Punctuated aridity in southern Africa during the last glacial cycle: The chronology of linear dune construction in the northeastern Kalahari

P AI O ELSEVIER

Palaeogeography, Palaeoclimatology, Palaeoecology 137 (1998) 305 322

Punctuated aridity in southern Africa during the last glacial cycle: The chronology of linear dune construction in the northeastern Kalahari S. Stokes a,,, G. Haynes b, D.S.G. Thomas c, J.L. Horrocks M. Malifa d

a,1

M. Higginson

a,2

a School of Geography and Research Laboratory for Archaeology, Oxford University, Mansfield Road, O.~ford, OXI 3Q J, UK b Department of Anthropology, University of Nevada, Reno, USA c Sheffield Centre Jot International Dryland Research, Department of Geography, Sheffield University, Sheffield, UK Hwange National Park, Research Branch, Sinamatella Camp, Zimbabwe Received 16 September 1996; received in revised form 26 June 1997; accepted 1 July 1997

Abstract

The Mega Kalahari of central southern Africa is one of the most extensive Quaternary desert basins. On a regional scale, present-day aeolian activity is restricted to episodic dune crest reactivation in the most arid southwestern desert core. There is, however, abundant evidence of former periods of both more arid and more humid conditions, many of which have little or no chronological control. We have employed optical dating of quartz sand grains to develop a chronology of arid intervals as recorded by phases of linear dune construction in the northeastern sector of the Mega Kalahari. We identify repeated phases of aeolian deposition during the last interglacial-glacial cycle, at ca. 95-115, 41-46, 20-26 and post-20 ka, which are separated by depositional hiatuses that we infer to correspond to more humid periods. These aeolian depositional events correlate with and are inferred to relate to millennial-scale cold sea surface temperature events in the southeast Atlantic which have been linked to sub-milankovitch climate changes recognised in northern hemisphere oceanic and cryospheric environmental archives covering the same time period. While the present landscape is the product of either post-20 ka (Hwange National Park dune field) or 20 30 ka (Victoria Falls dune field) aeolian activity and subsequent erosion and reworking, much of the vertical expression of the larger dune forms corresponds to the earlier periods of activity. The linear ridges of the area are rich archives of late Quaternary terrestrial climate change. © 1998 Elsevier Science B.V.

Keywords." Kalahari Desert; Optical dating; Linear dunes; Pleistocene; Geochronology; Palaeoenvironmental reconstruction

* Corresponding author. E-mail: [email protected] 1Present address: Department of Earth Sciences, University of Waikato, Private Bag, Hamilton, New Zealand. 0031-0182/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PH S0031-0182(97)001 06-5

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S. Stokes et al. / Palaeogeography, Palaeoelimatology, Palaeoeeology 137 (1998) 305 322

I. Introduction

Many research programmes have in the last two decades revealed evidence of palaeoclimatic changes which have taken place in the desert regions occupying the large continental land masses straddling the equator. These have integrated a wide range of data sources including studies focusing on chemical and physical characteristics of sediments in adjacent oceans (e.g., Parkin and Shackleton, 1973; Sarnthein et al., 1981; Prell and Van Campo, 1986; De Menocal et al., 1993), and those which have attempted to access terrestrial archives of environmental changes (e.g., Warren, 1970; Street and Grove, 1976; Ritchie et al., 1985; Lancaster, 1990; Kropelin and Soulie-Marsche, 1991; Gasse, 1994). Of this latter category, studies of closed-lake basins of the northern hemisphere have provided perhaps the clearest picture of climatic changes which have taken place over the past 18 kyr B.P. (Petit-Maire and Riser, 1981; Street-Perrott et al., 1989; Gasse et al., 1990; PetitMaire, 1993; Van Campo and Gasse, 1993; StreetPerrott, 1994). As a results of these studies two main models have emerged: a 'passive' African climate model which identifies independent climate changes at high latitudes as being the significant determinant of African palaeoenvironments; and an 'active' African climate model which considers changes in the location and abundance of rainfall driven by low latitude insolation levels as being the significant determinant. A third model which identifies both factors playing deterministic roles during discrete climatic periods has also been described (Prell and Van Campo, 1986). The general absence of suitably widespread and well studied closed-lake basins south of the equator, particularly within the African continent, has forced inferential interpretations of low latitude environmental changes which have in general suggested a close mirroring of southern hemisphere palaeoclimates to those established for the northern hemisphere (e.g., Heine, 1982; Van Zinderen Bakker, 1982). This view has recently been questioned on the basis of radiocarbon-dated evidence from lake and cave sequences in central southern Africa (Shaw et al., 1988; Shaw and Thomas, 1996), but is generally supported by analysis of

deep sea cores from high southern latitudes (Charles et al., 1996). Use of evidence of environmental changes recorded within the aeolian sequences that dominate the landscapes of both hemispheres in low latitude continental settings has, until recently been severely limited by difficulties in establishing appropriate chronological control. This problem particularly applies to the period beyond the time range of the radiocarbon method (Shaw et al., 1988; Partridge et al., 1990; Thomas and Shaw, 1991b; Lancaster, 1995). Despite these difficulties, a model of changes in the extent of later Quaternary aeolian sequences, and corresponding widespread aridity, was developed by (Sarnthein, 1978). The model proposes that the extent of global low latitude deserts was at a maximum at the time of the last glacial maximum (ca. 18-21 kyr B.P.), covering 50% of the land area between latitudes 22~N and 22°S. This model is a widely cited view which has strongly influenced interpretations of global desert sequences (e.g., Bradley, 1985; Dawson, 1992; Williams et al., 1993). The arid to semi-arid Mega Kalahari region of central southern Africa (Fig. l), covers an area in excess of 2.5 million km 2 and comprises the largest continuous sand sea on earth (Thomas and Shaw, 1991a). It has long been recognised as preserving a record of extensive continental aridity, which is most clearly manifested via an abundance of palaeolake basins, ephemeral stream channels and widespread, presently inactive, and in places extensively degraded, aeolian dune forms. By far the most abundant dune forms are of the linear variety. A combination of excessive annual rainfall and insufficient wind energy preclude the occurrence of widespread contemporary aeolian activity on the linear dunes of the Kalahari, even in the driest, southwestern area of the sand sea (Breed et al., 1979; Wiggs et al., 1995). Linear dunes of the Kalahari have been classified into three discrete dunefields (Fig. 1; Lancaster, 1989; Thomas and Shaw, 1991a): The southern field occurs south of latitude 23"~'S, and is centred between 18 and 2 T E . This area comprises the driest part of the Kalahari, with the aeolian landscape dominated by well preserved, partially vegetated, steep-sided (mean slope - 1 3 . 4 " (Lancaster,

S. Stokes et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 137 (1998) 305 322

I

] UUU Km 10 ° I

J

8OO 20 ° I

307

30 ° I

Fig. 1. The extent of the Mega Kalahari (adapted from T h o m a s and Shaw, 1991a). Northern (N), eastern (E) and southern (S) sectors of Kalahari desert are delineated. A n n u a l rainfall distributions for Southern Africa (isohyets in m m ) are also plotted.

