Holocene environmental change in the Okavango Panhandle, northwest Botswana

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Quaternary Science Reviews 25 (2006) 1302–1322

Holocene environmental change in the Okavango Panhandle, northwest Botswana David J. Nasha,, Michael E. Meadowsb, Vana L. Gullivera a

School of the Environment, University of Brighton, Lewes Road, Brighton BN2 4GJ, UK Department of Environmental and Geographical Science, University of Cape Town, Rondebosch 7701, South Africa

b

Received 6 March 2005; accepted 12 November 2005

Abstract This paper documents the first Holocene palaeoecological record for the Okavango Delta, northwest Botswana. Sedimentological, stable carbon isotope and palynological data, supported by conventional and AMS radiocarbon assays, are presented from coring sites at Gauxa Lagoon and the Ncamasere and Tamacha valleys along the western margin of the Okavango Panhandle. Earliest Holocene vegetation patterns are not readily distinguished at the three sites, owing to poor pollen preservation conditions. A wet phase or period of enhanced Okavango flooding is tentatively identified around 9000 BP on the basis of increased accumulation of organic matter within floodplain sediments. Palynological and sedimentological data, combined with stable carbon isotope analyses, suggest that relatively dry conditions extended from 7000 to 4000 BP (punctuated by a wet phase at around 6000 BP). This is interpreted as indicating reduced rainfall over the Okavango headwaters in Angola. Conditions from around 4000 BP became progressively wetter, initially in response to increased water throughputs via the Okavango system. Wettest conditions occurred from 2300 to 1000 BP due to a combination of increased regional rainfall and raised Okavango flood levels. Conditions approach those of the present day after this time. A major shift from grass- to sedge-dominated vegetation communities, apparent at all three sites in the past thousand years, is attributed to anthropogenic disturbance. These changes are subsequently discussed in light of regional continental and marine palaeoenvironmental records, and the implications for the future management of the Okavango River considered. r 2005 Elsevier Ltd. All rights reserved.

1. Introduction The dryland climate and dominance of sand-sized sediments across much of central southern Africa provides relatively few sites suitable for the reconstruction of vegetation histories, mainly because pollen preservation conditions are poor. As a result, our understanding of Late Quaternary environmental changes in regions such as the Kalahari Desert (Fig. 1) has had to rely heavily upon geomorphological and sedimentological, as opposed to palaeoecological, evidence (Thomas and Shaw, 1996). Attempts to reconstruct former vegetation communities have been based upon less conventional methods such as the analysis of pollen incorporated into archaeological deposits, coprolites or within cave sediments (Scott, 1987, 2000; Burney et al., 1994). To date, with the exception of Corresponding author. Tel.: +44 1273 642423; fax: +44 1273 642285.

E-mail address: [email protected] (D.J. Nash). 0277-3791/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2005.11.004

work on pollen extracted from a single speleothem at Drotsky’s Cave in Botswana (Burney et al., 1994) and from sediments within a submerged sinkhole at Lake Otjikoto in Namibia (Scott et al., 1991), all detailed palynological investigations have been restricted to sites around the southern rim of the Kalahari in South Africa (e.g. Beaumont et al., 1984; Butzer, 1984a, b; Scott, 1987). Wetland areas marginal to the Okavango Delta in northwest Botswana, however, offer the potential to redress this spatial and methodological imbalance, and provide an insight into Quaternary vegetation changes in the Middle Kalahari. The aim of this paper is to present the results of coring investigations undertaken at three sites along the western margin of the Okavango ‘Panhandle’ (Fig. 2). Palynological, sedimentological and stable carbon isotope data, supported by calibrated conventional and AMS radiocarbon dates, are utilised to construct the first continuous record of Holocene vegetation change for the region. Preliminary sedimentological and geomorphological

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Fig. 1. Key locations in the Kalahari region for which Holocene palaeoenvironmental data are available (dune and drainage distribution after Thomas and Shaw, 2002).

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22°E

23°E

24°E

Mohembo

ha

n Pa 0

40

le

nd

GAUXA

Fa ul t

km

n va ka go

Sepupa

G

O

om ar e

NCAMASERE

TAMACHA 19°S

19°S

Nqoga

o Ja

Mbo

Maunachira

lt

roga e

la

G om ot i

Th

an nt

am

Sa

ro

al

Bo

ak

an

e

nd

lt

Is

Ku ny er

f’s

Fa u

ie

Fa u

Ch

Gomare

ibe

ad

Th ao

Maun

Th

am

al

ak

an

e

ge

Coring sites 20°S

20°S

Locations mentioned in text Bote ti

Permanent swamp

e ab Nh

Seasonal marshes Perennial and seasonal river

Lake Ngami

Selected geological faults 22°E

23°E

24°E

Fig. 2. Location of the Gauxa, Ncamasere and Tamacha sampling sites within the wider context of the Okavango Delta region (distribution of swamps, major channels and faults after McCarthy and Ellery, 1998).

analyses have already been provided for one of the coring sites, the Ncamasere valley (Nash et al., 1997), but this study extends the palaeoenvironmental investigations to new sites at Gauxa and Tamacha and includes the first palynological data for all three locations. Following the presentation of data for the three sites, the results are interpreted in the light of our understanding of Holocene environmental changes in central southern Africa, as recorded in continental and marine records. 2. The environment of the Okavango Panhandle 2.1. Morphology, hydrology and sediment supply At over 40,000 km2, the Okavango Delta (Fig. 2) is one of the largest inland delta-fans in the world (McCarthy, 1993; Gumbricht et al., 2004, 2005). It supports the most extensive permanent wetland in southern Africa

(McCarthy and Ellery, 1998) and is one of the richest botanical areas in the subcontinent (SMEC, 1989). The Delta consists of two distinct sections. At its upstream end is the 90 km long by 15 km wide ‘Panhandle’, occupied by the Okavango River. Much of the Panhandle consists of permanent swamps, with seasonal marshes fringing the drier floodplain margins and at locations where ‘fossil’ drainage lines intersect the floodplain (Nash et al., 1994, 1997; Nash, 1996). At the southern end of the Panhandle, the Okavango River divides into three distributaries, the Nqoga, Jao-Boro and Thaoge, to form a delta-fan. This extends for 160 km from its apex to its distal outflows, the Boteti, Thamalakane and Nhabe rivers (McCarthy et al., 1997). Areas of permanent swamp occur close to the main distributaries, with other regions occupied by seasonal marshes. The Delta as a whole is situated within a subsiding half-graben at the southern end of the East African Rift system (McCarthy et al.,

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1993), with the margins of the Panhandle also controlled by perpendicular conjugate fault sets (McCarthy et al., 1998, 2002). The delta-fan receives its water from two sources. On average, 6140 million m3 yr
1 is supplied by local precipitation falling directly onto the fan surface during the austral summer months (McCarthy et al., 1998). However, the majority of water (9200 million m3 yr
1; McCarthy et al., 1998) is supplied by the Cubango and Cuito tributaries which rise in the highlands of central Angola. Rainfall in the Cubango and Cuito catchments typically peaks in the late summer (January–March; Wilson and Dincer, 1976), generating a flood pulse (Junk et al., 1989) which usually reaches Mohembo at the northern tip of the Panhandle in April (McCarthy et al., 2003). The flood wave passes through the delta-fan over a period of 4–5 months, with maximum flooding of the seasonal swamps occurring in July or August. High rates of potential evapotranspiration (ca 1400 mm p.a.; McCarthy et al., 1998) result in only 1.5% of the total input to the delta-fan leaving via the distal Boteti river, with less than 2% estimated to be lost to groundwater outflow (McCarthy and Ellery, 1998). 2.2. Vegetation communities The distribution of vegetation communities within the Okavango Delta is controlled by two factors. First is the seasonal variability of water supply, determined by climatic conditions and the depth and duration of annual flooding. Potential evapotranspiration exceeds annual precipitation (ca 490 mm p.a.; McCarthy et al., 2000) by a factor of

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three, leading to a high water deficit (McCarthy and Ellery, 1998). This is offset in permanent swamp areas by fluctuations in water level during the annual flood; levels in the Panhandle rise by up to 2 m at the flood peak (Sefe, 1996; McCarthy and Ellery, 1998). The amplitude of elevation of flooding decreases down the Panhandle and into the delta-fan, where fluctuations of less than 15 cm occur (Wilson and Dincer, 1976). The second factor influencing delta-fan ecology is nutrient availability. The Okavango River rises in, and flows through, areas dominated by quartz-rich sandstones, mudstones and Kalahari Group sediments (Andersson et al., 2003). Limited chemical weathering occurs, resulting in low solute loads (McCarthy and Metcalfe, 1990). Suspended load is also low, with most clastic sediment consisting of fine quartz sand transported as bedload (McCarthy et al., 1991). Okavango water is extremely nutrient poor (Cronberg et al., 1995) and the Okavango ecosystem as a whole is characterised by low nutrient concentrations. Fall-out and redistribution of aerosols may, however, contribute significantly to overall nutrient budgets (Garstang et al., 1998; Krah et al., 2004). Vegetation communities within the Panhandle can be divided into four groups (Table 1): permanent swamp, seasonal marshes, riverine forest and savanna woodland (SMEC, 1989; Thomas and Shaw, 1991). In the permanent swamp, channels are flanked by tall grass and sedge communities (Ellery et al., 2003; Mladenov et al., 2005), with open water areas dominated by floating-leaved and submerged species. Semi-aquatic trees and emergent grasses, together with sedges, rushes and allied plants

Table 1 Vegetation communities in the Okavango Panhandle (species information from Weare and Yalala, 1971; Smith, 1976; SMEC, 1989; Thomas and Shaw, 1991; Ellery and Ellery, 1997; McCarthy and Ellery, 1998; Roodt, 1998; Ellery et al., 2003) Major vegetation communities Permanent swamp

Environmental characteristics and dominant species

 Open water areas—floating-leaved (Brasenia schrebrei, Nymphae spp., Nymphoides indica) and submerged (Ceratophyllum demersum, Lagarosiphon ilicifolius, Najas pectinata) plants, with spike-rushes (Eleocharis spp.)

