An investigation of climatic change in the holocene epoch using archaeological charcoal from Swaziland, Southern Africa

Journal of Archaeological Science 1985,12,457475

An Investigation of Climatic Change in the Holocene Epoch using Archaeological Charcoal from Swaziland, Southern Africa J. Prior” and D. Price Williamsb (Received I1 December 1984, accepted 6 March 1985) A methodology is described for the analysis of Holocene charcoals excavated from a rock shelter in the Lubombo Mountains of northeast Swaziland. Scanning electron microscopy was used to compare these with modern reference woods. Of the ancient material 96.6% could be identified, in some casesat specific level. It is in such a subtropical area, where the woody flora is so rich, that assemblagesof local taxa can be used in palaeoclimatic reconstructions. The changing taxa indicated by the charcoal fragments from the rock shelter clearly reflect minor shifts in Holocene climate, from moist to dry and back to moist in recent times. This is of relevance to the fluctuations in Stone Age populations in southern Africa. The wider use of such evidence to complement other palaeoenvironmental and archaeological data is advocated. Keywords: SOUTHERN AFRICA, SWAZILAND, QUATERNARY, LATE STONE AGE, HOLOCENE, PALAEOCLIMATE, CHARCOAL IDENTIFICATION, SCANNING ELECTRON MICROSCOPY.

Introduction The Late Quaternary vegetational history of southern Africa is not well known, though recent palynological studies in the Transvaal by Scott (1982a, b) and the first South African charcoal studies in the Cape by Deacon (1979; Deacon et al., 1983) provide a framework for further research which amplifies the pioneering work of van Zinderen Bakker (1967) and Coetzee (1967). Research to date emphasizes the diversity of this large subcontinent and the need for an expansion of these and complementary lines of study into other areas, so that the environment in which so many of the important stages of Man’s material culture developed can be more fully understood. Swaziland lies between latitudes 25”4O’S and 27”2O’S, 250 km south of the Tropic of Capricorn. Within a surface area of only 17,565 km2 four major geographical and hence vegetational zones are represented. The great diversity of the present flora renders Swaziland an ideal area for studying changes in past vegetation, themselves a reflection of past climatic conditions. The research described here is concerned with the identifi“Department of Pure and Applied Biology, Imperial College of Science and Technology, Prince Consort Road, London SW1 2BB. %waziland National Trust Commission, P.O. Box 100, Lobamba, Swaziland, Southern Africa.

457 0305-4403/85/060457+

19 $03.00/O

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cation of charcoal fragments excavated from a rock shelter in the Lubombo mountains of northeast Swaziland, which possesses an extensive archaeological sequence dating from the Late Pleistocene to recent times. Only the Holocene levels are described in this paper; excavation of earlier levels is not yet complete. -The charcoal pieces represent the inert, charred remains of trees and shrubs burnt locally in fires by successive populations of hunter-gatherers. The position of the shelter in what is today a richly wooded area would imply that it was local trees and shrubs that were selected by each small, discrete group of inhabitants, either for firewood or for other utilitarian purposes. The assemblages of taxa represented by the charcoals in successive levels demonstrate the minor climatic shifts which have occurred through the Holocene, emphasizing the potential of this type of approach in the wider context of palaeoclimatic change. Regional Setting The Siphiso Rock Shelter is located at 26”18’5O”S, 31”58’3O”E on a steep north facing slope of the Lubombo Escarpment at an altitude of 320m a.m.s.1. (see Swaziland 1:50,000 topographic sheet 2631 BD, 1967) and approximately 75 km west of the Indian Ocean. The site is at the head of a narrow valley from which flow the headwaters of the Siphiso River, a tributary of the Mlawula and ultimately the Mbuluzi Rivers. The Lubombo Escarpment consists of rhyolites and rhyodacitic ignimbrites belonging to the Drakensberg Volcanics of the Karroo Supergroup (Cleverly & Bristow, 1979; Tankard et al., 1982). Within the successive rhyolite flows are small sedimentary basins and it is the differential weathering of one of these sedimentary pockets which has formed the Shelter (R.R. Maud, pers. comm.). The rainfall at Siphiso is approximately 550 mm per annum, 80% of which falls during the summer months when mean temperatures usually exceed 20°C. Under such conditions, evaporation and transpiration greatly exceed precipitation throughout the year (Murdoch, 1970). Soils developed locally vary from lithosols on the exposed slopes to hydromorphic bottomland soils, usually calcareous and often vertisols (Murdoch, 1970). In suitable areas, these are capable of supporting a dense cover of sweet grasses with a high grazing potential. The grassland is interspersed with a mosaic of deciduous and xeromorphic trees typical of the Bushveld as described by Acocks (1953). The Bushveld and the other three vegetational zones of Swaziland are part of the Zambezian domain of the wide-ranging Sudano-Zambezian region described by Werger (1978). A survey of the trees and shrubs growing today in the immediate vicinity of the Shelter shows a clear differentiation between those growing amongst the grasses on the sparse, raw mineral soil (pH 5.5-6.0) and those restricted to the deeper alluvium adjacent to the stream. Only woody taxa described by Coates Palgrave (1977) were included in the survey. Commonest trees on the slopes include the broad-leaved Combretum apiculatum, Spirostachys africana, Acacia nigrescens, Peltophorum africanurn and Ziziphus mucronata, with a sparse understorey scatter of the shrubs Euclea crispa, Euclea schimperi, Ximenia americana, Maytenus heterophylla, Grewia hexamita, Grewia monticola and Coddia rudis. A few trees of Afzelia quanzensis grow in the dry area at the base

