete heat treatment should not be added as a new trait on the list of behaviours that characterise the MSA of the southern African subcontinent.
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going the distance mapping mobility in the Kalahari Desert during the middle stone age through multi site geochemical provenancing of silcrete artefacts
Journal of Human Evolution, 2016Co-Authors: David J Nash, Sheila Coulson, Sigrid Staurset, Mosarwa Babutsi, John Ullyott, Martin SmithAbstract:Abstract This study utilises geochemical provenancing of silcrete raw materials, in combination with chaine operatoire analyses, to explore lithic procurement and behavioural patterns in the northern Kalahari Desert during the Middle Stone Age (MSA). New data from the sites of Rhino Cave, Corner Cave, and ≠Gi in northwest Botswana, combined with earlier results from White Paintings Shelter, reveal that the long distance transport of silcrete for stone tool manufacture was a repeated and extensively used behaviour in this region. Silcrete was imported over distances of up to 295 km to all four sites, from locations along the Boteti River and around Lake Ngami. Significantly, closer known sources of silcrete of equivalent quality were largely bypassed. Silcrete artefacts were transported at various stages of production (as partially and fully prepared cores, blanks, and finished tools) and, with the exception of ≠Gi, in large volumes. The import occurred despite the abundance of locally available raw materials, which were also used to manufacture the same tool types. On the basis of regional palaeoenvironmental data, the timing of the majority of silcrete import from the Boteti River and Lake Ngami is constrained to regionally drier periods of the MSA. The results of our investigation challenge key assumptions underlying predictive models of human mobility that use distance–decay curves and drop-off rates. Middle Stone Age peoples in the Kalahari appear to have been more mobile than anticipated, and repeatedly made costly choices with regard to both raw material selection and items to be transported. We conclude that (i) base transport cost has been overemphasised as a restrictive factor in predictive models, and (ii) factors such as source availability and preference, raw material quality, and potential sociocultural influences significantly shaped prehistoric landscape use choices.
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drainage development neotectonics and base level change in the Kalahari Desert southern africa
South African Geographical Journal, 2016Co-Authors: David J Nash, Frank EckardtAbstract:The Kalahari Desert contains extensive networks of ephemeral and fossil drainage which are potential indicators of past and present neotectonic activity and climate-driven environmental change. An absence of topographic data has hindered our understanding of their development. We present long-profile information for twenty-nine valley networks derived from Shuttle Radar Topographic Mission (SRTM) digital elevation data. In total, 8354 km of valley talweg was measured for x, y and z information. Most valleys exhibit concave-up profiles. Fifty-five previously unknown knickpoints were identified. The majority coincide with lithological boundaries or fractures, but many developed in response to Neogene uplift and/or downwarping or occur where valleys cross palaeolake shorelines. The headwaters of four valleys cross the Kalahari–Limpopo drainage divide and predate the presumed Miocene uplift of the Kalahari–Zimbabwe axis, suggesting that they are of considerable antiquity.
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of dunes depressions and dry valleys the arid landscapes of the Kalahari Desert
2015Co-Authors: David J NashAbstract:The combination of vegetated orange–red dunes , seasonal pans and dry valleys in the Kalahari creates a landscape with outstanding scientific and aesthetic value. This chapter describes the geomorphological features of the Kalahari Desert within South Africa and adjacent areas of Botswana and Namibia, with a special emphasis on aspects that make the landscape unique. The Kalahari is an arid to semi-arid region underlain by Cretaceous to recent Kalahari Group sediments , including a surface blanket of unconsolidated Kalahari sands . The landscape is dominated by three sets of landforms: (a) dry valley systems, including the Auob , Nossob , Kuruman and Molopo river s; (b) partially vegetated linear dunes , which stretch in a broad zone from Upington on the Orange River into Botswana and Namibia; and (c) seasonally flooded pan s. The importance of the long-term geological history of the Kalahari for understanding the present landscape is also discussed.
