Air Mixing in Firn and the Age of the Air at Pore Close-Off

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Abstract
The air trapped in the bubbles of natural ice is not the same age as the surrounding ice. This is due to the fact that the air is enclosed in isolated bubbles only at the depth of the firn–ice transition. Within the overlying porous firn layer the air is able to mix and to exchange to a certain degree with the atmosphere. The age difference between ice and air is given by the age of the ice at pore close-off, less the mixing delay. Also, there is an age distribution due to diffusive smoothing and due to the gradual enclosure of the air at the firn–ice transition. Knowledge of this age relation is necessary for the interpretation of climatic parameters measured on ice cores. This work concentrates on the effect of diffusive mixing. We report on measurements of the diffusivity of CO2 and O2 (in N2) in firn samples from Siple Station, Antarctica. It is shown that the dominant mixing process is molecular diffusion. The diffusion coefficient depends approximately linearly on the porosity. A one-dimensional diffusion model has been used to calculate the air mixing in firn at Siple Station (Antarctica), at the South Pole, and at Station Crête (Greenland). An exchange time of between 10 and 50 years is obtained.