Meltwater-induced relocation of chemical species in Alpine firn

Abstract

In order to interpret glaciochemical records with respect to the history of air pollution, an understanding of post-depositional processes taking place in firn and ice is crucial. In a 13-m firn section of an Alpine ice core, we observed a drastic disturbance of the concentration records of certain ionic species which we attribute to the inflow of meltwater. This observation opened up the possibility to investigate the effects of leaching processes on the chemical composition of a natural firn layer. Species were leached with different efficiencies: Whereas the normal seasonal pattern of the concentrations of NH4+, F-, C1-, and NO3- and of the δ18O values was preserved, concentrations of K+, Na+, Mg2+, Ca2+, and SO42- were significantly decreased. From the patterns of the concentration ratios the elution sequence SO42- > Ca2+ ~ Mg2+ > K+ ~ Na+ ≫ NO-3 > NH4+ ~ F- > Cl- was established, which could be explained by ion rearrangement during snow metamorphism. An incorporation of the species Cl-, F-, NH4+, and NO3- into the ice lattice during grain growth caused their least efficient scavenging by percolating meltwater. In contrast, the predominate exclusion of the ions SO42-, Ca2+, Mg2+, K+, and Na+ from the ice lattice due to their low solubility in ice and the subsequent segregation at grain surfaces led to the fast removal of these species from the firn layer.