CO sub(2) isotopes as tracers of firn air diffusion and age in an Arctic ice cap with summer melting, Devon Island, Canada

Firn air and ice have been sampled and analyzed for trace gases (CO sub(2), N sub(2)O, CH sub(4), and CO) and isotopes ( super(14)C, super(13)C, and super(18)O of CO sub(2); super(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 super(14)CO sub(2) profile from the permeable firn includes the 1963 thermonuclear peak at a depth of 53.9 plus or minus 1.5 m. The twofold increase and rapid decay that characterize the recent atmospheric history for super(14)CO sub(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 super(3)H peak. The average ages for CO sub(2) in the sampled firn air profile were determined by a new method based on the rate of super(18)O exchange between CO sub(2) and the ice matrix. Calibrated with the 1963 peak for thermonuclear super(14)CO sub(2), a 21.2-year reaction halftime is calculated for exchange taking place at the firn temperature of -22.8 degree C on Devon. This gives an average age of 54.9 (+6.0/-12.0) years for firn air at 60 m depth in 140-year-old ice. Thus CO sub(2) has a mean age 85 years younger than associated ice at the point of occlusion. The measured delta super(18)OCO sub(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.