MODELING AIR MOVEMENT AND BUBBLE TRAPPING IN FIRN

A finite difference model for gas diffusion and bubble trapping in fir n is described. The model uses prescribed profiles of density, open an d closed porosity, and diffusivity to determine the diffusion and trap ping processes. The model is calibrated and tested by using measured a ir composition in the firn at the DE08-2 site on Law Dome, Antarctica. In particular, we focus on carbon dioxide (CO2), methane (CH4), and s ulfur hexafluoride (SF6), which have well-determined atmospheric recor ds (CO2 since 1958, CH4 since 1983, and SF6 since 1978). These trace g ases are used to tune the diffusivity-porosity relationship, which is the most uncertain of the model inputs. Modeled trace gas profiles in the DE08-2 firn are improved if allowance is made for reduced diffusio n through the most prominent DE08-2 melt layer from the summer of 1989 /1990. The relatively rapid growth rate of SF6 in the atmosphere permi ts good definition of the diffusion reduction due to the melt layer (a bout 80%). The model quantifies the smoothing effect of the firn diffu sion and bubble trapping on atmospheric signals. Gravitational separat ion in the firn is investigated by comparison of modeled delta(15)N(2) With observations. The model is used to calculate the isotopic diffus ion correction for delta(13)CO(2) and delta(13)CH(4). This corrects fo r the fractionating effects of the firn diffusion process on the diffe rent isotopes. The diffusion and gravitational corrections are critica l at the measurement precision currently being obtained; for delta(13) CO(2) the diffusion correction is up to about 10 times the current mea surement precision. The diffusion correction is even more significant for delta(13)CH(4); at over 1 parts per thousand, at the bottom of the firn it is more than double the change over the last decade. The full y corrected delta(13)CO(2) record from the DE08-2 firn is compared wit h the history of Cape Grim direct atmospheric measurements with excell ent agreement.