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 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.
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