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University of California, Berkeley (2009)

Soil Formation and Transport Processes on Hillslopes along a Precipitation Gradient in the Atacama Desert, Chile

Owen Justine J

Titre : Soil Formation and Transport Processes on Hillslopes along a Precipitation Gradient in the Atacama Desert, Chile

Auteur : Justine J Owen

Université de soutenance : University of California, Berkeley

Grade : Doctor of Philosophy (PhD) 2009

Résumé partiel
The climate-dependency of the rates and types of soil formation processes on level landforms has been recognized and documented for decades. In contrast, methods for quantifying rates of soil formation and transport on hillslopes have only recently been developed and the results suggest that these rates are independent of climate. One explanation for this discrepancy is that hillslopes and their soil mantles are dynamic systems affected by local and regional tectonic effects. Tectonics can change local or regional baselevel which affects the hillslope through stream incision or terrace formation at its basal boundary. Another explanation is that in most of the world hillslope processes are biotic, and biota and their effects vary nonlinearly with climate. The effects of both tectonics and life can obscure climatic effects. Recent studies have been made to isolate the climatic effect on hillslope processes, but they are few and focus on humid and semiarid hillslopes. In order to isolate the effects of boundary condition, precipitation, and life, I studied pairs of hillslopes in northern Chile in semiarid, arid, and hyperarid climates. In each pair, one hillslope was bounded by an incising (bedrock-bedded), first-order channel, and the other was bounded by a low-slope, non-eroding surface. This precipitation gradient spans the transition from biotic to abiotic landscapes. The guiding framework for this study is a hillslope soil mass balance model in which the soil mass is controlled by the balance of soil production from bedrock and from atmospheric input, and soil loss through physical and chemical erosion. My objectives were to quantify the components of the mass balance model, identify the processes driving soil production from bedrock and soil transport, and interpret this data in the context of climate and hillslope morphology. In the field, I made observations of the processes driving soil formation and transport, surveyed the hillslopes to produce high-resolution topographic maps, and sampled soils and rock for chemical analysis and particle size analysis. Dust collectors were erected to measure atmospheric input. Bedrock and surface gravel samples were collected in order to calculate the rate of soil production from bedrock, the incision rate of the channels, the age of the non-eroding surfaces, and the exposure history of surface gravels using the concentrations of in situ-produced 10Be and 26Al. Rates of physical and chemical erosion were calculated using the soil mass balance model, based on the rate of soil production from bedrock, the atmospheric deposition rate, and the concentrations of an immobile element in the soil, bedrock, and atmospheric input. In addition, to understand the effect of precipitation on the landscape and to quantify the infiltration rate of the soil, sprinkling experiments were conducted in each climate region and infiltrometer measurements were made in the hyperarid region.

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Page publiée le 31 décembre 2011, mise à jour le 13 septembre 2017