Late Quaternary paleoenvironmental changes in East … · Africa was generally drier than present during the Medieval Warm Period ... [email protected]. 634 Late Quaternary paleoenvironmental

Progress in Physical Geography 30, 5 (2006) pp. 633–658

© 2006 SAGE Publications 10.1177/0309133306071146

I IntroductionOrganic and inorganic sediments, archivednaturally over millennia in environments such

as lakes, swamps, bogs, and ice caps, providean excellent source of data for paleoenviron-mental reconstruction. In particular, pollen

Late Quaternary paleoenvironmentalchanges in East Africa: a review ofmultiproxy evidence from palynology, lake sediments, and associated records

Lawrence M. Kiage1* and Kam-biu Liu2

1Department of Geography and Anthropology, Louisiana State University, 227Howe-Russell Geoscience Complex, Baton Rouge, LA 70803-4105, USA2Department of Oceanography and Coastal Sciences, School of Coast andEnvironment, Louisiana State University, 1002-Y Energy, Coast andEnvironment Building, Baton Rouge, LA 70803, USA

Abstract: This paper presents an overview of paleoenvironmental changes in East Africa during thelate Quaternary based on evidence from pollen, diatoms, microscopic charcoal, and lake level recordsand associated proxies. The paleoenvironmental records derived from different proxies complementeach other to provide a more accurate and complete assessment of the paleoenvironmental changesin East Africa. The records show that the period prior to c. 42,000 14C yr BP was characterized bywarm climatic conditions similar to the present. This was followed by a change to cold dry conditionsfrom 42,000 to 30,000 14C yr BP, and cold and moist conditions from 30,000 to 21,000 14C yr BP.Temperatures during the latter period leading to the Last Glacial Maximum (LGM) were probably 2to 4.1°C lower than the present. Between c. 21,000 and 12,500 14C yr BP East Africa’s environmentwas generally cool, punctuated by two significant episodes of prolonged desiccation. Warm and moistconditions punctuated by rapid climatic changes prevailed in the region during the deglacial and middleHolocene period. Ice core records document two significant and abrupt drought events in the region,one at ~8300 14C yr BP and the other at 5200 14C yr BP. The onset of a longer and more extensivedesiccation period commencing ~4000 14C yr BP was registered in nearly all sites. The climate of EastAfrica was generally drier than present during the Medieval Warm Period (MWP) while fairly wetconditions prevailed during the Little Ice Age (LIA) interrupted by three episodes of aridity, moresevere than those of more recent times. Whereas this review advances our understanding of climateand vegetational changes in East Africa beyond the Last Glacial Maximum, it also highlights limitationsof the paradigms that explain the forcing mechanisms behind the changes. However, unequivocalinterpretation of the multiproxy data from East Africa with respect to paleoenvironmental changesbecomes extremely complex and challenging especially when the anthropogenic input is considered.

Key words: East Africa, multiproxy evidence, paleoenvironmental changes, paleovegetation,palynology.

*Author for correspondence. Email: [email protected]

634 Late Quaternary paleoenvironmental changes in East Africa

records from lake sediments can providedetailed records of past paleoecological andclimatic changes (eg, Liu and Colinvaux,1988; Maley and Brenac, 1998; deMenocalet al., 2000; Gasse, 2000; Salzmann, 2000;Salzmann et al., 2002; Peyron et al.,2000; Wooller et al., 2000; Behling, 2002;Darbyshire et al., 2003; Lamb et al.,2003; Behling et al., 2004). In addition topollen, other proxies such as charcoal(Burney, 1987a; 1987b; Long et al., 1998;Whitlock and Millspaugh, 1996), phytoliths(Piperno, 1988; Mworia-Maitima, 1997;Piperno and Pearsall, 1998), diatoms (Gasseet al., 1997; Barker et al., 2001), and grasscuticles (Mworia-Maitima, 1997; Wooller etal., 2000) have proven to be very useful inpaleoenvironmental reconstruction.

The use of multiproxy approachesenables paleoecologists to more accuratelyand completely reconstruct the paleoenvi-ronment (Boyd and Hall, 1998). Since theenvironment affects different proxies differ-ently, the deficiencies in one proxy may becompensated for by another. For instance,prolonged droughts might compromise thepreservation of biological proxies such aspollen due to the lowering of lake levels, yetpaleoenvironmental conditions that pre-vailed during periods of low lake levels canstill be recorded through geochemical and/orisotopic proxies such as total organic content(TOC), hydrogen index (HI), �18O, �13C,and �15N, among others.

We review different proxy evidences of cli-matic changes in East Africa from beyond42,000 yr BP to the present millennium.Although we focus on the three East Africancountries of Kenya, Uganda, and Tanzania(Figure 1; Table 1), we include relevant infor-mation from tropical Africa as a whole. Theregion has considerable topographic relief, withstepped plateaus of 900–2000 m or more, andhigh mountains such as Mounts Kenya, Elgon,Kilimanjaro, and the Ruwenzoris. We examinebiological proxy records such as pollen,diatoms, and microscopic charcoal, as well as

paleohydrological evidences for lake level fluc-tuations. Where appropriate, these records arecomplemented by chemical and isotopic prox-ies. Although we try to cover the entirebreadth of the region we are limited by datagaps, especially from areas lacking suitablearchiving environments. Ages referred to in thereview are given in 14C years before present(BP), unless otherwise noted. Our review dif-fers from those of Peyron et al. (2000),Marchant and Hooghiemstra (2004), andthose in Battarbee et al. (2004), which, thoughexcellent, are either too focused on a particulartime period or too broad in regional coverageto give the complete picture of paleoenviron-mental changes in East Africa through theperiod reviewed herein.

Figure 1 Map showing East Africaand the neighboring countries. Theapproximate locations of some of thesites discussed in the review are alsoshown in the dots numbered 1 to 21,and cited in Table 1

Lawrence M. Kiage and Kam-biu Liu 635

II Modern climate and vegetationpatternsThe climatic patterns in East Africa are verycomplex and change rapidly over shortdistances. This is attributed to large-scaleclimatic controls that include majorconvergence zones which are superimposedupon regional factors associated with lakes,

topography, and maritime influences(Nicholson, 2000). Generally, East Africa hasa semi-arid to subhumid climate. However,the highlands and areas close to large lakestend to be wetter (Hamilton, 1982;McGregor and Nieuwolt, 1998; Nicholson,2000). The mean annual rainfall ranges fromwell over 1200 mm in Uganda and the

Table 1 Sites discussed in the text and plotted in Figure 1, with references toarticles with original data and interpretations

Site Location Elevation (m) References

1 Lake Albert, Uganda 619 Beuning et al. (1997)2 Ahakagyezi Swamp, Uganda 1830 Hamilton et al. (1986), Taylor (1990)3 Muchoya Swamp, Uganda 2256 Hamilton (1982), Taylor (1990)4 Lake Mahoma, (Ruwenzori 2960 Livingstone (1967)

