Can phytoliths provide an insight into past vegetation of the Middle Kalahari palaeolakes during the late Quaternary?

Journal of Arid Environments 82 (2012) 156e164

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Can phytoliths provide an insight into past vegetation of the Middle Kalahari palaeolakes during the late Quaternary? S.L. Burrough a, *, E. Breman b, C. Dodd a a b

School of Geography and the Environment, University of Oxford, South Parks Road, Oxford OX13QY, UK Oxford Long-term Ecology Laboratory, Biodiversity Institute, Department of Zoology, Tinbergen Building, South Parks Road, Oxford OX1 3PS, UK

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 April 2011 Received in revised form 27 December 2011 Accepted 30 January 2012 Available online 24 February 2012

The Middle Kalahari is characterised by significant regional scale geomorphic activity and landscape change during the late Quaternary period. Very little however, is known about vegetation dynamics over this period due in part to the absence of well-preserved organic records. Here we test the application of phytolith analyses to sandy shoreline deposits of megalake Makgadikgadi, one of the sumps of the Okavango delta, routinely sampled and dated as part of a separate systematic geomorphological analysis. We confirm the presence of both an abundant and diverse assemblage of diagnostic phytoliths within these sand-dominated samples. The phytolith record reveals significant differences in the savanna vegetation through time with the composition of shoreline vegetation during different lake high-stand events was found to vary. Lake high stands are characterised by a coherently grassland dominated signal as well as a general trend towards more mesic and C3 prominent taxa during lake events after w40 ka. We suggest that phytolith analyses, whilst far from a perfect proxy, provide the potential to offer an important insight into long-term changes in Kalahari savanna, critical for understanding the response of regional vegetation to climatic and hydrological change both in the past and under future climate change scenarios. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Geoproxy Kalahari Ngami Okavango Phytoliths Quaternary Southern Africa Vegetation

1. Introduction Establishing the envelope of past environmental variability in dryland environments is a challenging undertaking due in part to the rarity of anaerobic lake and swamp deposits preserving organic material which, in other regions, has provided an insight into environmental dynamics on Quaternary timescales. In dryland environments, information on past environmental states is more typically found encoded in archives generally described as ‘geoproxies’ (Thomas and Burrough, 2012). Within the Kalahari basin, which extends across the interior of Southern Africa, a wide range of geoproxies are found including extensive fossilised dune-fields, speleothems, fluvial channels and palaeolake shorelines. These relict features preserve snapshots of extreme ‘wet’ and ‘dry’ conditions that suggest the amplitude of environmental oscillations in this region during the late Quaternary was much greater than that observed in the historical record. Here we report in full on the findings of a pilot study to investigate the potential of phytolith assemblages from sandy depositional geoproxies for providing

* Corresponding author. Tel.: þ44 7855214064. E-mail address: [email protected] (S.L. Burrough). 0140-1963/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.jaridenv.2012.01.018

a concurrent but independent proxy of vegetation in these environments at the time of sediment deposition.

1.1. Study area Central to the Middle Kalahari (Passarge, 1904) ecosystem of northern Botswana is the Okavango River, its associated alluvial fan (the Okavango Delta) and the terminal sumps of the hydrological system (the Ngami, Mababe and Makgadikgadi basins) (Fig. 1). This endohoreic wetland system forms the core of one of the largest wetland areas of international importance protected under the RAMSAR convention. The study region is characterised by savanna vegetation, where over 75% of all species are herbaceous plants, dominated by grasses (Poaceae) and sedges (Cyperaceae) (Ramberg et al., 2006). Permanently flooded areas within the Delta and the Panhandle support a variety of plant communities, including tall emergent communities dominated by Cyperus papyrus, Phragmites, Typha and Miscanthus junceus. In the seasonal swamps sedges such as Pycreus nitidus are common in regularly inundated areas and short grasslands, mainly composed of Imperata cylindrical (C4), dominate less frequently flooded areas. In addition to grasses, the alluvial sediments close to the flooded zone tend to be characterised by Colophospermum mopane whilst sandier regions like

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Fig. 1. Site locations and sample context from within the palaeolake Makgadikgadi system (inset shows regional location). 157

