Evidence for Megalake Chad, North-central Africa

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Evidence for Megalake Chad, north-central Africa,during the late Quaternary from satellite data

Marc J. Leblanc a,*, Christian Leduc b, Frank Stagnitti c, Peter J. van Oevelen d,Chris Jones e, Linus A. Mofor f , Moumtaz Razack g, Guillaume Favreau b

a

School of Geosciences, Monash University, Clayton 3800, Australia b IRD-UMR Hydrosciences, BP 64501, 34394 Montpellier cedex 5, Francec School of Ecology and Environment, Deakin University, Warrnambool 3280, Australia

d European Space Agency, Keplerlaan 1, Postbus 299, 2200 AG Noordwijk, The Netherlandse School of Computer Science, Cardiff University, Newport Road, PO Box 916, Cardiff CF24 3XF, UK

f School of Technology, University of Glamorgan, Pontypridd CF37 1DL, UK g University of Poitiers, 40 avenue du Recteur Pineau, 86022 Poitiers Cedex, France

Received 22 September 2004; received in revised form 21 July 2005; accepted 27 July 2005

Abstract

The existence of a very large Lake Chad during the late Quaternary, Megalake Chad, has long been questioned. A MegalakeChad would present strong evidence for climatic fluctuations of great magnitude during the Holocene in tropical Africa. In this paper we used satellite data from Landsat and Modis sensors to collect and analyse new information on landforms in a 2000000km2 region of the Lake Chad Basin. We detected 2300 km of remains marking the ancient shoreline of Megalake Chad. Thesatellite data also indicated many Saharan rivers and relict deltas leading to the long paleoshoreline. Large dunefield flatteningswere observed and interpreted as the result of wave-cut erosion by the paleolake. Similarities were noticed between the landformsobserved along the paleoshoreline of Megalake Chad and that of the former Aral Sea. This finding has significant consequencesfor reconstructing paleohydrology and paleoenvironments through the Lake Chad basin, and continental climate change.D 2005 Elsevier B.V. All rights reserved.

Keywords: Lake Chad; Paleohydrology; Remote sensing; Geomorphology; Climate change; Ancient shoreline

1. Introduction

The Lake Chad Basin in north-central Africa is theworlds largest endorheic basin, covering an area of

2 500 000 km 2 (Fig. 1). The central part of this basin began to fill with sedimentary deposits during theCretaceous period ( Genik, 1992 ). It is now mainlycovered with unconsolidated Quaternary sediments.Presently, most of the northern portion of the basinis dry. Originating in the south, the Chari and theLogone rivers provide most of the inflow to LakeChada vast and shallow fresh water lake bordering

0031-0182/$ - see front matter D 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.palaeo.2005.07.016

* Corresponding author. Fax: +61 399 054 903. E-mail address: [email protected]

(M.J. Leblanc).

Palaeogeography, Palaeoclimatology, Palaeoecology 230 (2006) 230242

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the Sahara Desert. Due to its endorheic nature and because it is located in a region of little relief, climatefluctuations have a great impact on the areal extent of Lake Chad. For example, from 1960 to 1990 the levelof the lake fluctuated approximately between 277 and284 m amsl and its area from 6000 km 2 to 25000 km 2

(Olivry et al., 1996 ). During the last millennium major floods occurred on several occasions. During theseevents water levels rose above 286 m amsl and theLake expanded to the north-east, partly flooding the

Bahr el Ghazal valley ( Maley, 1981; Olivry et al.,1996 ). Although large portions of the Bahr el Ghazalwere inundated during these major floods, the water never flooded the Lowlandsa vast region of north-ern Chad about 115 m below the current level of LakeChad and linked to Lake Chad through the Bahr elGhazal valley ( Fig. 1).

