Some new insights into palaeoenvironmental ...

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Zbl. Geol. Paläont. Teil I, Jg. 2014, Heft 1, 387–403 Stuttgart, November 2014

Some new insights into palaeoenvironmental dynamics and Holocene landscape evolution in the Nigrian Central Sahara (Ténéré, Erg of Ténéré, Erg of Fachi-Bilma) Roland Baumhauer Department of Geography and Geology, University of Würzburg, Am Hubland, 97074 Würzburg, Germany; [email protected]

With 4 figures Abstract: Palaeoecological studies of terminal Pleistocene and Holocene deposits in the Central Sahara allow some new insights of the palaeoenvironmental development of the vast Ténéré sand plain, Erg of Fachi-Bilma and Erg of Ténéré region of Central Sahara, NENiger. The environmental changes are deduced from geomorphological, sedimentological and biological remains as well as from palaeolimnological and prehistoric evidence. Comparing the lacustrine sediments, three stages of water bodies with different conditions can be distinguished. Topographical factors, geomorphological evidence and biological remains (e.g. Lates niloticus) suggest that from 11.5 ka cal BP onwards a widespread lacustrine landscape existed with perennial deep-water lakes changing in size, water level and water balance. Following a change between 7.5 ka cal BP and 6.8 ka cal BP in the northern and central parts of Ténéré, and 6.5 ka cal BP in the southern part of the region studied, desiccation of the by now shallow permanent freshwater lakes began, coinciding with the establishment of seasonal ponds characterized by low water level and high pH. From 6.0 ka cal BP onwards in the north and 5 ka cal BP onwards in the south a more swampy environment developed finally. Concerning the Early Holocene, and towards the Mid-Holocene, the findings suggest an increasingly steep precipitation gradient from SW to NE as well as a more pronounced seasonality, indicating a weakening contact between the summer monsoon front and the westerlies. Key words: Global change, palaeoenvironment, palaeolakes, Central Sahara, Holocene

1. Introduction During the past ten years, research on the palaeoenvironments of tropical and northern subtropical Africa has progressed, mainly based on lake (e.g. Guo et al. 2000, Gasse 2000, Hoelzmann et al. 2004, Pachur 2001, Pachur & Altmann 2006, Russel & Johnson 2005, Kuper & Kröpelin 2006), fluvial (e.g. Gumnior 2008, Lespez et al. 2011) and marine records (e.g. Lézine et al. 2005, Haslett & Smart 2006, Marret et al. 2006), which provide evidence of the high degree of climatic variability during the Holocene (e.g. Gasse 2000, Hoelzmann et al. 2004). Nevertheless research activities on the palaeoenvironments are rare in the Central Sahara. The few comparable studies were accomplished during the 1970s and 1980s. This paper presents a synthesis of the research conducted on the lake system changes in the extensive flat landscapes of the central and eastern Niger part of Central Sahara. It is partly based on already published data (e.g. Baumhauer 2004, Baumhauer et al. 2009) completed by new investigations on the diatom flora of © 2014 E. Schweizerbart‘sche Verlagsbuchhandlung, Stuttgart, Germany

DOI: 10.1127/zgpI/2014/0387-0403

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Holocene swamp and lacustrine sediments. In conjunction with palaeobiological data, e.g. pollen, and palaeopedological data available from this area (Baumhauer et al. 2004, Schulz et al. 2009, Felix-Henningsen 2004, Felix-Henningsen et al. 2009), it is possible to reconstruct the palaeoenvironments in this part of Central Sahara since 10 ka cal BP on a solid base. Finally, the results are discussed in comparison with palaeoenvironmental studies available from elsewhere in Sahara and surrounding areas.

