Observing and modeling the influence of layering on bubble trapping in polar firn

Abstract

Interpretation of ice core trace gas records depends on an accurate understanding of the processes that smooth the atmospheric signal in the firn. Much work has been done to understand the processes affecting air transport in the open pores of the firn, but a paucity of data from air trapped in bubbles in the firn-ice transition region has limited the ability to constrain the effect of bubble closure processes. Here we present high-resolution measurements of firn density, methane concentrations, nitrogen isotopes, and total air content that show layering in the firn-ice transition region at the West Antarctic Ice Sheet (WAIS) Divide ice core site. Using the notion that bubble trapping is a stochastic process, we derive a new parameterization for closed porosity that incorporates the effects of layering in a steady state firn modeling approach. We include the process of bubble trapping into an open-porosity firn air transport model and obtain a good fit to the firn core data. We find that layering broadens the depth range over which bubbles are trapped, widens the modeled gas age distribution of air in closed bubbles, reduces the mean gas age of air in closed bubbles, and introduces stratigraphic irregularities in the gas age scale that have a peak-to-peak variability of~10 years at WAIS Divide. For a more complete understanding of gas occlusion and its impact on ice core records, we suggest that this experiment be repeated at sites climatically different from WAIS Divide, for example, on the East Antarctic plateau.

Analytical constraints on layered gas trapping and smoothing of atmospheric variability in ice under low-accumulation conditions

“…5). The weaker-than-expected smoothing during DO-17 at Vostok could be due to the presence of a strong layering preventing air renewal and mixing, as suggested in Mitchell et al (2015).…”

“…For example, pore closure anomalies have been associated with tortuosity anomalies, with more tortuous layers closing earlier (Gregory et al, 2014), or to density anomalies, with denser layers closing earlier Mitchell et al, 2015;Rhodes et al, 2016). In this section we used the latter hypothesis supported by observed relationships between local density and closed porosity (e.g., Stauffer et al, 1985;Mitchell et al, 2015) to test whether density-driven anomalies could result in artifacts as observed in the Vostok methane record.…”

“…Due to heterogeneities in firn density and porosity, an ice layer may undergo early gas trapping Rhodes et al, 2013;Mitchell et al, 2015;Rhodes et al, 2016). Thus during gas trapping, the corresponding layer is at a more advanced state of closure than the surrounding bulk layers.…”

“…For modern ice cores, GADs can be estimated with gas transport models constrained by firn air composition data Witrant et al, 2012). However, the results directly depend on the closed versus total porosity parameterization used, which is insufficiently constrained (e.g., Mitchell et al, 2015). We performed a comparison of our optimized GAD for Vostok during DO-17 with the lognormal fit to a GAD constrained with modern-condition firn air measurements at Vostok (Witrant et al, 2012, in yellow in Fig.…”

“…Working on ice cores and firn from highaccumulation sites, Etheridge et al (1992), Mitchell et al (2015), and Rhodes et al (2016) have discussed the influence of centimeter-scale physical variability in firn on recorded gas concentrations. They argue that physical heterogeneities can lead to variations in closure depth for juxtaposed ice layers.…”