Isotope evidence of palaeorecharge and palaeoclimate in the deep confined aquifers of the Chad Basin, NE Nigeria

Groundwaters from the Quaternary and Continental Terminal Formations in the Nigeria sector of the Chad Sedimentary Basin (CSB) together with rain and surface waters have been chemically and isotopically analyzed in order to investigate sources and ages of waters, possible modern renewal and mixing of the deep groundwaters, and to infer palaeoclimate incidences. Most of the waters are slightly to moderately mineralized and are of Na-HCO3 type induced mainly by Na-feldspar weathering and ion exchange reactions. The wide range of the δ18O and δ2H values and 3H contents in the upper aquifer indicate replenishment with modern meteoric water. However, the deep system (middle and lower aquifers) with a narrow range of depleted stable isotope values and low 14C activities indicates that these waters have a palaeometeoric origin. The period of infiltration was within the humid and cooler period (35 to 40 ka BP) prior to the Last Glacial Maximum. In addition, the isotope compositions of the deep system show no mixing with modern waters. These results are in agreement with other palaeorecord studies in the Sahel zone during this period.