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Earth-Science Reviews 93 (2009) 31–45
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Earth-Science Reviews
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Evaluating the use of dune sediments as a proxy for palaeo-aridity: A southernAfrican case study
Brian ChaseArid Environmental Systems Research Group, School of Geography and the Environment, University of Oxford, South Parks Road, Oxford, OX1 3QY, United KingdomDepartment of Environmental and Geographical Science, University of Cape Town, Rondebosch 7701, South Africa
E-mail address: [email protected].
0012-8252/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.earscirev.2008.12.004
a b s t r a c t
a r t i c l e i n f oArticle history:
The dominance of dryland Received 22 February 2008Accepted 19 December 2008Available online 10 January 2009Keywords:quaternarydunespalaeoclimateSouthern Africaluminescence dating
environments in the Southern Hemisphere makes the study of these regions ofcritical importance for the development of regional, hemispheric and global models of environmentalchange. Unfortunately, the wetting and drying cycles associated with semi-hyperarid climates are notconducive to the preservation of traditional organic proxy data sources. The last decade, however, has seenthe development of a number of alternative archives including the application of luminescence datingtechniques to dunes and other aeolian deposits. In continental situations, the existence of relict dune fieldshas long been thought to be evidence of drier conditions during the Pleistocene, and direct ages from thesefeatures have been interpreted almost exclusively as indicating phases of aridity. However, an increasingnumber of ages from a broader range of environments are calling into question the assumption that aeolianactivity can be simply equated with aridity. Presented here is a comparison of dune ages that have beenobtained from across southern Africa with a range of proxies from both terrestrial and marine records. Takenas a whole, three primary phases of activity can be identified at ~60–40, 35–20 and 17–4 ka. The frequentdiscordance with other terrestrial records indicating coeval increases in humidity and the close correlation ofthese 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 sedimentaryarchives needs to be reassessed.
© 2009 Elsevier B.V. All rights reserved.
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312. Dune ages and interpretations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333. Dune activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344. Dune form and sediment dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345. A new perspective on Kalahari linear dune ages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 366. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
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1. Introduction
Within the context of the developing global initiative to improveour understanding of global climate systems and dynamics, the needfor well-resolved palaeoenvironmental records has perhaps neverbeen so important. Terrestrial archives from the Southern Hemi-sphere, however, are relatively scarce. Lacking the extensive humid
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temperate regions of the Northern Hemisphere, the focus of asignificant proportion of palaeoenvironmental work on the SouthernHemisphere's continents has necessarily been in subtropical regions.While these regions are sensitive to large-scale environmental change,their semi- to hyper-arid climates are not conducive to the preserva-tion of organic material. The majority of terrestrial records fromsouthern Africa are compromised by these climatic controls either interms of their length (most are discontinuous, recording informationonly from discrete periods), their chronologies (many rely on open-system carbonates for chronologic control, e.g. Geyh and Eitel, 1998),
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or the archives available for analysis (silts, for example) that haveambiguous palaeoclimatic signals (Lancaster, 2002). While high-latitude Northern Hemisphere insolation and ice volume variations(e.g. Hays et al., 1976) may dominate the evolution of global climateover glacial–interglacial cycles (Barker and Gasse, 2003), the timing ofenvironmental change in the Southern Hemisphere cannot be fully oradequately explained either by Northern Hemisphere mechanisms orlocal insolation forcing alone (Partridge et al., 1997; Vandergoes et al.,2005; Chase and Meadows, 2007), and a clear need exists for moreterrestrial palaeoenvironmental archives.
Amajor development in terms of our ability to explore past drylandenvironments has been the application of luminescence datingtechniques to sand dunes and other aeolian landforms. In southernAfrica, the direct dating of aeolian sediments has allowed for asignificant refinement of the timing of episodes of aeolian activityacross the subcontinent (e.g. Stokes et al., 1998, 1997b; Thomas et al.,2000, 1998; Bateman et al., 2003; Chase and Thomas, 2006, 2007;Telfer and Thomas, 2007; Stone and Thomas, 2008), and the regionalvariations suggested by Lancaster (1981) based on early radiocarbonages from associated geomorphic features are becoming more tightlyconstrained (Fig. 1).
