French National Centre for Scientific Research
Department:Institut des Sciences de l'Evolution
Position:Chargé de recherche 1ère classe
Brian started following the work of Simon Haberle, The Australian National University, Archaeology and Natural History.
Brian started following the work of 2 people.
Brian added 2 papers
Papers
Mid-Holocene background climate in the south-eastern Pacific and its influence on South America
Carré, M., Azzoug, M., Bentaleb, I., Chase, B. M., Fontugne, M., Jackson, D., Ledru, M.-P., Maldonado, A., Sachs, J. P. (in press). Quaternary International.
The eastern tropical Pacific plays a key role in the tropical atmospheric circulation and in the global carbon cycle, and assessing the sensitivity of this region to global climate changes is a major challenge facing climatologists. Provided here is a synthesis of proxy records of the mean climate of the mid-Holocene (5–8 ka) along the south eastern Pacific margin and four regions of South America. These regions were selected for the strength and stability of ENSO teleconnections, and located outside the direct influence of the intertropical convergence zone or the southern westerlies in order to avoid the overprinting signal of their insolation-related variations and focus on the relationship between the eastern tropical Pacific and South America. This study is based on a review of published multiproxy data as well as new isotopic data from the Peruvian and Chilean coast. The available evidence indicates that sea-surface temperatures were ∼1–4 °C cooler from the Galapagos to the southern Peruvian coast as a result of increased coastal upwelling forced by changes in longshore windfields. The mean La Niña-like conditions in the eastern South Pacific were associated to aridity in southern Brazil and along the whole South American Pacific coast from central Chile to the Galapagos, and to wetter conditions on the western central Andes. This regional synthesis provides a coherent picture of the South American mean climate that is very similar to the modern precipitation pattern observed during La Niña conditions, suggesting that atmospheric teleconnections linking the South Eastern Pacific to these continental areas were similar in the middle Holocene.
A 19.5 kyr vegetation history from the central Cederberg Mountains, South Africa: palynological evidence from rock hyrax middens
Quick, L. J., Chase, B. M., Meadows, M. E., Scott, L., and Reimer, P. J. (2011). Palaeogeography, Palaeoclimatology, Palaeoecology, 253-270.
In arid and semi-arid areas such as southern Africa, rock hyrax (Procavia capensis) middens represent an exceptionally valuable source of late Quaternary palaeoenvironmental information. Pollen and stable isotope data derived from two rock hyrax middens extracted from De Rif in the Cederberg Mountains of the southwestern Cape, casts new light on climatic changes that occurred across the Last Glacial–Interglacial Transition (LGIT) and the Holocene (19.5–0.7 cal kBP) and how the region's vegetation responded to these changes. Significant changes in vegetation community composition would be expected in response to environmental changes characteristic of the contrast between ‘glacial’ and ‘interglacial’ climates. However, the pollen assemblages in general indicate that mountain fynbos remained dominant throughout the record and that most of the pollen taxa exhibited only muted frequency variations. This finding could be considered to be inconsistent with indications of marked climatic variations recorded in the stable isotope records for the same midden. An analysis of the geological setting and the bioclimatic affinities of the taxa suggest that the explanation for this inconsistency may lie in the dominating influence of sandstone substrates and the relative resilience of mountain fynbos vegetation to climate change.
Catastrophic Drought in the Afro-Asian Monsoon Region During Heinrich Event 1
Stager, J. C., Ryves, D. B., Chase, B. M., and Pausata, F. S. R. (2011), Science 331, 1299-1302.
Between 15,000 and 18,000 years ago, large amounts of ice and meltwater entered the North Atlantic during Heinrich stadial 1. This caused substantial regional cooling, but major climatic impacts also occurred in the tropics. Here, we demonstrate that the height of this stadial, about 16,000 to 17,000 years ago (Heinrich event 1), coincided with one of the most extreme and widespread megadroughts of the past 50,000 years or more in the Afro-Asian monsoon region, with potentially serious consequences for Paleolithic cultures. Late Quaternary tropical drying commonly is attributed to southward drift of the intertropical convergence zone, but the broad geographic range of the Heinrich event 1 megadrought suggests that severe, systemic weakening of Afro-Asian rainfall systems also occurred, probably in response to sea surface cooling.
