Search

Antecedent hydrological conditions are recorded through the evolution of dissolved lithium isotope signatures (δ7 ${\delta }^{7}$Li) by juxtaposing two storm events in an upland watershed subject to a Mediterranean climate. Discharge and δ7 ${\delta }^{7}$Li are negatively correlated in both events, but mean δ7 ${\delta }^{7}$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 δ7 ${\delta }^{7}$Li are not reproduced. To reconcile the stability of the subsurface solute weathering profile with our observations of dynamic stream δ7 ${\delta }^{7}$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 δ7 ${\delta }^{7}$Li‐discharge patterns distinguished by the antecedent conditions of the watershed. Plain Language Summary: Storm events often cause characteristic shifts in stream solute chemistry. Interpreting these signals offers insight into the water‐rock interactions occurring within watersheds. Here, we use lithium stable isotopes and reactive transport modeling to relate how long water spends in a catchment, or how deep water infiltrates through a catchment, to the extent of chemical weathering. We show that the first significant storm after a dry season exports more chemically evolved water, while a wet season storm releases less evolved, shallower, and younger water. Our results indicate that stream flow δ7 ${\delta }^{7}$Li in small watersheds offers a sensitive record of hydrological conditions prior to the storm, reflecting subtle shifts in the efficiency of the Critical Zone to generate, transport, and ultimately export solutes. Key Points: Stream lithium stable isotope ratios (δ7Li) recorded at high frequency over storm events are sensitive to antecedent conditionsA reactive transport model cannot produce observed shifts in stream chemistry through variations in flow rate aloneFlux‐weighting of model fluid outputs based on time‐varying fluid transit time distributions describes stream δ7Li over storm hydrographs [ABSTRACT FROM AUTHOR]

• An intense storm was sampled as it infiltrated a steep canyon hillslope and stream. • Rock moisture water isotopes track mixing of storm water into shallow subsurface. • Stream δ 7 Li tracks delivery of solutes from further upslope at higher flow rates. • Reactive transport modeling of stream solute stable isotope ratios during storms. Long-term relationships between stream chemistry and discharge are regulated by watershed subsurface structure and biogeochemical functioning. The extent to which these mechanisms are expressed and may be explored in the geochemical response of streams during storm events remains an open question. Here, we monitor an intense storm as it infiltrated an upland hillslope draining into a small steep canyon stream that is typified by chemostatic concentration-discharge relationships in rock-derived solutes. Our approach couples a high-frequency record of stable lithium isotope ratios (δ 7 Li) in the stream with novel sampling of rock moisture within the hillslope. At peak discharge, lithium-sodium ratios (Li/Na) increased from 0.58 μM/mM to 0.82 μM/mM and δ 7 Li decreased from + 28.9 ± 0.1 ‰ to + 26.4 ± 0.4 ‰ in the stream. Hillslope hydrologic monitoring reveals that the rainwater infiltrated the subsurface, yet attenuated breakthrough of the heavily depleted δ D signal of this storm (as low as −86‰) only reached the upper 3-4 meters of the vadose zone. These δ D data show that the storm water mixed with previously stored rock moisture and displaced stored fluid to deeper depths, causing an observable rise in the water table. Groundwater 87Sr/86Sr and δ 7 Li demonstrate consistency in the fluid-rock interactions that occur below the water table prior to and during the storm. In total, these observations indicate that the transfer of fluid and generation of solutes through the interior of the hillslope produce the variability of Li/Na and δ 7 Li within the stream during the storm, and support application of a previously established 1-D reactive transport model framework developed for the evolution of lithium within the hillslope to this extreme hydrologic event. Based on the model, both Li/Na and δ 7 Li versus discharge relationships reflect an overall shorter transit time of fluid through the interior of the hillslope. These model results are consistent with our hydrologic observations and indicate that Li from further upslope (where the vadose zone becomes thicker) contributes to stream solute chemistry at the height of the storm. We conclude that in this system, stream lithium isotope signatures record the routing of water and generation of solutes within the hillslope even during intense storm events. [ABSTRACT FROM AUTHOR]

• Reactive transport modeling of lithium in fluids draining a first-order hillslope. • Fluid δ 7 Li and Li/Na ratios distinguish compartments of the weathering profile. • Contemporaneous formation of multiple secondary phases governs fluid δ 7 Li. • Weathering continues below the water table as groundwater laterally drains to stream. The stable isotopes of lithium (Li) serve as a robust proxy of silicate weathering. The fate and transport of these isotopes in the dissolved load of major rivers have been characterized to infer changes in both contemporary weathering regimes and paleo-conditions. In this contribution, we deconvolve this integrated signal into the individual processes that fractionate Li at the inception of silicate weathering by directly measuring Li isotope ratios of waters (δ 7 Li) transiting through a rapidly eroding first-order hillslope. We use these data to develop a multicomponent reactive transport framework, which shows that net dissolution of weathered material generates light δ 7 Li signatures (as low as −9.2‰) in the shallow portion of the vadose zone. An increase in δ 7 Li deeper into the vadose zone (as much as +18‰) reflects an increasing contribution of secondary mineral formation. Below the water table, congruent weathering occurs and imparts elevated cation concentrations and bedrock δ 7 Li. Silicate weathering continues within the saturated zone as groundwater travels downslope (δ 7 Li = +13 to + 24‰) to the stream. The stream signatures (δ 7 Li = +28 to +29‰) reflect the terminus of this network of silicate weathering reactions and the relative magnitude of each contributing process (e.g., transitions in secondary mineral formation, dissolution of weathered material). We show that fluid progressing through the weathering profile of this first-order hillslope is distinguished by a sequence of characteristic Li isotope signatures, which can be reproduced in a forward, process-based model framework. This model development offers an improved quantitative basis for the use of metal(loid) stable isotopes in disentangling catchment-scale chemical weathering fluxes. [ABSTRACT FROM AUTHOR]