Model calculations of the age of firn air across the Antarctic continent

The age of firn air in Antarctica at pore close-off depth is only known for a few specific sites where firn air has been sampled for analyses. We present a model that calculates the age of firn air at pore close-off depth for the entire Antarctic continent. The model basically uses four meteorological parameters as input (surface temperature, pressure, accumulation rate and wind speed). Using parameterisations for surface snow density, pore close-off density and tortuosity, in combination with a density-depth model and data of a regional atmospheric climate model, distribution of pore close-off depth for the entire Antarctic continent is determined. The deepest pore close-off depth was found for the East Antarctic Plateau near 72degrees E, 82degrees S, at 150+/-15m (2sigma). A firn air diffusion model was applied to calculate the age of CO2 at pore close-off depth. The results predict that the oldest firn gas (CO2 age) is located between Dome Fuji, Dome Argos and Vostok at 82degrees E, 83degrees S being 156+/-22 (1sigma) years old with an age distribution of 103 years. At this location an atmospheric trace gas record should be obtained. In this study we show that methyl chloride could be recorded with a predicted length of 187 years (mean age of methyl chloride) as an example for trace gas records at this location. The longest record currently available from firn air is derived at South Pole, being 80 years. Sensitivity tests reveal that the locations with old firn air (East Antarctic Plateau) have an estimated uncertainty (2sigma) for the Modelled mean CO2 age at pore close-off depth of 30% and of about 35% for locations with younger firn air (mean CO2 age typically 40 years). Comparing the modelled age of CO2 at pore close-off depth with directly determined ages at ten sites yielded a correlation coefficient of 0.90 and a slope close to 1, suggesting a high level of confidence for the modelled results in spite of considerable remaining uncertainties.

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