Solute Isotope Fractionation during Fluid Transit

ended

The chemical dissolution (weathering) of continental silicate rocks is a crucial Earth System process that makes nutrients available to ecosystems and consumes atmospheric CO2, affecting Earth’s climate and habitability. The mechanistic controls on chemical weathering are still poorly understood, partly due to lack of integration between geochemical and hydrological concepts. Here, I propose to test a key hypothesis that silicate weathering fluxes are primarily controlled by how long water spends in contact with rocks, before being exported via rivers. To do this, I will use novel tracers of chemical silicate weathering - the stable isotope ratios of dissolved silicon (δ30Si) and lithium (δ7Li) in a project that couples controlled lab weathering experiments with a watershed-scale field study. I will use column flow-through and batch reactor experiments to simulate in isolation the effect of variable water-rock interaction times on dissolved δ30Si and δ7Li. I will then compare these results with natural weathering observed in a study watershed, recording the response of riverine δ30Si and δ7Li to variable hydrological conditions. The water transit time variations in the watershed will be constrained using water hydrogen and oxygen isotope ratios (δD, δ18O, 3H). Finally, I will synthesize the experimental and field results to build a novel reactive transport framework that will incorporate a robust representation of hydrological variability in determining weathering fluxes and isotopic riverine signatures. The results of this project will have important implications for our understanding of the links between climate and chemical weathering. In turn, this will enable a better prediction how nutrient and carbon cycles, driven by weathering, will respond to anthropogenic perturbation of atmospheric composition, the hydrological cycle, and natural ecosystems, which is a major European and global environmental research priority.