Assessment of diffusive isotopic fractionation in polar firn, and application to ice core trace gas records

During rapid variations of the atmospheric mixing ratio of a trace gas, diffusive transport in the porous firn layer atop ice sheets and glaciers alters the isotopic composition of that gas relative to the overlying atmosphere. Records of past atmospheric trace gas isotopic composition from ice cores and firn need to be corrected for this diffusive fractionation artifact.We present a novel, semi-empirical method to accurately estimate the magnitude of the diffusive fractionation in the ice core record. Our method (1) consists of a relatively simple analytical calculation; (2) requires only commonly available ice core data; (3) is not subject to the uncertainties inherent to estimating the accumulation rate, temperature, close-off depth and depth–diffusivity relationship back in time; (4) does not require knowledge of the true atmospheric variations, but uses the smoothed records obtained from ice cores; (5) arguably gives more accurate results than a combined firn densification–firn air transport modeling study would.We apply the method to records of CH4, CO2 and N2O mixing ratios, and we find that the correction is particularly important for δ13C–CH4δ13C–CH4. We apply the correction to δ13C–CH4δ13C–CH4 records over the last glacial termination and the 8.2 ka event. In both cases the diffusive signal exceeds the analytical precision of the data, and has a significant impact on the observed isotopic trends. For the 8.2 ka event the corrected data show an isotopic enrichment in δ13C–CH4δ13C–CH4 for the duration of the event, consistent with reduced wetland emissions.

► We derive an analytical description of diffusive isotopic fractionation in firn. ► We present a new and convenient framework for correcting ice core data for DF. ► We calculate the diffusive fractionation for CO2, CH4 and N2O ice core records. ► We correct δ13C–Cδ13C–C of CH4 records for the deglaciation and the 8.2 ka event for DF. ► The δ13C–CH4δ13C–CH4 signal of the 8.2 ka event is consistent with reduced wetland emission.