Fluorocarbon Tracers of the Age of Air in Alpine Firn

As snow accumulates on ice sheets it is slowly compressed and sintered, ultimately forming solid ice, typically at depths of between 50 and 100 m. Above this depth, in the unsolidified firn, the interstitial air is still in connection with the surface atmosphere. Molecular diffusion, however, retards the attainment of equilibrium at the base of the firn with any changes in surface concentration (Raynaud et al. (1993); Schwander et al. (1993)). As a result, concentrations at depth in the firn may reflect atmospheric conditions years, or even decades, earlier.