The palaeoenvironmental history of a ~0.1 km2 delta-shaped tufaceous spring mound at Barcavecchia... more The palaeoenvironmental history of a ~0.1 km2 delta-shaped tufaceous spring mound at Barcavecchia, on the southwest flank of Etna, is investigated using biota assay, sedimentology and geochemistry. Dating control has been provided by radiocarbon and optically stimulated luminescence (OSL). The Barcavecchia sediments, comprising a lower lacustrine unit, and an upper marsh unit, provide a late Quaternary history of local conditions following drainage disruption of the Simeto river by hawaiite lavas and the outflow of carbonate rich groundwater from the Adrano Escarpment after 24 ka. Initially a small, saline lake formed before 17 ka, which dried out around 14 ka leaving a depositional hiatus. Subsequent sedimentation in a saline marsh environment preserved a sequence of tephra layers of local, hawaiite origin, of which the lowest (and thickest) was deposited between 8-6 ka, coincident with the recurrent activity of the Leone eruptive center. The upper sections preserve further tephra layers and a discontinuous pollen record, and probably terminates around 3-2 ka. These ages accord well with those obtained by ROMANO et alii (1987) from radiometric dating. The Holocene vegetation of the site mirrors that of the contemporary upper marsh, although tree species largely disappeared in the late Holocene.
The arid zone contains few perennial lakes; examples such as the Caspian Sea, the Dead Sea and La... more The arid zone contains few perennial lakes; examples such as the Caspian Sea, the Dead Sea and Lake Aral invariably have inflowing rivers rising in fringing uplands or distant humid areas. Instead, the majority of arid and semi-arid re-gions are characterised by endoreic ( ...
Large lake basins, now almost entirely devoid of water in the Kalahari's semi arid environment, c... more Large lake basins, now almost entirely devoid of water in the Kalahari's semi arid environment, can be clearly identified from Landsat imagery of northern Botswana. These basins are delineated by a series of beach ridges marking the former shorelines of an inundated area that, at its largest extent, encompassed 60,000 km 2. In the last 30 yr over 150 radiocarbon ages have been produced from three component basins that make up the Palaeo-Makgadikgadi system (consisting of the Ngami, Mababe and Makgadikgadi basins). The paucity of well preserved organic material in the region has led to these 14 C ages being derived from inorganic carbonates whose open system behaviour and polygenetic origin frequently result in unreliable dates from uncertain contexts and then only within the temporal limits of the radiocarbon method. The application of Optically Stimulated Luminescence (OSL) dating to beach ridge sediments from Lake Ngami [Shaw, P.A., Bateman, M.D., Thomas, D.S.G., Davies, F., 2003. Holocene fluctuations of Lake Ngami, Middle Kalahari: chronology and responses to climatic change. Quaternary International 111 (1), 23-35] improved chronological resolution, but was limited by the use of hand dug pits. The new application of OSL to samples derived from deep coring of beach ridges has allowed access to a much longer, three dimensional, geomorphic record than previously available. Seventyfour ages new OSL ages from shorelines are presented allowing, for the first time, the opportunity to unravel the complex history of lake stages in the Ngami basin. Multiple lake highstands are identified in the period ∼ 0.3 ka to ∼ 140 ka, with several causes of these events, including tropical and local climates and upstream channel avulsion, identified. To unravel the full paleaoclimatic significance of these events it will be necessary to identify the timing of high stands in the other terminal basins within the Okavango system.
... Southern Kalahari, southern Africa PAUL A. SHAW' School of Geology and Envir... more ... Southern Kalahari, southern Africa PAUL A. SHAW' School of Geology and Environmental Science,Luton College of Higher Education, Luton, England DAVID SC THOMAS and DAVID J. NASH Department of Geography, University of Sheffield, Sheffield S10 2TN, England ...
