The age of the air in the firn and the ice at Summit, Greenland

Abstract

In the framework of the Eurocore drilling project 1989 at Summit, Central Greenland, air samples were collected from a drill hole in the firn at several depth levels. The samples have been analyzed for 85Kr, CO2, CH4, CFC's (F-11 and F-12) contents and for the isotopic composition of nitrogen and oxygen. The measured data are compared with the results of a diffusion model. CO2, for example, takes 12 years (with a standard deviation of 7.5 years) to diffuse from the surface to the firn-ice transition under the present climatic conditions at Summit. The difference between the age of the ice and the mean age of the occluded air is approximately 210 years. An enrichment of the heavier isotopes due to separation by gravity has been observed. Molecular diffusion alone suffices to explain the observed concentration trends in the firn column. We can, however, not exclude the existence of a convective zone below the surface. We have also measured the density and closed porosity along the firn core that has been retrieved when drilling the hole. The porosity data reveal that the air is occluded into bubbles mainly between 65 and 80 m, corresponding to densities of 790 and 830 kg/m 3, respectively.

Polar firn air reveals large-scale impact of anthropogenic mercury emissions during the 1970s

“…Note that this removed air has to be replaced by air coming from the upper parts of the firn, thus creating a downward flux. We set temperature and accumulation rate to their present-day values [respectively, 241°K (20) and 224 kg m Ϫ2 year Ϫ1 (22)] and assumed these rates were constant throughout the model run. We used firn structure parameters (density and closed porosity) from the EUROCORE drilling (20) (see Fig.…”

“…Due to physical effects and, in some instances, chemical effects, the composition of firn air at any depth does not correspond exactly to the atmospheric composition at some time in the past (20). Thus, evaluation of all physical and chemical processes that may affect GEM between the shallow snowpack and the deep firn is required to correctly infer an atmospheric history from firn air concentrations (reported in Fig.…”

“…At still greater depths, when the overburden is sufficient, all pores become closed, and air can no longer be extracted. This defined the transition from firn to ice also referred to as close-off and is located at Ϸ79.5 m depth at Summit Station (20).…”

Constraining N<sub>2</sub>O emissions since 1940 using firn air isotope measurements in both hemispheres

“…Information on the locations is provided in Table 1. The firn air collection procedure is described in detail by Schwander et al (1993). Here a brief description is presented.…”

Post-coring entrapment of modern air in some shallow ice cores collected near the firn-ice transition: evidence from CFC-12 measurements in Antarctic firn air and ice cores

“…The decrease in permeability is most rapid near the bottom of the firn, in the lock-in zone, where the bubble close-off process starts and the permeability drops to near zero. At some depth below the lock-in zone, the transition of firn to impermeable ice is complete and all air is occluded in bubbles (Battle et al, 1996;Schwander et al, 1993;Severinghaus and Battle, 2006). This full close-off Published by Copernicus Publications on behalf of the European Geosciences Union.…”