The hypothesis of a much larger ancient Lake Chadduring the late Quaternary, Megalake Chad, remainscontroversial. Doubt about the existence of Megalake

Fig. 1. Study area and the remains of Megalake Chad detected with satellite data. Black rectangles indicate the location of the other figuresmentioned in the text. Double circles represent the sites whose lithology was determined using spectral signature.

M.J. Leblanc et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 230 (2006) 230242 231


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Chad has largely resulted from a paucity of geomorphological mapping, especially at regional scale, duein part to the vast expanse of the Lake Chad Basin anddifficulties in accessing sites from the ground. How-ever, remote sensing data provide an excellent newsource of information. Satellite imagery also enablesvery large landforms to be examined entirely and incontext with the surrounding environment. Recent studies in other parts of the world have demonstratedthe value of remote sensing and GIS to reconstruct paleolakes hydrology (e.g. Pachur and Rottinger,1997; Komatsu et al., 2001; DeVogel et al., 2004 ).In the study area, which covers the c entral and north-ern part of the Lake Chad Basin ( Fig. 1), we used

Landsat TM and MSS and Modis data to analyse largegeomorphological and hydrological landforms.In the first part of this paper we review the pre-

vious research conducted on Megalake Chad. We thendetail how we used remotely sensed data to detect landforms in the study area and gather new informa-tion. Subsequently, each type of landform identified isdetailed and its potential link with Megalake Chadanalysed. In the discussion, we compare the evidencegathered in this study with the arguments for a neo-tectonic hypothesis. We also compare the landformsobserved in the Lake Chad Basin with those of theformer shoreline of the Aral Sea. Finally the implica-tions of our findings are discussed in regards to theregional paleohydrology and continental climatechange.

2. Review of previous work

The possible existence of a very large ancient LakeChad was noted by Tilho (1925) . Later, Pias andGuichard (1957) reported that a vast, ancient Lake

Chad was responsible for a long sandy ridge that theyobserved in Cameroon and Chad, and which coincideswith an outlet to the Gulf of Guinea through the MayoKebbi river. More features of this old, sandy, lacus-trine ridge were recognised by geologists, geomor- phologists and pedologists during early cartographicsurveys in the 1950s and 1960s, in Nigeria, Camer-oon, Chad and Niger ( Pias and Guichard, 1957;Grove, 1959; Pirard, 1967 ). Schneider (1967) synthe-sised these works together with archeological data andtopographic cross sections and interpreted the sandy

ridge as the perilacustrine ridge of Megalake Chadwith a consta nt elevation of 3 25 m amsl. The map published by Schneider (1967) was the first to exhibit rightly the paleoshoreline of Megalake Chad. Thestudy of diatoms and pollens in lacustrine sediments,together with stratigraphical (including 14 C dating)and sedimentological methods, confirmed the pre-sence of a large and deep ancient Lake Chad in th eHolocene ( Maley, 1981; Servant and Servant, 1983 ).The diatom and po llen data were synthesised byMaley (1981, 2004) and showed that the last mainextension of Megalake Chad intervened between 7700and 5500 14 C years BP with 3 successive sub-phases.Recently, an erosion platform and conglomerates were

identified around the Hadjer el Hamis inselbergs andwere interpreted as the result of wave erosion whent he inselbergs formed islands within Megalake Chad(Schuster et al., 2003 ).

However, some researchers argued against theexiste nce of a giant paleola ke. Durand (1982, 1995)and Durand et al. (1984) reported that landformsthought to be part of Megalake Chad were in fact of neotectonic origin. Major arguments are as follow.Firstly, it was reported that, across the basin, thedistribution of the paleoshoreline is not continuousand shows major gaps, some in wide areas. Secondly,it was assessed that the elevation of the ancient ridgeis not constant. Thirdly, it was argued that landformsinitially considered to be the remains of the paleoshoreline have limited orientation and thereforeare neotectonic features following the direction of major structural faults which were also indicated bygravimetric anomalies ( Durand, 1982; Durand et al.,1984; Durand, 1995 ).

