Late Pleistocene-Holocene vegetation and climate change in the Middle Kalahari, Lake Ngami, Botswana

Quaternary Science Reviews 171 (2017) 199e215

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Late Pleistocene-Holocene vegetation and climate change in the Middle Kalahari, Lake Ngami, Botswana Carlos E. Cordova a, *, Louis Scott b, Brian M. Chase c, Manuel Chevalier d a

Department of Geography, Oklahoma State University, Stillwater, OK 74078, USA Department of Plant Sciences, University of the Free State, Bloemfontein, South Africa c Centre National de Recherche Scientifique, UMR 5554, Institut des Sciences de l’Evolution de Montpellier, Universit
e Montpellier, Bat. 22, CC061, Place Eug ene Bataillon, 34095 Montpellier, Cedex 5, France d Institute of Earth Surface Dynamics, Geopolis, University of Lausanne, CH-1015 Lausanne, Switzerland b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 March 2017 Received in revised form 26 June 2017 Accepted 30 June 2017

Pollen, spores, and microscopic charcoal from a sediment core from Lake Ngami, in the Middle Kalahari, reflect paleovegetation and paleoclimatic conditions over the last 16,600 cal years BP. The location of Lake Ngami allows for the receipt of moisture sourced from the Indian and/or Atlantic oceans, which may have influenced local rainfall or long distance water transport via the Okavango system. We interpret results of statistical analyses of the pollen data as showing a complex, dynamic system wherein variability in tropical convective systems and local forcing mechanisms influence hydrological changes. Our reconstructions show three primary phases in the regional precipitation regime: 1) an early period of high but fluctuating summer rainfall under relatively cool conditions from ~16,600e12,500 cal BP, with reduced tree to herb and shrub ratio; 2) an episode of significantly reduced rainfall centered around c. 11,400 cal BP, characterized by an increase in xeric Asteraceae pollen, but persistent aquatic elements, suggesting less rainfall but cool conditions and lower evaporation that maintained water in the basin; and 3) a longer phase of high, but fluctuating rainfall from ~9000 cal BP to present with more woody savanna vegetation (Vachellia (Acacia) and Combretaceae). We propose a model to relate these changes to increased Indian Ocean-sourced moisture in the late Pleistocene due to a southerly position of the African rain belt, a northerly contraction of tropical systems that immediately followed the Younger Dryas, and a subsequent dominance of local insolation forcing, modulated by changes in the SE Atlantic basin. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Pollen Vegetation dynamics Kalahari Southern Africa Late Pleistocene Holocene Paleoclimates

1. Introduction Lake Ngami is located at the southern boundary of the Okavango Delta region in the Middle Kalahari of northern Botswana (Fig. 1), a region that lies at the modern juncture of the major tropical convergence systems of the Congo Air Boundary (CAB) and the Intertropical Convergence Zone (ITCZ) (Fig. 2), and is thus a key region for studying the long-term dynamics of southern tropical African climate systems. However, while it has been the focus of study for many years, the region's landscape and climate do not favor the preservation of organic material or long, continuous sedimentary sequences. As a result, most of the available data are from geological and geomorphological archives (Brook et al., 1992,

* Corresponding author. E-mail address: [email protected] (C.E. Cordova). http://dx.doi.org/10.1016/j.quascirev.2017.06.036 0277-3791/© 2017 Elsevier Ltd. All rights reserved.

1996; Burrough et al., 2007, Burrough and Thomas, 2008, 2009; Cooke, 1975; Heine, 1982; Huntsman-Mapila et al., 2006; Lancaster, 1981; O'Connor and Thomas, 1999; Ringrose et al., 2008; Shaw and Cooke, 1986; Shaw et al., 2003; Thomas and Shaw, 2002; Thomas et al., 2003). While these data highlight the complex evolution of the landscape during the late Quaternary, their significance as a coherent record of climate change has remained difficult to define (Burrough and Thomas, 2013; Chase, 2009; Thomas and Burrough, 2012). Although lacustrine deposits do exist in the Kalahari, most of them are either logistically difficult to study effectively, ephemeral, or are located in areas of the salt pans where wind erosion, exposure to alkaline conditions and the weather prevent the preservation of pollen. Abandoned channels filled with peat in the Okavango Delta are another source of pollen, but they tend to be shallow, and may reflect a dynamic sequence of depositional and erosive phases (Nash et al., 2006). Thus, long records of past

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Fig. 1. Location of Lake Ngami in the context of the Okavango Delta and Botswana.

vegetation change are largely absent from the region, with the only existing pollen records being a sequence of pollen from speleothems in Drotsky's Cave (Burney et al., 1994), and three discontinuous Holocene cores from alluvial wetlands in the Okavango Panhandle (Nash et al., 2006) (locations on Fig. 1). In this study, we present sediments from Lake Ngami, one of the region's larger water bodies, which have been shown to contain organic sediments dating back to the Last Glacial Maximum (~22,000 cal BP; Huntsman-Mapila et al., 2006). The basin of Lake Ngami has been the subject of several studies focusing primarily on the reconstruction of lake level changes across the late Quaternary (Burrough and Thomas, 2008; Endfield and Nash, 2002; McCarthy et al., 2000; Nash and Endfield, 2002; Hamandawana and Chanda, 2013; Huntsman-Mapila et al., 2006; Shaw, 1985a, 1985b; Shaw and Cooke, 1986; Shaw et al., 2003). Combined, this evidence suggests that considerable changes in lake level have occurred both in the recent past and the late Quaternary, with inferences that the lake may have at times covered as much as 2600 km2 (Burrough et al., 2007), but at times also dried out almost completely (Shaw et al., 2003). In general, such significant changes in lake levels would not normally indicate this as a favorable environment for pollen preservation. Nonetheless, previous studies have suggested that there is relatively good organic preservation in the Lake Ngami lacustrine sediments (Huntsman-Mapila et al., 2006) including pollen and spores (Oboh-Ikuenobe, 2005). Here, we present a pollen record spanning the past 16,600 cal years BP, including results of PCA analysis (Principal Component Analysis) and reconstructions of summer rainfall obtained using the CREST software package (Chevalier et al., 2014). As ancillary information,

