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Geomorphic evidence for the late Quaternary palaeoclimates of the middle Kalahari of northern Botswana
CATENA
Vol. 13, 349-359
Braunschweig 1986
GEOMORPHIC EVIDENCE FOR THE LATE QUATERNARY PALAEOCLIMATES OF THE MIDDLE KALAHARI OF NORTHERN BOTSWANA P.A. Shaw and H.J. Cooke, Gaborone SUMMARY Geomorphological studies of caves and lakes in northern Botswana since 1973 have yielded a suite of 80 radiometric dates which permit palaeoenvironmental reconstruction of the landscape over the last 45,000 years. The unique characteristics of the landforms, when interpreted with caution, yield palaeoclimatic information on both northern Botswana and the catchments of the Okavango River in the Angolan Highlands. Several major shifts in precipitation are indicated for the period 45,000-20,000 BP, with dry conditions prevailing at 25,000 BP. The Late Glacial was characterised by humid conditions, especially from 16,00013,000 BP, whilst the early Holocene was semi-arid, with precipitation peaks circa 6,0005,000 BP and 2,000 BP. The results contrast sharply with some southern African palaeoclimatic descriptions in print, where many of the inferences have been drawn from studies of the northern hemisphere inter-tropical zone. The implications of the study are briefly discussed in relation to palaeoclimatic models of the whole African continent.
1.
INTRODUCTION
Recent attempts to model the late Quaternary palaeoclimates of Africa have been profoundly affected by the dearth of published data for the region between 0 ° and 30°S, itself resulting from a lack of research input and suitable palaeoenvironmental sites. In the context of southern Africa hypothetical reconstructions have arrived at widely varying conclusions (VAN ZINDEREN BAKKER 1976, 1980, 1982, HEINE 1978, 1982) whilst studies on the continental scale (STREET & GROVE 1976, NICHOLSON & FLOHN 1980) have relied extensively on data from the northern and eastern parts of the continent, leading to an assumption that the low latitudes of the southern hemisphere mirror their northern counterparts. This assumption persists in the literature at the present time (WARREN 1985, WILLIAMS 1985, 231). Even where proxy data sets exist, as with lake level studies, the synthesis suffers from data selectivity or over-simplification of the climatic and hydrologic controls (STREET 1981, STREET-PERROTT et al. 1985). The dangers inherent in continental palaeoenvironmental reconstruction have been voiced clearly by BUTZER (1984a), who advocates a return to small scale studies. Nevertheless, in areas where detailed stratigraphic sites are lacking, meso-scale studies can be of prime importance, as BUTZER has himself demonstrated in East Africa (BUTZER et al. 1972). This paper describes a meso-scale study of a region infrequently referred to in the literature, the middle Kalahari* of northern Botswana. Here geomorphological investi* The middle Kalahari is defined after PASSARGE (1904) as the region between the Zambezi and the Kalahari Rise (approximately 17°30' - 22°S, 20° - 27°E). The southern Kalahari occupies the watershed between the Orange River system and the Makgadikgadi, whilst the northern Kalahari consists of the ancient ergs of Zambia, Angola and Zaire. ISSN 0341-8162 © Copyright 1986 by CATENA VERLAG, D 0341-8162/86/5011851/US $ 2.00 + 0.25
3302 Cremlingen-Destedt, W.Germany
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gations of lakes, rivers and caves since 1973 have yielded a suite of 72 C 14 dates and 8 U/Th dates which, for the first time, permit an overview of the late Quaternary history of the region. 75% of these dates have appeared in recent detailed studies (COOKE 1984, COOKE & VERSTAPPEN 1984, SHAW 1985, 1986) whilst the remainder arise from current research. Problems of palaeoclimatic interpretation arising from specific materials and site conditions are discussed, and parallels are drawn from other significant studies carried out in Botswana. Discussion of the results within the context of the African inter-tropical zone suggests that the palaeoclimatic conclusions drawn by large scale studies are over-simplified. In particular the concepts of climatic symmetry about the equator, and of Late Glacial aridity in equatorial Africa, must be questioned.
2.
PALAEOHYDROLOGICAL STUDIES
The major landforms of northern Botswana are shown in Figure 1. Palaeohydrological studies have concentrated on the Ngami, Mababe and Makgadikgadi Basins, together with their associated fluvial links. The region has a complex hydrological history, involving the interaction of the three basins with the major rivers of the region, the Okavango, Chobe and Zambezi, under conditions of changing climate and neo-tectonism. The climatic signal is often difficult to define without good topographic and budgetary studies, and for this reason climatic assumptions made solely on the basis of lake level (STREET-PERROTT et al. 1985) should be avoided. Common to all three basins is a shoreline complex at 940-945 m. asl. known as Lake Palaeo-Makgadikgadi (GREY & COOKE 1977), enclosing a formerly inundated area of some 60,000 km 2 including the lower Okavango Delta, and with access to the Chobe and Zambezi systems. The Makgadikgadi also contains well defined shoreline features at 920 m and 912 m, both indicative of a plentiful water supply to a closed basin stage, whilst the present bed level lies at 890 m in Sua Pan. The Ngami and Mababe basins contain concentric series of shorelines within their closed basins from a level of 932 m down to a sump at 919 m, representing the final stages of desiccation of each lake. A well defined ridge complex at 936 m indicates the coalescence of the two lakes along the Thamalakane axis at the foot of the Okavango Delta. Such a 936 m lake would have had substantial overflows to the Zambezi via the Chobe, and to the Makgadikgadi via the Boteti River, and thus represents an important stage in the dynamics of the Okavango system. Budgetary studies have shown that the palaeoenvironmental interpretation of the lake stages is not straightforward. In the first instance input is derived not only from local precipitation but from the catchment of the Okavango River, which rises in the Angolan Highlands at 12°S. Whereas deltaic landforms associated with the 945 m and 936 m levels suggest increased local input during times of high lake level there is no means of quantifying this, and comparison with data from the adjacent Drotsky's Cave becomes important. Furthermore, little is known of the role of the channels peripheral to the lake complex, such as the fossil Groote Laagte and Okwa drainage, and the smaller rivers of the eastern Makgadikgadi. Also inherent in lake level studies is the masking of earlier levels by subsequent rise. Thus time spans indicated for a specific level will probably include several major fluctuations that remain undetected with current techniques. The 936 m lake has been shown to be relatively stable in tectonic terms; its budget could be maintained by a 160-225% increase in precipitation (SHAW 1985), although less would be
