GeoJournal7.4 369-374/1 983 369 • t; AkademischeVerlagsgesellschaft Wiesbaden

Paleo-Environmental Perspectives on the Sahel Drought of 1968-73

Butzer,K.W., Prof.,Dept. of Geogr.,The Universityof Chicago, 5828 S UniversityAve., Chicago, IL 60637, USA

Abstract: A review of stream, dune and lake phenomena in several well-studied sectors of the Sahel indicates an informative record of repeated environmental change. Blowing sand encroached far into the now semi-arid savanna between approximately 20,000 and 12,500 years ago, while lakes had substantially greater volumes and rivers higher discharges between 9000 and 5000 years ago. But rapid change and high-amplitude variability have always been the rule. During the last 2500 years conditions fluctuated about the modern median, with strong short and medium-term oscillations. The Sahel drought after 1968 falls within the norm of at least 6 earlier, dry anomalies verfied since 1400 AD. Whether or not the coin- cidence of cumulative overgrazing and population pressure with severe drought has triggered a géomorphologie response that is unique in the record, can only be decided after much more systematic work has been carried out.

One of the more catastrophicand well-documentedrecent belt centered around latitude 15°N. But nowhere else in famineswas unleashed by the severe droughtthat affected Africa is the surface recordof ancient climatesso extensive the southern Saharan borderlands during 1968-73. and conspicuous. Relativelylittle attention has been devoted to whetherthis Three basic categories of evidence can be examined: droughtwas unique, as a consequence of recentchanges of (1) stream deposits, exposed in well-studiedriver valleys land use patternsand population growth,or whetherit was and in the offshoresediment cones of the principalrivers; but anotherexample of severeand recurrentenvironmental (2) fossil sands, in vegetated dune fields and as struckby stress in the area. This question can be addressed by a cores beneath Lake Chad or in the deep sea; and (3) lake paleo-environmentalreview, to place the 1968-73 drought beds and abandoned shorelinesfound in the in an historicalcontext. related, major tectonic depressions,as well as in the swales between fixed In fact, no other regionof Africa has as conspicuous a or active dunes. record of environmentalchange as the semiarid and sub- arid zone along the southern margins of the Sahara, This primaryrecord is complemented by a numberof between the Senegal and Nile Rivers. This record is best paleosols, by fossilpollen, and by lake microorganismssuch expressed in great expanses of active or immobilizedsand as diatoms. It has been reasonably well fixed in temporal dunes on the one hand, and dry lake beds on the other. terms by an unusually large numberof radiocarbon dates. Such evidence is not unique to this Sahel zone, a designa- The emphasis of the discussion is on the amplitude and tion of Arabic originfor the associated vegetationof grass wavelengthof environmentalchange, not on paleoclimatic and thorn-treesavanna that spans the 100-500 mm rainfall reconstructionor interpretation.

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Fig 1 The Sahel Zone of Africa