3. Methodology

To cover the entire study area, which spans over 2000000 km 2, five large mosaics of Landsat TM datawere obtained from NASA Earth Science Enterprise.Each mosaic was created from 20 to 29 orthorectifiedand contrast adjusted Landsat TM scenes which werecollected between 1986 and 1990 ( Table 1). Themosaics provide spectral information in 3 bands (b7-MIR, 2.082.35 Am; b4-NIR, 0.760.90 Am; b2-vis,0.520.60 Am) with a spatial resolution of 30 m. In thesouth, dense vegetation may mask the underlying

M.J. Leblanc et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 230 (2006) 230242232


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structures because the Landsat TM scenes of themosaics were taken at or close to the peak of thegrowing season. For this reason, in the south TMdata were supplemented with Landsat MSS selectedfrom the dry season taken between January 1973 and

December 1975 ( Table 1 ). These dry season imagesavoid interference by cloud and vegetation, thusenhancing the interpretation of geology, geomorphol-ogy and pedology.

Visual interpretation of the satellite images reliedon the analysis of essential surface characteristicswithin the regional context: image tone, texture, pat-tern, shape, size and site location. However this pro-cess is often subjective. Therefore, digital image processing techniques were also used in order to per-form a more rigorous analysis of the Landsat TM and

MSS data. A principal component analysis (PCA) wasused to compress the multi-spectral data into black and white images and to highlight parts of the regionalspectral variation ( Rees, 2001 ). The bands of PCAdata are independent and therefore non-correlated.

This leads to greater interpretability compared to thesource data. Successive principal components of MSSand TM data were generated by calculating newcoordinate systems in which much of the redundant information was removed. A standard deviation con-trast stretch was applied to the derived images for visual enhancement. This technique is widely usedin remote sensing to improve the visual interpretabil-ity of an image. It amplifies the spectral differences between the features of an image to increase their visual distinction. A Sobel edge detection and filtering

Table 1List of remote sensing data and maps

Data type Data ID Date Source

Remotesensing

Landsat TM mosaic(Lake Chad Basin)

N32-15 All images taken between 1986and 1990

NASA Earth Science Enterprise N33-05 N33-10 N33-15 N14-15

Landsat TM mosaic(Aral Sea)

N40-40 All images taken between 1986and 1993

NASA Earth Science Enterprise N40-45 N41-40 N41-45

Landsat MSS(Lake Chad Basin)

P197-R50 23/01/1975 USGSP197-R51 29/01/1973P197-R52 15/02/1975P198-R50 17/02/1973P198-R51 12/01/1973P198-R52 07/03/1973P199-R51 31/01/1973P199-R50 31/01/1973P200-R50 01/02/1973P200-R51 03/12/1975

MODIS (surface reflectance) MOD09A1.A2003041.h19v07.004.2004015043655

10/02/2003to 17/02/2003

NASA

Maps Topographic maps Topographic maps series of centraland west Africa at 1:200000. IGN,Paris

Geological maps International Geological Map of Africa Sheet no. 2. 1:5000000.Choubert, G. Faure-Muret, A.

Commission for the GeologicalMap of the World. UNESCO: Paris.1987Carte Geologique de la Republiquedu Tchad. 1:1500000. Wolf, J.P.BRGM. 1964



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In Cameroon, the PCA of Landsat TM datashows Megalake Chad ridge as a regular andslightly curved band, appearing in grey tones over a darker surface (r in Fig. 4). To the east, satellitedata indicate that the beach ridge stops abruptly at the edge of the Lake Chad Basin just before adjoin-ing with the Mayo Kebbi river, whose drainage

network is also clearly visible in the image (s inFig. 4). Therefore, a considerable discontinuity inMegalake Chad shoreline coincides with a substan-tial spillway, a former outlet to the sea, via theMayo Kebbi river and the Benue Basin. Satelliteimages thus confirm that this vast paleolake Chadwas no longer a closed lake.

