EXPERIMENTAL-DETERMINATION OF THE DIFFUSION RATE OF DEUTERATED WATER-VAPOR IN ICE AND APPLICATION TO THE STABLE ISOTOPES SMOOTHING OF ICE CORES

The stable isotope records of delta D and delta(18)O in ice cores show that the isotopic gradients are smoothed out with time by diffusion. Transport of hydrogen and oxygen atoms through solid ice is slow where as water vapor diffusion through the interconnected porosity is much f aster. Smoothing occurs preferentially in the upper layers of firn, wh ere the density is lowest, and is responsible for the gradual alterati on of the isotopic stratigraphy. Results for the diffusion of water va por in ice are presented. They were obtained from laboratory experimen ts with diffusion couples prepared using artificial snow with differen t D/H values. The samples were allowed to diffuse for about one year a t a controlled temperature before being cut into thin sections and ana lyzed with a mass spectrometer. The effects of both temperature and de nsity were investigated. The measured diffusion coefficients are fully consistent with water vapor diffusion through the ice porosity. The c omputed values are less than the diffusion coefficient of HDO in free- air and imply a tortuosity factor in the range 3.2-6.5 depending on th e sample density. The influence of the grain sizes on the timescale of the isotopic homogenization between the vapor phase and the ice matri x was studied. We show that for grain sizes up to 1 mm in diameter, so lid diffusion within the grains is not a limiting factor and therefore , the isotopic equilibrium between the vapor and the solid phases can be considered as immediate. The diffusion model developed to compute t he diffusion coefficients was further applied to investigate the real case of isotope smoothing in ice cores. The smoothing rate is highly d ependent on the wavelength of the isotopic signal, linked to the accum ulation rate, and on temperature. Finally, the model was applied to tr itium deposition in Antarctica, from a continuous record of artificial fallout at the South Pole (1954-1978). The result shows that in spite of large gradients, the initial distribution underwent only limited s moothing that did not exceed 10%. (C) 1998 Elsevier Science B.V. All r ights reserved.