While the development of sand dunes is a function of variations insediment supply, wind strength, and vegetation (generally taken to bean indicator of precipitation), it is this latter element, and itsimplications for patterns of palaeoclimatic variability that have beenthe focus of interpretation (e.g. Lancaster, 1981; Stokes et al., 1997b,1998; Thomas et al., 2000; Bateman et al., 2003; Lomax et al., 2003;Munyikwa, 2005b; Fitzsimmons et al., 2007). To date, however, thevalidity of the assumption that aeolian activity and dune developmentcan be interpreted as a proxy for aridity has yet to be adequatelytested. In southern Africa, the limited number of palaeoenvironmentalrecords has hampered direct correlations of multiple proxies, anduntil recently the limited number of luminescence ages from duneshas made it difficult to distinguish strong signals of aeolian activity/deposition. With improvements in optically stimulated luminescence(OSL) measurement techniques such as single aliquot regenerativedose (SAR) and standardised growth curve (SGC) protocols (e.g.Murray andWintle, 2000; Roberts and Duller, 2004), more precise andreliable ages can be obtained, and detailed chronologies can beconstructed more quickly (e.g. Chase and Thomas, 2006; Telfer andThomas, 2007; Telfer et al., 2008). The number of ages now availablefrom southern Africa is allowing for a more accurate identification ofphases of dune development, and more precise correlations withother proxy records possible.
2. Dune ages and interpretations
In total, 219 luminescence ages have been published from theextensive linear dune fields of the Kalahari, with 78 coming from thenorthern Kalahari (Stokes et al., 1998, 1997b; O'Connor and Thomas,1999;Munyikwaet al., 2000; Thomas et al., 2003, 2000), and141 comingfrom the southwestern Kalahari (Stokes et al., 1997a; Blümel et al., 1998;Thomas and Shaw, 2002; Bateman et al., 2003; Telfer and Thomas, 2007;Stone and Thomas, 2008). Other ages have been obtained from theelevated coastal platform along the western margin of South Africa atthe transition from the southern Namib Desert to the subhumidenvironments of the Cape (Chase and Thomas, 2006, 2007). While thisregion has been labelled the west coast, the dunes which have beendirectly dated are composed of continental sediments, derived fromfluvial rather than marine sediment sources. Considered regionally,these southern African dune ages indicate the episodic nature of dune
Fig. 1. The major dated dune fields of southern Africa with linear dune orientations derived freach dune field have been organised in ranked plots with five-point moving averages being uthe Kalahari dunefields identified by Lancaster (1981) and estimates of their ages based on
development in the region (Fig. 1). Recent work in the central NamibDesert (Bristow et al., 2007, 2005) is beginning to explore the history ofdune development in the region with direct dating, but the limitednumber of dunes sampled presently precludes comparison the morespatially extensive datasets presented here.
It is worth noting that it has been argued that all of the dune agesthat were obtained using thermoluminescence (TL) or multiple orsingle aliquot additive-dose OSL techniques (MAAD and SAAD) shouldbe excluded from further consideration (Telfer and Thomas, 2007;Thomas, 2007). This assertion is supported by studies of linear dunesin Tasmania, from which strikingly different results have beenobtained using MAAD and SAR techniques (Duller and Augustinus,2006). Ages from the SW Kalahari, however, (the only dunefield thathas been extensively dated using TL, MAAD, SAAD, SAR and SGCtechniques) indicate a very close correspondence between clusters ofages obtained using SAR/SGC (Telfer and Thomas, 2007; Stone andThomas, 2008) and TL/MAAD/SAAD techniques (Stokes et al., 1997a;Blümel et al., 1998; Thomas and Shaw, 2002; Bateman et al., 2003)(Fig. 2). While further work may indicate that a re-assessment isrequired, based on the evidence presently available, a wholesalerejection of these ages would appear to be a premature.
Taken as a whole, three primary phases of Kalahari dune develop-ment can be broadly identified at ~60–40, 35–20 and 17–4 ka (Fig. 1).What is immediately evident about this distribution from a palaeocli-matic point of view is the existence of both glacial and interglacialphases of dune development. This patterning is not consistent with thesimplistic paradigm of cold and warm periods being linked with aridityand humidity respectively (e.g. Sarnthein, 1978; Partridge, 1993;Partridge et al., 1999), suggesting that either the paradigm, or theinterpretationof dunes as anaridproxy, orboth, need to be re-evaluated.Chase and Meadows (2007) have reviewed the existing marine andterrestrial palaeoenvironmental records from southwestern Africa, andconcluded that interpretations of colder andwarmer conditions asbeingarid and humid respectively do not adequately represent the variationsthat are evident across southern Africa during the late Quaternary.Detailed comparisons of the terrestrial palaeoenvironmental recordsavailable from regions adjacent to the northern and southwesternKalahari dunefields (Fig. 3, sections (A) and (B) respectively) confirmsthat environments cannot be divided simply into glacial and interglacialend-members.
It should be clearly understood that the reliability of many of theserecords is compromised by imperfect chronologies, their oftendiscontinuous nature, and in some case difficultieswith interpretation.Considered regionally, however, the context created by the considera-tion of all available sites allows for an evaluation of the overall climatesignal (Chase and Meadows, 2007). Compared in such a way, outlierscan be identified, and the number of records indicating increasedhumidity/aridity can be taken to some extent as an additional measureof reliability, especially when multiple proxies indicate similar trends.