1988); heights ranging from 5 to 25 m) asymmetrical linear dunes that predominantly trend northwest southeast, and sporadic pans with associated lunette dunes (Bullard et al., 1995). The northern dune field occurs north of latitude 23°S and consists almost exclusively of linear dune forms and is bounded by Etosha Pan to the west and the Okavango Delta to the East (Fig. 1 ). Linear dune trend lines form an arc which varies from a W N W - E S E to WSW E N E orientation. The dunes are vegetation covered, and are frequently extensively degraded, although in places dune heights may reach up to 25 m (Grove, 1969; Thomas and Shaw, 1991a). Occurring in an area where rainfall currently exceeds 400 mm p.a. and increases northwards to 1200 mm p.a., these aeolian features have frequently been described as some of the strongest

evidence for previous periods of enhanced aridity (Thomas, 1983). The dunes of the eastern dune field, occupying the area north of 23°S and west of the Okavango Delta and eastwards beyond the Gwayi River (Fig. 1) have similarly been identified as strong evidence for past periods of enhanced aridity e.g. (Bond, 1948; Flint and Bond, 1968; Thomas, 1984). Annual present-day rainfall totals in this region of up to 800 mm have resulted in an extensively degraded dune landscape recognisable for the most part by contrasting dune and interdune vegetation communities. As an area marginal to the core of southern African aridity, which is centred on the Namib and southwest Kalahari Deserts, desiccation during arid periods was probably late to arrive and relatively short-lived; being replaced

308

S. Stokes et al. /Palaeogeography, Palaeoel#natology, Palaeoeeology 137 (1998)305 322

by more humid conditions which accentuate pedogenesis, vegetation and dune stabilisation. This should therefore be an environment highly sensitive to changes in moisture availability, and therefore an area which should provide a useful archive of past periods of desert expansion in southern central Africa. This study provides a reconnaissance chronology of linear ridges in the eastern dune field, based on optical dating of quartz sand grains, and provides for the first time directly dated evidence of late Quaternary aeolian activity in this portion of the Mega Kalahari. Our data allow an independent assessment, based on direct age determinations, of the models of climate changes in Africa south of the equator. This study complements concurrent chronological investigations of other portions of the Mega Kalahari (Stokes et al., 1997a; Thomas et al., 1997).

2. Study area

The extent of Kalahari sediments and linear dune ridges of the northeastern dune field is depicted in Fig. 2. The parallel ridges are clearly visible in satellite imagery (Fig. 3), where the actual topographic relief upon the ridges is most clearly reflected in contrasting dune ridge and interdune trough vegetation patterns (Bond, 1948). Previous descriptions of these features have been provided by (Bond, 1948; Flint and Bond, 1968; Thomas, 1983; Thomas and Shaw, 1991a; Lancaster, 1995). The dune ridges are generally broader and more widely spaced than the linear dunes of the southwest Kalahari. Dune spacing ranges from 1500 to 2500 m, and individual ridge widths range from 500 to 2500 m (Thomas and Shaw, 1991a). Heights of the ridges above the interdune troughs are typically low ( < 1 0 m ) , especially given the considerable width of the features, although some have been recorded up to a maximum height of 2 2 . 5 m (Thomas, 1984). Dune ridge orientation varies in a systematic arclike pattern across the dune field from a W N W - E S E orientation in the east and north of the field, to an E N E - W S W orientation to the west where the ridges intersect the Botswana border

(Fig. 2). Subtle (5 °) contrasts in ridge orientation from south (Hwange) to north (Victoria Falls) were used in combination with other morphological parameters by (Thomas, 1984) to infer genetic differences between the two areas. While rainfall levels in the region are presently relatively high and constitute part of a rainfall gradient increasing from southwest to northeast, they are highly seasonal with 80% of the total rains falling during the summer ( O c t o b e r ~ p r i l ) season. Low relative humidity and high year round temperatures result in annual potential evapotranspiration which exceeds 2000mm (Thomas and Shaw, 1991a). Annual wind roses and potential sand flow patterns for nearby stations are plotted in Fig. 2. Definitive statements on the wind regime are problematic due to the paucity of data (for discussion see Thomas, 1984). A complex wind regime is, however, apparent, exhibiting net resultant sand drift directions from east-northeast to west-northwest, a direction close, but not identical to the trend of the linear dunes. Total drift potential of the current wind field is low (Fryberger and Dean, 1979) and incapable of significant aeolian sand transportation (Heine, 1982; Thomas, 1984). All previous studies have inferred extensive degradation of the initial linear dunes to generate the subdued ridges that are present today. Within the dune ridges primary sedimentary structures have been destroyed. Active agents in this degradation process include sheet wash erosion (Flint and Bond, 1968), and bioturbation by both large herbivores and carnivores, and burrowing by smaller animals and termites (Thomas, 1984). Where exposed in section, the dune sediment appears as largely structureless red sand interrupted only by scarce, laterally continuous zones of charcoal, and at depth in some pits, by post depositional wavy non-parallel laminations which in part mimic former primary sedimentary structures. Post-depositional reddening of the sand has also occurred, the degree of reddening varying systematically from the south (7.5YR-10YR) to the north (2.5YR-5YR) within the study area. This has previously been used to infer a N-S-directed (older-younger) relative dune chronosequence (Thomas, 1984). The aeolian sediments preserved in the vicinity

S. Stokes et aL / Palaeogeography, Palaeoclimatology,

Palaeoecology 137 (1998) 305 322

309

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Fig. 2. The northeastern Kalahari showing sampling localities and major dune ridges in Hwange and Victoria Falls. Inset shows annual potential sandflow rose constructed from wind data at nearest meteorological station (Bulawayo Airport). Data redrawn from (Thomas, 1984). Arrow identifies resultant sand drift direction (RDD); numerator identifies resultant sand drift potential (RDP); denominator identifies total sand drift potential (DP). For details see Fryberger and Dean (1979).

of Victoria Falls do not define discrete linear ridges such as those exposed to the south. Instead, extensive mounds of sediment are observed which typically express a N E - S W orientation. These frequently partially infill palaeo-valley features or hug local topographic highs, while in some locations the orientations of vleis developed between the ridges confirm the orientation of the original dune landscape. In this study samples were collected for optical dating during two field seasons, in 1992 and 1995. The distribution of sampling localities is shown in Fig. 2. Summary locality and stratigraphic data are also provided in Table 1. While the main focus

of the study was to establish a chronology of dune building phases, a small number of samples was also collected from adjacent pan and ephemeral fluvial channel sediments. During the initial reconnaissance (samples 889-896) an emphasis was placed on establishing spatial patterns in near surface (0 2 m depth) samples, while the latter collections (950-952, 1003-1005; Fig. 4) focused on observing variations in dune age with depth (down to a maximum depth of 6 m). While this represents a substantial improvement on the collection of materials only from shallow (ca. < 2 m) pits, it is noted that the majority of the vertical expression of these dune forms remains unsampled.

310

S. Stokes et al. /Palaeogeography, Palaeoelimatology, Palaeoeeology 137 ( ] 998) 305 322

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Fig. 3. Satellite image of a portion of northwestern Zimbabwe and its borders with Botswana, showing the extensive linear dune field which occupies much of the Hwange National Park and passing across (SW) into Botswana. The less pronounced aeolian features preserved around Victoria Falls are observable in the top left portion of the image. A topographically high area of basement rocks defines an irregular margin for the Kalahari sediments across much of the upper centre and right of the image. Image area is approximately 160 km 2.