 Channel-margins—tall grasses (Pennisetum glaucocladum, Phragmites mauritianus) and sedges (Cyperus papyrus), with semiaquatic trees (Ficus verruculosa, Phoenix reclinata)

 Emergent zones—grasses, sedges (Pycreus nitidus), rushes and allied plants Seasonal marshes

 Lower elevation areas—floating-leaved and submerged aquatic species similar to permanent swamp  Primary floodplain—rushes and sedges (Cyperus articulatus, Schoenoplectus corymbosus, Scirpus inclinatus)  Secondary floodplain—grasses (Chloris gayana, Imperata cylindrical, Panicum repens, Setaria anceps, Sorghastrum friesii)

Riverine forest

 River and island margins—dense, evergreen woodland (Diospyros mespiliformis, Ficus sycamorus, F. natalensis, F. thonningi, Garcinia livingstonii) giving way laterally to deciduous woodland (Acacia nigrescens, A. nilotica, Combretum hereroense, C. imberbe, Croton megalobotrys, Hyphaenae ventricosa, H. petersiana, Kigalia africana, Lonchocarpus capassa)

Savanna woodland

 Larger islands and Kalahari sandveld tongues—open deciduous woodland (Acacia erioloba, A. giraffe, Combretum hereroense, Lonchocarpus capassa, Terminalia sericea) with understory grasses

 Areas surrounding the Panhandle—Ngamiland Tree Savanna and Northern Kalahari Tree and Bush Savanna, including grassland, bush, scrub and woodland (Acacia spp., Burkea africana, Combretum zeyheri, Croton spp., Grewia spp., Lonchocarpus nelsii)

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(hereafter referred to as sedges), also occur (Ellery and Ellery, 1997). Within the seasonal marshes, low-lying areas are dominated by aquatic species similar to those in the permanent swamp. Marginal floodplains also occur and can be divided into primary (inundated for a few months; sedge-dominated) and secondary (inundated only during large floods; grass-dominated) types (Smith, 1976; Ellery and Ellery, 1997). Riverine forests occur along most river and island edges, and consist of an evergreen woodland zone in which grasses are generally absent that gives way to deciduous woodland. Larger islands and Kalahari sandveld tongues support shrubland and woodland savanna vegetation, usually as open deciduous woodland with grasses in the understory (Ellery and Ellery, 1997; Roodt, 1998). In the area surrounding the Panhandle, these communities merge with savanna grasslands, scrub and woodland (Weare and Yalala, 1971).

3. Coring sites and methodology 3.1. Characteristics of coring sites Three areas along the western margin of the Okavango Panhandle were selected for investigation: Gauxa Lagoon and the Ncamasere and Tamacha valleys (Fig. 2). The Gauxa Lagoon site (Fig. 3a) was situated at the edge of a water-filled embayment at the terminus of an indistinct dry valley. The site is inundated annually and consists of seasonal swamps and primary floodplain. The Ncamasere and Tamacha sites were situated within the backflooded sections of two dry valleys which intersect the Okavango floodplain. The Ncamasere (Fig. 3b; Fig. 4) is the more extensive of the two systems, rising in Namibia and extending 150 km through vegetated dunefield. The valley is also backflooded to a greater distance, with waterlogged conditions occurring along 7 km of valley floor during the annual flood (Nash et al., 1997). The backflooded section grades up-valley from perennial swamp, to seasonal swamp, to primary and secondary floodplain. The Tamacha valley (Fig. 3c) rises close to the Tsodilo Hills and extends for 50 km through vegetated dunefield. The distal end of the valley does not flood to the same extent as at Ncamasere, but instead contains a string of pans and waterlogged valley sediments fed by sub-surface water seepage from the Okavango. Much of the lower valley is occupied by primary and secondary floodplain. Neither the Ncamasere or Tamacha systems have been known to flood during the historical period (Nash and Endfield, 2002). However, the upper Ncamasere contained sufficient water in the late Holocene to allow the development of peat deposits (Brook, 1995), and the neighbouring Xaudum contained standing water during the Late Quaternary (Shaw et al., 1992; Nash et al., 1994). This suggests that water from both systems may have flowed into the Okavango in the past.

Fig. 3. (A) The Okavango River immediately adjacent to coring site GL2 at Gauxa Lagoon; (B) view along the Ncamasere Valley, looking east from coring site NV2; (C) coring site TL2 at Tamacha.

3.2. Methodology In total, 15 cores were extracted using a vibracorer (Lanesky et al., 1979), two each at the Gauxa and Tamacha sites, seven in the Ncamasere Valley and a further four in a small pan immediately south of, and adjoining, the Ncamasere Valley (Table 2; Fig. 4). All sediment cores remained sealed in the field and were split and logged at the University of Cape Town, where major sediment colour and texture characteristics were recorded (Fig. 5). Sixteen

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Fig. 4. Aerial photograph showing the locations of coring sites NL1-4 and NV1-7 within the Ncamasere Valley.

Table 2 Locations of coring sites with details of pollen analysis and chronological control Location

Core

Latitude

Gauxa Lagoon Gauxa Lagoon Ncamasere Valley Ncamasere Valley Ncamasere Valley Ncamasere Valley Ncamasere Valley Ncamasere Valley Ncamasere Valley Ncamasere Valley Ncamasere Valley Ncamasere Valley Ncamasere Valley Tamacha Valley Tamacha Valley

GL1 GL2 NL1 NL2 NL3 NL4 NV1 NV2 NV3 NV4 NV5 NV6 NV7 TL1 TL2

181 181 181 181 181 181 181 181 181 181 181 181 181 181 181

230 230 360 360 360 360 350 350 350 350 360 350 350 500 510

4600 4600 1300 1400 1700 1500 2300 4900 5800 5900 0000 5600 4800 4500 0400

S S S S S S S S S S S S S S S

relatively organic-rich samples from ten of the cores were selected for radiocarbon age determination, four by conventional 14C dating at the CSIR Radiocarbon Laboratory at Pretoria (lab code Pta-, previously reported in Nash et al., 1997) and the remainder by AMS dating at the University of Arizona NSF-AMS Facility (lab code AA-) under NERC 14C Dating Allocation 732/0498. Reported ages are shown in Table 3 and expressed at the 71s level for overall analytical confidence. To allow comparison with recent luminescence and U/Th chronologies for the Kalahari (Thomas and Shaw, 2002), radiocarbon ages were calibrated using the CALIB REV 4.4.2

0211 0211 0221 0221 0221 0221 0221 0221 0221 0221 0221 0221 0221 0221 0221

510 510 010 010 010 010 000 010 010 010 020 030 040 170 160

1800 1800 3500 3200 3400 3200 4800 1100 4200 1700 5300 5600 0600 1300 4900

14

Pollen analysis

Y Y Y Y Y Y N N N N Y N Y Y Y

N Y N Y Y Y N N N N N N N N Y

C dating

Longitude E E E E E E E E E E E E E E E

programme (Stuiver and Reimer, 1993) run with the southern hemisphere calibration data set (shcal02.14c; McCormack et al., 2002). Calibrated ages are shown in Table 3, rounded to the nearest 10 years and expressed at the 71s level. Stable carbon isotope determinations were obtained as a by-product of the radiocarbon age assays, with additional samples subject to mass spectrometry employing standard procedures. Stable isotope ratios are reported in values per mil in the d notation relative to the PDB standard: d13C (%) ¼ (Rs/Rref
1)  1000, where R¼13 C=12 C. Stable carbon isotope data for cores NL2, GL2 and TL2 are shown

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1308 NV1

NV3

NV2

NV5

NV4

NV6

NV7

Compressed depth (cm)

0

0

Modern (Pta-6728)

100

100

900-980 (Pta-6724)

200

200

300

0

300

NL1

NL3

NL2 Modern (Pta-6719)

GL1

NL4 Modern (AA-30930)

GL2

TL1

TL2

0

Modern (AA-30931) 980-1030 (AA-30932)

1510-1630 (AA-30936)

900-960 (AA-30933)

2300-2350 (AA-30937)

100

100

Compressed depth (cm)

2150-2210 (AA-30927)

200

1810-1950 (AA-30934)

2310-2380 (Pta-6722)

300

Modern (AA-30935)

5990-6150 (AA-30928)

9260-9480 (AA-30929)

400

2310-2380 (AA-30938)

200

300

Organic-rich turf layer

Clay with organic material

Undifferentiated organic layer

No organic material

Organic mottles

Occasional organic mottles

400

Calibrated radiocarbon age range (cal yrs BP) 500

500

Fig. 5. Logs of all sediment cores, including variations in clay and organic matter content and calibrated radiocarbon ages.

Table 3 Calibrated conventional (laboratory code Pta-) and AMS radiocarbon (laboratory code AA-) assays from Ncamasere, Gauxa and Tamacha Site

Sample code

Sample depth (cm)

Uncalibrated (yr BP)

Ncamasere Ncamasere Ncamasere Ncamasere Ncamasere Ncamasere Ncamasere Ncamasere Ncamasere Gauxa Gauxa Gauxa Gauxa Tamacha Tamacha Tamacha

NL1-10 NL1-189 NV5-187 NV7-15 NL2-143 NL2-313 NL2-383 NL3-43 NL4-38 GL1-83 GL2-78 GL2-158 GL2-205 TL1-78 TL2-93 TL2-210

5–15 189–197 187–207 12–19 143–147 313–317 383–387 43–47 38–42 83–87 78–82 158–162 205–209 78–82 93–98 210–213

11170.6 33307150 1070770 10370.6 2250750 5365760 83907100 10470.62 10170.64 1200750 1065745 1965775 10770.78 1695775 2345750 2380750

14

C age

Laboratory code

Calibrated age (cal yr)

Pta-6719 Pta-6722 Pta-6724 Pta-6728 AA-30927 AA-30928 AA-30929 AA-30930 AA-30931 AA-30932 AA-30933 AA-30934 AA-30935 AA-30936 AA-30937 AA-30938

Modern 2310–2380 900–980 Modern 2150–2210 5990–6150 9260–9480 Modern Modern 980–1030 900–960 1810–1950 Modern 1510–1630 2300–2350 2310–2380

All ages are calibrated using the CALIB REV 4.4.2 programme (Stuiver and Reimer, 1993) run with the shcal02.14c calibration dataset (McCormack et al., 2002) and reported to 71s level.

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in Fig. 6. Organic carbon content was determined for selected samples using the loss on ignition method, with particle size analyses for the same samples undertaken using a settling column. Sediment data for cores NL2, GL2 and TL2 are shown in Tables 4–6. Pollen preserved in sediments from cores NL2, NL3, NL4, GL2 and TL2 was concentrated using standard techniques (Faegri and Iversen, 1989) involving the removal of extraneous particulate and dissolved organic matter via both physical

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(sieving) and chemical (NaOH and acetolysis) means. Inorganic particulates were removed through a combination of sieving and HF digestion. Pollen counts were made on the basis of relative concentrations using a Zeiss microscope at magnifications of  400 or  630. Pollen data were plotted using the PSIMPOLL 4.10 programme, with zonation undertaken using the binary splitting by sum-of-squares technique (Bennett, 2002). Pollen data are shown in Figs. 7–11. 4. Results 4.1. Chronology

Fig. 6. Stable carbon isotope ratios for cores NL2, GL2 and TL2. Calibrated radiocarbon ages: Core NL2, a ¼ 926029480 cal yr BP (AA-30929), b ¼ 599026150 cal yr BP (AA-30928), c ¼ 215022210 cal yr BP (AA-30927); Core GL2, a ¼ Modern (AA-30935), b ¼ 181021950 cal yr BP (AA-30934), c ¼ 9002960 cal yr BP (AA-30933); Core TL2, a ¼ 231022380 cal yr BP (AA-30938), b ¼ 230022350 cal yr BP (AA-30937). Stable carbon isotope determinations were conducted at the Archaeometry Laboratory, University of Cape Town on a VG 602E mass spectrometer. All values were consistently calibrated against standards to ensure adequate combustion and are recorded relative to Vienna PDB.