Although records older than 40 ka are scarce due to the limits ofradiocarbon dating, records from the northern Kalahari as a wholeindicate increased humidity at ~43–36, 35–26, 22–10 and 6–1 ka, whilerecords from the southern Kalahari and neighbouring regions indicateincreased humidity at ~37–17 and 12–6 ka, broken perhaps by drierepisodes from ~10–9 and 4–2 ka. The underlying forcing mechanismsdriving these patterns are still largely a matter for speculation, but itevident that when compared to the timing of dune development inthese respective regions an inverse relationship between dune ages andhumidity doesnot exist asmight be predicted (Fig. 3). This highlights theshortcomings that are evident in applying a model restricted tocorrelations with episodes of increased aridity, and it is clear that
om Landsat imagery and luminescence sampling sites indicated. Luminescence ages forsed to highlight clusters of ages and periods of increased dune development. Inset showsthe radiocarbon dating of associated sediments.
Fig. 2. Comparison of OSL ages from the SW Kalahari constructed using single aliquot regenerative dose (SAR) and standardised growth curve (SGC) protocols (Telfer and Thomas,2007) with ages constructed using pre-SAR/SGC techniques (thermoluminescence (TL) (Blümel et al., 1998) ormultiple or single aliquot additive-dose OSL techniques (OSLMAAD andSAAD) (Stokes et al., 1997a; Thomas et al., 1998; Bateman et al., 2003).
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other controls need to beexplored to explain the timingof cycles of duneconstruction.
3. Dune activity
The potential for dune development is determined by sedimentsupply, sediment availability (the susceptibility of sand to be deflated,determined by vegetation, soil moisture, and the position of the watertable amongst other factors), and the transport capacity of thewind (c.f.Kocurek,1998; Kocurek and Lancaster,1999). Avariety of equations havebeen proposed to calculate dune activity/mobility indices and explorethe relative effects of variations in transport capacity and climatevariables such as precipitation and potential evaporation (e.g. Talbot,1984; Lancaster, 1988; Bullard et al., 1997; Knight et al., 2004; Thomaset al., 2005).While thesehave been shown toperform reasonablywell atregional scales (Lancaster and Helm, 2000; Wang et al., 2007), theyrequire quantitative data to inform them, the quality of which can bequestionable even for present-day conditions. Applying dune activityindices to both future and past climate scenarios has incorporated eitherGCM results (e.g. Muhs and Maat, 1993; Thomas et al., 2005), or roughcalculations of potential wind strength (Lancaster, 1989). In consideringpast climate scenarios, evidence for which is largely qualitative innature, critical elements such as changes in rainfall gradients,seasonality and external sediment supply (Rendell et al., 2003) andthe effect of CO2 on vegetation (Hesse et al., 2003) overmulti-millennialtimescales are rarely available. Given this, dune activity indices may beused to explore hypothetical wind strength or precipitation/potentialevaporation scenarios, but the results must be properly recognised ascoarse estimations.
Using dune activity index (DAI) equations based on the equationapplied in Thomas et al. (2005) and CRU climatology (New et al., 1999,2002) and soil data taken from Zobler (1986), it can be seen that muchof southern Africa presently has limited potential for dune activity(Fig. 4). Of the dunefields considered here, it is apparent that thesouthwest Kalahari is themost prone to reactivation, but, as suggestedby Bullard et al. (1997) the system is presently primarily wind ratherthan precipitation limited. Applying the Last Glacial Maximum windspeeds that Lancaster (1989) used for his study of dune activity in the
southwestern Kalahari (117% of modern values, derived from roughestimates calculated by Newell et al. (1981)), it can be seen that eventhis modest increase is sufficient to induce significant dune activity inthe southwestern Kalahari under present precipitation regimes. Thedunefields of the northeastern Kalahari, however, require either anestimated 200% increase in wind speed or a 90% reduction inprecipitation to significantly reactivate. The dunefields of the northernKalahari exist in an exceptionally low wind environment, and evenwith Lancaster's 117% proposed wind speeds (1989), they wouldrequire a reduction in precipitation from 500–700 mm to 15–20 mm,3% of present day values. While similarly massive reductions of 85%have been suggested by Thomas et al. (2000), these are deemed hereto be unrealistic in the light of the other available evidence from theregion (Fig. 3), and it is suggested that variations in other forcingmechanisms need to be considered more explicitly.