3. Optical dating methods S a m p l e s were collected by h a m m e r i n g lightp r o o f P V C cylinders o f k n o w n v o l u m e (ca. 500 cm 3) h o r i z o n t a l l y into the vertical walls o f freshly cleaned e x p o s u r e s p r e p a r e d at each site. T h e ends o f the cylinders were sealed with b l a c k t a p e a n d

p l a c e d in b l a c k p o l y t h e n e bags for t r a n s p o r t a t i o n to the d a t i n g l a b o r a t o r y in Oxford. In the l a b o r a tory, all s a m p l e s were processed u n d e r s u b d u e d red light. D e t a i l s o f the optical d a t i n g m e t h o d a n d related p h e n o m e n o n are p r o v i d e d elsewhere ( A i t k e n , 1985, 1989, 1992; H u n t l e y et al., 1985; Smith et al.,

S. Stokes et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 137 (1998) 305-322

311

Table 1 S u m m a r y data for optical dating samples Site ref.

Latitude

Longitude

Locality/description (name o f pan/site)

Depositional environment

889 890

18 ° 55'S 18 ° 45'S

26 ° 25'E 25 ° 45'E

gleyed pan sand alluvial channel sands

2

891

18 ° 50'S

25 ° 50'E

degraded linear dune

2

82 894 895 896 950

19 ° 15'S 18 ° 55'S 18 ° 43'S 18 ° 00'S 17°58'S

26 ° 22'E 26 ° 41'E 26 ° 57'E 25 ° 50'E 25°38'E

degraded linear degraded linear degraded linear degraded linear linear dune

2

951

18°02'S

25°41'E

952

17°55'S

25°28'E

1003

18°35'S

25°54%

1004 1005

19°24'S 15°51 'S

26°45'E 26°56'E

N e h i m b a Pan, Hwange National Park abandoned channel, Shabi Shabi, Hwange National Park Border security road (Botswana/Zimbabwe), Hwange National Park, south of Mitswiri, Hwange National Park Giraffe Springs, Hwange National Park Hwange Maincamp, Hwange National Park Victoria Falls rubbish dump, Victoria Falls C h a m a b o n d o Vlei, Zambesi National Park, Victoria Falls C h a m a b o n d o Vlei, Zambesi National Park, Victoria Falls Kazungula Road, Zambesi National Park, Victoria Falls 10 k m N of Robins Camp, Hwange National Park Josibannini, Hwange National Park Dopi Pan, Hwange National Park

1990; Stokes, 1994). A portion of each sample was wet-sieved to separate the 90-150 I~m size fraction and immersed for two days in 1 N HC1 to remove carbonate, followed by two days immersion in H202 to remove organic matter. Heavy minerals (density >2.72 g cm 3) were removed from the treated sample fraction by magnetic and heavy liquid (sodium polytungstate) separations. The samples were then treated with 48% H F for 60 minutes, and HzSiF 6 for 4 days in order to further concentrate quartz grains. A further heavy liquid separation and a second stage of dry sieving through a 75 ~tm sieve was also undertaken to obtain the final quartz concentrate used for dating. At each stage of the separation procedure samples were generously rinsed with distilled water. The quartz separates were then mounted as monolayers (approximately 5 mg per disc) onto 10 mm diameter stainless steel discs using a silicone spray adhesive (Silkospray). A number of the prepared discs were tested for contaminating feldspathic or other non-quartz grains by infra-red light exposure (Stokes, 1992). Palaeodoses were calculated using the multiple aliquot dose method (Aitken, 1992). The aliquots

dune dune dune dune l

No. of samples 1

1 1 1

linear dune

1

linear dune

3

linear dune

2

linear dune degraded linear dune

12 16

were exposed to an argon laser (Coherent 2W), operated at an emission wavelength of 514.5 nm and at a power output level at the sample of 40 m W cm -2. The resulting sample OSL emissions were detected using a photomultiplier filtered by BG-39 and Corning 7-51 glass filters. Prior to OSL measurements, aliquots were pre-heated to remove geologically unstable charge populations created during laboratory irradiation procedures. The preheat procedure involved heating the discs at 160°C for 16 h, or 220°C for 5 min. Normalisation of growth curve data was undertaken either by sampling small portions (emitted following ca. 0.4 mJ cm-2 of laser stimulation) of natural OSL prior to dosing and pre-heating (so called natural normal isation), or by measuring the OSL responses to a test dose following a series of bleaching and preheating procedures (so-called equal total dose normalisation). Details of these methods are given in (Stokes, 1994). Generally the resulting palaeodoses exhibit good agreement at one standard deviation errors, independent of the normalisation procedure used (see Table 2 and below). OSL growth curves were generated for progressive durations of laser exposure and palaeo-

S. Stokes et al. / Palaeogeography, Palaeoelimatology, Palaeoeeology 137 (1998) 305 322

312 ii~~: ~ &i~;iii~ili
i=!~iiii= ¸¸¸=¸¸9¸

a

b

Fig. 4. Exposures of linear dune sediments. (a) Site 100K massive red sands underlie a moderately developed dune soil. (b) Upper portion of site 1004 showing typical structureless red sands with extensive root penetration and small-scale bioturbation features (note sampling holes spaced at 50 cm intervals).

doses were extrapolated for a range of exposure periods. Palaeodose estimates quoted in Table 2 are based on the light from integrated laser exposure at ca. 3 J c m - 2 and using a saturating exponential curve fitting procedure (Brumby, 1992). Where multiple palaeodose estimates were available a weighted average (Bevington, 1969) of all individual estimates was used for the calculation of sample ages. Dose rate estimation was undertaken both in the field via portable g a m m a spectrometry and in the laboratory via instrumental neutron activation analysis. Sample splits for lab-based dose rate determinations were crushed and homogenised by ring milling for 1 h. Conversion from concen-

trations to dose rate followed the procedures outlined in (Aitken, 1985). We chose to utilise the field g a m m a spectrometry measurements to estimate g a m m a and cosmic dose rates and the neutron activation analyses to estimate beta dose rates. The optical dates calculated incorporate both random and systematic errors (Aitken and Alldred, 1972), and are quoted to _+ 1 standard deviation.

4. Results

G r o w t h curves generated in most cases exhibited relatively low levels of disc-to-disc scatter (typ.

S. Stokes et al. /Palaeogeography, Palaeoclimatology, Palaeoecology 137 (1998)305-322

313

Table 2 Palaeodose determinations for samples collected from the northeastern Kalahari Sample

Number of aliquots

Pre-heat procedure

Normalisation a

Individual palaeodose estimate (Gy)

889/1 889/1 889/1 890/1 890/1 890/2 890/2 890/2 890/2 891/1 891/1 891/1 891/2 891/2 892/1 892/1 892/1 892/1 892/2 892/2 892/2 894/1 894/1 894/1 894/1 895/1 895/1 896/1 896/1 950/1 951/1 952/1 952/2 952/3 1003/1 1003/2 lO04/a lO04/b 1004/c lO04/d lO04/e lO04/f lO04/g lO04/h 1004/i lO04j lO04/k 1004/1 lO05/a lO05/b 1005/c lO05/d lO05/e lO05/f

26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50

160°C, 160°C, 220°C. 160°C 160°C. 160°C. 160°C. 220°C. 220°C. 160°C. 160°C. 220°C. 160°C. 220°C. 160°C. 160°C. 220°C. 220°C. 160°C. 160°C. 220°C. 160°C. 160°C. 220°C 220°C, 160°C, 160°C. 160°C 160°C 160°C 160°C 160°C 160°C 160°C 160°C, 160°C 160°C, 160°C 160°C 160°C 160°C 160°C 160°C 160°C 160°C 160°C 160°C 160°C 160°C 160°C 160°C 160°C 160°C 160°C

nn etd nn nn etd nn etd nn etd nn etd nn nn nn nn etd nn etd nn etd nn nn etd nn etd nn etd nn etd nn nn nn nn nn nn nn nn nn nn nn nn nn nn nn nn nn nn nn nn nn nn nn nn nn

