Experimental determination of the diffusion rate of deuterated water vapor in ice and application to the stable isotopes smoothing of ice cores

Details Experimental determination of the diffusion rate of deuterated water vapor in ice and application to the stable isotopes smoothing of ice cores Experimental determination of the diffusion rate of deuterated water vapor in ice and application to the stable isotopes smoothing of ice cores

May 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998e%26psl.158...81j&link_type=abstract

Earth and Planetary Science Letters, vol. 158, Issue 1-2, pp.81-90

Computer Science

4

Deuterium, Diffusion, Ice Cores, Tortuosity, Tritium

Scientific paper

The stable isotope records of D and 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, whereas water vapor diffusion through the interconnected porosity is much faster. Smoothing occurs preferentially in the upper layers of firn, where the density is lowest, and is responsible for the gradual alteration of the isotopic stratigraphy. Results for the diffusion of water vapor in ice are presented. They were obtained from laboratory experiments with diffusion couples prepared using artificial snow with different D/H values. The samples were allowed to diffuse for about one year at a controlled temperature before being cut into thin sections and analyzed with a mass spectrometer. The effects of both temperature and density were investigated. The measured diffusion coefficients are fully consistent with water vapor diffusion through the ice porosity. The computed 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 the sample density. The influence of the grain sizes on the timescale of the isotopic homogenization between the vapor phase and the ice matrix was studied. We show that for grain sizes up to 1 mm in diameter, solid 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 the diffusion coefficients was further applied to investigate the real case of isotope smoothing in ice cores. The smoothing rate is highly dependent on the wavelength of the isotopic signal, linked to the accumulation rate, and on temperature. Finally, the model was applied to tritium 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 smoothing that did not exceed 10%.

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