4. Dune form and sediment dynamics
In terms of the interpretation of dune ages, it is important toconsider the sediment dynamics of the type of dune that the age wasobtained from. Broadly speaking, accumulating dunes, particularlytopographic dunes, have the potential to accumulate stacked records ofmultiple phases of activity and deposition as their development isdetermined by sediment blowing from high to low energy environ-ments (e.g. Chase and Thomas, 2007). Depending on the site specificparameters of the sediment trap such as relief, sediment supply andtransport potential, once deposited these units are more or lessprotected fromsubsequent deflation.Migrating dunes, such as barchanor transverse dunes develop in regions with limited sediment supplyand generally unimodal wind regimes. With the crest of the duneoriented perpendicular to the prevailing wind, the sediment body as awhole ‘rolls’ with the wind. In terms of the ages derived from suchforms, this sediment recycling and systematic self-cannibalisationresults in a constant resetting of the dune's luminescence clock. Agesobtained from dormant migrating dunes therefore do not usuallyindicate aeolian activity, but rather the onset and development offactors such as increased vegetation cover or reduced wind speed thatresulted in the dune becoming ‘fixed’.
Fig. 3. Correlation of OSL age distributions from linear dunes from the N and NE Kalahari (A) and the SWKalahari (B) with other palaeoenvironmental records from each region. In the north, records are shown fromDrotsky's Cave (1: (Shaw andCooke, 1986); 2: (Brook et al., 1998)), the Tsodilo Hills (Thomas et al., 2003), Lake Ngami (6: (Burrough et al., 2007); 7: (Huntsman-Mapila et al., 2006); 8: (Shaw et al., 2003)), the Lake Makgadikgadi basin (9: (Shaw,1985; Shaw et al., 1997); 10:(Ringrose et al., 2005)), 1: Gi Pan (Helgren and Brooks, 1983) and 12: Gobabis (Butzer,1984). In the south, records are shown for Equus Cave (13: (Scott, 1987); 19: (Klein, 1986)), 14:Wonderwerk Cave (Beaumont et al., 1984),15: Klipfonteinrand(Avery, 1993), 16: Kathu Pan (Beaumont et al., 1984), 17: Stampriet Aquifer (Stute and Talma, 1998), 18: Alexandersfontein (Butzer et al., 1973), 20: Apollo 11 Cave (Thackeray, 1979), 21: Aminuis Pan (Beaumont, 1986), 22: Breek Been Kolk(Beaumont, 1986), 23: Bullsport (Heine, 1998), 24: the Gaap Escarpment (Butzer et al., 1978),25: Hoezar Oost (Beaumont, 1986), 26: Kannikwa (Beaumont, 1986), 27: Koichab Pan (Deacon and Lancaster, 1988), 28: Klein Awas Pan (Heine, 1982),29: Otjimaruru Pan (Lancaster, 1986; Deacon and Lancaster, 1988), 30: Urwi Pan (Lancaster, 1979), 31: Lobatse II Cave (Holmgren et al., 1995), and 32: Auob-Nossob (Heine, 1982). Dune ages and errors are givenwith lines indicating five-pointmoving averages calculated from the ages grouped into 1 ka bins.
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Exploring the variations that would be predicted to exist in theages obtained from different dune forms from one region Chase andThomas (2006, 2007) studied the distribution of ages obtained fromaccumulating and migrating dune forms along the west coast of SouthAfrica (Chase and Thomas, 2006). In aggregate, phases of aeolianactivity preserved in accumulating dune forms date to 74.5–61, 48.5–41, 33–31, and 23.5–16 ka (Chase and Thomas, 2007) (Fig. 1). Based onpalaeoenvironmental records from the region, the phases correlatenot with aridity, as might be predicted, but with increased humidity,windiness and fluvial sediment supply corresponding with invigo-rated glacial circulation systems (Little et al., 1997a,b; Birch et al., 1991;Parkington et al., 2000; Shi et al., 2001; Stuut et al., 2002; Pichevinet al., 2005). In contrast, ages from migrating dunes are clustered at17–12 and 8–4 ka (Fig.1), and represent a complex history of dunefielddevelopment (Chase and Thomas, 2006). The 17–12 ka phase ofmigrating dune development occurred during the still humid, butnotably less windy Lateglacial period, and rather than representing‘aeolian activity’, dune sediments from this phase most likelyrepresent the transition to dormancy of a highly mobile dunefield.The visibility of this transitional period in the aeolian record can beattributed to lower wind strength during the Holocene, and thuslimited potential for subsequent erosion and sediment recycling.These data corroborate the prediction of Nanson et al. (1992) thatplots of luminescence ages from dune systems that were prone toreworking would exhibit an exponential reduction with age.
