Abstract

Rates of chemical weathering in two forested Adirondack watersheds were determined from mineral and elemental depletion trends in soil profiles and from input/output budgets based on precipitation and surface-water chemistry. Long-term rates of weathering have averaged about 500 to 600 eq/ha·yr for both watersheds since the glaciers retreated from the region about 14,000 yr ago. Present-day denudation rates average 1679 eq/ha·yr in the Panther Lake watershed and only 198 eq/ha·yr in the Woods Lake watershed.

Mineral weathering reactions in both watersheds involve primarily the dissolution of plagioclase, potassium feldspar, and hornblende. Mass balance calculations, however, indicate that hornblende weathers in disproportionately large amounts in the Panther Lake watershed relative to its abundance in soils and till. Cation exchange in the glacial till mantling the Panther Lake watershed may also play an important role in controlling surface-water chemistry in the basin.

In the Panther Lake watershed, the current rate of weathering, which is about a factor of 3 greater than the long-term average, may reflect a recent adjustment for higher hydrogen ion fluxes brought about by acid deposition. The capacity of the thick till deposits in the Panther Lake watershed to neutralize acidity is ultimately reflected by the circum-neutrality of Panther Lake. In the Woods Lake watershed, the soils and thin till deposits cannot effectively neutralize current acid loadings and, therefore, Woods Lake is acidic with a pH below 5.

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