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those of the lake basin shorelines are more characteristically occupied by Acacia, Combretum and Terminalia species, and in the North, around the Mababe depression, by Baikaea plurijuga. Under contemporary climatic conditions, this semi-arid, deepsand environment is subject to a strongly seasonal rainfall regime, lying within the influence of the southern hemisphere summer rains which fall between November and March. Substantial interannual flood regime variability has a significant effect on ecological processes. Annual inflow ranges from 7000 to 15,000 million m3 and the area under flood has varied in recorded history between a dry season minimum of 4000 km2 and a wet season maximum of 13,000 km2 (including both the permanent and seasonal wetlands) (UNDP/FAO, 1977). The sedimentary deposits of the alluvial fan, however, cover an area of approximately 30,000 km2 (Gumbricht et al., 2004) suggesting the contemporary flooding regime does not capture the full magnitude of the envelope of hydrological variability in the past. Significant to interpreting palaeoenvironmental records associated with the wetland system, is the configuration of the hydrological system itself. In Northern Botswana, 95% of the Okavango’s flow is derived from a region constituting only 25% of the total catchment area, largely situated within Angola (Andersson et al., 2003) approximately 1000 km to the north. Each of the basins lying at the end of the system is delineated by sandy wave-built beach ridges prominent in STRM and satellite imagery. Historically these shorelines have been the focus of comment and investigations on past regional hydrological change (e.g. Livingstone, 1858; Grove, 1969; Cooke, 1984; Shaw, 1985; Shaw et al., 2003; Ringrose et al., 2005; Burrough and Thomas, 2009). The application of high resolution optically stimulated luminescence (OSL) dating to beach ridge sediments from the Ngami (Shaw et al., 2003; Burrough et al., 2007), Mababe (Burrough and Thomas, 2008) and Makgadikgadi basins (Burrough et al., 2009a, b) has allowed a detailed (though discontinuous) geomorphic record of shoreline accretion to be established both within each sub-basin and across the whole system (the latter referred to as ‘megalake’ events). These records suggest that past lake high stands were driven by a complex set of interacting global, regional and local forcing factors (Burrough et al., 2009b). Whilst such geoproxies provide a valuable (albeit discontinuous) chronological record of landscape dynamics, they offer little information on the nature, stability or dynamics of the ecological systems associated with the changing landscape. HadCM3 and BIOME4 model simulations suggest that vegetation is potentially strongly affected by past increases or decreases in surface water presence, and could itself play a key role in altering the local and regional climate via feedback mechanisms (Burrough et al., 2009b). Whilst recurrent megalake events within palaeolake Makgadikgadi (the lacustrine system associated with the Okavango Delta during the late Pleistocene) may have left a similar geomorphological signature within the landscape, their cause, evolution and duration may have been profoundly different between lake stages. Testing the validity of such an hypothesis is extremely challenging without greater palaeoenvironmental information. Fossil pollen, when found, has provided an important insight into local environmental conditions (Nash et al., 2006), but within the Okavango delta itself, the sporadic and chaotic reorganisation of the alluvial fan complicates the interpretation of pollen records. Outside the contemporary boundary of the Okavango Delta, sites for potential pollen preservation are rare and pollen is poorly preserved due to sediment desiccation and oxidation under dry conditions. At the sub-continental scale, there is little independent evidence on the response of vegetation communities to environmental perturbations since Quaternary vegetation records are generally used to