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required were distant inflow to be increased or lower evaporation rates prevail. The 945 m • • stage, however, would requxre an input of some 50 km 3 p .a. (GROVE 1969), a figure that lies beyond budgetary credibility without substantial flow from the Zambezi system. As this may involve tectonism it is difficult to speculate upon the Lake Palaeo-Makgadikgadi stage in purely climatic terms. A total of 37 C 14dates are available for the palaeohydrological studies, of which 33 have been published (COOKE 1984, COOKE & VERSTAPPEN 1984, SHAW 1985). Four dates obtained during the course of recent investigations are shown in Table 1, and include two dates from a terrace of the Chobe River at Serondella, close to the confluence with the Zambezi River. Most of the dates are from calcretes, with peat, shell and lithified wood as less common indicators. As the palaeoclimatic significance of calcrete is a matter of some debate (RUST et al. 1984), the material has been used purely in a stratigraphic context throughout, without palaeoclimatic interpretation. One assumption made is that calcrete formation on the scale found in the vicinity of these lakes is most likely to occur during phases of falling water levels and thus immediately postdates the high level in question. The Makgadikgadi Basin has revealed the most complete sequence to date. From low levels circa 46,000 BP the lake rose to 945 m between 40,000-35,000 BP before returning to low level conditions at 35,000-26,000 BP. Between 26,000-10,000 BP the Makgadikgadi was occupied by a succession of lakes at the 920 m level, interspersed with reduced levels, as at 21,000 BP and 19,000 BP, when the 912 m level is indicated. A similar range of fluctuations between desiccation and 912 m occupies the last 10,000 years, with pronounced 912 m levels at 3,000 BP and 1,700 BP. Dating of the 945 m level in the Ngami and Mababe Basins has proved inconclusive so far, though theoretically they should correlate with the Makdagikgadi. High levels at 936 m however, are indicated for 17,000-12,000 BP and c2,000 BP, which would provide overflow to the Makgadikgadi. Of interest in this context are the two dates from Serondella shown in Table 1 which indicate the presence of an extensive sheet of water at the Chobe-Zambezi confluence at 16,000-11,000 BP which would link to the Ngami-Mababe system. The significance of this link is being investigated at the present time. Low levels of the Ngami-Mababe system are indicated c25,000 BP and 8,000-6,000 BP, although the level of Lake Ngami was rising by the mid-Holocene. Other dates for the lakes are found in the literature but have not been incorporated into this study. Thus HELGREN (1984) provides two dates associated with LSA settlement on the 912 m level of the Makgadikgadi. HEINE (1978, 1982) has published 16 dates for the
Tab. 1: FURTHER DATES RELATING TO THE PALAEOHYDROLOGY OF NORTHERN BOTSWANA (To be read in conjunction with COOKE 1984, COOKE & VERSTAPPEN 1984, SHAW 1985) Laboratory no. Altitude (m) GrN 13191 933 m GrN 13192 933 m GrN 13193 934 m
Site
Material
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GrN 13194
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Terrace calcrete Shell (spp Lymnaeae) Lithified wood (Acacia tortillis) sandy lacustrine calcrete
930 m
Date (BP) 11,550 __ 110 15,380 + 140 1,900 + 40 9,580 _+ 90
QUATERNARY PALAEOCLIMATES OF NORTHERN BOTSWANA
353
greater Makgadikgadi area. This latter sequence is potentially of great value, but in the absence of locational or altitudinal data it is not possible to ascertain the relevance of the dates.
3.
SPELEOLOGICAL STUDIES
Studies of Drotsky's Cave in the Kwihabe Hills of northwest Botswana (COOKE 1975) (Figure 1) have revealed a sedimentation sequence involving the deposition and resolution ofsinter in the form of stalagmites, stalactites and a variety of flows. Also deposited within the cave are sand and material released by the dissolution of dolomite, rotting shale bands and a variety of organic debris left by living organisms which have inhabited the cave. These sedimentary processes are climatically controlled and can be related to the evolution of landforms in the adjacent Kwihabe Valley. A series of stages can be identified: (1) Solution of bedrock within the cave occurs during periods of high water table, with the cave itself in the phreatic zone. This water table is controlled by the level of the river in the Kwihabe Valley, itself consequent upon high rainfall in the catchment of the Aha and Kwihabe Hills. (2) The water table lowers due to incision of the Kwihabe Valley, leading to vadose conditions in the cave. Continuing high local rainfall supplies sufficient water for the dissolution of the bedrock in the vadose zone and the deposition of massive sinter formations within the cave void. (3) Arid conditions result in the deposition of Kalahari sand within the cave following the opening of the cave by collapse at two points. At the same time the Kwihabe Valley is aggraded and infilled by blown sand. (4) Increasing rainfall leads again to the re-establishment of river flow in the valley, with re-excavation of the valley form. Initial high water tables return the cave to phreatic conditions, leading to renewed solution of bedrock and older sinters. (5) Conditions return to stage (2), with river incision, falling water table, and the deposition of fresh sinter. A cyclical pattern is thus apparent, and there is evidence for a number of these cycles in the cave. It is also evident that the climatic controls of the cycle are local in nature, and may not reflect the conditions found in the Makgadikgadi-Okavango system to the east, which is influenced by the climate of a much wider area. It has been estimated (COOKE 1975, 443) that periods of rapid sinter deposition may represent phases with rainfall ofupto 300% of the present mean. Dating of the sinter growth in the cave and calcrete formation in the adjacent valley is provided by 25 C 14dates published in final form in 1977 (COOKE & VERHAGEN 1977) and shows periods ofsinter deposition at 45,000-37,000 BP, 34,000-29,000 BP and 16,000-13,000 BP. The latter sequence, comprising 8 dates from 3 stalagmites, is impressive evidence for higher local precipitation at the time. Holocene sinter formation is indicated for 2,000 BP and 750 BP. Recent research in Drotsky's Cave has yielded a further sequence of 10 C 14dates from 3 stalagmites previously undated, together with 8 U/Th dates on the same samples to test dating accuracy. These dates are shown in Table 2. Good correlation was achieved between the C 14and U/Th series, with stalagmite 1 confirming the conclusions drawn previously. Further mid-Holocene humid intervals are indicated by stalagmites 2 and 3 at 6,000-5,000 BP and c 4,000 BP.
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Tab. 2: FURTHER DATES FROM SINTER, DROTSKY's CAVE, KWIHABE HILLS. (To be read in conjunction with COOKE & VERHAGEN 1977, COOKE 1984) Laboratory no. Hv 13288 Hv 13289 Hv 13290 Hv 13291 Hv 13292 Hv 13293 Hv 13294 Hv 13295
Sample description Stalagmite 1: sample a ,, sample b ,, sample c Stalagmite 2: sample a ,, sample b ,, . sample c Stalagmite 3: sample a ,, sample b
C ~4 date 16,190 + 15,600 + 14,125 + 5,860 + 5,060 + 3,665 + 5,375 + 1,185 +
U/Th date 19,400 + 900 18,000 + 800 15,600 + 1,000 6,900 + 400 5,800 + 400 4,500 + 400 6,900 + 400 1,800 + 400
200 190 175 105 100 125 100 105
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Fig. 2: Summary ofpalaeoclimatic data, Botswana. Sources: (a) COOKE & VERSTAPPEN 1984, COOKE 1984; (b) SHAW 1985, this paper; (c) This paper; (d) COOKE & VERHAGEN 1977, COOKE 1984, this paper; (e) this paper; (f) LANCASTER 1979; (g) HEINE 1982; (h) HELGREN & BROOKS 1983.
4.