9.0_+5.9 12.9 _+ 1.8 9.1 +2.1 21.1_+1.3 26.4+3.5 18.7+4.4 11.0_+0.7 14.1 _+4.2 16.8_+ 1.4 16.5_+2.0 12.7+ 1.4 15.4-+4.0 11.7_+ 1.3 10.0_+2.3 12.2_+ 1.3 8.8_+2.1 8.6+ 1.3 9.1 + 2.4 13.3-+3.7 7.4_+0.3 13.2 + 3.9 6.3_+2.3 7.9+ 1.9 6.8 _+0.6 7.9 + 1.4 8.1 +3.0 12.3+3.3 15.6_+5.0 18.0_+5.0

16 h 16 h 5 min 16 h 16 h 16 h 16 h 5 min 5 min 16 h 16 h 5 min 16 h 5 min 16 h 16 h 5 min 5 min 16 h 16 h 5 min 16 h 16 h 5 min 5 min 16 h 16 h 16 h 16 h 16 h 16 h 16 h 16 h 16 h 16 h 16 h 16 h 16 h 16 h 16 h 16 h 16 h 16 h 16 h 16 h 16 h 16 h 16 h 16 h 16 h 16 h 16 h 16 h 16 h

Final palaeodose b (Gy)

11.2 _+ 1.3

21.7_+ 1.2

12.3_+0.6

14.1_+ 1.1

11.3_+1.1

10.0_+0.8

7.5_+0.3

7.0 _+0.5

10.0+2.2 16.8_+3.5 17.5_+3.4 16.1 _+2.3 37.8_+4.6 24.6_+2.9 71.0_+11 12.7_+ 1.5 22.0-+ 1.9 9.6_+2.5 13.1 ___2.5 16.2_+2.4 22.2_+3.3 24.3_+3.5 25.7_+2.8 21.8_+ 1.8 28.0_+2.9 26.7_+3.1 52.4_+5.5 54.7_+5.9 53.4_+6.5 4.3_+0.8 14.1 _+2.2 10.1 _+2.1 19.4_+2.0 16.6_+ 1.8 18.9_+2.1

ann = natural normalisation; etd = equal total dose normalisation. b Where multiple palaeodose estimates are available for a given sample the final palaeodose estimate used in the age evaluation is a weighted average of the individual estimates.

314

S. Stokes et aL/ Palaeogeography, Palaeoclimatology, Palaeoecology 13 7 (1998) 305 322

palaeodose estimates, coupled with the stratigraphically sensible ordering of most of the final age estimates, leads us to conclude that bioturbation of the dune sands has not resulted in large scale mixing and vertical relocation of sediments deposited at differing times. Despite exhibiting relatively low concentrations of all isotopes measured (mean K20 ca. 0.1%; mean U ca. 0.6 ppm; mean Th ca. 1.6 ppm; mean dose rate 0.55 Gy ka 1) the degree of agreement between the two dosimetric methods, particularly for U and Th, is good (Table 3). Potassium deter-

< 15%), and were found to be well modelled by a single saturating exponential regression line (Fig. 5). As a result, all sample palaeodoses were modelled by extrapolation of single exponential fits (Table 2). In many cases a combination of preheating and signal normalisation procedures were attempted. In most such cases, it was found that individual palaeodose estimates did not exhibit statistically significant and consistent differences, and final palaeodose estimates were calculated from a weighted average of the individual estimates (Table 2). The relatively low levels of scatter within

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Added dose (Gy) Fig. 5. Examples of growth curve data generated for selected samples•

200

depth (m)

1.4 3.0 2.0 1.8 0.9 1.2 0.7 1.2 1.0 1.6 1.3 1.2 1.8 3.0 4.5 1.0 2.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 0.5 1.0 1.5 2.0 2.5 3.0

ID

889/1 890/1 890/2 891/1 891/2 892/1 892/2 894/1 895/1 896/l 950/1 951/1 952/1 952/2 952/3 1003/1 1003/2 1004/a 1004/b 1004/c 1004/d 1004/e 1004/f 1004/g 1004/h 1004/i 1004/j 1004/k 1004/1 1005/a 1005/b 1005/c 1005/d 1005/e 1005/f 1.77±0.17 1.98__+0.18 1.92±0.18 2.17±0.20 1.68±0.16

1.40+0.14 1.56±0.15 1.60±0.16 1.65±0.16 1.50±0.15 1.58+0.15 1.47 ±0.15 1.58+0.15 1.46±0.15

0.96 ± 0.07 1.04±0.07 -0.47±0.04 0.44±0.04 0.72 ±0.07 0.65±0.06 0.59±0.06 -0.46 ±0.05 0.55_+0.05 0.44±0.04 0.55±0.05 0.58±0.06 0.58±0.06 0.67 ±0.07 0.61 ±0.06 0.57±0.06 -0.62±0.06 0.56 ± 0.06 0.71 ±0.07 0.78±0.08 0.87±0.09 1.02+0.10

0.09 ± 0.004 0.08±0.003 -0.03 ±0.003 0.06±0.005 0.01 ±0.001 n.d. n.d. --0.11 ±0.01 0.10±0.01 0.12±0.01 0.12+0.01 0.13±0.01 0.13±0.01 0.11 ±0.01 0.10±0.01 0.13±0.01

-0.11 ±0.01 0.04±0.003 0.03 ±0.003 0.03 ±0.003 0.04±0.003 0.04+0.004

-1.65±0.16 1.41 ±0.14 1.43 ±0.14 1.42±0.14 1.45±0.14 1.62±0.15

2.51 ±0.18 1.89±0.12

1.98+0.19

0.48±0.05

0.08 ±0.006

1.63±0.12 0.68 ±0.06 1.35±0.11 1.45±0.13

Th (ppm)

0.58__+0.04 0.33±0.03 0.48 ±0.04 0.50±0.05

U (ppm)

0.04±0.002 0.02±0.002 0.05±0.004 0.12±0.009

KzO (%)

Portable gamma spectrometry

n.d. = not detected above background. a Assumed value.

