Technical Note: Isotopic corrections for the radiocarbon composition of CO 2 in the soil gas environment must account for diffusion and diffusive mixing

Abstract. Earth systemscientists working with radiocarbon in organic samples use a stable carbon isotope ( δ13C) correction to account for mass-dependent fractionation, but it has not been evaluated for the soil gasenvironment, wherein both diffusive gas transport and diffusive mixing are important. Using theory and an analytical soil gastransport model, we demonstrate that the conventional correction is inappropriate for interpreting the radioisotopic composition of CO2 from biological production because it does not account for important gas transport mechanisms. Based on theory used to interpret δ13Cof soil production from soil CO2 , we propose a new solution for radiocarbon applications in the soil gasenvironment that fully accounts for both mass-dependent diffusion and mass-independent diffusive mixing.