Lithium isotopes fractionation in Amazon river-borne material

Silicate weathering is a key geological process controlling the long-term climatic cycle through the consumption of atmospheric CO2. The determination of paleo-weathering conditions is therefore critical to understand past climatic variations. For this purpose, there is evidence from numerous recent studies that Li isotopes in river-dissolved load could be a potentially good silicate weathering proxy because (i) Li is derived predominately from silicates rocks (ii) Li is mobile during chemical weathering and (iii) the water/rocks interactions induce the fractionation of Li isotopes with the preferential enrichment of 7Li in the dissolved phase. However, the respective influence of factors and weathering regime on the Li isotopes fractionation during chemical weathering is still poorly understood. In this study we investigate the Li isotopic fractionation in river-borne material of the largest river system on Earth, the Amazon River. This river system offers the possibility to study Li isotopes fractionation by chemical weathering at a continental scale, in a relatively unpolluted area with equatorial conditions and, to compare weathering and transport limited regimes of erosion. First results show that about 5 to 15% of the total lithium is transported in the dissolved phase. The dissolved Li isotopes composition (δ7Li) ranges from +6.4‰ to +30.8‰ and is much heavier than corresponding sand and suspended sediments samples (-3.6‰ to +4‰). Rio Negro have the lowest δ7Li values (+6.4‰) while Mamore River in the Madeira basin have the heaviest δ7Li composition (+30.8‰). Mass budget calculation between dissolved and particulate phases provides insights about the respective influence of parameters controlling the extent of Li isotopes fractionation during chemical weathering.