Fig. 2. Examples of the remains of Megalake Chad shoreline detected in the Landsat data. A: Landsat TM data, PCA 1st component. The imagereveals a very long continuous band of deposits with smooth curvatures (r). Note the Angamma cliff (c) to the south of this feature. B: Sobeledge detector for the PCA 1st component of Landsat TM data. The edge detector identifies this long band of deposits as a major discontinuity inthe landscape. C: Landsat TM data, PCA 1st component. The image shows a sub-continuous group of long, aligned bands of deposits (r). Notealso contemporary streams (s) cutting through the bands, some of which are feeding Lake Fitri (f). D: Sobel edge detector for the PCA 1st component of Landsat TM data. The edge detector identifies the long bands of deposits as a significant limit in the Landsat data. E: Landsat TMdata, PCA 1st component. The image shows a sharp break in the landscape (r) marking the limit between the Kanem dunefield (d) and a nearlyfeatureless, vast plain of unconsolidated sediments (p). F: Landsat MSS data during the dry season, PCA 1st component. The image reveals acontinuous sand band (r) associated with a sharp break in the landscape separating dunefields (d) to the west and a vast plain (p) to the east. Notethe presence of similar sandy bands (rr) closer to the shoreline of the d modern T Lake Chad (lc). These later sandy bands have been identified asthe remains of the shoreline corresponding to the last transgression of Lake Chad.



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Spectral signatures

0

1000

2000

3000

4000

5000

6000

7000

1 2 3 4 5 6 7

MODIS bands

D N

Site 1

Site 2

Site 3

Site 4

Site 5

Site 6

Site 7

siliceous sand of dunefields

mean basalt

clayey sands

ferrallitic sandstone

salt deposits

crystalline basement

sandy grey clay of oases

urban

vertisoils

(620-670nm) (841-876nm) (459-479nm) (545-565nm) (1230-1250nm) (1628-1652nm) (2105-2155nm)

Location ofthese sitesis reported

in Fig. 1

Fig. 3. Spectral signatures of typical geological deposits in the Lake Chad Basin and of the relict ridge of Megalake Chad.

Fig. 4. Landsat TM data, PCA 1st component. The satellite data confirm that a significant discontinuity in the paleoshoreline (r) coincides with aspillway to the sea, the MayoKebbi (s). Note the current course of the Logone river (l). The image also reveals that the paleoshoreline (r) isassociated with a sharp break in the landscape separating a vast plain (p) to the north and a dunefield (d) to the south in the region of Kalfou.



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In the Saharan portion of the Lake Chad Basin,the Landsat TM data reveal large drainage networks.In Niger, the Landsat TM data show that the Dillialandforms, sometimes regarded as corridors of aeo-lian erosion along neotectonic structures ( Durand,1995), are in fact interconnected channels forminga large dr ainage network that extends over 350 km.In Fig. 5A, the PCA of Landsat TM data sh owsdendritic channels upstream in the Dillia (d in Fig.5A), connecting to parallel channels (p in Fig. 5A)and the main river channel (m in Fig. 5A). Simi-larly, in northern Chad, the Landsat TM data indi-cate a series of large drainage networks, of anaverage length of ~250 km, with their dendritic

shape upstream (d in Fig. 5B) leading to mainchannels (m in Fig. 5B). These drainage networksare not compatible with modern climate. The present annual rainfall decreases from about 900 to 15 mmyear

1 between latitudes 10 8 N and 188 N, respec-tively (210 mm year

1 at the northern side of the present Lake Chad) and is unable to produce asignificant long distance runoff in a sandy flat environment, even when exceptionally heavy loca-lised rainstorms occur. These ancient drainage net-works all lead toward the paleoshoreline of Megalake Chad ( Fig. 1).