Sampling

Sample

Table 3 Summary of dosimetry and other age-related information

0.05±0.01 0.04±0.01 0.08±0.01 0.17±0.02 0.17±0.02 0.07±0.03 0.07±0.02 0.07±0.03 0.10±0.03 0.27±0.05 0.03±0.001 0.05±0.002 0.03±0.001 0.03±0.001 0.03±0.001 0.41±0.02 0.41±0.02 0.17±0.01 0.20±0.01 0.20±0.01 0.17±0.01 0.17±0.01 0.18±0.01 0.19±0.01 0.16±0.01 0.29±0.01 0.16±0.01 0.17±0.01 0.21±0.01 0.08±0.003 -0.08±0.003 0.04±0.003 0.09±0.004 0.09±0.004

(Gy ka l) 0.18±0.002 0.14±0.002 0.14±0.003 0.16±0.004 0.18" 0.18±0.004 0.18" 0.19±0.003 0.18±0.001 0.18 ~ 0.17±0.004 0.17±0.004 0.17±0.004 0.17±0.004 0.18±0.004 0.18 ~ 0.15 a 0.21±0.005 0.18±0.004 0.16±0.004 0.15±0.004 0.14±0.004 0.13±0.004 0.13±0.004 0.12±0.004 0.11±0.003 0.10 ~ 0.10 ~ 0.09±0.003 0.20±0.005 0.18±0.004 0.16±0.004 0.14±0.004 0.14±0.004 0.14 ~

K20

(%)

D . . . . ic

0.5±0.06 0.3±0.05 0.7±0.06 0.5±0.06 0.5±0.06 0.6±0.07 0.6±0.07 1.0±0.08 2.3±0.10 0.7±0.08 0.6±0.13 0.7±0.14 0.7±0.13 0.6±0.13 0.5±0.13 0.8±0.13 1.1±0.15 0.5±0.10 0.6±0.10 0.7±0.12 0.5±0.11 0.8±0.12 0.7±0.12 0.7±0.10 0.7±0.09 0.6±0.09 0.6±0.11 0.6±0.10 0.6±0.10 0.5±0.07 -0.7±0.11 0.7±0.13 1.0±0.12 0.7±0.11

(ppm)

U

Neutron activation analysis

1.6±0.19 1.3±0.19 1.5±0.18 1.5±0.16

1.6±0.10 0.5±0.10 1.4±0.10 1.3±0.10 1.2±0.10 1.7±0.10 1.6±0.10 2.5±0.10 1.5±0.10 2.6±0.10 1.8±0.17 1.8±0.17 1.8±0.17 1.9±0.18 1.5±0.17 2.3±0.21 2.8±0.22 1.0±0.15 1.3±0.18 1.4±0.24 1.6±0.24 1.5±0.24 1.6±0.22 1.6±0.16 1.6±0.18 1.5±0.17 1.9±0.22 1.6±0.17 1.7±0.17 1.2±0.11

Th (ppm) 0.47±0.06 0.30±0.04 0.46±0.05 0.52±0.06 0.53±0.04 0.52±0.08 0.51±0.06 0.69±0.12 0.81±0.14 0.59±0.07 0.47±0.06 0.50±0.06 0.51±0.06 0.50±0.06 0.45±0.06 0.88±0.08 0.97±0.08 0.55±0.08 0.57±0.08 0.57±0.08 0.53±0.08 0.55±0.07 0.55±0.07 0.55±0.07 0.52±0.06 0.56±0.07 0.52±0.05 0.50±0.04 0.52±0.07 0.49±0.0 0.50±0.06 0.51±0.07 0.47±0.06 0.57±0.07 0.49±0.04

(Gy ka 1)

Dose rate

11.2±1.3 21.7±1.2 12.3±0.6 14.1±1.1 11.3±1.! 10.0±0.8 7.5±0.3 7.0±0.5 10.0±2.2 16.8±3.5 17.5±3.4 16.1±2.3 37.8±4.6 24.6±2.9 71.0±11 12.7±1.5 22.0±1.9 9.6±2.5 13.1±2.5 16.2+2.4 22.2±3.3 24.3±3.5 25.6±2.8 21.8±1.8 28.0±2.9 26.7±3.1 52.4±5.5 54.6±5.9 53.4±6.5 64.3±1.8 14.1±2.2 10.1±2.1 19.4±2.0 16.6±1.82 18.9±2.1

(Gy)

Palaeodose

023.7±4.0 73.0±11 26.9±3.4 26.9±3.8 21.3±2.6 19.3±3.5 14.7±1.8 10.1±1.9 12.3±3.4 29.0±7 37.5±8.8 32.3±6.2 75±13 49.2±8.5 157.0±33 14.5±2.1 22.7±2.7 17.4±5.1 22.9±5.3 28.5±5.8 42.2±8.8 43.8±8.7 47.0±8.1 39.7±5.9 53.5±8.5 47.2±8.2 101.0±14 109.0±15 102.0±18 8.8±2.0 28.5±5.4 19.9±4.9 41.4±7.1 9.2±4.7 38.3±5.1

(ka)

Age

txa

I

,-.q

%

316

S. Stokes et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 137 (1998) 305 322

minations exhibit some differences between the two methods, in some cases beyond the quoted errors. This most likely relates to the low absolute values for K20, many of which are below the range over which the portable gamma spectrometer is calibrated in the laboratory (Stokes, 1994). Typically the most significant single contribution to the overall dose rate is derived from cosmic radiation (mean ca. 30%), with some samples receiving in excess of 50% of the total dose via the cosmic contribution. The resulting estimates of dune sediment deposition range from 8.8 to 157 ka (Table 3). When observed independent of location or sampling depth the data set is suggestive of a virtual continuum of dune activity spanning much of the latter half of the last glacial period (ca. 10 50 ka), with evidence of less frequent activity between 50 and 100 ka. Examination of the chronology of the dune deposits within a stratigraphic framework provides a clearer picture of dune age (Fig. 6). Interpretation of the vertical sequence sampled within the Victoria Falls dune field (site 952) is complicated by stratigraphic inversion of the upper two samples. The upper sample appears to be anomalously old as sample 952/2 at 1.8 m depth exhibits a depositional age close to that of the main period of linear dune construction identified within the Hwange dune field. Sample 952/3 may identify a phase of aeolian deposition relating to the penultimate glacial cycle (ca. 160 ka). The large one sigma errors on the single age preclude definitive conclusions at this stage and we intend to undertake further analysis and sampling to confirm this observation. Other localities sampled at depths of up to 2 m from the current dune surface within the Victoria Falls dune field produced statistically indistinguishable ages ranging from 29_+ 7 to 38_+ 8 ka. There is no evidence of dune activity post-dating ca. 30 ka in the Victoria Falls area. The Hwange dune field to the south was more extensively sampled in both the number and depths to which the dunes were excavated (Fig. 6). With the exception of sample 1005c, age estimates are in sensible stratigraphic order. Sampling pits on dunes within one or two metres of the present land surface indicated two-phases of dune activity at

ca. 20 3 0 k a a n d c a . 12 15 ka which are in most cases statistically distinct. Evidence of activity during the latter period being absent from the Victoria Falls dune field. When the dates from sites 1004 and 1005 are grouped into statistically distinct sets, three discrete phases of Pleistocene aeolian activity are recognised occurring at 95 115, 38-42 and 20 26 ka (Fig. 7). The ages demonstrate that dune construction in this area has taken place episodically, with relatively short-lived (5-20 ka) periods of dune development interspersed with extended (20-40 ka) periods of non-deposition. Each depositional phase resulted in the accumulation of between 1 and 2.5 m of sediment. These 'active' depositional phases were presumably punctuated by more humid periods during which time rainfall was greater (stabilising dune activity via changing soil and plant ecosystems) and/or wind speed were reduced (prohibiting active sediment transportation). The present period of inactivity appears to have commenced at or near to the Pleistocene-Holocene boundary and only two samples yielded early Holocene ages. The two samples of gleyed, fine-grained, waterlain sediments derived from pan and dry valley (vlei) settings (889/1, 890/1) produced age estimates of 23.7 _+4.0 and 73 _+ 11 ka, respectively. The former age is not significantly different from a period of widespread aeolian activity. This result might indicate that sampling actually took place within pre-existing aeolian strata which have undergone post-depositional bleaching, or that the pan depressions may have held water for some period of the year even during this phase of widespread aeolian activity (and presumably reduced rainfall ). Further sampling of such deposits is required to fully explain these data. The age of ca.73 ka for 890/1 is likewise somewhat problematic. Concomitant aeolian activity might be indicated by the age of ca. 75 ka obtained for the uppermost sample of the dune profile at site 952. As the age estimated for the dune is stratigraphically out of context, and is the only aeolian sample to indicate aeolian reworking and deposition at this time we are inclined to reject it and infer that fluvial deposition (and enhanced rainfall) may have occurred during this period.

