Presentation and Applications of Mixing Elements ANd Dissolved Isotopes in Rivers (MEANDIR), a Customizable Model for Monte Carlo Inversion of Dissolved River Chemistry

The dissolved chemistry of rivers reflects weathering processes within the critical zone and, more broadly, the alkalinity and carbon fluxes that couple landscape development with the global carbon cycle. When using measurements of river dissolved load to study chemical weathering in the critical zone, inversion models with Monte Carlo propagation of uncertainty are a common tool for distinguishing among chemical contributions from end-members or processes with distinct elemental and/or isotopic compositions. However, the methods underlying such river inversion models have typically been opaque. Here we present Mixing Elements ANd Dissolved Isotopes in Rivers (MEANDIR), a new open-access MATLAB model for customizable inversion of dissolved river chemistry. MEANDIR provides new insight into the chemical reactions that accompany the transformation of bedrock into soil by allocating the products of chemical denudation into fractional contributions from distinct reactions, such as weathering of silicate, carbonate, and sulfide minerals, as well as organic carbon oxidation and the formation of secondary phases. Among other functionality, MEANDIR is able to implement chlorine critical values, optimize new cost functions, normalize to the sum of dissolved variables, quantify river sulfate sourced from pyrite oxidation, account for isotopic fractionation during clay formation, and calculate the net impact of weathering on atmospheric carbon dioxide. We apply MEANDIR to five previously published datasets to demonstrate the sensitivity of results to parameter choices; we invert data from two global compilations of river chemistry (Gaillardet et al., 1999; Burke et al., 2018), the major element chemistry and sulfate sulfur isotope ratios of rivers in Peru (Torres et al., 2016), the major element chemistry of rivers in Iceland (Gíslason et al., 1996), and the major and trace element chemistry of water samples from the Mackenzie River (Horan et al., 2019).