infer climate conditions rather than assess the biotic response to them. The savanna ecosystem of the Kalahari lends itself to utilising the phytolith record (which predominantly provides information on the variability in grasses and sedges which use different photosynthetic pathways). Using these silica fossils from Kalahari shoreline deposits, we aim to establish whether vegetation communities during megalake high stand events in the late Quaternary were significantly different from each other or from vegetation under contemporary savanna conditions in the Middle Kalahari. 1.2. Phytolith analysis Phytoliths are plant cell microfossils composed of hydrated silicon dioxide (SiO2$nH2O). Uptake of solute monosilicic acid from within the sediment matrix leads to silica precipitation within the cell lumen and around the cell walls of plants via metabolic and evaporative processes (Piperno, 2006). The silica precipitated within the cellular matrix may assume the size, shape, and configuration of the cell in which it forms, which itself can be highly diagnostic of the plant taxa to which those cells belong. Whilst many plant families produce and accumulate phytoliths, some taxa produce cells that are more diagnostically taxon specific than others. In particular, highly diagnostic shapes (short-cell phytoliths) form by incomplete silica accumulation within specialised cells and intra-cellular spaces (Piperno, 2006). This occurs in a range of shrubs and herbs and monocotyledons including Poaceae (grasses), Cyperaceae (sedges) and Palmae (palms) (Piperno, 2006). After death, these cellular silicon dioxide precipitates are deposited back into the sedimentary environment. Unlike pollen, their inorganic nature makes them extremely robust to oxidation. Analyses of the morphological variation in phytoliths extracted from sedimentary deposits lacking organic material can provide a helpful means by which to explore past vegetation in savanna dominated regions such as the Kalahari, providing important information on variations in Poaceae, at the time of deposition. Phytolith analysis became established in the 1970s in mesic and wet temperate and tropical regions (e.g. Twiss et al., 1969). Since then the application of such analyses to Quaternary deposits has been increasingly used in palaeo-ecology (e.g. Parker et al., 2004; Alexandre et al., 1997; Barboni et al., 1999). Despite the importance of such analyses in arid environments, they remain rare (e.g. Boyd, 2005). Most studies tend to be based on phytoliths extracted from pan, lake and peat deposits in semi-arid regions (e.g. Parker et al., 2004; Scott and Rossouw, 2005; Telfer et al., 2009; Breman, 2010), while those using sandy deposits tend to focus on coastal environments (e.g. Horrocks et al., 2000). 2. Materials and methods 2.1. Phytolith preparation To test the application of phytolith analysis from shoreline geoproxy landforms five sites were selected from the sub-basins of the palaeolake Makgadikgadi system (Fig. 1). From these sites, dated by OSL, a total of 19 samples were selected based on the timing of lake high stand events (within errors) both within and between selected sites, and the quality of the additional data available (i.e. omission of samples with large dating uncertainties). Thirteen samples were taken from the Dautsa and Kareng shoreline ridges of Lake Ngami from three drill core locations (KAL/05/3, KAL/ 05/5 and KAL/05/7). A further three samples were taken from the Magikwe ridge of the Mababe depression (MAB/06/1) and three from a site on the western Gidikwe ridge of the Makgadikgadi basin (MAK/07/7) (Fig. 1).

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Phytolith preparation was based on Piperno (2006, p.90) and adapted to sandy sediments with low organic and clay content. Dry sediment sub-samples (w5 g) were sieved using a 2 mm diameter mesh to remove gravel and larger organic components. Carbonate nodules and coatings, iron and aluminium oxides were removed using 10% HCl and organic material was subsequently oxidised using 30% H2O2. Samples were then washed in hot water (w70  C) with a dispersing agent to remove soluble calcium ions. Density separation using sodium polytungstate at 2.35 g/cm3 was used to extract the lighter biogenic siliceous microfossils from the sample. Clays were removed from the supernatant using sodium hexa-metaphosphate dispersant and gentle shaking for 5 min followed by vacuum pump filtration through 5 mm nylon mesh. At all stages supernatant was microscopically analysed to confirm negligible phytolith loss during sample preparation. The phytolith residue was retained, dehydrated and stored under 2 ml of 100% ethanol. Samples were mounted on to glass microscope slides using Canada balsam and identified at 400 and 1000 magnifications using a Nikon Eclipse E400 light microscope. The phytolith sum ranged between 931 and 2857 per sample, and morphotypes were compared with modern reference materials collected in the field and published phytolith reference keys (e.g. Twiss et al., 1969; Brown, 1984; Twiss, 1992; Barboni et al., 1999). For each sample a minimum of 200 diagnostic short-cell morphotypes were counted to ensure statistical significance (Alexandre et al., 1997; Barboni et al., 1999). Diagnostic types consisted of trapezoids and elipses for Pooids; saddles for Chloridoids; and short-shanked dumbbells, polylobates and crosses for Panicoids (Fig. 2). 2.2. Phytolith indices Phytolith indices were used to investigate the density of woody vegetation cover (D/P) (Alexandre et al., 1997), aridity (Iph) (DiesterHass et al., 1973) and climate (Ic) (Twiss, 1992). The D/P ratio for woody vegetation density is based on the ratio of ligneous dicotyledon morphotypes (spherical rugose/nodulose) (D) to all Poaceae morphotypes (P) (Alexandre et al., 1997). Savanna environments are characterised by values less than 1 (Alexandre et al., 1997). The Iph (%) aridity index (Diester-Hass et al., 1973) utilises an ecological preference within C4 grasses for different levels of water availability, and is based on the relative abundance of Chloridoid (xerophytic short grass) to Panacoid (mesophytic long-grass)