Mountains), Uganda5 Kamiranzovu, Rwanda 1950 Roche and Bikwemu (1989)6 Kashiru, Burundi 2240 Bonnefille and Riollet (1988)7 Lake Tanganyika, Tanzania 773 Vincens (1989a; 1989b; 1993)8 Lake Rukwa, Tanzania 793 Butzer et al. (1972), Haberyan (1987),

Talbot and Livingstone (1989)9 Lake Masoko, Tanzania 770 Vincens et al. (2003)

10a Mount Kilimanjaro, Tanzania 2650 Coetzee (1967)10b Mount Kilimanjaro, Tanzania 5893 Thompson et al. (2002)

11 Lake Simbi, Kenya 1143 Mworia-Maitima (1997)12 Pekington Bay, Lake Victoria, 1134 Talbot and Livingstone (1989),

Uganda Stager et al. (2003)13 Lake Naivasha, Kenya 1890 Mworia-Maitima (1991), Verschuren

(1999), Verschuren et al. (2000)14a Sacred Lake, Mount Kenya, 2400 Coetzee (1967), Olago (1995),

Kenya Street-Perrott and Perrott (1993), Street-Perrott et al. (1997)

14b Simba Turn and Small Hill Turn 4595 Barker et al. (2001)(Mount Kenya, Kenya)

14c Simba Turn and Small Hill Turn 4289 Barker et al. (2001)(Mount Kenya, Kenya)

15 Lake Bogoria, Kenya 990 Tiercelin and Vincens (1987)16 Loboi Swamp, Kenya 1080 Ashley et al. (2004)17 Cherangani Hills, Kenya 2900 Bakker (1964)18 Lake Kimilili, Mount Elgon, 4150 Hamilton and Perrott (1978; 1979)

Kenya19 Laboot Swamp, Kenya 2880 Hamilton (1982)

20a, b Lake Turkana, Kenya 360 Bonnefille (1976), Mohammed et al. (1995)

21 Lake Abiyata, Ethiopia 1820 Lézine (1982), Gasse et al. (1995), Legesse et al. (2004)


highlands to less than 200 mm in the arid andsemi-arid parts of Kenya (Figure 2) (cf.Nicholson, 2000). The climate of the region isgoverned by air streams and convergencezones that are linked to the seasonal shifts ofthe subtropical high pressure cells situatedabout 20–30° north and south of the equator(McGregor and Nieuwolt, 1998; Nicholson,1996; 2000). The air streams include theCongo air with westerly and southwesterlyflow, the northeast monsoon (NE tradewinds), and the southeast monsoon (SE tradewinds) (Figure 3). The air streams are sepa-rated by two surface convergence zones, thatis, the Inter-tropical Convergence Zone(ITCZ) and the Congo Air Boundary (CAB).The former separates the two monsoons,the latter the easterlies and westerlies(Nicholson, 2000).

Synoptically, the climate of East Africa isdominated by the interplay between thenortheast monsoon in January and the south-east monsoon in July. Unlike the southwestmonsoon of Asia, the monsoons that affectEast Africa are thermally stable, hence

mostly dry. The northeast monsoon, comingfrom the Arabian Desert, is a dry wind. Thesoutheast monsoon, coming off the IndianOcean, constitutes the dominant air streamover East Africa in July. Although deep andmoist, it is not associated with much rainfallbecause the winds are divergent (McGregorand Nieuwolt, 1998). However, the Congoair, which originates in the south Atlantic, ishumid, convergent, and thermally unstable. Itis associated with rainfall in the northwesternpart of the region (Boucher, 1975; Hamilton,1982; McGregor and Nieuwolt, 1998;Nicholson, 2000).

The vegetational distribution pattern inEast Africa (Figure 4) closely mimics the rain-fall and topographic conditions as well as thelength of the dry season. Six major floristicregions can be delineated in East Africa (Lindand Morrison, 1974; White, 1983) (Figure 4).(1) Lowland and montane forests occur inpatches in Uganda and Kenya. These semi-deciduous and evergreen forests are made upmainly of Celtis spp., Urticaceae, Myrtaceae,Croton, Holoptelea, Prunus, Podocarpus, Ilex,

Figure 2 Schematic of the general patterns of winds, pressure and convergence overAfrica during the months of January and July. The dotted lines indicate the inter-tropical convergence zone (ITCZ), dashed lines, the Congo Air Boundary (CAB)Source: Modified from Nicholson (2000).


Acacia, and Olea, among others. TheAfroalpine belt occurs between 1600 and3000 m and is characterized by Prunusafricana, Olea, Hagenia abyssinica, Juniperusprocera, and Podocarpus. The Ericaceous beltdominated by Philipia and Erica is foundabove 3000 m. (2) The drier savanna types

(deciduous bushland and thicket), mainly con-sisting of Acacia and Lommiphora, are com-mon in Kenya, Ethiopia, and Somalia. (3) Thegrasslands, which often occur in the sameecological regions with deciduous bushland,are not widespread in East Africa. (4) Thewetter savanna types, mainly made of

Figure 3 The spatial distribution of rainfall in East AfricaSource: Modified from Hamilton (1982).


Figure 4 The spatial distribution of the major vegetation types of East AfricaSource: Modified from Lind and Morrison (1974).

Combretaceae and Acacia savannas, arewidely distributed in Uganda, Tanzania, andKenya (Figure 4). (5) In the southern parts ofTanzania, the wetter Zambezian deciduous

woodlands (mainly miombo) occur. Thesedeciduous woodlands are basically a moistsavanna type that is dominated by fairly tall(20 m) and densely spaced leguminous trees


belonging to such genera as Brachystegia,Julbernardia, Isoberlinia, and Uapaca. (6) Thesemi-desert and desert vegetation occurs innorthern Kenya and southern Ethiopia.

III Pollen records from East AfricaInvestigations of East Africa’s paleoclimateand paleoecology commenced in the 1960slargely due to pioneering studies byLivingstone (1962; 1967), Bakker (1964),Coetzee (1964; 1967), Morrison (1968), andKendall (1969), and have continued to thepresent. Pollen analysis from the early studiesprovided the first evidence to reveal that thehighlands of equatorial Africa, like otherparts of the world, were affected by glacia-tion and that glacier retreat was roughly contemporaneous with those in Europe andNorth America (Hamilton, 1982). Thelongest continuous records retrieved fromEast Africa are from Sacred Lake, MountKenya (Coetzee, 1967; Olago, 2001; Street-Perrott et al., 1997), covering the period from115,000 yr BP to the present. A record fromtwo swamps (Muchoya and Ahakagyezi) inthe Rukiga Highlands, Uganda (Taylor, 1990),has a basal date of �42,000 yr BP whilethose from Kashiru, Burundi (Bonnefille and Riollet, 1988) and Cheshi, Zambia(Stager, 1988) provide basal dates ofc. 40,000 yr BP.