this study presents and discusses the distribution of microscopic charcoal and coprophile spores (Sporormiella, Sordaria, and Podospora), which relates to local fire and herbivore incidence. 2. Lake Ngami and its environment 2.1. Climatic context Lake Ngami (20
28.8170 S; 22
45.650’ E) has a warm semi-arid climate with an average precipitation 450 mm/year concentrated in the austral summer (Fig. 2). Rainfall in the region is influenced by the seasonal migrations of tropical systems related to the African rain belt and convergence systems such as the ITCZ and the CAB (Fig. 2). The mean position and influence of these systems have fluctuated over time, and have acted in concert with changing conditions in the adjacent Indian and Atlantic oceans (Chase et al., 2010, 2015; Nicholson, 2000; Schefub et al., 2005). This has created a strong northeast-southwest rainfall gradient in the Kalahari, from around 600 mm/year along the Botswana-Namibia border along the Chobe River to less than 200 mm/year in southwestern Botswana (Fig. 2). The weather station of Sehitwa (931 m.a.s.l.), a village on the northern shore of Lake Ngami, registers a mean annual temperature of 22.3
C; and January and July mean annual temperatures of 26.7
C and 15.8
C, respectively. Frosts occur occasionally during the months of June, July and August, but are very rare (Cole and Brown, 1976). Dominant winds are normally from the east and southeast and vary in intensity throughout the year, with the strongest speeds at the beginning of the rainy season in October and November (Adringa, 1984).

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Fig. 2. (a) Summer rainfall patterns in southern Africa and location of Lake Ngami and the Cubango-Cuito Basin. (b) Climograph and basic meteorological data from Sehithwa, on the northern shore of Lake Ngami.

2.2. Geomorphologic and hydrologic contexts Lake Ngami is located at elevations between 919 m.a.s.l. (lake sump) and 932 m.a.s.l. (highest strandlines) (Burrough et al., 2007; Shaw et al., 2003). The lake occupies the tectonic depression that holds the Okavango Delta system (Fig. 1). Its southern end is limited by the escarpment formed by the Kunyere fault, which also controls the Kunyere River, the main feeder of the lake at present (Kinabo et al., 2007; McCarthy, 2013; Riedel et al., 2014; Shaw et al., 2003). East of lake Ngami, the Thamalakane fault controls the course of the Nghabe River, which drains the lake or captures its incoming waters directly from the Kunyere River (Riedel et al., 2014; Shaw, 1985a; Shaw et al., 2003). Depending on the amount of water in the system, the Nghabe River at times functions as feeder into the lake, and at times as an outlet into the Boteti River, which connects the system with the Makgadikgadi Pan system in central Botswana (Shaw, 1985a; Robbins et al., 2008, 2009). Lake Ngami water levels have varied considerably during the Holocene and late Pleistocene (Burrough et al., 2007; Huntsman-

Mapila et al., 2006; Murphy et al., 1998; Robbins et al., 2008, 2009; Shaw et al., 2003) and in historical times (Nash and Endfield, 2002; Endfield and Nash, 2002; Hamandawana and Chanda, 2013; Shaw, 1985b). These variations have generally been related to local and regional climatic influences, as well as the influx of water from Angola through the Okavango Delta (Burrough et al., 2007; McCarthy, 2013; Shaw, 1985b; Shaw et al., 2003; Wilson, 1973), although tectonic adjustments are also thought to have played a role in the amounts of water feeding into the lake (Moore et al., 2012; Nash and Eckardt, 2016) (Fig. 2). Today, the majority of water that comes into the lake arrives via the Kunyere River and occasionally by the Tamalakane-Nghabe River system (McCarthy and Ellery, 1998). This water originates in the basin of the Cuito-Cubango River system in Angola and flows to the lake towards the end of the rainy season. Today, local/regional precipitation constitutes about 20% of the amount of water coming into the lake annually (Shaw, 1985b). Historical data (Endfield and Nash, 2002; Nash and Endfield, 2002; Shaw, 1985b) and oral tradition (Hamandawana and

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Chanda, 2013; Robbins et al., 2009; Wilson, 1973) suggest a relatively large Lake Ngami from about 300 cal BP until the early decades of the 1800s. After this time, lake levels began to drop, reportedly because the flow of the Thaoge channel, emptying on the north-western part of Lake Ngami (Fig. 1), became erratic and diminished, before drying out completely by 1900 (Nash and Endfield, 2002; Shaw, 1985b; Wilson, 1973). During the 20th century, the lake basin remained dry, or with very low levels, except during particularly wet years (Hamandawana and Chanda, 2013; Robbins et al., 2009; Shaw, 1985b). In the present century, flooding in 2004 replenished part of the basin, but by 2007 the lake had been reduced to a very low level again. The situation changed when the delta flooded the basin again between 2009 and 2011, creating a relatively high level that was practically unseen for decades. Although waters have receded, as of August 2014, many fields and roads were still under water. These were extreme events that combine local and regional precipitation, with the water coming via the Okavango Delta. It is important to note that although these extreme events change the conditions of the lake and the vegetation, they are short-lived, and may not be reliable analogs for interpreting the centennial and millennial scale signals observed in this study. 2.3. Vegetation context In the broader context of African biomes, Lake Ngami is located on the arid margin of the Savanna Biome. The classification used in the Vegetation Map of Botswana (Bekker et al., 1991), shows several plant associations around the Lake Basin, including several types of savanna and riparian, and aquatic vegetation types (Fig. 3). The exposed areas of the Lake Ngami basin support a savanna type vegetation defined by Weare and Yalala (1971) as Lake Ngami Savanna, equivalent to type D12a in Bekker et al. (1991) (Fig. 3). This type of savanna is dominated by grasses, of which Cynodon dactylon (Bermuda grass) is the most important, and scattered Vachellia (i.e. Acacia) trees. This savanna vegetation occupies the low-lying areas formerly flooded during the highest levels. However, after the lake transgression of 2009e2011, most of this area flooded, drowning many of the acacias. Immediately north and west of the lake basin, associations of woodland and wooded savannas (A3a, A3b, and A3c associations in Fig. 3) dominate the environment with the general occurrence of trees such as Terminalia sericea, Lonchoracpus nelsii, and Vachellia erioloba, and more localized trees such as Burkea africana, Combretum zeyheri, Vachellia fleckii, Boscia albitrunca, Grewia flava, G. flavescens, Vachellia mellifera subsp. detnens, among others (Bekker et al., 1991; Weare and Yalala, 1971), and grasses such Aristida stipitata, Stipagrostis uniplumis, Anthephora pubescens (Weare and Yalala, 1971). Immediately south of the Lake Ngami basin, a different type of savanna exists (type B9a), which is also referred to as Ghanzi Bush Savanna (Weare and Yalala, 1971). Although it shares many species with the vegetation types A3a, A3b and A3c, it has several other dominant tree species such as Vachellia melifera, V. erioloba, and Terminalia prunoides. The Ghanzi Ridge (Fig. 1) is perhaps better described as a low plateau with some minor hills composed by folded and eroded sandstones and other rocks and capped in some places with calcrete. The different lithologies and geological structures of this area have created a complex mosaic of vegetation associations (Cole and Brown, 1976), but Kalahari sands have mantled the calcrete and other formations in some areas. The vegetation types to the east of Lake Ngami vary depending on substrate and water availability. The most common is the riparian to aquatic vegetation of the southwestern margin of the Okavango Delta and the Kunyere River (types F14c and F15a, Fig. 3).