In the absence of closed sites, fossiliferous strata and regional or even local stratigraphic fra... more In the absence of closed sites, fossiliferous strata and regional or even local stratigraphic frameworks, Quaternary scenarios for the Kalahari have been developed only in the past 20 years based on deductive geomorphological studies. Landforms studied include palaeo-lakes, caves, pans, dry valleys and spring deposits, all indicative of wetter episodes. Attempts to establish the presence of episodes of aridity from the vast linear dune fields have not been successful, primarily because the function of linear dunes has been misunderstood. A chronological framework for the past 40,000 years has been established, based on a corpus of 239 radiocarbon dates, of which nearly half, using calcrete as the dating medium, require careful interpretation. One of the most important findings is that the entire Kalahari experienced a wetter episode from 16,000-13,000 B.P., followed by a period of lowering water tables and return to the present climatic mean. The Kalahari is thus out of phase with Africa north of the Equator, and with other major subtropical deserts. This has implications for global palaeo-climatic modelling, although the phenomenon could be explained by the orbital forcing hypothesis. The recent use of U/Th dating on cave sinters suggests this pattern extends back into earlier glacial/interglacial cycles.
Silcrete-calcrete intergrade duricrusts (surface or near-surface chemically precipitated crusts w... more Silcrete-calcrete intergrade duricrusts (surface or near-surface chemically precipitated crusts with a cement comprising a mixture of silica and CaCO3) have been widely identified in the geological, geomorphological and pedological literature, but have not, to date, been systematically described or classified. This paper presents a review of previous definitions of the end members of the silcrete-calcrete spectrum and subsequently identifies the major silica-carbonate relationships within intergrade types are identified on the the Kalahari of Botswana and Namibia. Three main intergrade types are identified on the basis of silica-carbonate associations: duricrusts where secondary silica occurs within a calcareous matrix; varieties where secondary carbonate occurs within a siliceous matrix; and materials where silica and carbonate matrix cements appear to have been precipitated contemporaneously or in close succession. Within each of these three groups, sub-types are identified dependent upon whether secondary materials have replaced or been emplaced within a pre-existing duricrust. Finally, a practical procedure for the simple definition of silcrete-calcrete intergrade duricrusts is suggested based upon a combination of bulk chemical and thin-section analyses.
Transactions of the Institute of British Geographers, 1985
ABSTRACT Lake Ngami and the Mababe Depression contain series of concentric shoreline features con... more ABSTRACT Lake Ngami and the Mababe Depression contain series of concentric shoreline features controlled by volume: area thresholds in the Okavango Delta region. Of particular interest are the 945 m (Lake Palaeo-Makgadikgadi) and the 936 m shorelines, the latter ...
... for the historical period in figure 2. The Okavango Delta covers an area of approximately 22,... more ... for the historical period in figure 2. The Okavango Delta covers an area of approximately 22,000 km2, receiving an average annual inflow of 10,5 x 109 m3 (UNDP/FAO vol 1:1). About 95% of this water is lost within the Delta by evapo-transpiration or groundwater recharge, the ...
The sediments and landforms at the Tsodilo Hills, in the northwestern Kalahari desert, provide an... more The sediments and landforms at the Tsodilo Hills, in the northwestern Kalahari desert, provide an opportunity to directly investigate the late Quaternary wetting and drying of the region from evidence at a single site. Lacustrine carbonates, including incorporated molluscs and diatoms, a lake shoreline feature and stabalised linear dunes were investigated for their constituent palaeoenvironmental signals. Chronometric control is provided by calibrated 14 C; AMS and OSL dating. The evidence suggests that linear dune construction has not occurred since the Last Glacial Maximum, with particular development from 36 to 28 ka: Lake stands indicating wetter regional conditions than present occurred at 40-32 ka; with more seasonal conditions from 36 ka; and at 27-12 ka with a possible drying out at 22-19 ka: Data are consistent with other independent studies from the region, and with recent evidence obtained from Atlantic cores off the coast of Namibia. It is concluded that careful consideration of multi-proxy data from a single location can assist in resolving discrepancies that arise from independent studies of lake, cave and dune records in the Kalahari.
Little is known about the timing of dune-building episodes in the southwestern Kalahari Desert. P... more 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 t...
