Isotopic Fractionation in Non-Equilibrium Diffusive Environments: Implications for Chamber- Based Studies of Soil Gas Fluxes

Isotopic studies are frequently carried out in environments where diffusion is the dominant mode of transport, including the ocean, freshwaters, ice, solid earth, shallow subsurface, and within terrestrial vegetation. While researchers develop novel ways of exploiting isotopic fractionations as a way to track biochemical or physical processes, exploration of basic fractionation mechanisms associated with diffusive transport lags behind. This study examines the implications of diffusive-transport induced fractionations on the interpretation of isotopic processes in the geophysical environment using the example of gaseous δ13C-CO2 flux from soils. We use a simple iterative diffusion model to simulate isotopic transport of δ13C-CO2 from soil into a headspace chamber, followed by laboratory measurements using large and small chambers. Unlike existing models, we treat 12C-CO2 and 13C-CO2 as separate gas species that always travel at their respective theoretical diffusivities. In both model runs and laboratory measurements, the combined presence of isotopic and concentration gradients results in an observed fractionation of smaller average magnitude than the accepted theoretical diffusion fractionation for these species of 4.4 permil. The observed fractionation is not constant, but falls continuously during the headspace equilibration period, which means that isotopic data from a non-steady state diffusive environment can be misinterpreted when steady state diffusion models are applied. These results may also be extended to other gas flux environments including near- surface boundary layers where non-equilibrium diffusive environments are present due to the non-infinite nature of sources and/or sinks.