Abstract

The most important weathering reaction that neutralizes incoming atmospheric acidity in the South Cascade Lake basin is weathering of calcite, which occurs in trace amounts in veins, on joint surfaces, and as a subglacial surficial deposit. Although the basin is underlain by igneous and high-grade metamorphic rocks, weathering of plagioclase is quantitatively negligible; the principal silicate weathering reaction is alteration of biotite to vermiculite. These conclusions are based on mass-balance calculations involving runoff compositions and on mineralogical observations.

For predictive modeling of the effects of increased acid deposition, it is essential to identify the relevant weathering reactions. Feldspar weathering is commonly not an important source of solutes in alpine basins underlain by granitic rocks.

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