CO₂ isotopes as tracers of firn air diffusion and age in an Arctic ice cap with summer melting, Devon Island, Canada

Abstract

[1] Firn air and ice have been sampled and analyzed for trace gases (CO 2 , N 2 O, CH 4 , and CO) and isotopes ( 14 C, 13 C, and 18 O of CO 2 ; 3 H of ice) at 3 m intervals from the surface to the depth of closure at 60 m on the Devon Island Ice Cap, a low-elevation permanent glacier in the Canadian Arctic Islands, to investigate firn diffusion and the effects of summer melting. The 14 CO 2 profile from the permeable firn includes the 1963 thermonuclear peak at a depth of 53.9 ± 1.5 m. The twofold increase and rapid decay that characterize the recent atmospheric history for 14 CO 2 provide a robust atmospheric scenario that is used with a firn air diffusion model to inversely construct the firn diffusivity profile. The results show a permeable but essentially nondiffusive zone from 50 to 60 m depth. A firn-ice age profile was produced from density measurements, and accumulation rates were calibrated with the depth of the 1963 thermonuclear 3 H peak. The average ages for CO 2 in the sampled firn air profile were determined by a new method based on the rate of 18 O exchange between CO 2 and the ice matrix. Calibrated with the 1963 peak for thermonuclear 14 CO 2 , a 21.2-year reaction halftime is calculated for exchange taking place at the firn temperature of À22.8°C on Devon. This gives an average age of 54.9 (+6.0/À12.0) years for firn air at 60 m depth in 140-year-old ice. Thus CO 2 has a mean age 85 years younger than associated ice at the point of occlusion. The measured d18 O CO 2 in firn air provides no indication of alteration by summer melting, which is attributed to a high degree of convective and diffusive flushing of the upper firn as shown by diffusion modeling. This suggests that ice sheets with summer melt layers can reliably preserve atmospheric trace gas signals.

Can the carbon isotopic composition of methane be reconstructed from multi-site firn air measurements?

“…Numerous melt layers have been detected in the DI firn column (e.g. Clark et al, 2007;Martinerie et al, 2009), which strongly reduce molecular diffusivity with depth. The SH data show larger differences between the sites, because the meteorological/glaciological variables (temperature, snow accumulation, snow morphology, etc.)…”

The isotopic record of Northern Hemisphere atmospheric carbon monoxide since 1950: implications for the CO budget

“…The only available firn air CO measurements from the Northern Hemisphere are from samples collected from the summit of Devon Island Ice Cap, Nunavut, Canada (75 • 20 N; 82 • 08 W; 1929 m a.s.l.) in April 1998 (Clark et al, 2007). However, they showed the existence of in-situ CO production at depth, which is likely related to the relatively high temperature of this site, resulting in systematic summer melting and thick melt layers in the firn column, as well as relatively high levels of impurities in Devon Island ice (Clark et al, 2007).…”

A theoretical analysis of the neutron scattering method of measuring snow and ice density

“…It was profiled with fully offset probe in 2000. The densities of core segments taken from a nearby second borehole [ Clark et al , 2007] are plotted against count rates measured at the same depth. They are consistent with the theoretical calibration curve for T = −10°C with R 1 = 0.055 m and an offset factor of 0.88.…”