Based on the sixteen calibrated radiocarbon ages, the sediments in the Panhandle wetlands mostly accumulated during the late Holocene (Table 3; Fig. 5). The exception is the 500 cm core NL2 from Ncamasere, where near-basal sediments may date to the end-Pleistocene or early Holocene. The lack of older sediments is mainly a product of vibracorer penetration depth. The radiocarbon dates are in the correct chronostratigraphic sequence, with the exception of the lowest date for core GL2 which yields an anomalously modern age. The basal part of core TL2 also yields two similar ages (ca 2300 BP) for sediments deposited 100 cm apart, suggesting either rapid sedimentation or, more probably, sample contamination. The calibrated ages are clustered. Five samples, including many of the more organic near-surface facies, yield modern ages, three samples are dated to 900–1000 BP, two to between 1510 and 1950 BP and four to 2100–2400 BP. There are also outliers, both in core NL2, of around 6000 BP and 9300 BP. On the basis that all radiocarbon age determinations were conducted on more organic facies that accumulated under waterlogged conditions, it is possible

Table 4 Sedimentary analyses for Ncamasere Valley core NL2 (OM—organic matter) Depth (cm)

10 20 40 80 130 160 200 235 260 285 300 325 350 375 420 470

% OM

3.7 0.77 1.02 1.39 0.29 0.43 0.24 0.23 0.4 0.23 0.1 0.27 0.13 0.12 0.33 0.21

% Silt+Clay

8.30 2.23 7.98 6.61 4.71 4.57 5.76 3.77 0.60 1.77 0.90 1.73 2.87 5.88 6.67 9.79

% Sand

80 97 91 92 95 95 94 96 99 98 99 98 97 94 93 90

Sand fraction % Coarse

% Medium

% Fine

9.98 4.20 5.00 5.13 7.35 3.76 5.35 6.87 9.75 3.53 4.48 4.99 13.76 5.37 9.72 6.19

68.57 59.18 55.27 61.21 61.88 50.28 54.96 58.45 74.80 71.82 76.38 78.77 78.32 75.95 53.81 58.69

21.45 36.62 39.73 33.66 30.77 45.96 39.69 34.68 15.45 24.65 19.14 16.24 7.92 18.68 36.47 35.12

Sorting

Skewness

0.54 0.48 0.55 0.50 0.55 0.56 0.59 0.52 0.45 0.41 0.39 0.37 0.42 0.40 0.65 0.54

0.14 0.04 0.04 0.03 0.06 0.07 0.10 0.02 0.04 0.05 0.01
0.02
0.13 0.01 0.05 0.02

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Table 5 Sedimentary analyses for Gauxa core GL2 (OM—organic matter) Depth (cm)

10 25 40 45 50 60 80 95 110 140 155 175 195

% OM

18.30 2.64 0.85 0.33 0.16 0.10 0.09 0.12 0.13 0.27 0.12 0.19 0.25

% Silt+Clay

17.70 10.36 4.15 3.67 4.84 0.90 2.91 1.88 0.87 1.73 2.88 2.81 3.75

% Sand

50 87 95 96 95 99 97 98 99 98 97 97 96

Sand fraction % Coarse

% Medium

% Fine

7.21 25.26 13.58 8.32 5.62 8.66 8.12 7.16 6.50 6.59 6.63 7.23 6.23

50.67 65.68 68.59 63.07 55.19 66.77 54.02 53.84 56.39 46.64 50.88 46.05 43.98

42.12 9.06 17.83 28.61 39.19 24.57 37.86 39.00 37.11 46.77 42.49 46.72 49.79

Sorting

Skewness

0.67 0.53 0.50 0.52 0.53 0.49 0.61 0.58 0.55 0.59 0.55 0.61 0.60

0.09
0.05
0.03
0.01
0.02
0.05 0.01
0.01
0.01
0.05
0.07
0.06
0.05

Sorting

Skewness

0.57 0.56 0.56 0.56 0.59 0.57 0.61 0.59 0.57 0.61 0.62 0.60


0.01
0.02 0.02 0.06
0.01 0.03
0.01 0.03 0.02 0.02
0.03
0.02

Table 6 Sedimentary analyses for Tamacha core TL2 (OM—organic matter) Depth (cm)

10 25 40 60 70 80 100 115 130 150 170 210

% OM

0.92 0.56 0.61 0.24 0.41 1.68 1.06 0.69 1.45 1.27 0.92 0.12

% Silt+Clay

2.08 2.44 2.39 3.76 8.59 8.32 5.94 9.31 5.55 9.73 13.08 14.88

% Sand

97 97 97 96 91 90 93 90 93 89 86 85

Sand fraction % Coarse

% Medium

% Fine

9.91 10.41 5.99 6.95 10.33 9.37 7.08 7.48 6.07 13.26 11.38 18.20

59.30 60.10 55.29 59.18 57.43 60.20 49.61 54.82 53.57 60.37 54.95 61.92

30.79 29.49 38.72 33.87 32.24 30.43 43.31 37.70 40.36 26.37 33.67 19.88

that these age clusters represent periods of enhanced moisture availability during which biological productivity was greater and/or there was a combination of reduced microbial action and fewer or cooler dry season fires. If this assumption is valid, this would indicate wetter conditions in the Panhandle region around 1000 BP, between 1500 and 1900 BP and again from 2100 to 2400 BP. There may have been earlier periods of greater moisture availability at around 6000 BP and 9300 BP, but these early phases are more tenuous. 4.2. Stable carbon isotope stratigraphy Fig. 6 reveals that there is little variation in the stable carbon isotope values within cores, with most samples yielding d13C values between
16.0 and
24.0 per mil. This result is consistent with the carbon being derived from a mixture of C3 (more negative, dominated by trees and shrubs in savanna environments; Lee-Thorp and Beaumont, 1995; Scott, 2002) and C4 (more positive) plant

species, similar to the situation today (Ellery and Ellery, 1997). Nevertheless, some trends in the data can be detected. In general, surface and near-surface sediments have more positive d13C values than sediments at depth. This may indicate a greater prominence of C4 organics in recent sediments, as most grasses and many of the dominant sedges in the Panhandle are C4 species (Ellery et al., 1993). In other parts of all cores, more organic facies are associated with more negative (i.e. more C3) d13C values. There are two notable outliers in core NL2, where sediment from 60 cm depth (approximately 800 BP) yields a strongly negative excursion (
35 per mil) and there is also a departure at 310 cm (ca 6200 BP,
26 per mil). The first of these values is likely to be an anomaly, since it exceeds the typical C3 plant signature, although there are negative excursions at similar depths and ages in cores GL2 and TL2. The older negative departure is consistent with 100% of the organic carbon being derived from C3 plants. Sediments at Tamacha are associated with more negative

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Fig. 7. Pollen recovery for cores NL2, GL2 and TL2 as measured by the numbers of pollen grains counted in five microscope traverses at  400 magnification.

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suggesting that the material at all three sites is derived from a combination of down-valley, local and Okavango flood-derived sources. Few trends can be identified within the data, other than in core NL2 where the percentage silt and clay is at its lowest between around 350 and 250 cm depth (approximately 7500–4500 BP). This may represent decreased inputs from Okavango floodwaters over this period. Organic content (Tables 4–6) is low and, apart from at the surface, loss on ignition rarely exceeds 1% by weight. This, taken in tandem with the radiocarbon age determinations, supports the contention that the absence of perennial water for much of the Holocene prevented the development of the thick peat layers that occur in the Okavango delta-fan (Nash et al., 1997). This points to rapid and efficient organic matter recycling (Mladenov et al., 2005) under otherwise oligotrophic conditions. Limited dating control makes discussion about rates of sedimentation tentative, and the possibility exists that sedimentation is spatially and temporally discontinuous. Organic sediments (including those subject to radiocarbon dating) do not appear to have developed continuously at the three sites, and the accumulation and preservation of sediments with greater organic content appears to have been intermittent throughout the Holocene. Nevertheless, sedimentation rates between these organic phases vary little within or between cores. On the assumption that inorganic sediments accumulate uniformly between the more organic facies, the calibrated radiocarbon ages can be used to interpolate minimum rates of sedimentation. These suggest rates for core NL2 of 0.2 mm yr
1 from the base to around 6000 BP, increasing to 0.4 mm yr
1 from 6000–2200 BP and to 0.7 mm yr
1 from 2200 BP onwards. The latter rate compares well with sediment accumulation of 0.8 mm yr
1 from 2300 BP onwards in the adjacent core NL1. In GL2, sedimentation rates of 0.9 mm yr
1 occur from the basal core until 900–960 BP, decreasing to 0.6 mm yr
1 to the present day, whilst core TL2 maintains a rate of 0.4 mm yr
1 throughout the record. Accumulation rates for the main Ncamasere Valley are higher, exceeding 2.0 mm yr
1 over much of core NV5. 4.4. Pollen analysis

d13C anomalies throughout the core (
20 to
23 per mil compared with
18 to
20 per mil elsewhere), although the reasons are unclear. 4.3. Particle size and organic matter determinations The sediments in all fifteen cores are typical of those of the surficial Kalahari, being dominated by medium sand. Data for only three cores are presented here (Tables 4–6) but these are considered representative. In comparison with the surrounding sandveld, the cored sediments contain more fines, including layers where silts and clays approach 10% and where fine sands approach 50 wt%. Sediments are less well sorted than the surrounding surface sands,

Pollen preservation, as might be expected in a semi-arid environment with strongly seasonal flooding, is not good, and pollen frequencies are at best modest. Relatively large masses of sample were required to yield sufficient pollen grains to facilitate counting. Fig. 7 illustrates pollen recovery for cores NL2, GL2 and TL2 as measured by the numbers of pollen grains counted in five microscope traverses at  400 magnification. This suggests that pollen preservation is compromised in samples below approximately 1.25 m depth (corresponding to around 2000 BP in core NL2). Nonetheless, sufficient pollen was recovered from the cores to warrant description of changing frequencies over time.

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Fig. 8. Percentage pollen diagram for Ncamasere Valley core NL2. Calibrated radiocarbon ages: (a) 9260–9480 cal yr BP (AA-30929); (b) 5990–6150 cal yr BP (AA-30928); (c) 2150–2210 cal yr BP (AA-30927).