of the slope. Taxa restricted to the comparatively deep, coarse-grained alluvial soil include Acacia robusta, Mimusops zeyheri, Vepris reflexa, Terminalia phanerophlebia, Kraussia Jloribunda,

Bridelia cathartica, Syzygium guineense, Breonadia-microcephala

and Ziziphus rivularis. Woody scramblers such as Acacia schweinfurthii and Rhoicissus rhomboidea are a common occurrence amongst the riverine vegetation. Site Description and Stratigraphy A detailed plan of the Siphiso Rock Shelter is shown in Figure 1. The contours relate to an arbitrary datum. The “previous excavation” refers to an initial examination of the site

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conducted over a number of days in the mid-1960s when “70 cubic feet” were removed (Beaumont, n.d.). At that time the findings were inconclusive, though two radiocarbon determinations from the middle of the deposit gave dates of around 10,000 bp. The shaded areas in Figure 1 represent metre squares excavated during four 12 week seasons (1981-84) from primary, in situ material. It is the charcoal from four of these square metres, F8, G8, H8 and G9, and from Holocene levels only, that is discussed in this paper. Excavations are still continuing and none of the squares has been excavated fully to bedrock. When excavations are complete, the site is expected to yield a human record spanning the Holocene and part of the Late Pleistocene (Price Williams & Barham, 1982). Each horizon is exposed by airbrushing and all material remains are recorded in three-dimensional plots. The minutely divided horizons are then grouped together into strata according to the similarity of their stratigraphic, sedimentological and artifactual character. The strata thus represent the major cultural and chronological divisions of the site. All strata yielded charcoal fragments and some contained other macroscopic plant remains such as seeds of Sclerocarya birrea, the marula, and Grewiu spp. A full description of the stratigraphy and cultural content of the site, including the detailed sediment analyses and artifact inventories, will appear elsewhere (Barham & Price Williams, in prep.). Only a brief account of the Holocene Strata l-6 is given here (see Figure 2). Stratum I. This surface horizon represents an accumulation during the last few decades. Quantities of roof spa11aremixed with modern remains such as undecorated pot-sherds, lead pellets, fragments of fabric, wooden pins and wood chips cut with a metal blade. The roof spa11 is the natural product of pressure release in the surrounding rock, accelerated by the efflorescence of salts contained within it.

617 8 0

unexcavated

1

I

I

I

Figure 2. Section across part of Siphiso showing Holocene strata, HS/H7.