The pollen records from the region havebeen drawn from diverse environments(Figure 1; Table 1) including: small lakes onmountains and mountain ranges, eg, MountKilimanjaro (Coetzee, 1967), Mount Kenya(Coetzee, 1967; Olago, 2001; Street-Perrottet al., 1997), Ruwenzori (Livingstone, 1967),Cherangani (Bakker, 1964), and Mount Elgon(Hamilton and Perrott, 1978; 1979); largelakes, eg, Lake Victoria (Kendall, 1969;Stager, 1984; Stager et al., 1997), Lake Albert(Beuning et al., 1997), Lake Naivasha(Mworia-Maitima, 1991), Lake Turkana(Bonnefille, 1979; Mohammed et al., 1995),Lake Tanganyika (Vincens, 1989; 1993), andLake Rukwa (Butzer et al., 1972); and mires,eg, Rukiga Swamp (Taylor, 1990), Ahakagyezi

Swamp (Hamilton et al., 1986; Taylor, 1990),and Laboot Swamp (Hamilton, 1982), amongothers. However, most of the pollen recordsonly extend back to 30,000 14C yr BP(Figure 5). Longer records, with a fewexceptions, tend to be characterized by hia-tuses (Vincens, 1991; Bonnefille et al., 1995;Flenley, 1998; Elenga et al., 2000; Peyron etal., 2000; Wooller et al., 2000; Olago, 2001).The pollen records from this vast regionreveal a remarkably high diversity of bothpaleoclimate and paleovegetation types evenover short distances, probably due to thepresence of many mountainous areas (cf.Peyron et al., 2000). Pollen analysis from thedifferent sites has illuminated the paleoenvi-ronmental dynamics of the region beyond theLast Glacial Maximum (LGM).

1 Period 42,000 to 30,000 14C yr BPThe period prior to c. 42,000 14C yr BP ischaracterized by warm climatic conditionssimilar to those that presently abound in EastAfrica. During this period the vegetation inAhakagyezi and Muchoya Swamps, Uganda,was dominated by moist lower montane for-est, where Urticaceae, Myrtaceae, Croton,Minulopsis, Ilex, Macaranga, Neoboutonia,Nuxia, and Filcahoa occurred in theAhakagyezi and Muchoya catchment at thesame elevations as today (Figure 6) (Taylor,1990). Contemporaneous records fromSacred Lake, Mount Kenya (Coetzee, 1967;Olago, 2001) and Kamiranzovu Swamp(Hamilton, 1982) confirm similar trends.Between 42,000 14C yr BP and 30,000 14C yrBP cold dry conditions engulfed East Africa.Consequently, high-altitude vegetation beltsdescended to lower elevations. Evidencefrom charred grass cuticles from Sacred Lake,Mount Kenya, shows a general increase in thepercentages of Bambusoid/Arundinoid-typegrasses as well as a Panicoid(Andropogonid)-dominated assemblage usually associatedwith mesophytic conditions. The source ofthe fire that left the charred particles at theSacred Lake site is likely to have been naturalconsidering that humans had not established


themselves in the lake catchment (Wooller et al., 2000).

In the Rukiga Highlands, the vegeta-tion that constitutes the Ericaceous belt (eg, Anthospermum, Stoebe, Artemisia, andCliffortia) typically at c. 3300 m today wasfound in Muchoya Swamp at an elevation of2260 m above sea level (Figure 6) (Taylor,1990). The present conditions in theMuchoya catchment suggest that it wouldhave required a much colder and drier envi-ronment than present, equivalent to an altitu-dinal depression of up to 1000 m, for the

Ericaceous belt to thrive. Similar observa-tions are made from Mount Kenya (Olago,2001; Street-Perrott and Perrott, 1993), LakeAbiyata (Ethiopia) (Lézine, 1982), andKashiru Swamp (Burundi) (Bonnefille andRiollet, 1988). In Kashiru Swamp there was adecline in forest taxa and an increase inGramineae (Bonnefille and Riollet, 1988;Aucour et al., 1994), also suggesting cold, dryconditions. The Mount Kenya record duringthis period shows Hagenia dominated vegeta-tion coexisting with the Ericaceous belt components.

Figure 5 A generalized chronology of inferred environmental changes in EastAfrica for selected sites discussed in the text. The darker shades represent conditionsthat were colder or wetter than present conditions


2 Period 30,000 to 21,000 14C yr BPCool and moist conditions prevailed in EastAfrica in the period leading up to the LGMperiod of the Northern Hemisphere (c.30,000 14C yr BP to 21,000 14C yr BP). Thereappears to have been a slight increase in tem-perature and precipitation given that uppermontane forest replaced the ericaceous scrubin the Muchoya catchment (Figure 6) (Taylor,1990). Other montane sites in the region,including the Abadares, Kenya (Perrott andStreet-Perrott, 1982), Mount Kenya(Coetzee, 1967; Olago, 2001), andCherangani Hills (Coetzee, 1967), are char-acterized by more abundant arboreal pollentaxa. Charred grass cuticles from MountKenya show fire- and mesophytic-relatedtaxa with high percentages of Phalaris-typegrasses and Pooid grass types that usuallycorrespond to cool and moist conditions(Wooller et al., 2000). The cool wet condi-tions are consistent with those of WestAfrica inferred for the same period (Maleyand Brenac, 1988; Maley et al., 1990;Salzmann et al., 2002). However, the

increase in precipitation is not recorded at allsites in East Africa. For instance, Lake Albertwas surrounded at this time by an openwooded grassland assemblage (Gramineaeover 40%) (Beuning et al., 1997), suggesting adry climate. The absence of semi-deciduousforest taxa, the abundance of Combretaceaeand Chenopodium-type, and a generalincrease in herbaceous pollen types duringthat period further affirm the dry climate sce-nario. The temperatures were between 2 and4.1°C lower than present (Coetzee, 1967;Olago, 2001).

3 Period 21,000 to 12,500 14C yr BPBetween c. 21,000 and 12,500 yr BP EastAfrica’s environment was characterized bycool conditions, punctuated by episodes ofprolonged desiccation (cf. Kendall, 1969;Street and Grove, 1979; Beuning et al., 1997).During this period most sites are character-ized by poor pollen preservation, probablydue to low lake levels and subsequent oxida-tion in these environments (although in somesites pollen was well preserved). In the Rukiga

Figure 6 Fluctuations in % pollen and spores with depth in the Muchoya Swampcore MC2Source: Modified from Taylor (1990).


highlands the Ericaceous belt became re-established in the Muchoya Swamp catch-ment until c. 15,700 yr BP, which is coevalwith a substantial reduction in temperatureand precipitation in the region (Taylor, 1990).Lake level records from Lake Victoria andLake Albert identify at least two separateepisodes of extreme aridification during thisperiod (Talbot and Livingstone, 1989; Beuninget al., 1997).

