Holocene lake deposits and palaeoenvironments in Central Sahara, Northeastern Mali

Palaeogeography, Palaeoclimatology, Palaeoecology, 35 (1981): 45--61

45

Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

HOLOCENE LAKE DEPOSITS AND PALAEOENVIRONMENTS IN CENTRAL SAHARA, NORTHEASTERN MALI

N. PETIT-MAIRE ~ and J. RISER 1 With contributions by: M.F. BONIFAY, P. CARBONEL, G. DELIBRIAS, C. HILLAIREMARCEL, J.C. ROSSO, E. SCHULZ and I. SOULIE

Laboratoire de G~ologie du Quarternaire, CNRS--Luminy, Marseille (France) (Received March 2, 1981) ABSTRACT Petit-Maire, N. and Riser, J., 1981. Holocene lake deposits and palaeoenvironments in central Sahara, northeastern Mali. Palaeogeogr., Palaeoclimatol., Palaeoecol., 35: 45--61. Extensive and fossiliferous lacustrine deposits have been found in a now hyperarid area south of the Tanezrouft (20°30--21°N; 0--1°W), between the Erg Ine Sakane and the Djebel Takabart. The assemblage of fish, molluscs, ostracods and charophytes (among which Lychnothamnus is observed in Africa for the first time) testify to fresh-water conditions. However, stable carbon and oxygen isotopic values provide evidence for phases of strong evaporation. The presence of Limicolaria sp., a terrestrial pulmonate first observed in the Sahara, indicate permanent soil humidity around the palaeolake(s). The remains of large, steppe-type mammals (elephant, antelopes, buffalo) as well as Neolithic sites and burials in brown soils, also contribute to evidence a northward shift of the Sahel belt, 3--4 ° in latitude. This period is radiocarbon-dated from 6130 _+180 B.P. to 7520 -+ 70 B.P. on molluscan shells. The h u m a n material could be younger (3750 ± 100 B.P. on charcoal). INTRODUCTION

The area located between the Timetrine to the east, the Azawad to the south (Poussibet, 1961} and the Tanezrouft to the north constitutes the easternmost part of the Majab~t-al-Koubrfi hyper-arid desert (Fig.l). To the eastern margin of this region, Monod (1958) observed, overlying the sandyclays of the "Continental Terminal", a Quaternary sequence ending with lake deposits containing diatoms as well as prehistoric sites and vertebrate remains. However, no specific research of past environments has ever been made in this region that remains to this day a blank on palaeoclimatic maps. A short reconnaissance completed in February--March 1980 was aimed at reducing this lack of information.

'Participants in reconnaissance trip. 0031-0182/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company

46

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Fig.1. General location map of the study area in northeastern Mali: 1 = highlands; 2 = m a i n e s c a r p m e n t s ; 3 = p a l a e o l a c u s t r i n e d e p o s i t s ; 4 = e r g s ; 5 = T i l e m s i trail. PRESENT

CONDITIONS

The prospected area lies between 20 ° and 21°N and 0° and l°W (Fig.2). The geological structure of this region conforms t o the general structure of the Sahara, built up of broad upwarps and large basins separated by thresholds, and constitutes the northeastern part of the basin lying to the west of Adrar des Iforas. Formations attributed to the "Continental Intercalaire" and t o the Upper Cretaceous outcrop widely and are covered with a "Continental Terminal" veneer. These sandstone, gypsum, clayey or calcareous series are somewhat deformed; their position is synclinal along a NNE--SSW axis, between the Timet~ine to the east and the Erg Ine Sakane to the west. North of the Timetrine and of the Adrar Tidjerazrazine, they dip slightly to the north. These deformations have favoured differential erosion which resulted in a cuesta relief. Gentle dips and reduced thickness of beds lead to its dis-

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F i g . 2 . G e o m o r p h o l o g i c a l s k e t c h Of t h e E r g I n e S a k a n e a r e a : 1 = c u e s t a s c a r p ; 2 = c u e s t a o u t l i e r ; 3 = i n c l i n a t i o n ; 4 = c a p p e d b u t t e a n d h i l l o c k ; = b u t t e s z o n e o f r e d d i s h g y p s e o u s s a n d ; 6 = w h i t e silt d e p o s i t s ; 7 = r e d d i s h l o a m y s a n d d e p o s i t s ; 8 , 9 = p a l u s t r i n e o r l a c u s t r i n e shell b e d s ; 1 0 = E r g I n e S a k a n e ; 11 = m a j o r N e o l i t h i c s i t e s .