0.25m

1

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CHANGE

461

Stratum 2. This is a shallow but persistent horizon of calcined dung which probably accumulated since Stone Age cultures disappeared from Swaziland some few hundred years ago. It is thought either to be the result of using the site as a stock-pen or, more likely, to be the residue of a wild animal lair. In G8, H8 and G9 it acts as a clear marker horizon, delineating the recent debris from the prehistoric levels beneath. In F8, it is indistinguishable from Stratum 1, probably because of prolonged leaching caused by the proximity of this square to the drip line. Stratum 3. Quantities of roof spa11 are combined with considerable amounts of lithic debris, artifactual material and bone fragments relating to a portion of the African Late Stone Age. Preliminary morphometric analysis highlights the numbers of convex steepnosed scrapers, mainly made from chalcedony, which are typical of the Wilton Industry as defined by J. Deacon (1972, 1974, 1978). The apparent lack of backed elements and the relative position of the Stratum suggests that it might be later in the Wilton sequence, and whilst no absolute date from the Stratum is available, a period between 2000 and 4000 bp is suggested. Stratum 4. Very obviously different from 3, Stratum 4 is made up of a series of compacted light grey ash lenses, with a marked decrease in roof spa11 representing either different climatic conditions, or a greater concentration of human debris, or both. The artifactual material is clearly Wilton and the size ratios and ranges of the scrapers accord well with the subdivision of the period known as the Climax Wilton (J. Deacon, 1972) or Classic Wilton (Sampson, 1974). Comparative chronologies from elsewhere in southern Africa would favour a mid-Holocene date for this phase, prior to 4000 bp. (J. Deacon, 1974, 1978). Stratum 5. The sediment of Stratum 5 is not dissimilar to 3, a loose, grey spall-rich series of horizons with much lithic debris and bone remains. Preliminary analysis of the artifacts again demonstrates a Wilton Industry which would fit chronologically earlier in the Holocene, at or after 8000 bp. This is confirmed by a radiocarbon determination of bone from Stratum 5 in H8 of 7600 + 80 bp (Pta-3533). Stratum 6. A major stratigraphic and cultural break can be discerned between Stratum 5 and Stratum 6. The sediment is compacted and dark grey, and the artifactual material is not Wilton. The formal chalcedony scrapers which typified the Wilton are absent, the Stratum being characterized by less formally made, broad-edged rhyolite flakes with use scars. Provisional analysis demonstrates that there are in fact two separate phases of this pre-Wilton industry: an earlier, large flake element is followed by one of the same type, but smaller. The subdivision of Stratum 6 is fully reported by Barham & Price Williams (in prep.). For the purposes of the present paper, the two subphases are combined. The industry is equated with that described by Sampson (1974) as Oakhurst, by H. J. Deacon (1979) as Albany or what was once known as Smithfield A (Louw, 1960, to name but one). Dates at sites with material comparable to that from Siphiso range from about 9000 bp to 12,000 bp, which accords with a radiocarbon determination of bone from Stratum 6 of 8700 + 120 bp (Pta-3540). On the basis of correlation with the deep-sea isotope stages suggested by Shackleton (1977), it would seem that all the material analysed so far falls within his Isotopic Stage I and belongs to the Holocene era. Material of Late and Terminal Pleistocene age will be described in due course.

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Charcoal Retrieval, Sampling and Laboratory Procedure For some years it has been possible to retrieve biological remains from archaeological sites by froth flotation (Jarman et al., 1972) or by wet sieving. Water was not used in charcoal extraction from Siphiso’ since wetting and drying might preferentially damage those fragments with a high percentage of thin-walled cells or those most fragile when excavated. Each fragment, which might vary in size from several centimetres to less than 5 mm, was individually lifted and stored in an airtight container. Although unbiased sampling of large quantities of charcoal presents problems, as discussed by Wilcox (1974), the comparatively small size of the excavated area facilitated an analysis of all available fragments. Some fragments had deteriorated beyond recognition and a few were too small to process, but many had been hardened by impregnation with precipitates resulting from post-depositional migration of chemicals through the strata and could be handled easily. Each charcoal fragment was scored with a scalpel and manually fractured in three planes in relation to the wood structure: transverse, tangential/longitudinal and radial/ longitudinal. Fresh fractures were essential for subsequent examination, though in the case of the smallest fragments only one or two planes could be obtained. The fractured pieces were mounted on aluminium stubs with Durofix and desiccated for at least 24 h. They were then coated with 40nm gold in an’atmosphere of argon and examined by scanning electron microscopy (SEM) using magnifications from 50 to 6400 x so that the maximum number of anatomical features could be photographed. This technique provides a high resolution coupled with an appreciable depth of field when applied to charcoals, as emphasized by several workers (e.g. McGinnes et al., 1974). The alternative technique of incident light microscopy requires a flat surface, often difficult to obtain, and provides mainly two-dimensional images. Further, the magnifications that can be achieved by this method are often too low for some of the diagnostic features to be visible. Archaeological charcoals can only be satisfactorily identified when compared with charred reference material, similarly examined by SEM. A number of changes occur in the structure of the woods during charring (Kollman & Sachs, 1967; McGinnes et al., 1971) and the nature of these changes can vary greatly in woods of widely divergent anatomical structure (Prior & Alvin, 1983). Also, pyrolysis results in a considerable amount of shrinkage which can affect both cell size and shape. In the present study, a reference collection was made over a period of time from 1977. Swaziland, with its strong environmental diversity, has today approximately 600 different species of trees and shrubs and as many of these as possible were collected. Twig and trunk wood samples were included for each specimen as the anatomy of juvenile and mature woods may differ (Jane, 1970). Trunk wood samples had a diameter of at least 30-40 mm to ensure that their anatomical structure was mature. Many of the species occur in characteristic assemblages indicative of particular edaphic or climatic conditions and a sound knowledge of the contemporary flora is a prerequisite of any palaeoenvironmental interpretation. In a few instances, samples of a single taxon were collected from more than one geographical zone since the wood anatomy may vary with distribution (cf. Robbertse et al., 1980). Such variation may be either genotypically or environmentally induced. All reference specimens were charred by embedding in crucibles of washed s&er sand and heating in a muffle furnace at 450°C for 30 min. They were left in the sand until cold in order to minimize superficial ash formation which makes subsequent fracturing difficult. Electron micrographs of all planes of fracture were taken and these photographs, together with reference publications such as Kromhout (1975), Miles (1978), van Vliet (1979) and Butterfield & Meylan (1980) were used to identify the ancient taxa. The use of feature sheets (cf. Brazier & Franklin, 1961; Barefoot & Hankins, 1982) or the full range