Common to both accumulating and migrating dunes is a mid-Holocene phase of activity that occurs during a period of low windstrength and potentially limited sediment supply (Birch et al., 1991;Stuut et al., 2002). In this case, the widespread reactivation of aeoliandeposits is most likely to have occurred as a response to the period ofincreased aridity that is recorded in the palaeocological proxies fromthe neighbouring Elands Bay region (Meadows et al., 1996; Parkingtonet al., 2000; Meadows and Baxter, 2001).
By considering the region's palaeoenvironmental context and thesediment dynamics of different dune forms, studies of the west coastdunefields have been able to arrive at conclusions that contrastsharply with palaeoclimatic interpretations that simply attributeaeolian activity to increased aridity, and indicate the importance ofdefining and incorporating other factors known to influence dunedevelopment, in the interpretation of aeolian archives.
Considering the linear dunes of the Kalahari as a whole, it appearsthat they fall somewhere between pure migrating and pure accumulat-ing dune forms in terms of their potential to preservemultiple episodesof aeolian activity and dune development. While there remains somedebate over the dynamics of linear dune development (c.f. Wang et al.,2004; Munyikwa, 2005a), they are generally believed to form as aresponse to bi-modalwind regimes, and extendparallel to thedominantaveraged direction of wind flow (Tsoar, 1983; Livingstone, 1989; Tsoaret al., 2004). Variations in atmospheric circulation patterns, however,can shift this balance and result in asymmetrical patterns of sedimentdeposition and deflation across the dune body, and even lateralmigration of the dune as a whole (Rubin and Hunter, 1985; Rubin andIkeda, 1990; Bristow et al., 2000, 2005, 2007; Rubin et al., 2008).
These complex sediment dynamics need to be considered when OSLages from linear dunes are analysed. While they may provide “stacked”records of aeolian activity/deposition (O'Connor and Thomas, 1999), thisshould not be understood to imply that the record is complete,continuous or even representative of phases of activity. Under conditionsof unlimited sediment supply in source proximal locations and withconsistent wind regimes, linear dune sediments may in theory preservecomplete records of dune development, much like pure accumulatingdune forms. Limited sediment supplyanderraticwind regimes, however,can result in linear dunes behavingmore like migrating dunes, recyclingand sometimes entirely reworking the sediment body (Bateman et al.,2003; Fitzsimmons et al., 2007), and only depositing material whenerosive forces abate or the dunes become dormant. Over multi-
millennial timescales, linear dune sediments are thus likely to preservesediments marking episodes of both enhanced accumulation as well asreductions in transport capacity. In this light, the patterning of ages fromthe Kalahari's linear dune can perhaps be more accurately interpreted.
5. A new perspective on Kalahari linear dune ages
Across southern Africa, the patterning of luminescence ages fromlinear dunes, taken either regionally or as a whole, displays two broadprimary characteristics: 1) a decrease in the number of ages withincreasing antiquity, and 2) clusters of ages at ~60–40, 35–20 and 17–4 ka.
The first of these characteristics is almost certainly not necessarilyrelated to any real progressive increase in the intensity or extent ofdune development, but is more a reflection of sampling techniques,dune dynamics and the nature of the aeolian sediment record. Interms of sampling, the unconsolidated nature of most of the aeoliansediments in the Kalahari has resulted in the existence of very fewnatural exposures. In lieu of such features, many of the luminescencesamples from the Kalahari were obtained by digging pits in the dunecrests by hand, and are thus often limited to the uppermost 1–2 m ofsediment (e.g. Stokes et al., 1997b; O'Connor and Thomas,1999). Giventhat the sampled dunes are usually 4–10 m in height, there is aconsistent and potentially important bias towards younger samples,although as indicated by Bristow et al. (2007) ages from these near-surface samples cannot be taken as the minimum age of the dune as awhole.
The preponderance of younger ages may also be attributed to theimperfect preservation of sedimentary records in linear dunes. As hasbeen discussed, under conditions of optimal wind regimes andsediment supply linear dunes may preserve stacked records of dunedevelopment, but they may also, through intense activity and/orlimited sediment supply, be reworked and the sediment record erased(e.g. Bateman et al., 2003). This possibility highlights an importantaspect of the nature of dune records: that there is an improvedlikelihood of the preservation of sediment from phases of dunedevelopment during periods of progressively decreasing aeolianactivity. If trends of decreasing activity are broken by higher intensityphases, however, erosion of the record is possible with the degree ofdisruption varying according to the duration and intensity of thephase, as well as the amount and balance of sediment available fromboth internal and external sources. This recycling may explain whythere are so few dune ages from the last interglacial compared to theHolocene, and why more glacial age samples have been obtained fromthe northern Kalahari as compared to the drier and more activesouthwest Kalahari.