The Fluvial Record About 6000 BP riveractivity had stabilizedand soils began to develop on the uplands. Renewed, increasinglyswampy In the Senegal River valley the datum for Late Quaternary deposition is indicated 3300-1750 BP and duringthe last events is provided by gravellyalluvium, 2 m above modern 900 years or so. floodplain,related to a relativelyhigh world sea level before This pattern is pinpointed in deep-sea core M-12345, 31,000 years ago (Michel 1973, Beaudet et al. 1976). Sub- off the Senegal delta, where windbornesands and dust with sequently, sea level fell and the riverwas able to remove Saharan pollen were deposited between 16,300 and 12,500 much of this gravel infillingand cut down into its bedrock BP after which riverdischarge contributedabundant sedi- several floor, suggestingperiodically strong floods. But eventually ment, with tropical pollen, particularly during A similar the lower valley was drowned by dunes advancing south- millennia after 8600 BP (Sarntheinet al. 1982). the westwardout of the Saharan fringe,building up sand forms pattern is indicated in core KW-31 off Niger Delta, to with a reliefof 30 m and extendingout onto the edge of where dischargewas very low about 20,000 15,000 BP; with brieffluctua- the continentalshelf; the Senegal Riverwas unable to reach thereafterflow increased markedlyand, until about 4000 BP et al. the coast at this time. Later, a deep red soil, indicative tions, remainedhigh (Pastouret of a complete vegetative mat and intensive biochemical 1978). the White Nile to weathering,developed on these dunes while the Senegal At about the same latitude, appears have been of runoff between about and other riversonce again cut throughthe dune field to deprived Ugandan and so that a of saline water reach the sea. Between 5000 and 4000 years ago (BP) a 25,000 12,500 BP, body still, was in the lowerWhite Nile floodplain (Williams risingsea level flooded into the new Senegal delta and the ponded up Adamson massive White Nile reduced rivergradient favoured sandy alluviationas much and 1980). Subsequently, floods built silts to 4 m above modern until as 250 km upstream. Sea level then dropped a little and, up floodplain, when declined and dunes were from3000-1650 BP, was again slightlyhigher, resulting in 11,400 BP, dischargeagain to move across the onto the east bank. Stronger development of a series of beach ridges along the delta, able valley 8400 when local watercoursesalso while the riverhad a regimesimilar to that of today. floods resumendby BP, more active than dischargeis This pictureof alternatingdrier and wetterconditions became today. Relativelyhigh identifiedin the later record ca. 7000, 5500, 3000, 2700, is parallelled furthersouth, in central Sierra Leone (8° and 2000-1500 BP. The Blue Nile record,and that of the latitude), where dating is somewhat more precise (Thomas combined Nile below the Atbara confluence and Thorp 1980). In this wetter environment,broad drainage and Adamson Butzer 1980a), reflect swampy valleys with poorly defined channelswere flooded (Williams 1980, on trendsin Ethiopia and are peripheralto by high and protracted,monsoonal runofffrom before mainly upland the discussion. 36,000 until about 20,500 BP. A subsequent absence of present fluvial indic- sediments and organic matter indicates drier conditions In terms of preliminaryevaluation, the climate in withoutforest. About 12,500 BP riverdischarge increased, ators reviewed speak for an exceptionally dry when the firsteroding bedrock, then depositinggravelly alluvium by the Sahel between about 20,000 and 12,500 BP, of the Sahara. Sub- 9000 BP, increasingly interbedded with swamp clays. region evidently was an extension

