Publications des scientifiques de l'IRD

Dellinger M., Gaillardet J., Bouchez J., Calmels D., Louvat P., Dosseto A., Gorge C., Alanoca L., Maurice Laurence. (2015). Riverine Li isotope fractionation in the Amazon River basin controlled by the weathering regimes. Geochimica et Cosmochimica Acta, 164, 71-93. ISSN 0016-7037.

Titre du document
Riverine Li isotope fractionation in the Amazon River basin controlled by the weathering regimes
Année de publication
2015
Type de document
Article référencé dans le Web of Science WOS:000358021900005
Auteurs
Dellinger M., Gaillardet J., Bouchez J., Calmels D., Louvat P., Dosseto A., Gorge C., Alanoca L., Maurice Laurence
Source
Geochimica et Cosmochimica Acta, 2015, 164, 71-93 ISSN 0016-7037
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.
Plan de classement
Hydrologie [062] ; Géologie et formations superficielles [064]
Description Géographique
ANDES ; PEROU ; BOLIVIE ; BRESIL
Localisation
Fonds IRD [F B010064818]
Identifiant IRD
fdi:010064818
Contact