The extensive shoreline deposits of Lake Chilwa, southern Malawi, a shallow water body today cove... more The extensive shoreline deposits of Lake Chilwa, southern Malawi, a shallow water body today covering 600 km 2 of a basin of 7500 km 2 , are investigated for their record of late Quaternary highstands. OSL dating, applied to 36 samples from five sediment cores from the northern and western marginal sand ridges, reveal a highstand record spanning 44 ka. Using two different grouping methods, highstand phases are identified at 43.7-33.3 ka, 26.2-21.0 ka and 17.9-12.0 ka (total error method) or 38.4-35.5 ka, 24.3-22.3 ka, 16.2-15.1 ka and 13.5-12.7 ka (Finite Mixture Model age components) with two further discrete events recorded at 11.01 AE 0.76 ka and 8.52 AE 0.56 ka. Highstands are comparable to the timing of wet phases from other basins in East and southern Africa, demonstrating wet conditions in the region before the LGM, which was dry, and a wet Lateglacial, which commenced earlier in the southern compared to northern hemisphere in East Africa. We find no evidence that wet phases are insolation driven, but analysis of the dataset and GCM modelling experiments suggest that Heinrich events may be associated with enhanced monsoon activity in East Africa in both timing and as a possible causal mechanism.
High-resolution δ18O and δ13C analyses of a stalagmite from Lobatse II Cave reveal late Pleistoce... more High-resolution δ18O and δ13C analyses of a stalagmite from Lobatse II Cave reveal late Pleistocene environmental changes in Botswana. Large shifts in δ18O and δ13C are observed between two main periods of deposition. The first period, between 51,000 and 43,000 yr B.P., was warm and humid and may have been associated with some C3 vegetation. The second period, between 27,000 and 21,000 yr B.P., had temperatures approximately 2°C lower, and vegetation dominated by drought-adapted C4 plants. The intervening period, between 43,000 and 27,000 yr B.P., bounded by two major hiatuses in stalagmite growth, produced discontinuous speleothem formation, probably under dry conditions. New 230 Th/234U mass spectrometry age determinations for the stalagmite generally agree with previously measured 230 Th/234U ages.
The palaeoenvironmental history of a ~0.1 km2 delta-shaped tufaceous spring mound at Barcavecchia... more The palaeoenvironmental history of a ~0.1 km2 delta-shaped tufaceous spring mound at Barcavecchia, on the southwest flank of Etna, is investigated using biota assay, sedimentology and geochemistry. Dating control has been provided by radiocarbon and optically stimulated luminescence (OSL). The Barcavecchia sediments, comprising a lower lacustrine unit, and an upper marsh unit, provide a late Quaternary history of local conditions following drainage disruption of the Simeto river by hawaiite lavas and the outflow of carbonate rich groundwater from the Adrano Escarpment after 24 ka. Initially a small, saline lake formed before 17 ka, which dried out around 14 ka leaving a depositional hiatus. Subsequent sedimentation in a saline marsh environment preserved a sequence of tephra layers of local, hawaiite origin, of which the lowest (and thickest) was deposited between 8-6 ka, coincident with the recurrent activity of the Leone eruptive center. The upper sections preserve further tephra layers and a discontinuous pollen record, and probably terminates around 3-2 ka. These ages accord well with those obtained by ROMANO et alii (1987) from radiometric dating. The Holocene vegetation of the site mirrors that of the contemporary upper marsh, although tree species largely disappeared in the late Holocene.
The arid zone contains few perennial lakes; examples such as the Caspian Sea, the Dead Sea and La... more The arid zone contains few perennial lakes; examples such as the Caspian Sea, the Dead Sea and Lake Aral invariably have inflowing rivers rising in fringing uplands or distant humid areas. Instead, the majority of arid and semi-arid re-gions are characterised by endoreic ( ...