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sequently, conditions have been relativelymoist, but sub- The surface of modern Lake Chad had a fluctuating ject to considerablevariation. The details of this latervaria- area of 10,000 to 20,000 km2, with a mean depth varying bilityare only poorly resolvedin the fluvialrecord. from 3 to 7 m during the last millennium.The lake is separated from the lowest point of the basin, the Bodélé- Djourab depression some 500 km to the northeast,by a low divide; shorelines at +4 m are at an elevation (286 m The Dune Record above sea level) that would permitwater to overflowinto this depression. The highest shorelines are however at The eolian sands of the Sahel ¡nterdigitatewith the record +40m, correspondingto the Paleo-Chad maximum. This of increasingand decreasing stream activity,to help date great inland water body of about 350,000 km2 was almost the expanse of immobileand vegetateddune fieldsor sand comparable in size to the modern Caspian Sea. It was sheets so common throughmuch of the Sahel (Grove and primarilysupplied by the waters of the Chari and Logone Warren1968, Mainguet et al. 1980, Talbot 1980). In nor- rivers,draining what in historicaltimes were forestedwater- thern Nigeria such dunes are well developed in areas with sheds at latitudes 6-10°, and also by rains falling in the as much as 750 mm of rainfalltoday, although few active Sahel zone. Despite a vast evaporatingsurface, the Paleo- sand fields can now be identifiedin areas with more than Chad maintained various high levels between 7500 and 150 mm. This argues for desertconditions an averageof 3° about 5000 BP. It not only supporteda substantiallyhigher latitude furthersouth than is the case today. But such watertablethroughout its basin,but probablyalso indirectly extreme aridity has not been experienced during the last fed innumerableseepage lakes between the dune fields to 12,500 years. the west and north. Dust of Saharan origin firstbecame abundant off the The historyof Chad lake episodes firstacquires detail Senegal coast during two protracted episodes between with intermediateto high levels about 30,000-20,000 BP. about 7.5 and 5.5 millionyears ago (Samthein et al. 1982). During the firstlacustrine phase ca. 30,000-26,000 BP, the After 2.5 million years windborne dust fluctuated con- associated diatoms were of montane or high-latitude siderably, but generally remained quite prominent,sug- environments,but the pollen data indicate a desert vegeta- gesting an arid Sahara comparable to that of the present tion, with very few tropical taxa. Later, about 25,000- day. Deep cores under the Chad Basin record a dominance 20,000 BP, the "cool" diatoms persistedbut in association of clays, lake diatomites,and alluvial sands from the late with several tropical species; the pollen indicate a strong Oligocene or early Miocene until the end of the Pliocene increase in tropicalelements and a correspondingdecline of (Servantand Servant-Vildary1980). Thereaftereolian sands deserttaxa. are dominant, but repeatedly interbeddedwith lenses of During the period of maximumaridity between 20,000 lake deposits. and 13,000 BP, the southernChad tributarieswere charac- The dune sands and windbornedust consequentlyserve terized by perennial discharge and, after 17,000 BP, by a to document the developmentof the Sahara Desert at the major extension of swampy floodplains. North of the lake, onset of the Quaternaryera, some 2 million years ago. As lacustrine deposits, mainly of saline facies, reappear interfingeredwith marine, stream,and lake deposits they between 13,000 and 1 2,500 BP and probablyreflect in part subsequently provide a fundamentalindicator of relatively a positivemovement of the watertable,fed froma southerly dry climate, as a deterioratingvegetative cover repeatedly source. Between 12,500 and 9200 BP several modest allowed eolian processes to advance into what is now a lacustrine phases were interruptedby dry interludesca. savanna environment.The last major advance of dunes and 12,400-12,000, 10,600-10,200, and 9600-9400 BP. A sand fields,across a belt up to 500 kmwide, predates 1 2,500 rapid rise of the lake to an intermediatelevel began about BP, and latitudinalshifts since about 7000 BP have been 9200 BP, lastinguntil about 8500 BP. From about 12,500 comparativelylimited in extent. until 7000 BP the diatom fauna indicates cool waters and oligotrophicconditions. Aftera fall of the lake level to its modern position 7400-7000 BP, the Paleo-Chad maximum was achieved at +40 m, fluctuatingbelow this high level The Lake Record until 5200-5000 BP. The associated diatoms indicatetem- peraturessimilar to those to today, with eutrophic waters. The of lake beds in the Sahel and even in distribution dry Aftera long and importantregression, centered about 4000 and the parts of the Sahara is quite remarkable, provides BP, a last high lake level is dated between 3500 and 3000 most detailed record of climatic fluctuations during the BP. Subsequent levels fluctuated around those of the last past 12 millennia.The best-studiedrecord, serving as a key millenium. indicator of Sahelian climate, is that of the Chad Basin Palynological study of various lacustrinesedimentary (Servant 1973, Servant-Vildary1978, Servant and Servant- sequences (Maley 1981) shows that Sahelian formsbegan Vildary1980, Maley 1981, 1982). to slowly replace desert shrub after 12,500 BP, gaining