It contains several trees such as Combretum imberbe, Vachellia spp. and Colophospermum mopane (mopane), and a variety of grasses. Further east and along the southern edge of the Okavango Delta extends the savanna dominated by Colophospermum mopane (types H17d and H17e, Fig. 3). This association is referred to also as the Ngamiland Tree Savanna (Weare and Yalala, 1971) and is also found in some drier parts of the delta, particularly in the largest island (Chief's Island) and around the Okavango Delta margins. In this type of savanna, in addition to the dominant Colophospermum mopane, other tree species include Vachellia erioloba, Grewia flava, Terminalia prunoides, Ziziphus mucronata and Combretum mosambicense. The Okavango Delta wetlands (types I20a and I21b, Fig. 3) include a large number of trees and herbs, most of which are adapted to moisture. The islands are characterized by Hyphaene petersiana along with several trees adapted to more mesic conditions (e.g., Sclerocarya birrea) or riparian environments (e.g. Ficus verruculosa). A number of graminoids occupy the channels, particularly Cyperus papyrus, and grasses such as Miscanthus junceus, Phragmites australis, and Imperata cylindrica among others (Heath and Heath, 2009; Roodt, 2011). The Angolan Highlands, which serves as the headwaters of the Cuito-Cubango River system, is characterized by Miombo woodland vegetation (Huntley, 1974). Under the region's relatively high rainfall (800e1400 mm/yr), and the vegetation is dominated by trees of the Fabaceae family, particularly Brachystegia, Julbernardia, and Isoberlinia (Huntley, 1974; Frost, 1996). 3. Materials and methods 3.1. Coring and sub-sampling The core was taken from the north-central part of Lake Ngami, southwest of Sehithwa village (Fig. 4), in October 2007 using a Livingstone piston corer during one of the historical lows of the lake. This low lake stand allowed access by foot to the lower parts of the lake basin. Prior to coring, the top 20 cm of sediment was dug out and discarded because it contained large amounts of loose plant material mixed with unconsolidated sediments. The top of the core, at 20 cm depth from the modern surface, is hereafter referred to as 0 cm. Sub-sampling was undertaken in the lab at 5 cm intervals (Fig. 5). Each subsample was split into separate aliquots for the analysis of pollen and spores, and charred particles (microscopic charcoal), grain size analysis, organic and inorganic carbon, and magnetic susceptibility. 3.2. Chronology Nine bulk organic sediment samples were taken for AMS radiocarbon dating (Fig. 5). These analyses were undertaken at the Arizona Radiocarbon Lab (Table 1). The calibration and age models (Table 1; Fig. 6) were carried out using the SHCal13 calibration data (Hogg et al., 2013) and Bacon 2.2 software (Blaauw and Christen, 2011), with the assumption, based on a radiocarbon age at 60 cm of 214 ± 41 14C yr BP, that the core top sediment was of broadly modern age. The top age at 40 cm of 3359 ± 39 14C yr BP was not used in the age model because it was assumed to be contaminated with older organics, probably through re-deposition of organic matter and animal borrowing. 3.3. Pollen, spores and charcoal processing The microfossil samples were processed and analyzed at the Applied Geoarchaeology and Paleoecology Lab at Oklahoma State University. The procedure for pollen extraction consisted of the

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Fig. 3. Vegetation associations. Modified from Bekker et al. (1991).

conventional use of HCl for removing carbonates, acetolysis for excess organics, and heavy liquid using sodium polytungstate at density 2.0 for separating pollen grains. Pollen determination and counting was carried out using the reference collection at the University of the Free State, Bloemfontein, South Africa. Pollen diagrams were plotted using Tilia Graph v. 2.0 using the age-depth model described above. CONISS cluster analysis using the application in Tilia Graph was used as a general guide for the vertical zoning of the pollen sequence. Coprophile spores were determined and counted on the same slides prepared for pollen. They are proxies to determine herbivore incidence, as these spores thrive in dung (Richardson, 2001). Their identification in this study follows the references of ascomycete spores by Gelorini et al. (2011) and van Geel et al. (2011). As suggested by several studies, not all the ascomycete spores are

uniquely associated with dung (Graf and Chmura, 2006; Richardson, 2001; van Geel et al., 2011). The most closely associated with dung were Sporormiella, Podospora and Sordaria. The rest of the species were put together with other non-coprophile fungal spores under the category “other ascospores.” Microscopic charred particles (i.e., microscopic charcoal) were recorded on microscope slides by area and estimated in relation to sediment weight, i.e. mm/gram of sediment. The dataset was originally split into two groups: <125 mm and 125e500 mm, but the latter having limited quantities, was not presented in the graphs. 3.4. Sedimentological data The sedimentological data obtained include magnetic susceptibility, total organic and inorganic carbon, and particle size data

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Fig. 4. Superimposed Google Earth images showing the location of the LN-16 core and the fluctuations of the lake between 2007 (low lake level) and 2013 (high lake level).

Fig. 5. Core LN-16, lithology and radiocarbon dates.

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Table 1 Radiocarbon age determination and calibration information. Lab Id

d13C

14C age BP

Error

Depth

95.4% (2s) cal age ranges (cal BP)

Relative area under distribution

AA86266

23.3

3359

39

60

AA90128

17

214

41

80

AA86267

21

3191

39

105

AA86268 AA95805

18.9 18.4

6291 7859

61 55

135 145

AA98420 AA98421 AA90129 AA86269

19.6 18.1 17 19

7556 11244 11274 13580

62 60 66 220

150 190 205 225

3450e3640 3671e3678 2e27 58e117 135e234 237e303 3246e3312 3316e3454 6989e7305 8429e8776 8836e8857 8925e8025 8191e8412 12897e13208 12967e13269 15738e17000

0.940 0.010 0.064 0.122 0.514 0.249 0.193 0.756 0.950 0.938 0.011 0.001 0.950 0.950 0.950 0.950

category (125e250 mm). Although some samples had gravel, none of the grains were larger than 2.5 mm in diameter. 3.5. Pollen data analysis

Fig. 6. Age-depth models for the Lake Ngami core calculated using the Bacon 2.2 software package (Blaauw and Christen, 2011).