HOLOCENE Table

1. Features

CLIMATIC

CHANGE

used in rhe identification

463 of ancient

taxa

Vessels

Shapeand arrangement. Round, angular or oval, solitary or grouped. If grouped, whether in pairs, radial, oblique or tangential rows or clusters. In fragments with obvious growth rings, any variation in vessel shape and arrangement in areas of early and late wood were recorded. Storied or non-storied. Detailed structure. Comparative wall thickness and range of diameter, type & detail of perforation plates, nature & arrangement of pits, presence or absence of gums or other inclusions in lumina, presence or absence of tertiary spiral thickening.

Fibres

General distribution pits.

Axial parenchyma

Arrangement. Whether paratracheal, apotracheal or marginal. If paratracheal, whether scanty, abaxial, vasicentric, aliform or confluent. If apotracheal, whether diffuse, diffuseaggregate or banded. If marginal, relative position and extent of cells. Storied or non-storied. Derailed structure. Comparative shape of cells and relative thickness of walls, presence or absence of septa, presence or absence of cell inclusions and nature and distribution of these.

Rays

Shape and arrangement. Comparative height, whether uni-, bi- or multiseriate, presence or absence of sheath cells, whether homo- or heterocellular, storied or non-storied. Detailedstructure. Relative distribution of upright and procumbent cells (cf. Kribs, 1968), relative thickness of cell walls, presence or absence of inclusions and nature and distribution of these, presence or absence of schizogenous ducts, nature of ray to vessel pitting.

and presence or absence of gelatinous variant, type and arrangement of

Other features Presence or absence of included phloem, presence or absence of ducts and if present, position and nature of these.

of characters specified by the International Association of Wood Anatomists (I 98 1) was not thought practicable for a number of reasons. First, fragments were often too small for some of the diagnostic features to be discerned. Second, charring in open fires occurs over a wide temperature range, from 350°C to more than 800°C. The amount of shrinkage in the wood varies widely over the range, hence characters such as vessel diameter and fibre wall thickness also vary; even the chemical composition and structure of fibre walls within a single species may cause a differential response to heat (Prior & Alvin, 1983). Third, features such as the presence or absence of growth rings are “. . . of little taxonomic significance because of the belief that their formation is a response to environmental conditions rather than genetic control” (Metcalfe & Chalk, 1983: 45). Fourth, though detailed features such as the type of cross-field or intervessel pitting are sometimes diagnostic, in the case of some charred material pitting may be profoundly altered. In other cases, relevant details are obscured either by gums present in the original wood or by secondary deposits of substances within the vessel elements. The invariable features used in the identification of each of the ancient taxa are listed in Table 1. Results: Identification of Taxa Results of the analyses are shown in Tables 2 and 3. The preservation of the charcoal was such that almost every fragment permitted identification after treatment by the methods described. Of a total of 497 pieces, only 17 were unidentifiable owing to a lack of sufficient structural detail. Some of these were tiny twigs with poorly differentiated parenchyma and fibres or an atypical vessel arrangement (cf. Jane, 1970). Of the remaining 480 fragments, 25 are classified as “Unidentified l-l 1” in Table 2. These are wellcharacterized woods and their present “unidentified” status is due either to a lack of a

Table 2. Siphiso

Rock

Shelter;

charcoal

identiJications

from

Holocene

levels

Strata 1+2 Recent

3 Late Holocene

4 MidHolocene

5 Early Holocene

6 Early Holocene

261 112 12 8 23 8 8 12 3 2

134 64 36 8 2 5

32 14 14

30 17 3 3

40 19 3 2 3 1

2

1

No. of fragments Leguminosae Combretum apicularum Androstachys johnsonii Maytenus spp. Diospyros spp. Ziziphus spp. Grewia ssp. Euclea spp. Rubiaceae Ochna natalitia Bridelia Rhus Unidentified 1 Unidentified 2 Unidentified 3 Unidentified 4 Unidentified 5 Unidentified 6 Unidentified 7 Unidentified 8 Unidentified 9 Unidentified 10 Unidentified 1I Unidentified owing to lack of structural detail

4 1 2

2

1

1

1 3 1

1

7

1 1 1 6

Table 3. Horizontal

8

3

comparability

across Figure

Taxa No. of fragments Leguminosae Combretum apiculatum Androstachys johnsonii Maytenus spp. Diospyros spp. Ziziphus spp. Grewia spp. Euclea spp. Rubiaceae Ochna natalitia Unidentified 1 Unidentified 2 Unidentified 3 Unidentified 4 Unidentified owing to lack of structural detail

the site of Siphiso 3 notation

(square

no.)

