Stable isotope records of 18O in ice cores showed that isotopic gradients were smoothed out, in time, by diffusion. The transport of H and O atoms through solid ice was slow, whereas water-vapor diffusion through porosity was much faster. Smoothing occurred preferentially in the upper layers of firn; where the density was lowest. It was responsible for a gradual alteration in isotopic stratigraphy. Results were presented on the diffusion of water vapor in ice. They were obtained using laboratory experiments which involved diffusion couples between artificial snows with different D/H values. The samples were allowed to diffuse for about 8750h at a controlled temperature, cut into thin sections and analyzed using a mass spectrometer. The measured diffusion coefficients were consistent with water-vapor diffusion through pores. The values were lower than the diffusion coefficient of HDO in free air, and implied a tortuosity factor of 3.2 to 6.5; depending upon the sample density. It was shown that, for grain sizes up to 1mm, solid diffusion within the grains was not a limiting factor. Therefore, isotopic equilibration between vapor and solid could be considered to be immediate. The diffusion model which was developed in order to calculate the diffusion coefficients was also used to investigate isotope-smoothing in ice cores. The smoothing rate was highly dependent upon the wavelength of the isotopic signal, which was linked to accumulation rate and temperature. The model was then applied to T deposition in Antarctica. The results showed that, in spite of large gradients, the initial distribution underwent only a limited smoothing (less than 10%).

Experimental Determination of the Diffusion Rate of Deuterated Water Vapor in Ice and Application to the Stable Isotopes Smoothing of Ice Cores. P.Jean-Baptiste, J.Jouzel, M.Stievenard, P.Ciais: Earth and Planetary Science Letters, 1998, 158[1-2], 81-90