Late glacial interhemispheric climate dynamics revealed in South African hyrax middens
Chase, B. M., Quick, L., Meadows, M. E., Scott, L., Thomas, D. S. G., and Reimer, P. J. (2011). Geology, 39, 19-22.
Central to understanding changes in the global climate system is our ability to identify the timing and extent of past major climate fluctuations. Of the events that have occurred in recent geological time, the Younger Dryas (YD, 13 – 11.5 ka), an abrupt return to near-glacial conditions during the Last Glacial – Interglacial Transition (LGIT, ~18 – 11.5 ka), is one of the most widely reported. While this event is apparent throughout the northern hemisphere (Peteet, 1995), evidence for its occurrence in the southern hemisphere remains equivocal due to a lack of well-dated terrestrial records. Here we report high resolution stable carbon and nitrogen isotope records obtained from a rock hyrax midden, revealing the first unequivocal terrestrial manifestation of the YD from the southern African subtropics. These results provide key evidence for the relative influence of the YD, and suggest that a subtropical – temperate transition zone existed along the oceanic Subtropical Front (~41°S) across the southern hemisphere, with the northern hemisphere exerting a strong influence on all but the higher latitudes of the southern hemisphere after Heinrich Stadial 1 (15 ka).
Evidence for Holocene aridification in southern Africa recorded in Namibian hyrax middens: implications for African Monsoon dynamics and the ‘African Humid Period’
Chase, B. M., Meadows, M. E., Carr, A.S., and Reimer, P. J. (2010). Quaternary Research 74, 36-45.
Presented here are stable nitrogen isotope data from a rock hyrax (Procavia capensis) middens from northwestern Namibia that record a series of rapid aridification events beginning at ca. 3800 cal yr BP, and which mark a progressive decrease in regional humidity across the Holocene. Strong correlations exist between this record and other terrestrial and marine archives from southern Africa, indicating that the observed pattern of climate change is regionally coherent. Combined, these data indicate hemispheric synchrony in tropical African climate change during the Holocene, with similar trends characterising the termination of the ‘African Humid Period’ (AHP) in both the northern and southern tropics. These findings run counter to the widely accepted model of direct low-latitude insolation forcing, which requires an anti-phase relationship to exist between the hemispheres. The combined dataset highlights: 1) the importance of forcing mechanisms influencing the high northern latitudes in effecting low-latitude climate change in Africa, and 2) the potential importance of solar forcing and variations in the Earth's geomagnetic shield in determining both long-term and rapid centennial-scale climate changes, identifying a possible mechanism for the variations marking the AHP termination in both the southern and northern tropics.
South African palaeoenvironments during marine oxygen isotope stage 4: a context for the Howiesons Poort and Still Bay industries
Chase, B. M. (2010). Journal of Archaeological Science 37, 1359-1366.
Spanning the period from ~74 – 58 ka, and being broadly coeval with marine oxygen isotope stage 4 (MIS 4), the Still Bay and Howiesons Poort industries represent important phases in the development of the material culture of early modern humans. How and why these industries occurred when they did, however, remain matters for speculation. A key element of this debate has been the question of what role environmental change during the early stages of the last glacial period may have had on influencing human activity. Unfortunately, well-dated palaeoenvironmental evidence from this period is limited, and the debate has, out of necessity, been driven by the application of conceptual models of environmental change, many of which associate colder and warmer global climates with increased aridity and humidity respectively. In this paper, the regional marine and terrestrial evidence is synthesised to create an evidence based palaeoenvironmental context. Based on the evidence available to date, it appears that rather than being characterised by dry conditions, MIS 4 was a period of relatively cool, moist climates, the ubiquity of which transcends the modern regional climate boundaries. It is suggested that orbital parameters at this time created greater interaction between tropical and temperate circulation systems, bringing more moisture to the subcontinent. These findings highlight the distinct nature of southern African climate systems, and caution against the application extra-regional conceptual models or inferring linear relationships with polar records.
