The Saharan lakes of Ounianga Serir (NE Chad): a unique hydrogeological system
The deflational basin of Ounianga Serir in remote and almost rainless northeast Chad contains the Sahara’s largest bodies of water. The lakes are almost exclusively fed by the regional aquifer which was last recharged during the early Holocene humid phase and which continuously replaces the massive evaporation losses (> 6 m / yr). During the last millennia, the constant trade winds have driven dunes into the depression that have subdivided the once continuous freshwater lake into separate compartments presently hosting 10 lakes with a total surface of about 20 km2. With the exception of the central salt lake, Lake Teli, they are largely covered by thick mats of floating reed which significantly reduces evaporation.
The exposed central lake therefore undergoes the major evaporation and thus functions as a evaporation pump which affects the lowest lake level. As a consequence of the resulting gradient, freshwater is drawn from the more elevated peripheral lakes through the permeable dune barriers before they become saline. This very special mechanism results in persisting freshwater lakes – a paradox under hyperarid conditions. There are no comparable freshwater systems in the Sahara or any other desert. Only the combined geological, hydrological, climatic and biological factors, i.e. (a) a vast fossil groundwater reserve; (b) the specific position, morphology and orientation of the lake basin; (c) continuous winds from a perpendicular direction; (d) a source of aeolian sand; (e) extreme evaporation driving the central evaporative pump; and (f) the floating reed covers that receive their nutrients from aeolian dust; have created the unique ecological system of Ounianga Serir. It has conserved the genetic heritage of the Sahara’s humid past over more than 3000 years of dryness, including several species of fish and gastropods.
Due to severe aeolian erosion, only very limited remains of the deposits of earlier lake stages are preserved at positions up to 80 m above the present lake bottom. They have been radiocarbon-dated to the early Holocene and mainly consist of thinly laminated diatomites and mollusc-bearing carbonates. These high-resolution sedimentary archives are expected to correlate with the sub-bottom varve records of Lake Yoa at Ounianga Kebir (Kröpelin, Verschuren, Lézine, Francus et al. in prep.). Differential precision surveying of the uppermost lacustrine deposits and “virtual flooding” of digital elevation models allow to outline the extent of the Ounianga palaeolake during the early and mid-Holocene.