Quaternary Science Reviews 22 (2003) 1059–1065

The timing of climbing dune formation in southwestern Niger:fluvio-aeolian interactions and the r #ole of sand supply

Helen M. Rendella,*, Mich"ele L. Clarkeb, Andrew Warrenc, Adrian Chappelld

a Department of Geography, Loughborough University, Loughborough, Leicestershire LE11 3TU, UKb School of Geography, University of Nottingham, University Park, Nottingham, NG7 2RD, UK

c Department of Geography, University College London, 26 Bedford Way, London WC1H 0AP, UKd School of Environment & Life Sciences, University of Salford, Salford, M5 4WT UK

Abstract

Contemporary gully erosion has exposed sections in a climbing dune which is banked up against ferricrete terraces along the

southern bank of the Niger River in southwestern Niger. The main sand transport direction in this area is from northeast to

southwest, and the immediate source of the dune sand is the Niger River. Dune stratigraphy contains evidence of episodic, fluvially

controlled accretion, separated by two palaeosols. Channel fills and stone stringers suggest occasional alluvial and colluvial

reworking. Infra-red stimulated luminescence dating of the aeolian sands shows that dune development occurred episodically during

the African Humid Period (15–5 ka), probably in response to an increase in sediment supply from the Niger River. Soil development

occurred during the relatively short-lived period of enhanced aridity associated with the Younger Dryas, driven by weakening of the

southwesterly monsoon circulation. Climate-driven dune accretion and further soil development occurred during the Holocene

period.

r 2003 Elsevier Science Ltd. All rights reserved.

1. Introduction

The Late Glacial and Holocene palaeoclimatic recordof Africa inferred from the analysis of lake levels andlake cores, and supported by evidence from deep-seacores from the continental margins, is characterised bythe alternation of arid and humid phases reflectingchanges in monsoon strength and penetration. Thisrecord is one of abrupt rather than gradual change(Street-Perrott and Perrott, 1990; Gasse, 2000), with thetransition between humid and arid conditions occurringover time periods as short as several centuries (deMe-nocal et al., 2000). Actual records of environmentalchange from the Sahelian region of West Africa aresparse and mostly derived from the analysis of lakecores in Ghana, Nigeria, Chad and Mali (Talbot andJohannessen, 1992; Gasse, 2000), supplemented by datafrom deep-sea cores off the Mauritanian coast of WestAfrica (deMenocal et al, 2000) and the Niger delta(Pastouret et al., 1978; Zabel et al., 2001).

This paper focuses on the record of sand dunedevelopment in southwestern Niger where the southernbank of the Niger River is flanked by climbing dunesthat partly cover a series of river terraces (Dubois et al.,1984; Bergoing and Gilliard, 1997). These dunes lie nearthe southern limit of Sahelian fixed dunes (Talbot,1984), in a climatically sensitive zone where the strengthof Hadley Cell circulation determines seasonal move-ments of the Intertropical Convergence Zone (ITCZ),influencing the patterns of monsoonal rainfall, wind-speeds and dominant wind directions (Gasse, 2000). Theinterpretation of the palaeoclimatic significance of dunebuilding episodes is based on developing an under-standing of how the controlling factors of sand supply,windspeed and vegetation cover have interacted at localand regional levels.

2. Site description

Karey Gorou village (N13�340, E02�050) lies immedi-ately to the south of the Niger River and to the west ofNiamey (Fig. 1). Climbing dunes can be clearly seen,banked up against a low ferricrete plateau that has been

*Corresponding author.

E-mail address: h.m.rendell@lboro.ac.uk (H.M. Rendell).

0277-3791/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved.

doi:10.1016/S0277-3791(03)00026-X

mapped as terrace 3 of the Niger River (Willems andBergoing, 1994). Contemporary gully incision hasexposed sections in excess of 10 m thick in the climbingdunes to the southeast of the village, revealing strati-graphic evidence of episodic accretion punctuated bypalaeosol formation and talus stringers. Samples of sandwere collected from two exposed faces (Fig. 1) forluminescence dating, to provide absolute ages for sanddune development and inferred ages for periods ofstability, soil formation and colluvial activity. The baseof the composite section (Fig. 2) comprises severalmetres thickness of talus-free, yellow to pale yellowsand and the upper part of this unit contains a well-developed soil horizon. The sharp junction between thesoil and the overlying yellowish-red sand indicates anerosive contact and upper soil truncation. A prominentstone line within the yellowish-red sand divides this unitinto two, the lower part contains abundant evidence ofcolluvial activity whereas the upper part is stone-free

and is terminated by a thin soil horizon. This soil isoverlain by 0.8 m of reddish-yellow sand. The currentsurface of the dune is strewn with talus derived from thelower ferricrete plateau.

