UVM Theses and Dissertations
Format:
Print
Author:
Jungers, Matthew C.
Dept./Program:
Geology
Year:
2008
Degree:
MS
Abstract:
I seek to quantify sediment production and transport rates on steep, soil-mantled hillslopes. Specifically, I am using the activity of ¹⁰Be produced by cosmic ray bombardment, measured in both discrete and amalgamated transect samples of hillslope sediment (an extension of the method of Nichols et al., 2002, 2005), in conjunction with simple models of hillslope behavior, to better understand the patterns and rates of sediment production, as well as rates of sediment movement downslope. I have collected suites of samples (n = 96) from hillslope transects across varied climatic and tectonic settings. I focus on my most complete dataset from the Great Smoky Mountains, NC, for this thesis. These initial data clearly show that the spatial distribution of ¹⁰Be in hillslope soil is systematic and thus interpretable. Nuclide concentrations indicate the extent of soil stirring and are consistent with down-slope soil transport.
Sample sites include north-central Pennsylvania, New Zealand's North Island, Great Smoky Mountains National Park, the Oregon Coast Range, and the central plateau of Madagascar. Field and isotopic data from these hillslope samples is being considered along with cosmogenic data from river sediment samples collected near each site. This pairing provides context for the results of my new application of cosmogenic nuclides, and adds breadth and depth to the relevancy of this work and that of our collaborators. The importance of the link between hillslope processes and inferred basin-scale erosion rates is often cited (Bierman and Steig, 1996; Brown et al., 1995; Matmon et al., 2003), but rarely explored quantitatively (Heimsath et al., 2005). My project explicitly makes this link.
The stated goal of this thesis was to develop a method for quantifying rates of sediment production and transport on hillslopes using ¹⁰Be; that goal was successfully achieved. My work relies upon the power of amalgamation to both investigate and to smooth spatial variance in ¹⁰Be concentrations. Using cosmogenically-produced isotopes as a geomorphic tracer allows me to both quantify soil production and track soil as it is transported downslope. In the Great Smoky Mountains, best-fit models of the data require soil velocities of 1-3 cm·yr⁻¹ in the well-mixed active layer of the soil. The thickness of this active transport layer is dependent on the rooting depth of trees on the slope and the related depth of soil turnover and mixing due to treethrow. Modeled soil fluxes range from 55-165 cm³·yr⁻¹ depending on whether soil velocity or the thickness of the actively transported layer is assumed to be constant downslope. Diffusion coefficients calculated from these flux rates range from 82-247 cm³·yr⁻¹·cm⁻¹. This range of diffusion coefficients is most likely related to climatic variation. This hillslope does not appear to be in steady state, calling into question what constitutes a state of steady erosion for an ancient mountain range such as the Appalachians.
Sample sites include north-central Pennsylvania, New Zealand's North Island, Great Smoky Mountains National Park, the Oregon Coast Range, and the central plateau of Madagascar. Field and isotopic data from these hillslope samples is being considered along with cosmogenic data from river sediment samples collected near each site. This pairing provides context for the results of my new application of cosmogenic nuclides, and adds breadth and depth to the relevancy of this work and that of our collaborators. The importance of the link between hillslope processes and inferred basin-scale erosion rates is often cited (Bierman and Steig, 1996; Brown et al., 1995; Matmon et al., 2003), but rarely explored quantitatively (Heimsath et al., 2005). My project explicitly makes this link.
The stated goal of this thesis was to develop a method for quantifying rates of sediment production and transport on hillslopes using ¹⁰Be; that goal was successfully achieved. My work relies upon the power of amalgamation to both investigate and to smooth spatial variance in ¹⁰Be concentrations. Using cosmogenically-produced isotopes as a geomorphic tracer allows me to both quantify soil production and track soil as it is transported downslope. In the Great Smoky Mountains, best-fit models of the data require soil velocities of 1-3 cm·yr⁻¹ in the well-mixed active layer of the soil. The thickness of this active transport layer is dependent on the rooting depth of trees on the slope and the related depth of soil turnover and mixing due to treethrow. Modeled soil fluxes range from 55-165 cm³·yr⁻¹ depending on whether soil velocity or the thickness of the actively transported layer is assumed to be constant downslope. Diffusion coefficients calculated from these flux rates range from 82-247 cm³·yr⁻¹·cm⁻¹. This range of diffusion coefficients is most likely related to climatic variation. This hillslope does not appear to be in steady state, calling into question what constitutes a state of steady erosion for an ancient mountain range such as the Appalachians.