Origin of the Basal Ice Layer from Alpine Glaciers Indicated by its Chemistry

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Abstract
The basal ice layer of Alpine glaciers, the thickness of which can reach more than one metre, generally appears as a sequence composed of layers of bubble-poor ice 0.5-3 cm thick and dirt layers containing dispersed rock fragments. Locally, debris-free bubbly ice is also present in the sequence. This basal layer exists on the lee side of rock protuberances or along the marginal part of the glacier. Three geochemical characteristics shed some light on its origin: (a)The high calcium content of basal ice sliding over limestones from the north side of Glacier do Tsanfleuron can be successfully predicted from considerations of the phase relations in regelating subglacial solutions in approximate equilibrium with subglacial calcite deposits. (b)The (Na + K)/(Ca+Mg) ratio of bubble-poor basal ice from Glacier de Tsijiore Nouve which flows over gneissic rocks is significantly higher than this ratio in other samples taken in the same environment (glacier ice, melt water). This may be explained by refreezing of squeezed water, the ionic composition of which is governed by selective diffusion of ions during squeezing and/or by the water being forced through mud layers as demonstrated by us at the base of Glacier d'Argentière (Souchez and Lorrain. 1975). (c)The Lead-210 activity per kg of basal ice from the south side of Glacier de Tsanfleuron is greater than that of the glacier ice immediately above, which indicates that it is younger despite percolation effects. This can be explained if recent ice accretion has occurred at the base of relatively old glacier ice. Accretion of blowing snow has been found to occur under the southern margin of the glacier.