From approximately 55 ka, there is an increase in the number ofages from each of the Kalahari linear dunefields (Fig. 1). It may beworth considering that the first evidence for the phase of subconti-nent-wide dune development begins first in the southwest Kalahari(60.2 ka), and is later recorded in the north (57 ka) and northeast(53.5 ka) dunefields. While the errors associated with the datingtechniques may in fact mean that all of these age relate to the sameevent, it is also possible that the progressive trend in activity frommore arid to more humid regions and that the relative amount ofprecipitation that each region received may have influenced thespatiotemporal patterning of dune development; either as a responseto progressive aridification along a SW–NE gradient, or the develop-ment of a subcontinental/hemispheric-scale driver such as atmo-spheric circulation intensity and wind strength. Considering theevidence cited for increasing humidity in the Kalahari after ~40 ka,however, aridity is unlikely to have been the primary forcingmechanism driving aeolian activity during the last glacial period.The potential importance of wind strength, however, is perhapshighlighted by the strong relationship that existed between increasedaeolian activity in the Kalahari and an intensification in atmospheric
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Fig. 4. Modern precipitation, potential evaporation, wind speed and the modern maximum month dune activity index (DAI) calculation (top row) based on the equation outlined in Thomas et al. (2005) and using CRU climatology(New et al., 1999, 2002) and soil data taken from Zobler (1986). Modern monthly DAIs indicate the limited spatial and temporal range in southern Africa today, and calculations based on 50% of modern precipitation, 200% modernwind speed,117%wind speeds based on Lancaster's (1989) application of Newell et al.'s (1981) estimates, and 10% of modern precipitation based on Thomas et al.' s (2000) interpretations, indicate the extent to which these variableswould need to change to significantly reactivate the dune fields discussed in this paper.
Fig. 5. Correlation of OSL age distributions from migrating and accumulating dunes from the west coast of southern African and linear dune fields of the Kalahari with proxies forupwelling intensity and wind strength from marine cores off the Namibian coast as indicated by percentages of N. pachyderma (Little et al., 1997b) and by the ratio of coarse to fineaeolian dust (Stuut et al., 2002). Dune ages and errors are given with lines indicating five-point moving averages calculated from the ages grouped into 1 ka bins. The shaded areasindicate the main phases of dune development identified at ~60–40, 35–20 and 17–4 ka.
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circulation systems as inferred from upwelling and wind strengthproxies from SE Atlantic marine cores GeoB1711-4 (Little et al., 1997b)and MD962094 (Stuut et al., 2002) respectively (Fig. 5).
Interestingly, the correlationbetween Little et al.'s (1997b) upwellingrecord and the periodicity of aeolian activity in the Kalahari has beennoted by a number of researchers in the region (e.g. Stokes et al., 1997b,1998; Thomas and Shaw, 2002). Their interpretations, however, focus onthe role that reduced SE Atlantic sea-surface temperatures (SSTs), andparticularly the effect that the temperature differential between the SEAtlantic and SW Indian oceans would have on precipitation acrosssouthern Africa, with the suggestion being that these factors result inmore arid conditions in the Kalahari (Stokes et al., 1997b). Over multi-millennial time-scales, however, comparisons of SE Atlantic SSTreconstructions from GeoB1711-4 (Kirst et al., 1999) with those from aSW Indian ocean coreMD79257 (Sonzogni et al.,1998), indicate a steadydecline in the temperature differential between the two oceans from45–8 ka, a pattern that is inconsistent with increases in dune deve-lopment from ~35–20 and 17–4 ka.
Inconsistencies in interpretations based on SST differentials mayreflect the fact that Little et al.'s (1997b) study of foraminifera is notprimarily an SST record, but rather an indicator of coastal upwellingintensity, and more fundamentally, of trade wind strength associatedwith shifts and intensifications in the South Atlantic Anticyclone. Thisinterpretation is supported by the record from neighbour marine coreMD962094 (Stuut et al., 2002), which interprets ratios of fine to coarseaeolian sediments as an indicator of wind strength, and shows apattern similar to Little et al.'s upwelling proxies. Similarities betweenthe upwelling and wind strength proxies from SE Atlantic and phasesof dune development (Fig. 5) are thus most likely the result of thegeneral intensification of atmospheric circulation cells during the lastglacial period, and the coeval intensification of the SE Atlantic andsouthern African continental anticyclones. While this intensificationmay have had the effect of blocking the incursion of moisture-bearingeasterly systems, records of increased humidity during the last glacialperiod (Fig. 3) suggest that the resulting reductions in precipitation, ifthey occurred at all, are likely to have acted largely as an enablingfactor of secondary importance, and that increasedwind strengthmayhave had an important role in defining the timing and extent ofaeolian activity across southern Africa.
