Showing papers by "Anais Orsi published in 2011"

Controls on the movement and composition of firn air at the West Antarctic Ice Sheet Divide

Abstract: We sampled interstitial air from the perennial snowpack (firn) at a site near the West Antarctic Ice Sheet Di- vide (WAIS-D) and analyzed the air samples for a wide vari- ety of gas species and their isotopes. We find limited convec- tive 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 begin- ning at 67 m, and a total firn thickness consistent with pre- dictions 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 accumula- tion rate has a minor impact on firn air composition, caus- ing changes that are comparable to other modeling uncertain- ties and intrinsic sample variability. Furthermore, estimates of 1age (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.