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morphotypes (Diester-Hass et al., 1973; Twiss, 1992; Alexandre et al., 1997; Bremond et al., 2005). The Iph thus provides a measure of xeric/mesic conditions in grassland, with a high index value (>30%) indicative of drier conditions (Alexandre et al., 1997). The climatic index (Ic) (Twiss, 1992) provides an indication of climate induced fluctuations between C3 and C4 grass dominance. It is calculated as the ratio of C3 (Pooid) to C4 (Chloridoid and Panicoid) phytolith morphotypes, and has been used to distinguish cooler conditions (C3 dominant, values >60%) from warmer (C4 dominant, values <40%) conditions (Barboni et al., 2007). 2.3. Sedimentological analyses The carbonate and organic content of the sediment were determined using sequential loss on ignition at 550  C and 950  C (described in Heiri et al., 2001). Sediment colour was determined both in the field and after drying in the laboratory using standard Munsell colour charts. Particle size analysis (2e2000 micron grains) was carried out using a Malvern Laser Particle Size Analyser (Hydro 2000MU) and sediment statistics calculated using the Folk and Ward formulae (Folk and Ward, 1957). 2.4. Optical dating Optical dating was carried out on palaeo-shoreline samples. Equivalent dose (De) determination was undertaken using multigrain single aliquot regeneration at the Oxford Luminescence Dating Laboratory. Radionuclide concentrations within the sedimentary matrix were determined using inductively coupled plasma mass spectrometry (ICP-MS). Dose rates were calculated using the conversion factors of Adamiec and Aitken (1998) and beta attenuation factors of Mejdahl (1979) and Adamiec and Aitken (1998). Cosmic dose was determined using an iterative modelling procedure. A full description and analysis of the OSL dating procedure can be found in Burrough et al. (2007), Burrough and Thomas (2008) and Burrough et al. (2009a, b). 3. Results and discussion 3.1. The phytolith yield from sandy Kalahari sediments 68e96% of the biogenic siliceous microfossils counted were phytoliths, the remainder was accounted for by aquatic biogenic

Fig. 2. Diagnostic short-cell phytoliths for Pooids (aeb, wavy trapezoids), Chloridoids (c, saddles), Panicoids (dee, short-shank dumbbells, f, crosses) and Arundinoids (g, long-shank dumbells). All photos at 100 magnification, Meiji light microscope.

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3.2. The phytolith record: interpretations and limitations Reconstructing past vegetation with precision at these sites during lake high stands is currently not possible due to a lack of detailed modern analogue studies. The composition of grassland matrix at these sites and specifically the changes in C3 and C4 Poaceae abundance can, however, be further investigated using phytolith assemblage-based palaeoenvironmental indices (see Section 2.2). The low resolution of the phytolith data and the varying temporal length and resolution of each site renders intersite comparisons challenging. Analysis of variance (ANOVA) and homogeneity of variance (Levene statistic) tests were undertaken to test the hypothesis that inter-site indices were not significantly different as to consider each site as having a distinct phytolith assemblage. This hypothesis was found to be true for both the climatic and aridity indices where the mean and variance of indices was similar for each site. Tree cover density, however, showed inter-site variability significant at the 95% confidence interval. This may reflect the slower dispersal rates and more patchy nature of colonisation by trees dispersed in the landscape, particularly those associated with a landform that is likely to be subject to active geomorphological processes at the time of deposition. The phytolith indices plotted in Fig. 4 are, therefore, differentiated by site and by basin. Tree and/or shrub cover density values remain extremely low in all in samples (<0.05) suggesting a coherently grassland dominated signal across the region during lake high stands (e.g. Boyd, 2005). Peak D/P ratios occur in samples KAL/05/3/2 (0.05; 4.9  0.4 ka), KAL/05/3/4 (0.05; 13.6  1.3 ka) and KAL/05/3/6 (0.04;