(Fig. 7). High frequency magnetic susceptibility (chf) was measured on dry samples using a Bartington MS2 magnetic susceptibility meter with an MS2B dual-frequency sensor (e.g., Ellwood et al., 2008). The total carbon (TC) was measured by combustion at 950
C and the total inorganic carbon (TIC) as determined by acidification with 2 M HClO4. The total organic carbon (TOC) was determined by subtracting the TIC from the TC. The TC and TOC measurements were made using a CM5014 Coulometer (UIC Inc.). The standard deviation of replicates was between ±0.03 w. % and ±0.1 wt % for calcium carbonate standards and samples, respectively. Particle size distribution was obtained using wet sieving to separate sands from fines (<63 mm) and from gravel (>2 mm). The fine fraction was not separated to differentiate clay and silt. Fines generally composed at least 80% of the total weight (Fig. 7). The sand fractions were separated into different categories, i.e., very fine þ fine, medium, coarse and very coarse. The very-fine sand fraction (63e125 mm) category was merged with the fine sand

3.5.1. Principal components analysis Principal Components Analysis (PCA) was used to determine trends of terrestrial vegetation change in the area surrounding Lake Ngami. PCA summarizes all the information contained in a dataset into a reduced number of orthogonal axes of maximum variance (e.g. Legendre and Legendre, 2012), which can then be interpreted in terms of environmental change by analyzing the relative loadings of the different taxa (e.g. Scott, 1999; Scott et al., 2003; 2012). We used the PAST software (Hammer and Parker, 2006) to run our PCAs on a correlation matrix. To extract a regional signal, we only used taxa related to the vegetation of the Lake Ngami region, while aquatic elements, which may relate to hydrologic conditions in the Okavango system itself, as well as taxa with overproduction and widespread dispersal were not included (Fig. 8). A separate second PCA was done to include only these local elements. While we recognize that local and regional elements cannot be distinguished with absolute certainty, we believe that the selection of taxa can be broadly attributed to each category on the basis of where each taxon is most usually found. Only a single grain of Brachystegia e the dominant taxon of the Miombo woodlands of the Angolan Highlands - was found in the whole of the sequence, suggesting that the identified assemblages have not been significantly influenced by input from this extra-regional source. 3.5.2. CREST The CREST method provides the opportunity to obtain quantitative climate reconstructions of specific climatic parameters from pollen data (Chevalier et al., 2014; Chevalier and Chase, 2015, 2016). To do so, it uses modern plant distributions to create probability density functions (pdfs) that represent the probabilistic link between pollen and climate. The combination of these pdfs allows the quantification of different climatic and/or environmental parameters along with the complete distribution of their uncertainties. The ensemble of pollen types observed at Lake Ngami suggested strong sensitivity to summer precipitation amount, and we thus chose to reconstruct the precipitation of the wettest quarter (PWetQ, Hijmans et al., 2005). As with the PCAs, pollen types that could be primarily associated with inflow from the Okavango system were excluded. To improve the accuracy of pollen-based climate reconstructions, it is usually recommended to reduce the ensemble of observed pollen types to a subset of PWetQ-sensitive indicators

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Fig. 7. Core LN-16, total organic carbon, magnetic susceptibility, particle size distribution, percent of pollen deterioration.

(Chevalier and Chase, 2015; Juggins et al., 2015). Therefore, we only used the following taxa for our analysis: Acalypha, Aizoaceae, Bauhinia, Bignoniaceae, Capparaceae, Cardiospermum, Cassia, Cissus, Colophospermum, Combretaceae, Euphorbiaceae, Grewia, Ipomoea, Lentibulariaceae, Nyctaginaceae, Onagraceae, Oxygonum, Rhamnaceae, Rhynchosia, Syzygium, Tarchonanthus, Vachellia (Acacia) and Vernonia. To improve the signal over noise ratio of the reconstructed curve, we also used a Monte-Carlo framework that integrates and combine climatic and temporal uncertainties into a more robust interpolated curve by randomly generating a large number (n ¼ 500,000) of alternative scenarios. More details about this methodology can be found in Chevalier and Chase (2015, 2016). 4. Results 4.1. Chronology, sedimentation rates, and age reversal The age-depth model (Fig. 6) suggests that sedimentation rates were fairly constant throughout the sequence, particularly below 80 cm (~220 cal BP), where sedimentation rates are for the most part between 0.05 and 0.1 mm/year. Accumulation rates increase to 0.9 mm/year above 80 cm and above that they fluctuate between 3 and 4 mm/year.

The reversal of the AMS date at 60 cm (AA86266, 3559 ± 39 14C cal BP) presents a marked stratigraphical inconsistency (Fig. 5), possibly reflecting the reworking of older organic sediment from higher up in the drainage system, as high total organic carbon and magnetic susceptibility values in relation to the rest of the profile (see 4.2) suggest a rapid translocation of sediment at this time (Fig. 7). 4.2. Core stratigraphy and properties The lithology of the core is variable, but consists mainly of silt and clay with abundant diatoms, particularly in the lower half, and organics in the upper half (Fig. 7). Throughout the section, finesediments dominate. Of the sand fraction, the very-fine to fine sand fraction is the most prominent. Peaks of coarse sand and gravel occur at 75e85 and 110 cm, probably coinciding with pulses sedimentation from a nearby river. At present, the mouth of the nearest river, the Nghabe River, is located about 10 km to the east (Fig. 1), but at lower lake levels the mouth of the Nghabe would have been closer to the site. Increases in gravel do not correlate with increases in sand, suggesting that the fine sand, characterized by highly rounded grains, has an aeolian source. Magnetic susceptibility also increases considerably above 110 cm, peaking at ~85 cm (Fig. 7) and together with TOC coincides

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Fig. 8. Pollen summaries and zones with PCA scores curve.