T H E P A L A E O C L I M A T I C I N T E R P R E T A T I O N OF THE DATA
The results from the northern Botswana studies are summarised in Figure 2. Also shown are the results of other studies carried out in Botswana, including the investigations at Gi Pan in the D o b e Valley o f northwest Botswana ( H E L G R E N & BROOKS 1983) and at Urwi Pan close to the Namibia border ( L A N C A S T E R 1979). A series o f 8 C 14 shell dates representing perennial flow in the Molopo River are taken from H E I N E (1982). Current research in caves in the Transvaal dolomite near Lobatse in southeast Boswana, an area as
QUATERNARY PALAEOCLIMATES OF NORTHERN BOTSWANA
355
yet uncharted in palaeoclimatic studies (DEACON et al. 1984) has provided six initial sinter dates. The present semi-arid climate is characterised geomorphologically by dry lake beds and sporadic small scale sinter deposition, although precipitation, averaging 500 mm p.a. at Maun, is sufficient to stabilise dune systems. The circulation is dominated by a sub-tropical high pressure cell centered over the Transvaal. During the summer months low pressure develops over the interior of the sub-continent leading to incursions of moist air from the Atlantic and Indian Oceans; these incursions account for some 95% of the total rainfall of northern Botswana. In palaeoclimatic terms, however, the region is essentially transitional, liable to intensification of the present system, or to a possible increase in winter rainfall from a northward shift and intensification of the Westerlies. The study of sinter growth and lake levels yields complementary data sets which need not show a high degree of correlation. As cave studies provide information on local conditions whilst lake levels respond to both local and upper catchment inputs, it is possible to postulate climatic patterns for the region as follows: (1) High precipitation in both the Angolan Highlands and in northern Botswana, leading to high lake levels and corresponding flow along local drainage lines. In caves flooding, with phreatic conditions, occurs, accompanied by re-solution of sinter. (2) High local precipitation with low Angolan input leads to rapid fluctuation of the lakes over a wide range of levels in response to short term and seasonal oscillations in rainfall. In caves the high local rainfall causes sinter deposition under vadose conditions. (3) High Angolan input with drier local conditions leads to high lake levels with ephemeral conditions in local rivers. Caves dry out, with sand infill replacing sinter formation as the dominant process. Calcrete formation at lake edges could be a response to this climatic type. (4) Arid conditions throughout are indicated by low lake levels, dune migration and the infllling of drainage channels with blown sand. Again sinter growth is replaced by sand infill in the cave complex. Bearing in mind these possibilities, the data presented suggests the following conclusions:
45,000-20,000 BP Initial arid conditions are indicate at c 45,000 BP for the Makgadikgadi, but must be regarded with caution due to the limitations of C 14dating. High precipitation prevailed both locally and in the Angolan catchment at 40,000-35,000 BP. As the lakes stood at the Lake Palaeo-Makgadikgadi 945 m stage some of the hydrological variation must be attributed to non-climatic factors. However, any input from the Zambezi would require increased flows, and thus greater precipitation between 12°-18°S. Between 35,000-25,000 BP conditions varied between patterns 2 and 4. Increased local precipitation is indicated at Drotsky's Cave about 30,000 BP and ar nearby Gi Pan at 35,000 BP. The Makgadikgadi Basin was generally dry, although higher levels may well be masked by subsequent events. By 25,000 BP both the Ngami and Mababe Basins were also desiccated, suggesting conditions at least as dry as at present. From 25,000-20,000 BP the Makgadikgadi fined to 920 m on a number of occasions, interspersed with periods of low levels. The high levels were likely to have been supported by increased inflow, although Gi Pan shows locally wetter conditions at 25,000-22,300 BP. In southeast Botswana, Lobatse Cave shows renewed sinter deposition at 26,000-23,000 BP.
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SHAW & COOKE
20,000-10,000 BP: The Late Glacial Stage
At the Late Glacial Maximum of c 19,000 BP low lake levels occured, although cooler conditions and adequate local rainfall permitted the formation of peat near Tsoi, close to the lower Boteti (GrN 9719). This stage was followed by a profound increase in local moisture availability. Massive sinter growth occured at Kwihabe between 16,000-13,000 BP, whilst the Ngami and Mababe Lakes rose to their 936 m level. Overflow occured along the Boteti to flood the Makgadikgadi to the 920 m level, and to the Zambezi system, where water was ponded back upstream of the Mambova Falls. The humid conditions of the Late Glacial are also evident from adjacent sites. Sinter formation was renewed at Lobatse c 18,000 BP, whilst lacustrine conditions occured in Urwi Pan c 16,000 BP. HEINE's Molopo River dates indicate perennial flow from 19,000-12,500 BP. To the west Etosha Pan (HEINE 1982) contained water at 13,700 BP, whilst in the Cape Province BUTZER et al. (1973) record the existence of an impressive lake at Alexandersfontein between 16,100-13,600 BP. Although the lower temperatures of the Late Glacial undoubtedly reduced evaporation rates, the wealth of geomorphological data indicates increased precipitation throughout the Kalahari during this period. From 16,000-13,000 BP this increase was particularly effective, and was supplemented in the lake system by an increase in inflow. Decrease in precipitation appears to have set in about 12,000 BP. The series of fluvial calcretes in the Kwihabe Valley and at Savuti River dated at 11,000-9,500 BP indicate diminishing fiver flows and a falling lake level in the Mababe Depression. 10,000 BP - Present: The Holocene
By 9,500 BP conditions essentially similar to the present had been established. Semiaridity prevailed until c 6,000 BP, when the level of Lake Ngami rose to overflow level and sinter deposition was re-established in Drotsky's Cave. A number of cyclical increases in rainfall are indicated by sinter growth at 6,000-5,000, 4,200-3,600 and 2,500-2,000 BP. The latter is coincident with the last major fluctuations of the Ngami-Mababe Lake to 936 m and the Makgadikgadi to 912 m, again as a response to increased precipitation. Within the last two milennia conditions approximating to the present have prevailed, with low amplitude fluctuations in precipitation indicated from Drotsky's Cave and from the archaeological record (DENBOW & WILMSEN 1986).
5.
DISCUSSION
The palaeoclimatic conclusions drawn in the previous section have varying degrees of confidence, dependent upon the limitations imposed by C 14 dating techniques, and the range of variables encountered in the geomorphic systems involved. For the period 45,00020,000 BP the results will remain equivocal until the influence of tectonism and 'hidden' lake levels are established by further research, particularly core analysis, over the range of available sites. However, the palaeoclimatic conditions encountered in northern Botswana between 20,000 BP and the present are established, in broad terms, with greater certitude. It is important to note that these conditions are at variance with the inter-tropical zone north of the equator, where palaeo-lake studies at a number of sites (STREET-PERROTr & ROBERTS 1983) have established minimal levels, with cool dry conditions, from 17,000-12,500 BP.
