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 Li-6 and Li-7 associated with clay uptake. However, factors controlling the Li isotopic composition (delta Li-7) of river water, especially those with a water-bedrock delta Li-7 difference higher than that of the equilibrium fractionation associated with clay formation (Delta(W-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 delta Li-7 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 delta Li-7, resulting in higher delta Li-7 values in the residual water than Delta(W-C). We also provide evidence from the Chinese Loess Plateau, where an evaporative effect readily explains the observed delta Li-7. The influence of evapotranspiration on riverine delta Li-7 implies that changes in aridity may partly explain the variations of seawater delta Li-7. The same principle may also apply to other stable isotopic systems whereby incorporation into secondary precipitates controls the isotopic fractionation.

Automated summary

Lithium isotopes have the potential to trace Earth surface processes. The study proposes an in situ mechanism involving evapotranspiration to explain the formation of high δLi-7 values and provides evidence from the Chinese Loess Plateau. The research suggests that the influence of evapotranspiration on riverine δLi-7 may partly explain the variations of seawater δLi-7.