DownloadThe potential of plant biomarker evidence derived from rock hyrax middens as an indicator of palaeoenvironmental change
Carr, A. S., Boom, A., Chase, B. M. (2010). Palaeogeography, Palaeoclimatology, Palaeoecology 285, 321-330
Hyrax middens are unique environmental archives with the potential to provide unprecedented high-resolution palaeoenvironmental records, particularly in the arid regions of southern Africa. This study provides the first detailed characterisation of the organic matter composition of hyraceum and aims to identify biomarker evidence capable of providing new or supplementary palaeoenvironmental data from these novel archives. Pyrolysis gas chromatography mass spectrometry reveals hyraceum to be dominated by nitrogen-containing aromatic compounds, notably benzamide. This is almost certainly derived directly from the hyrax urine and is probably the main source of nitrogen as measured in bulk δ15N measurements. Solvent-extractable lipids comprise homologous suites of long-chain n-alkanes (C24–C34) and n-alkanols (C16–C26), characteristic of higher plant leaf waxes, along with an abundance of animal-derived sterols, higher plant sterols and terpenoids; as well as the ubiquitous benzamide.
n-alkane distributions and compound specific δ13C clearly differentiate samples from the C3 vegetation dominated Cape Floristic Region, and the more arid, C4 grass-rich savannas of central Namibia (Klein Spitzkoppe). Distinct changes in n-alkane distribution and δ13C are observed within the Spitzkoppe midden; most notably the mid to late Holocene period (c. 6000–2000 cal yr BP), which records a progressive reduction in grass-derived organic matter inputs. Based on the animal's feeding behaviour and contemporary site ecology, a phase of increasing aridity is inferred, which is consistent with other proxy data from this site (e.g. bulk δ15N). The excellent preservation of plant and animal biomarkers suggests that there is significant potential for midden-derived biomarkers to provide long-term palaeoenvironmental proxies.
Molecular fingerprinting of lake sediment organic matter: an example from the southern Cape coastline of South Africa
Carr, A. S., Boom, A., Chase, B. M., Roberts, D.L., Roberts, Z.E. (2010). Journal of Paleolimnology. 44, 947-961
Pyrolysis–gas chromatography mass spectrometry (py-GC/MS) allows the characterisation of complex macromolecular organic matter. In lakes and wetlands this can potentially be used to assess the preservation/diagenesis and provenance of sediment organic matter. It can complement palaeoenvironmental investigations utilising ‘bulk’ sediment variables such as total organic carbon (TOC) and TOC/total nitrogen ratios. We applied py-GC/MS analyses to a ~32,000-year sediment record from the southern Cape coastline of South Africa. We used the results to evaluate the sources and extent of degradation of organic matter in this semi-arid environment. Marked down-core changes in the relative abundance of multiple pyrolysis products were observed. Correspondence analysis revealed that the major driver of this down-core variability in OM composition was selective preservation/degradation. Samples comprising highly degraded OM are primarily confined to the lower half of the core, older than ~12,000 years, and are characterised by suites of low-molecular-weight aromatic pyrolysis products. Samples rich in organic matter, e.g. surface sediments, are characterised by products derived from fresh emergent or terrestrial vegetation, which include lignin monomers, plant-derived fatty acids and long-chain n-alkanes. Pyrolysates from the late glacial-early Holocene period, approximately mid-way down the core are characterised by distinct suites of long-chain n-alkene/n-alkane doublets, which may reflect the selective preservation of recalcitrant aliphatic macromolecules and/or enhanced inputs of the algal macromolecule algaenan/polymerised algal lipids. Increased TOC, lower δ13C and increased abundance of more labile lignin and fatty acid products at the same depths suggest this period was associated with increased lake primary productivity and enhanced inputs of terrestrial OM. TOC is the only ‘bulk’ parameter correlated with the correspondence analysis axes extracted from the py-GC/MS data. Distinct fluctuations in TOC/total nitrogen ratio are not explained by variation in organo-nitrogen pyrolysis products. Notwithstanding, the study suggests that py-GC/MS has potential to complement palaeolimnological investigations, particularly in regions such as southern Africa, where other paleoenvironmental proxy variables in sediments may be lacking or equivocal.
Holocene palaeoenvironments of the Cederberg and Swartruggens mountains, Western Cape, South Africa: Pollen and stable isotope evidence from hyrax dung middens
Meadows, M.E., Chase, B. M., Seliane, M. (2010). Journal of Arid Environments 74, 786-793.
