Articles | Volume 13, issue 21
Atmos. Chem. Phys., 13, 11141–11155, 2013

Special issue: Firn air: archive of the recent atmosphere

Atmos. Chem. Phys., 13, 11141–11155, 2013

Research article 15 Nov 2013

Research article | 15 Nov 2013

Kinetic fractionation of gases by deep air convection in polar firn

K. Kawamura1,2, J. P. Severinghaus3, M. R. Albert4,5, Z. R. Courville4,5, M. A. Fahnestock6, T. Scambos7, E. Shields3, and C. A. Shuman8 K. Kawamura et al.
  • 1National Institute for Polar Research, Tachikawa, Tokyo, Japan
  • 2Institute of Biogeosciences, Japan Agency for Marine-Earth Science and Technology, Natsushima-cho, Yokosuka, Japan
  • 3Scripps Institution of Oceanography, University of California, San Diego, CA, USA
  • 4Thayer School of Engineering, Dartmouth College, Hanover, NH, USA
  • 5Cryospheric and Terrestrial Sciences Division, Cold Regions Research and Engineering Laboratory, Hanover, NH, USA
  • 6Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK, USA
  • 7National Snow and Ice Data Center, Boulder, CO, USA
  • 8Cryospheric Sciences Branch, NASA Goddard Space Flight Center, Greenbelt, MD, USA

Abstract. A previously unrecognized type of gas fractionation occurs in firn air columns subjected to intense convection. It is a form of kinetic fractionation that depends on the fact that different gases have different molecular diffusivities. Convective mixing continually disturbs diffusive equilibrium, and gases diffuse back toward diffusive equilibrium under the influence of gravity and thermal gradients. In near-surface firn where convection and diffusion compete as gas transport mechanisms, slow-diffusing gases such as krypton (Kr) and xenon (Xe) are more heavily impacted by convection than fast diffusing gases such as nitrogen (N2) and argon (Ar), and the signals are preserved in deep firn and ice. We show a simple theory that predicts this kinetic effect, and the theory is confirmed by observations using a newly-developed Kr and Xe stable isotope system in air samples from the Megadunes field site on the East Antarctic plateau. Numerical simulations confirm the effect's magnitude at this site. A main purpose of this work is to support the development of a proxy indicator of past convection in firn, for use in ice-core gas records. To this aim, we also show with the simulations that the magnitude of the kinetic effect is fairly insensitive to the exact profile of convective strength, if the overall thickness of the convective zone is kept constant. These results suggest that it may be feasible to test for the existence of an extremely deep (~30–40 m) convective zone, which has been hypothesized for glacial maxima, by future ice-core measurements.

Final-revised paper