Laboratory study of the migration of methane sulphonate in firn

Two 10 cm lengths of firn core from the Dyer Plateau (70°31 ′S, 65°01 ′W), Antarctic Peninsula, were used to carry out a laboratory experiment to investigate the migration of methane sulphonate, the anion of methane sulphonic acid (MSA), in natural firn. Each length was cut vertically into four pieces, and a dopant solution containing Cl − , NO 3 − , SO 4 2− , F − and MSA − pipetted onto the top of three sections, the fourth being kept as a blank. The doped sections were stored vertically, with the doped end uppermost, for 8 months at a range of temperatures (nominally –5°, –10° and –22°C) before subsampling at 1 cm resolution and analysis by ion chromatography. The two firn lengths were treated identically and the results were consistent. Profiles of the doped firn sections showed that C1 − , NO 3 and SO 4 2− remained in the uppermost subsample, although the NO 3 − concentrations were variable compared to the blank. The F − profile shows slightly elevated concentrations in the second sample down compared to the blank, at temperatures of -10°C and above. The MSA − showed higher concentrations in the second, third and fourth samples down at -10°C and above, which indicates that some percolated downwards from its original position at the top of the core. This experiment shows that MSA is mobile in warm firn even over a short period of time. We propose that the mechanism for the mobility of MSA − in natural firn is via liquid MSA drainage, though we cannot yet discount vapour phase transport.