Large-scale fractionation of lithium isotopes during continental weathering and erosion: Insights from the Amazon River basin

Abstract:

The increase of 9‰ of the lithium isotope composition of the ocean since 60 Ma has been interpreted as reflecting a change of the silicate weathering regime with more rapid CO₂ consumption due to increased tectonic uplift (Misra and Froelich, 2012). However, the small number of case studies limits our current understanding of the parameters controlling the Li isotope composition of large rivers discharging to the ocean. To characterize Li isotope fractionation at the continental scale, we report the Li isotope composition of river-borne material in the largest Earth’s River system, the Amazon River basin. The sample set includes rivers from various geomorphological settings, notably rivers draining the Andes and the lowlands. As a result of silicate weathering, the Li isotope composition (δ⁷Li ) of the dissolved load is fractionated toward heavy values (from +1.2 to +32‰) compared to the upper continental crust (0 - 2‰) and the suspended sediments (-6.8 to -0.5‰). We show that despite having very contrasted weathering and erosion regimes, both Andean headwaters and lowland rivers share a similar range of Li isotope composition (+1.2 up to +18‰). The correlation of the dissolved δ⁷Li with the Li/Na and with the Li/Mg ratios suggests that the fraction of Li incorporated in secondary minerals during weathering is the main parameter controlling the Li isotope composition at the whole Amazon basin scale. Both Rayleigh distillation and batch fractionation models satisfactorily explain the data with fractionation factors between weathering products and dissolved load ranging from 0.982 to 0.991. The fraction of Li incorporated in secondary minerals is controlled by the weathering regime and therefore is linked to the denudation rates. However, we show that Li incorporation in floodplains also influences the δ⁷Li of rivers at their mouth. These results have important implications for the understanding of past ocean δ⁷Li and stress the need to characterize at the global scale what are the parameters controlling the proportion of Li incorporated in secondary minerals.