Percentage of phytolith sum

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silica, notably sponge spicules with some diatoms. Samples from the Makgadikgadi basin shorelines had a very low abundance of short-cell diagnostic morphotypes with only 87 and 64 counts being obtained (statistically insignificant). These samples were therefore omitted from further analyses. Long-cell phytoliths characteristic of grasses dominated the phytolith assemblages at all sites, with bulliforms being the most common type (Fig. 3). Short-cell phytoliths accounted for 6e36% of the phytolith sum. Within samples, Poaceae account for >90% of short-cell phytoliths, ligneous dicotyledons <5%, and Palmae and Cyperaceae (Fig. 3) are poorly represented (<2%) probably due to susceptibility to dissolution and fragmentation (Alexandre et al., 1997; Barboni et al., 1999). Pooid types (C3) dominated the Poaceae short-cell signal, followed by Panicoids and Chloridoids (both C4). Rondels were not included in the Pooid total (Twiss, 1992), due to the possibility of these types being produced by other taxa (e.g. Fredlund and Tieszen, 1994), thus the Pooid abundance reported here is unlikely to be an overestimate, and represents a real Pooid signal. Arundinoids were scarce at all sites. The Poaceae phytolith assemblage was remarkably consistent in all samples from the Mababe basin. From w36 ka to present Pooid phytolith represent 50e60% of the short-cell assemblage during lake high stands, together with approximately equal parts of Panicoid and Chloridoid phytoliths. Assemblages from the Ngami basin show greater variability between high-stand samples. Pooid abundance has been more prominent since w35 ka, accounting for w80% of short-cell phytoliths, Panicoid abundance peaked around 56  5 ka, and Chloridoid abundance was greatest w140 ka, and has increased since w2 ka (Fig. 3).

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Fig. 4. Phytolith index records in relation to regional and global scale records climatic proxy records. a.) Mean luminescence ages and errors (1 standard error) for megalake phases in the palaeolake Makgadikgadi system (Burrough et al., 2009a, b) also shown by horizontal grey shading; b.) Phytolith aridity index, Iph (%) (this study); c.) Phytolith climatic index, Ic (%) (this study); d.) Phytolith tree cover density (%) (this study); [Threshold values for each index indicated (see text).] e.) Individual luminescence shoreline ages with errors for sub-basin samples used in this study; f) Antarctic temperature variation indicated by Epica Dome C dD (&) (EPICA Community Members, 2004); g.) Composite Vostok CO2 (ppmv) record (Luthi et al., 2008); h.) Insolation (W/m2) at 15oS (Jan) (Berger and Loutre, 1991). Symbol shading indicates different site locations (see legend). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

15.1  1.8 ka). This site specific peak might be indicative of more amenable conditions for the colonisation of woody species at times of shoreline stability at this ridge relative to other sites, or a difference in the successional stage of the vegetation between sites that in turn reflects the relatively stability of the hydrological conditions at these times, thus introducing a spatial bias towards high D/P ratios via localised landscape stability. In addition, spherical rugose morphotypes are likely to be produced in limited numbers and preferentially degraded over time in relation to Poaceae short-cell morphotypes causing a temporal bias in the record towards the under-representation of tree-cover density in older samples (Bremond et al., 2004; Boyd, 2005). We therefore argue that in this particular context, D/P should only be used in its crudest sense to test the dominance of woodland, an ecological state that is clearly not locally apparent for the duration of the Middle Kalahari highstand shoreline phytolith record. Variation in moisture requirements exists between grass tribes following the C4 photsynthetic pathway, with Chloridoid grasses occurring in areas of lower water availability than Panicoid grasses (Tieszen et al., 1979). The Iph aridity index provides a measure of the

relative abundance of xeric Chloridoid to mesic Panicoid grasses with low % indicating the dominance of mesic Panacoid taxa. There is an expectation that the signal may itself be biased by the localised mesic system and not be entirely representative of the broader Kalahari environment, since i) by their nature all samples are taken from periods of high lake stands and ii) many of the Panicoid taxa may derive from annual short grasses adapted to the riparian corridors of the Okavango Delta system. However, significant variation within the record was observed. The lowest individual Iph value within the record is 10% at 32.2  2.8 ka (KAL/05/5/8). Values less than 30% may indicate mesic taxa dominance with cool/humid conditions and/or high available soil moisture. The sediment from which these counts were obtained was noted for its abnormal iron oxide component often strongly associated with soil formation and consistent with a period of stable lake levels and humid conditions at Lake Ngami. The lower limit of xeric species dominance obtained from phytolith assemblages within a number of different grassland environments range from 30% (Senegal and Congo; Alexandre et al., 1997) to 45% (North American prairies; Fredlund and Tieszen, 1994). Sites with aridity indices above these values may indicate

