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Enhanced rock weathering (ERW) is proposed as a carbon dioxide removal (CDR) strategy that sequesters carbon through the carbonic acid-promoted dissolution of ground silicate rocks. Studies have explored the efficacy of ERW through geochemical models and bench-scale reactors, but field-scale experimentation is limited. A year- long, replicated study was conducted at the Landscape Evolution Observatory (LEO) at Biosphere 2 to quantify basaltic CDR at the hillslope scale. LEO comprises three mesoscale surfaces (each 330 m2) with 1 m depth of granular basalt. We subjected these structures to three 30 d irrigation events followed by progressively lengthened dry periods. Aqueous discharge was collected bihourly for major and trace chemistry, and subsurface interactions were observed at 15 min intervals through distributed sensors enabling continuous monitoring of PCO2, volumetric water content, and total hillslope mass. This approach enabled closing of the carbon and water mass balance of the system for the duration of the experiment. CDR was quantified through direct monitoring of bicarbonate (HCO3) concentrations as validated through the charge balance of non-hydrolyzing cations and strong-acid anions. Concentration-discharge relations for HCO3 showed dilution trends with clockwise hysteresis, while a decrease in CO2 uptake occurred with increased hillslope water saturation (Shydro). The CDR rate, normalized to the specific surface area of the basalt, was -13.45 log10 moles C m 2 s 1, while other studies report CDR rates from -14 to -10 log10 moles m 2 s 1. We found that basalt CDR rates were impacted by depletions of PCO2 upon hydrologic infiltration, variable Shydro, and incongruent dissolution. 

Abstract Antecedent hydrological conditions are recorded through the evolution of dissolved lithium isotope signatures (Li) by juxtaposing two storm events in an upland watershed subject to a Mediterranean climate. Discharge and Li are negatively correlated in both events, but mean Li ratios and associated ranges of variation are distinct between them. We apply a previously developed reactive transport model (RTM) for the site to these event‐scale flow perturbations, but observed shifts in stream Li are not reproduced. To reconcile the stability of the subsurface solute weathering profile with our observations of dynamic stream Li signatures, we couple the RTM to a distribution of fluid transit times that evolve based on storm hydrographs. The approach guides appropriate flux‐weighting of fluid from the RTM over a range of flow path lengths, or equivalently fluid residence times. This flux‐weighted RTM approach accurately reproduces dynamic storm Li‐discharge patterns distinguished by the antecedent conditions of the watershed. 

Abstract Lithium isotope ratios (δ⁷Li) of rivers are increasingly serving as a diagnostic of the balance between chemical and physical weathering contributions to overall landscape denudation rates. Here, we show that intermediate weathering intensities and highly enriched stream δ⁷Li values typically associated with lowland floodplains can also describe small upland watersheds subject to cool, wet climates. This behavior is revealed by stream δ⁷Li between +22.4 and +23.5‰ within a Critical Zone observatory located in the Cévennes region of southern France, where dilute stream solute concentrations and significant atmospheric deposition otherwise mask evidence of incongruence. The water‐rock reaction pathways underlying this behavior are quantified through a multicomponent, isotope‐enabled reactive transport model. Using geochemical characterization of soil profiles, bedrock, and long‐term stream samples as constraints, we evolve the simulation from an initially unweathered granite to a steady state weathering profile which reflects the balance between (a) fluid infiltration and drainage and (b) bedrock uplift and soil erosion. Enriched stream δ⁷Li occurs because Li is strongly incorporated into actively precipitating secondary clay phases beyond what prior laboratory experiments have suggested. Chemical weathering incongruence is maintained despite relatively slow reaction rates and moderate clay accumulation due to a combination of two factors. First, reactive primary mineral phases persist across the weathering profile and effectively “shield” the secondary clays from resolubilization due to their greater solubility. Second, the clays accumulating in the near‐surface profile are relatively mature weathering byproducts. These factors promote characteristically low total dissolved solute export from the catchment despite significant input of exogenous dust. 

To date, the vast majority of studies seeking to link discharge to solute concentrations have been based on representations of fluid age distributions in watersheds that are time-invariant. As increasingly detailed spatial and temporal datasets become available for weathering-derived riverine solute concentrations, the capacity to link this mass flux to transient routing of reactive fluids through Critical Zone environments is vital to quantitative interpretation. Relationships between fluid age distributions and the stable isotope ratios of these geogenic solutes are even less developed, yet these signatures are vital to parsing the suite of water-rock-life interactions that create concentration-discharge relationships. Here we offer the first merging of a 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 $\delta$ ³⁰Si 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 is the result of a stronger link between isotope ratios and the age distribution parameters describing a given watershed. This novel modeling framework is used to provide a quantitative basis for the interpretation of SiO_2(aq)and $\delta$ ³⁰Si in six low-order streams spread across a diversity of climates, geologies, and ecosystems. To our knowledge, this is the first forward and process-based model to describe the isotopic signatures of solutes derived from weathering reactions in watersheds subject to time-varying discharge.