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48

section into tabular buttes, such as the Takabart or in the Ine Kousam~ne (Fig.2). The climate is characterized by sparse precipitation; it averages 30 mm, but is quite irregular (9 mm in 1979), being due to an episodic advance of the intertropical convergence. Violent winds, oriented north to south, are the most frequent in March, April and May (Monod, 1958; Dubief, 1963). The region is hyper-arid and eolian deflation results in large regs and dune fields. R u n o f f is rare, scarce and of very short duration (Dewolf et al., 1972). The vegetation is very limited, i.e. restricted to wadis and depressions, inferring presence of groundwater. In some wadis of the Timetrine going toward the Erg Ine Sakane, one finds scattered Acacia, whereas the rest of the region only shows some assemblages of tuff grasses (Stipagrostis sp., Panicum sp.) together with small shrubs of Fagonia, Cornulaca, Pergularia and others. THE LACUSTRINE DEPOSITS

Between 2 0 ° 3 0 ' - - 2 0 ° 5 0 ' N and I ° W - - 0 ° 4 0 ' E , a string of lacustrine formations lie along the border of the Erg Ine Sakane for some 60 km by 5 km. For the m o m e n t their thickness is n o t known, since vertical sections are rare. However, a 3-m section cutting into a characteristic part of the formation was recorded (Fig.3). The sedimentation is of superimposed beds (grey or ashy to black colour) of silty or clayey sand with a fossil molluscan content. The structure is generally horizontal (Plate I) although a 10 ° dip was occasionally observed.

1111111 ' -f.lllllllllllllll

2 Y

3

4 ~5

Fig.3. S e c t i o n in l a c u s t r i n e d e p o s i t s i n t h e Erg I n e S a k a n e ( h e i g h t 3 m ) : 1 = l o a m y - s a n d y bar c o n t a i n i n g shells, w i t h o c h r e grading at t h e s u r f a c e ; 2 = light-grey l o a m y - s a n d y b a r w i t h shells; 3 ffi dark grey l o a m y - s a n d y bar w i t h shells; 4 = grey l o a m y - s a n d y b a r w i t h shells a n d fish r e m a i n s ; 5 = irregular shell b a r w i t h c o m p a c t e d s t r a t a ; 6 = grey argillaceouss a n d y b a r w i t h o u t shells; 7 = b l a c k i s h b a r w i t h shells; 8 = grey s a n d w i t h shells at t o p .

49 Quartz dominates the mineral content (dull, rounded, pitted sand grains) which is quite homogeneous. Feldspar, gypsum and a small quantity of dolomite are the other components. THE LACUSTRINE FAUNA (Plate I) In these sediments, the following aquatic fauna has been found:

Fish The remains are quite numerous and mostly consist of vertebrae; their size varies from a few millimetres to about 10 cm. (Plate I). These large specimens belong to Lates niloticus, observed by Monod (1958) in the Arawan area. Their presence implies the former existence of permanent deep water (det. M. Gayet).