QUATERNARY PALAEOCLIMATES OF NORTHERN BOTSWANA
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Conversely high lake levels were experienced at c 9,000 BP and 7,300-6,800 BP, before contemporary conditions were established in the mid to late Holocene. These fluctuations are best explained by a northerly shift of the ITCZ and an enhancement of the monsoonal circulation at times of lake highs (KUTZBACH 1983). The study by STREET-PERROTr & ROBERTS found insufficient evidence to comment on the southern hemisphere circulation, so it is pertinent to examine the chequered history of the middle Kalahari in the palaeoclimatic literature, with particular reference to the Late Glacial. An early attempt to reconstruct the climate of southern Africa (VAN ZINDEREN BAKKER 1976) was based on evidence outside the Kalahari, and led to a provisional assumption that the middle Kalahari would experience arid conditions during glacial phases, attributable to reduced evaporation and increased wind strengths. Subsequent publications (VAN ZINDEREN BAKKER 1980, 1982, HEINE 1978a, 1978b, 1979, 1982) have reinforced this view, although allowing for the incidence of higher summer rainfall in the southern Kalahari suggested by BUTZER et al. (1973) and LANCASTER (1979). HEINE substantiated the scenario by the interpretation of his 16 C 14dates for the Makgadikgadi area as indicative of dry bed conditions between 19,000-12,000 BP, following a palaeo-lake phase at 30,000-19,000 BP, and succeeded by a further lacustrine episode during the Holocene climatic optimum. The imprecise context of the dated samples has led to criticism (COOKE 1979, BUTZER 1984a, HELGREN 1984, RUST et al. 1984). To explain the contrast between arid Late Glacial conditions in northern Botswana, and more humid conditions in the southern Kalahari and at Etosha Pan, HEINE suggested an intensification rather than a shift of the general circulation patterns, stronger upwelling of the Benguella Current, and a strengthening of the west-east temperature gradient to give higher wind velocities to initiate dune movement. Thus, whilst the southern Kalahari received greater rainfall, high evaporation accounted for the desiccation of the Makgadikgadi and greater aridity in Namibia at 19,000-13,000 BP. Conversely the review of 98 C 14dates for southwest Africa for the 30,000-5,000 BP time span by RUST et al. (1984) concluded that conditions in both the Namib and Kalahari were distinctly more humid between 16,000-13,000 BP. Drawing on the suggestions of LANCASTER (1979) for greater summer rainfall due to a more southerly position of the ITCZ, and BUTZER et al.'s (1973) alternative option of more effectivewinter cyclonic rains, RUST et al. propose that the increased humidity was dependant on the variable interaction of increased winter and summer rains on a season to season basis. Given the transitional characteristics of the region we can only concur with this view, although recognising, in the absence of definitive evidence (BUTZER 1984b), that the mechanisms remain speculative. Such a pattern has relevance for more generalised climatic modelling. The evidence presented here suggests that the southern hemisphere intertropical zone was not merely a parallel of its northern counterpart, with overall reductions in precipitation during glacial phases. On the contrary, there is evidence of strong but opposite precipitation characteristics to the northern hemisphere, a possible attribute of the meridional shifts of the meteorological equator proposed in some models (NICHOLSON & FLOHN 1980, KUTZBACH 1983), but not pursued to their logical conclusion. However, such a generalisation ignores the true complexity of the field evidence encountered in northern Botswana. Overall patterns of palaeoclimatic data distribution in southern Africa give credence to BUTZER's (1984a) assertion that continent-wide palaeometeorological hypothesising is best postponed until the regional patterns are better understood.
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ACKNOWLEDGEMENTS Radiometric dating was carried out by Prof. W.G. Mook of the University of Groningen, Prof. M.A. Geyh of the Nieders/ichsisches Landesamt t'tir Bodenforschung, Hannover, Dr. B.Th. Verhagen of the University of the Witwatersrand, and Dr. A.S. Talma of CSIR, Pretoria. Financial assistance was provided by the University of Botswana, NUFFIC and De Beers (Botswana) Ltd. Logistical support on the field was provided by the Department of Water Affairs and Department of Lands and Surveys, Government of Botswana. The illustrations were drawn by Mr. B. Makwiti. The authors wish to thank all of them for their assistance.
BIBLIOGRAPHY
BUTZER, ILW. (1984a): Archaeology and Quaternary environment in the interior of Southern Africa. In: R. Klein (ed.): Southern African prehistory and palaeoenvironments. A.A. Balkema, Rotterdam. 404 pp. BUTZER, K.W. (1984b): Late Quaternary environments in South Africa. In: J. Vogel (ed.): Late Cenozoic palaeoclimates of the southern Hemisphere. A.A. Balkema, Rotterdam, 520 pp. BUTZER, K.W., FOCK, G.J., STUCKENRATH, tL & ZILCH, A. (1973): Palaeohydrology of Late Pleistocene Lake Alexandersfontein, South Africa. Nature 243, 328-330. BUTZER, K.W., ISSAC, G.L., RICHARDSON, J.L. & WASHBORN-KAMAU, C. (1972): Radiocarbon dating of East African Lake levels. Nature 175, 1069-1076. COOKE, H.J. ( 1975): The palaeoclimatic significance of caves and adjacent landforms in western Ngamiland, Botswana. Geogr. Jour. 141, 430-444. COOKE, H.J. (1979): K. Heine: Radiocarbon chronology of Late Quaternary Lakes in the Kalahari, Southern Africa. A discussion. CATENA 6, 107. COOKE, H.J. (1984): The evidence from northern Botswana of climatic change. In: J. Vogel (ed.): Late Cenozioc palaeoclimates of the southern Hemisphere. A.A. Balkema, Rotterdam. 520 pp. COOKE, H.J. & VERHAGEN, B.Th. (1977): The dating of cave development - an example from Botswana. Proc. 7th. Intern. Spel. Cong. (Sheffield). ~OOKE, H.J. & VERSTAPPEN, H.Th. (1984): The landforms of the western Makgadikgadi Basin in northern Botswana, with a consideration of the chronology of the evolution of Lake PaleoMakgadikgadi. Z. Geomorph. NF. 28, 1-19. DEACON, J., LANCASTER, N. & SCOTT, L. (1984): Evidence for late Quaternary climatic change in southern Africa: Summary of the proceedings ofthe SASQUAWorkshop held in Johannesburg, 1983. In: J. Vogel (ed.): Late Cenozoic Paleoclimates ofthe southern Hemisphere. A.A. Balkema Rotterdam, 520 pp. DENBOW, J. & WlLMSEN, E. (1986): After the flood: a preliminary account of recent geological, archaeological and liguistic investigations in the Okavango region of northern Botswana. Essays in honour of Oswin K6hler. K61n University. GREY, D.1LC. & COOKE, H.J. (1977): Some problems in the Quaternary evolution of the landforms of northern Botswana. CATENA4, 123-133. GROVE, A.T. (1969): Landforms and climatic change in the Kalahari and Ngamiland. Geogr. Jour. 135, 191-212. HEINE, K. (1978a): Jungquart~re Pluviale und Interpluviale in der Kalahari (Siadliches Afrika). Palaeoecology of Africa 10, 31-39. HEINE, K. (1978b): Radiocarbon chronology of the Late Quaternary Lakes in the Kalahari. CATENA 5, 145-149. HEINE, K. (1979): Reply to Cooke's discussion of"K. Heine: Radiocarbon chronology of the Late Quaternary Lakes in the Kalahari". CATENA 6, 259-266. HEINE, K. (1982): The main stages of the Late Quaternary evolution of the Kalahari region, southern Africa. Palaeoecology of Africa 15, 53-76. HELGREN, D.M. (1984): Historical geomorphology and geoarchaeology in the southwestern Makgadikgadi Basin, Botswana. Ann. Assoc. Amer. Geogr. 74, 298-307. HELGREN, D.M. & BROOKS, A. (1983): Geoarchaeology ofGi, a Middle Stone Age site in the northwest Kalahari. Jour. Arch. Sci. 10, 181-187.