4.4.1. Core NL2 The sediments in this 5 m core (Fig. 8) represent much of the Holocene and may well extend back to 14,000 BP, although there are no age determinations below the approximately 9300 BP date at around 380 cm. The sequence is dominated by Poaceae and Cyperaceae pollen which, at every level, constitute at least 75% of the pollen spectra, and, in the lower half of the core, often make up more than 90%. The core can be divided into five pollen zones: Zone O-1: 495–380 cm, accumulating until around 9300 BP. The zone is characterised by very high frequencies of grass and sedge pollen, and dominated in particular by Poaceae which achieve percentages in excess of 80% at the base. There are few other pollen types present, other than monolete spores from unidentified fern(s). Zone O-2: 380–340 cm, 9300 BP to approximately 7000 BP. There is a sharp decline in Poaceae pollen associated with a peak in the monolete spores and the first signs of both Asteraceae and Chenopodiaceae pollen, including a marked peak in Chenopodiaceae towards the top of the zone coupled with a pronounced decline in Poaceae pollen. Zone O-3: 340–60 cm, approx. 7000 BP to around 1000 BP. High grass pollen percentages characterise this zone, which peak at around 7000 BP but gradually decline after 2000 BP. Sedge pollen appears consistently at

moderate percentages, fluctuating around 20%, although there is a peak at 6000 BP which coincides with a potential period of increased moisture availability. The remainder of the spectrum, less than 30% in all, is constituted by monolete fern spores and small, but consistent percentages of Aizoaceae, Asteraceae and Chenopodiaceae, the latter being prevalent until around 4500 BP. Fabaceae and, to a lesser extent, Euphorbiaceae pollen also appear from around 4000 and 2200 BP, respectively. Zone O-4: 60–20 cm, 1000 BP to around 150 BP. Grass pollen percentages decline from the start of this zone to around 60% at 20 cm depth. Several other pollen types achieve their highest percentages in this zone, for example, Aizoaceae at around 40 cm, Asteraceae, Malvaceae and Fabaceae at 60 cm, Acanthaceae at around 50 cm, and Combretaceae at 25 cm. Zone O-5: 20–0 cm, representing conditions prevailing for approximately the past 150 years. There is a sharp decline in Poaceae and corresponding increase in sedge pollen, with few other pollen types except for Aizoaceae, Asteraceae, Combretaceae and Fabaceae present. 4.4.2. Cores NL3 and NL4 Cores NL3 and NL4 are both approximately 1 m length (Fig. 9), and represent the immediate historical record.

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Fig. 9. Percentage pollen diagram for Ncamasere Valley cores NL3 (top) and NL4 (bottom). Calibrated radiocarbon ages: (a) Modern (AA-30930); (b) Modern (AA-30931).

Fig. 10. Percentage pollen diagram for Gauxa Lagoon core GL2. Calibrated radiocarbon ages: (a) Modern (AA-30935); (b) 1810–1950 cal yr BP (AA-30934); (c) 900–960 cal yr BP (AA-30933).

Both sequences are consistent with the spectra from the upper part of adjacent core NL2 in demonstrating a steep decline in grass pollen mirrored by an increase in sedge

pollen towards the top of the cores. In core NL3, there are measurable percentages of Aizoaceae, declining from a minor peak at 60 cm immediately prior to the change from

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Fig. 11. Percentage pollen diagram for Tamacha Valley core TL2. Calibrated radiocarbon ages: (a) 2310–2380 cal yr BP (AA-30938); (b) 2300– 2350 cal yr BP (AA-30937).

grass- to sedge-dominance. Core NL4 also contains evidence of peaks in Aizoaceae, Combretaceae and Fabaceae shortly before the shift from Poaceae to Cyperaceae dominance occurs. 4.4.3. Core GL2 The core, extending to just over 2 m depth, has sediments laid down over approximately the past 2000 years. Two pollen zones are discernible (Fig. 10): Zone G-1: 200–60 cm, approximately 2200–900 BP. The zone is dominated by grass pollen at frequencies exceeding 70%, with smaller percentages of Cyperaceae pollen. Other pollen types occur intermittently, with peaks in Asteraceae pollen at the base of the zone, in Aizoaceae pollen at 130 cm and a rise in Fabaceae percentages at the top of the zone. Zone G-2: 60–0 cm, around 900 BP to present. The zone is characterised, as in the near-surface zone of the other cores, by a decline in grass pollen and corresponding increase in sedges to reach a peak of around 70% at the surface. There is a small, but marked, peak in Chenopodiaceae near the bottom of this zone. Acanthaceae, Aizoaceae, Arecaceae and Combretaceae all reach their maximum (low) percentages in the top 50 cm of sediment. 4.4.4. Core TL2 This core (Fig. 11) is just over 2 m in length and represents the last approx. 2300 years of sedimentation in the Tamacha valley. Two zones are again identified: Zone T-1: 210–50 cm. The chronology for this core is problematic as there are two similar age determinations for sediments sampled over 100 cm apart. On the basis of the better-resolved chronology at GL2, it is reasonable to

assume that the date for sediments at around 100 cm is contaminated by older material and that the age for the basal sediments (around 2300 BP) is more likely correct. Given this, the sediments of zone T1 probably accumulated between around 2300 BP to approximately 700 BP. The zone is characterised by high grass pollen percentages, exceeding 80%, and fluctuating, if minor, quantities of pollen of other varieties, including Asteraceae, Combretaceae, Fabaceae and Monolete spores (all of which peak here) and Chenopodiaceae, which reaches a maximum at 180 cm. Zone T-2: 50–0 cm, approximately 700 BP (interpolated) to present. The zone displays the now familiar shift in grass and sedge dominance, associated with small but significant quantities of Aizoaceae, Asteraceae, Combretaceae and Fabaceae pollen. 5. Discussion 5.1. Sequences of Holocene environmental change in the Okavango Panhandle The pollen spectra in Figs. 8–11 represent the first palaeoecological data for the Okavango system and, as such, are important archives of vegetation history. Relatively poor pollen preservation, low taxonomic resolution and the availability of only relative pollen counts mean that palaeoenvironmental interpretation needs to proceed with caution. Any moisture signal in the pollen record is difficult to resolve, given that grasses and sedges which dominate all spectra are co-dominants in contemporary Panhandle floodplains. Establishing whether vegetation shifts reflect regional climatic variability or changes in

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Okavango flood levels in response to catchment conditions is similarly problematic. Nevertheless, there are consistencies in the patterns of pollen abundance that, when viewed in the context of existing palaeoenvironmental evidence, can be used to suggest changes in vegetation dynamics and regional climate during the Holocene. To aid discussion, a chronology of Holocene environmental changes for central southern Africa is presented in Fig. 12, compiled from all U-series, luminescence and radiocarbon ages published for the Kalahari to date. All radiocarbon assays are calibrated (Section 3.2), with uncalibrated ages shown in parentheses. Establishing a clear palaeoclimatic signal for the earliest Holocene is problematic, since this time interval is represented only in core NL2 (Fig. 8). Pollen frequencies are low towards the base of the core (Fig. 7) and the nearabsence of palynomorphs from taxa other than Poaceae and Cyperaceae may be an artefact of poor preservation as opposed to reflecting actual vegetation patterns. There is evidence for the enhanced accumulation of organic sediments at around 9300 BP, which may indicate either elevated Okavango flood levels or a wetter climatic phase at this time. Higher flood levels are most likely, given that a shallow lake, fed mainly by inflow from the Okavango system, was present in the Ngami basin until 7.5 kyr (Shaw, 1985a; Shaw et al., 2003). Marine records also indicate increased run-off from the Angolan Highlands commencing in the early Holocene (Gingele, 1996). Any period of enhanced flow may have been short-lived, as it is not apparent in the pollen record. It is also likely to have been a response to conditions over the upper Okavango catchment as opposed to local rainfall. Pollen spectra from Drotsky’s Cave indicate the presence of Commiphora, several genera of Leguminosae, Capparis, Brachystegia and low values of Combretaceae (Burney et al., 1994) in the early Holocene, with U-series dating of speleothems within cenotes near Otavi (Brook et al., 1997, 1998, 1999) also suggesting drier conditions. Aeolian activity has been recognised in Zambia (O’Connor and Thomas, 1999) and Zimbabwe (Stokes et al., 1997a, 1998) until 8 kyr, although this may have been localised given the absence of synchronous dune activation at Tsodilo Hills and in northern Namibia (Thomas et al., 2000, 2003). Dune activity is, however, evident in the southwest Kalahari (Eitel and Blu¨mel, 1997; Stokes et al., 1997a, b; Thomas et al., 1997; Blu¨mel et al., 1998), with reduced moisture levels identified at Equus Cave (Beaumont et al., 1984; Scott, 1987; Johnson et al., 1997). Marine records confirm this pattern, with maximum percentages of desert and semi-desert pollen and low values for dry-forest woodland recorded in cores off southwest Africa for 11–8.9 kyr (Shi et al., 2000). Conditions from 8000 BP onwards can be interpreted with greater confidence. Most notable in the pollen record is the marked peak in Chenopodiaceae pollen at around 7000 BP in core NL2. The return of Chenopodiaceae shortly thereafter, coupled with increased frequencies of

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Aizoaceae is, perhaps, indicative of reduced moisture availability, since chenopods are more abundant in the drier floodplain habitats of the contemporary Panhandle (Ellery and Ellery, 1997; Roodt, 1998). The event is accompanied by a pronounced increase in grass pollen and decline in sedge pollen, and is also associated with a lighter stable carbon isotope signal (Fig. 6). The negative shift in stable carbon isotope values is consistent with an interpretation of drier conditions. Many of the most prominent sedges in the Panhandle region (e.g. Cyperus papyrus) are C4 species and would be expected to decline with lower moisture availability. Furthermore, the chenopods, which increase at the same time, are CAM plants and typically have d13C values around 22 per mil. A reduced frequency of C4 sedges and increased abundance of CAM plants would cause the stable carbon isotope values of associated sediments to decline. Elevated levels of Chenopodiaceae pollen extend until around 4000 BP and may indicate continuing drier conditions in the Ncamasere Valley, although a wetter period around 6000 BP may have punctuated this dry phase. The period of reduced moisture availability from 7000 to 4000 BP is out of phase with the palaeoenvironmental sequences from Drotsky’s and Bone caves (Burney et al., 1994; Railsback et al., 1994, 1999; Brook et al., 1996, 1997, 1998; Robbins et al., 1996) and the Tsodilo Hills (Robbins et al., 1994) in the Middle Kalahari, and the StamprietAuob aquifer (Stute and Talma, 1998) and Kathu Pan (Beaumont et al., 1984; Butzer, 1984b) records from the Southern Kalahari. Together, evidence from these sites suggests wetter conditions over much of central southern Africa during the Holocene Altithermal (Partridge et al., 1990, 1999), extending to 4000 BP in northwest Botswana. The onset of the dry period is also out of phase with records from Wonderwerk (Avery, 1981; Beaumont et al., 1984; Butzer, 1984a, b; Thackeray, 1984; Thackeray and Lee-Thorp, 1992; van Zinderen Bakker, 1982) and Equus (Beaumont et al., 1984; Scott, 1987) caves in the Southern Kalahari. These indicate wetter conditions during the Altithermal followed by a return to drier conditions around 5 kyr, though more recent work by Johnson et al. (1997) at Equus Cave conflicts with this pattern. However, the reduced moisture levels between 7000 and 4000 BP coincide with drier conditions to the west and north of the Panhandle. Palynological investigations in Windhoek (Scott et al., 1991) indicate a decline in Cyperaceae pollen coupled with an increase in Chenopodiaceae–Amaranthaceae pollen at this time. Enhanced aeolian activity is also recognised on lunette dunes marginal to Etosha Pan (Buch and Zo¨ller, 1992; Buch et al., 1992) and on palaeodunes in western Zambia (O’Connor and Thomas, 1999). More significantly, there is a hiatus in the palaeolake record from the Ngami basin (Shaw et al., 2003) which suggests reduced inflow from the Okavango system. Taken as a whole, these independent proxies indicate that, even though Middle Kalahari palaeoenvironmental signals imply wetter conditions, the drier period evident in the Panhandle from 7000