317

S. Stokes et al. /Palaeogeography, Palaeoclimatology, Palaeoecology 137 (1998) 305 322 952

896

895 75 ± 13

29-+7

9 5 0 ~ rm:-~

49.2 ± 8.5

R ~l

'"

R

~

157

-+ 33

~ , Duneridges 1~) :~:':: Kalahar~Beds

ZAMBIA

R ~ 3 7 "-+ 8.8 5

12.3 ± 3.4

~ 1005

8.8±2.0

28.5-+5.4 19,9-+4.9 41.4¢7,1

Vlcto#a Fails

951

R loll"

29.2±4,7 38,3+5.1

o Hwange Town

~.3-+6.2

7

1003 -0 4.5+2.1 -1 -2

894

R ~ I" 22"7± 2"7

89o(1)

~

J004

89o(2)~ ~ -3

R

-4

73+ 11

£

C~1:21"3±2"6

42.2-+8.8 ~.8-+8.7 47,0+8.1

26.9+3.8

c Charcoal R Root penetration B Burrowing / bioturbation

14.7-+1.8

C

19.3-+3,5

C

10,1 + 1.9

17.4-+5.1 22.9±5.3 28.5±5.8

i

39.7-+5,9 53.5±8.5 47.2±8.2 101 -+ 14 109+ 15 102 -+ 18

Fig. 6. Optical dates for samples analysed in this study plotted within a schematic stratigraphic framework.

5. Discussion Our study provides a useful numerical chronology for dune construction within the extensive relict linear dune fields of western Zimbabwe and the Victoria Falls. Clearly the linear dunes in this area are composite features which provide an extended record of environmental change. Some specific implications of our dune chronology for palaeoenvironmental research in central southern Africa are discussed separately below. Central to our palaeoclimatic interpretations from the dune chronosequence is the assumption that periods of aeolian accumulation are contemporaneous with increased aridity. The controls on aeolian sedimentation are complex, particularly in the case of linear dune development (Lancaster, 1987;

Livingstone and Thomas, 1993). Key environmental limitations on aeolian activity include windiness, moisture and vegetation cover (Ash and Wasson, 1983). In the northeastern Kalahari, mean rainfall today exceeds 400 mm per year with the surfaces of degraded linear dunes undergoing pedogenesis and covered in mixed woodland. The climate would have to be significantly more arid than at present for aeolian processes to operate. Past changes in aeolian dynamism must be related to changes in rainfall intensity in the area, which in turn relates to the large scale atmospheric changes influencing the incursion of summertime convection which is supplied from the east and reduces southwards and westwards (Fig. 1). Modelling studies have indicated that southeast Atlantic SSTs strongly influence present-day

318

S. Stokes et aL/ Palaeogeography, Palaeoclimatology, Palaeoecology 137 (1998) 305 322

Age(ka) 100

0 .

.

.

.

i

.

.

200 .

.

iI iI

\

\

\N \\ 5

Depth ~ . (m)

6

Fig. 7. Sampling depth versus dune sediment age for sites 952 (~), 1004 (0) and 1005 (~). regional summertime rainfall levels (Stokes et al., 1997b). 5.1. Causes o f environmental change

Our data indicate that widespread aridity and linear dune emplacement occurred during at least three periods since the last interglacial at ca. 95-115, 38-42 and 20-26 ka, and more localised reworking has taken place in the Hwange dune field since that time. The arid phases are shortlived (5 20 ka) in comparison to the intervening humid periods (20-40 ka) and appear to be restricted almost exclusively to pre-Holocene times. Recent studies by Little et al. (1997) have provided a detailed record of palaeo-sea surface temperatures (SSTs) in the adjacent southeastern

Atlantic Ocean over the same period which they relate to the changing position of oceanic convergence zones, and demonstrate that such changes are teleconnected to equatorial Atlantic temperature seasonality, trade wind intensity and zonality, and high latitude palaeoclimates. We consider it likely that past changes in southeast Atlantic SSTs may have likewise influenced regional aridity. They identify nine periods of cold southeast Atlantic SST's and enhanced upwelling during the past 140 ka in the southeast with an average spacing of 10 ka during marine isotope stages (MIS) 2-4 and longer (ca. 20 ka) spacing during MIS 5. They termed these cold periods PS events, and related them to times of enhanced trade winds and reduced rainfall across southern Africa. These were also correlated, incorporating a 2-3 ka S N-directed phase lag, with Heinrich events and Dansgaard Oeschger Cycles identified, respectively, in the Northern Atlantic Ocean and Greenland ice cap (Little et al., 1997). Four of the five most extreme of these PS events compare closely in duration and timing to the identified periods of linear dune activity in the northeastern Kalahari. It would therefore appear that changes in aridity within the northeastern Kalahari are closely related to global scale changes in glacial phase palaeoclimates which may be collectively caused by changes in equatorial and mid-latitude oceanic and atmospheric processes (Little et al., 1997). In the case of the Kalahari, neither the 'passive Africa-high latitude forcing' nor the 'active Africa precessional forcing' models account for the observed arid humid transitions. Instead, a model which identifies the Southern African continental land mass playing a passive role, but in turn being strongly influenced by mid to low latitude, southern hemisphere oceanic-atmospheric processes (changing SST's, cyclogenesis and trade wind intensity) may be more appropriate. 5.2. The nature and extent o f humid environmental conditions

The date on the vlei sands from site 890 is suggestive of fluvial activity (?humid period) preceding the mid-glacial (ca. 38-42 ka) phase. While

S. Stokes et al. / Palaeogeography, Palaeoclimatology, Palaeoeeology 137 (1998) 305-322

not beyond the individual error limits of the respective age estimates we infer a break in aeolian deposition between the two latter phases. This inference is based on both evidence of stratigraphic discontinuity and the observation that the age-depth profiles of the deeper sequences exhibit uniformity of ages between the breaks over thickness' of the order of metres. We recognise the break stratigraphically as either a zone of charcoal and rare insect coprolites (site 1004), or a zone of postdepositional wavy non-parallel laminations. There is no similar stratigraphic break recorded between the first and second phases of aeolian activity at site 1004. Charcoal-bearing zones recognised at other levels may relate to fire damage to root structures, the incarceration of burned stands of vegetation which occupied the dune areas during short-lived humid phases, or additional breaks in dune accumulation which occurred on shorter time-scales than that which can be recognised by our present chronology. 5.3. Dune construction and preservation

While much of the upper few metres of the aeolian landscape preserved within the Hwange National Park was emplaced or reworked during the later portion of the last glacial period (ca. 10-20 ka) we note that the period spanning ca. 40-50 ka is more extensively preserved at depth. It is not yet clear whether this reflects widespread degradation of dunes constructed during the later phase, or whether the middle and/or earlier phases were more arid, more windy or more long-lived, contributing to the high volume of aeolian accumulation. What is, however, apparent is that widespread continental aridity punctuated by, in some cases long-lived, more humid periods appears to typify the environment of the northeastern Kalahari during glacial times. The absence of the youngest (post 20 ka) phase of activity in the Victoria Falls dune field in the north of the study area may represent a significant contrast from the dunes 100-200 km to the south. Given the more degraded and weathered appearance of these features, and the existence today of a rainfall gradient which reduces at a rate of ca. 1 mm k m - 1 southwards, it is possible that we have