1+2a(F8)

1+2b(G9)

1+2c(H8)

44

167

50

80 46 1 17 4 4 4 3 2 1

17 24 1

5

2 I 1 1 3

3

Totals 497 226 128 21 28 14 8 12 10 3 1 2 2 2 2 3 1 1 1 11 1 1 1 1 17

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CHANGE

I

Figure 3. Principal coordinate analysis of proportions Siphiso.

of taxa in Strata l-6,

full set of characters required for secure identification or a failure so far to match convincingly with charred reference material. As further work proceeds, the identity of most if not all of these should be resolved. In Table 3, additional information is given concerning recent strata; results from each of the squares F8, G9 and H8 (see Figure 1) were treated separately in the numerical analysis in order to test horizontal comparability from the front to the rear of the site. As a result of the close correlation between electron micrographs of ancient and modern material, 12 taxa could be securely identified (Table 2). This correlation was equally applicable to all three fracture planes, as shown in Figures 47. The quality of fractures appears to be a feature of the anatomical structure of the original woods and their subsequent depositional history rather than their age, since much of the charcoal from the older Strata 5 and 6 was well preserved, whereas that found in the more recent Stratum 2 amongst the calcined dung was more difficult to identify. In this case excessive secondary intracellular precipitation of crystals had often distorted cell structure and obscured anatomical details. Of the three taxa identified at specific level (Table 2) Androstachys is a unispecific genus, Combretum apiculatum has a highly characteristic anatomical structure and Ochna natalitia, found only in the more recent strata, is the only Ochna species currently growing in the region. Seven further taxa were identified at generic level. Grewia only occurs in Strata 1 and 2 and is therefore likely to be Grewia monticola, a common understorey shrub around the Shelter, or Grewia hexamita, also found close by. The BrideZia present in the Late Holocene sample from Stratum 3 is probably either Bridelia cathartica or Bridelia micrantha, the two species recorded in Swaziland today (Coates Palgrave, 1977). Similarly, the recent pieces of Ziziphus must either originate from Ziziphus mucronata or Ziziphus rivularis. Since Maytenus, Diospyros, Euclea and tius are all taxonomically complex genera, the anatomical structure of their charcoal is unlikely to be resolved at specific level. Two groups cannot so far be resolved below family level, Leguminosae and Rubiaceae. Legumes are one of the most important and numerous components of typical Bushveld vegetation (Acocks, 1953). Large trees of Afielia quanzensis are indicators

Figure 4. (a) Transverse fracture of Ziziphus charcoal from Stratum 1+2b G9, showing numerous crystals in ray cells, surrounded by vasicentricqarenchyma and radial rows of vessels (large pores); x 135. (b) Transverse fracture of modern, charred Ziziphus mucronaru; x 135. (c) Radial fracture of Ziziphus charcoal from Stratum 1+2a F8, showing a simple perforation plate across the vessel (left) and shape and nature of ray cells (right); x 135. (d) Radial fracture of modern, charred Ziziphus mucronata; x 135. (e) Tangential fracture of legume charcoal from Stratum 1+ 2a F8, showing vessel (left of centre) with inclusions in lumen and lenticular homocellular rays of various heights and widths; x 68. (f) Tangential fracture of modern, charred Dichrosrachys cinerea; x 68.

Figure 5. (a) Transverse fracture of Combretum upiculatum charcoal from Stratum 3 F8, showing thick-walled oval solitary vessels and phloem island (top left); x 68. (b) Transverse fracture of modern, charred Combreturn apiculatum; x 68. (c) Tangential fracture of Bridelia charcoal from Stratum 3 F8, showing vessels with simple, oblique perforation plates (right) and heterocellular rays of varying widths (centre/left); x 135. (d) Tangential fracture of modern, charred Brideliu catharticn; x 135. (e) Intervessel pits in legume charcoal from Stratum 3 F8, showing remains of vesturing around pit aperture; x 2430. (f) Vestured intervessel pits in modern, charred Lonchocarpus cupassa; x 2430.

Figure 6. (a) Transverse fracture of Ochna charcoal from Stratum 1+2b G9, showing angular vessels (large pores) and groups of thick-walled fibres; x 135. (b) Transverse fracture of modern, charred Ochna natalifia; x 135. (c) Tangential fracture of Ochna charcoal from Stratum 1 + 2b G9, showing high, muhiseriate rays; x 68. (d) Tangential fracture of modern, charred Ochna natulitiu; x 68. (e) Transverse fracture of Maytenus charcoal from Stratum 1 + 2a, F8, showing numerous small vessels and characteristic banding (top to bottom) of parenchyma and fibres; x 68. (f) Transverse fracture of modern, charred Muytenus heterophyllu;

x 68.