In three locations, dense, braided and bifurcatedchannel networks forming fan shaped structures,which are typical of deltas, are detected in the Landsat

TM data. The largest paleo delta is that of the Chaririver (sd1 in Figs. 1 and 6A). In the Sahara, paleo-deltas are observed in the Angamma region (sd2 inFig. 1) and at the terminus of the wadi Achim (sd3 inFigs. 1 and 6B). The Landsat data show that the paleodeltas are positioned along the outer edge of Megalake Chad shoreline.

Regions where the shoreline of Megalake Chadsharply limits the extent of old d unefields were pre-sented above ( Figs. 2E, 2F and 4 ). On the Lake Chadside, these old dunefields are replaced by large plains.Given the regional extent, clear layout, and distribu-tion along Megalake Chad shoreline, we interpret these patterns as dunefield flattenings resulting from

wave-cut action by Megalake Chad.Further traces of wave erosion b y Megalake Chadare revealed in the satellite data. Fig. 7A is a PCAimage of Landsat TM data for the Harr dunefield, aregion located inside the limits of Megalake Chad.The Landsat data and topographic maps of this areareveal that the higher part of the Harr dunefield, between 300 and 310 m, is homogeneously flat. Onthe PCA image of the Landsat TM data, this flattenedarea appears as a homogenous surfac e forming anearly featureless sand plain (fd in Fig. 7A). Incontrast, in the lower belt surrounding this region,the dunefield regains its typical form and topography.On the PCA of the Landsat data, this is indicated by aseries of dark and bright tones (respectively, oases and

Fig. 5. Saharan rivers detected in the Landsat TM data. A: Landsat TM data, PCA 3rd component. The image reveals that the Dillia is a paleoriver. It shows the Dillias large drainage network with dentritic drainage patterns upstream (d), parallel channels (p) following theunderlying geology of longitudinal sand dunes (s), and the main channel (m). Note that the long sand dunes (s), with abundant supply of sand,are parallel to the direction of the prevailing wind. B: Landsat TM data, PCA 3rd component. The image shows large Saharan rivers in north-east Chad with dendritic drainage networks upstream (d) leading to main channel (m).



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sand dunes) that are arranged in a regular patterncharac teristic of a well-developed dunefield (d inFig. 7A). We interpret this large dunefield flatteningas the result of wave-cut action by Megalake Chad.

Fig. 7B shows a series of contemporary streams incentral Chad cutting through a 35 km length of the

relict beach ridge of Megalake Chad. Their dispositionis particularly revealing. They all flow in the samedirection and are perpendicular to the relict ridge. Thisis an indication that in this area the relict ridge is sub-horizontal and that elevation is lower on the LakeChad side. Just after crossing the beach ridge these

Fig. 7. A: Landsat TM data, PCA 1st component. The image shows the flattening of the Harr dunefield in its higher part (fd) surrounded, at alower elevation, by typical sand dunes (d). Note the Bahr el Ghazal valley (v) which links Lake Chad to the Lowlands region in north Chad. B:Landsat TM data, PCA 1st component. The image reveals a series of small contemporary streams (s), all cutting the relict beach ridge (r) in thesame direction and perpendicularly. Just after the relict beach ridge these small streams generate alluvial fans (af) which indicates that a flat topography is bordering the shoreline of Megalake Chad.

Fig. 6. Paleodeltas detected in the Landsat data. A: Landsat TM data, PCA 1st component. The image shows the large paleodelta of the Chari(sd1) that coincides with the paleoshoreline of Megalake Chad (r). Note the current course of the Chari (c) and Logone (l) rivers. B: Landsat TMdata, PCA 1st component. The image shows the main channel of wadi Achim (m) leading to a paleodelta (sd3) at the border of Megalake Chadshoreline (r).



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small streams spawn into alluvial fans (af in Fig. 7B).This indicates that a flat, sub-horizontal area is bor-dering the inner side of the paleoridge and can beinterpreted as a shoreline terrace.

