The firn zone: Transforming snow to ice
In the firn zone, the air can move relatively freely and therefore exchanges with the atmospheric air. This has two important consequences:
The concentration of a gas species in the firn changes relative to the atmospheric concentration due to physical processes (and in case of reactive gases also chemical processes).
The age of the gas in an occluded air bubble is less than the age of the surrounding ice. This age difference (the so-called Δage) depends on temperature and the amount of snowfall. The value of Δage can range from a few hundred years to several thousand years.
When interpreting ice core measurements performed on the ice itself together with measurements on the gas in the bubbles, knowledge of Δage is important. Dedicated firn densification models, based on empirical studies of present-day Greenland and Antarctic conditions, are used to calculate Δage. These models use information about how the transformation from snow to ice takes place.
The density of the firn at the surface is around 350 kg/m3 which correspond to a porosity of about 60-70% (meaning that 60-70% of the volume is air). The firn is compacted by the weight of the overlying layers and as a result of water vapour diffusion. From the surface and until a density of about 550 kg/m3 is reached, the transformation of snow to ice is dominated by the rearrangement of the firn grains in order to get to a more dense packing. At even larger depths (and densities), simple rearrangement of the grains leads to no significant density increase. There sintering and plastic deformation become the most important transformation processes. When a density of 800 kg/m3 is reached, the pores are gradually pinched off and form bubbles in the ice. This zone is called firn-ice transition and spans about the lowest 10% of the total firn column. Depending on the site (especially the temperature and amount of snowfall), the firn zone can be between 50 and 150 m thick, and Δage can range from a few hundred years (e.g. in Southern Greenland) to several thousand years (Central Antarctica). Both the firn layer thickness and Δage were significantly larger during periods of colder climate with reduced accumulation and temperature.
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