3. Luminescence dating

A set of seven samples were taken from the exposedsections of the climbing dune by cleaning back thesections with a spade and then hammering lengths ofopaque plastic tube into the sections. All samplepreparation and measurements for luminescence datingwere undertaken in the laboratory under subduedorange light to prevent bleaching of the naturalluminescence signal from the samples. Approximately30 g of each sample was removed for measurement ofthe field moisture content. Once dry, this fraction wasthen ground in a ball mill to a mean diameter of less

Fig. 1. Location of the Karey Gorou climbing dune.

H.M. Rendell et al. / Quaternary Science Reviews 22 (2003) 1059–10651060

than 10 mm for use in dosimetry measurements (seebelow). The remainder of the sample was sieved and the180–212 mm fraction was chosen for mineralogicalseparation. The chosen fraction was then treated with0.0032 M HCl and 30% H2O2 to remove carbonates andorganic material. Potassium-rich feldspars were thenseparated from the polymineral sands using solutions ofthe heavy liquid sodium polytungstate. Quartz andheavy minerals were separated from the lighter feldsparsusing a density of 2.62 g cm�3 and the potassiumfeldspars were subsequently separated from the sodiumand plagioclase feldspars using a density of 2.58 g cm�3.Sub-samples of approximately 10 mg of the potassiumfeldspars were mounted using silicone oil on 1 cmdiameter aluminium discs for use in luminescencemeasurements.

Luminescence measurements were undertaken in anautomated Ris^ TL-OSL Reader mounted with anintegral 90Sr–90Y beta source. Stimulation was achievedusing an array of 31 TEMT484 infra-red diodes with apeak emission wavelength at 880780 nm delivering apower of 40 mW cm�2 to the sample. The detectionsystem consisted of an EMI 9635QA photomultipliertube filtered with a combination of Schott BG39 and

Corning 7–59 colour glass filters. The ED was deter-mined on between 9 and 12 aliquots of each sampleusing a single aliquot additive dose (SAAD) r!egimeemploying infra-red stimulation for 0.5 s at 50�C and apreheat of 220�C for 10 min. Use of this preheat hasbeen shown to remove any unstable component of thesignal from orthoclase and microcline, potassium-richfeldspars (Clarke and Rendell, 1997). Six aliquots wereused to correct for loss of signal due to repetitive preheatand measurement. Routine fading tests revealed no lossof signal on storage.

The alpha dose was determined using a Daybreak 582Thick Source Alpha Counter, with pairs countingemployed to ascertain uranium and thorium concentra-tions (ppm). The external beta dose was determinedfrom 15 g of the sample using an SURRC Thick SourceBeta Counter with a Shap granite standard andmagnesium oxide as a background. The internal betadose from the decay of 40K within feldspar grains wasobtained in one of two ways. Potassium-rich feldsparseparate (100 mg) was mounted in resin and thechemical contents of individual grains were determinedwith a Cameca SX50 electron microprobe using threewavelength dispersive spectrometers. Between 55 and 61

Fig. 2. Composite stratigraphy through the climbing dune showing location of IRSL samples.

H.M. Rendell et al. / Quaternary Science Reviews 22 (2003) 1059–1065 1061

feldspar grains were analysed from each sample using anaccelerating voltage of 20 kV, a current of 2� 10�8 Aand with the beam defocused to 10 mm, and the meanK2O% concentration was calculated. This was con-verted in K% using the formula K%=K2O%/1.2046for use in calculation of the infra-red stimulatedluminescence (IRSL) age. The K% of three of the same100 mg separates had been previously measured using aRis^ GM Gas Flow Beta Counter, in which the K% isderived from the bulk sample using magnesium oxide asbackground and 100 mg of NIST70a potassium feldsparstandard.

4. Results

The single aliquot ED values are given in Table 1together with the natural brightness (In) of each aliquot.Using the criteria of Clarke (1996), none of the samples

measured here are poorly bleached although they doshow some degree of scatter, which could result from therelatively short transport distance from the source in theNiger River, which currently lies a few hundred metresnorth of the sample site. An additional source of scattermay derive from colluvial grus, which has been shown tohave affected the scatter in luminescence signal fromsand ramps and climbing dunes in the Mojave Desert(Clarke et al., 1996; Rendell and Sheffer, 1996).