5. Discussion

Data presented here show that the major tectonicsarguments used to refute the existence of MegalakeChad require re-ex amination. Firstly, it was reported by Durand (1995) that the traces of Megalake Chadshoreline were discontinuous with areas showing widegaps. However, many more traces were clearly

detected in the satellite data resulting in an increaseof 75% in the total length of the paleosho reline compared to the mapping by Durand (1982) . Our studyalso shows that the remains of the paleoshoreline donot form d isconnected st raight lines as previouslyreported in Durand (1982) but rather long continuouscurves (Fig. 1).

Secondly, the rose diagram ( Fig. 8) indicates that the directions of the paleo shoreline are far morenumerous than reported in Durand (1982) . Neotec-tonics affecting unconsolidated sediments could not display so many orientations. It is also extremelyunlikely that tectonic features with so many orienta-tions would result in such a singular arrangement

aligned in a very large closed loop. Neoctectonicfeatures are often secant. However here, none of theadjoining landforms detected in the satellite dataextend beyo nd their juncti on as faults would.

Thirdly, Durand (1995) suggested that on the basisof topographic maps the elongated features formingthe ridge are not at constant elevation, at least in Niger. However, close examination of the best topo-graphic maps available across the entire study areareveals that topographic information in the region isstill sketchy and provides no clear evidence either way. The Landsat data, on the other hand, clearlydemonstrate that all the rivers and paleorivers runningthrough or leading to the ridge drain towards the

ridge, indicating that the inside of the loop has alower eleva tion than the o utside.Finally, Durand (1995) reported that the Dillia and,

to the west of Termit, the Dillia Achetinamou werelarge corridors of aeolian erosion that follow tectonicdirections. The extensive parallel features, inte rpretedas neotectonic evidence ( Durand et al., 1984 ), are infact ancient parallel river channels that follow theunderlying geology which is composed of a seriesof long parallel sand dunes oriented north-east/ south-west according to prevailing wind of the region,the Harmattan (s in Fig. 5A).

In the region of rift systems that is the Lake ChadBasin, tectonics has played an important role sinceMesozoic and is probably still active. Therefore,tectonics could have influenced hydrographicforms, present or past. For example, todays direc-tion of the Dillias main channel follows that of amajor rift, while the location of Lake Chad coincideswith the junction of rift systems (e.g. Burke, 1976;Genik, 1992 ). However, a very active neotectonicsduring the Holocene, as required by Durand (1982) ,was never demonstrated and cannot be at the origin

of the variety of forms and directions met in the basin. The hydrological origin for the landformsherein identified is therefore the only pertinent explanation.

The limits of the paleoshoreline mapped by the present study are in rather good agreement with themap proposed by Schneider (1967) , even if slightlymodified in many places.

Unlike todays Lake Chad, Megalake Chad was not a closed lake and the stabilisation of its maximumwater level by the sill of the Mayo Kebbi allowed the

Fig. 8. Rose diagram showing the orientations of the remains of Megalake Chad shoreline mapped in this study. Linear sections of the paleoshoreline were grouped in 5 8 classes. The total length of the sections in each class was then reported on the rose diagram (seemethodology).



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development of a well-defined shoreline system that survived in the landscape.

Our findings strongly support the view that hydro-climatic fluctuations in tropical Africa durin g theHolocene were of great magnitude (e.g. Gasse,2000, 2002 ). Following the very harsh phases of late Pleistocene, a wetter climate was marked bylacust rine deposits and Sahelian pollens. Satellitedata (Figs. 2E, 2F and 4 ) indicate wave erosion byMegalake Chad of old dunefields that were formed or

reworked during the las t major arid phase, 20000 13000 14 C years BP (Servant and Servant, 1983;Maley, 2004 ). Megalake Chad reached its maximumfilling for the Holocene between 7700 and 550 0 14 Cyears BP, with three major phases identified by Maley(2004) as 77007400, 71006600 and 6000550014 C years BP. Climatic conditions deteriorated pro-gressively after 5500 14 C years BP till a relativelyshort but intens e arid event between 4000 and 380014 C years BP (Maley, 1997; Gasse, 2000 ). The last

