Water flow exerts a strong control on weathering reactions at the Earth’s surface, hence on CO2 fluxes and climate. Identifying the mechanisms controlling the coupling between hydrology and chemical weathering in the Critical Zone is thus essential. This thesis addresses this question through theoretical and field work on the relationships between stream chemistry (concentration, concentration ratios and isotope ratios) and discharge (“x-q relationships”) over short (flood events) and long (seasonal) times scale in the small, granitic, forested, instrumented catchment of Sapine, located in southern France (Ce?vennes).
We develop a reactive transport model, accounting for the behaviour of trace elements and their isotopes, which predicts a higher variability in x-q relationships during flood events than those induced by simple hydrological mixing. At Sapine, field investigations and isotope analyses show that the soil chemistry is significantly influenced by dust inputs. High-frequency measurements of the stream dissolved chemistry during flood events reveal the important role of this soil layer on the stream solute export. In addition, a higher variability of x-q relationships is observed for flood events with wet antecedent conditions, compared to those occurring after dry conditions. Dissolved lithium (Li) and its isotopes show the strong imprint of weathering reactions on the solute export. During flood events, simple hydrological mixing suffices to explain variations in the stream chemistry, whereas variations at the seasonal time scale are best explained by a reactive framework and a control through the residence time of water in the subsurface.
Key words: Concentration-discharge relationships, flood events, Lithium isotopes, Critical Zone, weathering, reactive transport modelling.