The mean values for composition of the feldspargrains analysed by electron microprobe are shown inTable 2, while the results of the comparison with theRis^ GM Gas Flow Beta Counter are given in Table 3.In the case of these samples, the determination of %Kfor the potassium-rich feldspar separates results in anunderestimation of %K of the individual potassiumfeldspar grains within these separates, and therefore anoverestimation of the age of the samples. The microp-robe results also confirm that the quartz grains present

Table 1

Equivalent dose (ED) and natural brightness (In) measurements for each aliquot including mean (w), standard deviation (sn�1) and bleaching

parameters (SN)

Aliquot number LB-050 LB-051 LB-052 LB-053

ED (Gy) In (cps) ED (Gy) In (cps) ED (Gy) In (cps) ED (Gy) In (cps)

1 0.514 240 5.184 4584 15.369 6210 19.233 21876

2 0.924 264 4.577 2218 9.440 3718 20.496 27390

3 0.325 106 3.894 2342 14.190 4890 19.001 19806

4 1.597 346 4.159 1806 9.125 3398 19.865 16172

5 0.515 230 4.696 2632 14.715 3934 19.021 19220

6 1.458 726 3.464 2344 13.447 6664 17.527 13362

7 0.329 156 6.323 3804 13.075 5030 18.375 24274

8 0.604 288 3.402 2876 13.590 3228 17.876 22930

9 0.513 282 3.186 2120 10.322 3098 16.477 17388

10 0.379 164 3.812 2824 11.898 3628 17.984 22058

11 0.584 236 5.645 3956 12.765 4582 17.856 18598

12 0.844 210 4.349 2780 17.740 19388

w 0.715 271 4.391 2857 12.54 4398 18.452 20205

sn�1 0.422 157 0.953 838 2.105 1201 1.116 3780

SN 0.589 — 0.217 — 0.116 — 0.060 —

LB-054 LB-055 LB-056 LB-057

ED (Gy) In (cps) ED (Gy) In (cps) ED (Gy) In (cps) ED (Gy) In (cps)

1 15.999 1582 13.994 1228 27.814 43954 14.847 4328

2 16.188 2418 13.644 834 28.200 30322 14.080 6032

3 17.302 2910 14.218 2306 25.432 32754 12.758 3850

4 16.903 1930 20.510 3082 25.668 40652 13.661 4814

5 19.468 1902 17.084 876 26.482 30084 12.499 4124

6 17.493 1786 19.540 1420 28.173 50380 9.841 4734

7 15.766 1520 16.249 1358 23.485 39500 11.619 10208

8 22.651 2082 16.845 940 23.867 37164 14.093 4242

9 17.161 2714 16.106 1270 26.321 43130 15.213 4182

10 19.669 728 24.712 25308 12.670 3186

11 16.286 698 25.107 43910 13.462 4824

12 28.367 65616

w 17.659 2093 16.740 1340 26.136 40231 13.159 4956

sn�1 2.170 488 2.347 735 1.714 10757 1.528 1881

SN 0.123 — 0.140 — 0.066 — 0.116 —

H.M. Rendell et al. / Quaternary Science Reviews 22 (2003) 1059–10651062

do not contain any feldspar inclusions. The attenuateddosimetry data and the final ages for the samplesanalysed are given in Table 4. The ages obtained rangefrom 13,17071100 yr, near the base of the compositesection, to the present. The two palaeosols are bracketedby ages of 12,96072250 and 11,75071690 yr, and32807740 and 4807280 yr, respectively.

5. Discussion

Climbing dunes require a source of sand, which isexposed to deflation in a regime of strong, persistentwinds, and the presence of a topographic barrier againstwhich the sand can accumulate. Dune building has oftenbeen associated with aridity but it has been shown that

high wind speeds may result in active sand movementunder high rainfall conditions (Talbot, 1984). Apersistent increase in rainfall can also result in anincreased supply of sand through fluvio-aeolian inter-actions, hillwash and colluvial sediment reworking(Clarke and Rendell, 1998). Given the close proximityof the Niger River to the Karey Gorou site, sandaccretion could occur if perennial flow in the riverperiodically faltered (as it has done during the period ofinstrumental records, most recently in 1984), allowingsand exposed on the dry bed of the river to be blown in asouthwesterly direction from the river bed and upagainst the ferricrete terrace. Alternatively, the sandmay have derived from overbank flood depositsassociated with a wetter phase; the sand being reworkedby the wind, once the floods (such as those that occurredin 1998) had receded. Whichever of these situationspertained, sand movement is likely to have been episodicand to have reflected seasonal contrasts in moisture andwind regimes driven by changes in the relative strengthand dominance of the monsoonal circulation (L!ezineand Casanova, 1989; Gasse, 2000).

