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

[摘要] The age of firn air in Antarctica at pore close-off depth is only known fora few specific sites where firn air has been sampled for analyses. Wepresent a model that calculates the age of firn air at pore close-off depthfor the entire Antarctic continent. The model basically uses fourmeteorological parameters as input (surface temperature, pressure,accumulation rate and wind speed). Using parameterisations for surface snowdensity, pore close-off density and tortuosity, in combination with adensity-depth model and data of a regional atmospheric climate model,distribution of pore close-off depth for the entire Antarctic continent isdetermined. The deepest pore close-off depth was found for the EastAntarctic Plateau near 72° E, 82° S, at 150±15 m(2σ). A firn air diffusion model was applied to calculate the age ofCO₂ at pore close-off depth. The results predict that the oldest firngas (CO₂ age) is located between Dome Fuji, Dome Argos and Vostok at43° E, 78° S being 148±23 (1σ or 38 for2σ) years old. At this location an atmospheric trace gas recordshould be obtained. In this study we show that methyl chloride could berecorded with a predicted length of 125 years as an example for trace gasrecords at this location. The longest record currently available from firnair is derived at South Pole, being 80 years.Sensitivity tests reveal that the locations with old firn air (EastAntarctic Plateau) have an estimated uncertainty (2σ) for themodelled CO₂ age at pore close-off depth of 30% and of about 40%for locations with younger firn air (CO₂ age typically 40 years).Comparing the modelled age of CO₂ at pore close-off depth with directlydetermined ages at seven sites yielded a correlation coefficient of 0.90 anda slope close to 1, suggesting a high level of confidence for the modelledresults in spite of considerable remaining uncertainties.

[学科分类] 大气科学