with the influx of coarse sands that suggest they originated from erosion. The presumable peak of magnetic susceptibility coincides with a peak of coarser sand and some small gravel, suggesting perhaps a rapid discharge of a nearby stream. 4.3. Pollen and spores Significant variability is observed in the pollen record, particularly in Amaranthaceae, Poaceae and the aquatic groups (Fig. 8). Tree pollen frequencies, in contrast, are generally lower, and less variable. Considered broadly, the percentages of herbs and shrubs are prominent from ~16,600 to 9100 cal BP. Their decline is marked by an increase in Amaranthaceae pollen, lasting until ~4600 cal BP, and a then a subsequent increase in arboreal and Poaceae pollen for the remainder of the record. 4.3.1. Pollen zone N6 (16,600e14,800 cal BP) The lowest part of the record is primarily characterized by the scarcity or absence of many of the tree taxa that characterize the modern environment, such as Combretaceae and Fabaceae (e.g., Bauhinia, Burkea, and Pterocarpus) (Fig. 9). In contrast, many Euphorbiaceae (including Acalypha, Allophylus and Phyllanthus) constitute most of the woody vegetation pollen, although in relatively low numbers compared to herbaceous and aquatic pollen. The only appearance of Hyphaene pollen in the entire record appears towards the top of this zone. Podocarpus appears in this zone, but given its potential for long-distance transport, it may not have existed locally. In terms of non-Poaceae herbaceous pollen, zone N6 is characterized by Solanaceae, Asteraceae, and Amaranthaceae, among other taxa (Fig. 10). However, the most distinctive characteristic of the herbs-shrubs vegetation is the presence of Stoebe-type pollen, which is associated with cool climates, and does not currently grow

in Botswana. Practically all types of aquatics are present, and conspicuously the amount of fungal spores is high (Fig. 11). 4.3.2. Pollen zone N5 (14,800 to 12,200 cal BP) The second section of the Ngami record is characterized by an increase in Amaranthaceae pollen at the expense of all other pollen groups (Figs. 8 and 10). Although there is a reduction of tree pollen, some taxa such as Combretaceae, Burkea, Capparidaceae, Vachellia (Acacia), and Tharchonantheae appear in some samples (Fig. 9). Rhamnaceae, which most likely represents the locally abundant Ziziphus mucronata, remains very high, constituting one of the highest frequencies among the woody pollen taxa. The shrub-andherb vegetation in zone N5 is dominated by the Amaranthaceae, which indicates evaporative lake floor/margin conditions that are supported by the appearance xeric taxa such as Tribulus and Nyctaginaceae. The latter is a group that, although rare today in the area, occupies exposed rock, sand and loams in Namibia and elsewhere Botswana (Struwig et al., 2011). Despite the overall reduction of aquatic pollen, this zone has representation of the same taxa of the previous zone. Cyperaceae and other aquatics such as Juncaceae and Persicaria occur together with halophytic Amaranthaceae. 4.3.3. Pollen zone N4 (12,200e7400 cal BP) At the bottom of pollen zone N4 Stoebe-type pollen appears for the last time. The zone shows a decline of Amaranthaceae, and the increase of grasses and aquatics, especially Typhaceae early in the zone (Fig. 8). The frequency of tree pollen increased a little in the middle of the zone, but generally remains low, while the frequencies of shrubs and herbs remain the same as in the previous zone. The tree pollen shows a relatively significant change evident in the switch from the dominance of Rhamanceae pollen to a codominance of Combretaceae, Euclea, Vachellia (Acacia), and Dichrostachys (Fig. 11). Tarchonantheae as in the previous zone is

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Fig. 9. Arboreal taxa pollen spectra.

Fig. 10. Terrestrial non-arboreal pollen spectra.

present in this zone. An unusual characteristic of this zone is the only appearance of Proteaceae pollen in the core. Today, only one species of Proteaceae, Protea gaguedi, is reported in northwestern Botswana. The other nearest South African type that this pollen may represent is Faurea saligna (Coates-Palgrave, 2005; Scott 1982). Protea gaguedi is the most tolerant of warm conditions, spreading across in tropical Africa, and but absent in the Fynbos Biome where most Proteaceae are found (Beard, 1958; Coates-Palgrave, 2005).

The herb and shrub vegetation is still dominated by grasses, several Asteraceae and Amaranthaceae, although the frequencies of latter vary from 0 to 40% (Fig. 10). Tribulus and Nyctaginaceae are still persistent throughout the zone. The aquatics become dominated by Typhaceae, although many other taxa are still present at moderate frequencies (Fig. 11). The frequencies of spores in general remain similar to those of the previous zone. At this time, mycorrhizal hyphae appear in the record.

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Fig. 11. Aquatic and semi-aquatic pollen and spores and fungal spectra.

4.3.4. Pollen zone N3 (7400e4200 cal BP) Zone N3 is generally marked by the increase in Amaranthaceae at the expense of other groups, particularly the grasses (Fig. 10). The aquatics are still prominent at the beginning of the zone and decrease considerably towards its end. Despite the presence of some xeric elements, several tree species of relatively humid environments appear or increase, as is the case of Burkea, Combretaceae, Pterocarpus, Celastraceae, and Colophospermum mopane, which in very small amounts has its only appearance in the entire core (Fig. 9). Among the shrubs and herb taxa, despite the overwhelming dominance of the Amaranthaceae, many taxa remain present in the environment, particularly Asteraceae (Fig. 10). Tribulus and Nyctaginaceae reach very low frequencies and disappear from the record. The aquatics are again dominated by Cyperaceae followed by Typhaceae and Persicaria (Fig. 11). No major changes occur in the spores, except for a slight reduction of inaperturate spores.

4.3.6. Pollen zone N1 (1100 cal BP to present) Pollen zone N1, which extends from 1100 to the present, sees a continuation of many of the general traits of the previous zone, but with changes in taxonomic composition. Amaranthaceae pollen increases slightly at the expense of grasses while the other groups remain practically constant (Fig. 9). The tree pollen composition and frequencies vary little from the previous zones, but Burkea, Morella, and Sizygium disappear and Vachellia (Acacia) increases considerably particularly to the top of the zone (Fig. 11). In addition to the reduction of grasses and increase in Amaranthaceae, the herb and shrub taxa, sees some changes in composition. The Solanaceae, Oxygonum, and Vernonia become important, and new species in the varia group increase (Fig. 10). The aquatics are in general decline, but some increase again to the top, particularly Cyperaceae, Juncaceae, Nymphaeae, Ponteridaceae and Haloragaceae (Fig. 11). The most conspicuous change in the spores is the unprecedented increase in all fungal spores including Epicoccum and Sporormiella.