The use of pollen analysis of herbivore middens in arid and semi-arid areas has facilitated palaeoenvironmental reconstruction and interpretation in places where this may otherwise be impossible due to the absence of organic sediments. This paper presents evidence based on pollen analysis and stable isotope data of hyrax (Procavia capensis) dung middens in the reconstruction and interpretation of the Holocene vegetation history of the Cederberg and Swartruggens mountains. Hyrax middens from upland sites in the Cape Fold Mountains, specifically at Katbakkies Pass (KB) in the Swartruggens and Truitjes Kraal (TK) in the Cederberg, are used to reconstruct past vegetation and palaeoenvironments. Radiocarbon dating of a 20 cm sample at Katbakkies confirms a late Holocene sequence, with sedimentation commencing around 3700 cal yr BP and terminating within the last 600 years. At Truitjes Kraal, a smaller (11 cm) section appears to represent much of the Holocene, with accumulation spanning the period from around 9500 to 1300 cal yr BP. Stable isotope and pollen analyses concur in indicating only relatively subtle changes in vegetation conditions over the Holocene, a conclusion that is compared with and supported by other palaeoecological evidence from the region.
A record of rapid Holocene climate change preserved in hyrax middens from SW Africa
Chase, B. M., Meadows, M. E., Scott, L., Thomas, D. S. G., Marais, E., Sealy, J., and Reimer, P. J. (2009). Geology 37, 703-706
The discovery of sensitive paleoenvironmental proxies contained within fossilized rock hyrax middens from the margin of the central Namib Desert, Africa, is providing unprecedented insight into the region’s environmental history. High-resolution stable carbon and nitrogen isotope records spanning 0–11,700 cal (calibrated) yr B.P. indicate phases of relatively humid conditions from 8700–7500, 6900–6700, 5600–4900, and 4200–3500 cal yr B.P., with a period of marked aridity occurring from 3500 until ca. 300 cal yr B.P. Transitions between these phases appear to have occurred very rapidly, often within <200 years. Of particular importance are: (1) the observed relationship between regional aridification and the decline in Northern Hemisphere insolation across the Holocene, and (2) the signifi cance of suborbital scale variations in climate that covary strongly with fluctuations in solar forcing. Together, these elements call for a fundamental re-examination of the role of orbital forcing on tropical African systems, and a reconsideration of what factors drive climate change in the region. The quality and resolution of these data far surpass any other evidence available from the region, and the continued development of this unique archive promises to revolutionize paleoenvironmental studies in southern Africa.
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Evaluating the use of dune sediments as a proxy for palaeo-aridity: a southern African case study
Chase, B. M. (2009). Earth-Science Reviews 93, 31-45.
The dominance of dryland environments in the Southern Hemisphere makes the study of these regions of critical importance for the development of regional, hemispheric and global models of environmental change. Unfortunately, the wetting and drying cycles associated with semi-hyperarid climates are not conducive to the preservation of traditional organic proxy data sources. The last decade, however, has seen the development of a number of alternative archives including the application of luminescence dating techniques to dunes and other aeolian deposits. In continental situations, the existence of relict dune fields as long been thought to be evidence of drier conditions during the Pleistocene, and direct ages from these features have been interpreted almost exclusively as indicating phases of aridity. However, an increasing number of ages from a broader range of environments are calling into question the assumption that aeolian activity can be simply equated with aridity. Presented here is a comparison of dune ages that have been obtained from across southern Africa with a range of proxies from both terrestrial and marine records. Taken as a whole, three primary phases of activity can be identified at ~60–40, 35–20 and 17–4 ka. The frequent discordance with other terrestrial records indicating coeval increases in humidity and the close correlation of these phases with wind strength proxies suggest that aridity is unlikely to be the sole, or even primary, forcing mechanism for aeolian activity in the region, and the palaeoclimatic significance of these sedimentary archives needs to be reassessed.
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Last Glacial Maximum dune activity in the Kalahari Desert of southern Africa: observations and simulations
Chase, B. M. and Brewer, S. (2009). Quaternary Science Reviews 28, 301-307.
