Firn Densification: An Empirical Model

An empirical model of firn densification from the surface to the zone of pore close-off has been constructed. Fundamental rate equations have been derived for the first two stages of densification. In the first stage, for densities less than 0.55 Mg m−3, the densification rate is proportional to the mean annual accumulation times the term (ρ i − ρ), where ρ is the density of the snow and ρ i is the density of pure ice. The densification rate in the second stage, where 0.55 Mg m −3 < ρ < 0.8 Mg m−3, is proportional to the square root of the accumulation rate and to (ρi− ρ). Depth–density and depth–age calculations from this model are compared with observation. Model accumulation rates are within about 20% of values obtained by other techniques. It is suggested that depth intervals of constant density in some Antarctic cores may represent a synchronous event in the 1880 ’s when ten times the normal accumulation fell within a year or two.