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significant grassland adaptation to warm, dry (xeric) conditions with increasing relative abundance of short Choloridoid grasses. In Ngami (where the records are longer than those of the Mababe basin) xeric taxa were dominant (>30%) during lake events prior to w38 ka (Fig. 4). The dominance of mesic taxa after this time may be caused by cooler temperatures during the peak of the last glacial, though this does not explain a continued mesic signal through the Holocene. The mesic vegetation signal seen in this particular record would additionally have been enhanced by moisture availability increases due to permanent sub-basin water bodies at the time of deposition. Xeric C4 dominated vegetation did, however, reestablish again in Lake Ngami at 15.1  1.8 ka. This is during the megalake event, recorded in Makgadikgadi shoreline construction, when all three basins were known to contain large lakes. This particular megalake event has been hypothesized to have spanned the period between 19 and 13 ka (Burrough et al., 2009a). No regional temperature records currently exist in the Middle Kalahari that encapsulate this period making it difficult to separate whether moisture availability or temperature (or both) could have driven this vegetation change. It is possible this xeric signal was at least in part driven by less stable (i.e. inter-annually or centennially fluctuating) lake status suggesting the megalake phase was not a single stable event. It should also be noted that the Iph index exhibits significant spatial (site specific) variability. 3.2.1. Potential drivers of vegetation change In contemporary environments, C3 plants tend to dominate in cool, mesic temperate regions (Tieszen et al., 1979; Livingstone and Clayton, 1980) whilst tropically adapted resource-efficient C4 species can photosynthesize at higher temperature and with reduced availability of CO2 and water (Ellis et al., 1980). In a semiarid environment like the Kalahari, the competitive balance between C3 and C4 grasses is often dominantly driven by the minimum growing season (wet season). C3 abundance becomes significant in cool environments where effective precipitation is relatively high, and thus under contemporary conditions C3 dominance is confined to the winter rainfall region or high altitude locations within the summer rainfall zone (Scott, 2002). In the continental interior of the Kalahari, 86% of grassland species are classified as C4 (Vogel et al., 1978). In contrast, measured as the Ic index (C3 Pooid morphotypes as a percentage of sum of Pooid, Chloridoid, Panicoid and Arundinoid types), phytolith assemblages suggest dominance of C3 grasses during lake high stands, indicated by index values of >60% (Barboni et al., 2007). Whilst at the time of deposition water availability and/or effective precipitation is expected to be greater than average, the production of C4 Chloridoid type phytoliths by the C3 Arundinoid genus Phragmites (common to the Okavango delta and identified in their fossilised form in both Ngami and Makgadkigadi) (Shaw et al., 2003; Ringrose et al., 2005)) may in fact bias the Ic index towards underestimating the true magnitude of the C3 signal. C4 prominence (<40%) is only observed once within the high stand vegetation record of Lake Ngami at 56  5 ka and not at all in the Mababe depression. Hydrological and biophysical feedback mechanisms explored in previous studies using the Hadley Centre coupled oceanatmosphere general circulation model (HadCM3), suggest that high lake stands would potentially increase local rainfall and amplify the seasonal cycle of precipitation-evapo-transpiration (Burrough et al., 2009b). As the seasonal precipitation regime is anti-phased to the peak hydrological inflow due to the nature and orientation of the catchment, this would effectively increase water availability throughout the year. The subsequent impact of these changes on local vegetation was simulated using an equilibrium terrestrial biosphere model (Biome 4) and predicted a first order response to a large surface water body to be an increase in primary

