On the composition of firn air and its dependence on seasonally varying atmospheric boundary conditions and the firn structure

Weiler, Karin (2008). On the composition of firn air and its dependence on seasonally varying atmospheric boundary conditions and the firn structure (Unpublished). (Dissertation, Universität Bern, Philosophisch–naturwissenschaftliche Fakultät, Physikalisches Institut, Abteilung für Klima– und Umweltphysik)

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The dependence of the firn air composition on seasonally varying atmospheric boundary conditions and the firn structure has been studied based on firn air samples from two Antarctic sites, Halley and Kohnen.
The Halley data set consists of six profiles sampled in a two month resolution in the year 2003. Settling of the firn and snow accumulation led to a shift of the depth levels of both, the firn air sampling setup and the thermistors. Prediction of this sample level shift with a 1–dimensional box model developed in the frame of this study coincides with the decrease of the analyzed diurnal temperature cycle.
By forcing the temperature–diffusion model with the surface temperature history of Halley, temperature oscillations in the firn column originating from seasonal atmospheric temperature variations are reasonably well reproduced. Thus, the dependence of the thermal conductivity on firn density as derived by Schwander et al. [1997] is sufficient also on the seasonal scale.
In order to simulate thermal fractionation of the different gas species, thermal diffusion factors αT are determined based on two different model approaches and the firn air data set: αT = 3.727 · 10−3 for the fractionation of 15N14N in N2, αT = 12.480·10−3 for 18O16O in air, αT = 7.321·10−3 for 16O2 in air, αT = 32.241·10−3 for 36Ar in air, αT = 45.516 · 10−3 for 40Ar in air, and 57.371 · 10−3 ≤ αT ≤ 67.811 · 10−3 for CO2 in air.
Seasonal oscillations observed in the firn air profiles of CO2 and the isotopic species δ15N, δ18O, and δ36Ar act as expected from theory during the entire year. The imprint of both, seasonal atmospheric temperature variations and atmospheric concentration oscillations is well reproduced by the diffusion–temperature model. Neither a seasonal variability of ±0.5 ppmv in the atmospheric CO2 concentration nor thermal diffusion forced by seasonal atmospheric temperature oscillations will lead to considerable changes of the CO2 concentration in air bubbles finally occluded in the ice.
An overestimation of the measured February amplitude of the isotopic profiles (δ15N, δ18O δ36Ar) by the diffusion–temperature model can be explained by surface convection. Based on a parametrisation approach of Severinghaus et al. [2001] and Kawamura et al. [2006], a convective column height of 0.28 ≤ zconv ≤ 0.43 m is derived for Halley firn of 2003. Convection turned out to be active between end of December and mid of February. The calculation of surface convection as Darcian air flow through the firn driven by a pressure field originating from dunes on the surface [Colbeck, 1989] confirms the size of the convective layer. However, the temporal forcing and monthly mean wind speeds measured in Halley do not clearly coincide.
Within the frame of thesis, firn and firn air have been sampled at the deep drilling site Kohnen in Austral summer 2005/06. Independently from the container type samples have a good quality with respect to the already analyzed species (isotopes, elemental ratios, CO2, noble gases and ozone depleting substances). The usage of a bladder tube of distinctly smaller diameter (by 15 mm) than the hole turned out to be problematic and thus, is not recommended for further experiments.
From the average of the 20 m temperature obtained from five borehole temperature profiles measured between November 2005 and January 2006, an annual mean temperature of –44.4°C could be derived for the Kohnen site.
Forcing the temperature model with atmospheric temperature data obtained from an automatic weather station near Kohnen Station leads to a good agreement between simulations and the bore hole temperatures. CO2 firn air data agree well with simulations based on the atmospheric CO2 history from South Pole and the measured temperatures. The best match between modeled and measured CO2 profiles was obtained by selecting the tortuosity parameters α and β to α = 0.85 and β = 2.5. The close–off density is determined to ρco = 825 kg m−3, which is slightly lower than the mean air isolation level of 833.4 kg m−3 derived according to Martinerie et al. [1992, 1994]. ρco corresponds to a close–off depth zco = 87.6 m. Effective diffusivity is determined to become terminated at the lock–in depth zli = 87.4 m. Kohnen firn has a non–diffusive zone of about 7 m length, where the deepest firn air sample is at zmax = 94.9 m.
CO2 firn air data agree with the simulation disregarding thermal diffusion rather than showing a thermal signal. Thus, Kohnen firn seems to have a convective zone attenuating the seasonal thermal amplitude.
The mean gas age at the close–off level is estimated by combining the mean gas ages of air originating from the open pore space and the air occluded in air bubbles. In order to determine the amount of occluded air, a mass balance is implemented in the diffusion–temperature model. Air enclosure is simulated from two different temporally constant enclosure functions. According to this approach a combined mean CO2 gas age at the close–off level of (65 +75/–50) or (74 +85/–56) years, respectively, is retrieved for present–day Kohnen firn. Depending on the enclosure functions the width of corresponding age distribution functions representing air occluded in air bubbles lies between ≈ 47 and ≈ 52 years. The width of the age distribution of the air originating from the open pore volume is ≈ 17 years.
Forcing effective diffusivity De with the open porosity sop instead of the total porosity s, leads to about 2 ppmv lower CO2 concentrations simulated for the depth range around and below the close–off region. Accordingly, the mean gas age is calculated to be 2 to 5 years lower than for De ∼ s.
The ice age at the close–off level is determined to (905 ± 7) years from assigning documented volcanic events to peaks in the electrical conductivity record of the ice. Based on this depth–to–age relation an annual mean accumulation of 65 kg m−2 a−1 could be derived for the Kohnen site.
The ∆age, i.e. the difference between ice age and combined mean gas age, varies between (831 +85/–56) and (840 +76/–51) years for Kohnen firn subjected topresent–day conditions.
To the best of current knowledge Kohnen firn provides the oldest firn air that has ever been sampled so far. The age of the firn air at zmax = 94.9 m is determined to (114 +13/–10) years. The occurrence of such old firn air is attributed to a distinct stratification of Kohnen firn, which originates from a wind driven surface orography and is preserved down to the close–off region.

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