First Lithium Isotope Data from Rivers and Subsurface Water in the Pamirs

Abstract Lithium and its isotopes has generally been used as a proxy for silicate weathering and only in a few case studies investigated the link between Lithium isotope systematics of surface and ground waters with respect to residence times in crustal reservoirs and aquifers. However, the processes controlling the Lithium isotope fractionation in such systems are not fully understood, yet. In order to gain a better understanding of how Lithium isotopes are fractionated in large catchments we analyzed Lithium concentrations and Lithium isotope compositions of different water reservoirs (rivers, groundwater, thermal water and lakes) of a high alpine granitic catchment in the Pamir Mountains. We also determined the Lithium concentration and Lithium isotope composition of the (granitic) basement as well as the suspended particulate matter (SPM) in the waters. We observe large isotopic variations in the water samples (from δ 7 Li = +3 to +36‰) and light isotopic compositions for the bedrock and SPM (δ 7 Li = -14 to +1.5‰). The wide span of Li isotope composition in the water samples reflects the weathering of silicate rock along the flow path of the (sub) surface water within the catchment.