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

• Main Authors: Buizert, Christo, Sowers, Todd, Blunier, Thomas
• Other Authors: College of Earth, Ocean, and Atmospheric Sciences
• Format: Article in Journal/Newspaper
• Language: English; unknown
• Published: Elsevier
• Subjects: ice core
• Online Access: https://ir.library.oregonstate.edu/concern/articles/8w32rb45v

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–CH₄. We apply the correction to δ¹³C–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–CH₄ for the duration of the event, consistent with reduced wetland emissions. KEYWORDS: firn air, isotopic fractionation, 8.2 ka event, greenhouse gas, ice core