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
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
Abstract. 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 fractionation process.