Date of Final Oral Examination (Defense)

9-2008

Type of Culminating Activity

Dissertation

Degree Title

PhD in Geophysics

Department

Geosciences

Major Advisor

Dr. John Bradford

Abstract

Seasonal thaw depth beneath arctic streams significantly impacts physical and biological processes within arctic stream environments. The impact of greater seasonal thaw for extended periods of time can alter ecosystems that have, in the past, resulted from more prevalent permafrost environments. Effects of climatic change on arctic stream environments necessitate the need for more information on characteristics of seasonal thaw and processes that occur within the thawed layer. Multiple ground-penetrating radar (GPR) methods and one-dimensional (1D) thermal modeling were used to investigate seasonal thaw beneath arctic streams and determine the dominant thermal process. Study sites were selected to include stream reaches that span a range of geomorphologic conditions in rivers and streams on Alaska’s North Slope. Results from seasonal time-lapse common-offset GPR transects, gathered throughout the summer season of 2004, illustrated that low-energy stream environments react slowly to seasonal solar input and maintain thaw thicknesses longer throughout the late season. Thaw depths beneath high-energy streams respond quickly in the beginning of the season and appear to decrease just as quickly over the late season period. Continuous multi-offset (CMO) GPR method improves the quality of subsurface images through stacking and velocity filtering and provides measurements of vertical and lateral velocity distributions. Detailed velocities were estimated from CMO transects, gathered in August 2005, using reflection tomography processing methods. Resulting velocity tomograms were then used to estimate water content and porosity using the Topp equation. Porosity estimates were then used to help constrain a 1D finite-difference thermal model. Within the high-energy stream environments three-dimensional (3D) GPR data illustrate greater thaw depths beneath riffle and gravel bar features relative to the neighboring pool features. Due to differences in thermal properties the low-energy stream sites indicate the opposite: greater thaw depths beneath pools and thinner thaws beneath the connecting runs. Results provide detailed 3D geometry of active layer thaw depths that can greatly improve hydrological studies seeking to quantify transport and biogeochemical processes that occur within the hyporheic zone. Using the finite-difference approach a 1D heat transport model with phase change was developed to estimate seasonal temperatures beneath two arctic streams to determine the dominant thermal process occurring beneath the stream sites. The model was driven by stream water or near-surface temperature data recorded at select stream sites near Toolik Field Station, AK, for the summer months of 2004 and 2005. Model temperatures were calibrated to measured temperatures at corresponding depths and evaluated against interpreted thaw depths from cross-sectional GPR images gathered over the 2004 summer season. Results are in reasonable agreement with observed temperatures and GPR thaw depth estimates and imply thaw processes dominated by thermal conduction. Discrepancies between model and observed values are likely due to homogenous soil property assumptions, oversimplified convection influence assumptions, and deviations from the 1D model.

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