%0 Journal Article
%9 ACL : Articles dans des revues avec comité de lecture répertoriées par l'AERES
%A Dellinger, M.
%A Gaillardet, J.
%A Bouchez, J.
%A Calmels, D.
%A Louvat, P.
%A Dosseto, A.
%A Gorge, C.
%A Alanoca, L.
%A Maurice, Laurence
%T Riverine Li isotope fractionation in the Amazon River basin controlled by the weathering regimes
%D 2015
%L fdi:010064818
%G ENG
%J Geochimica et Cosmochimica Acta
%@ 0016-7037
%K ANDES ; PEROU ; BOLIVIE ; BRESIL
%K AMAZONE BASSIN
%M ISI:000358021900005
%P 71-93
%R 10.1016/j.gca.2015.04.042
%U https://www.documentation.ird.fr/hor/fdi:010064818
%> https://www.documentation.ird.fr/intranet/publi/2015/08/010064818.pdf
%V 164
%W Horizon (IRD)
%X We report Li isotope composition (delta Li-7) of river-borne dissolved and solid material in the largest River system on Earth, the Amazon River basin, to characterize Li isotope fractionation at a continental scale. The delta Li-7 in the dissolved load (+1.2 parts per thousand to +32 parts per thousand) is fractionated toward heavy values compared to the inferred bedrock (-1 parts per thousand to 5 parts per thousand) and the suspended sediments (-6.8 parts per thousand to -0.5 parts per thousand) as a result of the preferential incorporation of Li-6 into secondary minerals during weathering. Despite having very contrasted weathering and erosion regimes, both Andean headwaters and lowland rivers share similar ranges of dissolved delta Li-7 (+1.2 parts per thousand to +18 parts per thousand). Correlations between dissolved delta Li-7 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 delta Li-7(diss) 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 (alpha(sec-dis)) of 0.983 +/- 0.002. 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 delta Li-7. Apart from the black waters, the delta Li-7 of Andean and lowland rivers is negatively correlated to the denudation rates with the lowest delta Li-7 corresponding to the rivers having the highest denudation rates. In contrast, the main tributaries draining both the Andes and the lowlands have higher delta Li-7 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 delta Li-7 along the course of these rivers. Unlike other rivers, the dissolved delta Li-7 in the main tributaries is best described by a Rayleigh fractionation model with a fractionation factor alpha(sec-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 delta Li-7 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 delta Li-7 and its use as a paleo weathering proxy.
%$ 062 ; 064