The attenuation of fast atmospheric CH4 variations recorded in polar ice cores

To reconstruct fast atmospheric trace gas variations from polar ice cores it has to be considered that their amplitudes are attenuated during the enclosure process in the ice. Relevant processes for the attenuation are the molecular diffusion in the open pores of the firn column and the gradual bubble close off in the depth of the transition from firn to ice. These processes depend mainly on temperature and accumulation rate and lead e.g. to a strong attenuation for cold sites with low accumulation rates. With a diffusion and enclosure model it is possible to calculate the attenuation for a single event and to compare ice core records from different sites. We investigate the atmospheric methane (CH4) variation during the cold event 8200 years ago and calculate that its amplitude as recorded in the EPICA Dome C ice core is attenuated to a magnitude between 34% and 59%.