Northward Transport of Saharan Dust Recorded in a Deep Alpine Ice Core

Abstract

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 (δ¹⁸O, δD) 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 δ¹⁸o and deuterium excess values. They contribute substantially to the overall cation and SO₄ ²⁻ 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 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 SO₄ ²⁻ 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 SO₄ ²⁻ originates from anthropogenic sources.