Abstract
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.
Highlights
Air extracted from the perennial snowpack is scientifically interesting for two reasons: it provides insight into the processes that determine the composition of air, both in the firn, and in the bubbles trapped in the underlying ice, and it provides a record of atmospheric composition in the recent past.In December of 2005, we traveled to the US West Antarctic Ice Sheet Divide (WAIS-D) research station, where a deep ice coring program was commencing
The primary goal of this work was to quantify the nature of firn air movement at the site, thereby aiding interpretations of trapped gases extracted from the deep core
Uncertainties associated with the tuning of the diffusivitydepth profile can be reduced by more sophisticated multispecies techniques (Buizert et al, 2011), but real variability in the composition of firn air extracted from the lock-in zone is a more difficult problem
Summary
Air extracted from the perennial snowpack (firn) is scientifically interesting for two reasons: it provides insight into the processes that determine the composition of air, both in the firn, and in the bubbles trapped in the underlying ice, and it provides a record of atmospheric composition in the recent past. The measurements provide atmospheric histories of intermediate length and resolution for a wide range of species, allowing comparisons with existing records from other sites and emissions models. In the manuscript that follows, we focus on the use of gas measurements to illuminate the physical processes controlling the composition of air in the firn and in bubbles in the underlying ice. We begin with an overview of air movement in the firn and describe the sampling site, collection methods, and modeling approach. We consider fractionation of gases during the formation of bubbles, and briefly discuss the relation between the formation of bubbles and determination of the gas age/ice age difference ( age)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
