Lithium isotopic composition of soil pore water: Responses to evapotranspiration

Lithium isotopes show great potential to trace Earth surface processes due to the large mass discrimination between 6Li and 7Li associated with clay uptake. However, factors controlling the Li isotopic composition (δ7Li) of river water, especially those with a water-bedrock δ7Li difference higher than that of the equilibrium fractionation associated with clay formation (ΔW-C), have not yet been fully resolved. Traditional interpretation involves the Rayleigh distillation, but it unrealistically separates the stage of clay formation from that of silicate dissolution using fractionation factors that are much lower than laboratories can constrain. We propose an in situ mechanism that simulates high δ7Li by evapotranspiration. A model with coupled mineral dissolution and clay precipitation shows that evaporative enrichment of pore-water Li progressively increases the incorporation of Li into clays with light δ7Li, resulting in higher δ7Li values in the residual water than ΔW-C. We also provide evidence from the Chinese Loess Plateau, where an evaporative effect readily explains the observed δ7Li. The influence of evapotranspiration on riverine δ7Li implies that changes in aridity may partly explain the variations of seawater δ7Li. The same principle may also apply to other stable isotopic systems whereby incorporation into secondary precipitates controls the isotopic fractionation.