Figure 2.

δ¹⁵N (Left) and CH₄ (Right) vs. depth in firn air from the GISP2 site at Summit, Greenland. The subsurface maximum in δ¹⁵N is due to thermal fractionation (¹⁵N is enriched at 5- and 10-m depth because the firn at these depths, which remains at the mean annual temperature, is colder than air at the surface during summertime, when sampling was done). The increase below 20-m depth is due to gravitational fractionation. CH₄ decreases very slowly to the top of the bubble closeoff zone at 70-m depth. Below it decreases very rapidly because gases cannot migrate vertically and the age of the gas in the firn increases as rapidly as the age of the ice (about 4 yr/m).