Li and Mg adsorption onto soil phyllosilicates and oxides

Continental silicate weathering is of great importance for global climate regulation as it represents one the most important atmospheric CO2 removal mechanism over long timescales. In this regard, many geochemical tools have been developed over the last decades to trace and quantify continental weathering, and amongst them lithium and magnesium isotopes are considered as robust proxies that provide complementary information. For example, lithium isotopes (i.e., δ7Li) are robust weathering tracers due to the narrow range of δ7Li values of the silicate continental primary rocks compared to the isotope fractionation generated by weathering processes and subsequent secondary mineral formation (e.g., clays, oxides) affecting the isotope composition of the rivers reaching the oceans (e.g., Pogge von Strandmann et al., 2012). Magnesium isotopes (i.e., δ26Mg) are also robust proxies because δ26Mg composition of silicates are rather homogeneous and significantly different from secondary carbonates. Therefore, δ26Mg values of rivers can provide information on the silicate/carbonate ratio of the altered source rock, in addition to being affected by secondary mineral formation (e.g., Tipper et al., 2006, 2008). Beyond this secondary mineral formation, the effects of many interactions occurring within the critical zone on the isotope fractionation of Li and Mg remain poorly constrained. Amongst these interactions, adsorption onto silicate minerals and oxides is one of the most important elemental retention mechanisms within soils and is controlled by numerous environmental parameters (e.g., pH, elemental concentration in the soil solution, solid-solution ratio). Adsorption often generates isotope fractionation, therefore influencing the isotope budget of river waters. The aim of this study is first to explore the adsorption behaviour of Li and Mg on commonly encountered minerals within soil environments (smectite, chlorite, vernadite, gibbsite, and ferrihydrite), and second to determine the isotope fractionation generated during adsorption. Adsorption experiments were performed in batch reactors over a large range of pH and initial cation concentration providing new insights on the dependence of Li and Mg adsorption on (i) the mineralogy, and (ii) solution composition (e.g., pH-dependent speciation). These results will provide insights into the processes controlling Li and Mg mobility and isotope fractionation in soils, and their ultimate release into rivers. Tipper, E. T., Galy, A., & Bickle, M. J. (2006). Riverine evidence for a fractionated reservoir of Ca and Mg on the continents: implications for the oceanic Ca cycle. Earth and Planetary Science Letters, 247(3-4), 267-279. Tipper, E. T., Galy, A., & Bickle, M. J. (2008). Calcium and magnesium isotope systematics in rivers draining the Himalaya-Tibetan-Plateau region: Lithological or fractionation control? Geochimica et Cosmochimica Acta, 72(4), 1057-1075. Pogge von Strandmann, P. A., Opfergelt, S., Lai, Y. J., Sigfússon, B., Gislason, S. R., & Burton, K. W. (2012). Lithium, magnesium and silicon isotope behaviour accompanying weathering in a basaltic soil and pore water profile in Iceland. Earth and Planetary Science Letters, 339, 11-23.