HR: 1340h
AN: PP33C-1590    [Abstracts]
TI: Krypton and Xenon as Indicators of Convective Zone Thickness in Firn at Megadunes, Antarctica
AU: * Kawamura, K
EM: kkawamura@ucsd.edu
AF: Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0244 United States
AU: Severinghaus, J P
EM: jseveringhaus@ucsd.edu
AF: Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0244 United States
AB: Air trapped in ice on the Antarctic plateau (Vostok, Dome Fuji and Dome C) during glacial periods exhibits unexpectedly low gravitational fractionation, as inferred from δ¹⁵N and δ⁴⁰Ar. The deduced thickness of the ``diffusive zone'', in which molecular diffusion dominates as gas transport mechanism, is thinner by 30-40 m than the total firn thickness estimated with densification models. It has been hypothesized that this apparent inconsistency is due to a thickened near-surface ``convective zone'', in which convective mixing overwhelms molecular diffusion and thus prevents isotopic fractionation. However, there is currently no means to estimate the convective zone thickness in the past. In studies of today's firn, a significant convective zone has been found at low accumulation sites (Megadunes, 20 m; Vostok, 13m; Dome Fuji, 9 m) and at a very windy site (YM85, 14 m). The deep convection at the Megadunes site is probably attributable to the very low accumulation rate, which leads to very high permeability by making large firn grains and deep surface cracks. In this study, we measured ⁴⁰Ar/³⁶Ar, Kr/Ar and Xe/Ar ratios of the Megadunes firn air, in order to test the effect of deep convection on gravitational fractionation of heavy noble gases. In theory, the dominance of convection over molecular diffusion is larger for heavier gases than for lighter gases due to their low diffusivity. As a result, the gravitational fractionation of heavier gases should be smaller than that of lighter gases (i.e. the diffusive zone is thinner for heavier gases) if a deep convective zone exists. Our result indeed shows smaller gravitational fractionation of Kr/Ar and Xe/Ar than of ⁴⁰Ar/³⁶Ar when normalized by the mass differences for each gas pair. A preliminary measurement of ⁸⁶Kr/⁸²Kr also suggests smaller gravitational fractionation. Possible future application includes the reconstruction of the thickness of past convective zones by measuring deep ice core noble gases. This topic is important for validating densification models for the glacial periods, which may help constrain the phase relationship between climate and greenhouse gas variations.
DE: 0724 Ice cores (4932)
DE: 0736 Snow (1827, 1863)
DE: 0776 Glaciology (1621, 1827, 1863)
DE: 3344 Paleoclimatology (0473, 4900)
DE: 9310 Antarctica (4207)
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005