MAJOR AND TRACE-ELEMENTS OF RIVER-BORNE MATERIAL - THE CONGO BASIN

The Congo river Basin is the second largest drainage basin in the worl d, after the Amazon. The materials carried by its main rivers provide the opportunity to study the products of denudation of a large fractio n of the upper continental crust of the African continent. This paper presents the chemical composition of the different phases carried in t he Congo rivers and is followed by a companion paper, devoted to the m odelling of major and trace elements. The Congo river between Bangui a nd Brazzaville as well as its main tributaries, including a few organi c-rich rivers, also called Black rivers, were sampled during the 1989 high water stage. The three main phases (suspended load, dissolved loa d, and bedload) were analysed for twenty-five major and trace elements . Concentrations normalized to the upper continental crust show that i n each river, suspended sediments and dissolved load are chemical comp lements for the most soluble elements (Ca, Na, Sr, K, Ba, Rb, and U). While these elements are enriched in the dissolved loads, they are con siderably depleted in the corresponding suspended sediments. This is c onsistent 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 sedimen ts and zircons which are concentrated in bedloads. Compared to other r ivers, 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), espe cially 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 sys tematically normalized to the suspended loads for each river, in order to remove the variations of the element abundances owing to source ro ck variations. Normalized diagrams for REEs are presented and extended to the other elements. They strongly support the argument that the ap parent higher solubility of trace elements in the Congo waters is due to the presence in the dissolved load of a colloidal phase (as a resul t of 0.2 mu m 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 transp ort fluxes of each element in the Congo Basin rivers shows that, altho ugh the proportions of, for example, the REEs in the dissolved loads o f 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.