Northward transport of Saharan dust recorded in a deep Alpine ice core

Large amounts of mineral dust are regularly deposited over the Alps in the course of episodic Saharan air mass advections. To establish the long term chronology of this phenomenon, firn cores and one deep ice core recovered from a non-temperated glacier in the Monte Rosa summit range (Colle Gnifetti, 4500m a.s.l.) have been continuously analysed for their chemical (major ions) and isotopic (delta(18)O, delta D) stratigraphy. The present Saharan dust impact on the ion chemistry of alpine winter snow packs is also documented by a multi-year snow pit study of high elevation snow fields in the Monte Rosa region. Saharan dust bearing snow layers are specifically marked by extremely high CaCO3 contents as well as by increased delta(18)O and deuterium excess values. They contribute substantially to the overall cation and SO42- inventories of the seasonal snow pack, and thus, decrease its present acid load by roughly 60%. The Ca2+ ice core record dating back to 1600 A.D., approximately, is dominated by Saharan dust related spikes, explaining close to 70% of the total Ca2+ inventory of the glacier, and much of the the large interdecadal variability seen over the whole period. The Ca2+ record exhibits a yet unexplained increase of the background level by a factor of two, approximately, confined to the 20(th) century. The SO42- to Ca2+ ratio observed in Saharan dust layers deposited in the pre-industrial era is only slightly depleted with respect to the modem period indicating that, on the average, only 14% of the Saharan dust related SO42- originates from anthropogenic sources.