PALEORECHARGE BY THE NIGER RIVER (MALI) DEDUCED FROM GROUNDWATER GEOCHEMISTRY

Stable isotopes, major elements, trace elements, and noble gases in groundwater from northern Mali permit a reconstruction of the paleohydrology of the Sahelian and sub-Saharan regions of west Africa. Major floods of the Niger River during the more humid episodes of the Holocene, probably associated with northward migration of the interior river delta (flood zone), were responsible for groundwater recharge. A marine aerosol imput, somewhat modified by rock-water interaction, dominates the groundwater chemistry. Stable isotope compositions distinguish the Saharan zone groundwater from waters recharged in the Sahelian zone south of the Azaouad ridge. Carbon 14 dating shows that whereas the latter are modern, the Saharan zone was recharged in the Early to Middle Holocene. Noble gas contents suggest that recharge north of Azaouad occurred under cooler climatic conditions than presently prevail.