The Impact of Early-Season Acidic Snowmelt on the Magnitude
of Chemical Erosion in a Glacierized Alpine Catchment
Giles H. Brown 1
Two major types of chemical weathering reaction control base cation acquisition by meltwaters
beneath alpine glaciers, namely simple dissolution and acid hydrolysis. Protons to fuel acid hy-
drolysis reactions are primarily derived from two main sources: the oxidation of sulfide minerals
such as pyrite, and the dissolution and dissociation of atmospheric CO . In addition, acidity to
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perform chemical erosion may also be derived from the dissolution of acids (e.g., H SO and
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HNO ) in the seasonal snowpack, and significant contributions of H+ from this source have been
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shown to be limited to the early melt season.
The average measured NO and SO 2 concentrations in the 1990 snowpack overlying Haut
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4
Glacier d'Arolla, Switzerland, were 3.52 and 3.74 (eq l-1 respectively, which inputs 545 and 449
kg of NO and SO 2, respectively, when the supraglacial snowpack is eradicated between 2560
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and 3100 m). If 100% eradication of the supraglacial snowcover is invoked, these fluxes increase
to 1097 and 905 kg, respectively. However, snowpack fluxes of NO and SO 2 are considerably
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lower than those presented in the literature, which may suggest significant snowpack-derived sol-
ute contributions from above 3100 m on the glacier surface, extraglacial snowmelt inputs to the
hydroglacial system, or interseasonal storage of snowmelt-derived ions in the subglacial hydro-
logical system.
Assuming eradication of the supraglacial snowcover between 2560 and 3100 m, snowpack-de-
rived acidity liberates 0.23 and 0.31% of the total 1990 lithogenic Ca + and Mg2+ fluxes when
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atmospheric H+ is attributed solely to carbonate weathering. These carbonate-derived fluxes of
Ca2+ and Mg2+ are reduced to 0.12 and 0.26% when atmospherically derived H+ is also appor-
tioned to the weathering of silicates, which contribute 0.04 and 0.39% of the total Ca2+ and Mg2+
fluxes, and 0.64 and 0.30% of the seasonal Na+ and K+ fluxes. More significant contributions of
lithogenic solute liberated by snowpack acidity are only possible if the maximum recorded snow-
pack NO3 and SO 2 concentrations are used in the calculations, and 100% eradication of the
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supraglacial snowcover is invoked.
These estimates of crustal weathering associated with neutralization of snowpack-derived acidity
suggest that actively glaciated Alpine headwater catchments are more than capable of neutraliz-
ing atmospherically derived acidity, due to the abundant supply of freshly ground, geochemically
reactive lithogenic material.
1 Centre for Glaciology, Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, Cere-
digion, SY23 3 DC, Wales, UK
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