A GLOBAL GEOCHEMICAL MASS BUDGET APPLIED TO THE CONGO BASIN RIVERS - EROSION RATES AND CONTINENTAL-CRUST COMPOSITION

Rivers carry the products of continental denudation either in a dissol ved form (chemical erosion) or in a solid form (physical erosion). We focus in this paper on the relationship between physical erosion and c hemical erosion. We establish the mass budget of the Congo Basin River s using chemical complementarities between river suspended sediments, sandy bedload, and dissolved load of the Congo Basin rivers reported i n a previous paper (Dupre et al., 1995). A series of equations are pre sented, assuming that the physical and chemical erosion processes are in a steady state during one year. The total mass of river-home materi al (dissolved and particulate) transported in the river over a given p eriod of time should balance the mass of upper continental crust erode d during this time. We show that the local continental crust on each d rainage basin can be estimated and solve our steady-state weathering m odel using an inversion procedure. The very good agreement between mod elled and measured values of the river-suspended sediment concentratio ns validates the steady-state hypothesis in this wet tropical area. Co nsequently, in this area, the sediment yield provide a good estimate o f the rates of mechanical denudation. This result also validates the c alculation of the chemical and isotopic composition of the local conti nental upper crust using the bulk river load. Erosion rates for the si licate upper crust and thus independent of the lithological variabilit y (silicates, evaporites, and carbonates) of the drainage basins are c alculated. Mechanical erosion rates and chemical erosion rates for the Congo Basin at Brazzaville are 8 t/km(2)/y and 5 t/km(2)/y. The corre sponding consumption of atmospheric CO2 by weathering process is estim ated to 51 x 10(3) mol/km(2)/an. These weathering and consumption rate s are low in spite of the severity of the weathering conditions, of th e high soil temperature, and of the intensity of precipitations. These conclusions indicate the limiting influence the dynamic equilibrium o f soils for silicate weathering. Finally, by estimating the local cont inental crust chemical composition before the onset of erosion process es, especially for the most soluble elements, we can test the model of Taylor and McLennan.