It has long been understood that as ephemeral landscape features sand dunes are highly sensitive to environmental change, and thus their distribution and the timing of their development may provide clues to past climate dynamics. The relationship between climate and dune activity, however, is neither simple nor straightforward, with a range of controls affecting the balance between erodibility (the availability of sediment for deflation) and erosivity (the potential for sediment transport). To explore such complex systems over large spatial and temporal scales, a number of dune activity indices (DAI) have been created that incorporate wind speed and moisture balances to calculate the potential for, and degree of dune mobilisation. Using modern weather station data, these indices have generally been shown to provide reasonable indications of dune activity potential. Until recently, however, the detailed quantitative data required to inform these equations has not been available for past climate scenarios, and attempts to determine the relative importance of the various controls of dune activity have relied on rough estimations of climatic parameters. This paper combines data from monthly general circulation model (GCM) outputs from the coupled Ocean-Atmosphere GCMs for 21 ka with the most detailed DAI equation presently available to calculate the potential for dune reactivation in southern Africa during the Last Glacial Maximum (LGM, 18–24 ka). Based on these data and calculations it is indicated that there was significantly less potential for dune activity across southern Africa at 21 ka. When compared to the aeolian sediment records from the region, this study poses serious and fundamental questions about: 1) the reliability of the model outputs, 2) the degree to which DAIs are able to account for the complexity and dynamics of aeolian systems, and/or 3) the interpretation of dune records as palaeoclimatic proxies at millennial time scales.
Speciation and radiations track climate transitions since the Miocene Climatic Optimum: a case study of southern African chameleon
Tolley, K. A., Chase, B. M., and Forest, F. (2008). Journal of Biogeography 35, 1402-1414.
Aim The high amount of species diversity concentrated in southern Africa has been attributed to palaeoclimatic factors, and the timing of radiations in some taxa corresponds to global palaeoclimatic trends. Using dwarf chameleons (Bradypodion: Chamaeleonidae) as a model system, we explored the relationship between palaeoclimatic fluctuations and cladogenesis with respect to both temporal and spatial patterns in an effort to understand the process of speciation in southern Africa.
Location South Africa, with particular emphasis on the Cape Floristic Region and the Maputaland–Pondoland–Albany hotspot.
Methods Mitochondrial sequence data (ND2 and 16S) were used to estimate the timing of major radiations and to examine the number of lineages through time. A dated phylogeny was constructed using Bayesian phylogenetic reconstruction, and a Bayesian relaxed molecular clock was used to estimate divergence times. Spatial data and lineage-through-time plots were used to identify geographic regions that underwent diversification in connection with major climatic events. Both parsimony and likelihood optimizations of habitat type on the phylogeny were used to determine whether major habitat shifts have occurred. On a coarse scale (half-degree grid cells), phylogenetic diversity (sum of the branch lengths linking terminals) was compared with species richness (absolute number of species) to identify areas of conservation importance.
Results The complete species phylogeny of dwarf chameleons shows that the timing and mode of diversification exhibit spatio-temporal patterns that link to phases in the evolution of southern Africa’s climate over the last 14 Myr. Optimizations of habitat on the phylogenetic tree show a progression from closed to open habitats since the Mid-Miocene, corresponding to the shift from C3 to C4 environments, and later with the development of south-western Africa’s winter-rainfall regime. These shifts are not simultaneous across the region, with different geographic centres of diversity generated during different time periods.
Main conclusions Regions that are prominent centres of chameleon diversification are encompassed by the current biodiversity hotspots as shown by chameleon species richness and phylogenetic diversity. Diversity within the Cape Floristic Region appears to be the result of a Late Pliocene radiation, whereas the diversity encompassed within the Maputaland–Pondoland–Albany hotspot is an aggregate of asynchronous radiation events, probably influenced by lineage losses. Overall, dwarf chameleons have experienced a shift in habitat types, with recent radiations occupying open habitats, and older lineages persisting in relictual forested habitats, corresponding to the continental shift of vegetation types since the Miocene Climatic Optimum.
Testing the applicability of a Standardized Growth Curve (SGC) for quartz OSL dating: Kalahari dunes, South Africa
Telfer M, Bateman M. D., Carr A, Chase B. M. (2008). Quaternary Geochronology 3, 137-142.