2. Regional setting The Ténéré is situated in the northern part of the Chad basin, comprising the central and also the largest part of the south-western central Sahara, with an E-W-extension of about 500 km from the Bilma escarpment to the Aïr Mountains. Northward the sand plain extends to the Algero-Nigrian border, southward to about 16°N (cf. Fig. 1). The northern part of the Ténéré – the Ténéré du Tafassasset – is a perfectly levelled serir plain only interrupted by a few small ergs, and without any evidence of former lacustrine deposition. In contrast, the central and southern parts of the Ténéré, between the chain of oases of the Kawar and the south-easterly outliers of the Aïr-Mountains, is structured by shallow irregular basins varying in diameter between 0.2 km and 2 km. Situated within older Pleistocene sand dunes, they may extend across shallow swells, forming depression areas of several square kilometres. With the exception of the Aïr-Mountains foreland, evidence of present or former fluvial processes is lacking. To the south and southeast the depressions are more and more re-shaped by recent aeolian sand deposition. Then another type of aeolian landforms dominates: the longitudinal and older erosional dune systems of the Erg du Fachi-Bilma, Erg du Ténéré, and the Grand Erg de Bilma. In many of the intra- and interdune basins lake sediments do still exist, present deflation taking a heavy toll. In the central and eastern parts of the Ténéré a second type of endorheïc depressions are detected, of which the most interesting ones in the present context are those of Achegour, Ezerza, Fazei, Kafra and Fachi. All of them occur as scarpfoot depressions in the western forelands of the N-S-oriented escarpments largely embedded into isolated plateaus of Cretaceous sand-, silt- and clay stones of the western shoulder of the tectonic basin of Bilma overlying the Precambrian basement (cf. Faure 1966). Lake deposits are frequent in these depressions, some of them more than 2 km wide, originated as Neogene sandstone karst landforms and only partly reshaped by wind erosion (Busche & Sponholz 1998). The western rise towards the Ténéré plain level generally appears in several distinct steps. The hydrological situation of the area is largely unknown. There is only one meteorological station – Bilma – within the total Saharan region of north-eastern Niger. There, the annual precipitation is 16 mm; with a class A pan evaporation of 2.700 mm/y. Traces of surface runoff are restricted to parts of the escarpment foot. 3. Previous research Publications on the stratigraphy of lacustrine deposits of the terminal Pleistocene and Holocene from north-eastern Niger and its palaeoclimatic and palaeoenvironmental interpretation were first presented by Servant (1973), who connected his studies in with Cenozoic continental sedimentation and climate history based on the works of Faure (1965, 1966, 1969) and Faure et al. (1963). Servant-Vildary

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(1978) outlined the history of the lakes interpreting their diatom floras, and Maley (1980) presented the climatic history of the region during the last 30,000 years based on palynological data. The palaeoenvironmental evidence obtained by Baumhauer from the Ténéré, the escarpment forelands and the ergs to the south is in good accordance with the chronology of the outgoing Pleistocene and Holocene climate on a millennium scale as well as with landscape changes detected in the southern Sahara (cf. Baumhauer 1984–2009, Gasse 1988, Maley 1980, Servant 1973, Servant-Vildary 1978). With regard to lake formation, however, it seems that there have been sufficiently humid conditions from the end of the Pleistocene until the Mid-Holocene only, with a decreasing tendency from 6.5 to 5.5 ka cal BP, interrupted by several short arid phases. Fully arid conditions occur since 5 ka cal BP at the latest. This is in good concordance with the present evidence from the dispersal of Holocene soils in eastern Niger (Felix-Henningsen, several publications between 1992–2004, FelixHenningsen et al. 2009). He concluded that the palaeosoils formed on the ancient dunes of the region developed during semi-humid conditions. His macro- and micromorphological, soil-physical, geochemical and clay-mineralogical analyses allow the reconstruction of a sequence of processes. The formation of brunified to partly rubyfied horizons is a major diagnostic attribute for Pleistocene and Holocene chromi-cambic Arenosols; they originated from silicate weathering as part of a pedogenetic sequence, preceded by desalinization, decarbonatization and acidification due to leaching by percolating soil water in a more humid climate. Similarly, Baumhauer et al. (2004) conclude from the palynological analysis of limnic sediments that, during the late Early Holocene at least northward to that latitude of 20° N a kind of sahelian savanna vegetation existed, which experienced a change towards Saharan floral elements at around 7 ka cal BP. The aridification shown in the pollen spectrum, however, is partly attributed to anthropogenic interference (grazing, fire) in the contact region between Sudanian and Saharan vegetation at the beginning of the Neolithic. This, together with the climatic deterioration, led to the development of the Sahelian savanna vegetation, possibly expedited by the human interaction (Baumhauer et al. 2004).

4. Material and methods As stated above, pre-existing lake sediments appear in the sand dune depressions of the Ténéré, they are ubiquitous in the interdune depressions of the ergs, and occur also in the scarpfoot depressions. Their facies is diatomitic or gleyic, partly silty, casually diataomitic reworked dune sands of varying thickness. They are minor due to the original sedimentation but to deflation and corrasion. Care was taken to the detection of the transition towards underlying Pleistocene dune sand or the bedrock base of the depressions. 69 palaeolake deposit sections were taken in the study region. The 14C yr. BP dates were calibrated for 95% confidence intervals using the Cal Pal-2007 software (Weninger et al. 2007) with the calibration data set CalPal-2007 Hulu (Weninger & Jöris, 2008).