4.3.5. Pollen zone N2 (4200e1100 cal BP) Pollen zone N2 marks a conspicuous change in the pollen record as the tree and grass pollen frequencies increase considerably at the expense of Amaranthaceae (Fig. 10). The aquatics increase, and overall the record becomes more diverse. The dominant tree taxa in this zone are Burkea, Combretaceae, Grewia, Pterocarpus and Euclea. Other prominent taxa are Rhamnaceae, Celastracaeae, and Dichrostachys. Morella and Sizygium occur in low numbers representing riparian vegetation. The shrubs and herbs taxa show little change in N2 (Fig. 10). The aquatics are at first dominated by Cyperaceae and Typhaceae, but the latter declines quickly in favor of a variety of other taxa. The fungal spores, particularly Epicoccum, begin to increase in the middle of this zone.

4.4. Principal Component Analysis (PCA) The first principal component (PCA1, 18.47% variance) for the regional elements (Fig. 8) cannot be clearly interpreted in terms of specific climate variables, due to different sensitivities of taxa moisture and temperature that makes it difficult to distinguish between these parameters. The contrast between loadings seems, however, to predominantly reflect temperature, with positive values for warm climate trees (e.g. Vachellia (Acacia), Burkea, Capparidaceae, Celastraceae, Combretaceae, Diospyros, Euclea, Grewia, and Pterocarpus) against the negative values for smaller more hardy plants like Asteraceae (including Stoebe type) and Tribulus that are associated with cooler and/or drier conditions (Fig. 12a). With the exception of an early peak around 14,000 cal BP, the fluctuating

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Fig. 12. a) PCA1 loadings for selected regional and local pollen taxa; b) PCA1 model for selected taxa; c) 10e90% climatic range estimated from the probability density functions (pdfs) of the summer-rainfall sensitive taxa; d) CREST-based reconstruction of precipitation of the wettest quarter.

PCA1 curve is negative in the earlier phases lasting until c. 11,400 cal BP but gradually become more positive from the early Holocene onwards, with a first peak at ~9000 cal BP and maximum values later between 4000 and 2000 cal BP (Fig. 12b). For the local elements, the PCA1 (22.24% variance) (Fig. 12a) suggests a contrast between the most prominent taxa, Amaranthaceae (halophytes associated with dry lake floor) (negative) and Cyperaceae and Typhaceae (semi-aquatic and/or aquatic) (positive). The significance of this, however, is difficult to attribute to a simple variable such as lake level as their presence may be related other factors such as seasonality of rainfall, potential evapotranspiration, rather than depth, of the water body. 4.5. CREST-based reconstruction of precipitation of the wettest quarter Our reconstruction of wet season (austral summer) precipitation using the CREST method (Chevalier et al., 2014) indicates three primary phases in the regional precipitation regime. From the beginning of the record at ~16,600 cal BP until 12,500 cal BP rainfall was relatively high, similar to present day, as indicated by the presence/abundance of Acalypha, Asteraceae (Vernonia and Pluchea types) and Olacaceae (Fig. 12c). The end of this period is marked by a presence of Nyctaginaceae and Tribulus, indicating a strong decline in precipitation to approximately half that received in the terminal Pleistocene. This episode of reduced rainfall is defined by a single sample, so it robustness and duration are difficult to assess, but our reconstruction indicates that by 10,000 cal BP a new phase of high/increasing rainfall was established (Fig. 12d). Initially defined by Pluchea, Croton-type, Diochrostachys and Combretaceae, its later phases are determined by percentages of Grewia and Cassia, and the absence/disappearance of any truly xeric elements. This period of higher rainfall persisted for the remainder of the sequence. While these changes may reflect conditions in the broader Okavango system, including its remote headwaters, the

vegetation of the Angolan Highlands is Miombo woodland, dominated by Brachystegia. If there was significant pollen input from the headwaters, the expectation would be that Brachystegia pollen would be generally abundant, or at least present, whereas it is absent from the record, with the exception of one sample at 5200 cal BP. 4.6. Paleofire and paleograzing The concentration of microscopic charred particles shows that the incidence of fires through the period encompassed by the core has varied considerably showing peaks at different times (Fig. 13). The first prominent peak, and afterwards the concentration remains low until around 3400 cal BP, where the most prominent peak of the sequence occurs. Afterwards, the concentration of microscopic charred particles suggests that fire incidence increased again during the last 2000 years. The percent of coprophile spores varies throughout the section with an almost total absence in the early Holocene (N4; Fig. 13). Sporormiella is the best represented of the spore types, but variations in its abundance are similar to the other ascospores. All the coprophile spores and ascospores become significantly more prominent in the last 2000 years. 5. Discussion 5.1. General reconstruction and correlation with regional paleorecords A fundamental consideration is the source of the pollen identified in the Lake Ngami core. Considering that a significant proportion of the water entering the basin is sourced in the Angolan Highlands, and flows through the Okavango Delta to Lake Ngami (McCarthy and Ellery, 1998), the potential exists for a larger and more complex source region for the pollen. While we can't exclude

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Fig. 13. Microscopic charcoal concentration by area on slide per gram of sediment and coprophile spores as a percent of all pollen and spores.

the possibility of such long distance fluvial transport, dominant elements of Angolan Highland (e.g. Brachystegia) and Okavango Delta vegetation (e.g. Hyphaene) are almost entirely absent in the Lake Ngami pollen record. Thus, while the record presented here may reflect conditions in the northern Okavango River Basin generally, it is more likely that the signal is dominated by local and regional pollen sources. Considering the two pollen-based reconstructions and the charred particle proxy for paleofires from Lake Ngami, the period between 16,600 to about 12,500 cal BP appears to have been generally humid, with relatively high rainfall (CREST reconstruction) under perhaps cooler conditions (regional pollen PCA1) (Figs. 12 and 14). During this time, fire incidence (charred particles) is low (Figs. 13 and 14). Interpreted as primarily reflecting changes in temperature, the Lake Ngami regional pollen PCA1 curve shows higher values between 14,500 and 13,500 cal BP. While it may reflect warmer conditions relating to the Bølling-Allerød interstadial, the resolution of the record is insufficient to draw any robust comparisons. Lake levels are generally high across this period (Burrough et al., 2007), consistent with our reconstruction of high rainfall under cooler conditions during the late Pleistocene. Records of dune activity suggest relative stability during this period, with the exception of the 14e15 ka period when activity is recorded in both the northern (O'Connor and Thomas, 1999) and northeastern (Stokes et al., 1997; Munyikwa et al., 2000) Kalahari dunefields. The period from 12,500 to 10,000 cal BP is marked by a significant decrease in rainfall, and an absence of shoreline ages between 12.4 ± 1.4 ka and 10.5 ± 1.0 ka, suggesting a substantial decline in lake levels (Burrough et al., 2007). A concurrent increase in aquatic pollen during this phase likely reflects shallow lake conditions (local pollen PCA1). A general absence of dune ages from this period runs counter to interpretations that dune ages are a direct proxy for aridity (cf. Chase et al., 2009; Stokes et al., 1997). The early Holocene sees a marked recovery in precipitation (Fig. 14d) and a commensurate increase in Lake Ngami shoreline