Standardized growth curves (SGCs) have recently been developed as a means to reduce measurement times when determining palaeodoses in the standardized luminescence (OSL) dating of quartz, but this approach is not without its critics. We test the performance of SGCs derived from samples from a range of environments in southern Africa and Florida (United States of America). Data presented here suggest that the method performed well when determining low (<50 Gy) equivalent dose (De) values from samples with different geological provenances within South Africa. However, the application of an SGC built from South African data performed poorly when used to estimate the palaeodoses of samples from Florida, with seven out of ten samples failing to replicate the single aliquot regenerative (SAR)-derived De value to within ±10%. We thus advocate the use only of regionally based SGCs for full De determination, although SGCs can also be utilized as a way of quickly generating SAR ‘range finders’ irrespective of sample origin. Furthermore, one sample from the Florida site did not show regenerative growth in accordance with any samples from either the South Africa or Florida data, resulting in significant De underestimation when using the SGC. Although the reasons for such behaviour are not yet understood, this re-iterates the recommendation of Burbidge et al. [2006. De determination for young samples using the standardised OSL response of coarse-grain quartz. Radiation Measurements 41(3), 278], to incorporate a single regenerative step in SGC protocols in order to check consistency.
The Late-Devensian pro-glacial Lake Humber: new evidence from littoral deposits at Ferrybridge, Yorkshire, England
Bateman, M. D., Buckland, P. C., Chase, B., Frederick, C. D., Gaunt, G. D. (2008 . Boreas 37, 195-210.
Proglacial Lake Humber is of UK national significance in terms of landscape drainage and development of the British Ice Sheet (BIS) during Marine Isotope Stage 2 (MIS 2), yet it is poorly understood in terms of its dynamics and timing. Sands and gravels exposed at Ferrybridge, West Yorkshire, UK, are interpreted as part of the Upper Littoral sands and gravels related to a high-level Lake Humber, which inundated the Humber Basin to ∼30 m OD during MIS 2. Excavations exposed well-rounded gravels of local origin extending downslope from the 27.5 m OD contour and interbedded sands and fine gravels, which are interpreted as the coarse littoral deposits and nearshore associated deposits. A sample from the distal sands returned an Optically Stimulated Luminescence age of 16.6±1.2 kyr, providing the first direct age for the high-level lake and for when North Sea Basin ice must have blocked the Humber Gap. An underlying sequence included a diamicton dated to after 23.3 ±1.5 kyr and before 20.5±1.2 kyr, indicating that the Late Devensian ice reached at least 15 km south of the Escrick Moraine prior to the high-level lake. Previous to both the high-level lake and this ice advance, loess found at the two sites investigated indicates a long period of loess deposition earlier in MIS 2. These new data for the history of Lake Humber are discussed in the context of ice-marginal oscillations in both the Vale of York and the North Sea Basin.
Late Quaternary dynamics of southern Africa's winter-rainfall zone
Chase, B. M., and Meadows, M. E. (2007). Earth-Science Reviews 84, 103-138.
Variations in the nature and extent of southern Africa's winter rainfall zone (WRZ) have the potential to provide important information concerning the nature of long-term climate change at both regional and hemispheric scales. Positioned at the interface between tropical and temperate systems, southern Africa's climate is influenced by shifts in the Intertropical Convergence Zone, the westerlies, and the development and position of continental and oceanic anticyclones. Over the last glacial–interglacial cycle substantial changes in the amount and seasonality of precipitation across the subcontinent have been linked to the relative dominance of these systems. Central to this discussion has been the extent to which the region's glacial climates would have been affected by expansions of Antarctic sea-ice, equatorward migrations of the westerlies, more frequent/intense winter storms and an expanded WRZ.
This paper reviews the developing body of evidence pertaining to shifts in the WRZ, and the evolution of ideas that have been presented to explain the patterns observed. Dividing the region into three separate axes, along the western and southern margins of the continent and across the interior into the Karoo and the Kalahari, a range of evidence from both terrestrial sites and marine cores is considered, and potential expansions of the WRZ expansions are explored. Despite the limitations of many of the region's proxy records, a coherent pattern has begun to develop of a significantly expanded WRZ during phases of the last glacial period, with the best-documented being between 32–17 ka. While more detailed inferences will require the recovery and analysis of longer and better-dated records, this synthesis provides a new baseline for further research in this key region.
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Multiphase late Quaternary aeolian sediment accumulation in western South Africa: timing and relationship to palaeoclimatic changes inferred from the marine record
Chase, B. M. and Thomas, D. S. G. (2007). Quaternary International 166, 29-41.
