Col du Dôme (Mt Blanc Massif, French Alps) suitability for ice-core studies in relation with past atmospheric chemistry over Europe

Abstract

The site of Col du Dôme glacier, located at 4250 m a.s.l. nearby the Mont Blanc summit (French Alps), was investigated for its suitability for reconstructing the anthropogenic perturbation of the atmosphere chemistry over Europe via glacio-chemical ice core studies. For this purpose, a 126 m long ice core drilled close to bedrock has been dedicated for glacio-chemical studies. Major ions (Na⁺, NH₄⁺, K⁺, Mg2⁺, Ca2⁺, Cl^-,>NO₃^-, and SO₄^2-) and D/H isotope ratios have been measured with high seasonal resolution along the upper 60 m of this core (covering 13 years). For dating by annual layer counting, a highly resolved ammonium profile was obtained for the entire core. To assess the spatial representativity of the chemical signals obtained from this core, additional chemical profiles were obtained from two shallow firn cores (13 and 20 m long) drilled at about 100 m from the deep ice core. All ice core parameters show regular seasonal variations with low winter and high summer values. Mean summer to winter ratios (averaged over the period 1981 to 1994) are close to 4 for NO₃^-, SO₄^2- and Ca2⁺, but reach 14 for NH₄⁺. While the shape of the mean seasonal cycles of NH₄⁺, NO₃^-, and SO₄^2- show a flat winter minimum followed by persistently high summer concentrations, more flickered variations are observed in the case of Ca²⁺. In contrast to the chemical species, δD shows a smoothed seasonal cycle. The chemical impurity levels and the δD content in snow deposits from Col du Dôme have been compared with those from Colle Gnifetti (4450 m a.s.l., located in the Swiss Alps, 80 km east of Col du Dôme) over a time period of 10 years. This comparison suggests that the two sites may experience similar atmospheric pollution conditions throughout the whole year, at least for NH₄⁺, NO₃^-, and SO₄^2-. Precise dating of the ice core drilled in 1994 was achieved by annual layer counting using the NH₄⁺ stratigraphy. The latter reveals that the glacier of Col du Dôme records well preserved snow deposits, arising from summer as well as from winter precipitation, over, at least, the last 75 years. However, the seasonal signal of the δD content appears to be disturbed at increasing depth, in particular below of 115 m. A systematic decrease in the ratio of the winter to summer net snow accumulation found with increasing depth is shown to exert an important influence on mean temporal ice core trends for parameters underlying a strong seasonal variation. The comparison of chemical impurities between Col du Dôme and Colle Gnifetti indicates that glacio-chemical ice core records from Col du Dôme will provide seasonally resolved records over the 20th century which are at least representative on a regional scale.