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The plot shows inert-gas air composition of the firn versus depth. The firn in polar regions is porous and permeable, allowing the air in the pore spaces to mix slowly with the atmosphere by molecular diffusion. Two physical processes cause the composition to change with depth (fractionate): gravitational settling and thermal diffusion. Gravity causes the heavier gases to settle to the bottom of the firn layer. This process is called the barometric equation, which predicts the composition very precisely. That prediction is shown by the model curves (solid lines).
Thermal diffusion is the result of heavier gases migrating down temperature gradients in the firn, which arise from seasonal temperature changes at the surface. This process affects gases only in the top 20 m or so, because no temperature gradients exist below that. Note that the opposite effect is observed in winter as in summer.
In the chart above, δ15N is the relative deviation of the 15N/14N ratio with respect to the atmosphere, multiplied by 1000 (units of per mil). δ40Ar is the corresponding quantity for the 40Ar/36Ar ratio. δKr is for the Kr/Ar ratio.