productivity of both tree and grass plant functional types (PFTs) by up to 50% (Burrough et al., 2009b). The observed dominance of mesic and C3 prominent taxa during high lake stands in this study, suggests a significant departure from prevailing C4 dominated savanna vegetation and is consistent with model predictions for increased water availability and longer growing seasons. Assemblages that exhibit a xeric and C4 prominent taxa (which take up carbon more effectively than C3 grasses during warm summer rainfall growing seasons) suggest conditions similar to present day Kalahari basin environments where lake levels are extremely low and seasonally dry. This supports an assertion that at 56  5 ka and during the post LGM lake phase at 15.2  1.8 ka, lake high stands were briefer, or lake level variability was greater, than during other recorded periods of lake presence. Alternatively, or coincidently, the dominance of xeric C4 taxon at these times could have been driven by warmer growing season conditions or reduced CO2 (Fig. 4). A sensible means to evaluate the observed vegetation response is to assess the record presented here within the context of local records of Quaternary vegetation change. The availability of independent palaeoecological records from the Kalahari is, however, extremely scarce. The latter exists, to our knowledge, only from three sites that lie to the west of the Okavango panhandle and includes a discontinuous Holocene pollen record from Drotsky’s Cave speleothem (Burney et al., 1994); faunal remains from archaeological sites (Helgren and Brooks, 1983; Robbins et al., 1996) and a more complete Holocene pollen and isotope record from the Okavango panhandle (Nash et al., 2006). The contrasting site localities between the wetland ecosystem of the Panhandle region and the drier, rain-fed shoreline environment (see Section 1) may introduce some expected taxa specific variation between records although broadly, successional transitions from deep water to shallow water communities, would also be expected to show a C3 dominant to a C4 dominant trend (Ellery et al., 1992). Nash et al. (2006) observe increased Chenopodiaceae and Aizoaceae pollen from 7000 to 4000 BP. Chenopods are usually more abundant in the drier floodplain habitats of the contemporary Panhandle (Ellery and Ellery, 1997), but a dry period is out of phase with other palaeoenvironmental indicators in the Middle Kalahari (Burney et al., 1994; Robbins et al., 1996). They thus interpret this transition to represent a wetland ecosystem response to lower flood levels in the Okavango Delta. This is consistent with the phytolith record observed here which shows a peak in C4 taxa at 4.9  0.4 ka. However, the geomorphological record from Ngami infers that this was a time of lake high stand conditions and thus suggests that either lake filling was highly variable and/or that increased growing season temperature in the subtropics was also a significant driver of vegetation change at this time. Nash et al., 2006 suggest a trend to increased flow in the Okavango system from 4500 BP, a trend marked in the lake-sumps to the south by shoreline construction at both Lake Ngami and Mababe and a phytolith signal from this study that suggests relatively mesic C3 dominated grassland systems. The pollen and isotope data from the Panhandle indicate a move towards increased regional rainfall between 3000 and 1000 BP after which there is a striking shift from grass to sedge dominance which may be due to a number of factors including climate and hydrological change as well as human impact. The resolution of the phytolith record here is too low and discontinuous to make direct comparisons to these records, though the shift towards C3/mesic conditions observed in the Ngami basin between 4.9  0.4 and 1.4  0.2 ka is consistent with a return to wetter conditions observed by Nash et al. (2006). 3.2.2. The limitations and scope of geoproxy phytolith records Using phytolith assemblages as independent environmental proxies in the context of the low resolution and discontinuous

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record presented here should be undertaken with caution due to a number of significant limitations. The stability of geoproxy landforms and the active processes operating on them (in this instance the accumulation of lake shoreline ridges) may be variable in both time and space and may account for some degree of site specific and event-specific variability. This is a characteristic true for most sedimentary deposits in dryland environments and remains a challenging limitation to Quaternary Scientists working in these areas. Such variability can to some extent be controlled for by utilizing multiple sites within the region of interest through inter-site comparison, though this is dependent on high precision chronological control and may well be limited by the spatially discreet and discontinuous nature of dryland sedimentary deposits. The spatially variable and generally low temporal resolution of the analyses carried out in this pilot study is currently insufficient to conclusively establish how environmental variability differs between lake high stand events, although it does appear to indicate that such variation occurs. The chemical and physical robustness of phytoliths also allows the possibility of persistence and reworking within the environment. Whilst this is true for all Quaternary phytolith records, it is a particular concern in the context of geoproxy landforms where sediment is by definition accumulated during periods of landscape instability and active fluvial, aeolian or lacustrine processes. In this particular instance the large catchment of the Okavango system allows the possibility that the record from the shoreline phytolith assemblages incorporates both locally deposited phytoliths and those reworked from further afield (see Section 1) and thus could be the cumulative integration of multiple vegetation signals over a considerable spatial and temporal scale. There are several possible ways in which this issue could be addressed: i) Comparative analyses of sites from within the catchment may offer the potential to ascertain the scale and consistency of the vegetation record; ii) careful examination of the state of phytoliths at the individual level in terms of pitting, abraision, smoothness etc that might provide some indication (albeit subjective) of long distance fluvial transport; iii) in addition, the collection and analysis of phytolith assemblages from modern analogue vegetation both locally and regionally (i.e. further upstream within the hydrological system) would also provide a greater level of robustness of any interpretations of the fossil phytolith record and may help to establish the degree of reworking within the modern system. 4. Conclusions Phytolith records from shoreline samples have the potential to provide information about local conditions of the lacustrine environment at the time of deposition. This study confirmed the presence of both an abundant and diverse assemblage of phytoliths within samples from sand-dominated shoreline ridges primarily used as geoproxies for high lake stands. Short-cell diagnostic phytolith morphotypes were found to be remarkably well preserved within all samples analysed in this study. This is the first such record from this type of environment, where usually diagnostic phytolith types are rare or absent. With the exception of two samples, phytoliths were found to exist in ample abundance allowing statistically significant analyses of the phytolith record to be made. This is an important finding and demonstrates the potential of this long-term ecological proxy to shed light on past vegetation assemblages by unconventionally utilizing routine geomorphological investigations that simultaneously produce records of landscape dynamics. This is particularly important in a region where very little is known about the impact of hydrological and climatic change on local and regional ecosystems and where understanding the response of vegetation to climatic and