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of hot, dry areas, Acacia nigrescens favours clay soils with a high water-holding capacity, Dichrostachys cinerea and Acacia nilotica are bush encroachment species tending to favo&impoverished ground; many other legumes are equally ecologically distinctive. Their anatomical structure appears to differ widely (see Figure 7) yet in the present study secure generic identification was not possible. This was due in part to the small size of some of the fragments which made details such as storied parenchyma in Lonchocarpus or the adjacent strands of chambered parenchyma of Acacia nigrescens difficult to see. However, the main cause was the wide range of structural detail present within some of the species. Charred reference specimens of Dichrostachys cinerea collected from trees growing in differing geographical areas showed major differences in the presence or absence of growth rings, the diameter of vessels and the relative amounts of parenchyma and fibres. Intraspecific variation in Acacia karroo has been noted in other studies (Robbertse et al., 1980). Until more is known of the scale and ecological implications of these variations, legume charcoals cannot be more precisely identified from ancient contexts. The three fragments identified as Rubiaceae are most similar to the genus Canthium, but more reference material of this widespread family needs to be charred before this tentative identification can be substantiated. Figure 7 also illustrates some of the difficulties precluding the use of feature sheets. Legume charcoals commonly contain gelatinous as well as non-gelatinous fibres. Differences in the chemical composition of the wall layers of each fibre type result in differential shrinkage, making conclusions based on wall thickness invalid. The charcoals of Androstachys johnsonii often show extreme shrinkage and homogenization of intercellular material which prevents any meaningful estimate of quantitative features such as ray or vessel frequency being made. Numerical Analysis

The results were used in a principal coordinates analysis (Gower, 1966) using a Fortran 77 program following the outline described in Legendre & Legendre (1983). The program included the calculations of the correlation coefficients between the descriptors (taxa) and the normalized eigenvectors for the similarity matrix between individuals (strata). The similarity index used was the complement of Whittaker’s Index of Association (metric distance D9 in Legendre & Legendre, 1983). This was selected because of its suitability in transforming counts of charcoal fragments to proportions of the various taxa. Analysis of these proportions rather than absolute data was considered more informative in the present study. The results of the principal coordinates analysis are shown in Figure 3. The correlation coefficients identify the importance of taxa relative to each axis. 1 + 2a, 1 + 2b and 1+2c represent the recent strata in squares FS, G9 and H8 respectively. Clustering shows horizontal comparability from the front to the rear of the site. The gradient drawn reflects the extent of taxa shared between axes I and II. A clear affinity exists between Strata 1, 2 and 3 on the one hand and Stratum 5 and 6 on the other. Stratum 4 shows little affinity with either of the two groups, a feature which is consistent with the contrasting character of the deposits. Discussion and Conclusions

The reconstruction of past climates is a complex problem involving many disciplines. In southern Africa, geomorphological studies have concentrated on tufa cycles (Butzer et al., 1978), on fluctuating lake basins (Heine, 1981), on desert expansions (Lancaster, 1981; Thomas & Goudie, 1984) and on the history of slope sediments (Price Williams et al., 1982; Watson et al., 1984). Palaeozoological studies by Klein (1979) and marine core evidence (Prell et al., 1980; Martin, 1981) have enhanced our view of the scale and

