Controls on the movement and composition of firn air at the West Antarctic Ice Sheet Divide 1Department of Physics and Astronomy, Bowdoin College, 8800 College Station, Brunswick ME, 04011, USA
08 Nov 2011
2Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093-0244, USA
3Department of Earth System Science, University of California, Irvine, CA 92697-3100, USA
4National Oceanic and Atmospheric Administration, Earth System Research Laboratory, Global Monitoring Division, Boulder, CO 80305, USA
5Earth and Environment Systems Institute, Pennsylvania State University, University Park, PA 16802, USA
*now at: Dept. of Atmospheric Sciences, University of Washington, Seattle, WA 98195, USA
Received: 02 Jun 2011 – Published in Atmos. Chem. Phys. Discuss.: 30 Jun 2011Abstract. We sampled interstitial air from the perennial snowpack (firn)
at a site near the West Antarctic Ice Sheet Divide (WAIS-D) and analyzed
the air samples for a wide variety of gas species and their isotopes. We find
limited convective influence (1.4–5.2 m, depending on detection
method) in the shallow firn,
gravitational enrichment of heavy species throughout the diffusive column
in general agreement with theoretical expectations, a ~10 m thick
lock-in zone beginning at ~67 m, and a total firn
thickness consistent with predictions of
Kaspers et al. (2004). Our modeling work shows that the air has an age spread
(spectral width) of 4.8 yr for CO2 at the firn-ice transition.
We also find that advection
of firn air due to the 22 cm yr−1 ice-equivalent accumulation rate has
a minor impact on firn air composition, causing changes that are comparable
to other modeling uncertainties
and intrinsic sample variability.
Furthermore, estimates of Δage
(the gas age/ice age difference) at WAIS-D
appear to be largely unaffected by bubble closure above the lock-in zone.
Within the lock-in zone, small gas species and their isotopes show evidence
of size-dependent fractionation due to permeation through the ice lattice
with a size threshold of 0.36 nm, as at other sites.
We also see an unequivocal and unprecedented
signal of oxygen isotope fractionation within the lock-in zone,
which we interpret as the mass-dependent expression of a size-dependent
Revised: 07 Oct 2011 – Accepted: 10 Oct 2011 – Published: 08 Nov 2011
Citation: Battle, M. O., Severinghaus, J. P., Sofen, E. D., Plotkin, D., Orsi, A. J., Aydin, M., Montzka, S. A., Sowers, T., and Tans, P. P.: Controls on the movement and composition of firn air at the West Antarctic Ice Sheet Divide, Atmos. Chem. Phys., 11, 11007-11021, doi:10.5194/acp-11-11007-2011, 2011.