Modelling the isotopic signatures of solutes derived from weathering reactions

High-resolution water chemistry records in rivers typically show that the routing of reactive solutes through the Critical Zone is a dynamic process that can change drastically across hydrologic responses. Quantitative transient models are needed to interpret these riverine solute measurements as emergent signatures of coupled geochemical and ecohydrological functioning. In this context, the stable isotope signatures of geogenic solutes offer a unique opportunity to disentangle processes such as the dissolution or primary minerals, precipitation of secondary phases and ecological nutrient cycling. Here, we describe the first merging of a parsimonious hydrological model featuring time-variant fluid age distributions with a geochemical model for isotopically fractionating weathering reactions. Using SiO_2(aq) and the corresponding silicon isotope ratio δ³⁰Si as an example, we show that the stable isotope signatures of riverine solutes produced by weathering reactions reflect a component of the fluid age distribution that is unique to the corresponding solute concentrations. This distinct sensitivity offers a novel diagnostic tool to interpret the SiO_2(aq) and δ³⁰Si dynamics recorded in six low-order streams spread across a diversity of climates, geologies, and ecosystems.