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 er osion 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 Bas in, both for water and suspended sediment. We have also analyzed 30 st reams draining monolithological terranes. A systematic study of precip itation has permitted the estimation of a good rain correction factor. Sr isotopic ratios have shown that the seawater input correction base d on riverine Cl content is not valid in the Congo Basin because a lar ge part of the Sr, Ca and Mg come from a terrestrial source. The conve ntional atmospheric input correction by reference to the marine ratios underestimates the real atmospheric input because of the crustal elem ents carried by rainwaters. Different erosion source parameters have b een obtained for carbonates, evaporites and silicates. An inversion sc heme has been developed to compute the multimixing equations and allow s -the quantification of the input of each main reservoir (atmosphere, carbonates, evaporites and silicates) for each tributary and each ele ment. For Ca and Mg, rainfall and carbonate dissolution are the main i nputs. For Sr, the input is mainly controlled by rains and silicate we athering. By using Sr isotopic systematics we have calculated the Sr i sotopic composition of the silicate weathered crust for each of the ma in tributaries of the Congo Basin. We obtain uniform values for the ma in tributaries ranging between Sr-87/Sr-86 = 0.7195 +/- 0.001 and 0.72 51 +/- 0.005. These results allow the calculation of strontium model a ges 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 e stimates for the average silicate crust. The discrepancy between T(Sr) and T(Nd) may be linked to the vegetation impact which fractionates R b and Sr.