Atmos. Chem. Phys., 13, 11141-11155, 2013
www.atmos-chem-phys.net/13/11141/2013/
doi:10.5194/acp-13-11141-2013
© Author(s) 2013. This work is distributed
under the Creative Commons Attribution 3.0 License.
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
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.

Citation: Kawamura, K., Severinghaus, J. P., Albert, M. R., Courville, Z. R., Fahnestock, M. A., Scambos, T., Shields, E., and Shuman, C. A.: Kinetic fractionation of gases by deep air convection in polar firn, Atmos. Chem. Phys., 13, 11141-11155, doi:10.5194/acp-13-11141-2013, 2013.
 
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