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hydrological change has significant implications for predicting the resilience of the prevailing ecosystem under future climate change scenarios. The application of HadCM3 and BIOME4 models to the changing hydrological status of megalake systems in the Middle Kalahari (Burrough et al., 2009b) predicts that increased surface water at times of high lake stands would significantly impact regional vegetation systems. The results from this study which examine lake high stand vegetation are consistent with this prediction, showing a significant departure from contemporary C4 dominated savanna (under seasonally dry lake conditions). Phytolith assemblages indicate tree cover density was extremely low (<0.05%) throughout the record suggesting a coherently grassland dominated signal (c.f. Boyd, 2005) across the region through time during high stand lake events. However, whilst all samples analysed in this study were derived from late Quaternary lake high stand phases within the lacustrine basins of the Middle Kalahari, there were notable temporal differences in the vegetation response during these events. In Lake Ngami, the phytolith record indicates a general trend towards more mesic and C3 prominent taxa during lake events after w40 ka. This shift in grassland species could be driven by cooler conditions or less seasonal water availability. The exception to this is found at 15.1  1.8 ka when xeric C4 taxa become abundant, a trend that we tentatively suggest may reflect either a warmer growing season within the subtropics or more pronounced seasonality in water availability. These interpretations however, are made with extreme caution. The small number of sites and samples used in this study, persistence and reworking of phytoliths, the influence of landform instability, and the effects of a low and inconsistent resolution render anything more than broad interpretations less than robust. The use of phytolith records as an independent vegetation proxy in the context of routinely analysed geoproxy landforms such as sandy shorelines opens up temporal and geographic areas previously beyond the boundaries of more conventional palaeoecological techniques, and offers the potential to provide important insights into the response of vegetation to independently recorded extreme environmental conditions. Whilst there is certainly a well preserved record within these sediments we suggest three key criteria need to be established before more significance can be attached to these findings: i) C3/C4 characterisation and phytolith signatures of modern analogue vegetation at site need to be established; ii) records should be sub-sampled at intervals that provide a greater (and directly dated) temporal resolution and iii) sample sites should incorporate a wider spatial resolution both within and outside the hydrological system to test the site to site variability of the record, this is particularly important in heterogeneous environments such as Northern Botswana where wetland and dryland vegetation lie in close proximity. Acknowledgements We thank David Thomas and two anonymous reviewers for helpful comments on this manuscript. We also thank Ross Burrough and Ralph Bousfield for the collection and identification of field samples contributing to phytolith reference collections. References Adamiec, G., Aitken, M.J., 1998. Dose-rate conversion factors: update. Ancient TL 16, 37e50. Alexandre, A., Meunier, J.D., Lezine, A.M., Vincens, A., Schwartz, D., 1997. Phytoliths: indicators of grassland dynamics during the late Holocene in inter-tropical Africa. Palaeogeography Palaeoclimatology Palaeoecology 136, 213e229. 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

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