Large lake basins, now almost entirely devoid of water in the Kalahari's semi arid environment, c... more Large lake basins, now almost entirely devoid of water in the Kalahari's semi arid environment, can be clearly identified from Landsat imagery of northern Botswana. These basins are delineated by a series of beach ridges marking the former shorelines of an inundated area that, at its largest extent, encompassed 60,000 km 2. In the last 30 yr over 150 radiocarbon ages have been produced from three component basins that make up the Palaeo-Makgadikgadi system (consisting of the Ngami, Mababe and Makgadikgadi basins). The paucity of well preserved organic material in the region has led to these 14 C ages being derived from inorganic carbonates whose open system behaviour and polygenetic origin frequently result in unreliable dates from uncertain contexts and then only within the temporal limits of the radiocarbon method. The application of Optically Stimulated Luminescence (OSL) dating to beach ridge sediments from Lake Ngami [Shaw, P.A., Bateman, M.D., Thomas, D.S.G., Davies, F., 2003. Holocene fluctuations of Lake Ngami, Middle Kalahari: chronology and responses to climatic change. Quaternary International 111 (1), 23-35] improved chronological resolution, but was limited by the use of hand dug pits. The new application of OSL to samples derived from deep coring of beach ridges has allowed access to a much longer, three dimensional, geomorphic record than previously available. Seventyfour ages new OSL ages from shorelines are presented allowing, for the first time, the opportunity to unravel the complex history of lake stages in the Ngami basin. Multiple lake highstands are identified in the period ∼ 0.3 ka to ∼ 140 ka, with several causes of these events, including tropical and local climates and upstream channel avulsion, identified. To unravel the full paleaoclimatic significance of these events it will be necessary to identify the timing of high stands in the other terminal basins within the Okavango system.
... Southern Kalahari, southern Africa PAUL A. SHAW' School of Geology and Envir... more ... Southern Kalahari, southern Africa PAUL A. SHAW' School of Geology and Environmental Science,Luton College of Higher Education, Luton, England DAVID SC THOMAS and DAVID J. NASH Department of Geography, University of Sheffield, Sheffield S10 2TN, England ...
In the absence of closed sites, fossiliferous strata and regional or even local stratigraphic fra... more In the absence of closed sites, fossiliferous strata and regional or even local stratigraphic frameworks, Quaternary scenarios for the Kalahari have been developed only in the past 20 years based on deductive geomorphological studies. Landforms studied include palaeo-lakes, caves, pans, dry valleys and spring deposits, all indicative of wetter episodes. Attempts to establish the presence of episodes of aridity from the vast linear dune fields have not been successful, primarily because the function of linear dunes has been misunderstood. A chronological framework for the past 40,000 years has been established, based on a corpus of 239 radiocarbon dates, of which nearly half, using calcrete as the dating medium, require careful interpretation. One of the most important findings is that the entire Kalahari experienced a wetter episode from 16,000-13,000 B.P., followed by a period of lowering water tables and return to the present climatic mean. The Kalahari is thus out of phase with Africa north of the Equator, and with other major subtropical deserts. This has implications for global palaeo-climatic modelling, although the phenomenon could be explained by the orbital forcing hypothesis. The recent use of U/Th dating on cave sinters suggests this pattern extends back into earlier glacial/interglacial cycles.
Silcrete-calcrete intergrade duricrusts (surface or near-surface chemically precipitated crusts w... more Silcrete-calcrete intergrade duricrusts (surface or near-surface chemically precipitated crusts with a cement comprising a mixture of silica and CaCO3) have been widely identified in the geological, geomorphological and pedological literature, but have not, to date, been systematically described or classified. This paper presents a review of previous definitions of the end members of the silcrete-calcrete spectrum and subsequently identifies the major silica-carbonate relationships within intergrade types are identified on the the Kalahari of Botswana and Namibia. Three main intergrade types are identified on the basis of silica-carbonate associations: duricrusts where secondary silica occurs within a calcareous matrix; varieties where secondary carbonate occurs within a siliceous matrix; and materials where silica and carbonate matrix cements appear to have been precipitated contemporaneously or in close succession. Within each of these three groups, sub-types are identified dependent upon whether secondary materials have replaced or been emplaced within a pre-existing duricrust. Finally, a practical procedure for the simple definition of silcrete-calcrete intergrade duricrusts is suggested based upon a combination of bulk chemical and thin-section analyses.
