With the intention of contributing to a better understanding of snow depth profiles used in reconstructing the southern hemisphere nitrate background we have measured C1−, , and in firn cores from two coastal Antarctic locations (GvN = Ekströmisen 70°S, 8°W, and Filchner = Filchner-Ronne Ice Shelf 79°S, 57°W). The depth resolution chosen is 2 cm per sample (i.e. 36 and 14 samples per year, respectively).
The isotopie composition of the firn cores was concurrently measured, with equally high resolution (deuterium and 18O data, W. Graf, private communication). The GvN core yields an average accumulation rate of 35 cm H2O per year during the period 1979–86, while the Filchner core gives 14 cm H2O per year during the period 1955-80. The net snow accumulation being relatively high allows precise determination of the year to year boundaries, as well as the relative contribution of individual seasons to the total net accumulation. This was achieved by combining the stable isotope data with the chemical tracers nss-sulfate (high concentration in summer) and sea salt (high C1− in autumn and winter). For the two individual locations this procedure allowed assessment of the glacial nitrate concentration seasonality as well as comparing the yearly nitrate deposition.
The Filchner location shows a distinct seasonality with maximum concentrations in summer. For nearly half of the years covered we also find higher concentrations in winter. This higher nitrate in winter is always accompanied by high sea salt concentrations. We suggest therefore a mechanism making sea salt aerosol an additional deposition pathway for nitrate in winter. This means that we are not able to link the enhanced winter nitrate deposition to stratospheric denitrification directly. The GvN core does not show significant seasonality. This is presumably due to frequent snow drift events preventing signal conservation.
Although the individual accumulation rates differ by a factor of 2.5 the yearly nitrate deposition is in the same range (4-11 kg (km-2 a-1) at both sites. This is caused by the different seasonal modulation of the net snow accumulation. At GvN low nitrate autumn precipitation prevails and keeps the yearly nitrate level constantly low (18-22 ppb). At Filchner, however, a high contribution of summer snow brings the yearly concentration average up to about 30–70 pbb.