Two cores (Ibis 1 and Ibis 3) obtained fromLake Victoria at different depths, 32 m and

66 m respectively, reveal two distinct discon-tinuities that correspond to low lake levels(Talbot and Livingstone, 1989). The physicalproperties of Core Ibis 1 mark the first discon-tinuity at 8.2 m (~11,710 � 120 14C yr BP) andthe second at 8.85 m (~15,390 � 220 14C yrBP) (Figure 7). The two episodes are concur-rent with prominent layers of the littoral gas-tropod Bellamya unicolor, which is consistentwith desiccation and oxidation of sedimentsduring low lake levels (cf. Stager et al., 1986;Talbot and Livingstone, 1989). Traces of

Figure 7 Summary of Ibis core 1 from Lake Victoria, showing changes in thestratigraphy, especially sections indicative of significant desiccation periodsSource: Modified from Talbot and Livingstone (1989).


desiccation cracks and rootlets or burrowsextend c. 25 cm down into the mottled sedi-ments below the color change (Figure 7) sug-gesting exposure at these levels (cf. Stager et al., 1986).

The Lake Albert record is based on a 9.2 mcore (core F) collected from 46 m belowwater level yielding a record for the last30,000 14C yr BP (Beuning et al., 1997). Thecore consists of predominantly dark grey toblack, diatomaceous mud with a thin sandyzone at 6.6 m overlying drier, silty sedimentsto the base of the core (9.2 m) (see details inBeuning et al., 1997). The key to the sedimen-tation history of the core site is provided bythe sequence between 8.4 and 6.6 m wherelow water contents and presence of rootlettraces suggest terrestrial conditions. Thesequence is associated with calcite nodulescrumb structure, and vertical and oblique,

clay lined cracks are typical pedogenic fea-tures of terrestrial environments (Beuninget al., 1997). Such features are associatedwith cotton clays, a soil type that develops onfloodplains and lake margins in warm, season-ally contrasted climates (cf. Blodgett, 1985).

Further scrutiny by Beuning et al. (1997)revealed that there were, in fact, two suchlevels in the Lake Albert stratigraphy (similarto core Ibis 1, Talbot and Livingstone, 1989),one at 7.25 to 7.9 m and the other at 6.6 to6.9 m (Figure 8) separated by a zone contain-ing diatoms, Botryococcus, ostracods, fishfragments and rare pollen grains indicatingmore persistent aquatic conditions (Beuninget al., 1997). Therefore, the lowstands musthave been separated by at least one period oflake transgression.

The desiccation in East Africa must havebeen so severe that it led to a lowstand of at

Figure 8 Geochemical data from the core (core F) from Lake Albert. TOC � totalorganic content (wt%), HI � hydrogen index. All determinations on bulk organic matterSource: Beuning et al. (1997).


least 46 m below present levels at the coresite in Lake Albert. The timing of this periodof maximum aridity in Lake Albert is consis-tent with dated records of prolonged minimallake levels throughout East Africa and mostof the African continent from c. 20,000 to12,500 14C yr BP (eg, Livingstone, 1967; 1980;Kendall, 1969; Talbot and Livingstone, 1989;Street-Perrott and Robert, 1983; Taylor,1990). Rising water in Lake Albert at 12,50014C yr BP was coincident with severe floodingalong the main Nile (Said, 1993). The LakeAlbert record shows that deep-water condi-tions were restored soon after 12,500 14C yrBP although there is an apparent change insedimentation rate between 8000 and 340014C yr BP. During the latter period there wasan interruption of sedimentation at the coringsite that could be another, briefer period oflow water, due to dry conditions in the catch-ment (Beuning et al., 1997). This dry episodemay well be part of the mid-Holocene lowprecipitation episode that is recordedthroughout the tropics at c. 4000 yr 14C yr BP.

The pollen record from Lake Naivashashows a brief moist period between 17,000and 15,000 14C yr BP characterized by some-what increased percentages of forest taxa,Podocarpus, Olea, Celtis, and Hagenia(Mworia-Maitima, 1991). However, the LakeNaivasha record, though supported bydiatom and ostracod data that suggest highlake levels, is inconsistent with all the otherrecords from the region, which show aridconditions during this period (Bakker, 1964;Coetzee, 1967; Hamilton, 1982). The anom-alous Naivasha record is probably a result ofdating problems.

There was a slight amelioration of climatetoward more humid conditions after 12,500yr BP as evident by an expansion of montaneforest in the Cherangani Hills and also onMount Kenya (Coetzee, 1967; 1987;Hamilton, 1982). Evidence from SacredLake, Mount Kenya, suggests that the maxi-mum aridity phase occurred just before13,500 14C yr BP. During this period thecharred grass cuticle record from Mount

Kenya is characterized by a stable abundanceof Bambusoid/Arundinoid-type grassesreflecting the consistent presence of mon-tane forest grasses (Wooller et al., 2000).

4 Period 12,500 to 10,000 14C yr BPThe period after 12,500 14C yr BP and intothe onset of the Holocene basically marks theterminal phase of the last glaciation. Thistransition period was marked by an increasein temperature and moisture, as evidenced byan establishment of lower montane forest asindicated by a reduction of Hagenia andUrticaceae and increase in Podocarpus pollenat Muchoya Swamp (c. 11,000 14C yr BP) andAhakagyezi (c. 10,600 14C yr BP) (Taylor,1990). However, a major climatic fluctuationinvolving an abrupt return to arid conditionsoccurred at several sites just before 10,00014C yr BP (Figure 9), probably reflecting theYounger Dryas climatic reversal (cf. Coetzee,1967; Hamilton, 1982; Gillespie et al.,1983; Gasse et al., 1989; Roberts et al., 1993;Beuning et al., 1997; Johnson et al.,2000; Olago, 2001). Further evidence for theYounger Dryas event affecting East Africacomes from the Burundi highlands and theAberdare Mountains (Barker et al., 2004). Inthe Burundi highlands the abrupt aridity wasmarked by a shift to grassland pollen types(Bonnefille et al., 1995) while the Aberdarerecord shows cessation of peat growth(Street-Perrott and Perrott, 1990). At theLake Albert site there was an abrupt returnto drier conditions (c. 11,400 to 9900 14C yrBP), as reflected by a 40% increase in grasspollen contemporaneous with an independentreduction of all forest taxa percentages tonear zero (Beuning et al., 1997). Moist condi-tions resumed soon after c. 9900 14C yr BPbut were again interrupted by dry arid condi-tions at 8000 14C yr BP as evidenced by achange in sedimentation rate.

