Riverine Li isotope fractionation in the Amazon River basin controlled by the weathering regimes

International audienceWe report Li isotope composition (δ7Liδ7Li) of river-borne dissolved and solid material in the largest River system on Earth, the Amazon River basin, to characterize Li isotope fractionation arl a continental scale. The δ7Liδ7Li in the dissolved load (+1.2‰‰ to +32‰‰) is fractionated toward heavy values compared to the inferred bedrock (-1‰‰ to 5‰‰) and the suspended sediments (-6.8 to -0.5‰‰) as a result of the preferential incorporation of 66Li into secondary minerals during weathering. Despite having very contrasted weathering and erosion regimes, both Andean headwaters and lowland rivers share similar ranges of dissolved δ7Liδ7Li (+1.2‰‰ to +18‰‰). Correlations between dissolved δ7Liδ7Li and Li/Na and Li/Mg ratios suggest that the proportion of Li incorporated in secondary minerals during weathering act as the main control on the δ7Lidissδ7Lidiss across the entire Amazon basin. A ”batch” steady-state fractionation model for Andean and lowland rivers satisfactorily reproduces these variations, with a fractionation factor between weathering products and dissolved load (αsec-disαsec-dis) of 0.983. Two types of supply-limited weathering regimes can be identified for the lowlands: ”clearwaters” with dominant incorporation of Li in secondary minerals, and ”black waters” (e.g. Rio Negro) where dissolution of secondary minerals enhanced by organic matter produces low δ7Liδ7Li. Apart from the black waters, the δ7Liδ7Li of Andean and lowland rivers is negatively correlated to the denudation rates with the lowest δ7Liδ7Li corresponding to the rivers having the highest denudation rates. In contrast, the main tributaries draining both the Andes and the lowlands have higher δ7Liδ7Li compared to other rivers. We propose that part of the dissolved Li derived from weathering in the Andes is re-incorporated in sediments during transfer of water and sediments in floodplains and that this results in an increase of the dissolved δ7Liδ7Li along the course of these rivers. Unlike other rivers, the dissolved δ7Liδ7Li in the main tributaries is best described by a Rayleigh fractionation model with a fractionation factor αsec-disαsec-dis of 0.991. Altogether, the control imposed by residence time in the weathering zone and floodplain processes results in (i) a non-linear correlation between dissolved δ7Liδ7Li and the weathering intensity (defined as W/D) and (ii) a positive relationship between the dissolved Li flux and the denudation rate. These results have important implications for the understanding of past ocean δ7Liδ7Li and its use as a paleo weathering proxy