Kinetic fractionation of gases by deep air convection in polar firn

K Kawamura,M R Albert,Z R Courville,E Shields,T Scambos,M A Fahnestock,J P Severinghaus,C A Shuman

doi:10.5194/acp-13-11141-2013

Abstract

Abstract. A previously unrecognized type of gas fractionation occurs in firn air columns subjected to intense convection. It is a form of kinetic fractionation that depends on the fact that different gases have different molecular diffusivities. Convective mixing continually disturbs diffusive equilibrium, and gases diffuse back toward diffusive equilibrium under the influence of gravity and thermal gradients. In near-surface firn where convection and diffusion compete as gas transport mechanisms, slow-diffusing gases such as krypton (Kr) and xenon (Xe) are more heavily impacted by convection than fast diffusing gases such as nitrogen (N2) and argon (Ar), and the signals are preserved in deep firn and ice. We show a simple theory that predicts this kinetic effect, and the theory is confirmed by observations using a newly-developed Kr and Xe stable isotope system in air samples from the Megadunes field site on the East Antarctic plateau. Numerical simulations confirm the effect's magnitude at this site. A main purpose of this work is to support the development of a proxy indicator of past convection in firn, for use in ice-core gas records. To this aim, we also show with the simulations that the magnitude of the kinetic effect is fairly insensitive to the exact profile of convective strength, if the overall thickness of the convective zone is kept constant. These results suggest that it may be feasible to test for the existence of an extremely deep (~30–40 m) convective zone, which has been hypothesized for glacial maxima, by future ice-core measurements.

Highlights

Trapped air in polar ice cores provides a unique archive of past atmospheric composition, and permits comparison of the past burden of greenhouse gases and associated climatic variations
In contrast to gravitational and thermal fractionation, which are equilibrium processes, this third process arises from the disequilibrium caused by convective mixing. It is a form of kinetic fractionation that depends on the fact that different gases have different molecular diffusivities
Air samples were withdrawn from the Megadunes firn in January 2004 as part of a multi-investigator, multidisciplinary field campaign to better understand the genesis of megadunes and their possible implications for ice core paleorecords

Summary

Introduction

Trapped air in polar ice cores provides a unique archive of past atmospheric composition, and permits comparison of the past burden of greenhouse gases and associated climatic variations. Interpreting the gas records requires understanding the processes that modify the gas composition in the ∼100-m-thick permeable firn layer on top of polar ice sheets, because the gas trapped in bubbles originates as air in the base of the firn (Schwander, 1989) These processes include gravitational fractionation of heavy isotopes and gases, due to settling of the heavier components under the influence of molecular diffusion in the stagnant portion of the firn (Schwander et al, 1989; Sowers et al, 1989). A second diffusive process is thermal fractionation, in which gases separate due to temperature gradients in the stagnant air column (Severinghaus et al, 1998; Lang et al, 1999; Landais et al, 2004; Grachev and Severinghaus, 2005) This portion of the firn is known as the “diffusive column” (Sowers et al, 1992).

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