Open forest pollen types began to appearin Lake Tanganyika at c. 12,000 14C yr BP andattained maximum diversity at c. 10,000 14Cyr BP (Vincens, 1989a). The Zambezi wood-lands expanded and diversified while the


forests and Afro-alpine vegetation belt in theLake Tanganyika catchment abruptlyretreated upslope to levels similar to those ofthe present at the onset of the Holocene(Vincens, 1989b). These changes occur dur-ing a regime of rising temperatures and areobserved in many sites within the region,albeit with a few exceptions. For instance,

the Lake Naivasha record (Mworia-Maitima,1991) does not show evidence of vegetationchange around this time (10,000 14C yr BP).Similarly there is no evidence of temperaturechange in the Ruwenzori record (Livingstone,1967). Generally, there is no agreementamong the researchers as to whether the vegetation shifts evident during this period

Figure 9 Enlarged subset of the pollen diagram of core F from Lake Albert.Percentages of pollen types are based on percent total of all identified, non-aquaticpollen grains. Selected taxa most clearly depict the 1500-year arid interval around thelake from 11,400 to 9900 14C yr BP. This arid interval is divided into three zones with‘P’ (11,400 to 10,900 14C yr BP) and ‘B’ (10,400 to 9900 14C yr BP) being the most aridand a slightly moister phase during zone ‘A’ (10,900 to 10,400 14C yr BP). Calculationsof arboreal pollen percentages excluding Gramineae from the pollen sum produce similar reductionsSource: Beuning et al. (1997).


represents a temperature or moisture signal:it is probably a combination of both.

5 Early to middle HolocenePollen data from the East African region gen-erally suggest that the early Holocene waswarm and moist. In most sites forest pollentypes became dominant concurrent with areduction in Gramineae and an increase inPteridophytes (Coetzee, 1967; Bakker andCoetzee, 1972; Owen et al., 1982; Mworia-Maitima, 1991; Beuning et al., 1997; Olago,2001). Apparently there was montane forestand thicket taxa in the Lake Turkana basinduring this period (cf. Bonnefille, 1976; Owen et al., 1982). The Lake Turkana basinpresently lies in an arid environment(Mohammed et al., 1995) and these pollentypes may have been transported into thebasin by rivers flowing from the EthiopianHighlands, which suggests an increase in pre-cipitation. It is also likely that the highlandforests were more extensive at that time(Owen et al., 1982). At Lake Victoria foresttaxa became more abundant after a briefdecline at 10,000 14C yr BP, followed by a shiftfrom an evergreen to a more deciduous typebetween c. 7000 and 6000 14C yr BP (Kendall,1969). Although humans had establishedthemselves in the region by that time, theirimpact is assumed to be minimal meaningthat the changes were probably driven by cli-mate. The Lake Naivasha record reveals thatthe changes towards dry conditions occurredaround 6500 14C yr BP (Mworia-Maitima,1991), while in Kashiru forest extension waspronounced between 10,000 and 5000 14C yrBP and declined afterwards (Roche andBikwemu, 1989).

Oxygen isotope record from diatomic silicaconfirms that the early to middle Holocene inEast Africa was warm and moist, albeit withrapid climatic changes. Between c. 11,100 14Cyr BP and 5000 14C yr BP two major negativeshifts in �18Odiatom are evident in data fromtwo sites on Mount Kenya (Barker et al.,2001). The first occurred from 11,100 to 860014C yr BP, and the second shift between c.6700 14C yr and 5600 14C yr BP. Both

correspond with enhanced graminoid, shrub,and forest taxa pollen percentages, consistentwith wetter and/or warmer conditions(Barker et al., 2001). The negative shifts in�18Odiatom are contemporaneous with highlake levels throughout the region (Street andGrove, 1976; Street-Perrott and Perrott,1990). Further evidence of a warm and wetearly Holocene in the region is adduced fromice core records from Mount Kilimanjaro(Thompson et al., 2002). The periodbetween c. 11,000 and 4000 BP is character-ized by isotopic (�18O) enrichment (indicatingwarmer conditions) and reduced concentra-tions of major aerosol species (Ca2�, Mg2�,SO4

2�, and NO3�

, indicating wetter condi-tions). Within the early and middle Holocenethe Kilimanjaro ice core record documentstwo serious drought events for the region,one at ~8300 BP and the other at 5200 BP(Thompson et al., 2002). The Kilimanjaro icecore record parallels the high-resolutiondiatom record from Lake Victoria that identi-fies similar trends at ~8200 and ~5700 yr BP(Stager et al., 2003). However, except for theLake Naivasha records (Mworia-Maitima,1991), these abrupt changes are not conspicu-ous in pollen records from the region.

6 Middle to late HoloceneThere was a general reduction in moisture inEast Africa from the middle Holocene to thepresent. Interestingly, it is during this periodthat human impact on the environmentbegins to register in the regional pollensequences. The anthropogenic impact on theenvironment in the tropics during the mid-Holocene complicates paleoenvironmentalreconstructions by providing signals that aresometimes difficult to differentiate from theclimate signals. For instance, drought eventsare often accompanied by increases inGramineae pollen or high frequencies ofmicroscopic charcoal. Human activities suchas farming and/or pastoralism register similarsignatures (cf. Marchant et al., 2002).However, most sites in East Africa areunlikely to have experienced extensivehuman disturbance especially during the


mid-Holocene due to the delayed adoption ofiron technology (Hamilton et al., 1986).

A recent pollen record from LakeTanganyika documents increasingly arid con-ditions after 5000 BP with the probablereplacement of forest by open grassland inparts of the lake’s catchment (Msaky et al.,2005). The Lake Bogoria pollen sequenceshows an abrupt disappearance of high-altitude forest pollen such as Hageniaabyssinica, Hypericum, Stoebe, and Ericaceae,and increases in more drought-adapted taxasuch as Podocarpus, Juniperus, Acacia andDodonaea after 4500 14C yr BP (Vincens,1986). There was a sharp increase inPodocarpus after 4500 14C yr BP in MountKenya (Street-Perrott and Perrott, 1993) anda similar rise in Mount Elgon at c. 3500 14C yrBP (Hamilton, 1982), which signify the onsetand establishment of dry conditions in theregion. Of the four species of Podocarpusfound in East Africa, only two are widely dis-tributed – Podocarpus gracilior and P. malan-jianus, the former being more typical of drymontane forests than the latter (Hamilton,1982). During this same period there was arise in dry forest components such as Olea,Macaranga, Pygeum, Celtis, Syzygium,Neoboutonia, and Galiniera (Coetzee, 1967;Street-Perrott and Perrott, 1993; Olago,2001).