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dominance by 10,000 BP. At thistime the 12 Sahelian taxa as flood data for the Nigerand Nile rivers,and population verifiedinclude 5 tree species. Nonetheless,until 7000 BP, movements, to establish broadly synchronous climatic tree pollen account for only 1 % of the available spectra. anomalies for the Sahel. The lake level dropped some 5 m Thereaftertree pollen increaserapidly and toward 5000 BP withina few decades priorto 1450 AD, then recovered3 m they exceeded non-arborealtypes, clearly documentinga by 1500, only to fall again by 1550. A +5 m level was main- tree-savannaduring mid-Holocene times. This matches the tained throughoutthe 1 7th century.Subsequent levelshave sedimentaryrecord, which indicates maximum precipita- fluctuatedin the 0 to +3 m range,with two lows duringthe tion and Chari-Logonedischarge during the same period. 18th century,one ca. 1850, another after 1913 and again The high lake stands ca. 7500-5200 BP potentially duringthe 1970s. This scale of variationis more rapid than allow reconstructionof a hydrological budget, assuming that revealed by the older, generalizedlake trace, but cor- similar radiation, temperature, and wind conditions of responds better with the major oscillations of the parallel those of today. Kutzbach (1980) has proposed a rainfall pollen recordsthat were frequentlycompressed within one averaginga little over 650 mm per year across the basin, to threecenturies (Maley 1981). Even more detailed are the compared with only about 350 mm today. But he adopted smoothened discharge data of the Sahel rivers, which a non-empiricallake surfaceevaporation estimate of 1297 indicate major dry anomalies every 30 yearsor so since the mm per year,whereas in realitythis figure is over 2200 mm turnof the century(Faure and Gac 1981). In conjunction (J. Maley, pers. comm.). Perhaps more significantthan this with the historicalevidence and oral tradition(Nicholson conservativemodal value is the repeatedevidence for sharp 1979), this hydrologieinformation points up the significant periodicities in lake level. Servant and Servant-Vildary differences in scale resolution picked up by different (1980) suggest four scale-modelsfor such variability:fluc- researchmodes: tuationsof several tens of metersover time spans of several (a) Short-termsevere droughts such as those of 1968- millennia,of 1 to 5 m over several centuries,of up to 3 m 73 are reversedwithin less than a decade, but appear to be over several decades, and of about 50 cm between seasons. too brief to show up in most sedimentologicalor biotic This scale of change is broadly replicated in other, smaller records. lakes of the Chad Basin fed by groundwateror by a com- (b) Intermediate-scale,dry anomalies lasting several bination of runoffand watertable.That it is characteristic decades are more likelytobe revealedby detailed geological of a broad climatic belt spanning the African continent or biological studies but, even though their repercussions from west to east can be seen by comparingthe trace of on productivity,biomass and carryingcapacity may be Lake Rudolf (Turkana), which had a similarhistory to that severe, they most commonly representno more than tem- of Lake Chad since 10,000 BP, with similarabrupt fluctua- poraryoscillations in the highlyvariable climatic regime of tions (Butzer 1980b). monsoonal Africa(Butzer 1971). Comparison of measured stream discharge and lake (c) Long-termdrying trends over severalcenturies or levelchanges since 1900 (Faure and Gac 1981 , Maley 1981 ) millennia are primarilyresponsible for the simplifiedtrace indeed shows that the historyof Lake Chad is fullyrepres- of stream,dune and lake records reviewedhere; they com- entativeof hydrologicalchanges across the Sahel. Basic data prise many positive and negative,intermediate-scale oscilla- of the area west of Lake Chad are reviewed by Tal bot tions that cumulatively effect fundamental changes in (1980) and Maley (1981). Another, less precisely-dated, hydrology, biotic distributions, and regional géomor- local sequence has been reportedfrom the caldera of Gebel phologie thresholds.These are the magnitudesof change - Marra (13° latitude), in the southernSudan (Williamset al. in terms of amplitude and wavelength- reflectedin the 1980). Here a +25 m lake of considerable longevityshrank historyof Lake Chad over the last 12 millennia. about 19,000 BP, but was reconstitutedat +5 to +8 m The lake records discussed here are among the most about 19,000-16,000 BP. After another drop, the level detailed available for the Late Quaternaryof Africa. They regained+9 m ca. 14,000 BP, and then maintainedan inter- serve to show that much of the period 9000-5000 BP was mediate level until about 3000 BP; since then the level has substantiallywetter than today, providinga counterpoint fluctuated between -2 and +2.5 m. This record to some to the evidence for drier Sahelian climate in the period degree reflects special conditions, but it is interesting before 12,500 BP. They furtherplace recentclimatic oscil- because it shows that the time between about 20,000 and lations into context: anomalies comparable to the drought 12,500 BP was not uniformlyarid, confirminglocal lacus- after 1968 are verifiedon at least 6 occasions since 1400 trine beds from the Tibesti mountains that indicate that AD, and may have a recurrencefrequency of three times climate at higherelevations and higher latitudes was pre- per century. dominantlywetter since as earlyas 16,000 BP (Jäkel 1979), even while the Saharan lowlandsand the Sahel were desert. Concluding Discussion Lake Chad historyfor the last 800 years has been re- constructedin detail by Maley (1981: Fig 2.1) on the basis In paleoclimatic terms, the Sahel drought of 1968-73 of dune history,pollen and lake level fluctuations,as well evidentlyfalls well withinthe range of shortand medium-

