Model calculations of the age of firn air across the Antarctic continent 1Inst. for Marine and Atmospheric research Utrecht, Utrecht Univ., Princetonplein 5, 3584 CC Utrecht, The Netherlands
2Physics Institute, University of Bern, Switzerland
3British Antarctic Survey, Cambridge, UK
4Max Planck Institute for Chemistry, Mainz, Germany
Abstract. 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 72° E, 82° S, at 150±15 m
(2σ). 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
43° E, 78° S being 148±23 (1σ or 38 for
2σ) years old. 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 125 years 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 (2σ) for the
modelled 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 directly
determined ages at seven 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.
Citation: Kaspers, K. A., van de Wal, R. S. W., van den Broeke, M. R., Schwander, J., van Lipzig, N. P. M., and Brenninkmeijer, C. A. M.: Model calculations of the age of firn air across the Antarctic continent, Atmos. Chem. Phys., 4, 1365-1380, doi:10.5194/acp-4-1365-2004, 2004.