The travel time and pathways of water in the Critical Zone (CZ) influence the extent and style of water-rock interaction. As a consequence, the concentration and isotope composition of elements in the river dissolved load reflect processes whose dominance varies during seasonal changes, and even during single flood events. In particular, our working hypothesis is that these effects are mostly controlled by the depth at which the water flows, as vertical chemical gradients (in e.g. pH and redox conditions) are expected in the CZ. In addition to elemental abundances, isotope tracers can be used to better identify the sources of dissolved material and the weathering processes affecting elements on their way from rocks to rivers. As weathering budgets fundamentally result from the combination of mineral dissolution and precipitation, the abundance and isotopic composition of lithium (Li) which are sensitive to secondary mineral formation and dissolution in the subsurface (e.g. Ryu et al., 2014; Wimpenny et al., 2015) offer a promising tracer for these reactions. Therefore, time series of chemical and isotope composition of dissolved material in rivers should help us to understand the coupling between water transport and chemical reactions in the CZ, beyond the existing steady state models (e.g. Ibarra et al., 2016).
My goal is to establish how these processes and sources which influence the Li isotope ratios in the groundwater and then in the river, and therefore to develop a novel isotopic tracer of water-rock interactions at the catchment scale. The approach is based on small instrumented catchments with a simple lithology and vegetation type. Hence, the hydrology is the main control for solute export. I am focusing on time series of samples from different catchments and its compartments as river-, ground-, soil and rain water, as well as soil, bedrock, litter fall and suspended load. The investigations are being done at three catchments: (1) Sapine (Cévennes, France) has a granitic lithology and beech vegetation. (2) In Naizin (Bretagne, France) the bedrock consists of schists and the land is agriculturally used. (3) The fractured aquifer of the Elder Creek catchment (California, US) is characterized by mudstone conglomerate containing sandstones. At all these catchments, we focus on “hot moments”, i.e. flood events where discharge significantly changes over short periods of time, which leads us to consider the fundamentally transient aspects of water-rock interactions.
Ibarra D. E., Caves J. K., Moon S., Thomas D. L., Hartmann J., Chamberlain C. P., Maher K. and Ibarra D. E. (2016) Differential weathering of basaltic and granitic catchments from concentration-discharge relationships. Geochim. Cosmochim. Acta 190, 265–293.
Ryu J.-S., Vigier N., Lee S.-W., Lee K.-S. and Chadwick O. A. (2014) Variation of lithium isotope geochemistry during basalt weathering and secondary mineral transformations in Hawaii. Geochim. Cosmochim. Acta 145, 103–115.
Wimpenny J., Colla C. A., Yu P., Yin Q. Z., Rustad J. R. and Casey W. H. (2015) Lithium isotope fractionation during uptake by gibbsite. Geochim. Cosmochim. Acta 168, 133–150.