Nitrate trace determinations in snow and firn core samples of ice shelves at the weddell sea, Antarctica

Abstract

The definitive method of isotope dilution mass spectrometry was applied to determine nitrate traces in surface snow and firn core samples of different ice shelves along the Weddell Sea, West Antarctica and in precipitations near the Antarctic Peninsula. Three of a total number of seven depth profiles we analyzed showed weak seasonal variations with a trend to nitrate concentration maxima in summer and minima in winter. The average nitrate concentration of the depth profiles down to 220 cm lay in the range of 38–93 ng g −1 which agrees with other ice shelf analyses. The highest levels in our ice shelf depth profiles are in the same range as those analyzed at the South Pole. No marine influence has been found for the nitrate concentration in contrast to the situation for chloride. The mean nitrate concentrations in new snow, in old surface snow and in firn core samples were 176, 107, and 60 ng g −1, respectively, indicating a substantial decrease with time. The results clearly demonstrate that a re-emission of HNO 3 after deposition in surface snow by evaporation or photochemical decomposition must occur and that there is not a simple correlation between the nitrate concentration in Antarctic snow and in the Antarctic atmosphere. A relatively high nitrate concentration of 266 ng g −1 was found in a hoar-frost sample. The same range of nitrate concentrations as in surface snow samples was found in precipitations of snow around the Antarctic Peninsula and in precipitations over the South Atlantic at places without substantial anthropogenic influence. A similar nitrate ‘background’ exists in these regions and, therefore, possibly the same primary source in the troposphere is responsible for these comparable nitrate levels.