Flow paths, solute sources, weathering, and denudation rates: The chemical geomorphology of a small catchment.

摘要

This study explores chemical weathering processes in an 860 m{dollar}sp2{dollar} unchanneled valley in the Oregon Coast Range. Collaborative catchment-scale sprinkling experiments achieved relatively steady hydrologic conditions, and allowed me to manipulate chemical inputs and conduct tracer studies. With these experiments I address issues of inverse concentration-discharge relationships, subsurface water flow paths, runoff solute sources, and evolution of water in the catchment. Long-term effects of weathering were investigated with a mass balance analysis of chemical change in rocks and soils.; Tectonic uplift of the Coast Range drives river incision, creating steep, thinly soil-mantled slopes, and a weathering-limited landscape. The deepest manifestation of weathering observed in a 35 m core was open fractures. These provide avenues for water circulation, which oxidizes, but does not, over time, produce losses in the rock detectable with the mass balance technique. Physical processes, including tree-throw and burrowing, incorporate weathered rock into colluvium. Measured mass loss from the colluvium requires 3800-6800 years at present denudation rates, but its porosity derives from bioturbation rather than dissolution.; The catchment chemical denudation rate, 32 {dollar}pm{dollar} 10 t km{dollar}sp{lcub}-2{rcub}{dollar} yr{dollar}sp{lcub}-1{rcub},{dollar} is within the range in nearby catchments up to 10{dollar}sp6{dollar} times larger. Half the solutes came from colluvium, the rest from weathered rock. Percolating water leaches organic anions from the soil, and leads to soil water solute profiles identical to those in Spodosols. The effects of podzolization are not manifested in horizon development, however, due to homogenization by bioturbation and short residence times for colluvium.; The thickness of weathered bedrock tapers downslope. An analytical model of steady flow and solute acquisition suggests that chemical denudation is spatially uniform. This mismatch implies that physical rather than chemical processes are responsible for downslope tapering of the weathered profile. An additional result is that water flux dominates over chemical kinetics in chemical denudation.; Low mass losses in the weathered rock, the importance of bioturbation, and the model results all point to the importance of physical processes in this catchment. Although it is weathering-limited, erosion appears to be limited by physical rather than chemical weathering processes.