Mollusca Bivalvia. In the Erg Ine Sakane (E.I.S.) deposits, Aspatharia sp. (Krauss) has a maximal size of 64 mm. It requires permanent fresh water and occurs mainly in places with a strong bottom current. Their optimal biotope is the clayey--mud bottom of unconfined lakes. Corbicula africana (Krauss) is well represented and of medium size (max. 18 mm). This species has the same ecological requirements as Aspatharia and may reach high densities, as occurs in Lake Chad (Lev6que, 1967; Dupont and Lev6que, 1968). Gastropoda, Prosobranchia. Bellamya unicolor (Olivier), a polymorphic species, common in the Chad Basin, is known today throughout North Africa, from the Nile to the Senegal rivers. It was found in small numbers at E.I.S. These gregarious and ovoviviparous animals are gonochoric (separate sexes); they live in colonies among aquatic plants and are particularly fond of papyrus rhizomes (Lev6que, 1967). Their small number at E.I.S. could be explained either by low productivity of the waters, by important predation by malacophagic fish, or by an insufficient group effect (unfertilized females). The species Melania tuberculata (Miiller) may vary greatly in size, shape, ornamentation and test thickness. It has a large ecological tolerance and may adapt to different types of substratum. It normally requires warm, oligohaline fresh waters and outlives temporary emersion through the drying of its operculum and burying itself in the substratum. It may also stand important seasonal variations in salinity (Monteillet and Rosso, 1978). Gastropoda, Pulmonata. Limnaea natalensis (Krauss), Biomphalaria pfeifferi (Krauss) and Bullinus truncatus rohlfsi (Ciessin) are common in present-day

50

PLATE I

51

fresh-water sites, in association with aquatic (Ceratophyllum) or semi-aquatic (Cyperus, Vossia, Phragmites, Typha) plants, these protecting and feeding them (Lev~que, 1967, 1975). Limicolaria turriformis {Martens), a terrestrial pulmonate, is found in the black softs around the lake margins; it may be found in a density reaching 3 to 5 per m 2. It lives t o d a y around Lake Chad as well as in the Niger and Chari Basins (Germain, 1907, 1911). At E.I.S., two types are associated: L. t. typica (Martens) and L. t. obesa (Germain, 1906), differing in spire length. The ecological requirements of these organisms are still poorly known. In any case, the presence and the density of the genus Limicolaria in a present hyper-arid area is very significant, since its optimal biotope is tropical savannah and forests, with permanent soil humidity and vegetation.

Ostracoda Six species m a y be observed in E.I.S. lake deposits, b u t 70% of the ostracod fauna belong to the species Cyprideis gr. torosa, followed by Cypris sp. Cyprideis torosa seems to have been well represented all over the Sahara (Chamard, 1973; Petit-Maire, 1979; Carbonel and Pinson, 1979). They are very polymorphic and their ornamentation was proven to have ecological significance (Carbonel, 1980; Peypouquet, 1977). At E.I.S., nodes occur in 20--25% of the specimens according to localities, they infer oligohaline waters rich in organic matter (0--5%); a poorly marked reticulation indicates a good ionic balance; however, some localities show an ionic balance favourable to Ca2+and indicate concentration processes. The association of ornamented and smooth shells in the same deposits also testifies to this fact. Thus, the ostracod fauna at E.I.S. infers changing environments, under stress conditions. As a whole, the variety and density of the aquatic fauna in the E.I.S. lacustrine sediments, as well as its c o m m o n ecological requirements, testify to the past existence of extensive quasi-permanent fresh-water areas. They should have been large and deep enough to allow the settling and development of large bivalves (Aspatharia) and large fish. A marginal vegetation was required for the life of such epiphytic gastropods as Bellamya, Biomphalaria, Bullinus and Limnaea, while constant soil humidity was required for Limicolaria reproduction and growth.

Stable isotope analysis. Oxygen and carbon stable isotopes have been analyzed in mollusc shells from four localities situated along the E.I.S. deposits (cf. Fig.2). The results are shown in Table I.

PLATE I Lacustrine deposits at the Erg Ine Sakane. Mollusc shells and fish remains.

52 TABLE I Oxygen and carbon stable isotope analyses in mollusc shells from four Erg Ine Sakane localities

Localities from north to south

Analyzed species

Loc. 17 Loc. 19

Bellamya unicolor Melania tuberculata Corb icula africana Biomphalaria pfeifferi Limicolaria turriformis Limicolaria turriformis (typica) Limicolaria turriforrnis (obesa)

Loc. 20 Loc. 22

513CpDB

--5.59 --0.65 --2.18 --2.47 --6.15 --10.30 --10.34

61SOpDB

--10.12 --0.70 --2.81 ---6.23 --5.10 --6.42 --6.16

The data correspond with the ecological interpretation of the observed species, the salt-intolerant Limicolaria having the lowest values, together with the fresh-water Bellamya. On the contrary, the tolerant Melania provide evidence of phases of strong evaporation. TERRESTRIAL

