Evolution of Lake Chad Basin hydrology during the mid-Holocene: A preliminary approach from lake to climate modelling

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Abstract

During the mid-Holocene (6000 yr Before Present, hereafter yr BP) the Chad Basin was occupied by a large endoreic lake, called Lake Mega-Chad. The existence of this lake at that time seems linked to increased monsoonal moisture supply to the Sahel and the Sahara, which in turn was probably ultimately caused by variations in the orbital forcing and higher temperature gradients between ocean and continent. This study provides a synthesis of several works carried out on the Lake Chad Basin and analyses the results of a simulation of the mid-Holocene climate with an Atmosphere General Circulation Model (LMDZ for Laboratoire de Météorologie Dynamique, IPSL Paris), with emphasis on the possible conditions leading to the existence of Lake Mega-Chad. The aim is to define the best diagnostics to understand which mechanisms lead to the existence of the large lake. This paper is the first step of an ongoing work that intends to understand the environmental conditions that this part of Africa experienced during the Upper Miocene (ca. 7 Ma BP), an epoch that was contemporaneous with the first known hominids. Indeed, early hominids of Lake Chad Basin, Australopithecus bahrelghazali [Brunet, M., et al., 1995. The first australopithecine 2500 kilometers west of the Rift-Valley (Chad). Nature, 378(6554): 273–275] and Sahelanthropus tchadensis [Brunet, M., et al., 2002. A new hominid from the Upper Miocene of Chad, central Africa. Nature, 418(6894): 145–151; Brunet, M., et al., 2005. New material of the earliest hominid from the Upper Miocene of Chad. Nature, 434(7034): 752–755] are systematically associated with wet episodes that are documented for 7 Ma BP [Vignaud, P., et al., 2002. Geology and palaeontology of the Upper Miocene Toros-Menalla hominid locality, Chad. Nature, 418(6894): 152–155] and testified by extended lacustrine deposits (diatomites, pelites, various aquatic fauna). Because the mid-Holocene was the last such mega-lake episode, our aim here is to assess the simulated response of Lake Chad to the hydrologic changes caused by 6 kyr BP forcings (orbital variations, albedo, sea surface temperatures) as a test for a future use of the model for studies of the Miocene climate. We show that the induced northward shift of the simulated ITCZ, and the hydrological changes around the lake caused by this shift, are consistent with an increased water balance over the Lake Chad Basin 6000 yr ago. Water supply from the soil (runoff and river inputs) will have to be taken into account in further simulations in order to discuss the timing of the onset, expansion and decay of such a giant water surface in subtropical Africa.

Introduction

The western Djurab desert, located in the northern part of the Lake Chad Basin, yielded fossils that led to major improvements in the understanding of the evolution of the Upper Miocene and Pliocene fauna, in particular for the hominid clad with the discoveries of the first Australopithecus out the East african Rift System (Brunet, M., et al., 1995) and of the earliest hominid known in Toros-Menalla (Brunet, 2002, Brunet, 2005). The TM sedimentologic section shows a succession of aeolian, periclacustrine and lacustrine facies reflecting the extension of a large lake over a desertic environment (Vignaud et al., 2002). The same scheme is found in other sites through Upper Miocene to Pliocene (Schuster, 2002). They are supplied with paleontological record testifying the existence of an extended open water body inside the Lake Chad Basin. A similar sedimentary cycle exists in the Pleistocene and Holocene records, with the long Kanemian arid phase (20–13 kyr BP), followed by a wetter phase until 6000 yr BP (Maley, 2004), during which a giant paleolake called Lake Mega-Chad (Tilho, 1925, Schneider, 1967) was present in the basin. This large lacustrine episode is considered as a reliable analog to Mio-Pliocene paleoenvironmental changes in the Lake Chad Basin (Schuster et al., 2001). It appears therefore as an appropriate “model” to determine: (i) the climatic forcings that allowed the existence of such a large lake, and (ii) the impact of this potential moisture source on the regional and north-central Africa climate system (Ghienne et al., 2002). To achieve this goal, we have undertaken a preliminary study (with an atmospheric general circulation model (AGCM)) of the mid-Holocene climate in which we simulate the behaviour of the basin hydrology.

The Lake Chad Basin is an endoreic basin located in North Central Africa. It is bordered to the north by the Tibesti uplift (mainly Cenozoic volcanic rocks), to the east by the Paleozoic Ennedi sandstone plateau and the Precambrian shield of Ouaddaï, and to the south and west by tectonically active areas related to the Cretaceous–Cenozoic rifting events affecting Western and Central Africa (Guiraud et al., 1992).

The theoretical hydrological basin (Fig. 1) has a total surface area of 2.5 · 106 km2 and is divided into two sub-basins. The southern one corresponds in fact to the present-day Lake Chad active hydrological basin while the northern sub-basin is nowadays a large wind-deflated depression (< 300 m above sea-level, hereafter asl) in the Sahelian and Saharan areas of Chad (Mainguet and Chemin, 1990). Under present climatic conditions, the lake receives water mainly from the Chari–Logone river system draining the humid tropics (Sudano–Guinean belt). During the last few decades, the lake level varied between 275 and 283 m asl. This latter value, which was the maximum of the last century, was reached in 1963 and corresponded to a lake area of 22,000 km2 (Olivry et al., 1996).