319

identified the northern margin of significant latest Pleistocene aeolian activity, in a position bifurcating the two dunefields. The deep (ca. 3-6 m) sampling in the linear dune features has confirmed their potential as a long-term archive of aeolian activity and continental palaeoenvironments. This contrasts with many other (e.g., barchanoid) dune forms which undergo wholesale destruction and reconstitution during transportation (Cooke et al., 1993), preventing an extended environmental record. While near surface sampling provides insights into ultimate, and in some cases penultimate, phases of activity, establishing the time of post-depositional degradation of the dunefields, the deep profiles serve to identify precursory arid phases. We intend to undertake a drilling programme to probe deeper into the linear dunes. Given the relatively low levels of radiation dose in the study area it should be possible to establish a chronology which extends back ca. 500 ka. Our data both support previously inferred periods of aeolian activity in the southwest Kalahari (Lancaster, 1989), and identify additional older phases of activity. The three main ergs of the Kalahari have been active at different times during the later Quaternary and this activity has been preserved in a spatially variable manner. These new findings in part reflect the location of the field area studied. Being positioned at the northeast margin of the S W - N E regional rainfall gradient, away from the core of aridity, both reduces the likelihood of long-lived aridity causing wholesale erosion and reworking of pre-existent dunes, and enhances the extent of pedogenesis and dune stabilisation during humid periods. These two factors indicate that the linear dune system of the Northeastern Kalahari is a sensitive archive of long term changes in moisture balance.

6. Conclusions

(1) We have developed a direct chronology of the linear dunes preserved in the Hwange National Park and Victoria Falls dune fields on the northeastern Kalahari. Our chronology is based on the

320

S. Stokes" et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 137 (1998) 305 322

application of optical dating to sand-sized quartz separates from the dune bodies. (2) At least four phases of dune activity are identified (ca. 95 115, 41 46, 20-26 and post-20 ka), the most recent of which is not recognised within the Victoria Fall dune field. These phases of aeolian deposition and aridity may be linked to changes in southeast Atlantic SST's, which may in turn exert a strong influence on late Quaternary global climate changes. A possible older period of dune construction which took place during the penultimate glacial (marine isotope stage 6) is identified from a single locality within the Victoria Falls dune field. Aeolian deposition during full interglacial conditions has not been identified (i.e., during Holocene or MIS 5e). (3) Vertical accretion of the dunes during these periods was rapid, and punctuated by extended periods of inactivity which we correlate with humid periods. There is limited stratigraphic evidence within the dunes to support these findings. The timing of these humid episodes is in agreement with previous studies in the Mega Kalahari that have frequently focused upon direct determination of the timing of wetter episodes in the Pleistocene. (4) The surficial dune cover in the Hwange National Park dune field is predominantly the result of aeolian activity which has occurred since the Last Glacial Maximum, those to the north in the Victoria Falls dune field are the result of aeolian deposition ca. 20 30 ka. The period spanning the Last Glacial Maximum is not specifically recognised as the time of maximum aridity and aeolian deposition. (5) Our data set is based on samples collected from only the upper portions of large linear dune forms. Further analysis is likely to identify additional earlier periods of dune building which could provide further tests on our inferred model of causes of changes in the regional moisture balance. (6) Linear dune ridges provide a useful and extended archive of aeolian activity which may be broadly correlated with regional aridity. This is particularly the case at desert marginal localities where phases of aridity-driven aeolian activity may be separated by extended periods of inactivity during more humid phases.

Acknowledgements Financial assistance for this project was provided by the Leakey Foundation, the Royal Society, and the Trapnell Fund for Environmental Research in Africa. B. Adder, L. Thomas, P. Ngwenya, T. Tshuma and J. Klimowicz are thanked for field assistance. Professor M.S. Tite (RLAHA, Oxford) and M.J. Aitken are thanked for advice and support. Mr. M. Higginson assisted in some of the laboratory analysis. An earlier version of this manuscript was substantially improved by the review comments of K. Pye and N.A. Lancaster.

References Aitken, M.J., 1985. Thermoluminescence Dating. Academic Press, London, 395 pp. Aitken, M.J., 1989. Luminescence dating: a guide for non-specialists. Archaeometry 31 (2), 147 159. Aitken, M.J., 1992. Optical dating. Quat. Sci. Rev. 11, 127 131. Aitken, M.J., Alldred, J.C., 1972. The assessment of error limits in thermoluminescent dating. Archaeometry 14 (2), 257- 267. Ash, J.E., Wasson, R.J., 1983. Vegetation and sand mobility in the Australian desert dunefield. Z. Geomorphol. Suppl, 45, 7 25. Bevington, P.R., 1969. Data Reduction and Error Analysis for the Physical Sciences. McGraw-Hill, San Francisco, 336 pp. Bond, G., 1948. The direction of origin of the Kalahari sand of Southern Rhodesia. Geol. Mag. 85 (5), 305 313. Bradley, R.S., 1985. Quaternary Palaeoclimatology. Chapman and Hall, London, 472 pp. Breed, C.S. et al., 1979. Regional studies of sand seas, using Landsat (ERTS) imagery. In: McKee, E.D. (Ed.), A Study of Global Sand Seas. U.S. Printing Office, Washington, pp. 305 397. Brumby, S., 1992. Regression analysis of ESR/TL dose response data. Nucl. Tracks Radiat. Meas. 20 (4), 595 599. Bullard, J.E., Thomas, D.S.G., Livingstone, I.~ Wiggs, G.F.S., 1995. Analysis of linear sand dune morphological variability, southwestern Kalahari Desert. Geomorphology 11, 189-203. Charles, C.D., Lynch-Steiglitz, J., Ninnemann, U.S., Fairbanks, R.G., 1996. Climate connections between the hemispheres revealed by deep-sea sediment core/ice core correlations. Earth Planet. Sci. Lett. 142, 19 27. Cooke, R.U., Warren, A., Goudie, A.S., 1993. Desert Geomorphology. UCL Press, London, 526 pp. Dawson, A.G., 1992. Ice Age Earth: Late Quaternary Geology and Climate. Routledge, London, 293 pp. De Menocal, P.B., Ruddiman, W.F., Pokras, E.M., 1993.