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Fig. 12. Combined luminescence, U/Th and calibrated radiocarbon chronology of Holocene environmental change in the Kalahari, derived from data for the following locations: (a) Drotsky’s Cave (Wayland, 1944; Cooke, 1975, 1984; Cooke and Verhagen, 1977; Shaw and Cooke, 1986; Burney et al., 1994; Railsback et al., 1994, 1999; Robbins et al., 1996; Brook et al., 1996, 1997, 1998); (b) Tsodilo Hills (Robbins et al., 1994); (c) Upper Ncamasere and Dobe (Helgren and Brooks, 1983; Brook, 1995); (d) Lake Ngami (Shaw, 1985a, b; Shaw and Cooke, 1986; Robbins et al., 1998; Shaw et al., 2003); (e) Mababe and Makgadikgadi depressions (Helgren, 1984; Shaw, 1985a); (f) Chobe River (Shaw and Thomas, 1988); (g) Okavango River (Nash et al., 1997); (h) Zambia palaeodunes (O’Connor and Thomas, 1999); (i) Zimbabwe palaeodunes (Stokes et al., 1997a, 1998); (j) Etosha lunette dunes (Buch and Zo¨ller, 1992; Buch et al., 1992); (k) Otavi cenotes (Brook et al., 1997, 1998, 1999); (l) Drotsky’s pollen record (Burney et al., 1994); (m) Otjikoto pollen record (Scott et al., 1991); (n) Windhoek pollen record (Scott et al., 1991); (o) Gaap escarpment tufas (Butzer et al., 1978; Beaumont and Vogel, 1993); (p) Stampriet aquifer (Heaton et al., 1983; Stute and Talma, 1998); (q) Auob and Kuruman rivers (Heine, 1982; Shaw et al., 1992); (r) SW Kalahari palaeodunes (Eitel and Blu¨mel, 1997; Stokes et al., 1997a, b; Thomas et al., 1997, 1998; Blu¨mel et al., 1998; Lawson and Thomas, 2002; Bateman et al., 2003); (s) Kathu Vlei (Beaumont et al., 1984); (t) Wonderwerk Cave (Beaumont et al., 1984; Butzer, 1984a, b); (u) Equus Cave (Scott, 1987). Radiocarbon ages were calibrated using CALIB REV 4.4.2 (Stuiver and Reimer, 1993) run with the southern hemisphere calibration dataset (shcal02.14c; McCormack et al., 2002).

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to 4000 BP represents a wetland ecosystem response to lower flood levels in the Okavango River. Such low flood levels are most likely a product of reduced available moisture over the Cubango and Cuito catchments, as opposed to local climatic variability. This is further supported by the reduced silt/clay accumulation recorded in core NL2 over the same period (Table 4). Vegetation patterns from 4000 to 2300 BP are difficult to ascertain in core NL2 as no strong signal is apparent. The decline in Chenopodiaceae and Aizoaceae pollen after 4500 BP, coupled with the appearance and rise in levels of Fabaceae pollen from 4000 BP onwards, may represent increasingly wet ground conditions. The start of this relatively wet period is out of phase with the regional rainfall record from Drotsky’s and Bone caves. However, it coincides with high lake levels at 936 m asl in the Ngami basin (4–3 kyr; Shaw, 1985a, Shaw and Cooke, 1986; Robbins et al., 1998; Shaw et al., 2003) and at 912 m in the Makgadikgadi basin (radiocarbon dated to prior to 2965–3081 cal yr BP [2960750 yr BP]; Helgren, 1984), which occurred mainly in response to increased discharge through the Okavango system. The wetter conditions at Ncamasere until 3 kyr probably, therefore, reflect an enhanced Okavango flood. After this time, regional rainfall appears to have increased, as indicated by speleothem development in Drotsky’s and Bone caves (Burney et al., 1994) and lacustrine calcrete formation in the Etosha basin (Rust, 1984, 1985; Rust et al., 1984). The later Holocene is well-represented in cores NL2, GL2 and TL2. The period from 2300 to around 1000 BP is characterised by consistent or slightly increasing sedge pollen concentrations, declining grass pollen and lower chenopod percentages (although Aizoaceae remain prominent). There are also higher values for all arboreal pollen including Fabaceae, Combretaceae and several other tree families, which would appear to represent a moisture signal. In contrast with the 7000–4000 BP dry period, this wet phase accords with other palaeoenvironmental data, suggesting widespread enhanced moisture availability. For example, wetter conditions are indicated at Drotsky’s and Bone caves (Cooke, 1975; Burney et al., 1994; Brook et al., 1996, 1997, 1998), the upper Ncamasere valley (Brook, 1995), Lake Otjikoto (Scott et al., 1991), the Gaap Escarpment (Butzer et al., 1978; Beaumont and Vogel, 1993) and Wonderwerk Cave (Beaumont et al., 1984). More interesting, however, is the clustering of radiocarbon ages at 2400–2100, 1900–1500 and 1000 BP within the Panhandle cores. These may indicate elevated Okavango flood levels as they coincide with other evidence for enhanced flow. For example, McCarthy and Metcalfe (1990) have calculated that volumes of Na and K within the Okavango delta-fan would have taken 1400 and 2800 years, respectively, to accumulate, suggesting mid- to lateHolocene flushing of the system. The occurrence of flooded enlarged meanders in the Maunachira distributary has also been attributed to higher discharge during the late Holocene (McCarthy et al., 2002). Furthermore, a high

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lake level has been recognised in the Ngami basin from 2.8 to 2 kyr (Shaw et al., 2003), which coincides with high stands in the Mababe depression (Shaw, 1985a) and Chobe River (Shaw and Thomas, 1988). As such, the 2300–1000 BP wet phase appears to reflect a combination of locally wetter conditions superimposed on the effects of increased available moisture in the Cubango and Cuito catchments. The development of deltas within streams marginal to the most recent Ngami and Mababe high stands (Cooke and Verstappen, 1984; Shaw and Cooke, 1986) adds further support to this suggestion. By far the most striking feature of all five pollen records is the shift in dominance between grass and sedge pollen in the last thousand years. This is clearly a regional vegetation signal, as opposed to a palynological artefact, since it is evident in all cores. The trend is well dated in core GL2 to 900–960 cal yr BP (1065745 yr BP) and is interpolated to start at the same time in core TL2. In core NL2, the grassto-sedge shift may begin as recently as 200 BP, although dating control in the top sediments is poor. Certainly, the trend occurs over a shallower sediment accumulation. The presence of only ‘modern’ ages in cores NL3 and NL4 make the timing of the change impossible to determine, although the trend is so clearly evident in both cores that its commencement could arguably be used as a better chronological marker than the radiocarbon ages themselves. Stable carbon isotope values for the recent sediments are marginally more enriched, and this is consistent with a greater prominence of C4 sedges such as Cyperus papyrus and Pycreus nitidus which are common in later successional stages (i.e. shallower water areas) of the contemporary Panhandle (Ellery et al., 1993). As regards a mechanism underlying the shift in relative grass- to sedge-dominance at the top of the pollen spectra, there are a number of possibilities, including climate change, hydrological change and human impact, which may have acted in isolation or combination. On the basis of available evidence, climatic fluctuations in the Kalahari during the past 1000 years have been minor (Fig. 12), with palaeoprecipitation data from Equus Cave (Johnson et al., 1997) indicating that rainfall levels may have changed little since 6 kyr. Wetter phases have been identified at Drotsky’s and Bone caves from 1.6 to 0.5 kyr (Burney et al., 1994), at Gi Pan from 660–740 cal yr BP (810760 yr BP) to 490– 530 cal yr BP (495745 yr BP; Helgren and Brooks, 1983), and at the Gaap Escarpment from 1220–1300 cal yr BP (1375765 yr BP) to 450–560 cal yr BP (515780 yr BP; Butzer et al., 1978; Beaumont and Vogel, 1993). However, there is no evidence for the marked change in rainfall necessary to generate the increase in Cyperaceae seen in the pollen spectra. Similarly, the late Holocene palaeohydrological record for the Okavango system does not suggest increases in flow of the magnitude that would lead to expansion of the wetland settings favoured by Cyperaceae (Stock et al., 2004). Lake levels in the Ngami basin remained low (Shaw et al., 2003) and documentary evidence suggests periods of both increased and decreased

