Selectivity of Chemical Weathering in High Elevation Catchments of the Colorado Front Range

Mark W Williams
Department of Geography and Institute of Arctic and Alpine Research University of Colorado, Boulder

Platts-Mills, T
Harvard University and INSTAAR, University of Colorado, Boulder CB 450 Boulder, CO 80309 United States


Interest in the controls on chemical weathering in mountain environments has been renewed by the hypothesis that tectonic uplift could drive global climate change. We tested the hypothesis that selectivity of chemical weathering in the Colorado Front Range would be greater in recently deglaciated catchments compared to catchments that were unglaciated during the Pleistocene. Ten years of mass balance results from recently deglaciated and non-glaciated catchments showed that Ca/Na (p $<$ 0.001) and K/Na (p $<$ 0.01) molar ratios were significantly higher in the recently deglaciated basin. Geochemical reconstructions of the solute compositions of these catchments indicate that selective weathering of calcite and biotite can explain the differences in the solute compositions of the two catchments. Cathodoluminescence microscopy and coulometric titration of whole rock samples showed that calcite was present in all samples (n = 24), with the weight percent of calcite ranging from 0.01 to 0.11. Normalizing the amount of trace minerals weathered to the amount of plagioclase weathered in each catchment, calcite weathering in the recently deglaciated catchment was a factor of 2.28 greater than in the non-glaciated catchment, and biotite weathering in the recently deglaciated catchment was greater by a factor of 5.05. A synoptic survey of water chemistry from 75 catchments in the Front Range showed similar results; Ca/Na and K/Na molar ratios were significantly higher (p $<$ 0.01) in samples collected from surface waters draining recently deglaciated basins compared to non-glaciated basins. Rates of physical erosion appear to be greater on recently deglaciated areas, facilitating the weathering of reactive minerals and creating the conditions for selective weathering.