QUATERNARY PALAEOCLIMATESOF NORTHERN BOTSWANA
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KUTZBACH, J.E. (1983): Monsoon rains of the late Pleistocene and early Holocene: patterns, intensity and possible causes of changes. In: F. STREET-PERROTI" et al. (ed.): Variations in the Global Water Budget. Reidel, Dordrecht. 518 pp. LANCASTER, N. (1979): Evidence for a widespread late Pleistocene humid period in the Kalahari. Nature 279, 145-146. NICHOLSON, S.E. & FLOHN, H. (1980): African environmental and climatic changes and the general atmospheric circulation in late Pleistocene and Holocene. Climatic Change 2, 313-348. PASSARGE, S. (1904): Die Kalahari. Dietrich Riemer, Berlin. RUST, U., SCHMIDT, H.H. & DIETZ, K.R. (1984): Palaeoenvironments of the present day arid south western Africa 30,000-5,000 BP: results and problems. Palaeoecologyof Africa 16, 109-148. SHAW, P.A. (1985): Late Quaternary landforms and environmental change in northwest Botswana: the evidence of Lake Ngami and the Mababe Depression. Trans. Inst. Brit. Geogr. NS. 10, 333-346. SHAW, P.A. (1986): The palaeohydrology of the Okavango Delta - some preliminary results. Palaeoecology of Africa 17, 51-58. STREET, F.A. (1981): Tropical palaeoenvironments. Prog. in Phys. Geog. 5, 157-185. STREET, F.A. & GROVE, A.T. (1976): Environmental and climatic implications of late Quaternary lake-level fluctuations in Africa. Nature 261,385-390. STREET-PERROTI', F.A. & ROBERTS, N. (1983): Fluctuations in closed-basin lakes as an indicator of past atmospheric circulation patterns. In: EA. STREET-PERROTr et al. (eds.): Variations in the Global Water Budget. Reidel, Dordrecht. 518 pp. STREET-PERROTr, F.A., ROBERTS, N. & METCALFE, S. (1985): Geomorphicimplicationsof late Quaternary hydrological and climatic changes in the Northern hemisphere tropics. In: I. DOUGLAS & T. SPENCER: Environmental change and tropical geomorphology. Unwin, London. 378 pp. VAN ZINDEREN BAKKER, E.M. (1976): The evolution of Late Quaternary palaeoclimatesof southern Africa. Palaeoecology of Africa 9, 160-202. VAN ZINDEREN BAKKER, E.M. (1980): Comparison of Late Quaternary climatic evolutions in the Sahara and Namib-Kalahari region. Palaeoecology of Africa 12, 381-394. VAN ZINDEREN BAKKER, E.M. (1982): African palaeoenvironments 18,000 yrs BP. Palaeoecology of Africa 15, 77-99. WARREN, A. (1985): Arid geomorphology. Prog. in Phys. Geog. 9, 434-441. WILLIAMS, M.A.J. (1985): Pleistocene aridity in tropical Africa, Australia and Asia. In: I. DOUGLAS & T. SPENCER (eds.): Environmental change and tropical geomorphology. Unwin, London. 378 pp.
Address of authors: P.A. Shaw and H.J. Cooke, Department of Environmental Science, University of Botswana P/Bag 0022, Gaborone, Botswana
Vol. 13, 349-359
Braunschweig 1986
GEOMORPHIC EVIDENCE FOR THE LATE QUATERNARY PALAEOCLIMATES OF THE MIDDLE KALAHARI OF NORTHERN BOTSWANA P.A. Shaw and H.J. Cooke, Gaborone SUMMARY Geomorphological studies of caves and lakes in northern Botswana since 1973 have yielded a suite of 80 radiometric dates which permit palaeoenvironmental reconstruction of the landscape over the last 45,000 years. The unique characteristics of the landforms, when interpreted with caution, yield palaeoclimatic information on both northern Botswana and the catchments of the Okavango River in the Angolan Highlands. Several major shifts in precipitation are indicated for the period 45,000-20,000 BP, with dry conditions prevailing at 25,000 BP. The Late Glacial was characterised by humid conditions, especially from 16,00013,000 BP, whilst the early Holocene was semi-arid, with precipitation peaks circa 6,0005,000 BP and 2,000 BP. The results contrast sharply with some southern African palaeoclimatic descriptions in print, where many of the inferences have been drawn from studies of the northern hemisphere inter-tropical zone. The implications of the study are briefly discussed in relation to palaeoclimatic models of the whole African continent.
1.
INTRODUCTION
Recent attempts to model the late Quaternary palaeoclimates of Africa have been profoundly affected by the dearth of published data for the region between 0 ° and 30°S, itself resulting from a lack of research input and suitable palaeoenvironmental sites. In the context of southern Africa hypothetical reconstructions have arrived at widely varying conclusions (VAN ZINDEREN BAKKER 1976, 1980, 1982, HEINE 1978, 1982) whilst studies on the continental scale (STREET & GROVE 1976, NICHOLSON & FLOHN 1980) have relied extensively on data from the northern and eastern parts of the continent, leading to an assumption that the low latitudes of the southern hemisphere mirror their northern counterparts. This assumption persists in the literature at the present time (WARREN 1985, WILLIAMS 1985, 231). Even where proxy data sets exist, as with lake level studies, the synthesis suffers from data selectivity or over-simplification of the climatic and hydrologic controls (STREET 1981, STREET-PERROTT et al. 1985). The dangers inherent in continental palaeoenvironmental reconstruction have been voiced clearly by BUTZER (1984a), who advocates a return to small scale studies. Nevertheless, in areas where detailed stratigraphic sites are lacking, meso-scale studies can be of prime importance, as BUTZER has himself demonstrated in East Africa (BUTZER et al. 1972). This paper describes a meso-scale study of a region infrequently referred to in the literature, the middle Kalahari* of northern Botswana. Here geomorphological investi* The middle Kalahari is defined after PASSARGE (1904) as the region between the Zambezi and the Kalahari Rise (approximately 17°30' - 22°S, 20° - 27°E). The southern Kalahari occupies the watershed between the Orange River system and the Makgadikgadi, whilst the northern Kalahari consists of the ancient ergs of Zambia, Angola and Zaire. ISSN 0341-8162 © Copyright 1986 by CATENA VERLAG, D 0341-8162/86/5011851/US $ 2.00 + 0.25
3302 Cremlingen-Destedt, W.Germany
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gations of lakes, rivers and caves since 1973 have yielded a suite of 72 C 14 dates and 8 U/Th dates which, for the first time, permit an overview of the late Quaternary history of the region. 75% of these dates have appeared in recent detailed studies (COOKE 1984, COOKE & VERSTAPPEN 1984, SHAW 1985, 1986) whilst the remainder arise from current research. Problems of palaeoclimatic interpretation arising from specific materials and site conditions are discussed, and parallels are drawn from other significant studies carried out in Botswana. Discussion of the results within the context of the African inter-tropical zone suggests that the palaeoclimatic conclusions drawn by large scale studies are over-simplified. In particular the concepts of climatic symmetry about the equator, and of Late Glacial aridity in equatorial Africa, must be questioned.
