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

• Published in: Journal of Geophysical Research
• Main Authors: D. Clark, Ian, Henderson, Lori, Chappellaz, Jérôme, Fisher, David, Koerner, Roy, E. J. Worthy, Douglas, Kotzer, Tom, Norman, Ann-Lise, Barnola, Jean-Marc
• Other Authors: Ottawa-Carleton Geoscience Centre, University of Ottawa Ottawa, Laboratoire de glaciologie et géophysique de l'environnement (LGGE), Centre National de la Recherche Scientifique (CNRS)-Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire des Sciences de l'Univers de Grenoble (OSUG), Université Joseph Fourier - Grenoble 1 (UJF)-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP )-Institut national des sciences de l'Univers (INSU - CNRS)-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Université Savoie Mont Blanc (USMB Université de Savoie Université de Chambéry )-Centre National de la Recherche Scientifique (CNRS)-Université Joseph Fourier - Grenoble 1 (UJF)-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP )-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Université Savoie Mont Blanc (USMB Université de Savoie Université de Chambéry )-Centre National de la Recherche Scientifique (CNRS), Terrain Sciences Ottawa, Geological Survey of Canada Ottawa (GSC Central & Northern Canada), Geological Survey of Canada - Office (GSC), Natural Resources Canada (NRCan)-Natural Resources Canada (NRCan)-Geological Survey of Canada - Office (GSC), Natural Resources Canada (NRCan)-Natural Resources Canada (NRCan), Service Météorologique du Canada (SMC), Environment and Climate Change Canada, Canadian Light Source, University of Saskatchewan Saskatoon (U of S), Department of Physics and Astronomy Calgary, University of Calgary, European Commission : projects FIRETRACC/100 (ENV4-CT97-0406) and CRYOSTAT (EVK2-CT2001-00116)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2007
• Subjects: firn air diffusion; CO2; firn air age; [SDU.STU.GL]Sciences of the Universe [physics]/Earth Sciences/Glaciology; Arctic; Canada; Devon Island; glacier*; Ice cap
• Online Access: https://hal-insu.archives-ouvertes.fr/insu-00375906; https://hal-insu.archives-ouvertes.fr/insu-00375906/document; https://hal-insu.archives-ouvertes.fr/insu-00375906/file/ark%20_67375_WNG-9ZGN9ZV5-4.pdf; https://doi.org/10.1029/2006JD007471

International audience Firn air and ice have been sampled and analyzed for trace gases (CO2, N2O, CH4, and CO) and isotopes (14C, 13C, and 18O of CO2; 3H 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 14CO2 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 14CO2 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 3H peak. The average ages for CO2 in the sampled firn air profile were determined by a new method based on the rate of 18O exchange between CO2 and the ice matrix. Calibrated with the 1963 peak for thermonuclear 14CO2, a 21.2-year reaction halftime is calculated for exchange taking place at the firn temperature of −22.8°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 CO2 has a mean age 85 years younger than associated ice at the point of occlusion. The measured δ 18OCO2 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.