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HOLOCENE

CLIMATIC

CHANGE

471

amplitude of climatic change in the Late Pleistocene and Holocene in this part of the earth. Nevertheless, much remains to be studied. Our knowledge of former vegetation in southern Africa is “. . . far too limited” (van Zinderen Bakker, 1978: 135). This is partly explained by the scarcity of pollen-bearing deposits in this relatively dry part of the world (Scott, 19826), but is also due to the scattered nature of the few palaeobotanical records so far available. As long ago as 1941, Godwin & Tansley (1941) recognized the role of charcoals in assessing past vegetational change. Western (1969) argued for the increased use of evidence obtained from archaeological wood and charcoal to complement the wealth of palynological evidence pertaining to the vegetational history of northern Europe. Charcoals are some of the most commonly preserved remains in both geological and archaeological sites of the Quaternary period. They have been widely used as a source of radiometric dating, yet they have been largely ignored as indicators of vegetational history. Although charcoal found in archaeological contexts reflects a degree of human selection, where the woody flora is rich in species and where many elements of this are used for a wide range of medicinal, artifactual, magical and other utilitarian purposes, past assemblages containing the relevant indicator species whose ecological susceptibilities are well understood can provide much detailed climatic information. The value of such an approach in the Fynbos region of southern Africa has recently been demonstrated (Deacon et al., 1983; Tusenius, 1984). The present study illustrates the use of comparable techniques in a different vegetational zone. A high percentage of the fragments, 96.6% of the total available, has been identified. This emphasizes the efficacy of the methodology even where differences in juvenile and mature woods might have proved problematic. However, it seems that because it is woods with a compact anatomical structure that are best preserved as charcoals, certain more vulnerable taxa may be under-represented. This should not preclude palaeoclimatic interpretation based upon assemblages similar in size to those described in the present study. Fragmentation of the remains after charring will always represent a source of statistical error, though numbering each fragment and plotting it in three dimensions during excavation will reduce this problem. In a number of species, SEM has revealed that during the life of the plant, crystals were stored in axial strands of chambered parenchyma or in enlarged ray cells (many legumes and Combretum apiculatum respectively). In the aspen such concentrations are known to be associated with a wood-rotting fungus (Muhammad & Micko, 1984). Several earlier studies report substantially higher mineral concentrations in decayed as compared to healthy wood from the same tree (Wardell & Hart, 1973; Basham & Cowling, 1976). If a similar phenomenon is widespread among subtropical taxa, then the presence of large Figure 7. (a) Transverse fracture of Androstachys johnsonii charcoal from Stratum 5 H8, showing homogenization of intercellular material with resultant loss of structural detail; x 300. (b) Tangential fracture of Androstachys johnsonii charcoal from Stratum 5 H8, showing similar charring damage; x 150. (c) Transverse fracture of legume charcoal from Stratum 1+2b G9, showing two large vessels with inclusions in lumina and aliform parenchyma; x 75. (d) Transverse fracture of legume charcoal from Stratum 1 + 2b G9, showing numerous small vessels with inclusions in lumina and confluent parenchyma; x 75. (e) Thick-walled, gelatinous fibres of legume charcoal from Stratum 1 G9. In four cells in the upper half of the micrograph there is a large space between the thick, inner wall layers (probably S2 and S3) which have separated from the thinner outer layer (probably the primary wall and Sl); x 1200. (f) Thick-walled, non-gelatinous fibres of legume charcoal from Stratum 5 H8. In this case there is no separation of the wall layers; x 1200.

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amounts of such crystalline material in the ancient charcoal may indicate the preferred selection of decayed branches and twigs. In terms of palaeoclimatic interpretation, certain tentative conclusions may be reached for the HolQcene sequence at Siphiso. During early Holocene times, dated in Stratum 6 and Stratum 5, there is a relatively broad spectrum of taxa many of which occur locally in recent times. This would indicate moist conditions between about 9000 and 6000 bp. Stratum 4, which belongs some time in the mid-Holocene, is clearly different. The charcoal assemblage shows a noticeable reduction in the total number of taxa coupled with a lack of any riverine species. Severe climatic conditions are implied for this phase, with reduced tree cover; the substantial percentage increase in the amount of Combretum apiculatum also suggests a drier climate. A return to more moist conditions occurs within the past 3000-4000 years. The assemblage of woods in Strata 3,2 and 1 are similar to those in Strata 6 and 5, with a broad spectrum of taxa, all of which may be found in the area today. Botanical records elsewhere in southern Africa provide some corroboration for these perceived climatic fluctuations. At Wonderwerk Cave in the northern Cape Province, three climatic zones were distinguished on the basis of changing pollen spectra (van Zinderen Bakker, 1982). Zone I, spanning the period about 9200-5000 bp, is described as warmer and wetter, whereas the mid-Holocene Zone II, about 5000-2000 or 3000 bp, is said to be drier with a reduced tree cover. Since 2000 or 3000 bp in Zone III, the climate is described as slightly more humid than Zone II, nearly as today. There is a strong correlation between the palynological evidence from Wonderwerk and that from the adjacent site of Kathu Pan where, according to van Zinderen Bakker (pers. comm.), the changes are roughly parallel. He estimates the climate to be humid and warmer in the early Holocene, followed by a much drier phase in the mid-Holocene, itself succeeded by locally wet conditions in recent times. The chronology at Kathu Pan is not as precise as the Wonderwerk, though a date of 7350f90 bp is available from near the bottom of the earliest pollen zone. Another line of e’vidence from the same general area is provided by the tufa carapaces of the Gaap Escarpment (Butzer et al., 1978). Variations in Kalahari groundwater which are presumed to be correlated with long-term variations in rainfall indicate subhumid conditions between 9700 bp and 75Ot3-6500 bp (phase Via tufa). Subhumid conditions are again said to prevail for SOme time after 4500 bp, continuing sporadically until only a few hundred years ago (phases VIb and VIc tufas). A semi-arid period is inferred between the early and the late Holocene tufa emplacements. One feature which is common to all the sites mentioned, including Siphiso, is the clear indication of moist conditions in the early Holocene. Several lines of evidence suggest a climatic amelioration in the early Holocene (Lorius et al., 1979; Delmas et al., 1980; Salinger, 1981; Street, 1981). Optimum moist conditions were reached by about 9000 bp in the southern hemisphere, some 3000 years earlier than in areas north of the equator, and this is likely to be the reason behind the early Holocene vegetational pattern at Siphiso. A decline in these optimum conditions seems indicated in mid-Holocene times when the climate deteriorated as a result of a decrease in moisture availability. The modern vegetation seems to have been established some time since 4000 bp. Acknowledgements