Transactions of the Institute of British Geographers, 1985
ABSTRACT Lake Ngami and the Mababe Depression contain series of concentric shoreline features con... more ABSTRACT Lake Ngami and the Mababe Depression contain series of concentric shoreline features controlled by volume: area thresholds in the Okavango Delta region. Of particular interest are the 945 m (Lake Palaeo-Makgadikgadi) and the 936 m shorelines, the latter ...
... for the historical period in figure 2. The Okavango Delta covers an area of approximately 22,... more ... for the historical period in figure 2. The Okavango Delta covers an area of approximately 22,000 km2, receiving an average annual inflow of 10,5 x 109 m3 (UNDP/FAO vol 1:1). About 95% of this water is lost within the Delta by evapo-transpiration or groundwater recharge, the ...
The sediments and landforms at the Tsodilo Hills, in the northwestern Kalahari desert, provide an... more The sediments and landforms at the Tsodilo Hills, in the northwestern Kalahari desert, provide an opportunity to directly investigate the late Quaternary wetting and drying of the region from evidence at a single site. Lacustrine carbonates, including incorporated molluscs and diatoms, a lake shoreline feature and stabalised linear dunes were investigated for their constituent palaeoenvironmental signals. Chronometric control is provided by calibrated 14 C; AMS and OSL dating. The evidence suggests that linear dune construction has not occurred since the Last Glacial Maximum, with particular development from 36 to 28 ka: Lake stands indicating wetter regional conditions than present occurred at 40-32 ka; with more seasonal conditions from 36 ka; and at 27-12 ka with a possible drying out at 22-19 ka: Data are consistent with other independent studies from the region, and with recent evidence obtained from Atlantic cores off the coast of Namibia. It is concluded that careful consideration of multi-proxy data from a single location can assist in resolving discrepancies that arise from independent studies of lake, cave and dune records in the Kalahari.
Little is known about the timing of dune-building episodes in the southwestern Kalahari Desert. P... more 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 t...
The extensive shoreline deposits of Lake Chilwa, southern Malawi, a shallow water body today cove... more The extensive shoreline deposits of Lake Chilwa, southern Malawi, a shallow water body today covering 600 km 2 of a basin of 7500 km 2 , are investigated for their record of late Quaternary highstands. OSL dating, applied to 36 samples from five sediment cores from the northern and western marginal sand ridges, reveal a highstand record spanning 44 ka. Using two different grouping methods, highstand phases are identified at 43.7-33.3 ka, 26.2-21.0 ka and 17.9-12.0 ka (total error method) or 38.4-35.5 ka, 24.3-22.3 ka, 16.2-15.1 ka and 13.5-12.7 ka (Finite Mixture Model age components) with two further discrete events recorded at 11.01 AE 0.76 ka and 8.52 AE 0.56 ka. Highstands are comparable to the timing of wet phases from other basins in East and southern Africa, demonstrating wet conditions in the region before the LGM, which was dry, and a wet Lateglacial, which commenced earlier in the southern compared to northern hemisphere in East Africa. We find no evidence that wet phases are insolation driven, but analysis of the dataset and GCM modelling experiments suggest that Heinrich events may be associated with enhanced monsoon activity in East Africa in both timing and as a possible causal mechanism.
High-resolution δ18O and δ13C analyses of a stalagmite from Lobatse II Cave reveal late Pleistoce... more High-resolution δ18O and δ13C analyses of a stalagmite from Lobatse II Cave reveal late Pleistocene environmental changes in Botswana. Large shifts in δ18O and δ13C are observed between two main periods of deposition. The first period, between 51,000 and 43,000 yr B.P., was warm and humid and may have been associated with some C3 vegetation. The second period, between 27,000 and 21,000 yr B.P., had temperatures approximately 2°C lower, and vegetation dominated by drought-adapted C4 plants. The intervening period, between 43,000 and 27,000 yr B.P., bounded by two major hiatuses in stalagmite growth, produced discontinuous speleothem formation, probably under dry conditions. New 230 Th/234U mass spectrometry age determinations for the stalagmite generally agree with previously measured 230 Th/234U ages.
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