Antecedent Hydrologic Conditions Reflected in Stream Lithium Isotope Ratios During Storms

Abstract

Abstract Antecedent hydrological conditions are recorded through the evolution of dissolved lithium isotope signatures (δ7Li) by juxtaposing two storm events in an upland watershed subject to a Mediterranean climate. Discharge and δ7Li are negatively correlated in both events, but mean δ7Li 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 δ7Li are not reproduced. To reconcile the stability of the subsurface solute weathering profile with our observations of dynamic stream δ7Li 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 δ7Li‐discharge patterns distinguished by the antecedent conditions of the watershed.

Keywords

• critical zone
• hydrology
• geochemistry
• lithium isotopes
• solute export
• storms