MAMMALS

At the margins of the lacustrine deposits, either in situ in the m u d d y sediments or associated to the fireplaces of Neolithic sites, the b o n y remains of a large mammalian fauna were collected. Artiodactyls are dominant. A large-sized bovid is the most frequent. Its height of the withers was at least 1.50 m and the length of its metapods exceeded 200 mm. Domestic bovids do n o t reach this size and one may think rather of Syncerus caller, the wild buffalo, n o w extending throughout tropical East and Central Africa, with isolated groups reaching as far as the western littoral (Dorst and Dandelot, 1976). Medium-sized artiodactyls are also found, among which a very robust caprine, b e y o n d the domestic Capra sp. range. We may have here an Ammotragus laevia Pallas, still living in the mountainous Sahara. It could have lived in the Timetrine. Moreover, smaller-sized remains can be attributed to Gazella dorcas L., still numerous in the Azawad. The elephant Loxodonta africana Blumenbach is represented by five lamellar fragments from an immature tooth. Further field research should confirm this determination. An incomplete equid t o o t h may be attributed to the subgenus Hippotigris sp. (zebra}. This faunal assemblage (with its similarities to the Holocene Chami fauna, at the same latitude in northern Mauritania} evokes a Sahel-type biotope, with permanent water available. The presence of y o u n g animals (Ammotragus, Loxodonta) suggests herds rather than isolated animals. The large antelopes suggest the possibility of gramineae feeding. The buffalo requires water for frequent bathing and drinking.

53 REPTILES The lacustrine area is rich in remains of two fresh-water turtles: Pelosius castaneus castaneus and Cyclanorbis senegalensis. Both are essentially carnivorous and live nowadays in the arboreal savannah south of the Sahara (det. F. de Broin). At two localities, Crocodylus niloticus was found (det. E. Buffetaut). PALAEOFLORA

Numerous concretions after vegetation were observed in the greyish to blackish sediments along the lake(s) margins. Typha sp. is quite c o m m o n at some localities; it is well preserved, the stems still clearly showing their nodes and ribs. Two charophyte genera have been observed in the same layers: Chara represented by C. pappii Soulie-M~irsche, and Lychnothamnus. Chara pappii was first described from the Upper Pliocene in Greece (SoulieMirsche, 1979b). The apical periphery of the gyrogonites bears a very distinct groove; in the apical center, strong calcification of the lime-spirals form a rounded and protruding apical rosette (Plate II). A biometrical study cannot be made from the first material collected at E.I.S. However, the available gyrogonites average 575 pm in length and 4 2 0 / a m in width. These values are quite similar to the European ones. To this day, the species and its particular gyrogonites type are only found in the fossil state; considering the Holocene age of E.I.S. lacustrine sediments, one could suppose they could still exist in Africa. The ecological requirements of C. pappii are n o t known; one cannot deduce t h e m from a single (brackish} site. Lychnothamnus sp. is a remarkable element in E.I.S. palaeolake(s), since it has only been observed previously in Central Europe, Pakistan and India (Cotillion, 1957; Kargzmarz, 1967). A single living species is known, L. barbatus (Meyen) v. Leonh. Its gyrogonites show a characteristic globular general shape. The apical pole is flattened; the basal pole protrudes and the gyrogonites show typical apical and basal structures (Plate II). An apical outgrowth is present in one of our specimens; it shows apical swelling of the spiral cells at the top of the oogone, a characteristic of extant L. barbatus (Soulie-Mirsche, 1979a). The mean size of our specimens (medium length 1000/am, medium width 850/am) significantly differs from European populations (850/am and 700/am). We possibly have here a new species or subspecies. As this is the first observation of Lychnothamnus in Africa, one may wonder whether its presence, during the late Quaternary at E.I.S., suggests a very recent extinction or, more probably, merely a lack of research in appropriate biotopes on the African continent. The ecological requirements for L. barbatus are restricted to permanent fresh-water lakes, with an optimal depth of 3--5 m. According to Kargzmarz'