Of the threemajor phases of linear dune aeolian activity in southernAfrica (~60–40, 35–20 and 17–4 ka), both glacial age episodes showstrong correlations with records indicating increased wind strength.One episode that is notably absent from the linear dune archives (if acausal link is to be inferred between dune development and increasedwind strength) is the 73–62 ka episode identified in MD962094 (Stuutet al., 2002). Since this episode is strongly represented in theaccumulating dune deposits of the west coast, one explanation maybe that linear dunes did in fact develop in the Kalahari during this time,but the sediments were largely recycled during subsequent phases ofintense aeolian activity.
The Lateglacial–Holocene phase of aeolian activity from 17–4 ka isdistinguishable in all of the Kalahari dunefields, with the relativeincrease in dune development most notable in the southwest. Telferand Thomas (2007) identify the period from 15–9 ka as one ofsignificantly enhanced aeolian activity in the southwest Kalahari, andattribute it to what appears to have been a Lateglacial–early Holoceneperiod of progressive aridification (beginning at ~17 ka and peakingbetween 11 and 9 ka, Fig. 3). The evidence available from the regionsuggests that this was an important transitional period in the region'senvironmental history, when the increased influence of winter stormsduring oxygen isotope stages 2 and 3 (~35–17 ka) was declining, buthad yet to be replaced by enhanced summer rainfall during theHolocene (Chase andMeadows, 2007). Concurrent with this decline inwinter rain was a post-glacial reduction in atmospheric circulationintensity that is evident in the region's marine records (from ~18 ka,Fig. 5). Considering this, it is difficult to determine whether the
dramatic increase in dune ages from this time actually represent aperiod of increased activity (Telfer and Thomas, 2007), or alternatively aperiod of increased preservation of the sedimentary record as a result ofmarkedly reduced wind strength both during and after this period.This latter possibility was suggested by Bateman et al. (2003), whointerpreted the overall patterning of dune ages from the region asindicating the earlier widespread recycling of the linear dunes in theregion during the Last Glacial Maximum, and the preservation of asedimentary record occurring only with the onset of the less activepost-glacial period. Indeed, strong similarities are evident between thedistribution of these linear dune ages and ages from the west coast'smigrating dunes,which, as has beenmentioned, have been interpretedas representing the transition towards dormancy of a highly activedunefield (Chase and Thomas, 2006). Considering that this Lateglacial–early Holocene phase of dune development can also be identified(albeit to a lesser degree) in the northern and northeastern Kalaharilinear dunefields, and despite the opposing climatic trends evidentbetween the north and south (Fig. 3), it is possible that a commonsubcontinental-scale driver such as reduced atmospheric circulationintensity and decreasedwind strengthmayhave had an important rolein defining this period of sediment deposition.
A less equivocal link between precipitation and dune activity canperhaps be made during the mid-Holocene. Records from StamprietAquifer (Stute and Talma, 1998), Wonderwerk Cave and Kathu Pan(Beaumont et al., 1984) indicate that conditions during the mid-Holocene from ~8.5–5 ka were more humid than present day. Thecorrelation between the onset of these wetter conditions and thecurtailment of dune development may indicate, given the consistentlylow wind strength from ~10 ka to present, a causal link betweenprecipitation and aeolian activity during the early tomid-Holocene. Thisraises the possibility that while elevated wind strength may confoundthe use of dune sediments as a palaeo-aridity proxy during the lastglacial period, the reduced influence of wind during the Holocene andthe improved potential for preservation of the sedimentary record mayallow for clearer interpretations to bemade. The case has beenmade forthe role of aridity in the patterning of dune ages along the west coast ofSouth Africa during the Holocene Altithermal (Chase and Thomas,2006, 2007), and in the southwest Kalahari during the Lateglacial–earlyHolocene (Telfer and Thomas, 2007). However, this relationshipbetween aridity and dune development is less evident in the south-western Kalahari during the late Holocene, and in the northern andnortheastern Kalahari there is no clear correlation between dune agesand increased aridity at this time.
While the focus of this discussion has been on wind strength andprecipitation proxies, the importance of other controls on dunedevelopment, such as variations in external sediment supply (e.g.Rendell et al., 2003) and the effect of CO2 on tree/grass ratios (Ehleringeret al., 1991) and the impact of these vegetation changes on dune activity(Hesse et al., 2003), cannot be overlooked as potentially significantcontrols on the aeolian record. In the northern Kalahari, for instance,dune activity index calculations indicate that even with extremereductions in precipitation and significant increases in wind speed theregion's dunes would remain largely inactive. One possible explanationis that the increased humidity indicated by regional records resulted inenhanced fluvial activity and external sediment supply. Unfortunately,records of fluvial activity are limited and very poorly resolved. If it isinferred that the development of lacustrine features and sediments atLake Ngami relates to inflow from the Angolan Highlands, it is possiblethat enhanced/extreme sediment accumulation along the interveningfluvial networks resulted in dune development. However, comparisonsof ages from the region's aeolian and lacustrine features are inconclusive(Fig. 3), and until further data can be obtained this will remain a matterfor speculation.
