Résumé
Concentration-discharge relationships have been extensively utilized to infer the routing of water and pathways of (bio)geochemical reactions in the Critical Zone. To date, relatively little complementary development of stable isotope ratio - discharge relationships has been made, despite the fact that these tracers are commonly used to disentangle weathering reactions in the fluids draining from headwater systems. A process-based understanding of the extent to which fluid flow rates, antecedent hydrological conditions, and water age distributions impact these isotopic signatures in stream or river exports would present a pivotal advancement in the quantitative analysis of Critical Zone structure and function. Here, we explore how these factors regulate variations in stable lithium isotope ratios (expressed as δ<sup>7</sup>Li) of streamflow and the underlying water-rock interactions recorded by this signal during periods of hydrological transience. We present novel data sets collected during two distinct flooding events in Sapine Creek, a small, granitic catchment located on the southern flank of the Mont-Lozère, France and part of a Critical Zone Observatory within the French OZCAR Research Infrastructure. The data from this site are used to parametrize and validate an isotope-enabled, multicomponent reactive transport model capable of running transient simulations designed to mimic two sampling conditions at Sapine: (1) a storm preceded by a low-flow period and (2) a storm event during the wet season. The models are initiated from an unweathered granite subject to steady state uplift and infiltration. From this point, two synthetic storm events are simulated. Simulation of a storm event ending a period of dry conditions results in a net increase in streamflow lithium isotope ratios due to enhanced secondary mineral formation promoted by relatively long water residence times. In contrast, when the model is used to simulate a succession of relatively larger (~2 times greater in magnitude) storms during a wet season, a net decrease in δ<sup>7</sup>Li is observed. These lower isotope ratios are a consequence of attenuated secondary mineral formation. These preliminary results and trends demonstrate the influence of antecedent hydrological conditions and storm intensity on the magnitude and duration over which stream δ<sup>7</sup>Li is perturbed and, hence, the seasonal dependence of this signal as a record of transient behavior.</p>