ages (Fig. 14e, Burrough et al., 2007). This onset of wetter conditions may also be registered in the Okavango Delta, as inferred from the increased accumulation of organic material from 9000 cal BP (Nash et al., 2006). The remainder of the Holocene exhibits generally increasing, but variable precipitation. Lake shoreline ages are consistent with these data, with high numbers of Holocene ages, but again with significant variability. The resolution of both the pollen and shoreline age datasets does not allow for a detailed comparison, but the timing of increased precipitation and high lake levels is not inconsistent. Compared with the Okavango Delta pollen record of Nash et al. (2006), clear similarities are not immediately evident. Whereas progressively drier conditions are inferred in the delta from 7000 to 4000 cal BP, the precipitation reconstruction and lake shoreline data from Lake Ngami indicates more humid conditions. It may be that the delta is responding more strongly to conditions in the Angolan Highlands, and reflecting trends observed in the Congo Basin record (Schefub et al., 2005), but the nature of the Okavango sequences make direct comparison difficult. From 4000 cal BP, the Lake Ngami record indicates relatively high rainfall, in accordance with the more humid conditions that have been inferred from the Okavango records (Nash et al., 2006). This trend is similar to the regional PCA1 curve, suggesting higher rainfall under warmer conditions (Fig. 14c). Charcoal concentrations indicate a peak in burning at this time (Fig. 14b), perhaps resulting from an increase in plant biomass, and are consistent also with the Drotsky's Cave pollen record, which shows an increase in arboreal taxa (Burney et al., 1994). For the period that the Ngami and Drotsky's Cave pollen records overlap, strong similarities are apparent, particularly in the abundance of Combretaceae pollen, the most common tree taxa in the region. The advent of pastoralism in the region according to the archaeological records occurs around 2000 years BP (Denbow and Wilmsen 1986; Robbins et al., 2008). In the Lake Ngami record this event is marked by a relatively high concentration of microscopic charred particles and high percentages of coprophile spores

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geoarchaeological research (Murphy et al., 1998; Robbins et al., 2008). 5.2. Paleoclimatic implications of the Lake Ngami pollen record

Fig. 14. a) Summer (21 Nov - 20 Jan) insolation at latitude 20
S (Laskar et al., 2004); b) Lake Ngami charred particles concentration; c) regional and local (dashed) PCA1 curves; d) Lake Ngami wet season precipitation; e) Lake Ngami shoreline ages (Burrough et al., 2007); f) linear dunes ages in the northern (O'Connor and Thomas, 1999) and northeastern Kalahari (Stokes et al., 1997; Munyikwa et al., 2000). The Younger Dryas (YD) and Heinrich Stadial 1 (HS1) chronozones are shaded.

(Fig. 13). The CREST-based precipitation reconstruction suggests relatively higher precipitation compared to the present, and Burrough et al. (2007) also suggested that lake levels were apparently higher during the 2000-1000 cal BP period (Fig. 14), a matter that has been asserted through archaeological and

While the data and reconstructions presented here do provide a degree of support and clarity to the aggregate record of regional environmental change, the patterns observed are not readily attributed to any of the interpretive models that have been proposed for the region (cf. Burrough and Thomas, 2013; Chase and Meadows, 2007; Gasse et al., 2008; van Zinderen Bakker, 1976). As mentioned, three major phases are evident in the reconstruction of wet-season precipitation based on the Lake Ngami pollen data: 1) an early period of high rainfall from ~16,600e12,500 cal BP, 2) an episode of significantly reduced rainfall from ~12,500e10,000 cal BP, and 3) a subsequent period of high/increasing rainfall from ~10,000 cal BP to the present day. Considered in their broadest context, the Lake Ngami records indicate changes in temperature that reflect global conditions (regional PCA1; Fig. 14c), and that changes in summer precipitation are not inconsistent with local insolation forcing (Fig. 14d, a). However, examining the Lake Ngami records in more detail in their subtropical African context, similarities during these distinct phases in wet-season precipitation may be observed in records from both the Zambezi Basin (Schefub et al., 2011; Wang et al., 2013) and the Congo Basin (Schefub et al., 2005) as regional climates evolved since the late Pleistocene (Fig. 15). These similarities are consistent with the location of Lake Ngami just south of the modern austral summer position of the Congo Air Boundary (CAB) and to the southwest of the southernmost limit of the ITCZ, which is the core of the southern African monsoon region (Fig. 1). During the late Pleistocene, evidence from marine sediment cores off the Zambezi River mouth indicates that rainfall in the catchment was relatively high during periods of Northern Hemisphere cooling, particularly Heinrich stadial 1 (HS1) and the Younger Dryas (Schefub et al., 2011; Wang et al., 2013). This supports early propositions of a southward migration tropical systems such as the ITCZ (Johnson et al., 2002), and, within the context of the eastern African margin as a whole, highlights a zone of anomalously high rainfall when much of the rest of the continent was experiencing drier conditions (Chevalier et al., 2017; Chevalier and Chase, 2015; Di Nezio et al., 2016). The limited spatial resolution of the data available makes it difficult to fully assess the extent of this high-rainfall zone, but the evidence from Lake Ngami suggest that it may have extended far westward into the continental interior towards the Middle Kalahari. This may have been associated with a concomitant late Pleistocene shift sometime between 22 and 15 ka in the CAB, resulting in the relatively humid conditions in Angola, as suggested by the high Podocarpus pollen percentages observed in marine sediments off the coast (Dupont et al., 2008), or this drought-sensitive taxon may have been responding to lower potential evapotranspiration under cooler conditions (Chevalier and Chase, 2016). The mechanism driving the second phase in the evolution of wet-season precipitation is more difficult to identify. Whereas the high-resolution Zambezi record of Schefub et al. (2011) indicates a period of higher rainfall from 12,700e11,700 cal BP, coincident with the Younger Dryas chronozone, precipitation at Lake Ngami appears to decline from ~12,500 cal BP (Fig. 15). While this may relate to limitations imposed by the resolution and chronologic control of the record, it may also reflect the diminished impact of Younger Dryas cooling on the global climate system relative to the stronger HS1 cooling, as has been shown in other southern African records (Chase et al., 2015; Scott, 2016). The position of Lake Ngami may have placed it at the southern/western margin of increased HS1

