Weathering of floodplains and cratonic shields in large intertropical basins

Large intertropical rivers are the main sources of sediments and dissolved species delivered from the continents to the oceans. Since pioneering studies in the 90;s, it is considered that their weathering budget is mainly driven by the competition between erosion and weathering in mountains ranges. One controversy about this paradigm is that sedimentary plains and shields cratons, which dominate the intertropical region area, can also significantly contribute to the continental weathering budget. However the factors controlling their contribution are not well constrained. Here we describe the spatial and temporal variability in weathering intensity of two systems characteristic of these environments: the Amazon floodplain and the African cratonic shields (Oggooué river basin Gabon). Our question is: do these lowlands areas act as efficient weathering environments in relation with sediment deposition rate (floodplains) or erosion rates (shields)? To address this question, 1) we used Holocene sedimentary floodplain cores collected in lakes located in the Andes piedmont (Peru) and near the mouth of the Amazon basin (Curuai floodplain – Brazil) and 2) we sampled riverine water and sediments in the Ogooué basin (Gabon) and in 13 of its tributaries to explore the relationship between weathering budget, climate and intra-cratonic geomorphologic diversity. In both cases we analysed major and traces element ratios to estimate weathering intensity and the Sr and Nd isotopes to track the lithological origin of water (Ogooué) and sediments (in both cases). Moreover, in floodplain sediment, 7Li/6Li ratios were used to track potential role of floodplain sediment residence time in weathering. These results contribute to a better understanding of climate-weathering-erosion interactions at the continental scale, and of their implication for Earth global biogeochemical cycles.