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Okavango discharge during the nineteenth and 20th centuries (Wilson, 1973; Shaw, 1984, 1985b, 1986; Nash and Endfield, 2002). This leaves human impact as a possible influence. According to Denbow and Wilmsen (1986), pastoralism was introduced to the wider Kalahari from about 2000 years ago and increasing technological innovation associated with population growth characterised the Early Iron Age (AD 600–1100) and Later Stone Age (AD 1500 onwards; Thomas and Shaw, 1991). The use of fire as a management tool by these and later more sophisticated economies (Tlou, 1972), coupled with increased grazing pressure and harvesting of wetland plant species (as continues to the present day; Bernard and Moetapele, 2005), could have facilitated widespread vegetation change of the order of magnitude observed in the pollen record. The substantial landscape changes of the recent past (Sporton and Thomas, 2002) do not, however, appear to have been archived in the pollen spectra. 5.2. Climate forcing mechanisms The contention that conditions in the Panhandle during the Holocene reflected the dynamics of the Okavango catchment, as opposed to simply regional climatic conditions, requires a consideration of possible forcing mechanisms. Southern African rainfall patterns are regionally complex, but are a composite of several major rain-bearing systems (Tyson and Preston-Whyte, 2000): (a) easterly waves and lows (the southeast African Monsoon) emanating from the Indian Ocean and supplying summer rainfall to areas south of 201 S (i.e. south of the Okavango); (b) tropical lows (the so-called West African Monsoon) associated with the equatorial trough or inter-tropical convergence zone (ITCZ), deriving their moisture from the Atlantic and supplying rainfall to latitudes north of the Okavango (including the Cuito and Cubango catchments); (c) westerlies that now bring only occasional rainfall to the tropics but may have been more prominent during the Last Glacial (Shi et al., 2000, 2001) due to equatorward shifts in frontal belts associated with expansions of Antarctic sea ice (Stuut et al., 2002, 2004; Stuut and Lamy, 2004). Also of significance are subtropical anticyclone systems that induce atmospheric stability and aridity over much of the western parts of the subcontinent, especially in the austral summer. The early Holocene record in the Panhandle, with increased Okavango flooding in contrast to a regionally drier Kalahari, would require greater intensities of tropical lows associated with the equatorial trough, coupled with a reduced importance of easterly disturbances and/or increased intensity of subtropical anticyclones. Shi et al. (2000) attribute the anti-phase between wet inter-tropical Africa and dry subtropical southern Africa in the early Holocene to precessional forcing that shifted the ITCZ to the north. The drier phase from 7000 to 4000 BP is also in anti-phase, although in this case reduced Okavango flooding coincides with regionally wetter conditions and requires reduced importance of West African Monsoon systems

combined with greater prominence of easterly flows. Higher Okavango flood levels from 4000 to 2300 BP are in anti-phase with regional indicators of aridity and would require a climatic scenario similar to that of the early Holocene, although the forcing mechanism cannot be driven by precession in this instance. Finally, the wetter period from 2300 to 1000 BP, which is in phase with regional indicators, could be explained by increases in the intensity of both easterly flows and equatorial trough disturbances. Support for this interpretation exists in the marine record, including pollen evidence from the Cunene River mouth at approximately 171 S (core GeoB 1023-5; Shi et al., 1998, 2000; Dupont et al., 2004) and from the Walvis Ridge at around 231S (core GeoB 1711-4; Shi et al., 2001). The temporal resolution for the Holocene in these records is not always ideal, and the localities of the cores make it difficult to distinguish between moisture sources related to easterly disturbances or tropical lows. Nevertheless, there are indications of elevated levels of dry-forest pollen at both coring sites during the mid-Holocene, interpreted by Shi et al. (2001) as reflecting dry-forest expansion and increased easterly flow. This may corroborate the antiphase vegetation response in the Panhandle during the Holocene Altithermal, since easterly flows do not typically impact upon the upper Okavango catchment. Shi et al. (2000) also note a decrease in Afromontane forest pollen in sediments off the Cunene River mouth between 6300 and 4800 BP, which they attribute to an altitudinal shift in vegetation distribution under warmer and wetter conditions. It is intriguing to consider the impact of such variations in the extent of northern Kalahari forest cover upon discharges in the Okavango River. Increased forest cover during the Holocene Altithermal would, presumably, result in higher rates of evapotranspiration over the Okavango catchment, a greater draw on moisture sources, a less vigorous flow regime (even if increased rainfall occurred) and less extensive flooding in the delta-fan. It remains to be seen if similar effects are recorded in other endoreic rivers in the Kalahari. 6. Conclusions The sedimentological, stable carbon isotope, pollen and chronological data presented in this paper demonstrate that, despite the poor conditions for pollen preservation across much of the Kalahari, coring in wetland sites such as the swamps and marshes of the Okavango Panhandle offers considerable potential for unravelling sequences of past environmental change. Earliest Holocene vegetation patterns are not well-represented at Ncamasere, Gauxa and Tamacha, although a possible wet phase or, more likely, period of enhanced Okavango flooding has been identified around 9300 BP. There is more convincing evidence for drier conditions from 7000 to 4000 BP (punctuated by a wet phase around 6000 BP), at a time when other Middle Kalahari palaeoenvironmental records indicate wetter

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conditions. This is interpreted as an ecosystem response to lower flood levels resulting from reduced rainfall over the upper Okavango catchment. Conditions appear to have become progressively wetter from 4000 BP, initially in response to increased water throughputs via the Okavango system. The wettest conditions occurred from 2300 to 1000 BP, due to a combination of increased local rainfall and Okavango flood levels. After this time, conditions approached those of the present day, although a major shift from grass- to sedge-dominated vegetation communities is apparent, most probably in response to increased grazing and/or use of fire. Taken as a whole, the palaeoenvironmental record from the Panhandle wetlands provides potentially important lessons for the future environmental management of the Okavango delta-fan. The shift towards drier-climate taxa between 7000 and 4000 BP, despite widespread evidence for higher local rainfall at this time, implies that ecosystems in the Okavango Panhandle may have been considerably more sensitive to changes in the magnitude of the annual flood pulse during the Holocene than to fluctuations in regional climate. This suggestion is reinforced by the observation that wetter-climate communities only appear to have developed from around 4000 BP onwards in response to a combination of higher flood levels and enhanced local rainfall. This would suggest that any future efforts to utilise water from the Okavango catchment upstream of the delta-fan (Andersson et al., 2003; Swatuk, 2003; Mbaiwa, 2004) are likely to have major impacts on vegetation communities, not only in the main Okavango Delta, but also in the swamp and marsh environments of the Panhandle. The dramatic switch from grass- to sedge-dominated vegetation systems in the last thousand years of the palaeoenvironmental record provides perhaps the most graphic indication of the sensitivity of Okavango ecosystems to human disturbance. Acknowledgements Funding for preliminary work in the Ncamasere Valley in 1994 was generously provided by the Gilchrist Educational Trust. Further coring at Ncamasere, Tamacha and Gauxa in 1997 was funded by the universities of Brighton and Cape Town whilst VLG was in receipt of a University of Brighton studentship. Radiocarbon dating was funded by NERC 14C Dating Allocation 732/0498. The authors are indebted to Andrew Baxter, Bruce Dell, Bill Downey, Ambro Gieske, Alison Meadows, Ian Michler, Alexander Shaw, Paul Shaw and Brett Smith for their support in the field, to John Lanham for assistance in interpreting the stable carbon isotope data, and to Brian Chase for suggestions on climate forcing mechanisms. Thanks also to David Thomas and an anonymous referee whose comments greatly improved the manuscript. This project was carried out under kind permission of Botswana Government Research Permit number OP 46/1 XLIII (19).

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References Andersson, L., Gumbricht, T., Hughes, D., Kniveton, D., Ringrose, S., Savenije, H., Todd, M., Wilk, J., Wolski, P., 2003. Water flow dynamics in the Okavango River Basin and Delta—a prerequisite for the ecosystems of the Delta. Physics and Chemistry of the Earth 28, 1165–1172. Avery, D.M., 1981. Holocene micromammalian faunas from the Northern Cape Province, South Africa. South African Journal of Science 77, 265–273. Bateman, M.D., Thomas, D.S.G., Singhvi, A.K., 2003. Extending the aridity record of the Southwest Kalahari: current problems and future perspectives. Quaternary International 111, 37–49. Beaumont, P.D., Vogel, J.C., 1993. What turned the young tufas on at Gorrokop? South African Journal of Science 89, 196–198. Beaumont, P.D., van Zinderen Bakker, E.M., Vogel, J.C., 1984. Environmental changes since 32,000 BP at Kathu Pan, Northern Cape. In: Vogel, J.C. (Ed.), Late Cainozoic Palaeoclimates of the Southern Hemisphere. Balkema, Rotterdam, pp. 329–338. Bennett, K.D., 2002. Documentation for psimpoll 4.10 and pscomb 1.03. Department of Earth Sciences, Uppsala University. Bernard, T., Moetapele, N., 2005. Desiccation of the Gomoti River: biophysical process and indigenous resource management in Northern Botswana. Journal of Arid Environments 63, 256–283. Blu¨mel, W.D., Eitel, B., Lang, A., 1998. Dunes in southeastern Namibia: evidence for Holocene environmental changes in the southwestern Kalahari based on thermoluminescence data. Palaeogeography, Palaeoclimatology, Palaeoecology 138, 139–149. Brook, G.A., 1995. Quaternary environments in the Somali-Chalbi and Kalahari deserts of Africa. In: Abstracts of the International Conference on Quaternary Deserts and Climatic Change, December 9–11, 1995, Al Ain, United Arab Emirates University, UAE, p. 6. Brook, G.A., Cowart, J.B., Marais, E., 1996. Wet and dry periods in the southern African summer rainfall zone during the last 330 kyr from speleothem, tufa and sand dune age data. Palaeoecology of Africa 24, 147–158. Brook, G.A., Cowart, J.B., Brandt, S.A., Scott, L., 1997. Quaternary climatic change in southern and eastern Africa during the last 300 ka: the evidence from caves in Somalia and the Transvaal region of South Africa. Zeitschrift fu¨r Geomorphologie, Supplementband 108, 15–48. Brook, G.A., Cowart, J.B., Brandt, S.A., 1998. Comparison of Quaternary climatic changes in eastern and southern Africa using cave speleothem, tufa and rock shelter sediment data. In: Alsharan, A., Glennie, K.W., Whittle, G.L., Kendall, C.G.StC. (Eds.), Quaternary Deserts and Climatic Change. Balkema, Rotterdam, pp. 239–249. Brook, G.A., Marais, E., Cowart, J.B., 1999. Evidence of wetter and drier conditions in Namibia from tufas and submerged cave speleothems. Cimbebasia 15, 29–39. Buch, M.W., Zo¨ller, L., 1992. Pedostratigraphy and thermoluminescence chronology of the western margin- (lunette-) dunes of Etosha Pan, Northern Namibia. Wu¨rzburger Geographische Arbeiten 84, 361–384. Buch, M.W., Rose, D., Zo¨ller, L., 1992. A TL-calibrated pedostratigraphy of the western lunette dunes of Etosha Pan, northern Namibia: palaeoenvironmental implications for the last 140 ka. Palaeoecology of Africa 23, 129–147. Burney, D.A., Brook, G.A., Cowart, J.B., 1994. A Holocene pollen record for the Kalahari Desert of Botswana from a U-series dated speleothem. The Holocene 4, 225–232. Butzer, K.W., 1984a. Late Quaternary environments in South Africa. In: Vogel, J.C. (Ed.), Late Cainozoic Palaeoclimates of the Southern Hemisphere. Balkema, Rotterdam, pp. 235–264. Butzer, K.W., 1984b. Archeogeology and Quaternary environment in the interior of southern Africa. In: Klein, R.G. (Ed.), Southern African Prehistory and Palaeoenvironments. Balkema, Rotterdam, pp. 1–64. Butzer, K.W., Stuckenrath, R., Bruzewicz, A.J., Helgren, D.M., 1978. Late Cenozoic paleoclimates of the Gaap Escarpment, Kalahari margin, South Africa. Quaternary Research 10, 310–339.

