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

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  • 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.
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  • Mitchell, L. E., Buizert, C., Brook, E. J., Breton, D. J., Fegyveresi, J., Baggenstos, D., ... & Ahn, J. (2015). Observing and modeling the influence of layering on bubble trapping in polar firn. Journal of Geophysical Research: Atmospheres, 120(6), 2558-2574. doi:10.1002/2014JD022766
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  • This work was supported by NSF OPP grants 0538578, 0520523, 0538538, 0944343 (to J.P.S.), 0944078 (to M.R.A.), 1142166 (to J.R.M.), and 1043528 (to R.B.A.), the NASA/Oregon Space Grant Consortium grant NNG05GJ85H (to L.E.M.), the NOAA Climate and Global Change Fellowship Program, administered by the University Corporation for Atmospheric Research (to C.B.), and the Polar Academic Program (PAP, PD12010) of Korea Polar Research Institute (KOPRI); the University of Wisconsin-Madison Automatic Weather Station Program for the surface pressure observations from Kominko-Slade AWS (NSF grants ANT-0944018 and ANT-1245663); the WAIS Divide Science Coordination Office at DRI, Reno, Nevada, for the collection and distribution of the WAIS Divide ice core (Kendrick Taylor, NSF grants 0230396, 0440817, 0944348, and 0944266—University of New Hampshire); NSF OPP which funds the Ice Drilling Program Office and Ice Drilling Design and Operations group for coring activities; NSF which funds the National Ice Core Laboratory which curated and processed the core.
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