Migration of methane sulphonate in Antarctic firn and ice

We investigate the seasonal relationship of the sulphur-bearing anions methane sulphonate (MSA−) and non-sea-salt sulphate (nssSO42−) in sections of firn and ice cores from the Antarctic Peninsula and Weddell Sea region of Antarctica. In cores from Dolleman Island and Berkner Island, MSA− has clearly migrated from the summer snow layer, where it is initially deposited, to become concentrated in the winter layer. A similar behavior is evident in a core from the Dyer Plateau, though in deeper layers. Cores from Gomez Nunatak and Beethoven Peninsula show little evidence of relocation of MSA−, though migration at greater depth in the ice sheet cannot be ruled out. In contrast, in all these cores, non-sea-salt sulphate remains in the summer layer. From comparisons between the ice core characteristics and the migration behavior at these sites, we conclude that the movement of MSA− does not occur via percolation and refreezing of meltwater. Simple concentration-driven diffusion is also not a factor, as the MSA− peaks are sharp in the winter layer. The data presented indicate that the movement of MSA− in firn is likely to be linked to the concentration of other ionic species in the snowpack and to the snow accumulation rate. A possible mechanism for the migration of MSA− in the snowpack is via an initial diffusion in the liquid or vapor phase, which is halted by trapping in the winter layer when the MSA− forms a salt with a cation.