2.
PALAEOHYDROLOGICAL STUDIES
The major landforms of northern Botswana are shown in Figure 1. Palaeohydrological studies have concentrated on the Ngami, Mababe and Makgadikgadi Basins, together with their associated fluvial links. The region has a complex hydrological history, involving the interaction of the three basins with the major rivers of the region, the Okavango, Chobe and Zambezi, under conditions of changing climate and neo-tectonism. The climatic signal is often difficult to define without good topographic and budgetary studies, and for this reason climatic assumptions made solely on the basis of lake level (STREET-PERROTT et al. 1985) should be avoided. Common to all three basins is a shoreline complex at 940-945 m. asl. known as Lake Palaeo-Makgadikgadi (GREY & COOKE 1977), enclosing a formerly inundated area of some 60,000 km 2 including the lower Okavango Delta, and with access to the Chobe and Zambezi systems. The Makgadikgadi also contains well defined shoreline features at 920 m and 912 m, both indicative of a plentiful water supply to a closed basin stage, whilst the present bed level lies at 890 m in Sua Pan. The Ngami and Mababe basins contain concentric series of shorelines within their closed basins from a level of 932 m down to a sump at 919 m, representing the final stages of desiccation of each lake. A well defined ridge complex at 936 m indicates the coalescence of the two lakes along the Thamalakane axis at the foot of the Okavango Delta. Such a 936 m lake would have had substantial overflows to the Zambezi via the Chobe, and to the Makgadikgadi via the Boteti River, and thus represents an important stage in the dynamics of the Okavango system. Budgetary studies have shown that the palaeoenvironmental interpretation of the lake stages is not straightforward. In the first instance input is derived not only from local precipitation but from the catchment of the Okavango River, which rises in the Angolan Highlands at 12°S. Whereas deltaic landforms associated with the 945 m and 936 m levels suggest increased local input during times of high lake level there is no means of quantifying this, and comparison with data from the adjacent Drotsky's Cave becomes important. Furthermore, little is known of the role of the channels peripheral to the lake complex, such as the fossil Groote Laagte and Okwa drainage, and the smaller rivers of the eastern Makgadikgadi. Also inherent in lake level studies is the masking of earlier levels by subsequent rise. Thus time spans indicated for a specific level will probably include several major fluctuations that remain undetected with current techniques. The 936 m lake has been shown to be relatively stable in tectonic terms; its budget could be maintained by a 160-225% increase in precipitation (SHAW 1985), although less would be
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required were distant inflow to be increased or lower evaporation rates prevail. The 945 m • • stage, however, would requxre an input of some 50 km 3 p .a. (GROVE 1969), a figure that lies beyond budgetary credibility without substantial flow from the Zambezi system. As this may involve tectonism it is difficult to speculate upon the Lake Palaeo-Makgadikgadi stage in purely climatic terms. A total of 37 C 14dates are available for the palaeohydrological studies, of which 33 have been published (COOKE 1984, COOKE & VERSTAPPEN 1984, SHAW 1985). Four dates obtained during the course of recent investigations are shown in Table 1, and include two dates from a terrace of the Chobe River at Serondella, close to the confluence with the Zambezi River. Most of the dates are from calcretes, with peat, shell and lithified wood as less common indicators. As the palaeoclimatic significance of calcrete is a matter of some debate (RUST et al. 1984), the material has been used purely in a stratigraphic context throughout, without palaeoclimatic interpretation. One assumption made is that calcrete formation on the scale found in the vicinity of these lakes is most likely to occur during phases of falling water levels and thus immediately postdates the high level in question. The Makgadikgadi Basin has revealed the most complete sequence to date. From low levels circa 46,000 BP the lake rose to 945 m between 40,000-35,000 BP before returning to low level conditions at 35,000-26,000 BP. Between 26,000-10,000 BP the Makgadikgadi was occupied by a succession of lakes at the 920 m level, interspersed with reduced levels, as at 21,000 BP and 19,000 BP, when the 912 m level is indicated. A similar range of fluctuations between desiccation and 912 m occupies the last 10,000 years, with pronounced 912 m levels at 3,000 BP and 1,700 BP. Dating of the 945 m level in the Ngami and Mababe Basins has proved inconclusive so far, though theoretically they should correlate with the Makdagikgadi. High levels at 936 m however, are indicated for 17,000-12,000 BP and c2,000 BP, which would provide overflow to the Makgadikgadi. Of interest in this context are the two dates from Serondella shown in Table 1 which indicate the presence of an extensive sheet of water at the Chobe-Zambezi confluence at 16,000-11,000 BP which would link to the Ngami-Mababe system. The significance of this link is being investigated at the present time. Low levels of the Ngami-Mababe system are indicated c25,000 BP and 8,000-6,000 BP, although the level of Lake Ngami was rising by the mid-Holocene. Other dates for the lakes are found in the literature but have not been incorporated into this study. Thus HELGREN (1984) provides two dates associated with LSA settlement on the 912 m level of the Makgadikgadi. HEINE (1978, 1982) has published 16 dates for the
Tab. 1: FURTHER DATES RELATING TO THE PALAEOHYDROLOGY OF NORTHERN BOTSWANA (To be read in conjunction with COOKE 1984, COOKE & VERSTAPPEN 1984, SHAW 1985) Laboratory no. Altitude (m) GrN 13191 933 m GrN 13192 933 m GrN 13193 934 m
Site
Material
Chobe River - Serondella Chobe River - Serondella Ngami Basin Phatane Gap
GrN 13194
Mababe Basin - Savuti R.
Terrace calcrete Shell (spp Lymnaeae) Lithified wood (Acacia tortillis) sandy lacustrine calcrete
930 m
Date (BP) 11,550 __ 110 15,380 + 140 1,900 + 40 9,580 _+ 90
QUATERNARY PALAEOCLIMATES OF NORTHERN BOTSWANA
353
greater Makgadikgadi area. This latter sequence is potentially of great value, but in the absence of locational or altitudinal data it is not possible to ascertain the relevance of the dates.
3.
