Do Deep Convective Zones Exist in Low-Accumulation Firn?

Trapped air in deep ice cores from the Antarctic plateau (Vostok, Dome Fuji) exhibits anomalously low gravitational fractionation during glacial periods, as inferred from \delta¹⁵N and \delta⁴⁰Ar. These isotopic signals reflect the thickness of the stagnant air column in the firn, where molecular diffusion dominates over convection as a vertical transport mechanism. These signals have been used in prior studies as paleoindicators of the firn thickness at the time of bubble close-off. However, a near-surface layer of intense air convection may dominate molecular diffusion sufficiently that isotopic fractionation is prevented. This "convective zone" could explain the discrepancy seen in Vostok/Dome Fuji between expected firn thickness (from densification models) and inferred diffusive column thickness, if the convective zone were 30 m thick during glacial periods. Importantly, accumulation rates were very low during these times (1-2 cm/yr). The existence of deep convective zones has been debated, but not yet observed in firn air studies. Certain low-accumulation regions of the Antarctic plateau today exhibit very large grain sizes at and near the surface, as inferred from satellite passive microwave studies (Fahnestock et al., GRL). Permeability of the firn in these regions should be orders of magnitude higher than in normal firn, suggesting the hypothesis that deep convective zones exist in these regions. In January 2004 we sampled firn air for measurement of \delta¹⁵N and \delta⁴⁰Ar profiles at one such site, where snow megadunes exist. Initial results suggest a convective zone of approximately 20 m thickness.