Chemical and physical denudation in the Amazon River Basin

We present major and trace element data on the suspended and dissolved phases of the Amazon River and its main tributaries. The Sr isotopic composition of the dissolved load is also reported. Special attention is paid to the abundances of REE and to their fractionation between the dissolved and suspended phase. The rivers of the Amazon Basin are among the richest in dissolved REE and are similar to the rivers of the Congo system. However a greater range of fractionation between LREE and HREE is reported here. At a global scale the rivers have intermediate patterns between those of the Congo system and those of high pH rivers such as the Indus and Mississippi rivers. Only few elements (Rb, U, Ba, K, Na, Sr and Ca) are mobilized by silicate weathering. These elements are strongly depleted in the suspended phase with respect to upper continental crust. In the dissolved load, these elements are controlled by atmospheric inputs and the weathering of the main lithologies. We propose a model based on mass budget equations, that allow the proportions derived from the different sources to be calculated. As a consequence silicate, carbonate and evaporite weathering rates can be estimated as well as the consumption of CO2 by weathering of each of these lithologies. Physical weathering rates can be estimated by two complementary approaches. On the one hand, the multi-year average of suspended sediments yields can be used to estimate physical denudation. On the other hand, we have developed a steady-state model of erosion that allows us to calculate physical erosion rates on the basis of the dissolved load of rivers. A mean crustal composition is assumed in this model for the rock sources of the drainage basins. Comparison of the rates predicted by the model to the observed rates shows good agreement for the lowland rivers, but a strong discrepancy for the rivers derived from the Andes. Andean rivers (Solimoes, Madeira and Amazon) have observed sediment yields much greater than those predicted according to the steady-state model of chemical and physical weathering. Two interpretations can account for this inconsistency. The first is that these rivers are not in steady state and hence that the soils are being destroyed. The second requires that the local continental crust is different from the average continental crust of Taylor and McLennan, and contains a large proportion of sedimentary rocks. Using the measured sediment yields, and assuming a steady state, we can estimate the amount of sediment recycling for each drainage basin. For the Amazon at Santarem, we find that at least 25% of the mass of the upper continental crust of the Amazon drainage basin is constituted of recycled material.