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provenancing of silcrete raw materials indicates long distance transport to tsodilo hills botswana during the middle stone age
Journal of Human Evolution, 2013Co-Authors: David J Nash, Stewart J Ullyott, Laurence Hopkinson, Sheila Coulson, Sigrid Staurset, Mosarwa Babutsi, Martin SmithAbstract:Abstract Lithic artifacts from the African Middle Stone Age (MSA) offer an avenue to explore a range of human behaviors, including mobility, raw material acquisition, trade and exchange. However, to date, in southern Africa it has not been possible to provenance the locations from which commonly used stone materials were acquired prior to transport to archaeological sites. Here we present results of the first investigation to geochemically fingerprint silcrete, a material widely used for tool manufacture across the subcontinent. The study focuses on the provenancing of silcrete artifacts from the MSA of White Paintings Shelter (WPS), Tsodilo Hills, in the Kalahari Desert of northwest Botswana. Our results suggest that: (i) despite having access to local quartz and quartzite at Tsodilo Hills, MSA peoples chose to transport silcrete over 220 km to WPS from sites south of the Okavango Delta; (ii) these sites were preferred to silcrete sources much closer to Tsodilo Hills; (iii) the same source areas were repeatedly used for silcrete supply throughout the 3 m MSA sequence; (iv) during periods of colder, wetter climate, silcrete may have been sourced from unknown, more distant, sites. Our results offer a new provenancing approach for exploring prehistoric behavior at other sites where silcrete is present in the archaeological record.
Ian Livingstone - One of the best experts on this subject based on the ideXlab platform.
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dunefield activity and interactions with climatic variability in the southwest Kalahari Desert
Earth Surface Processes and Landforms, 1997Co-Authors: Joanna E. Bullard, Ian Livingstone, David Thomas, G S F WiggsAbstract:An analysis is undertaken of the temporal variability of climatic parameters that influence dunefield aeolian activity. Data from seven meteorological stations in the southwestern Kalahari Desert are used, spanning the period 1960–1992. Erosivity is considered through analysis of wind data, and erodibility through analysis of precipitation and potential evapotranspiration, which together influence dune surface plant growth. The data are integrated using Lancaster's ‘mobility’ index which provides a measure of potential dune surface sand transport. This is renamed ‘potential dune surface activity index’, to reflect the actual characteristic that is measured. The subsequent analysis indicates that dunefield activity is episodic and temporally variable, that both erosivity and erodibility vary through time, and that present levels of activity cannot be characterized by a single simple state. © 1997 by John Wiley & Sons, Ltd.
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wind energy variations in the southwestern Kalahari Desert and implications for linear dunefield activity
Earth Surface Processes and Landforms, 1996Co-Authors: Joanna E. Bullard, Ian Livingstone, David Thomas, Giles F.s. WiggsAbstract:The southwestern Kalahari linear dunefield, which displays marked morphological variability, possesses a partial but temporally and spatially variable vegetation cover and has frequently been described as a palaeodunefield. Palaeo status has been ascribed on the basis of several criteria including the presence of vegetation, but also because dunes are thought to be out of alignment with modern resultant potential sand-moving wind directions and because present-day wind energy is regarded as low. For the period 1960–1992, wind data from eight dunefield meteorological stations are analysed in detail to examine these assertions. Potential sand transport directions, including spatial and temporal variations, and potential drift directions for the windiest three month periods, are calculated and explained. It is concluded that the present-day potential sand transport environment is markedly variable from year to year and from place to place. While periods of low sand transport energy do occur, it is also noted that the 1980s possessed considerable potential for sand transport in the dunefield. Directional variability is also relatively high, perhaps exceeding that under which linear dunes can be expected to form. Because linear dune aeolian activity has a number of states, however, the present-day wind environment may allow dune surface aeolian activity to occur which does not alter the overall pattern of the dunes.