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1: Dune sand; 2: Dark gray ancient dune sand, 63-630 μm, organic matter; 3: White ancient dune sand, bleached by gleying; 4: Brownish ancient dune sand with mottles of goethite and lepidocrocite; 5: Diatomite, greyish; 6: White diatomite, stratified; 7: Calcareous crust; 8: Gypsum incrustation; 9: Rhizoconcretions; 10: Gastropoda; 11: Organic matter, charcoal.


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Fig. 1. Location of the investigated area, location and lithostratigraphy of the study sites discussed in this paper.

The stratigraphical position of the samples analyzed for diatom species is given in Fig.1-4. The diatom slides were prepared from ~0.5 g of dry sediment. At least 600 diatom valves were counted per sample with high diatom contents and 200-300 valves when it was low. The frustules are well preserved; however, fragmented valves are more abundant in the lower and upper part of the profiles. The diatom identification was based on Krammer & Lange-Bertalot (19861991), Schoeman (1973), Servant-Vildary (1978), Gasse (1980, 1986), Germain (1981) and Simonsen (1987). Concerning the reconstruction of palaeoecological conditions from the diatom flora (salinity and chemical facies as well as life forms and habitat), the classification system according to Gasse (1987) and Gasse et al. (1987) is used in this paper.

5. Results 5.1 Ténéré In the scarpfoot depressions of the west-facing escarpments of Achegour (sites 43-45, Fig. 1), Ezerza (site 46, Fig. 1), and Fazei (site 47, Figs. 1, 2) diatomites are ubiquitous. With some exceptions the diatom sites studied in these depressions either over- or underlie a carbonate crust only a few cm thick. Where exposed at the surface due to deflation of the once overlying diatomite, it may display a regular pattern of holes where the then still muddy material had been squeezed aside due to slumping in the overlying diatomitic mud. Even in these central parts of the Ténéré the lake sediments invariably overlie the groundwater-bleached old dune sand, partly containing mollusc shells, charcoal, carbonate concretions as well as signs of oxidation and reduction. The Fazei site comprises two main phases of diatomite sedimentation, dated at < 10.045 ± 338 cal a BP and < 7.505 ± 231 cal a BP (HV 15331, HV 15330). The two detritus-free diatomites are separated by a layer of gleyic dune sand (Fig. 2). The surface of the upper diatomite is scattered with Neolithic artefacts and potsherds. The site is identical with the Holocene sedimentary sequence at Fachi/Dogonboulo, about 80 km south of Fazei (Baumhauer 1990). The Early Holocene diatomites at Fazei (site 47, cf. Fig. 2) display pronounced oscillations throughout the site, both in species composition and number of individuals. Dominant are alkaliphilous to alkalibionthic forms. Like the secondary species, they indicate hydrological conditions of various chemical composition with high pH (zone I). The secondary species display a comparable palaeoenvironment and confirm both the chemical heterogeneity of the water and the existence of a swampy environment, with open water surfaces restricted to rather small ponds (zone II). It is debatable whether the time slice represented by the preserved diatomite only represents the oscillations of water supply and chemistry towards the beginning of the Holocene lacustrine depositional phase also known from other regions, or whether at Fazei all of the beginning Holocene was characterized by an unstable water body.

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Fig. 2. Site 47 (Fazei): Lithostratigraphy, cal ages BP (1950), habitat, pH and percentages of diatom taxa which reach frequencies >1% at least in one sample level.

The old dune sand overlying the diatomite is diatom-sterile, so that a concordance of the stratigraphic units cannot be verified. Clearly differentiated hydrological conditions are expressed in the spectra of the diatomite representing the second lake phase. All the assemblages are characteristic of the littoral of a perennial, oligotrophic freshwater lake. Higher pH values only occurred towards the end of the lake phase. Planktonic Aulacoseirae, with 45 to 65% are dominant in all sites taken at Achegour (site 44) and Ezerza (site 45). They are always associated with tychoplanktonic species such as Anomoeoneis sphaerophora (Schwabe & Simonsen 1961). These assemblages are indicative of hydrological conditions characterizing fluctuations in time, space and water chemistry. The latter are documented by alkalibiontic forms, secondary in number but ubiquitous. As indicated by the large number of epiphytic species, water depth remained quite low throughout. According to Howard-Williams & Ganf (1981) the diatom assemblages are characteristic of daily wind-induced mixing of the lake water, as is typical for most of the present shallow lakes of Africa.