In considering the development of dune fields in the now humidBlue Mountains of southeastern Australia, it has been suggested byHesse et al. (2003) that reduced atmospheric CO2 during glacial periods
43B. Chase / Earth-Science Reviews 93 (2009) 31–45
waroleKte(ddoCcbrdc
6
1
2
3
ould promote the spread of grassland, and that significantly increasednd more effective disturbance of the vegetation by fire could haveesulted in substantial dune development. Modelling studies focusingn 21 ka scenarios from Africa have indicated that despite lower CO2
vels forests may have been more extensive across the northernalahari as a result of more humid conditions (Cowling et al., 2008), buthat slow tree recovery rates after fires may result in their virtuallimination from at least some part of the southern African landscapeBond et al., 2003).While this is a viable hypothesis, andmay explain theevelopment of some apparently anomalous phases of glacial age duneevelopment, it does not explain the phases of dune development thatccurred after 11 ka, when CO2 had reach near pre-industrial levels.onsidering the potential importance of these variables, and theomplex dune–climate relationships discussed earlier, it is clear thatefore we can accurately interpret aeolian archives substantial workemains to be done on how vegetation, sediment and wind strengthynamics interact over multi-millennial timescales to create theomplex records preserved in dune sediments.
. Conclusions
. Taken either regionally or as a whole, the timing of episodes ofdune development in southern Africa does not show a clear,consistent relationship with aridity. While they do in some casesappear to support palaeoclimatic evidence derived from otherarchives and proxies (e.g. the mid-Holocene along the westernmargin of South Africa (Chase and Thomas, 2006, 2007) and theLateglacial/early Holocene in the southwestern Kalahari (Telfer andThomas, 2007), rarely are these correlations unequivocal, and it isoften difficult to determine the relative influence of precipitationversus other possible controls on the system. Based on the datasetpresently available, however, precipitation would appear to havebeen an enabling/limiting factor, with no obvious, consistent signalof its own being imprinted on the overall record.
. Based on the evidence outlined here, it is suggested that despitedifferences in regional manifestations, the southern African dune-fields broadly share glacial phases of dune development at ~60–40and 35–20 ka, corresponding with evidence frommarine records forincreased wind strength at ~57–39 and 36–18 ka (Little et al., 1997b;Stuut et al., 2002). It is not, however, being suggested that windstrength replace aridity as a blanket explanation for the timing ofphases of dune development. While wind strength may haveinfluenced the timing of dune development during the lateQuaternary in southern Africa, it clearly does not explain all of thevariability evident in the system.
. It is proposed here that the Lateglacial–early Holocene distributionof luminescence ages from the linear dunes of the southwesternKalahari exhibit strong similarities to the migrating dunes of thewest coast of South Africa, indicating the strong influence ofsediment recycling on the record, and marking the transition todormancy of a highly active dunefield. While the morphodynamicsof the southwestern Kalahari's linear dunes have allowed forgreater preservation of the ~35–20 ka phase of dune developmentcompared to the west coast's migrating dunes, the former haveevidently been close to the threshold of reactivation for at least thelast 100 ka, and have experienced significant reworking, withmuchof the pre-40 ka record being recycled. It was only with thereduction in wind strength after ~18 ka that sediment was able toaccumulate and preserve a record of dune development. It ispossible that this decrease in erosive potential allowed for thedunes to preserve evidence of the phase of increased aridity thatappears to have occurred in the region from ~15–9 ka, as suggestedby Telfer and Thomas (2007), but the coeval decreases in humidityand wind strength make it difficult to determine whether it wasaridity or simply a progressive decline in activity that has driventhis distribution of luminescence ages.
4. While this review has called the use of dune ages as palaeoclimaticproxy into question, the issue of how to interpret dune ages as afunction of environment remains unresolved, and will require asignificantly improved understanding of how all of the controls ondune development interact over multi-millennial timescales.
Acknowledgments
I would like to thank Simon Armitage, Simon Brewer, Joanne Bullard,AndrewCarr, JohnCompton, Frank Eckhardt,MichaelMeadows, RichardWashington and Giles Wiggs and two anonymous reviewers for theirassistance and feedbackon this paper and/or thedata it contains. Iwouldalso like to thank all of the authors of the papers cited herein for theenormous amount of time and effort they have put into studying thisregion, and for the data they have produced.
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