54

PLATE II

55

experiments (1967), these algae cannot normally fructify in a saline environment. One can infer from the excellent fructification of E.I.S. gyrogonites that the fundamental ecological requirements of the genus were fulfilled, even in the case of a different species: they testify to long or even permanent submersion in waters with a quite stable low or very low salinity. In the lacustrine sediments as well as in the brown soils underlying them, Neolithic remains and pollen grains were searched for. They are relatively poor and clearly influenced by local factors (Table II). Gramineae and Chenopodiaceae, including Cornulaca sp., dominate; additional elements mostly belong to Cyperaceae, Compositae and Malvaceae. Long-distance transportation is rare or negligible (Betula, Quercus, Corylus,

Pinus, Artemisia ). In one of the profiles, one notices an evolution from Gramineae dominance (lower layer) to Chenopodiaceae dominance (upper layer). This could mean that the corresponding sediments accumulated at different seasons and flowering periods (Cour and Duzer, 1978). The E.I.S. palaeoenvironment may therefore be seen as a scarce grass and Chenopodiaceae cover, mainly including tuff grasses of Stipagrostis and Panicum type. Other herbs, such as the ephemerous "acheb", may have been present. Nothing indicates a past terrestrial vegetation qualitatively differing from the present one. However, the mammalian c o n t e x t gives evidence of a much greater density, although vegetation may have been, as nowadays, limited to the most favourable spots. HUMAN LIFE

Prehistoric life is clearly associated with the lacustrine environment. Small Neolithic sites are scattered, as in the whole Sahara, all throughout the area, b u t they become much larger, richer in material and more dense as we approach the palaeolake(s) area. Apart from workshop areas, close to raw material outcrops (magmatic greenstone to the east, chert to the

PLATE II

Lychnothamnus sp. (nov. sp. ?) Erg Ine Sakane. Carbonate shells. 1. Lateral view with apical outgrowth. 2. Other specimen, lateral view. 3. Basal view. 4. Apical view. All x 37. 5,6 Lychnothamnus barbatus. Mature carbonate shells from herbarium specimen. 5. Lateral view, x 45.6. Apical view, X 55. 7--11 Chara cf. aspera, Tichet. Carbonate shells. 7, 8, 9. Lateral views. 10. Basal view of same specimen as 8, a pentagonal basal plug filling the whole of basal pore. 11. Apical view, lime spirals are not thickened but largely widened in the apical centre. X 75. 12, 13 Chara pappii, Erg Ine Sakane. Carbonate shells. 12. Lateral view. 13. Apical view showing apical rosette, x 60. 1--4

56 TABLE II Pollen spectra from E.I.S. lacustrine sediments Floristic elements

Temperate

Mediterranean Medit.-preSaharan

Saharo-Sindian

Sahel--Tropical

Sample no. Mali 15 subsoil Mali 13/6 (36 cm)

515

Mali 13/7

30

5

3

9 1

(50 cm) Mali 13/9 (76 cm)

Mali 13110 (96 cm)

5 1

1

1

Mali 17/4

4

1

1

(30 cm) Mali 17/6

1

1

7

1

1

5

5

2

(80 cm) Mall 22

Mali 4 soil

1

1

8

2

1

1

north), we observed large dwelling sites, with stone structures, fireplaces, lithics ceramics, .bone industry and adornments. The dominant impression is of poPulations living on Gramineae collecting (large ratio of grinding artefacts) and fishing (density of fish bones in the fireplaces), with occasional hunting (artiodactyl remains in the fireplaces). The c o n t e x t is clearly Neolithic; the ceramics would suggest early Neolithic (G. Aumassip, pers. comm.). Burial sites are also f o u n d in the same region. They are n o t isolated, b u t each grouped into a necropolis of a b o u t 10 to 50 tombs. At one such site, a b o u t 80 km NE of the E.I.S. deposits, a grave was excavated. The skeleton was found lying u p o n a brown soil, in a contracted position (Plate III; it was