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Fig. 3. Site 50 (Kafra): Lithostratigraphy, cal ages BP (1950), habitat, pH and percentages of diatom taxa which reach frequencies >1% at least in one sample level.

About 70 km west of the Bilma escarpment, and thus already within the easternmost part of the Ténéré, lies the escarpment of Kafra, dissected into several low plateaus. Diatomites directly overlying the bedrock floor line the almost sand free, 5 x 2 km large scarpfoot depression, which is situated in front of the central part of the N-S-oriented scarp. In its deepest part more than a dozen of diatomite yardangs, 2 to 5 m high, exist. Radiocarbon dates obtained from charcoal samples found at the base of site 50, about 10 cm above the deflation surface surrounding the yardang yielded ages of 8.362 ± 277 cal a BP, 6.543 ± 279 cal a BP and 5.114 ± 164 cal a BP (HV 15333, 15337, 15341) 2,72 m and 0,32 m under the top of the site. Numerous fragments of fish bones and small reptiles are scattered on top of the remaining diatomite inside the depression, intensively carbonized, corraded and thus no longer to be assigned to a specific species. The diatom assemblages of site 50 (Fig. 3) are characterized by a considerable variation in species composition and numbers, the latter varying between 102 to 106 valves per g-1. In the upper part in particular, the portion of corroded valves is very high, in places above 50%. At the base of the site planktonics are dominant, indicating an oligotrophic environment (zone Ia). Numerous Cyclotellae could not be fully identified, but are likely to be C. glomerata, which, according to Cholnoky (1968), thrive in slightly acidic waters. On the other hand, Rhopalodia gibba and Cyclotella ocellata are typical of an alkalophilous environment (cf. Gasse et al. 1987, Gasse 1987). In zone II planktonic species are largely replaced by benthics and epiphytics, but do

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not disappear completely. The variability of the diatomites, several intercalated carbonate crusts, as well as the host of silty fine-sand layers, suggests considerable oscillations in the water budget of the Kafra lacustrine environments. According to the diatom assemblages there is no evidence of any periodicity. Water depth remained low throughout the time of deposition (zone II and zone III a,b). Variations in water chemistry are only marginal. The species composition in zone III is indicative of a permanently slightly higher pH (zone III b). The lake sediments in the area of the gently rolling old dunes of the Ténéré are shallow throughout and partly corraded. Of particular significance is the basically identical stratigraphy of all sections studied, although the sites may be far disjunct: bedded detritus-free diatomites overlay grey, gleyic and unconsolidated sands (10 YR 6/1). The sands contain specks of humus, gastropod shells are frequent, and they are underlain by bleached fine sand (7.5 YR 8/2; cf. sites 55-56, Fig. 1). Except for site 57, all of the gastropods are Melanoides tuberculata, a highly adaptive species with a high propagation rate which, however, cannot survive in a periodically desiccated environment. In some sites the diatomites are separated from the underlying sand by a gypsum crust, 2–5 cm thick (cf. site 57, Fig. 1). In site 57, radiocarbon dating of fragments of Melanoides tuberculata and Biompholaria pfeifferi and unidentified bivalve fragments taken at 4 cm below the present deflation surface of a diatomite yielded an age of 6.376 ± 68 cal a BP (Hv 15239). For the reconstruction of the hydrological conditions the diatom assemblages within the dune sands underneath the diatomites are characterized by quite similar species. Another common feature is that numerous valves are corroded or broken. The corrosion features, in particular, are evidence of solution processes of varying intensity in extreme environmental conditions. The palaeoecological causes will be discussed in another paper in detail. Although the corrosion features observed may have a negative effect on the environmental interpretation, the assemblages suggest largely identical hydrological conditions throughout the study region: high pH, the water level was low pronouncing rapid changes of water chemistry. In addition to the diatomitic sediments another type of lacustrine deposits is frequently found at the bottom of small shallow basins or in fringes around major depressions: low ridges or surficial beds of isolated or interlaced iron rhizome concretions. On level ground they occur in places where wave action had bevelled sand-dune bodies, creating shallow-water areas among depressions. On the slopes of those depressions – the preserved parts of interdune basins – the swamp iron crusts tend to be preserved in fringes at various heights. In the western Ténéré and in the northern Erg du Ténéré, in particular, up to six of those fringes representing former lake levels may be traced for several kilometres along the slopes of depressions, some km² in size, at vertical intervals of 10 to 20 cm. In any case they form the uppermost horizon of gleyic old dune sands (7.5 YR 8/2), always follow contour lines, and may have a preserved length of several metres to kilometres, reaching thicknesses up to 50 cm. Depending on the slope angle of the depressions they may be up to 1 m wide. According to Felix-Henningsen (2004) the swamp ore mainly consists of goethite, partly with an outer layer of lepidocrocite, which has also impregnated the old dune sand. It was formed by the oxidation of Fe++-ions in the water of the palaeoenvironment, and their concentration within the root zone of near-shore vegetation.


