Here I propose that annual layering of some gas ratios is indeed present in glacial ice. The simplest interpretation of these data are that summer layers have lower density and retain permeability longer than winter layers, and thus permit air to be sampled in the transition region. Under this scenario, oxygen and argon are preferentially excluded from the bubbles as they close off, and accumulate in the residual firn air in the summer layers. When the summer layers ultimately close off, bubbles in the summer layers should be enriched in oxygen and argon. Samples of firn air from Siple Dome, Antarctica, and bubble air taken from ice samples from the same drill hole, lend support to this hypothesis.
Argon shows similar patterns with smaller magnitudes.
The mechanism of preferential exclusion is unknown but is probably dependent on molecular size, as O2 is smaller than N2, and Ar is intermediate.