The effect of postdepositional process on the chemical profiles of snow pits in the percolation zone

Two snow pits, 3 and 2.3 m in depth, were successively sampled at the same spot (4230 m a.s.l.) from Glacier no. 1 at the head of Ürümqi River, Tien Shan, China, on June 4 and 17, 1996, respectively. All samples were analyzed for major anions (SO42−, NO3− and Cl−) and oxygen isotope ratios (δ18O). Similar mean Cl− concentrations are observed for the two snow pits, but the NO3− and, to a less degree, SO42− concentrations increased considerably. The increase in NO3− can be attributed by anthropogenic emission, biomass burning, and soil emission resulted from the use of nitrogen fertilizers, while the increase in SO42− is suggested due to the chemical reactions of pyrite and carbonate in presence of meltwater. Moreover, all the major anion profiles were further modified by elution process, which leaches most of the ions from the uppermost part of the snow/firn to the bottom. The correlation coefficients among the major anions can also change owing to the postdepositional process. Change of the δ18O profiles as meltwater percolating through the snow/firn is less significant as compared with that of ionic profiles, consequently, causing a phasic discrepancy among the stable isotopic and major ionic profiles. Therefore, the ice core records are not only affected by the climatic and environmental conditions during precipitation, but also affected by the postdepositional process.