Kinetic Gas Fractionation by Air Convection in Polar Firn

Recent firn air studies at the Megadunes site in central Antarctica (80.78°S, 124.5°E) show the existence of a 20-m deep convective zone. Large cracks or fissures in the firn likely enable this deep convection (Courville et al., 2007). The existence of fissures appears to be due to near-zero accumulation rate, intense weathering, thermal-contraction cracking, and sublimation. Measured xenon and krypton isotopes are less gravitationally enriched than argon and nitrogen isotopes, and the xenon-nitrogen isotope difference has been explored as a possible paleo-convective zone thickness indicator for use in ice cores (Kawamura et al., 2006). Here we show using first-principles arguments that this phenomenon may be viewed as a special case of a more general type of kinetic gas fractionation. In concept, convective mixing continually disturbs gravitational equilibrium, and gases diffuse back toward equilibrium, with heavier gases diffusing more slowly. Thus a steady state persists in which heavy gases are depleted relative to diffusive equilibrium conditions. The prerequisite for this mode of fractionation is that convective (turbulent) mixing and gravitational unmixing are in competition with each other, such that the effective eddy diffusivity and molecular diffusivity are similar (Prandtl number near 1). The effect on deep firn gases (and hence bubble air) increases with the thickness of the layer in which the Prandtl number is near 1. At Megadunes the magnitude of the effect is 10 per meg for 40Ar/36Ar. The effect may be safely neglected in ice core studies for most biogeochemical purposes.