CONSTRAINTS ON THE NATURE AND TIMING OF RECHARGE OF THE FOSSIL AQUIFERS IN SAHARAN AFRICA AND SINAI

The isotopic composition of modern precipitation collected monthly from the International Atomic Energy Agency (IAEA) stations over North Africa is enriched (average δD ± 1δ: Algeria: –21.3‰ ± 8.81‰; Tunisia: –20.6‰ ± 4.23‰; Libya: –17.7‰ ± 2.72‰; Egypt: –11.7‰ ± 3.45‰) compared to fossil groundwater across North Africa and shows progressive west-to-east depletion indicative. A few monthly records showed isotopic depletions approaching those of the fossil groundwater in Egypt. For each of these events, we investigated the cloud propagation direction (from temporal Meteosat image products) in relation to the isotopic composition of precipitation as it travels over the African continent and surroundings. The extracted cloud propagation directions and the observed spatial variation in the isotopic compositions of the precipitating rain support a model that attributes previous wet climatic periods over North Africa to the intensification of palaeowesterlies. Additional evidences for this model comes from the analysis of sixteen water samples from productive wells tapping the Lower Cretaceous Nubian Sandstone and the fractured basement aquifers in Sinai that were analyzed for their stable isotopic compositions, dissolved noble gas concentrations (recharge temperatures), tritium activities, and ¹⁴C abundances. Results define two groups of samples, I and II. Our findings: older ages, lower recharge temperatures, and depleted isotopic compositions of Group I compared to Group II, (2) similarity of Group II isotopic compositions to average modern rainfall; and (3) measurable tritium activity in a sample from group II are consistent with: (1) the Nubian Aquifer being largely recharged during the Last Glacial Maximum (LGM)(represented by Group I waters), possibly through the intensification of paleowesterlies; and (2) continued recharge during the relatively dry and warmer interglacial period (represented by Group II waters) under conditions similar to those of the present.