SPELEOLOGICAL STUDIES
Studies of Drotsky's Cave in the Kwihabe Hills of northwest Botswana (COOKE 1975) (Figure 1) have revealed a sedimentation sequence involving the deposition and resolution ofsinter in the form of stalagmites, stalactites and a variety of flows. Also deposited within the cave are sand and material released by the dissolution of dolomite, rotting shale bands and a variety of organic debris left by living organisms which have inhabited the cave. These sedimentary processes are climatically controlled and can be related to the evolution of landforms in the adjacent Kwihabe Valley. A series of stages can be identified: (1) Solution of bedrock within the cave occurs during periods of high water table, with the cave itself in the phreatic zone. This water table is controlled by the level of the river in the Kwihabe Valley, itself consequent upon high rainfall in the catchment of the Aha and Kwihabe Hills. (2) The water table lowers due to incision of the Kwihabe Valley, leading to vadose conditions in the cave. Continuing high local rainfall supplies sufficient water for the dissolution of the bedrock in the vadose zone and the deposition of massive sinter formations within the cave void. (3) Arid conditions result in the deposition of Kalahari sand within the cave following the opening of the cave by collapse at two points. At the same time the Kwihabe Valley is aggraded and infilled by blown sand. (4) Increasing rainfall leads again to the re-establishment of river flow in the valley, with re-excavation of the valley form. Initial high water tables return the cave to phreatic conditions, leading to renewed solution of bedrock and older sinters. (5) Conditions return to stage (2), with river incision, falling water table, and the deposition of fresh sinter. A cyclical pattern is thus apparent, and there is evidence for a number of these cycles in the cave. It is also evident that the climatic controls of the cycle are local in nature, and may not reflect the conditions found in the Makgadikgadi-Okavango system to the east, which is influenced by the climate of a much wider area. It has been estimated (COOKE 1975, 443) that periods of rapid sinter deposition may represent phases with rainfall ofupto 300% of the present mean. Dating of the sinter growth in the cave and calcrete formation in the adjacent valley is provided by 25 C 14dates published in final form in 1977 (COOKE & VERHAGEN 1977) and shows periods ofsinter deposition at 45,000-37,000 BP, 34,000-29,000 BP and 16,000-13,000 BP. The latter sequence, comprising 8 dates from 3 stalagmites, is impressive evidence for higher local precipitation at the time. Holocene sinter formation is indicated for 2,000 BP and 750 BP. Recent research in Drotsky's Cave has yielded a further sequence of 10 C 14dates from 3 stalagmites previously undated, together with 8 U/Th dates on the same samples to test dating accuracy. These dates are shown in Table 2. Good correlation was achieved between the C 14and U/Th series, with stalagmite 1 confirming the conclusions drawn previously. Further mid-Holocene humid intervals are indicated by stalagmites 2 and 3 at 6,000-5,000 BP and c 4,000 BP.
354
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Tab. 2: FURTHER DATES FROM SINTER, DROTSKY's CAVE, KWIHABE HILLS. (To be read in conjunction with COOKE & VERHAGEN 1977, COOKE 1984) Laboratory no. Hv 13288 Hv 13289 Hv 13290 Hv 13291 Hv 13292 Hv 13293 Hv 13294 Hv 13295
Sample description Stalagmite 1: sample a ,, sample b ,, sample c Stalagmite 2: sample a ,, sample b ,, . sample c Stalagmite 3: sample a ,, sample b
C ~4 date 16,190 + 15,600 + 14,125 + 5,860 + 5,060 + 3,665 + 5,375 + 1,185 +
U/Th date 19,400 + 900 18,000 + 800 15,600 + 1,000 6,900 + 400 5,800 + 400 4,500 + 400 6,900 + 400 1,800 + 400
200 190 175 105 100 125 100 105
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4.
T H E P A L A E O C L I M A T I C I N T E R P R E T A T I O N OF THE DATA
The results from the northern Botswana studies are summarised in Figure 2. Also shown are the results of other studies carried out in Botswana, including the investigations at Gi Pan in the D o b e Valley o f northwest Botswana ( H E L G R E N & BROOKS 1983) and at Urwi Pan close to the Namibia border ( L A N C A S T E R 1979). A series o f 8 C 14 shell dates representing perennial flow in the Molopo River are taken from H E I N E (1982). Current research in caves in the Transvaal dolomite near Lobatse in southeast Boswana, an area as
QUATERNARY PALAEOCLIMATES OF NORTHERN BOTSWANA
355
yet uncharted in palaeoclimatic studies (DEACON et al. 1984) has provided six initial sinter dates. The present semi-arid climate is characterised geomorphologically by dry lake beds and sporadic small scale sinter deposition, although precipitation, averaging 500 mm p.a. at Maun, is sufficient to stabilise dune systems. The circulation is dominated by a sub-tropical high pressure cell centered over the Transvaal. During the summer months low pressure develops over the interior of the sub-continent leading to incursions of moist air from the Atlantic and Indian Oceans; these incursions account for some 95% of the total rainfall of northern Botswana. In palaeoclimatic terms, however, the region is essentially transitional, liable to intensification of the present system, or to a possible increase in winter rainfall from a northward shift and intensification of the Westerlies. The study of sinter growth and lake levels yields complementary data sets which need not show a high degree of correlation. As cave studies provide information on local conditions whilst lake levels respond to both local and upper catchment inputs, it is possible to postulate climatic patterns for the region as follows: (1) High precipitation in both the Angolan Highlands and in northern Botswana, leading to high lake levels and corresponding flow along local drainage lines. In caves flooding, with phreatic conditions, occurs, accompanied by re-solution of sinter. (2) High local precipitation with low Angolan input leads to rapid fluctuation of the lakes over a wide range of levels in response to short term and seasonal oscillations in rainfall. In caves the high local rainfall causes sinter deposition under vadose conditions. (3) High Angolan input with drier local conditions leads to high lake levels with ephemeral conditions in local rivers. Caves dry out, with sand infill replacing sinter formation as the dominant process. Calcrete formation at lake edges could be a response to this climatic type. (4) Arid conditions throughout are indicated by low lake levels, dune migration and the infllling of drainage channels with blown sand. Again sinter growth is replaced by sand infill in the cave complex. Bearing in mind these possibilities, the data presented suggests the following conclusions:
45,000-20,000 BP Initial arid conditions are indicate at c 45,000 BP for the Makgadikgadi, but must be regarded with caution due to the limitations of C 14dating. High precipitation prevailed both locally and in the Angolan catchment at 40,000-35,000 BP. As the lakes stood at the Lake Palaeo-Makgadikgadi 945 m stage some of the hydrological variation must be attributed to non-climatic factors. However, any input from the Zambezi would require increased flows, and thus greater precipitation between 12°-18°S. Between 35,000-25,000 BP conditions varied between patterns 2 and 4. Increased local precipitation is indicated at Drotsky's Cave about 30,000 BP and ar nearby Gi Pan at 35,000 BP. The Makgadikgadi Basin was generally dry, although higher levels may well be masked by subsequent events. By 25,000 BP both the Ngami and Mababe Basins were also desiccated, suggesting conditions at least as dry as at present. From 25,000-20,000 BP the Makgadikgadi fined to 920 m on a number of occasions, interspersed with periods of low levels. The high levels were likely to have been supported by increased inflow, although Gi Pan shows locally wetter conditions at 25,000-22,300 BP. In southeast Botswana, Lobatse Cave shows renewed sinter deposition at 26,000-23,000 BP.