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airflow and roughness characteristics over partially vegetated linear dunes in the southwest Kalahari Desert
Earth Surface Processes and Landforms, 1996Co-Authors: Giles F.s. Wiggs, Ian Livingstone, David Thomas, Joanna E. BullardAbstract:There is little understanding of the flow-field surrounding semi-vegetated linear dunes, and predictions of dune mobility are hampered by a lack of empirical data concerning windflow. In an attempt to characterize the near-surface airflow upwind of and over partially vegetated linear dunes in the southwest Kalahari Desert, this study presents measurements of vertical and horizontal wind velocity profiles across cross-sectional transects of seven partially vegetated linear dunes. Vegetation surveys combined with velocity measurements from vertical arrays of cup-anemometers, placed up to 2·3 m above the ground surface, were used to gain information concerning the modification of airflow structure caused by the intrusion of the dunes into the atmospheric boundary layer and to predict the variability of aerodynamic roughness (z0) from interdune to crest. The results suggest an acceleration of flow up the windward slopes of the dunes and, as such, the data correspond to classical theory concerning flow over low hills (essentially Jackson and Hunt (1975) principles). Where the theory is incapable of explaining the airflow structure and acceleration characteristics, this is explained, in part, by the presence of a spatially variable vegetation cover over the dunes. The vegetation is important both in terms of the varying aerodynamic roughness (z0) and problems concerning the definition of a zero-plane displacement (d). It is considered that any attempts to characterize surface shear stress over the Kalahari linear dunes, in order to predict sand transport and dune mobility, will be hampered by two problems. These are the progressively non-log-linear nature of the velocity profiles over the dunes caused by flow acceleration, and the production of thin near-surface boundary layers caused by areally variable aerodynamic roughness as a result of the partially vegetated nature of the dunes.
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Dune mobility and vegetation cover in the Southwest Kalahari Desert
Earth Surface Processes and Landforms, 1995Co-Authors: Giles F.s. Wiggs, David S.g. Thomas, Joanna E. Bullard, Ian LivingstoneAbstract:As part of a wider project investigating the palaeoenvironmental significance of partially vegetated linear dunes in the southwest Kalahari, data collected in the latter part of 1992 concerning dune movement and vegetation cover suggest that sediment transport is occurring on some dune surfaces, and that the majority of surface activity occurs on the crests and upper slopes of the dunes. The data suggest that the limiting variables on surface sediment movement vary on different parts of a dune. On interdunes and lower dune slopes the primary limiting variable is available wind energy, while on dune crests and upper slopes it is vegetation cover. Ground cover by litter has much greater importance in protecting the surface sediment from erosion than rooted vegetation. From individual data points, no evidence is found to support a threshold vegetation cover below which sediment movement occurs. Rather, a gradient of activity is suggested whereby a reduction in vegetation cover increases the potential for sediment movement and surface change. However, dunes with differing amounts of mean vegetation cover display differing degrees of surface activity, and at this scale, a vegetation cover threshold in the region of 14 per cent may be recognized.
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analysis of linear sand dune morphological variability southwestern Kalahari Desert
Geomorphology, 1995Co-Authors: Joanna E. Bullard, Ian Livingstone, David Thomas, Giles F.s. WiggsAbstract:Abstract Linear dunes are the most common Desert dune form, usually occurring in extensive dunefields rather than as isolated individuals. As part of a wider project investigating the dynamics and environmental significance of linear dunes, the extensive linear dunefield of the southwestern Kalahari Desert, southern Africa, was investigated for planimetric pattern variability. Considerable intradunefield variability was identified through aerial photograph analysis of a 4000 km 2 area, leading to the development of a five-class classification scheme. This scheme was validated statistically utilising data for key planimetric pattern variables: Y-junctions, termini, orientation range, and wavelength. The application of the classification scheme thoughout the dunefield permits the identification of trends in planimetric patterns. This provides a basis for first attempts to explain aspects of planimetric variability in terms of the behaviour of linear dunes and their responses to key environmental variables.