57

Multi-regional

O

10

2

15

15

1

1

1

14

100

22

4 4

2

204

67

1

285

4

200

13

325

18

445

18

15

1

1

2

1

36 1

8

1

3

7

4

48

2

1

9

1

1

1

696 263

6 7

18

3

82

1

6

348

2

3

232

8

423

7

490

1 4

7

25

associated with ostrich-egg shell beads and ochre grains. This c o n t e x t is typical Neolithic. These anthropological data for the E.I.S. area will be considered in more detail in another paper. They fit the data inferred from palaeontology and palynology. RADIOCARBON AGES

T w o radiocarbon dates were obtained from molluscs at E.I.S., one from the fresh-water species Biomphalaria pfeifferi: 7520 + 70 B.P. (Gif 5226), and another from the terrestrial snarl Limicolaria turriformis, at the lake margin: 6540 + 130 B.P. (Gif 5232).

58

PLATE III

59 T h e s e t w o d a t e s fit t h e o b s e r v a t i o n s o f high levels in T i r e r s i u m l a c u s t r i n e d e p o s i t s in M a u r i t a n i a (Delibrias e t al., 1 9 7 6 ) at t h e s a m e l a t i t u d e o f 2 1 ° N , a n d also m a t c h a p o s i t i v e oscillation in L a k e C h a d a t 1 4 ° N (Servant, 1974). C h a r c o a l f r o m o n e o f t h e fireplaces at a large N e o l i t h i c site, a s s o c i a t e d w i t h b o n e s o f n u m e r o u s fish a n d large bovids, was d a t e d 3 7 5 0 -+ 100 B.P. ( G i f 5228). This o c c u p a t i o n m a y c o r r e s p o n d e i t h e r to a final p h a s e o f the l a c u s t r i n e p e r i o d o r to t h e small positive oscillation r e c o r d e d e l s e w h e r e in t h e Sahara. O t h e r r a d i o c a r b o n analysis are in process. CONCLUSIONS T h e s e p r e l i m i n a r y o b s e r v a t i o n s a n d l a b o r a t o r y analyses o n Erg I n e S a k a n e l a c u s t r i n e d e p o s i t s p r o v i d e a c o n t r i b u t i o n to p a l a e o e n v i r o n m e n t a l studies in a n e a r l y u n k n o w n area: t h e y s h o w t h e o c c u r r e n c e , s o u t h o f t h e T a n e z r o u f t , o f e x t e n s i v e , u n c o n f i n e d f r e s h - w a t e r lakes a t 6 5 0 0 - - 7 5 0 0 B.P.. T h e s e were s u b j e c t e d t o o n e or m o r e s t r o n g e v a p o r a t i o n p h a s e s ( w h i c h c o u l d s o m e d a y be d a t e d b y c o m p a r e d 14 C analyses o f species w i t h d i f f e r e n t salt t o l e r a n c e ) . F u r t h e r studies s h o u l d s h o w w h e t h e r these lakes w e r e fed b y a rise o f a q u i f e r level, b y r u n o f f , or b y b o t h . T h e m a m m a l s t h e n living in t h e s a m e region t e s t i f y t o a v e g e t a t i o n a t least q u a n t i t a t i v e l y v e r y d i f f e r e n t f r o m the p r e s e n t one. T h u s , it s e e m s t h a t a p o s i t i v e oscillation o f h u m i d i t y c o r r e l a t e s all t h r o u g h o u t the w e s t e r n a n d c e n t r a l S a h a r a a l o n g 21°N. This will be c h e c k e d o u t f u r t h e r , b o t h to t h e w e s t a n d east. ADDENDUM We only dealt here with the Erg Ine Sakane lacustrine deposits. However, similar, less important, formations were also observed in the prospected area: at Tessounfat, Bit Ahmed and about 250 km to the SE at Tichet (Fig.l). At Tichet, the deposits are superficial and fill a small (1 km diam.) depression. Their faunal content is limited to Melania tuberculata and Cyprideis gr. torosa, typical freshwater species being excluded. M. tuberculata can normally stand salinities of ca. 10°]o0 C. torosa is always ornamented nodes occur in less than 10% of them, indicating concentrated water. Moreover, long cylindrical gyrogonites of charophytes with smooth spires were observed in association with them. Their length/width ratio (L/W) varies a great deal, from 1.6 to 2.2 (Plate II). They are quite similar to the present specimens of Chara aspera. However, we could also be dealing with Chara comrnersonii Nordstedt which also has morphological similarities with the few specimens at Tichet. Both species testify to a brackish environment. C. aspera is described from a water site in southern France with a salinity seasonally changing from 7 to 24%o (Tourenq, 1975; Soulie-M~irsche, 1979a). C. comrnersonii was observed in PLATE III Burials in the Erg Ine Sakane area. 1. A tomb before excavation. 2. Internal view of excavated pit. 3. The skeleton in situ upon a brown soft.