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The gleyic dune sand (7.5 YR 8/2) underlying the 10 cm thick iron crust of site 58 (Fig. 1) was found to contain less than 10² diatom valves x g-1, and thus not enough for a meaningful interpretation. Though, this deficit is compensated by numerous faunal remains both on the surface and within the sand in the immediate vicinity. The following findings were reported:  Molluscs: small bivalvia, unidentified, 1 fragment;  Fish: Polypterus sp., 1 fragment.; Clariidae, 156 fragments; Synodontis sp., 12 fragments Parachanna obscura, 31 fragments; Tilapiini, 8 fragments; Lates niloticus, 3 fragments;  Reptiles: Phyton sp., 3 fragments; Varanus sp., 3 fragments; Trionyx triunguis, 6 fragments; Pelusios sp., several fragments, at least one fragment, one fragment;  Birds: Struthio camelus, egg-shell fragment; a small Rallida, 1 fragment;  Mammals: large bovides, 3 fragments, 18 fish fragments, undetermined, and another 78 mammal fragments, undetermined. The age of the swamp ore formation, and consequently that of the faunal remains (on their palaeoecological interpretation cf. site 59) are difficult to determine. From the general knowledge on swamp ore formation Felix-Henningsen (2004) suggests extended periods of a stable lake level. Their occurrence at various levels above the former lake bottoms is seen as evidence of a generally sinking water level interrupted by longer phases of a stable one. Their topographic and geomorphological position suggests that swamp ore formation and diatom sedimentation took place simultaneously. After the lakes had terminally fallen dry, deflation largely removed the diatomitic sediments, whereas the swamp ores were preserved due to their mechanical resistance and thus still outline former shorelines and shallows within the lakes system.

5.2 Erg du Ténéré and Erg de Fachi-Bilma Two overlying different diatomites are ubiquitous for the transition area between the gently rolling old dune landscape and the Erg du Ténéré with its longitudinal dunes (e.g. site 59, Fig. 1), which either overly a thin horizon of light grey (10 YR 7/2), gley-spotted and sligthly humous silty sand, or a partly gley-spotted dune sand body. The depressions are deeper than in the Ténéré area, up to 20 m. The upper diatomite shows no bedding and is mainly 10 cm thick, with desiccation cracks at the surface. The underlying yellowish-brown diatomite is bedded, with cracks and, as the upper one, polluted by fine sand. The unconformity suggested by the different structure of the two diatomites is corroborated by their diatom assemblages. The basal diatomite has a very narrow spectrum of species (15–32 per sample). The dominant ones are planktonics, thriving in shallow and muddy water bodies with a high primary production. Accompanying epiphytics and tychoplanktonics are equally indicative of shallow water, macrophytes and frequent freshwater influx. The assemblages bear evidence for a large, but only shallow tropical lake with slightly alkaline waters (pH 7–8). This characterization of the environmental conditions is supported by the following list of faunal remains from the underlying silty sands:

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 Fish: Clariidae, 23 fragments; Auchenoglanis sp., 4 fragments; Lates niloticus, 153 fragments;  Reptiles: Crocodylus niloticus, 3 fragments and 5 coprolites; Trionyx triunguis, 92 fragments; Pelusios sp., 7 fragments;  Mammals: Hippopotamus amphibius, 1 fragment,  180 fish fragments and 8 more mammal fragments for which further determination was not possible due to their poor state of preservation. The species-rich freshwater fauna and the food-chain it forms presently occurs in the non-Saharan parts of Niger, in southern Lake Chad, and in the Nile region. The presence of the non-predatory genus Tilapia, living on plankton and water plants, only makes sense if quite large water bodies existed, and Lates niloticus clearly indicates relatively deep, perennial and oxygen-rich waters. In contrast, Claridae and Siluridae, which today occur in the desert environment of Borkou as Clarias lazera, may survive drought periods in small ponds or will leave them at night, protected by darkness, for moving overland to wetter places. They are thus often the only species to survive when a lake dries up (among others van Neer 1989). This catfish is not able, however, to survive in completey dried-up mud. The diatomite of site 59 overlying the silty sand with its rich fauna does not contain comparable assemblages. In contrast, the large number of alkalibiontics, together with alkalibiontic assemblages identified in the basal diatomite of sites 55–57 (Fig. 1) are also indicative of swampy hydrological conditions characterizing frequent changes of water level. The water level was shallow and the pH high. Towards the south and southeast the lacustrine sediments of the depressions between the groups of longitudinal dunes of the Erg de Fachi-Bilma tend to be thicker than in the gently rolling Ténéré, which is comprising diatomites overlying dark grey and humous to bleached dune sands (sites 60, 61, Fig. 1, Fig.4). Their geomorphic and topographic position evidences deposition in a dunes landscape which existed in that specific shape already before the Holocene, and which has hardly been modified since. Radiocarbon datings of gastropods from the base of the diatomites have cal a BP of 9.806 ± 300 BP (HV 15332 site 60, and 6.392 ± 284 BP (0.15 m under the top, HV 15336 site 60) and 10.052 ± 329 BP (HV 15334, site 61), thus determining the beginning of lake formation there. In the underlying dark-grey humous sand only the few highly weathered mollusc fragments of site 60 were found. Within them exclusively gastropods could be identified, mainly Melanoides tuberculata (60–70%) and Biompholaria pfeifferi (20–30%). Except for the occasional Lymnaea the other species could not be identified due to their poor preservation. All three species together indicate an environment of small to medium-size fresh-water lakes rich in macrophytes, both along the shore and on the lake bottom (cf. among others Böttcher et al. 1972, Gabriel & Kröpelin 1983). Disregarding the findings within the underlying sands which, with less than 10² diatom valves x g-1, do not allow any further interpretation, all the diatomites studied are characterized by two different diatom assemblages (Fig. 4): In zone I the alkalibionthics stand for the existence of hydrological conditions characterizing a small and shallow lacustrine environment with waters of various chemical composition and a temporarily very high pH (subzones Ia, Ib and Ic). Separated by a small transition zone (zone II), in the upper part of the site to its top (zone


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Fig. 4. Site 60: Lithostratigraphy, cal ages BP (1950), habitat, pH and percentages of diatom taxa which reach frequencies >1% at least in one sample level.

III) the planktonics are dominant. They make up 20 to 40% of the species and are accompanied by various freshwater to alkaliphilous species. For instance Cymbella ventricosa (5–12%) is described by Hustedt (1957) as a form, existing in oxygenrich waters with a pH of 7.7–7.8. Synedra acus has been described as living in the bottom mud of shallow and small equatorial lakes of East Africa (Gasse 1986a). According to Richardson (1964) and Servant-Vildary (1978) Synedra acus and Synedra ulna are indicative of relatively small, neutral to slightly alkaline waters “with fluctuations between the two indicting shifts a relatively more planktonic and a relatively more littoral environment [….]” (Richardson 1964, p. 252).

6. Discussion and conclusion Comparing the hydrological conditions, there are spatial variations, but they are merely gradual, mostly due to the geomorphic and topographic situation. Beginning with the terminal Pleistocene, the following range of lacustrine environments can be identified in the Ténéré, Erg du Ténéré and Erg de Fachi-Bilma: The first lake