ARTICLE IN PRESS 1320

D.J. Nash et al. / Quaternary Science Reviews 25 (2006) 1302–1322

Cooke, H.J., 1975. The palaeoclimatic significance of caves and adjacent landforms in western Ngamiland, Botswana. Geographical Journal 141, 430–444. Cooke, H.J., 1984. The evidence from northern Botswana of climate change. In: Vogel, J. (Ed.), Late Cenozoic Palaeoclimates of the Southern Hemisphere. Balkema, Rotterdam, pp. 265–278. Cooke, H.J., Verhagen, B.Th., 1977. The dating of cave development—an example from Botswana. In: Proceedings of the Seventh International Speleological Congress, Sheffield, UK, pp. 122–124. Cooke, H.J., Verstappen, H.Th., 1984. The landforms of the western Makgadikgadi basin, northern Botswana, with a consideration of the chronology of the evolution of Lake Palaeo-Makgadikgadi. Zeitschrift fu¨r Geomorphologie 28, 1–19. Cronberg, G., Gieske, A., Martins, E., Nengu, J.P., Stenstro¨m, I.-M., 1995. Hydrobiological studies of the Okavango Delta and the Kwando/Linyanti/Chobe River, Botswana. Botswana Notes and Records 27, 151–226. Denbow, J.R., Wilmsen, E.N., 1986. Advent and course of pastoralism in the Kalahari. Science 234, 1509–1515. Dupont, L.M., Kim, J.-H., Schneider, R.R., Shi, N., 2004. Southwest African climate independent of Atlantic sea surface temperatures during the Younger Dryas. Quaternary Research 61, 318–324. Eitel, B., Blu¨mel, W.D., 1997. Pans and dunes in the southwestern Kalahari (Namibia): geomorphology and evidence for Quaternary paleoclimates. Zeitschrift fu¨r Geomorphologie, Supplementband 111, 73–95. Ellery, K., Ellery, W.N., 1997. Plants of the Okavango Delta: A Field Guide. Tsaro Publishers, Durban, South Africa. Ellery, W.N., Ellery, K., McCarthy, T.S., 1993. Plant distribution on islands in the Okavango Delta: determinants and feedback interaction. African Journal of Ecology 31, 118–134. Ellery, W.N., McCarthy, T.S., Smith, N.D., 2003. Vegetation, hydrology, and sedimentation patterns on the major distributary system of the Okavango fan, Botswana. Wetlands 23, 357–375. Faegri, K., Iversen, J., 1989. Textbook of Pollen Analysis, fourth ed. Wiley, Chichester. Garstang, M., Ellery, W.N., McCarthy, T.S., Scholes, M.C., Swap, R.J., Tyson, P.D., 1998. The contribution of aerosol-borne nutrients to the functioning of the Okavango Delta ecosystem, Botswana. South African Journal of Science 94, 223–229. Gingele, F.X., 1996. Holocene climatic optimum in Southwest Africa: evidence from the marine clay mineral record. Palaeogeography, Palaeoclimatology, Palaeoecology 122, 77–87. Gumbricht, T., McCarthy, J., McCarthy, T.S., 2004. Channels, wetlands and islands in the Okavango Delta, Botswana, and their relation to hydrological and sedimentological processes. Earth Surface Processes and Landforms 29, 15–29. Gumbricht, T., McCarthy, T.S., Bauer, P., 2005. The micro-topography of the wetlands of the Okavango Delta, Botswana. Earth Surface Processes and Landforms 30, 27–39. Heaton, T.H.E., Talma, A.S., Vogel, J.C., 1983. Origin and history of nitrate in confined groundwater in the Western Kalahari. Journal of Hydrology 62, 243–262. Heine, K., 1982. The main stages of the Late Quaternary evolution of the Kalahari region, southern Africa. Palaeoecology of Africa 15, 53–76. Helgren, D.M., 1984. Historical geomorphology and geoarchaeology in the southwestern Makgadikgadi Basin, Botswana. Annals of the Association of American Geographers 74, 298–307. Helgren, D.M., Brooks, A.S., 1983. Geoarchaeology at Gi, a middle stone age and later stone age site in the Northwest Kalahari. Journal of Archaeological Science 10, 181–197. Johnson, B.J., Miller, G.H., Fogel, M.L., Beaumont, P.B., 1997. The determination of late Quaternary paleoenvironments at Equus Cave, South Africa, using stable isotopes and amino acid racemization in ostrich eggshell. Palaeogeography, Palaeoclimatology, Paleoecology 136, 121–137. Junk, W.J., Bayley, P.B., Sparks, R.E., 1989. The flood pulse concept in river-floodplain systems. In: Dodge, D.P. (Ed.) Proceedings of the

International Large River Symposium. Canadian Special Publication, Fisheries and Aquatic Science 106, pp. 110–127. Krah, M., McCarthy, T.S., Annegarn, H., Ramberg, L., 2004. Airborne dust deposition in the Okavango Delta, Botswana, and its impact on landforms. Earth Surface Processes and Landforms 29, 565–577. Lanesky, D.E., Logan, B.W., Brown, R.G., Hine, A.C., 1979. A new approach to portable vibracoring underwater and on land. Journal of Sedimentary Petrology 49, 654–657. Lawson, M.P., Thomas, D.S.G., 2002. Late Quaternary lunette dune formation in the southwestern Kalahari Desert: luminescence based chronologies of aeolian activity. Quaternary Science Reviews 21, 825–836. Lee-Thorp, J.A., Beaumont, P.B., 1995. Vegetation and seasonality shifts during the late Quaternary deduced from 13C/12C ratios of grazers at Equus Cave, South Africa. Quaternary Research 43, 426–432. Mbaiwa, J.E., 2004. Causes and possible solutions to water resource conflicts in the Okavango River Basin: the case of Angola, Namibia and Botswana. Physics and Chemistry of the Earth 29, 1319–1326. McCarthy, T.S., 1993. The great inland deltas of Africa. Journal of African Earth Sciences 17, 275–291. McCarthy, T.S., Ellery, W.N., 1998. The Okavango Delta. Transactions of the Royal Society of South Africa 53, 157–182. McCarthy, T.S., Metcalfe, J., 1990. Chemical sedimentation in the Okavango Delta, Botswana. Chemical Geology 89, 157–178. McCarthy, T.S., Stanistreet, I.G., Cairncross, B., 1991. The sedimentary dynamics of active fluvial channels on the Okavango fan, Botswana. Sedimentology 38, 471–487. McCarthy, T.S., Green, R.W., Franey, N.J., 1993. The influence of neo-tectonics on water dispersal in the northeastern region of the Okavango swamps, Botswana. Journal of African Earth Sciences 17, 23–32. McCarthy, T.S., Barry, M., Bloem, A., Ellery, W.N., Heister, H., Merry, C.C., Ruther, H., Stromberg, H., 1997. The gradient on the Okavango fan, Botswana, and its sedimentological and tectonic implications. Journal of African Earth Sciences 24, 65–78. McCarthy, T.S., Bloem, A., Larkin, P.A., 1998. Observations on the hydrology and geohydrology of the Okavango Delta. South African Journal of Geology 101, 101–117. McCarthy, T.S., Cooper, G.R.J., Tyson, P.D., Ellery, W.N., 2000. Seasonal flooding in the Okavango Delta, Botswana—recent history and future prospects. South African Journal of Science 96, 25–33. McCarthy, T.S., Smith, N.D., Ellery, W.N., Gumbricht, T., 2002. The Okavango Delta—semi-arid alluvial fan sedimentation related to incipient rifting. In: Renaut, R.W., Asley, G.M. (Eds.), Sedimentation in Continental Rifts. Special Publication of the Society for Sedimentary Geology (SEPM). Tulsa, Oklahoma, pp. 179–193. McCarthy, J.M., Gumbricht, T., McCarthy, T.S., Frost, P., Wessels, K., Seidel, F., 2003. Flooding patterns of the Okavango wetland in Botswana between 1972 and 2000. Ambio 32, 453–457. McCormack, F.G., Reimer, P.J., Hogg, A.G., Higham, T.F.G., Baillie, M.G.L., Palmer, J., Stuiver, M., 2002. Calibration of the radiocarbon time scale for the southern hemisphere: AD 1850–950. Radiocarbon 44, 641–651. Mladenov, N., McKnight, D.M., Wolski, P., Ramberg, L., 2005. Effects of annual flooding on dissolved organic carbon dynamics within a pristine wetland, the Okavango Delta, Botswana. Wetlands 25, 622–638. Nash, D.J., 1996. On the dry valleys of the Kalahari Desert: documentary evidence of environmental change in central southern Africa. Geographical Journal 162, 154–168. Nash, D.J., Endfield, G.H., 2002. Historical flows in the dry valleys of the Kalahari identified from missionary correspondence. South African Journal of Science 98, 244–248. Nash, D.J., Thomas, D.S.G., Shaw, P.A., 1994. Timescales, environmental change and dryland valley development. In: Millington, A.C., Pye, K. (Eds.), Environmental Change in Drylands: Biogeographical and Geomorphological Perspectives. Wiley, Chichester, pp. 25–41.

