Post depositional losses of methane sulfonate, nitrate, and chloride at the EPICA deep-drilling site in Dronning Maud Land, Antarctica

We quantifed post depositional losses of methanesulfonate (MSA-), nitrate, and chloride at the EPICA drilling site in Dronning Maud Land (DML, 75°S, 0°E). Analyses of four interme-diate deep firn cores and 13 snow pits were considered. We found that about 26±13% of the once deposited nitrate and typically 51±20% of MSA- were lost, while for chloride no signifi-cant depletion could be observed in firn older than one year. Assuming a first order exponen-tial decay rate, the characteristic e-folding time for MSA- is 6.4±3 a and 19±6 a for nitrate. It turns out that for nitrate and MSA- the typical mean concentrations representative for the last 100 years were reached after 5.4 a and 6.5 a, repectively, indicating that beneath a depth of around 1.2-1.4 m post depositional losses can be neglected. With a simple approach we esti-mated the partial pressure of the corresponding free acids MSA, HNO3, and HCl based on Henrys law assuming that ionic impurities of the bulk ice matrix are localised in a quasi brine layer (QBL). From this approach it stands to reason that a nearly quantitative loss of MSA-, NO3-, and Cl- should occur. We suggest that firn metamorphosis and fractional dissolution of electrolytes in in the QBL and the ice matrix are the decisive processes leading to the observed partial losses.