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phase characterized by diatomites sets in between 9.5 to 10 ka cal BP, above a sequence of fine sand to silty sediments. Humus particles, charcoal and mollusc shells (Melanoides tuberculata) evidence favourable hydrological conditions even before diatom sedimentation. Radiocarbon dates show that sediments of a lacustrine nature were deposited as early as 11.5 and 10 ka cal BP (cf., among others, Baumhauer 2009). The ecological data obtained from the Early Holocene show, that first there were shallow plant-rich environments, before they were transformed in the course of time into larger, perennial freshwater lakes up to several metres in depth. The lakes came into being in a sand-dune landscape of LGM/Kanemian age. These old dune sands underlie the lake deposits everywhere. In depressions and dune valleys they have invariably been hydromorphically affected. With increasing humidity in the transition phase from Pleistocene to Holocene soil formation began, simultaneously with the growth of the lakes, leading to region-wide formation of thick terrestrial chromi-cambic or cambic Arenosols which, along the shorelines of the palaeolakes, grade into hydromorphic gleyic Arenosols (Felix-Henningsen 2004, Felix-Henningsen et al. 2009). The problem when, and where at various localities, the Early Holocene lake phase came to its end remains unsolved. A regression phase as evidenced by the shoreline features of the scarpfoot depressions is not reflected in the diatomitic sediments with the possible exception of the irregularly intercalated layers of fine sand, without any grading or bedding. The vertical distribution of the diatoms identified might be explained by a phase of erosion in the upper parts of the Early Holocene deposits that took place before the onset of a later lake phase, as well as by the sediments which have been largely corraded or have been inextractably mixed with the older sediments. The rapid change, not only vertically, but also laterally (even within one and the same sample), of in places extremely different species assemblages may not necessarily be due to the location of the diatomitic sediments preserved in the marginal parts of the palaeolakes. It is also debatable if meromiktic lake types were much more frequent than so far assumed. If that should be the case, it would also have to be asked whether oscillations in time and space of water budget and water chemistry can really be deduced from detected diatom assemblages, which were supposed to stand for very specific ecological conditions. Meromixis may also have originated from freshwater input to an alkaline environment, resulting in a vertical differentiation of the water body and its ecological conditions. In the sediment record this would appear as a mixture of the different diatom assemblages. Meromixis is not exclusively the result of ektogenic causes in the sense of Hutchinson (1957), and is also not restricted to deep water. In the present shallow groundwater-fed lakes of the Kawar, at a water depth of only 40 – 60 cm, a measurable chemocline was recorded at a depth of 23 cm, explained by persistent meromixis due to groundwater inflow (Schultze et al. 1990). Evidence of a comparable situation for the Holocene lakes could only come from finely laminated sediments of the former profundal zone. Persistence can only be inferred from the assemblages if meromixis is accepted. In the monomolimnion waters the pH was high. If the lakes were holomictic, the differentiated species composition would reflect a considerable instability in time and space, with no evidence, however, as to the time frame in which chemical changes took place, or to any individual hydrological cycles or periodicity.


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According to the radiocarbon data obtained so far the shift in ecological conditions, namely the transformation of a landscape of lakes to a vast swamp in the southern part of the study region, and in the northern and central parts of the Ténéré the change from an overall vast swamp to small local swamps, should have taken place between 7.5 and 6.8 ka cal BP. At Kafra, the hydrological transition from an oligotrophic, quite open lake to shallow, swampy hydrological conditions with a high pH and a varying water level in the central part of the modern scarpfoot depression emerges in the diatom assemblages as early as 6.5 ka cal BP. At Fazei, hydrological conditions characterizing oligotrophic waters reappeared after 7.5 ka cal BP, the water level was high and the pH low. At Kafra the lacustrine environment outlined above persisted until 5 ka cal BP at least. The ubiquity and dense dispersal of Neolithic surface sites with artefacts and pottery on the floors of the former lakes, as well as the sequence of swamp ore fringing down towards the deepest parts of the basins are evidence for a trend of sinking lake water levels and thus the gradual deterioration of the lacustrine environment. Repeatedly the trend was stopped by stable humid phases, so that a new swamp ore fringe could be formed. According to Felix-Henningsen (2004) humid conditions persisted long enough so that a terrestrial Bv or cambic horizon could leave its imprint on the hydromorphic characteristics of the older swamp ore fringes. If this model is accepted, the swamp ores and the faunal remains of sites 58, 59 have to be ranged into the Early Holocene. Surprisingly the core period of Neolithic settlement, in contrast to the regions to the north and south, begins as late as about 5 ka cal BP (Busche & Heistermann 1992). It thus does not coincide with the peak, but with the end of the humid period. In a similar way, Roset (1987) reports that settlement in the eastern Aïr Mountain foreland peaked between 5 and 4 ka cal BP, being for him an inexplicable disagreement between the archaeological evidence and the “inclement” arid climate then. The contradiction is solved considering the changing palaeoenvironment and landforms of the region: It was not before 5 ka cal BP that the vast lake and swamp environment of the Ténéré, the Erg du Ténéré and the Erg de Fachi-Bilma had sufficiently fallen dry in its northern parts, and not before 4 ka cal BP farther south, so that there was a sufficiently large terrain for human activity. The peak of Neolithic settlement did not coincide with the climatic optimum with respect to humidity, but occurred in adaptation to the environment as a whole. For the people living in the region the increasing aridity since the Mid-Holocene caused an improvement of living conditions, with still enough water but, for the time being, an increasing pasture area on former lake and swamp land. A decrease of insect infestation and the diseases transmitted by them under less lacustrine conditions may also have been responsible for the short-term Late Neolithic population increase in the region. The general traits of the limnostratigraphy of the Saharan region studied are the same, but both for the Late Pleistocene and the Holocene lakes there are considerable differences between them. It appears that at any timeslice there were differences in ecological status, persistence and size which directly can be traced back to climatic causes, in contrast to the deeper, largely groundwater-fed lakes in the scarpfoot depressions of Bilma, Fachi or Dibella (Baumhauer et al. 2009). Combining all observations, the most likely climatic mechanism is seen inthe interaction of the monsoon front with Atlantic-Mediterranean depressions when the