The IRSL ages show that the base of the KareyGorou section dates back to the Late Glacial(13,17071100 yr) and that much of the dune developedtaking during the ‘African Humid Period (14,800–5500 cal yr BP) (deMenocal et al., 2000). Earlier phasesof dune development within West Africa have beendated to ca. 18,000 yr in the Erg of Bilma in northernNiger (V .olkel and Grunert, 1990) and to 21,500–13,000 yr in the Lantewa Dune field in northern Nigeria

Table 2

The mean chemical composition of the feldspars analysed from each

sample using an electron microprobe

Sample K2O (%) Na2O (%) CaO (%) n

LB-050 15.0271.76 0.8870.47 0.0170.02 59

LB-051 15.2970.79 0.7870.52 0.0170.01 59

LB-052 16.2770.47 0.7870.26 0.0170.02 58

LB-053 16.0370.50 0.7570.28 0.0170.02 61

LB-054 15.7670.49 0.8770.30 0.0170.01 60

LB-055 15.7571.35 0.9170.88 0.0170.04 57

LB-056 15.7970.45 0.7570.23 0.0070.01 60

LB-057 15.8570.44 0.7770.25 0.0070.01 55

n stands for the number of grains of feldspar used to calculated the

mean.

Table 3

A comparison of K% concentrations derived from the same aliquot of potassium-rich feldspar separates using an electron microprobe and Ris^ GM

beta counter

Sample K% derived

from electron

microprobe

K% derived

from Ris^ GM beta

counter

Ratio of K% Ris^/

microprobe

Age

overestimate

(%)

Age

overestimate (yr)

LB-050 12.4771.46 4.4070.44 0.353 135 190

LB-056 13.1170.37 4.5070.45 0.344 131 4050

LB-057 13.0870.41 4.1070.41 0.313 175 8560

Table 4

Attenuated dosimetry data, mean equivalent dose (ED) and IRSL ages for the samples: the water content used was 575%

Sample Internal

beta dose

rate (mGy/yr)

External

alpha dose

rate (mGy/yr)

External

beta dose rate

(mGy/yr)

Gamma

dose rate

(mGy/yr)

Cosmic

dose rate

(mGy/yr)

Total

dose rate

(mGy/yr)

ED (Gy) IRSL

age (yr)

LB-050 675794 125764 242745 270732 197710 15097127 0.71570.422 4707280

LB-051 687762 100752 171732 218725 16478 1340788 4.39170.953 32807740

LB-052 732758 94749 1907185 209724 12376 13487202 12.54072.105 10,16072290

LB-054 708757 122763 292766 301735 7974 15027113 17.65972.170 11,75071690

LB-055 708780 92747 217791 214725 6673 12927133 16.74072.347 12,96072250

LB-056 710756 244734 429747 540763 6173 19847103 26.13671.714 13,17071100

LB-057 713757 58730 149754 139716 8974 1148785 13.15971.528 11,47071580

H.M. Rendell et al. / Quaternary Science Reviews 22 (2003) 1059–1065 1063

(Thiemeyer, 1995; Stokes and Horrocks, 1998).However, in many areas of the Sahara, it has beenshown that sand deposited at the Last GlacialMaximum may have been reworked during the LateGlacial (Swezey, 2001). A 7 m thickness of sand wasdeposited at Karey Gorou from 13,17071100 to10,16072290 yr. At the same time, dune buildingcontinued within the Lantewa Dune field (Thiemeyer,1995; Stokes and Horrocks, 1998). It is notable that thestart of dune building at Karey Gorou coincides with adramatic decrease in the amount of terrigenous quartzrecorded in core CH22KW31 through Niger deltasediments ca. 13,000 cal yr BP (11,500 yr BP) (Pastouretet al., 1978).