The sharp increase in Podocarpus at c. 450014C yr BP at the Mount Kenya site (Street-Perrott and Perrott, 1993) and at c. 3500 14Cyr BP at Mount Elgon (Hamilton, 1982) corre-sponds with the third abrupt climate eventrecorded in the Mount Kilimanjaro ice corerecord (Thompson et al., 2002). In theKilimanjaro ice core record the desiccationepisode is marked by a visibly distinct (30 mmthick) dust layer containing high concentra-tions of other chemical species (F� and Na�),accompanying a possible hiatus in ice accumu-lation. Lake level records from Lake Rukwa,Tanzania (Talbot and Livingstone, 1989), andLake Cheshi, Zambia (Stager, 1988), show anabrupt dry episode at c. 3000 14C yr BP. Theabrupt dry event during c. 4500–3500 14C yrBP has now been recognized across tropical

Africa (Hamilton, 1982; Talbot, 1988; Street-Perrott and Harrison, 1985; Street-Perrottand Perrott, 1993; Elenga et al., 1994; Maleyand Brenac, 1998; Bonnefille and Chalié,2000; Barker et al., 2004). This abrupt aridityhas been associated with decline in thebase flow of the White Nile (Talbot andBrendeland, 2001) and has also been linked tothe collapse of the Old Kingdom in the NileValley (cf. Hassan, 1997; Barker et al., 2004).The abrupt shift to a drier and more seasonalenvironment at c. 4000 14C yr BP has alsobeen recorded at different sites in the islandof Madagascar, which lies to the southeast ofthe study region (Burney, 1993).

From 2500 14C yr BP there was progres-sive degradation of arboreal cover and a con-current increase in Gramineae pollen in thenorthern basin of Lake Tanganyika, which isprobably a consequence of both increasingaridity and human interference. There is evi-dence of human activities documented in thepollen sequence, such as the presence ofthe rare Elaeis guineensis (wild oil palm) in theMasoko record (Vincens, 1989a). Pollen andcharcoal records from Ahakagyezi Swampshow evidence of deforestation associatedwith agricultural activities before c. 4800 14Cyr BP (Hamilton et al., 1986). Unfortunatelymost pollen of cultivated plants has not beendefinitely identified (cf. Msaky et al., 2005).Environmental changes associated withdeforestation appeared in Muchoya Swampat c. 2500 yr BP and contemporaneouschanges are evident in the Batongo andKatenga mires (Morrison and Hamilton,1974; Taylor, 1990). At Lake Tanganyika thereis a consistent trend of a decrease in grasspollen, and increase in pteridophyte andforest indicator pollen during the last fewcenturies (Msaky et al., 2005). This trendsuggests an increase in soil erosion and recy-cling of abundant arboreal pollen followingland clearance for agriculture. Increase in sed-imentation rates during this period providesfurther evidence for increased soil erosion inthe Lake Tanganyika catchment (Cohen et al.,2005; Msaky et al., 2005; Palacios-Fest et al.,2005).


7 The last millenniumVerschuren et al. (2000) obtained a 6 m sedi-ment core from the partially submergedCrescent Island Crater (CIC) basin in LakeNaivasha, Kenya, to reconstruct the recordof rainfall and drought in equatorial EastAfrica during the past 1100 years. LakeNaivasha is a freshwater lake with subsurfaceoutflow and a short (�10 yr) water-residencetime (Ojiambo and Lyons, 1996). Strongevaporation and highly variable river inflowcause its surface elevation to fluctuate inresponse to changing climate regimes(Verschuren et al., 2000). Although the lakelevel record for Lake Naivasha is shorter than

that for Lake Victoria and Lake Albert it pro-vides high-temporal-resolution informationconcerning recent climate history, occasion-ally corroborated by oral tradition.

A combination of sediment-inferreddepth reconstruction and diatom composi-tion and distributions revealed that LakeNaivasha experienced a long saline lowstandfrom ~AD 1000 to 1270 interrupted by onefreshwater interval in the early part of themillennium (Figure 10). This lowstand wasfollowed from ~AD 1270 to 1550 by a mostlypositive water balance and the establish-ment of freshwater conditions, except forone recurrence of saline conditions dated

Figure 10 A plot of lake level and salinity record for Crescent Island Crater (LakeNaivasha) compared with the decadal record of atmospheric 14CO2 production as aproxy for solar radiation. The grey bars are the severe drought events as recountedfrom oral tradition. Bars A, B, and C represent the Wamara, Nyarubanga, andLapanarat-Mahlatule droughts, respectivelySource: From Verschuren et al. (2000).


around ~AD 1380 to 1420. The LakeNaivasha record is consistent with the LakeTanganyika record for this period, whichshows a peak in total inorganic content(TIC) and a rapid decline after AD 1100 con-current with a rise in charcoal mass accumu-lation rates (Cohen et al., 2005). A secondpronounced lowstand in Lake Naivasha isdated to ~AD 1560 to 1590, followed by ahighstand that lasted between AD 1670 and1770 with lake level continuously above thehistorical maximum reached in AD 1894.During the highstand period (AD 1670–1770)the diatom flora was dominated byAulacoseira ambigua, which, compared to A. granulata, thrives in higher light and lowernutrient environments.

Lake-level and salinity fluctuations weresuperimposed on a long-term fresheningtrend in the lake that started around AD 1270;evident in the gradual disappearance of salt-tolerant diatom and chironomid taxa and theexpansion of stenotrophic freshwater taxasuch as Psectrocladius viridescens. The paral-lels in the different proxy records are strik-ingly apparent. The fluctuations in lake levelsare indicative of reduced precipitation andextensive drought periods. This is confirmedby oral traditions (cf. Webster, 1979;Verschuren et al., 2000). Evidence fordrought-induced famine, political unrest, andlarge-scale migration of indigenous peoples isconcentrated in three periods around AD

1390–1420 (Wamara drought), AD 1560–1625(Nyarubanga drought), and AD 1670–1840(Lapanarat-Mahlatule drought) that matchthe reconstructed sequence of lowstands(Verschuren et al., 2000).

The Lake Naivasha record correspondswith the findings from Lake Victoria for thesame period. Stager et al. (2003) used adiatom record from Lake Victoria to recon-struct the paleoclimate of the region duringthe Holocene. Although they observe a gen-eral aridification trend over the last 3000 14Cyr BP, abrupt lake level decline is registeredbetween 1200 and 600 14C yr BP due toreduction in precipitation.

