Behavior of lithium isotopes in the Changjiang River system: Sources effects and response to weathering and erosion

Abstract

Lithium (Li) concentrations and Li isotopic compositions of the main channel of the Changjiang River and its main tributaries were measured to better understand the geochemical behavior of Li and its isotopic fractionation during weathering and erosion. The Li concentrations of the dissolved loads of the main channel range from 475 to 4570nmol/l and decrease from the upper reaches to the lower reaches; values are clearly higher than those reported for many of the world’s largest rivers. Samples with very high Li concentrations are affected by the dissolution of evaporites in the headwater region. The δ7Li values of the particulate material has a narrow range (−4.7‰ to +0.7‰), whereas the dissolved load has very variable δ7Li (+7.6‰ to +42.1‰), with values increasing from the upper reaches to the lower reaches. After correction for contributions other than silicate weathering, we show that the Li isotopic compositions of the dissolved load reflect the balance between dissolution of primary minerals and preferential incorporation of the light isotope into secondary products of erosion which enrich the solution in 7Li. The evolution of the isotopic composition from the Upper Reaches to the Lower Reaches is mainly controlled by the fraction of Li incorporated/adsorbed in/onto secondary minerals, and in fine by probably the difference in weathering regimes (i.e. kinetic limited vs transport limited). We also show that the magnitude of the apparent isotopic fractionation between the bedrock and dissolved Li for example are not always only due to variation of the isotopic fractionation factor but might also be due to composition (Li content of the bedrock) and mineralogical controls.The composition of the eroded silicate crust estimated by a steady state mass balance reveals the important contribution of sedimentary rocks (shales) and highlights the effects of sedimentary recycling on the composition of the continental crust.