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

Abstract

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.

Present and future variations in Antarctic firn air content

“…Also, in the current FDM the surface density is assumed to be constant, while it is uncertain whether and how the density of fresh snow will change in the future. For the present day, this approximation introduces uncertainty as not every accumulation event is the same, but, on average, the calculated values agree well with observations (Kaspers et al, 2004). The meltwater percolation and refreezing scheme in the current FDM does not include heterogeneous percolation ("piping"), a process that is known to be quite widespread on the Greenland ice sheet (Marsh and Woo, 1984;Harper et al, 2012).…”

“…yr −1 ), g is the gravitational acceleration, ρ i is the ice density (917 kg m −3 ), R is the gas constant, and E c and E g are the activation energy constants (Arthern et al, 2010). The density of fresh snow for every location is calculated using the local average surface temperature, accumulation, and wind speed, according to Kaspers et al (2004). A snowmelt module is included to simulate and calculate simple firn hydrology; the amount of percolation, retention, refreezing and runoff of meltwater.…”

An improved semi-empirical model for the densification of Antarctic firn

“…The fresh snow density parameterization of Kaspers et al (2004) is now used in both the steady state and the timedependent firn densification model. As the density of fresh snow is most likely not constant in time, this could be a point of improvement in future research of the time-dependent model.…”

“…New added surface snow is instantly treated as the upper layer of the vertical firn column. The fresh snow density for each grid point is determined by a parameterization of Kaspers et al (2004), based on average annual accumulation (ḃ in mm w.e. yr −1 ), 10 m wind speed (V 10 in m s −1 ) and surface temperature (T s in K), with a slope correction for Antarctica by Helsen et al (2008):…”

Towards quantifying the contribution of the Antarctic ice sheet to global sea level change

“…2 shows the resulting depth of the firn layer. Owing to the wide range of (near) surface climate conditions over the AIS, firn depth shows large spatial variability [10,28,29], which are well reproduced by the model. In the calm, dry and cold interior, densification is slow and the firn-layer thickness exceeds 100 m. In the windier, wetter and milder coastal zone, densification is more rapid and the firn layer shallower, typically 40 − 60 m. In regions with active katabatic winds and low precipitation rate, the firn layer may have been completely removed by snowdrift erosion and/or sublimation, exposing the glacier ice at the surface [25,30].…”