Above 285 m asl, the sub-basins are connected via the Bahr el Ghazal valley. Indeed, this valley, which is usually dry, can occasionally be flooded as shown by historical reports (Maley, 1981). During Mega-Chad episodes, initial spillover from the southern basin to the north is probable due to the typically higher moisture supply in the southern part of the lake. When the two lake basins are connected, the Mayo Kebbi outlet towards the Benue river then controls a potential maximum level of Lake Chad at 320 m asl (Schuster et al., 2003). This maximum level corresponds to a lake surface area of more than 350,000 km2.

Several lacustrine episodes are identified in the Lake Chad Basin during the Quaternary (Maley, 1977, Servant and Servant-Vildary, 1980). Based on the endoreic character of this intra-cratonic basin (Burke, 1976) and on the identification of long sandridges surrounding widespread lake deposits (Schneider, 1967), a Holocene Lake Mega-Chad (LMC) covering more than 350,000 km2 has been outlined. Although some authors (Durand, 1982) strongly rejected the LMC concept arguing that neotectonic faulting is locally responsible for the sandridge, recent geomorphic evidences confirm the occurrence of long-lasting LMC highstands (Ghienne et al., 2002, Schuster, 2003) and accurately defined its outlines (Schuster et al., 2005). The most recent LMC episode developed during the last Holocene climate optimum as testified for example by lake deposits over an area of more than 115,000 km2 (Kusnir and Moutaye, 1997). This last LMC episode started after 7 kyr BP (Maley, 1977, Gumnior and Thiemeyer, 2003) and corresponds to the Middle Holocene wetter conditions identified in northern and central Africa (e.g., Gasse, 2000). Radiocarbon ages from the Goz Kerki (eastern Djourab) area show that this LMC episode lasted until ca. 5.3–4.4 kyr BP (Schuster et al., 2005).

The mid-Holocene in Saharan and Sahelian Africa has been described several times as a very different period from now, with a high Precipitation minus Evaporation (P  E) balance (e.g. Gasse, 2000). Several high-level lakes and flooding episodes have been recorded in the northern monsoon domain during this African Humid Period (Gasse and Vancampo, 1994, Williams et al., 2000). Moreover grassland and xerophytic woodland/shrubland expanded up to the 23°N latitude (Hoelzmann, 1998, Jolly et al., 1998). Many AGCM studies have shown that high summer insolation during the mid-Holocene, in particular at 6000 yr BP (hereafter ky BP), leads to an enhanced African monsoon which may explain the observed hydrologic changes (Kutzbach, 1981, Kutzbach, 1985, Braconnot et al., 2000). This stronger summer insolation leads to enhanced land–sea temperature contrasts and to a northward shift of the Inter-Tropical Convergence Zone (hereafter ITCZ) over continents in summer. The summer monsoon over tropical Africa was therefore reinforced: Rainfall increased over the continent while it decreased over the oceans, because of the raise of wet oceanic air masses and large-scale advection of moisture towards the continent (Braconnot, 2004). Positive ocean (e.g., Kutzbach and Liu, 1997, Joussaume, 1999, Cook et al., 2003) and vegetation (Braconnot et al., 1999, Levis et al., 2004) feedbacks amplified this orbital forcing by enhancing water advection and local recycling of precipitation. However, models fail so far to shift the summer precipitation during the mid-Holocene as far north as suggested by the data.

Until now, although LMC was the largest fresh-water body that existed in Africa during the Quaternary, this giant lake on the southern fringe of the Sahara only received little attention from climate modellers, although open water bodies can have significant climatic impacts on the regional scale (e.g., Bonan, 1995, Krinner et al., 2004). As a first step of a deeper study about LMC–climate interactions, we therefore focus on the Lake Chad Basin to analyse whether a wetter regional context is sufficient to create a giant lake. The following section will describe a simple hydrological model of the Lake Chad. We then present our climate simulations for 6 ky BP and analyse these with respect to the hydrological balance of the Lake Chad Basin and its implications for the possible existence of Lake Mega-Chad during this period.

Section snippets

Modelling the LMC: the Gac hydrological model and its recent improved version

Few studies have dealt with the LMC, particularly because this concept was still contested until the recent studies of Ghienne et al. (2002). Kutzbach (1980) has depicted the “Paleolake Chad” system by modelling nonlinear processes between hydrological end energy balances. He proposed that the change in vegetation acted as an important feedback increasing the runoff and thus allowed the expansion of a giant lake. At the same period, Gac (1980) suggested a hydrological model of the basin of the

Modelling mid-Holocene climate: impact on the Lake Chad Basin

Here we use the three-dimensional Atmospheric General Circulation Model LMDZ4 from the LMD–IPSL (Li and Conil, 2003) to simulate the climate at mid-Holocene (hereafter 6K) and at present. The model has 144 gridpoints in longitude, 108 in latitude and 19 vertical layers. We use the model with a zoom yielding a typical horizontal resolution of 50 km over the Chad basin. The hydrodynamical equations are solved at a timestep of 40 s. The model is parameterised with a simple bucket soil scheme, and

Discussion

The Three Cones Model gives information about the potential rate of growth of the lake that are in good accordance with the rates proposed by Maley (2004) for different large lake episodes in this basin, and represents correctly the twofold process inside the basin. The interest of this basic model is that it only requires few parameters and is easy to run on personal computers. It provided good results that show that a large lacustrine episode, comparable to LMC episodes, can extend through

Acknowledgements

PS and MS wish to dedicate this paper to the memory of their recently deceased friend, the eminent Professor Dominique Jolly. As a specialist of African paleoflora, he had initiated the present collaboration when having some Gewurztraminer in his garden, by a sunny day of august 2003. We are grateful to the reviewers for their constructive review and their encouragements. This research benefited from the financial support of the ECLIPSE research program (CNRS–INSU).

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