Major and trace elements of river-borne material: The Congo Basin

Abstract

The Congo river Basin is the second largest drainage basin in the world, after the Amazon. The materials carried by its main rivers provide the opportunity to study the products of denudation of a large fraction of the upper continental crust of the African continent. This paper presents the chemical composition of the different phases carried in the Congo rivers and is followed by a companion paper, devoted to the modelling of major and trace elements. The Congo river between Bangui and Brazzaville as well as its main tributaries, including a few organic-rich rivers, also called Black rivers, were sampled during the 1989 high water stage. The three main phases (suspended load, dissolved load, and bedload) were analysed for twenty-five major and trace elements. Concentrations normalized to the upper continental crust show that in each river, suspended sediments and dissolved load are chemical complements for the most soluble elements (Ca, Na, Sr, K, Ba, Rb, and U). While these elements are enriched in the dissolved loads, they are considerably depleted in the corresponding suspended sediments. This is consistent with their high mobility during weathering. Another type of complementarity is observed for Zr and Hf between suspended sediments and bedload, related to the differential velocity of suspended sediments and zircons which are concentrated in bedloads. Compared to other rivers, absolute dissolved concentrations of Ca, Na, Sr, K, Ba, Rb, and U are remarkably low. Surprisingly, high dissolved concentrations are found in the Congo waters for other trace elements (e.g., REEs), especially in the Black rivers. On a world scale, these concentrations are among the highest measured in rivers and are shown to be pH dependent for a number of dissolved trace elements. The dissolved loads are systematically normalized to the suspended loads for each river, in order to remove the variations of the element abundances owing to source rock variations. Normalized diagrams for REEs are presented and extended to the other elements. They strongly support the argument that the apparent higher solubility of trace elements in the Congo waters is due to the presence in the dissolved load of a colloidal phase (as a result of 0.2 μm filtration). An important result is that these colloids are strongly depleted in Fe and Al with respect to the other elements. Finally, the comparison of the dissolved, suspended, and sandy transport fluxes of each element in the Congo Basin rivers shows that, although the proportions of, for example, the REEs in the dissolved loads of the majority of the Congo Basin rivers is close to 10% of the total transport flux, up to 80% of the REEs are transported by the so-called “dissolved”load in the Black rivers.