Quaternary climatic variability in the Tarat aquifer (Arlit, Niger)

The concentration of major elements and environmental isotopes in the Carboniferous (Guezouman and Tarat), Triassic (Teloua) and Quaternary aquifers allows to describe the groundwater flow and to define the upward leakage. O-18 and H-2 in water samples from Guezouman and some Tarat aquifers fall on a regressive line with the equation, delta(2)H = (8.01 +/- 0.50) delta(18)O + (3.85 +/- 3.56), similar to that found for old groundwater in the northern and southern Sahara. The isotopic difference between the Arlit area and the easterly Djado basin groundwater results from the combined effects of continentality and altitude on meteoric waters. The carbon 14 ages, the palaeo-temperatures evaluated from oxygen 18 and nitrate contents in the Tarat aquifer show climatic variations consisting of two humid periods at 10 000 and 30 000 years separated by a warmer period at 25 000. The mean temperatures of the palaeo-recharge, estimated using oxygen 18 data, are 18 degrees C during the Holocene, 20 degrees C at 25 000 years and 16 degrees C at 30 000 years. Water infiltrating during the warming period shows 79 mg.L-1 nitrates. High nitrates could be due to an important meteorological event which entrained and stored organic matter from the surface vegetation and soil by infiltration. ((C) Academie des sciences/Elsevier, Paris.)