60 Senegal brackish waters (CoriUion et Guerlesquin, 1972) under the name of C. zeylanica vat. diaphana. The Tichet water site was probably shallow, maybe temporary, thus submitting the organisms to heavy stress. The stable isotopic values in Melania shells are close to 0. (8~3CpDB - 1.24; ~8OpDs 1.60). They indicate major evaporation phases and correlate with the biological data. The radiocarbon date for the Tichet M. tuberculata (6130 + 180 B.P., Gif 5225) is somewhat younger than the dates for the Erg Ine Sakane shells, but fits an evaporation phase possibly following the main humid oscillation which was around 7000 years B.P. REFERENCES Carbonel, P. and Pinson, J., 1979. Les Cyprideis t~moins de l'~volution des sels en milieu laguno-lacustre sous climat semi-aride. In: Proc. 7th Int. Syrup. on Ostracods: Taxono m y , Biostratigraphy and Distribution of Ostracods. Serbian Geological Society~ Belgrade, pp. 211--217. Carbonel, P., 1980. Les Ostracodes et leur int~r~t dans la d~finition des ~cosyst~mes estuariens et de plateforme continentale. Essais d'application ~ des domaines anciens. Thesis, University Bordeaux I, Bordeaux 348 pp. Chamard, P., 1973. La Sebkha de Chemchane. Bull. IFAN, XXXV(2): 207--243. Corillion, R., 1957. Les Charophyc~es de France et d'Europe occidentale (~tude syst~matique, ~cologique, phytosociologique et phytog~ographique). Thesis, University of Toulouse, Toulouse, 499 pp. Cotillion, R. and Guerlesquin, M., 1972. Recherches sur les Charopbyc~es d'Afrique occidentale (Syst~matique, Ecologie et Phytog~ographie, Cytologie). Bull. Soc. Sci. Bretagne, 47: 1--169. Cour, P. and Duzer, D., 1978. La signification climatique, ~daphique et s~dimentologique des rapports entre taxons en analyse pollinique. Ann. Mines Belg., 7--8: 155--164. Delibrias, G., Ortlieb, L. and Petit-Maire, N., 1976. New ~4C data for the Atlantic Sahara (Holocene): tentative interpretation. J. Human Evol., 5: 535--546. Dewolf, Y., Joly, F., Raynal, R. and Ro~gerie, G., 1972. Premieres observations sur une travers~e du Sahara central. Bull. Assoc. G~ogr. Fr., 399: 191--211. Dorst, J. and Dandelot, P., 1976. Guide des grands Mammif~res d'Afrique. Delachaux & Niestl~, Neuch~tel, 286 pp. Dubief, J., 1963. Le climat du Sahara, II. Inst. Rech. Sahar., Alger, 275 pp. Dupont, B. and Lev~que, C., 1968. Biomasse en Mollusques et nature des fonds dans la zone Est du lac Tchad. Cah. O.R.S.T.O.M. (Hydrobiol.), 2(2): 113--126. Germain, L., 1906. Contributions ~ la faune malacologique de l'Afrique ~quatoriale. V. Sur les Mollusques recueillis par M. le capitaine Duperthuis dans la r~gion du Kanem (lac Tchad). Bull. Mus. Hist. Nat. Paris, 12(3): 166--174. Germain, L., 1907. Les Mollusques terrestres et fluviatiles de l'Afrique centrale franqaise. In: A. Chevalier, L'Afrique centrale franqaise (Mission Chari-lac Tchad). Challamel, Paris, 459--617. Germain, L., 1911. Etude sur les Mollusques terrestres et fluviatiles recueillis au cours de la mission de d~limitation du Niger--Tchad (Mission Tilho). Doc. Sci. Mission Tilbo, Paris, 2: 165--247. Kargzmarz, K., 1967. Variabilit~ et distribution g~ographique de Lychnothamnus barbatus (Meyen) Leonh. Acta Soc. Bot. Polon., 36/3: 431--439. Lev~que, C., 1967. Mollusques aquatiques de la zone Est du lac Tchad. Bull. Inst. Fond. Afr. Noire, A, 29(4): 1494--1533. Lev~que, C., 1972. Mollusques b enthiques du lac Tchad: ~cologie, ~tude des peuplements et estimation des biomasses. Cab. O.R.S.T.O.M. (Hydrobiol.), 6(1): 3--45. Lev~lue, C., 1975. Mollusques des herbiers ~ Ceratophyllum du lac Tchad: biomasses et variations saisonni~res de la densitY. Cab. O.R.S.T.O.M. (Hydrobiol.), 9(1): 25--31.

