Loss of volatile acid species from upper firn layers at Vostok, Antarctica

Significant natural artifacts in ice chemical records have been pointed out in recent preliminary glaciochemical studies carried out in central Antarc tic areas with very low snow accumulation rates (generally less than 5 g cm (-2) yr(-1)). Several deep drilling operations are underway in these region s for long-term paleoclimatic reconstructions. A detailed glaciochemical st udy has been carried out at Vostok Station in order to investigate post dep osition changes of ion concentrations in the snow and firn layers. The resu lts show that, in general, concentration profiles of species such as Cl, F, and NO3, partly deposited as gases, exhibit a rapid decrease in the first few meters, indicating that a fraction, sometimes major, of these compounds is expelled back in the atmosphere after deposition. Some redeposition pro cess of the gases is likely in the upper firn layers. Surprisingly, a simil ar effect is found for methanesulfonate (MS), suggesting that this compound could have a gaseous component in central Antarctic regions. The data also show that Cl, F, NO,, and MSA may be slowly but significantly displaced in the firn layers by high sulfuric acid levels of volcanic origin. The drast ic changes observed in the surface snow layers may severely question curren t interpretations of certain chemical data recovered in these areas and poi nt out an urgent need for new field and laboratory experiments on the air-t o-ice transfer processes prevailing under central Antarctic conditions.