EROSION SOURCES DETERMINED BY INVERSION OF MAJOR AND TRACE-ELEMENT RATIOS AND STRONTIUM ISOTOPIC-RATIOS IN RIVER WATER - THE CONGO BASIN CASE

Dissolved and suspended load river material represents the integrated products of the erosion of drainage basins. To enlarge the study of erosion processes we have determined Sr-87/Sr-86 ratios and the Cl, Na, Mg, Ca and Sr contents for the main tributaries of the Congo River Basin, both for water and suspended sediment. We have also analyzed 30 streams draining monolithological terranes. A systematic study of precipitation has permitted the estimation of a good rain correction factor. Sr isotopic ratios have shown that the seawater input correction based on riverine Cl content is not valid in the Congo Basin because a large part of the Sr, Ca and Mg come from a terrestrial source. The conventional atmospheric input correction by reference to the marine ratios underestimates the real atmospheric input because of the crustal elements carried by rainwaters. Different erosion source parameters have been obtained for carbonates, evaporites and silicates. An inversion scheme has been developed to compute the multimixing equations and allows -the quantification of the input of each main reservoir (atmosphere, carbonates, evaporites and silicates) for each tributary and each element. For Ca and Mg, rainfall and carbonate dissolution are the main inputs. For Sr, the input is mainly controlled by rains and silicate weathering. By using Sr isotopic systematics we have calculated the Sr isotopic composition of the silicate weathered crust for each of the main tributaries of the Congo Basin. We obtain uniform values for the main tributaries ranging between Sr-87/Sr-86 = 0.7195 +/- 0.001 and 0.7251 +/- 0.005. These results allow the calculation of strontium model ages T(Sr), which differ from neodymium model ages T(Nd). Using T(Nd), we have calculated the Rb-87/Sr-86 of the silicate weathered crust. We obtain homogeneous values close to 0.75, which is in agreement with estimates for the average silicate crust. The discrepancy between T(Sr) and T(Nd) may be linked to the vegetation impact which fractionates Rb and Sr.