61 Monod, T., 1958. M~jabfit-al-Koubr~ - - Contribution ~ l'~tude de l " ' e m p t y quarter" ouest-saharien. Inst. Fr. Afr. Noire, Dakar, 407 pp. (M~m. 52). Monteillet, J. and Rosso, J. C., 1978. R~partition de la faune testac~e actuelle (Mollusques et Crustac~s Cirrip~des) dans la basse vall~e et le delta du S~n~gal. Bull. Inst. Fond. Aft. Noire, A, 39(4): 788--820. Petit-Maire, N. (Editor), 1979. Le Sahara atlantique ~ l'Holoc~ne. Peuplement et ~cologie. Mem. C.R.A.P.E., Alger, No. XXVIII: 340 pp. Petit-Maire, N., 1980. Holocene biogeographical variations along the northwestern African coast (28°--19°N). Paleoclimatic implications. In: M. Sarnthein, E. Seibold, and P. Rognon (Editors), Sahara and the Surrounding Seas. Palaeoecology of Africa, 12. A. A. Balkema, Rotterdam, pp. 365--377. Petit-Maire, N., Delibrias, G. and Gaven, C., 1980. Pleistocene lakes in the Shati area, Fezzan (27 ° 30'N). In: M. Sarnthein, E. Seibold and P. Rognon (Editors), Sahara and the Surrounding Seas. Palaeoecology of Africa, 12. A.A. Balkema, Rotterdam, pp. 289--295. Peypouquet, J. P., 1977. Les Ostracodes et la connaissance des pal~omilieux profonds. Application au C4nozo]que de l'Atlantique Nord-Oriental. Thesis. University Bordeaux I, Bordeaux, 443 pp. Poussibet, F., 1961. Notes sur l'Azaouad. Bull. Inst. Fr. Afr. Noire, B, 23 (3--4): 573--595. Soulie-M~rsche, I., 1979a. Etude compar~e de gyrogonites de Charophytes actuelles et fossiles et phylog~nie des genres actuels. Thesis, Montpellier, 341, pp. (2 vols.). Soulie-M~rsche, I., 1979b. Charophytes fossiles des formations plioc~nes de l'Isthme de Megara (Grace). In: VIIth Int. Congr. Medit. N~og~ne, Athens, 1979. Ann. G~ol. Pays Hell., III: 1127--1136. Tourenq, J. N., 1975. Recherches ~cologiques sur les Chironomides (Diptera) de Camargue. Thesis, University of Toulouse, Toulouse, 424 pp. Servant, M., 1974. Les variations climatiques des r~gions intertropicales du continent africain depuis la fin du Pleistocene. 13 ° Journ~es Hydraul., Paris, I, Rapp. 8, 11 pp.