The lithium and magnesium isotope signature of olivine dissolution in soil experiments

This study presents lithium and magnesium isotope ratios of soils and their drainage waters from a well-characterised weathering experiment with two soil cores, one with olivine added to the surface layer, and the other a control core. The experimental design mimics olivine addition to soils for CO2 sequestration and/or crop fertilisation, as well as natural surface addition of reactive minerals such as during volcanic deposition. More generally, this study presents an opportunity to better understand how isotopic fractionation records weathering processes. At the start of the experiment, waters draining both cores have similar Mg isotope composition to the soil exchangeable pool. The composition in the two cores evolve in different directions as olivine dissolution progresses. Mass balance calculations show that the water δ26Mg value is controlled by congruent dissolution of carbonate and silicates (the latter in the olivine core only), plus an isotopically fractionated exchangeable pool. For Li, waters exiting the base of the cores initially have the same isotope composition, but then diverge as olivine dissolution progresses. For both Mg and Li, the transport down-core is significantly retarded and fractionated by exchange with the exchangeable pool. This observation has implications for the monitoring of enhanced weathering using trace elements or isotopes, because dissolution rates and fluxes will be underestimated during the time when the exchangeable pool evolves towards a new equilibrium.