The period from 13,000 to 10,000 yr also includes theYounger Dryas, a millennial scale global cooling eventdated at 12,800–11,500 cal yr BP. This event has beenassociated with increased aridity in sub-Saharan Africa(Street-Perrott and Perrott, 1990; Gasse, 2000). How-ever, within West Africa, the desiccation of Lake Bal(13�18.410N, 10�56.960E) in northern Nigeria dated at11,200 cal yr BP (Holmes et al., 1999) does not coincidewith either of the arid intervals at Lake Bosumtwi(1�250W, 6�300N), a closed lake basin in southernGhana (Talbot and Johannessen, 1992), dated at ca.14,000 and 12,500 cal yr BP (12,000 and 10,500 yr BP,respectively). At Karey Gorou the lower of the twopalaeosols has bracketing ages of 12,96072250 and11,75071690 yr which place the period of soil formationwithin the Younger Dryas. In a recent synthesis of directand indirect evidence of aeolian mobilisation orstabilisation in the southwest Sahara (south of 24�N),Swezey (2001) pointed to a phase of mobilisationcoinciding with the Younger Dryas, with other short-lived phases ca. 10,000, 8100 and 7500 cal yr BP.

After the Younger Dryas event, Zabel et al. (2001)note a decrease in the accumulation rate of Niger deltasediments in core GeoB 4901 between 11,000 and10,000 yr which they interpreted as reflecting an increasein vegetation cover within the Niger River catchment,associated with increased rainfall. Yet, at Karey Gorou,sand accumulation continued between 11,47071580and 10,16072290 yr, with evidence of reworking byfluvial and colluvial activity, as demonstrated by thestone stringers and small channel fills.

There is convincing evidence of late Holocene aridityin the West African record with increased Saharan dustflux in core 658C off the Mauritanian coast from5500 cal yr BP (deMenocal et al., 2000) and a desiccationsurface in Bal Lake dated at 3400 cal yr BP (Holmeset al., 1999). Aeolian activity within the Manga Grass-lands occurred during the periods 1700–1300 and 210 –170 yr (Holmes et al, 1999). In the Karey Gorou section,the upper palaeosol has bracketing ages of 32807740and 4807280 yr and is overlain by 0.8 m of sand. Thefinal phase of sand movement dated at this section falls

within the time period of the North Atlantic coolingepisode known as the Little Ice Age.

At Karey Gorou both of the palaeosols are dated toperiods which are associated with regional aridity, theYounger Dryas and the Late Holocene, whereas muchof the sand accumulation is associated with more humidconditions than at present. It appears that soil develop-ment occurred when sediment supply from the NigerRiver was limited and that dune building resumed whensupply was more abundant.

6. Conclusions

At Karey Gorou, dune accretion is controlled by theprovision of sand for entrainment through overbank,flood sedimentation associated with the Niger River,rather than by aridity. Sand is apparently blown fromthe floodplain and trapped against the ferricreteterraces. Colluvial and alluvial reworking of theseaeolian deposits in humid periods is reflected in theformation of channel fills and stone stringers. Most ofthe climbing dune developed in several episodes duringthe African Humid Period (15,000–5500 yr). A well-developed soil corresponds in age to the Younger Dryaswhich, in West Africa, is believed to have been anabrupt and short-lived arid phase. This arid phasereflects a weakening of the southwesterly monsoon, astall in the location of the ITCZ, a more pronouncedrainfall seasonality in the Sahel and, at Karey Gorou, alack of sediment supply for active dune building. Aweak soil, formed between 3000 and 500 yr ago,coincides with the onset of Late Holocene aridity andis buried by a final phase of dune building during theLittle Ice Age.

Acknowledgements

This work was partly funded by a NottinghamUniversity Research Scholarship to MLC. We wouldlike to thank Ann Wintle and Susan Packman of theUniversity of Wales, Aberystwyth for measurement ofthe feldspars in the Ris^ GM beta counter and bothMike Styles and Paul Wetton of the British GeologicalSurvey at Keyworth for the electron microprobeanalysis.

References

Bergoing, J-P., Gilliard, P., 1997. Geomorphologie des terrasses du

fleuve Niger "a la latitude du Parc National du W, Niger. Zeitschrift

f .ur Geomorphologie. N. F. 41, 491–504.

Clarke, M.L., 1996. IRSL dating of sands: bleaching characteristics at

deposition inferred from the use of single aliquots. Radiation

Measurements 26, 611–620.

H.M. Rendell et al. / Quaternary Science Reviews 22 (2003) 1059–10651064

Clarke, M.L., Rendell, H.M., 1997. Stability of the IRSL spectra of

alkali feldspars. Physica Status Solidi (b) 199 (2), 597–604.

