The transformation of snow to firn and eventually ice in polar regions involves the enclosure of atmospheric air, first in porous channels, before these form individual air bubbles. The hydrostatic pressure increases with depth and, depending on the temperature of the ice sheet, reaches a point from which on air clathrates are formed. In contrast to hexagonal ice containing air bubbles, in which air is in a gaseous state, clathrates are crystalline inclusion compounds consisting of a cubic lattice with a cage-like arrangement of water molecules and individual nitrogen and oxygen, presumably also trace gas molecules, occluded in the cages.

In a range of several hundred metres called the transition zone (700 - 1300 m of depth in the GRIP deep ice core), air bubbles and clathrates coexist due to the long period of clathrate nucleation, before air bubbles, which appear black in transmitted light, cease to exist. Clathrates are found as clear inclusions, which, due to their different refractive index from that of ice, can be found in microscopic images with diameters ranging from ca. 20 - 300 µm.
Presumably because of different pressure conditions between oxygen and nitrogen clathrates, oxygen is enriched in the clathrates at the beginning of the transition zone, whereas nitrogen is enriched in the remaining bubbles, as determined by Raman-spectroscopy. At the end of the transition zone, we observe nitrogen/oxygen ratios close the atmospheric value of 3.7 on average.1
Yet clathrates are not stationary after their first formation. They first appear opaque in transmitted light, preferentially round-shaped and bubble-sized (ca. 200 µm), before they segregate, forming smaller fragments, or metamorphose into clear specimens with facets and straight edges.

The implication of fractionation effects described above is that the air captured in clathrates is not necessarily representative of the atmosphere at the time of deposition of the surrounding ice. On the other hand, it has been shown that the number of clathrates is correlated with climatic events, with more and correspondingly smaller clathrates at colder stages.2 Thus, the interpretation of gas concentration from extraction procedures has to be undertaken with great care.