Sr-Nd-Pb isotope systematics in Amazon and Congo River systems: Constraints about erosion processes

Sr-87/Sr-86, Nd-143/Nd-144, Pb-206/Pb-204, Pb-207/Pb-204 and Pb-208/Pb-204 isotopic ratios and Rb, Sr, Sm, Nd, U, Pb and Th concentrations have been measured in the suspended loads of the Congo and Amazon rivers and their tributaries. In the dissolved load, Sr-87/Sr-86, Rb, Sr, Nd, Sm, U, Pb and Th concentrations are also reported. These results show that Nd, Sm, Th and Pb are almost insoluble and that their mass balance is controlled by particulates whereas Rb, Sr and U are fractionated between soluble and particulate phases. The Sr-87/Sr-86 ratios can only be interpreted after computing the amount of carbonate recycling and the partitioning for silicates between soluble and insoluble. This paper presents a method based on the lead isotopic system that can be used to test the steady state of erosion which is tacitly assumed in many river and erosion studies. The results presented show that the steady state is validated in each river of the Congo Basin and in the lowland rivers of the Amazon Basin, but in not verified in the rivers from the Andes (Rio Solimoes and Rio Madeira). Sr-87/Sr-86, Pb-206/Pb-204 and Pb-208/Pb-204 ratios are positively correlated and negatively correlated with Nd-143/Nd-144 ratios. A number of arguments indicate that these correlations are mixing lines, the end-members being orogenic and shield components, respectively. For the Amazon Basin, analysis of the river loads shows that the orogenic zone is favoured some 5 times more than the shield on an equal area basis. This leads to a reinterpretation of Sm-Nd data for shales and casts some doubts upon the proposition of secular variations in the Sm/Nd ratio of the continental crust.