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term variability directly documented for the last few controls,and local informants,was able to show thatabout centuriesand indirectlyshown for the last 12 millenniaor 1900 AD intermediate-orderstreams were perennial,with so. Kates (1981), in fact, argues that the human impact of low flood peaks, and deposited suspended sediment.As a a similardrought in 1913-14 was of similaror greaterpro- resultof overgrazingand degraded groundcover duringthe portions,although both the criteriaand data to make such 20th century, channels have been incised, and are now an evaluation are unsatisfactory.Other authors, however, lined with coarse bed load; they function intermittently, believe that colonial and post-colonialchanges in land use mainly duringflash floods followingrains. This set of pro- and sustained population growth since the turn of the cesses is familiar from several other contemporaryand centuryhave dramaticallyincreased pressureson the eco- historical settings, in the American Southwest, the Medi- system,rendering it unusually vulnerableto periodic stress terranean Basin, and upland Ethiopia (Butzer 1981), and (Glantz 1976, Dalby et al. 1977). Withoutaddressing this do not specificallyreflect the 1968-73 drought. underlyingissue, of whetheror not the rapid technological In central Sierra Leone, Thomas and Thorp (1980) and social transformationsdefined as "development" serve established several alluvial episodes that span the last to increasesystemic fragility, it is pertinenthere to explore 12,500 years. Although artifacts related to agricultural the degree to which recent desertificationhas had tangible settlementare evident since 4000 BP and implya measure geomorphicimpact. of deforestation,there is no qualitative differencein the It is reasonable to expect that overgrazing,devegeta- developmentof earlierand lateralluvial sedimentsuites. tion, soil erosion, and even secondarymeso-climatic change These examples do not do justice to the extent and will follow in thewake of ecological disbalance,accentuated severity of ecological degradation by rapid population of extreme Yet the by years drought. "historical", i.e. growthin West Africaduring the last 50 years. But theydo context for desertificationremains geomorphological hope- caution that environmentaltrends since the advent of agri- a few have been lessly fragmentary.Only examples pub- culture and pastoralism in the Sahel cannot automatically lishedthus far. be attributedto human intervention,without verification Near Talbot and Williams Niamey (Niger), (1979) by means of meticulouslocal studies.They also suggestthat studied the of erosion and that cycles deposition periodic- there is an urgent need for systematic géomorphologie account for small fans below ally buildingup gullysystems examination of modern desertificationprocesses in an his- that cut back into fixed dunes. The most recentsuch cycle torical context, in order to determineto what extentthese switched from fan accumulation to geomorphic stability are unique, or simplycyclical withinthe Holocene record. over 300 years ago, but stream incision is now active,with gullies cuttingback into the partly-devegetateddunes. Two earlier cycles of cuttingdo not seem to have proceded at differentrates, suggestingthat ecological recoveryis merely a matterof time. Acknowledgement On the Adamaua Plateau of north-centralCameroons, Jean Maley (Montpellier) provided valuable criticisms and sugges- Hurault (1975), based on airphoto interpretation,ground tions on an earlier version of the manuscript.

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