Testing the Steady State Assumption for the Earth's Surface Denudation Using Li Isotopes in the Amazon Basin

Abstract

In this contribution we test the steady-state assumption for the surface denudation in the rivers from the Amazon basin by calculating “predicted” sediment concentration for each river using the Li isotope composition of dissolved and solid river materials. This calculation relies on a good estimation of the Li isotope composition of the bedrock drained by each River. By using the mean Li isotope composition of the major rock types from the literature, in combination with suspended sediment depth-profile and Nd isotope composition, it is possible to obtain an accurate estimation of the Li isotope composition of the bedrock. Calculated “Predicted” sediment concentration by the steady-state model agree well within a factor of two with the suspended sediment concentration derived from both decadal sediment gauging measurements and cosmogenic nuclides implying that the steady-state assumption is valid for the Amazon Rivers studied here. This has important implications for estimating the long-term erosion rates and constraining the geomorphic dynamic of watersheds using “non-traditional” isotopes.