Isotopic hydrology and paleohydrology of the Madagi (Kenya)-Natron (Tanzania) basin during the late quaternary

Abstract

Three generations of stromatolites mark the recent extension of paleolakes in the Magadi-Natron basin on the border between Tanzania and Kenya. The last two generations are observed some 50 m above the modern lakes' level; they are respectively attributed to the beginning of the last interglacial and to the Pleistocene-Holocene transition. Analysis of the morphology and texture of the stromatolites reveals an ecological zonation between 0 and 10–14 m below the shoreline. Their relatively high δ 18O and δ 13C values reflect a long residence time for the paleolake water, which would have been favorable to the establishment of an isotopic equilibrium between atmospheric CO 2 and the dissolved inorganic carbon, despite deep carbon inputs in relation to carbonititic volcanism. A study of the 18O, 2H and 13C contents in the hydrothermal springs, perennial rivers and lacustrine brines indicates that, during the last period of significant renewal of the deep saline groundwater, precipitation was depleted in heavy isotopes when compared to modern precipitation. This difference corresponds to a drop of about 600–1000 m in altitudinal “hydroclimatic” zones. Finally, a comparison between the modern and paleohydrological data leads us to believe that the stabilization of the paleolake shorelines at the same altitude can be attributed to control by the water table level in the aquifer of the basaltic plateaus east of the basin.