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at Lake Ngami. For the remainder of the Holocene, the Lake Ngami record appears to be primarily driven by a combination of direct insolation forcing (resulting in a general long-term increase in precipitation) and more complex source-driven circulation dynamics. While the Zambezi data indicate that the early Holocene was particularly arid (Schefub et al., 2011, Fig. 15), a strong, rapid increase in precipitation from 11,500 cal BP to 9500 cal BP shows marked similarities with changes in the Congo Basin record, as do periods of increased humidity centered at approximately 9000, 7000 and 4000 cal BP (Fig. 16). In comparing these records, however, there is a clear difference in the overall Holocene trends in precipitation. Whereas the Congo Basin record shows a significant multi-millennial decrease in precipitation, the Lake Ngami record indicates a gradual increase over the same period (Fig. 16). Records from the subtropical Southeast Atlantic (Kim et al., 2003; Farmer et al., 2005) and the adjacent continental margin (Chase et al., 2009, 2010; Lim et al., 2016) indicate increasing southeast trade wind strength across the Holocene, and commensurate aridification as a result of increased atmospheric subsidence, and the blocking of moisturebearing systems, similar to the trends observed for the Congo Basin (Schefub et al., 2005). At Lake Ngami, in the continental interior, the impact of these dynamics is clearly muted, if not absent. Instead, as highlighted in southeast Africa (Schefub et al., 2011; Chevalier and Chase, 2015), as Northern Hemisphere ice sheets and their influence on low latitude circulation systems diminished, direct local insolation became a dominant driver of southern tropical climate regimes. The progressive multi-millennial scale increase in precipitation observed at Lake Ngami is consistent with this mechanism, while the coincidence in timing of millennial-scale episodes of increased rainfall in the Congo Basin and Lake Ngami records suggests that Atlantic climate dynamics still played an important, if secondary, role in determining the climate of the continental interior. 6. Conclusion

Fig. 15. a) Summer (21 Nov - 20 Jan) insolation at latitude 20
S (Laskar et al., 2004); b) Congo Basin dD record (Schefub et al., 2005); c) Lake Ngami wet season precipitation d); Zambezi Basin dD record (Schefub et al., 2011). The Younger Dryas (YD) and Heinrich Stadial 1 (HS1) chronozones are shaded.

rainfall associated with shifts of the ITCZ, and a less significant displacement of austral circulation systems during this time may have restricted the high rainfall zone to the east, leaving Lake Ngami relatively dry. Without the direct influence of the ITCZ, it may be expected that conditions at Lake Ngami would be most strongly related to that part of southeast Africa south of the ITCZ, which is dominated by easterly moisture transport from the Indian Ocean. Records from this region, however, do not exhibit clear similarities with the Lake Ngami record, either in terms precipitation (Chevalier and Chase, 2015) or aridity (Chevalier and Chase, 2016). Rather, the Lake Ngami record correlates most strongly with hydrologic changes in the Congo Basin, reflecting variations in the transport of moist Atlantic air to the continental interior (Schefub et al., 2005). Here, increased meridional tropicalsubtropical sea surface temperature gradients in the southeast Atlantic are thought to have intensified the southeast trade winds (Lindzen and Nigam, 1987) between ~12,500 cal BP and ~10,000 cal BP, blocking moisture transport and resulting in a period of marked aridity. Either through a direct local decrease in precipitation, or through decreased precipitation in the Angola Highlands, this dynamic seems to have a significant impact on hydrologic conditions

Using the pollen data from the Lake Ngami core, a reconstruction of wet season (austral summer) precipitation indicates three primary phases in the regional precipitation regime: 1) an early period of high rainfall from ~16,600e12,500 cal BP, 2) an episode of significantly reduced rainfall from ~12,500e10,000 cal BP, and 3) a subsequent period of high/increasing rainfall from ~10,000 cal BP to the present day. Considered in their broadest regional context, the Ngami record indicates the potential importance of direct insolation forcing in the continental interior, but also exhibits a temporally-evolving relationship with records from both the Zambezi Basin (Schefub et al., 2011; Wang et al., 2013), and the Congo Basin (Schefub et al., 2005), and associated Indian and Atlantic Ocean moisture sources. Questions remain as to whether the observed signal reflects changes in the Lake Ngami region specifically, or more generally in the Okavango River Basin, but a more spatially extensive network of sites will be required to further refine our conclusions. Acknowledgements Jo van As and Liesl van AS (University of the Free State) gave LS logistical support during fieldwork through access to research facilities in the Okavango Delta. Paul Grobler, Deidre West, Whitey Jordaan, and Hanlie Groenewald and helped with coring. P. Huntsman-Mapila is thanked for exploratory field-work assistance to LS, who was supported by the National Research Foundation of South Africa. We also thank Dr. Eliot Atekwana and Nicole Paizis (Oklahoma State University) for help with processing samples for

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Fig. 16. a) First-order variability observed in the Lake Ngami wet season precipitation reconstruction (PWetQ) is compared with b) summer (21 Nov - 20 Jan) insolation at latitude 20
S (Laskar et al., 2004). Removing this first-order trend from the Lake Ngami reconstruction (c), the pattern of second-order variability (c, d) is compared with the pattern of second-order variability in the Congo Basin dD record (e; Schefub et al., 2005).

high frequency magnetic susceptibility and total organic carbon. Any opinion, finding, and conclusion or recommendation expressed in this material is that of the authors, and the NRF does not accept any liability in this regard. BMC and MC's involvement was supported in part by the European Research Council (ERC) under the European Union's Seventh Framework Programme (FP7/ 2007e2013)/ERC Starting Grant “HYRAX”, grant agreement no. 258657. The authors also acknowledge the support of the South African National Botanical Institute (SANBI) in sharing their botanical data. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.quascirev.2017.06.036 References Adringa, J., 1984. The climate of Botswana in histograms. Botsw. Notes Rec. 16, 117e125. Beard, J.S., 1958. The Protea species of the summer rainfall area of South Africa. Bothalia 7, 41e65. Bekker, R.P., De Wit, P.V., Fernando, K.S., Tumisang, D.M., Radcliffe, D.J., Mphathi, M., 1991. Vegetation Map of the Republic of Botswana 1:2,000,000. Food and Agriculture organization and Ministry of Agriculture of Botswana, Gaborone. Blaauw, M., Christen, J.A., 2011. Flexible paleoclimate ageedepth models using an autoregressive gamma process. Bayesian Anal. 6, 457e474.

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