IV SynthesisOnly a handful of paleoenvironmental recordsfrom East Africa extend beyond 42,000 yr BP(Coetzee, 1967; Taylor, 1990; Street-Perrottet al., 1997; Olago et al., 1999). The few avail-able (mostly pollen records), however, pres-ent useful data for the reconstruction of EastAfrica’s paleoenvironment beyond the LGM.The period prior to c. 42,000 14C yr BP ischaracterized by warm climatic conditionssimilar to the present. The shifting of high-altitude vegetation belts to lower elevationsbetween 42,000 14C yr BP and 30,000 14C yrBP marks a change to cold dry conditions.Much of this evidence is obtained from pollenand diatom data as well as charred grass cuti-cles and is verified by carbon isotope (�13C)records from the region (Olago, 2001). Coolconditions prevailed in East Africa in theperiod leading to the LGM with temperaturesbeing 2–4.1°C colder than present (Coetzee,1967; Olago, 2001). Paleoenvironmentalproxies show that between c. 21,000 and12,500 14C yr BP East Africa’s environmentwas generally cool, punctuated by episodes ofprolonged desiccation (Kendall, 1969; Streetand Grove, 1979; Beuning et al., 1997).

The Lake Albert record documents twoextended episodes of lowstands of at least–46 m below the present lake levels during theLGM c. 18,000–20,000 and at 12,500 14C yrBP. Core Ibis 1 from Lake Victoria also docu-ments two episodes of lowstand, at ~15,390 �220 14C yr BP and ~11,710 � 120 14C yr BP.The aridity episode in Lake Victoria musthave reduced the lake level to at least –66 mbelow present at the site of core Ibis 3.

It is likely that the two lakes are recordingthe same desiccation episodes, albeit withdifferences in dating control. The dating shouldbe interpreted with caution because low lakelevels are likely to be associated with increasedmixing and bioturbation that could interferewith both the integrity and the time resolutionof climate-proxy records. Whereas the first episode at 18,000–20,000 14C yr BP or ~ 15,390(?) 14C yr BP (for Lake Albert and Lake Victoria, respectively) could be


synchronous with the LGM, the second aridityevent recorded in the two lakes (12,500 14C yrBP, 11,710 14C yr BP) could be associated with the Younger Dryas interval. Palynologicaland other proxy evidence from Lake Albert(Figure 3) further provides clear evidence of sig-nificant and abrupt drying of climate in theLake Albert region at ~12,500 14C yr BP. Theterminal phase of the last glaciation, from12,500 14C yr BP into the onset of theHolocene, was marked by an increase in tem-perature and moisture. However, more workneeds to be done before the occurrence of theYounger Dryas in East Africa can be confirmed.

Different proxies, eg, pollen, diatoms,charred grass cuticles, and oxygen isotoperecords from diatomic silica, suggest that EastAfrica was characterized by warm and moistconditions amid rapid climatic changes duringthe early to middle Holocene period. Theice core record from Mount Kilimanjaro(Thompson et al., 2002) documents two sig-nificant and abrupt drought events in theregion, one at c. 8300 BP and the other at5200 BP, which appear to correspond with anapparent change in sedimentation rate atLake Albert sometime between 8000 and3400 14C yr BP (Beuning et al., 1997).However, these drought events are notrecorded in the pollen sequence. This empha-sizes the importance of proxy sensitivity andsampling resolution as it affects paleoenvi-ronmental data interpretation. The ice corerecord provides higher-temporal resolutiondata than the lake sediment and pollenrecords can, and therefore is able to detectthe two abrupt drought events. The longerand more extensive drought event at c. 400014C yr BP (cf. Marchant and Hooghiemstra,2004) was registered in both the pollen anddiatom records at different sites (Hamilton,1982: Mworia-Maitima, 1991; Street-Perrottand Perrott, 1993; Thompson et al., 2002).

Lake level records from Lake Naivashaemphasize the significance of high-resolutionstratigraphic records in revealing decadal andcentury-scale paleoclimatic changes. TheNaivasha lake level record suggests that the

climate of equatorial East Africa was gener-ally drier than today during the MedievalWarm Period (MWP; ~AD 1000–1270), andthat fairly wet conditions during the LittleIce Age (LIA; ~AD 1270–1850) were inter-rupted around AD 1380–1420, 1560–1620 and1760–1840 by episodes of persistent aridity,which seem to be more severe than those ofmore recent times (Verschuren et al., 2000;Verschuren, 2004). Again these subtledecadal- and century-scale events are missingin the pollen record from the lake (cf. Mworia-Maitima, 1991), further emphasizing the roleof proxy sensitivity in revealing details of paleoenvironmental conditions.

That the oscillations in the lake levels andchanges in pollen and other proxy recordsreflect changes in the climate and vegetationof the region is not in doubt. However, thechallenge remains in the understanding of theforcing mechanism behind paleoenvironmen-tal changes in a tropical region. Although theMilankovich cycles have been advanced as apossible explanation for the changes (Shortet al., 1991; Tyson, 1999; Tyson et al., 2000;Olago, 2001; Stager et al., 2003; Verschurenet al., 2004), they probably are not responsi-ble for the centennial-scale and decadal-scalechanges such as those evident from the LakeNaivasha records. Therefore more studiesand thought ought to be given to possiblecausal mechanisms.

A number of hypotheses have beenoffered identifying these mechanisms. Oneexplanation is based on the intensity of thesoutheast summer monsoon and the Congoair stream that influence the amount and sea-sonality of rainfall in the East African region(cf. Hamilton, 1982; Nicholson, 1996;Nicholson and Yin, 2001; Vincens et al.,2003). Since the southeast summer monsoonacts in tandem with the seasonal movementof the Intertropical Convergence Zone(ITCZ), their character must have dramati-cally changed during the periods of intensearidity recorded in the East African lakes.

It is possible that the summer monsoon wasvery weak during the periods of intense


aridification (c. 20,000–18,000 14C yr BP, 12,500yr BP, and after ~4000 14C yr BP), resulting inintense droughts that reduced the lake levels tothose registered in Lakes Albert, Victoria, andNaivasha. Indeed, records from many sites interrestrial Asia and the Arabian Sea, whichcorrespond with those from terrestrial sites inAfrica, suggest the existence of significant century-scale monsoon weakening eventsespecially during the early to middle Holocene(Van Campo et al., 1982; Bryson, 1989; Street-Perrott and Perrott, 1990; An et al., 1993; VanCampo and Gasse, 1993; Gasse and VanCampo, 1994; Overpeck et al., 1996). Thesevariations in the summer monsoon may explainsome of the abrupt climatic changes evident inthe paleoecological records from East Africa.Changes in the summer monsoon are linked tochanges in global circulation patterns, especiallytemperature changes associated with eventssuch as the deglaciation at the end of the LGM,and Younger Dryas episodes in the mid-latituderegions of the Northern Hemisphere as well assea surface temperatures (SST). There is alsoincreasing evidence linking changes in the mon-soon and the ITCZ to SST changes (Gasse andVan Campo, 1994; Gasse, 2002) as well as thecharacter of the thermohaline circulation(THC) (Nyberg et al., 2002).