Clarke, M.L., Rendell, H.M., 1998. Climate change impacts on sand

supply and the formation of desert sand dunes. Journal of Arid

Environments 39, 517–531.

Clarke, M.L., Wintle, A.G., Lancaster, N., 1996. Infra-red stimulated

luminescence dating of sands from the Cronese Basins, Mojave

Desert. Geomorphology 17, 199–205.

deMenocal, P., Ortiz, J., Guilderson, T., Adkins, J., Sarnthein, M.,

Baker, L., Yarusinsky, M., 2000. Abrupt onset and termination of

the African Humid Period: rapid climate responses to gradual

insolation forcing. Quaternary Science Reviews 19, 347–361.

Dubois, D.M., Icole, M., Trichet, J., 1984. Evolution

g!eomorphologique de la vall!ee du Niger aux abords de Niamey

(R!epublique de Niger). Bulletin de la Societe Geologique de France

XXVI 6, 1305–1318.

Gasse, F., 2000. Hydrological changes in the African tropics since the

Last Glacial Maximum. Quaternary Science Reviews 19, 189–211.

Holmes, J.A., Street-Perrott, F.A., Perrott, R.A., Stokes, S., Waller,

M.P., Huang, Y., Eglinton, G., Ivanovich, M., 1999. Holocene

landscape evolution in the Manga Grasslands, NE Nigeria:

evidence from palaeolimnology and dune chronology. Journal of

the Geological Society of London 156, 357–368.

L!ezine, A-M., Casanova, J., 1989. Pollen and hydrological evidence for

the interpretation of past climates in tropical West Africa during

the Holocene. Quaternary Science Reviews 8, 45–55.

Pastouret, L., Chamley, H., Delibrias, G., Duplessy, J.C., Thiede, J.,

1978. Late quaternary climatic changes in western tropical Africa

deduced from deep-sea sedimentation off the Niger Delta.

Oceanologica Acta 1 (2), 217–232.

Rendell, H.M., Sheffer, N.L., 1996. Luminescence dating of sand

ramps in the eastern Mojave Desert. Geomorphology 17, 187–197.

Stokes, S., Horrocks, J., 1998. A reconnaissance survey of the linear

dunes and loess plains of northwestern Nigeria: granulometry and

geochronology. In: Alsharhan, A.S., Glennie, K.W., Whittle, G.L.,

Kendall, C.G.St.C. (Eds.), Quaternary Deserts and Climate

Change. Balkema, Rotterdam, pp. 165–174.

Street-Perrott, F.A., Perrott, R.A., 1990. Abrupt climate fluctuations

in the tropics: the influence of Atlantic Ocean circulation. Nature

343, 607–612.

Swezey, C., 2001. Eolian sediment response to late quaternary climate

changes: temporal and spatial patterns in the Sahara. Palaeogeo-

graphy, Palaeoclimatology, Palaeoecology 167, 119–155.

Talbot, M.R., 1984. Late Pleistocene rainfall and dune building in the

Sahel. Palaeoecology of Africa. 16, 203–214.

Talbot, M.R., Johannessen, T., 1992. A high resolution palaeo-

climatic records for the last 27,500 years in tropical West Africa

from the carbon and nitrogen isotopic composition of

lacustrine organic matter. Earth and Planetary Science Letters

110, 23–37.

Thiemeyer, H., 1995. Dating of palaeodunes from NE Nigeria with

thermoluminescence. Zeitschrift f .ur Geomorphologie Supplement-

band 99, 97–106.

V .olkel, J., Grunert, J., 1990. To the problem of dune formation and

dune weathering during the late pleistocene and holocene in the

southern Sahara and Sahel. Zeitschrift f .ur Geomorphologie N. F.

34 (1), 1–17.

Willems, L., Bergoing, J-P., 1994. Observations des coupe dans la

terrasse T3 vall!ee du Niger en aval de Niamey. Revue ed

G!eographie Alpine Au contact Sahara–Sahel 1, 47–64.

Zabel, M., Schneider, R.R., Wagner, T., Adegbie, A.T., de Vries, U.,

Kolonic, S., 2001. Late Quaternary climate changes in Central

Africa as inferred from terrigenous input to the Niger fan.

Quaternary Research 56, 207–217.

H.M. Rendell et al. / Quaternary Science Reviews 22 (2003) 1059–1065 1065