Fig. 9. Elements of comparison with landforms observed in the Aral Sea. Landsat TM data in the Lake Chad Basin (A and B) and in the AralSea (C and D). A: Landsat TM data, PCA 1st component, showing a detail of the paleoshoreline of Megalake Chad with sub-parallel marks (se).Location reported in Fig. 1. B: Landsa t TM data, PCA 1st component, showing a detail of the paleoshoreline of Megalake Chad sub-parallelmarks (se). Location reported in Fig. 1. C: Landsat TM data Sept. 1989, PCA 1st component, Aral Sea. The image shows sub-parallel marks (se) between the contemporary (1989) and former shorelines of the Aral Sea. These marks are paleostrandlines indicating lake level fluctuations withdeposits at successive elevations. Similar sub-parallel marks are observed along Megalake Chad shoreline (A and B). D: Landsat TM data June1987, PCA 1st component, Aral Sea. The former shoreline of the Aral Sea (r) is marked by a sharp break in the landscape between the dunefieldof the Kyzyl Kum desert (d) and costal sand and mud flats (f).



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important filling of the paleolake Chad occurred between 3700 and 3000 14 C years BP but did not reach the levels of the period 77005500 BP: thesandy ridge at about 28 8 m amsl pr obably corres- ponds to this last phase ( Maley, 2004 ).

In other close arid regions, especially in neighbour-ing southern Egypt and northern Sudan, the general pattern is identical (e.g. Pachur and Hoelzmann, 2000; Nicoll, 2004 ): the return of wetter conditions after thehyper-arid late Pleistocene was in fact a succession of episodes highly variable in rainfall distribution andduration. Large paleorivers were active during wetter periods, significantly more humid than present, andcould flow into the Nile. The West Nubian paleolake

reached a maximum extent of 5300 km2

, for a catch-ment area of about 78000 km 2 (Hoelzmann et al.,2001 ).

A comparison with another large closed lake of theworld is instructive. The Aral Sea, with a knownhistory of recent dramatic shrinkage, has left behindremains of its old shoreline and beach ridge. Theremains of the former Aral Sea shoreline have remark-able similarities with that of the paleos horeline iden-tified from the Lake Chad Basin ( Fig. 9). In particular a series of sub-parallel marks (se in Fig. 9C) corre-sponding to paleostrandlines marking the fluctuationsof the Aral Sea with deposits at successive elevationsis replicated in several locations along the paleoshore-line of Megalake Chad (se in Fig. 9A and B).

A potential next step of this research is the recon-struction of hydrological conditions allowing a Mega-lake Chad. The climatological regimes during the last wet periods in the Lake Chad Basin are not yet clearly identified and do not correspond with a sim- ple shift to the north of the present rainfall isohyets.Atmospheric circulation and ground conditions weresignificantly different from present. The estimation of

hydrological budget of Megalake Chad and its whole basin in these times is therefore far from an easy task,even when assuming a temporary equilibrium for some centuries. Relative contributions of northernand southern tributaries of the lake varied with timeand this variation, obvious between wet and arid phases, also existed between two wet periods com- parable in terms of lake levels. Our present assess-ment of the expanse of Megalake Chad is a first stepin the paleohydrological reconstruction of the regionduring the Holocene.

Acknowledgements

We wish to thank the Lake Chad Basin Commis-sion, the DHA (Ministry of Hydraulics and Water Treatment) in Chad and the DRH of Diffa (Ministryof Hydraulics) in Niger for their support. Landsat TMmosaics and Modis data were obtained from NASA.Partial funding by the French national programmeECCO-PNRH is also acknowledged.

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