ARTICLE IN PRESS D.J. Nash et al. / Quaternary Science Reviews 25 (2006) 1302–1322 Nash, D.J., Meadows, M.E., Shaw, P.A., Baxter, A.J., Gieske, A., 1997. Late Holocene sedimentation rates and geomorphological significance of the Ncamasere Valley, Okavango Delta, Botswana. South African Geographical Journal 79, 93–100. O’Connor, P.W., Thomas, D.S.G., 1999. The timing and environmental significance of Late Quaternary linear dune development in Western Zambia. Quaternary Research 52, 44–55. Partridge, T.C., Avery, D.M., Botha, G.A., Brink, J.S., Deacon, J., Herbert, R.S., Maud, R.R., Scholtz, A., Scott, L., Talma, A.S., Vogel, J.C., 1990. Late Pleistocene and Holocene climatic change in southern Africa. South African Journal of Science 86, 302–306. Partridge, T.C., Scott, L., Hamilton, J.E., 1999. Synthetic reconstructions of southern African environments during the Last Glacial Maximum (21–18 kyr) and the Holocene Altithermal (8–6 kyr). Quaternary International 57/58, 207–214. Railsback, L.B., Brook, G.A., Chen, J., Kalin, R., Fleisher, C.J., 1994. Environmental controls on the petrology of a late Holocene speleothem from Botswana with annual layers of aragonite and calcite. Journal of Sedimentary Research A 64, 147–155. Railsback, L.B., Brook, G.A., Webster, J.W., 1999. Petrology and paleoenvironmental significance of detrital sand and silt in a stalagmite from Drotsky’s Cave, Botswana. Physical Geography 20, 331–347. Robbins, L.H., Murphy, M.L., Stewart, K.M., Campbell, A.C., Brook, G.A., 1994. Barbed bone points, paleoenvironment, and the antiquity of fish exploitation in the Kalahari Desert, Botswana. Journal of Field Archaeology 21, 257–264. Robbins, L., Murphy, A., Stevens, N.J., Brook, G.A., Ivester, A.H., Haberyan, K., Klein, R.G., Milo, R., Stewart, K.M., Matthiesen, D.G., Winkler, A.J., 1996. Paleoenvironment and archaeology of Drotsky’s Cave: western Kalahari Desert, Botswana. Journal of Archaeological Science 23, 7–22. Robbins, L., Murphy, A., Campbell, A., Brook, G.A., Reid, D., Haberyan, K., Downey, W., 1998. Test excavations and reconnaissance palaeoenvironmental work at Toteng, Botswana. South African Archaeological Bulletin 53, 125–132. Roodt, V., 1998. Trees and Shrubs of the Okavango Delta. Shell Oil Botswana, Gaborone. Rust, U., 1984. Geomorphic evidence of Quaternary environmental changes in Etosha, South West Africa/Namibia. In: Vogel, J. (Ed.), Late Cenozoic Palaeoclimates of the Southern Hemisphere. Balkema, Rotterdam, pp. 279–286. Rust, U., 1985. Die Enstehung der Etoschapfanne im Rahmen der Landschaftsentwicklung des Etoscha Nationalparks (no¨rdliches Su¨dwestafrika/Namibia). Madoqua 14, 197–266. Rust, U., Schmidt, H., Dietz, K., 1984. Palaeoenvironments of present day arid south western Africa, 30,000–5000 BP: results and problems. Palaeoecology of Africa 16, 109–148. Scott, L., 1987. Pollen analysis of hyena coprolites and sediments from Equus Cave, Taung, Southern Kalahari (S. Africa). Quaternary Research 28, 144–156. Scott, L., 2000. Pollen. In: Partridge, T.C., Maud, R.R. (Eds.), The Cenozoic of Southern Africa. Oxford University Press, Oxford, pp. 339–350. Scott, L., 2002. Grassland development under glacial and interglacial conditions in southern Africa: review of pollen, phytolith and isotope evidence. Palaeogeography, Palaeoclimatology, Palaeoecology 177, 47–57. Scott, L., Cooremans, B., de Wet, J.S., Vogel, J.C., 1991. Holocene environmental changes in Namibia inferred from pollen analysis of swamp and lake deposits. The Holocene 1, 8–13. Sefe, F.T.K., 1996. A study of the stage-discharge relationship of the Okavango River at Mohembo, Botswana. Hydrological Sciences Journal 41, 97–116. Shaw, P.A., 1984. A historical note on the outflows of the Okavango Delta system. Botswana Notes and Records 16, 127–130. Shaw, P.A., 1985a. Late Quaternary landforms and environmental change in northwest Botswana: the evidence of Lake Ngami and the Mababe

1321

depression. Transactions of the Institute of British Geographers 10, 333–346. Shaw, P.A., 1985b. The desiccation of Lake Ngami: an historical perspective. Geographical Journal 151, 318–326. Shaw, P.A., 1986. The palaeohydrology of the Okavango Delta: some preliminary results. Palaeoecology of Africa 17, 51–58. Shaw, P.A., Cooke, H.J., 1986. Geomorphic evidence for the Late Quaternary palaeoclimates of the Middle Kalahari of northern Botswana. Catena 13, 349–359. Shaw, P.A., Thomas, D.S.G., 1988. Lake Caprivi: a late Quaternary link between the Zambezi and middle Kalahari drainage systems. Zeitschrift fu¨r Geomorphologie 32, 329–337. Shaw, P.A., Thomas, D.S.G., Nash, D.J., 1992. Late Quaternary fluvial activity in the dry valleys (mekgacha) of the Middle and Southern Kalahari, southern Africa. Journal of Quaternary Science 7, 273–281. Shaw, P.A., Bateman, M.D., Thomas, D.S.G., Davies, F., 2003. Holocene fluctuations of Lake Ngami, Middle Kalahari: chronology and responses to climate change. Quaternary International 111, 23–35. Shi, N., Dupont, L.M., Beug, H.-J., Schneider, R., 1998. Vegetation and climatic changes during the last 21,000 years in S.W. Africa based on a marine pollen record. Vegetation History and Archaeobotany 7, 127–140. Shi, N., Dupont, L.M., Beug, H.-J., Schneider, R., 2000. Correlation between vegetation in southwestern Africa and oceanic upwelling in the past 21,000 years. Quaternary Research 54, 72–80. Shi, N., Schneider, R., Beug, H.-J., Dupont, L.M., 2001. Southeast trade wind variations during the last 135 kyr: evidence from pollen spectra in eastern South Atlantic sediments. Earth and Planetary Science Letters 187, 311–321. SMEC (Snowy Mountains Engineering Corporation), 1989. Ecological zoning, Okavango Delta—Final Report. Government of Botswana Ministry of Local Government and Lands (Ngamiland District Land Use Planning Unit)/Kalahari Conservation Society, Gaborone. Smith, P.A., 1976. An outline of the vegetation of the Okavango drainage system. In: Symposium on the Okavango Delta and its Future Utilisation. Botswana Society, Gaborone, pp. 93–112. Sporton, D., Thomas, D.S.G. (Eds.), 2002. Sustainable Livelihoods in Kalahari Environments: Contributions to Global Debates. Oxford University Press, Oxford. Stock, W.D., Chuba, D.K., Verboom, G.A., 2004. Distribution of South African C3 and C4 species of Cyperaceae in relation to climate and phylogeny. Austral Ecology 29, 313–319. Stokes, S., Thomas, D.S.G., Washington, R., 1997a. Multiple episodes of aridity in southern Africa since the last interglacial period. Nature 388, 154–158. Stokes, S., Thomas, D.S.G., Shaw, P.A., 1997b. New chronological evidence for the nature and timing of linear dune development in the southwest Kalahari Desert. Geomorphology 20, 81–93. Stokes, S., Haynes, G., Thomas, D.S.G., Horrocks, J.L., Higginson, M., Malifa, M., 1998. Punctuated aridity in southern Africa during the last glacial cycle: the chronology of linear dune construction in the northeastern Kalahari. Palaeogeography, Palaeoclimatology, Palaeoecology 137, 305–322. Stuiver, M., Reimer, P.J., 1993. Extended 14C database and revised CALIB 3.0 14C age calibration programme. Radiocarbon 35, 215–230. Stute, M., Talma, A.S., 1998. Glacial temperatures and moisture transport regimes reconstructed from noble gases and d18O, Stampriet aquifer, Namibia. In: Isotope Techniques in the Study of Environmental Change (IAEA Report IAEA-SM-349/53). International Atomic Energy Agency, Vienna, pp. 307–318. Stuut, J.-B.W., Lamy, F., 2004. Climate variability at the southern boundaries of the Namib (southwestern Africa) and Atacama (northern Chile) coastal deserts during the last 120,000 yr. Quaternary Research 62, 301–309. Stuut, J.-B.W., Prins, M.A., Schneider, R.R., Weltje, G.J., Jansen, J.H.F., Postma, G., 2002. A 300 kyr record of aridity and wind strength in southwestern Africa: inferences from grain-size distributions of

ARTICLE IN PRESS 1322

D.J. Nash et al. / Quaternary Science Reviews 25 (2006) 1302–1322

sediments on Walvis Ridge, SE Atlantic. Marine Geology 180, 221–233. Stuut, J.-B.W., Crosta, X., van der Borg, K., Schneider, R.R., 2004. Relationship between Antarctic sea ice and southwest African climate during the late Quaternary. Geology 32, 909–912. Swatuk, L.A., 2003. State interests and multilateral cooperation: thinking strategically about achieving ‘wise use’ of the Okavango Delta system. Physics and Chemistry of the Earth 28, 897–905. Thackeray, J.F., 1984. Climatic change and mammalian fauna from Holocene deposits in Wonderwerk Cave, northern Cape. In: Vogel, J.C. (Ed.), Late Cainozoic Palaeoclimates of the Southern Hemisphere. Balkema, Rot0terdam, pp. 371–374. Thackeray, J.F., Lee-Thorp, J.A., 1992. Isotopic analysis of equid teeth from Wonderwerk Cave, northern Cape Province, South Africa. Palaeogeography, Palaeoclimatology, Palaeoecology 99, 141–150. Thomas, D.S.G., Shaw, P.A., 1991. The Kalahari Environment. Cambridge University Press, Cambridge. Thomas, D.S.G., Shaw, P.A., 1996. The Quaternary palaeoenvironmental history of the Kalahari, southern Africa. Journal of Arid Environments 32, 9–22. Thomas, D.S.G., Shaw, P.A., 2002. Late Quaternary environmental change in central southern Africa: new data, synthesis, issues and prospects. Quaternary Science Reviews 21, 783–797. Thomas, D.S.G., Stokes, S., Shaw, P.A., 1997. Holocene aeolian activity in the southwestern Kalahari Desert, southern Africa: significance and relationships to late-Pleistocene dune-building events. The Holocene 7, 271–279. Thomas, D.S.G., O’Connor, P.W., Stokes, S., 1998. Late Quaternary aridity in the southwestern Kalahari Desert: new contributions from

OSL dating of aeolian sediments. In: Alsharan, A., Glennie, K.W., Whittle, G.L., Kendall, C.G.StC. (Eds.), Quaternary Deserts and Climate Change. Balkema, Rotterdam, pp. 213–224. Thomas, D.S.G., O’Connor, P.W., Bateman, M., Shaw, P.A., Stokes, S., Nash, D.J., 2000. Dune activity as a record of late Quaternary aridity in the Northern Kalahari: new evidence from northern Namibia interpreted in the context of regional arid and humid chronologies. Palaeogeography, Palaeoclimatology, Palaeoecology 156, 243–259. Thomas, D.S.G., Brook, G., Shaw, P.A., Bateman, M., Haberyan, K., Appleton, C., Nash, D.J., McLaren, S., Davies, F., 2003. Late Pleistocene wetting and drying in the NW Kalahari: an integrated study from the Tsodilo Hills, Botswana. Quaternary International 104, 53–67. Tlou, T., 1972. The taming of the Okavango swamps—the utilisation of a riverine environment, 1750–1800. Botswana Notes and Records 4, 147–159. Tyson, P.D., Preston-Whyte, R.A., 2000. The Weather and Climate of Southern Africa. Oxford University Press, Cape Town. Van Zinderen Bakker, E.M., 1982. Pollen analytical studies of the Wonderwerk Cave, South Africa. Pollen et Spores 24, 235–250. Wayland, E.J., 1944. Drotsky’s Cave. Geographical Journal 103, 230–233. Weare, P.R., Yalala, A., 1971. Provisional vegetation map. Botswana Notes and Records 3, 131–152. Wilson, B.H., 1973. Some natural and man-made changes in the channels of the Okavango Delta. Botswana Notes and Records 5, 132–153. Wilson, B.H., Dincer, T., 1976. An introduction to the hydrology and hydrography of the Okavango Delta. In: Proceedings of the Symposium on the Okavango Delta and its Future Utilisation. Botswana Society, Gaborone, pp. 33–48.