Abstract EN:

Integrated in the context of the PALHYDAF program (PALaeoHYDogy of AFrica), the study of lake deposits lie in closed depressions along the northeastern margin of the Aïr mountains aims to a precise reconstitution of the Pleistocène and Holocene environments, for a better understanding of climatic oscillations in the Saharo-Sahelian border zone. The basic material for this research work consists of sediment samples collected by auger drilling or at natural outcrops ln four endoreic basins (17-2l°N, 8-ll°E), with different kinds of water supply (groundwater, surface water). A multidisciplinary study (sedimentology, geochemistry, isotopic geochemisty, palaeobiology) of the collected samples has provided detailed information about the palaeoenvironment such as, for example, variations in the rate of evaporation and their influence on the water salinity. In some favorable cases it's possible to reconstruct the fluctuation curve of Holocene lake levels. A comparison of the results from different sites shows that the Holocene period comprises a major humid phase which starts about 10000-9500 years BP and reaches a maximum 8000 years BP. This phase seems to end around 7500 years BP. In some of the basins another humid phase is observed approximately 5000 years BP. Concerning the Pleistocene period there is probably excess of water from 13- 12000 years BP, at least in the basins supplied by surface run-off (Adrar Bous). As regards more ancient conditions,analysis have shown that the calcite was diagenetic. Consequently, carbonate 14C ages (20-35000 years BP) reflect fluctuations ln the groudwater level and not the original age of the deposit. This conclusion is of great importance as it questions former age of the Upper Pleistocene period,"the humid phase 25-35000 years BP earlier recognized in the Sahellan zone, and the resulting palaeoclimatic scenarios. Several datations by Th/U suggest that the original age of the fluvio-lacustrine episode is dated around 90000 years.