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Chemical denudation and hydrology near tree limit, Coast Mountains, British Columbia Gallie, Thomas Muir


Major cation and silicon budgets from a 2.3 hectare, watershed are interpreted to determine environmental factors controlling chemical denudation rates on siliceous lithologies in the alpine-subalpine ecotone, Coast Mountains, British Columbia. Ionic source areas were sought by spatial and temporal solute sampling. The data were stratified by soil-vegetation complexes and components of the hydrologic cascade. The geochemical system has two major ionic sources. Soil solutions are sodium facies and are stoichiometrically balanced with volcanic glass probably because the glass fraction of volcanic ash is the primary weathering source in soils and is dissolving congruently. Streamflow is predominantly calcium facies because bedrock is composed of calcium-magnesium silicates and groundwater flow through bedrock discharges to phreatic soil water zones. The hydrologic system has two major components. Direct stormflow is generated by saturation overland flow from large contributing areas due to extensive soil water-repellency. Indirect stormflow is generated by saturated subsurface throughflow due, in part, to strong textural contrasts among soil parent materials. The actual basin drainage density is greater than is apparent superficially because preferred subsurface pathways are common. Annual runoff was very similar in the study years 1979 and 1980 (904 mm and 1027 mm respectively) but net major cation and silicon yields varied from 1.6 metric tons km⁻⁴ to 3.4 metric tons km⁻². The low 1979 yield was caused by segregated ground ice which limited soil-water contact, soil solution shunting, and groundwater flow. Differences in denudation rates between subcatchments were equally large because of topographically induced groundwater contributions to streamflow. A similar conclusion is reached for larger adjacent watersheds. Most mass is denuded along groundwater flowpaths in bedrock rather than from soils. Denudation from alpine surficial materials during 1980 was approximately 2 tons km⁻² yr⁻¹ which is an order of magnitude less than expected. Results imply that where groundwater contributions are not defined, watershed experiments can produce highly biased estimates of surficial denudation which confound process interpretations. Hydrologic pathways are more important than the inherent chemical reactivity of soils and bedrock in regulating chemical denudation rates in this landscape. Runoff mechanisms partition water between the surface, soil and groundwater reservoirs and control flowpaths, contact-times and shunting within each reservoir. Biopedochemical controls on denudation appear to be relatively less important than hydrologic controls because soil solutions are similar among alpine soil-vegetation associations. Soil leaching losses are small because soil water-repellency, preferred subsurface pathways and the discharge regime limit soil solution shunting.

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