Extensive aeolian deposits exist along the western margin of South Africa from Cape Town through to the Namib Sand Sea, and represent a significant palaeoenvironmental archive recording millennial-scale patterns of regional and hemispheric environmental change. To establish the timing of dune development, this study applies optically stimulated luminescence (OSL) dating techniques to the aeolian features of the west coast. In total, 35 samples were obtained from six dune cores along a north–south transect extending from Elands Bay (32.1260S, 18.114 E) to Kleinsee (29.114 S, 16.159 E). Ages from these dunes exhibit five distinct peaks, suggesting phases of activity/deposition at 4–5, 16–24, 30–33, 43–49 and 63–73 ka. The spatial and temporal extent of the data have allowed for correlations to be made with evidence from marine cores from the Southeast Atlantic, and a more coherent regional environmental history to be developed. Results suggest that: (1) environmental changes in this region are strongly linked to variations in wind strength and
atmospheric circulation systems during the late Quaternary; and (2) luminescence ages from aeolian deposits cannot necessarily be equated with periods of increased aridity, with other factors such as wind strength and sediment supply being of critical importance.
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Late Quaternary dune accumulation along the western margin of South Africa: distinguishing forcing mechanisms through the analysis of migratory dune forms
Chase, B. M. and Thomas, D. S. G. (2006). Earth and Planetary Science Letters 251, 318-333.
The west coast of South Africa is situated at a critical location between temperate and subtropical oceanic and atmospheric circulation systems, and palaeoenvironmental proxies from this region have the potential to elucidate issues concerning variations within these systems over glacial–interglacial cycles. While semi-arid climates have hindered the preservations of organic proxies, a variety of aeolian bedforms have been analysed in an effort to improve our understanding of environmental change in the region. Optically stimulated luminescence measurements of 51 samples from 15 reticulate dune sites along the west coast have enabled the identification of phases of aeolian activity, as well as periods of relative geomorphic stability. Combined with data derived from previous studies of the region's sediment accumulating deposits and other palaeoenvironmental proxies, periods of increased windiness are identified at 16–24, 30–33, 43–49 and 63–73 ka From approximately 17–12 ka, decreasing transport capacity resulted in the stabilisation of the west coast's dune fields. During the Holocene Altithermal (∼4–8 ka), despite reduced wind strength, increased aridity resulting from higher temperatures and a reduced influence of moisture bearing westerly systems appears to have trigged widespread remobilisation of the region's dune fields. The combination of ages from a variety of dune forms with different development mechanisms, and comparisons with a range of proxy data sources, have allowed for an enhanced interpretation of the region's aeolian archives, moving beyond simple correlations between dune activity and “aridity.”
Late Quaternary palaeoenvironments of the winter-rainfall zone of southern Africa: palynological and sedimentological evidence from the Agulhas
Carr, A. S., Thomas, D. S. G., Bateman, M. D., Meadows, M. E. and Chase, B. (2006). Palaeogeography, Palaeoclimatology, Palaeoecology, 239 (1-2), 147-165.
South Africa's winter-rainfall zone is a climatically sensitive region, lying at the interface of the earth's temperate and sub-tropical climatic systems. Its seasonally arid climate has generally prevented the preservation of long and organic-rich sedimentary deposits, producing a spatially limited and temporally biased Quaternary record. This paper seeks to address this issue further and develop a fuller understanding of the wider climatic changes in this region during the Late Quaternary. Modern climatic data does not support the definitions of the “winter-rainfall zone” presented in previous syntheses and this factor, amongst others, may explain difficulties in resolving the palaeoenvironmental record in this region. We present palaeoenvironmental evidence from the Agulhas Plain, an area marginal to the modern winter-rainfall zone. Cored sedimentological and palynological records from two pans are integrated with previously reported records of aeolian activity, providing evidence for phases of enhanced aridity and humidity during the Late Quaternary. The record is fragmentary, which may reflect the cycling of sediment between the pans and their accompanying lunette dunes. Lacustrine sedimentary sequences dating to c. 33 ka – > 47 ka provide evidence for enhanced humidity consistent with evidence in the winter and year-round rainfall zones. Increased humidity is also recorded from c. 2 ka, following drier conditions than the present c. 2.7–2.5 ka. Palynological evidence supports the sedimentological interpretations of pan water status and offers rare insights into the nature of the region's unique Fynbos vegetation during the Late Pleistocene.