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average position of the west-wind drift was still farther south than today, although there is as yet only some evidence for this hypothesis from palaeopedology and palynology (cf., among others, Baumhauer et al. 2004, 2009; Felix-Henningsen 2004, Felix-Henningsen et al. 2009, Pachur 2001, Pachur & Altmann 2006, Kuper & Kröpelin 2006, Schulz et al. 2009). Concerning the Early Holocene, and increasingly the Middle Holocene, the findings suggest an increasingly steep precipitation gradient from S (SW) to N (NE) and a more pronounced seasonality. Rainfalls appear to have been increasingly of the downpour type, indicative of weakening contacts between the summer monsoon front and the westerlies. From 6.0 ka cal BP in the north, and 5 ka cal BP in the farther southern ergs, only the monsoonal rainfall regime was still active. This provided no longer sufficient precipitation for maintaining a generally high groundwater level. Monsoonalwesterlies interaction depleted, which is the case today, when even now this will lead to exceptionally intensive but isolated rainfall events.

Acknowledgements Fieldwork was supported by the Deutsche Forschungsgemeinschaft (DFG) and the Institut de la Recherches des Sciences Humaines (IRSH) in Niger. Many thanks to W. Van Neer, Tervuren, S. Servant, Paris, P. Felix-Henningsen, Gießen, H. Hagedorn and D. Busche (†), Würzburg and all the team members for their assistance in the department and on the many expeditions in the Sahara. References Baumhauer, R. (1986): Zur jungquartären Seenentwicklung im Bereich der Stufe von Bilma (NE-Niger). – Würzb. Geogr. Arb. 65: 235 p. Baumhauer, R. (1988): Holozäne limnische Akkumulationen im Großen Erg von Bilma, NE-Niger. – Würzb. Geogr. Arb. 69: 137–148. Baumhauer, R. (1990): Zur holozänen Landschafts- und Klimaentwicklung in der zentralen Sahara am Beispiel von Fachi/Dogonboulo (NE-Niger). – Berliner Geogr. Stud. 30: 35–48. Baumhauer, R. (1991): Palaeolakes of south central Sahara – Problems of palaeoclimatological interpretation. – Hydrobiologica 214: 347–357. Baumhauer, R., Busche, D. & Sponholz, B. (1989): Reliefgeschichte und Paläo-klima des saharischen Ost-Niger. – Geogr. Rdsch. 41: 493–499. Baumhauer, R., Schulz, E. & Pomel, S. (2004): Environmental changes in the Central Sahara during the Holocene – The Mid-Holocene transition from freshwater lake into sebkha in the Segedim depression, NE-Niger. – Lecture Notes in Earth Sci. 102: 33–47. Baumhauer, R., Brauneck, J., Schulz, E., Sponholz, B., Faran Maiga, O. & Pomel, S. (2009): Holocene palaeoenvironmental changes in Central Sahara inferred from new records from Seggedim scarpfoot depression (NE-Niger). – Palaeoecology of Africa 29: 39–62. Baumhauer, R., Schütt, B. & Felix-Henningsen, P. (2009): Geomorphological Research in the South-Central Sahara in Review. – Palaeoecology of Africa 29: 1–22. Böttcher, U., Ergenzinger, P., Jäkel, D. & Kaiser, K. (1972): Quartäre Seebildungen und ihre Molluskeninhalte im Tibestigebirge und seinen Rahmenbereichen der zentralen Ostsahara. – Z. Geomorph. N.F. 16: 182–234.


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