The Origin of Strontium and the Strontium Isotope Budget of the River Nile

Abstract

The river Nile has been used as an example to study the evolution of the Sr content and the Sr isotope ratios in rivers in dependence on rock types of the catchment areas, the mixing of the waters from tributaries, the levelling out in a reservoir and evaporation losses. The Blue Nile, Sobat and Atbara rivers drain the Ethiopian Highlands, which consist partially of oceanic crustal rocks. Hence, the Sr content of these rivers is comparatively high and the Sr isotope ratio exhibits an old oceanic crustal signature. The tributaries of the White Nile, in contrast, drain mainly supracrustal metamorphic rocks with higher Sr isotope ratios. The seasonal change of the Sr contents in the Blue Nile and Atbari waters can be modelled using a factor deduced from published – experimental data for Sr contents in river waters at high and low stage conditions. It is demonstrated that the Aswan High Dam reservoir levels out seasonal fluctuations of the Sr contents and Sr isotope ratios. The rate of evaporation loss along the river was determined to be 0.038% per km. Contrary to the situation in highly industrialized urban areas, there is no measurable effect on the Sr isotope ratio from waste waters of Cairo.