Kinetics of air-hydrate nucleation in polar ice sheets

Nucleation of air clathrate hydrates in air bubbles and diffusive air-mass exchange between coexisting ensembles of bubbles and hydrate crystals are t he major interrelated processes that determine the phase change in the air- ice system in polar ice. In continuation of Salamatin et al. (J. Crystal Gr owth 193 (1998) 197; Ann. Glaciol. 29 (1999) 191) where the post-nucleation conversion of single air bubbles to hydrates was considered, we present he re a statistical description for transformation of air bubbles to air clath rate hydrates based on the general theory of evolution of these two ensembl es, including the gas fractionation effects. The model is fit to data on ic e cores from central Antarctica, and then compared to other ice-core data. The focus is on the rate of clathrate-hydrate nucleation, which is determin ed to be the product of the inverse relative bubble size raised to the powe r lambda approximate to 5.8 with the relative supersaturation to the power beta approximate to 2 The clathration-rate constant is k(0) approximate to3 .2-4.5 x 10(-6) yr(-1) at 220 K. The N-2- and O-2-permeation coefficients i n ice, at 220 K, are inferred to be D-N2(0) approximate to 1.8-2.5 x 10(-8) mm(2)yr(-1) and D-O2(0) approximate to 5.4-7.5 x 10(-8)mm(2)yr(-1), respec tively. Comparison of observations to simulations of bubble-to-hydrate tran sformation in Greenland ice sheer gave estimates for activation energies of hydrate formation and air diffusion of Q(J) approximate to 70 kJ mol(-1) a nd Q(d) approximate to 50 kJ mol(-1) respectively. (C) 2001 Elsevier Scienc e B.V. All rights reserved.