Positive correlation between Li and Mg isotope ratios in the river waters of the Mackenzie Basin challenges the interpretation of apparent isotopic fractionation during weathering

During chemical weathering, magnesium (Mg) is released by the dissolution of both carbonate and silicate sources. The degree to which solute concentrations and isotopic compositions of rivers reflect the relative proportions of these two inputs, or cycling by a series of processes associated with weathering is poorly constrained. In the river waters of the Mackenzie Basin (Canada), the Mg content is high and Mg isotope ratios (26Mg/24Mg expressed as δMg26) show in excess of one per mil variability. Part of this variability is attributed to the 3‰ range in the carbonate and silicate rocks drained. Despite this inherent lithological control on river water δMg26 values, there is also evidence for a fractionation control. A linear positive covariation between lithium (7Li/6Li, expressed as δLi7) and Mg isotope ratios in the river waters of the Mackenzie Basin is reported. This covariation is not expected because previously reported fractionation related to physicochemical processes associated with clays or oxides should induce a negative covariation with Mg isotope ratios.This continental-scale covariation can be resolved by either process-related fractionation or mixing. Evidence for fractionation associated with clays is provided firstly by comparing Mg and Li isotopes in both the waters and sediments carried in suspension. Secondly a linear covariation between the sediment concentrations of large ion lithophile elements caesium and rubidium (a proxy for clay content of the sediment) and δMg26 values of the water suggests that processes linked to clay, such as neoformation of clay, cation exchange or adsorption may be important. Simple models illustrate that if the covariation is induced by fractionation, there is either more than one process acting, or a single process is kinetically limited. Alternatively, the data can be reconciled by mixtures between at least three different water bodies, two of which have similar isotopic compositions but differing Li/Mg ratios. This intriguing data set highlights the challenges associated with distinguishing mixing from process with stable isotope data. Despite the complexity, the data question to what extent and by what mechanism clays mediate river water chemistry, at least in terms of the stable isotope compositions of Mg and Li. These questions are fundamental to the quantification of carbon dioxide consumption by silicate weathering and its role in climatic feedback.

► Mg and Li isotope ratios covary in the river waters of the Mackenzie Basin. ► This could either be induced by weathering processes or mixing between water bodies. ► There are links between the clay content of the sediment and isotopic composition of the water. ► This suggests clays might have a prominent control in the isotope composition of the water.