The authors are greatly indebted to the Swaziland National Trust Commission under whose auspices the research is being carried out. The Commission has financed the fieldwork in Swaziland, with additional grants from the Central Research Fund of the University of London. They are also extremely grateful to the Anglo American De Beers

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Chairman’s Fund which has supported the work at Imperial College. They would like to thank the Herbarium staff at the Royal Botanic Gardens, Kew and the Botanical Research Institute, Pretoria for the authentication of vegetative material, Dr K. L. Alvin of Imperial College for his help throughout the work, Dr D. H. Dalby of Imperial College for writing the computer program (incorporating a subroutine by Prof. R. G. Davies), Dr J. Chard of Edinburgh University for her assistance with the ecological survey and Mr N. Costa of Imperial College for his advice on the scanning electron microscopy and the photographic work. References Acocks, J. P. H. (1953). Veld Types of South Africa. Memoirs of the Botanical Survey of South Africa 28, 1-192. Barefoot, A. C. 8~Hankins, F. W. (1982). Identification of Modern and Tertiary Woods. Oxford: Clarendon Press. Basham, H. G. & Cowling, E. B. (1976). Distribution of essential elements in forest trees and their role in wood deterioration. In (G. Becker & W. Liese, Eds) Organismen und Holz. pp. 155-165. International Symposium Berlin-Dahlem 1975. Berlin: Duncker & Humblot. Beaumont, P. B. (nd.) Communication to the Swaziland National Trust Commission on sitesand samples from Swaziland. Brazier, J. D. & Franklin, G. L. (1961). Identification of hardwoods. A microscope key. Forest Products Research Bulletin 46, 1-19. Butterfield, B. G. 8cMeylan, B. A. (1980). Three Dimensional Structure of Wood, 2nd edition. London, New York: Chapman & Hall. Butzer, K. W., Stuckenrath, R., Bruzewicz, A. J. & Helgren, D. M. (1978). Late Cenozoic paleoclimates of the Gaap Escarpment, Kalahari Margin, South Africa. Quaternary Research 10,310-339. Cleverly, R. W. & Bristow, J. W. (1979). Revised volcanic stratigraphy of the Lebombo monocline. Geological Society of South Africa Transactions 82,227-230. Coates Palgrave, K. (1977). Trees of Southern Africa. Cape Town, Johannesburg: C. Struick. Coetzee, J. A. (1967). Pollen analytical studies in East and Southern Africa. Palaeoecology of Africa 3, 1-146. Deacon, H. J. (1979). Excavations at Boomplaas Cave-a sequencethrough the Upper Pleistoceneand Holocene in South Africa. World Archaeology 10,241-257. Deacon, H. J., Scholtz, A. & Daitz, L. D. (1983). Fossil charcoals as a source of palaeoecological information in the Fynbos region. In (H. J. Deacon, Q. B. Hendy & J. J. N. Lambrechts, Eds) Fynbos Palaeoecology: a Preliminary Synthesis, pp. 174182. South African National Scientific Programme Report 75. Deacon, J. (1972). Wilton: An assessmentafter fifty years. South African Archaeological Bulletin 27, l&48. Deacon, J. (1974). Patterning in the radiocarbon dates for the Wilton/Smithfield complex in southern Africa. South African Archaeological Bulletin 29,3-18. Deacon, J. (1978). Changing patterns in the late Pleistocene/early Holocene prehistory of southern Africa as seen from the Nelson Bay Cave stone artefact sequence. Quaternary Research (NY) 10,84-l 11. Delmas, R. J., Ascencio, J.-M. & Legrand, M. (1980). Polar ice evidence that atmospheric CO, 20,000 yr. BP was 50% of present. Nature 284, 155-157. Godwin, H. & Tansley, A. G. (1941). Prehistoric charcoals as evidence of former vegetation, soil and climate. Journal of Ecology 29, 117-126. Gower, J. C. (1966). Some distance properties of latent root and vector methods used in multivariate analysis. Biometrika 53,325-338. Heine, K. (198 1). Aride und pluviale Bedingungen wlhrend der letzten Kaltzeit in der Siidwest-Kalahari (siidliches Afrika). Zeitschrift fiir Geomorphologie 38, l-37. International Association of Wood Anatomists (198 1). Standard list of characters suitable for computerized hardwood classification. IA WA Bulletin 2,99-I 10.

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