S. Stokes et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 137 (1998) 305 322 Influences of high-and low-latitude processes on African terrestrial climate: pleistocene eolian records from equatorial atlantic ocean drilling program site 663. Paleoceanography 8 (2), 209 242. Flint, R.F., Bond, G., 1968. Pleistocene sand ridges and pans in Western Rhodesia. Geol. Soc. Am. Bull. 79, 299-314. Fryberger, S.G., Dean, G., 1979. Dune forms and wind regime. In: McKee, E. (Ed.), A Study of Global Sand Seas. US Geol. Surv. Spec. Publ. 12l, 137-169. Gasse, F., 1994. Lacustrine diatoms for reconstructing past hydrology and climate. NATO ISI Ser. (Long-term Clim. Var.) 122, 335-369. Gasse, F., Tehet, R., Durand, A., Gibert, E., Fontes, J.-C., 1990. The arid humid transition in the Sahara and the Sahel during the last deglaciaton. Nature 346, 141-146. Grove, A.T., 1969. Landforms and climatic change in the Kalahari and Ngamiland. Geogr. J. 135 (2), 91-212. Heine, K., 1982. The main stages of the late Quaternary evolution of the Kalahari region, southern Africa. Palaeoecol. Aft. 15, 53 76. Huntley, D.J., Godfrey-Smith, D.I., Thewalt, M.L.W., 1985. Optical dating of sediments. Nature 313 (5998), 105 107. Kropelin, S., Soulie-Marsche, I., 1991. Charophyte remains from Wadi Howar as evidence for deep Mid-Holocene freshwater lakes in the eastern Sahara of northwest Sudan. Quat. Res. 36, 210 223. Lancaster, N., 1987. Formation and reactivation of dunes in the southwest Kalahari: palaeoclimatic implications. Palaeoecol. Afr. 18, 103 110. Lancaster, N., 1988. Development of linear dunes in the southwest Kalahari, southern Africa. J. Arid Environ. 14, 233-244. Lancaster, N., 1989. Late Quaternary palaeoenvironments of the southwestern Kalahari. Palaeogeogr., Palaeoclimatol., Palaeoecol. 70, 367-376. Lancaster, N., 1990. Palaeoclimatic evidence from sand seas. Palaeogeogr., Palaeoclimatol., Palaeoecol. 76, 279-290. Lancaster, N.A., 1995. Geomorphology of Desert Dunes. Routledge, London, 290 pp. Little, M.G. et al., 1997. Trade wind forcing of upwelling, seasonality, and Heinrich events as a response to sub-Milankovitch climate variability. Palaeoceanography (in press). Livingstone, I., Thomas, D.S.G., 1993. Mode of linear dune activity and their palaeoenvironmental significance: An evaluation with reference to southern African examples. In: Pye, K. (Ed.), The Dynamics and Environmental Context of Aeolian Sedimentary Systems. Geol. Soc. London, pp. 91 101. Parkin, D.W., Shackleton, N.J., 1973. Trade wind and temperature correlations down a deep-sea core off the Saharan coast. Nature 245, 251-253. Partridge, T.C. et al., 1990. Late Pleistocene and Holocene climatic change in Southern Africa. S. Afr. J. Sci. 86, 302 306. Petit-Maire, N., 1993. Past global climatic changes and the tropical arid/semi-arid belt in the North of Africa. In: Thorweihe, U. and Schandelmeier, H. (Eds.), Geoscientific Research in Northeast Africa. A.A Balkema, Rotterdam, pp. 551 560. Petit-Maire, N., Riser, J., 1981. Holocene Lake Deposits and

321

palaeoenvironments in Central Sahara, Northeastern Mali. Palaeogeogr., Palaeoclimatol., Palaeoecol. 35, 45-61. Prell, W.L., Van Campo, E., 1986. Coherent response of Arabian Sea upwelling and pollen transport to late Quaternary monsoonal winds. Nature 323, 526-529. Ritchie, J.C., Eyles, C.H., Vance Haynes, C., 1985. Sediment and pollen evidence for an early to mid-Holocene humid period in the eastern Sahara. Nature 314, 352 355. Sarnthein, M., 1978. Sand deserts during glacial maximum and climatic optima. Nature 272, 43-46. Sarnthein, M., Tetzlaff, G., Koopmann, B., Wolter, K., Pilaumann, U., 1981. Glacial and inter-glacial wind regimes over the eastern subtropical Atlantic and North-West Africa. Nature 293, 193 197. Shaw, P.A., Cooke, H.J., Thomas, D.S.G., 1988. Recent advances in the study of Quaternary landforms in Botswana. S. Afr. Soc. Quat. Res. 19, 15 26. Shaw, P.A., Thomas, D.S.G., 1996. The Quaternary Palaeoenvironmental history of the Kalahari, Southern Africa. J. Arid Environ. 32, 9-22. Smith, B.W., Rhodes, E.J., Stokes, S., Spooner, N.A., 1990. The optical dating of sediments using quartz. Radiat. Prot. Dosim. 34 (1/4), 75 78. Stokes, S., 1992. Optical dating of young (modern) sediments using quartz: results from a selection of depositional environments. Quat. Sci. Rev. 11, 153 159. Stokes, S., 1994. Optical dating of selected aeolian sediments from the southwestern United States. Ph.D. Thesis, Oxford Univ., Oxford, 660 pp. Stokes, S., Thomas, D.S.G., Shaw, P.A., 1997a. New chronological evidence for the nature and timing of linear dune development in the southwest Kalahari Desert. Geomorphology (in press). Stokes, S., Thomas, D.S.G., Washington, R., 1997b. Multiple episodes of aridity in southern Africa since the last interglacial period. Nature 388, 154-158. Street, F.A., Grove, A.T., 1976. Environmental and climatic implications of late Quaternary lake-level fluctuations in Africa. Nature 261,385 390. Street-Perrott, F.A., 1994. Palaeo-perspectives: Changes in terrestrial ecosystems. Ambio 23 (1), 37 43. Street-Perrott, F.A., Marchand, D.S., Roberts, N., Harrison, S.P., 1989. Global lake-level variations from 18,000 to 0 years ago: A palaeoclimatic analysis. Tropical Palaeoenvironments Research Group, pp. 19-31. Thomas, D.S.G., 1983. Ancient ergs of the former arid zones of Zimbabwe, Zambia and Angola. Trans. Inst. Br. Geogr. 9, 75-88. Thomas, D.S.G., 1984. Late Quaternary environmental change in central southern Africa with particular reference to extensions of the arid zone. Ph.D. Thesis, Oxford Univ., Oxford, 306 pp. Thomas, D.S.G., Shaw, P.A., 1991a. The Kalahari Environment. Cambridge Univ. Press, Cambridge, 284 pp. Thomas, D.S.G., Shaw, P.A., 1991b. 'Relict' desert dune systems: interpretations and problems. J. Arid Environ. 20, 1 14.

322

S. Stokes et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 137 (1998) 305 322

Thomas, D.S.G., Stokes, S., O'Connor, P.W., t997. Late Quaternary aridity in the southwestern Kalahari Desert: New contributions from OSL dating of aeolian deposits: northern Cape Province, South Africa. In: Glennie, K.W., A1 Ashran, M.A. (Eds.), Geology of Quaternary Desert Margins. Balkema, Rotterdam. Van Campo, E., Gasse, F., 1993. Pollen- and diatom-inferred climatic and hydrological changes in Sumxi Co Basin ( Western Tibet) since 13,000 yr B.P.. Quat. Res. 39, 300-313. Van Zinderen Bakker, E.M., 1982. African palaeoenvironments 18000 years BP. Palaeoecol. Afr. 15, 77 99.

Warren, A., 1970. Dune trends and their implications in the Central Sudan. Z. Geomorphol. 10, 154-180. Wiggs, G.F.S., Thomas, D.S.G., Bullard, J.E., Livingstone, 1., 1995. Dune mobility and vegetation cover in the southwest Kalahari desert. Earth Surf Processes Landforms 20, 515 529. Williams, M.A.J., Dunkerley, D.L., De Deckker, P., Kershaw, A.P., Stokes, T., 1993. Quaternary Environments. Edward Arnold, London, 330 pp.