Earth and Planetary Sciences ETDs

Publication Date

2-9-2010

Abstract

In northeastern Arizona, on the Colorado Plateau, landscapes associated with weakly cemented sandstones are sensitive to Holocene climate changes on millennial to decadal scales. It is hypothesized that these climate changes affect processes of weathering, erosion and plant community establishment on hillslopes in this semiarid environment. In small basins formed at the base of Black Mesa escarpment west of Chinle, Arizona, Jurassic sandstones cemented mainly by clay minerals weather by hydration, favoring relatively high erodibility. Two end member slope forms related to aspect-induced topoclimates are present in this area: (1) transport-limited slopes mantled by 10-20 cm of weathered materials and (2) bedrock slopes lacking accumulations of weathered materials. In addition to being ideal end-members of a continuous hillslope morphology series, these two slope types and their associated plant communities can be considered end-member ecological systems. The zone of transition, or ecotone, between them represents a gradational change that, due to the slope form-aspect relations, may have shifted spatially during the late Holocene. We identified two sub basins that encompass the full range of aspects and analyzed the hillslopes by characterizing vegetation, measuring soil thicknesses and estimating erosion rates. In contrast to south-facing, more xeric slopes, north-facing, more mesic slopes have substantially thicker mantles, less exposed bedrock and the greatest tree and herbaceous cover. On the more transitional aspects, there is an intermediate amount of vegetation and bare bedrock cover and intermediate soil thicknesses compared to the aspect-related end member slope forms. Hillslope erosion rates vary by slope aspect and slope position. The soil geomorphic and dendrogeomorphic data indicate that, through feedback mechanisms linking weathering, erosion and vegetative growth, the hillslope transition zones are responding dynamically to late Holocene climate changes by shifting in the direction of bedrock-dominated slopes. Ultimately, this suggests that overall, the hillslopes are transforming away from the transport-limited end-member toward the bedrock-dominated end-member. With the predicted temperature increases and precipitation decreases in the desert southwest over the next 100 years, this slope transformation in areas with lithologically sensitive bedrock is most likely irreversible.

Degree Name

Earth and Planetary Sciences

Level of Degree

Masters

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Meyer, Grant

Second Committee Member

McAuliffe, Joe

Third Committee Member

Scuderi, Louis

Project Sponsors

Geological Society of America

Language

English

Keywords

hillslopes, topoclimate, aspect, soils, vegetation, erosion

Document Type

Thesis

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