The position of the ITCZ has been shownto be closely linked to the changes in the seasurface temperatures over the AtlanticOcean (Gasse and Van Campo, 1994; Sirockoet al., 1996; Marshall et al., 2001) and the ElNiño Southern Oscillation (ENSO) events(Nicholson, 1995; 1996; Hastenrath, 2002).Periods of lower precipitation in Africa havebeen associated with warm SST in the north-ern Atlantic Ocean and colder SST in thesouthern Atlantic Ocean. Gasse and VanCampo (1994) specifically link the weak mon-soons at c. 4000 14C yr BP to contemporane-ous decreases in the North Atlantic SST.Decrease in the latter would reduce moisturein the parts of Africa where monsoonal air-flow brings water vapour from the southernsubtropical Atlantic anticyclone (Marchantand Hooghiemstra, 2004).

At the centennial timescale, the lake levelfluctuations could also be a result of the solaractivity and atmospheric radiocarbon (14C)production (cf. Street-Perrott et al., 1989;Jouzel et al., 1993; Olago, 1995). The latterphenomena are negatively linked becausesolar activity (as measured by reducedsunspot numbers) increases cosmic ray bom-bardment in the upper atmosphere, whichenhances 14C production. Indeed a compari-son of the Lake Naivasha record with that ofreconstructed atmospheric 14CO2 productionreveals that the inferred MWP African aridityand all the low lake level episodes were coevalwith high solar activity, with the interveninghighstands contemporaneous with low solaractivity (cf. Street-Perrott et al., 1997; VanGeel et al., 2000; Verschuren et al., 2000;Olago, 2001; Thompson et al., 2002). Forinstance, the highest rainfall inferred fromLake Naivasha over the past millenniumwas contemporaneous with the ‘MaunderMinimum’ of solar radiation (Verschurenet al., 2000). Therefore, variation in solaractivity may have contributed to the changesin the paleoclimate of East Africa.

Another hypothesis for the paleoenviron-mental changes in East Africa is the possibleteleconnection between high and low lati-tudes during the early and mid-Holocene(Stager et al., 2003). This could be the forcingmechanism behind the abrupt dry episodes at~8300 and 5200 BP (Thompson et al., 2002)that were synchronous with major meltwaterand ice rafting phenomena in the NorthAtlantic (Bond et al., 1997). However, the8200 BP meltwater event was based oncalendar years (first detected in ice corerecords). Firm conclusions regarding the tele-connections hypothesis are not possible dueto uncertainties surrounding the radiocarbonchronologies (cf. Stager et al., 2003).

Human-induced vegetation changes inEast Africa, especially during the Holocene,cannot be ruled out. During this periodhumans progressed from being relativelyminor influences, restricted to a few ecosys-tems, to becoming a major force across even


remote and inhospitable environments(Hamilton, 1982). The records fromAhakagyezi (Hamilton et al., 1986; Taylor,1990), Muchoya (Taylor, 1990; Marchant andTaylor, 1998), Sacred Lake (Street-Perrottet al., 1997), Lake Tanganyika (Cohen et al.,2005; Msaky et al., 2005), and Masoko(Vincens et al., 2003) show reduction in arbo-real taxa, and an increase in fire episodes(charcoal peaks) after 5000 14C yr BP thathave been interpreted as human-induced.These interpretations are based on the pres-ence of high counts of microscopic charcoal(suggesting fire occurrence) concurrent withforest clearance. In some cases (eg, Vincenset al., 2003) forest clearance is accompaniedby increased pollen frequencies of cultivatedcrops (eg, Elaeis guineensis), Gramineae, anddisturbance markers (eg, Ricinus communisthat may indicate clearance for agriculturaland/or pastoral activities).

Rapid vegetation changes due to humanactivities have the potential of producingremarkable changes in land surface albedocapable of inducing feedback processes thatcould result in changes in climate (cf.Kutzbach et al., 1996; Doherty et al., 2000).Climate models have shown that vegetation-albedo changes provide positive feedbackeffects that cause African climate to switchabruptly between wet and dry conditions(Claussen, 1997; Claussen et al., 1999). It ispossible that the mid-Holocene aridificationmay have been stimulated by changes in land-surface conditions (cf. Lamb et al., 1989;DeMenocal et al., 2000). However, the fun-damental problem remains to separate purelybackground-level environmental or climaticchange from that caused by humans. Sincemost indigenous crop plants in East Africa(eg, cereals, cassava, banana, and legumes)have not left identifiable traces in the pollenrecord, the palynological identification of agri-cultural influence in the region becomes diffi-cult. Another problem is that natural droughtevents are often accompanied by increases ingrass pollen or high frequencies of micro-scopic charcoal. Human activities such as

farming and/or pastoralism register similarsignatures, further complicating the inter-pretation process. Therefore, wheneveranthropogenic disturbance is suspected inpaleoenvironmental changes, doubt can onlybe eliminated by thorough multiproxy evi-dences including archaeological findings.

Deciphering the Holocene anthropogenicfire-related vegetation changes should betreated with caution. Fire episodes in the EastAfrican records appear as early as ~30,00014C yr BP (Wooller et al., 2000) and may haveno anthropogenic link. Records from theisland of Madagascar (latitude 12°S to 26°S)(Burney, 1987a), similar to East Africa interms of climate and vegetation communities,document fire episodes well before the pre-sumed arrival of humans c. 1500–2000 14C yrBP (cf. Burney, 1987b; 1993; 1997). Clearly,not all paleoecologically recorded fires wereinduced by humans, since even in the absenceof humans wildfires occur when necessaryconditions of low fuel moisture and a naturalignition source (for instance, lightning and vol-canic activities) are met. However, changesin fire regimes and intensity may point to thehuman agency, as evident in the Masokorecords (Vincens et al., 2003). Unfortunatelymost of the records from tropical Africa havetemporal resolutions that are too coarse tofully investigate the anthropogenic effect onthe vegetation.

Unequivocal interpretation of the varioussedimentological, biological, and geochemicalclimate proxy data extracted from the lakesediments with respect to past climatic varia-tions is an extremely complex and challengingexercise. Whereas this review offers a con-tribution to our understanding of paleoenvi-ronmental changes in East Africa, it alsohighlights what is unknown. For instance,many of the accepted paradigms are based oncoarse-temporal-resolution analyses, whichmay miss subtle, yet important, changes invegetation and climate. Although humans andtheir ancestors are assumed to have inter-acted with East African environments longerthan in most other regions in the world, very


little is known about their impacts on theenvironment, especially during the Holocene,let alone the earlier periods. There is a needfor more higher-temporal-resolution palyno-logical studies with an emphasis on multi-proxy methodologies in order to improve ourknowledge of the human/climate impact inthe region. We still need to devise methods toseparate purely climatic and human-drivenenvironmental changes. Identification ofpollen of indigenous cultivated crops iscertainly part of the solution.

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