Weathering Incongruence in Mountainous Mediterranean Climates Recorded by Stream Lithium Isotope Ratios

Lithium isotope ratios (delta 7Li) 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 delta 7Li values typically associated with lowland floodplains can also describe small upland watersheds subject to cool, wet climates. This behavior is revealed by stream delta 7Li between +22.4 and +23.5 parts per thousand within a Critical Zone observatory located in the Cevennes 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 delta 7Li 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. Chemical weathering of silicate minerals by meteoric water serves as a long-term regulator of atmospheric CO2. The extent to which these reactions produce secondary minerals dictates the chemical composition of soils and rivers. The isotope composition of metals is a potent tracer of this chemical weathering and significant progress has been gained in application of lithium stable isotopes for this purpose. Here, we demonstrate that water draining a mountainous landscape subject to a wet, temperate climate in Southern France produces isotopically heavy lithium in stream water. These signatures suggest an environment in which chemical weathering rates keep pace with the rates of soil erosion. To reveal the underlying causes of this behavior, we develop a numerical model that simulates the flow of water and its interaction with rock in the subsurface using measured data from the field site, including soil, bedrock, and water chemistry. We show that new minerals formed in the subsurface resist dissolution and effectively retain lithium. This work leverages the sensitivity of lithium isotopes to analyze weathering conditions that typify upland watersheds subject to cool, wet climates. Small, upland watersheds under Mediterranean climate display characteristically high lithium stable isotope ratios (delta 7Li) In a watershed in southern France subject to strong atmospheric inputs, weathering incongruence leads to high stream delta 7Li values Isotope-enabled reactive transport modeling indicates that at this site soil clays retain lithium and are resilient to (re-)solubilization