Li+ speciation and the use of 7Li/6Li isotope ratios for ancient climate monitoring

•The choice of solvent model is observed to severely influence conclusions.•A tentative range of 1.00220 to 1.04448 is established for KOH−H2OL7/6i.•We establish an effective isotopic fractionation Δ7/6effLiCO32−−H2O containing speciation effects.

We have carried out a theoretical study of aqueous Li+ speciation including effects of ligand coordination, temperature and solution pH. We have calculated the isotope exchange equilibrium constant, KOH−H2O7/6Li, associated with the Li acid/base equilibrium and can constrain it to positive values. The consequences of this species dependent isotope fractionation are then studied using a model for Li-Carbonate coordination. We define an effective isotope fractionation, effΔ7/6LiCO32−−H2O, to model temperature and pH induced changes in Li isotope fractionation associated with the formation of Li-Carbonate species. We predict that, under normal oceanic conditions, effΔ7/6LiCO32−−H2O is not sensitive to pH indeed but may be significantly influenced (on the order of 12‰) by temperature. Since the isotope ratios found in Li containing calcium carbonate shells show no indications of variation in isotopic composition, we conclude that the mechanism by which Li is incorporated to this material is not dependent on aqueous Li speciation.