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
Abstract
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.