Résumé
This study describes the chemistry of the upper Niger river system upstream of the inland delta in Mali. It constitutes the first geochemical investigation of suspended and dissolved loads of this region of the world. The Niger, on its way between Banankoro and Ké-Macina and its tributary, the Bani river at Douna, were sampled during eight hydrological years (from 1991/1992 to 1997/1998). Major elements were determined for all 8 years and trace elements during 2 years (1996/1997 and 1997/1998). The total period of investigation is representative of the hydrological fluctuations over the last 25 years. Dissolved concentrations show that in comparison with other rivers, the upper Niger rivers are among the least mineralised and are strongly influenced by silicate rocks, which underlie almost the totality of the basin. The contribution of atmospheric precipitation to the rivers' chemistry (using the technique of marine contribution) appears very low. Normalised to the upper crust, the chemical composition of the different phases shows that in each tributary, suspended sediment and dissolved load are chemically complementary for the most soluble elements (Ca, Na, Sr, K, Ba, Rb and U). The depletion of these elements in the suspended phase (and the corresponding enrichment in the dissolved phase) is related to their high chemical mobility during silicate weathering. The variability over the hydrological year of trace elements in suspended matter has been attributed to the mixing between inputs of clay minerals and quartz. The distribution of the dissolved particulate ratio in the upper Niger basin leads to the conclusion that for the majority of elements, the sediment content controls the quantity of elements in the dissolved phase.
As carbonate outcrops are insignificant in the upper Niger basin, it is possible to calculate chemical denudation rates by correcting water quality for atmospheric inputs. For the studied period, the denudation rates found range from 0.2 to 1.9×10
6 tons/year, corresponding to 26×10
3 to 100×10
3 mol CO
2/km
2/year. Physical denudation rates were also estimated and range from 3.9 to 8.2 tons/km
2/year. It appears that the low values of these chemical and physical denudation rates are related to a number of factors such as the lithology (silicate rocks), the lack of tectonism (permitting thick soil formation that limits bedrock weathering) and low precipitation (limiting runoff). Finally, a simple steady state model has been applied to the chemical composition of dissolved and solid products of silicate weathering. This model shows a good agreement between the modelled and the measured values of the suspended sediment concentrations, thus confirming the existence of steady state conditions in this tropical area.