356
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20,000-10,000 BP: The Late Glacial Stage
At the Late Glacial Maximum of c 19,000 BP low lake levels occured, although cooler conditions and adequate local rainfall permitted the formation of peat near Tsoi, close to the lower Boteti (GrN 9719). This stage was followed by a profound increase in local moisture availability. Massive sinter growth occured at Kwihabe between 16,000-13,000 BP, whilst the Ngami and Mababe Lakes rose to their 936 m level. Overflow occured along the Boteti to flood the Makgadikgadi to the 920 m level, and to the Zambezi system, where water was ponded back upstream of the Mambova Falls. The humid conditions of the Late Glacial are also evident from adjacent sites. Sinter formation was renewed at Lobatse c 18,000 BP, whilst lacustrine conditions occured in Urwi Pan c 16,000 BP. HEINE's Molopo River dates indicate perennial flow from 19,000-12,500 BP. To the west Etosha Pan (HEINE 1982) contained water at 13,700 BP, whilst in the Cape Province BUTZER et al. (1973) record the existence of an impressive lake at Alexandersfontein between 16,100-13,600 BP. Although the lower temperatures of the Late Glacial undoubtedly reduced evaporation rates, the wealth of geomorphological data indicates increased precipitation throughout the Kalahari during this period. From 16,000-13,000 BP this increase was particularly effective, and was supplemented in the lake system by an increase in inflow. Decrease in precipitation appears to have set in about 12,000 BP. The series of fluvial calcretes in the Kwihabe Valley and at Savuti River dated at 11,000-9,500 BP indicate diminishing fiver flows and a falling lake level in the Mababe Depression. 10,000 BP - Present: The Holocene
By 9,500 BP conditions essentially similar to the present had been established. Semiaridity prevailed until c 6,000 BP, when the level of Lake Ngami rose to overflow level and sinter deposition was re-established in Drotsky's Cave. A number of cyclical increases in rainfall are indicated by sinter growth at 6,000-5,000, 4,200-3,600 and 2,500-2,000 BP. The latter is coincident with the last major fluctuations of the Ngami-Mababe Lake to 936 m and the Makgadikgadi to 912 m, again as a response to increased precipitation. Within the last two milennia conditions approximating to the present have prevailed, with low amplitude fluctuations in precipitation indicated from Drotsky's Cave and from the archaeological record (DENBOW & WILMSEN 1986).
5.
DISCUSSION
The palaeoclimatic conclusions drawn in the previous section have varying degrees of confidence, dependent upon the limitations imposed by C 14 dating techniques, and the range of variables encountered in the geomorphic systems involved. For the period 45,00020,000 BP the results will remain equivocal until the influence of tectonism and 'hidden' lake levels are established by further research, particularly core analysis, over the range of available sites. However, the palaeoclimatic conditions encountered in northern Botswana between 20,000 BP and the present are established, in broad terms, with greater certitude. It is important to note that these conditions are at variance with the inter-tropical zone north of the equator, where palaeo-lake studies at a number of sites (STREET-PERROTr & ROBERTS 1983) have established minimal levels, with cool dry conditions, from 17,000-12,500 BP.
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Conversely high lake levels were experienced at c 9,000 BP and 7,300-6,800 BP, before contemporary conditions were established in the mid to late Holocene. These fluctuations are best explained by a northerly shift of the ITCZ and an enhancement of the monsoonal circulation at times of lake highs (KUTZBACH 1983). The study by STREET-PERROTr & ROBERTS found insufficient evidence to comment on the southern hemisphere circulation, so it is pertinent to examine the chequered history of the middle Kalahari in the palaeoclimatic literature, with particular reference to the Late Glacial. An early attempt to reconstruct the climate of southern Africa (VAN ZINDEREN BAKKER 1976) was based on evidence outside the Kalahari, and led to a provisional assumption that the middle Kalahari would experience arid conditions during glacial phases, attributable to reduced evaporation and increased wind strengths. Subsequent publications (VAN ZINDEREN BAKKER 1980, 1982, HEINE 1978a, 1978b, 1979, 1982) have reinforced this view, although allowing for the incidence of higher summer rainfall in the southern Kalahari suggested by BUTZER et al. (1973) and LANCASTER (1979). HEINE substantiated the scenario by the interpretation of his 16 C 14dates for the Makgadikgadi area as indicative of dry bed conditions between 19,000-12,000 BP, following a palaeo-lake phase at 30,000-19,000 BP, and succeeded by a further lacustrine episode during the Holocene climatic optimum. The imprecise context of the dated samples has led to criticism (COOKE 1979, BUTZER 1984a, HELGREN 1984, RUST et al. 1984). To explain the contrast between arid Late Glacial conditions in northern Botswana, and more humid conditions in the southern Kalahari and at Etosha Pan, HEINE suggested an intensification rather than a shift of the general circulation patterns, stronger upwelling of the Benguella Current, and a strengthening of the west-east temperature gradient to give higher wind velocities to initiate dune movement. Thus, whilst the southern Kalahari received greater rainfall, high evaporation accounted for the desiccation of the Makgadikgadi and greater aridity in Namibia at 19,000-13,000 BP. Conversely the review of 98 C 14dates for southwest Africa for the 30,000-5,000 BP time span by RUST et al. (1984) concluded that conditions in both the Namib and Kalahari were distinctly more humid between 16,000-13,000 BP. Drawing on the suggestions of LANCASTER (1979) for greater summer rainfall due to a more southerly position of the ITCZ, and BUTZER et al.'s (1973) alternative option of more effectivewinter cyclonic rains, RUST et al. propose that the increased humidity was dependant on the variable interaction of increased winter and summer rains on a season to season basis. Given the transitional characteristics of the region we can only concur with this view, although recognising, in the absence of definitive evidence (BUTZER 1984b), that the mechanisms remain speculative. Such a pattern has relevance for more generalised climatic modelling. The evidence presented here suggests that the southern hemisphere intertropical zone was not merely a parallel of its northern counterpart, with overall reductions in precipitation during glacial phases. On the contrary, there is evidence of strong but opposite precipitation characteristics to the northern hemisphere, a possible attribute of the meridional shifts of the meteorological equator proposed in some models (NICHOLSON & FLOHN 1980, KUTZBACH 1983), but not pursued to their logical conclusion. However, such a generalisation ignores the true complexity of the field evidence encountered in northern Botswana. Overall patterns of palaeoclimatic data distribution in southern Africa give credence to BUTZER's (1984a) assertion that continent-wide palaeometeorological hypothesising is best postponed until the regional patterns are better understood.
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ACKNOWLEDGEMENTS Radiometric dating was carried out by Prof. W.G. Mook of the University of Groningen, Prof. M.A. Geyh of the Nieders/ichsisches Landesamt t'tir Bodenforschung, Hannover, Dr. B.Th. Verhagen of the University of the Witwatersrand, and Dr. A.S. Talma of CSIR, Pretoria. Financial assistance was provided by the University of Botswana, NUFFIC and De Beers (Botswana) Ltd. Logistical support on the field was provided by the Department of Water Affairs and Department of Lands and Surveys, Government of Botswana. The illustrations were drawn by Mr. B. Makwiti. The authors wish to thank all of them for their assistance.
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Address of authors: P.A. Shaw and H.J. Cooke, Department of Environmental Science, University of Botswana P/Bag 0022, Gaborone, Botswana