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We sampled interstitial air from the perennial snowpack (firn)at a site near the West Antarctic Ice Sheet Divide (WAIS-D) and analyzedthe air samples for a wide variety of gas species and their isotopes.  We findlimited convective influence (1.4–5.2 m, depending on detectionmethod) in the shallow firn,gravitational enrichment of heavy species throughout the diffusive columnin general agreement with theoretical expectations, a ~10 m thicklock-in zone beginning at ~67 m, and a total firnthickness consistent with predictions ofKaspers et al. (2004).  Our modeling work shows that the air has an age spread(spectral width) of 4.8 yr for CO<sub>2</sub> at the firn-ice transition.We also find that advectionof firn air due to the 22 cm yr<sup>−1</sup> ice-equivalent accumulation rate hasa minor impact on firn air composition, causing changes that are comparableto other modeling uncertaintiesand intrinsic sample variability.Furthermore, estimates of Δage(the gas age/ice age difference) at WAIS-Dappear to be largely unaffected by bubble closure above the lock-in zone.Within the lock-in zone, small gas species and their isotopes show evidenceof size-dependent fractionation due to permeation through the ice latticewith a size threshold of 0.36 nm, as at other sites.We also see an unequivocal and unprecedentedsignal of oxygen isotope fractionation  within the lock-in zone,which we interpret as the mass-dependent expression of a size-dependentfractionation process

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