3 research outputs found
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Trajectories of land surface evolution in polygonal tundra
In the past three decades, an abrupt acceleration in the thaw of ice wedges has spurred rapid surface deformation (i.e., thermokarst) in polygonal tundra landscapes spanning the Arctic. The ensuing conversion of low-centered polygons (LCPs) and flat terrain into high-centered polygons (HCPs) has profound impacts on regional hydrology and carbon fluxes between the soil and atmosphere. However, pathways of ice wedge degradation and the stability of the deformed terrain are uncertain, complicating efforts to project feedbacks on global climate change. In this dissertation, I explore trajectories of surface deformation in ice wedge polygons, using a combination of calibration-constrained numerical experiments, remote sensing, and machine learning. In the first two chapters, numerical simulations of the soil hydrologic and thermal regimes reveal that, relative to terrain unaffected by thermokarst, the permafrost beneath HCPs tends to be well-buffered against climate extremes, promoting landscape stability. Ice wedges at HCP boundaries are less vulnerable to thaw during warm summers, reinforcing prior field-based observations that thermokarst is typically a self-arresting process. Simultaneously, the cooling of thermokarst-affected ice wedges in winter tends to be inhibited by snow accumulation in degraded troughs, reducing the likelihood of renewed ice wedge cracking and restoration of LCP microtopography. Overall, these results indicate that the microtopography of polygons already affected by thermokarst will likely remain stable over the next century. In the second half of this dissertation, a novel machine-learning-based tool is introduced for delineating and measuring the microtopography associated with ice wedge polygons in high-resolution digital elevation models. The tool is used to map polygonal geomorphology across ~1,000 km² of tundra south of Prudhoe Bay, Alaska, visualizing in unprecedented detail the heterogeneous extent to which thermokarst has affected a modern polygonal landscape. This map of polygonal geomorphology provides useful context for upscaling point- to plot-scale observations of gas exchange in ice wedge polygons to landscape-scale estimates of carbon fluxes. It also provides an extensive baseline dataset for quantifying contemporary rates of land surface deformation, through future surveys at the site.Geological Science
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Simulation of ice wedge polygon geomorphic transition, Prudhoe Bay, Alaska
textA numerical model is presented to simulate the changes in topography associated with ice wedge polygon transition from low-centered to high-centered form. The model applies a hillslope diffusion equation to an eroding polygon using a finite-difference approach. It is calibrated using a LiDAR dataset from a site where low-centered polygons exist within meters of high-centered polygons, whose formation appears to have been triggered by construction of the Dalton Highway. The loss of hydrologic storage and the transport of soil from the polygon center into polygon troughs during transition are estimated from model simulations. Optimized values of the hillslope diffusion coefficient suggest that multiple physical processes, including frost heave and continuous soil creep, may drive lateral soil transport at the site. The optimized parameters, furthermore, capture the decreasing influence of anthropogenic disturbances (in this case, the Dalton Highway) on polygon form at distances greater than 35 meters. Overall, a match between the topography of simulated and observed high-centered polygons confirms that the hillslope diffusion paradigm approximates much of the complexity of polygon transition. Future refinements to the model should include more process-based treatment of the mechanisms that drive soil transport and control rates of polygon erosion.Geological Science
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Determining Wetlands Distribution, Lake Depths, and Topography Using Airborne Lidar and Imagery on the North Slope, Deadhorse Area, Alaska
On the Alaskan North Slope, a permafrost-dominated coastal plain above the Arctic Circle, the ability of lakes to support a fish population is a function of water depth, and wetlands classification is highly dependent on soil saturation (and thus microtopography). Using a new airborne lidar instrument that combines laser ranging at near-infrared wavelengths for topography and green wavelengths for bathymetry, we flew a pilot study over a 490-km2 area south of Prudhoe Bay in August 2012 to measure surface topography at a density of about 20 points/m2 and water-body depths at a density of about 2 points/m2.
High-resolution digital elevation models, having vertical accuracies of a few centimeters, have been generated from the topographic laser data that was acquired at a 400 kHz pulse rate. These models and associated point clouds can be used along with high-resolution color-infrared imagery to map permafrost landscape features such as soil/ice polygons and pingos and identify microtopographic features that influence soil moisture and consequently wetlands distribution.
Bathymetric lidar data, acquired at a pulse rate of 36 kHz, can be used to produce elevation models for the water surface and the water bottom, allowing water depths to be determined for shallow, freshwater lakes and anastomosing stream channels. Water penetration to depths greater than 6 m has been achieved in lakes with reported turbidities ranging from 0.7 to 4.3 nephelometric turbidity units (NTU).
Topographic and bathymetric lidar data were used to establish the boundaries, measure water depths, and estimate total and incremental water volumes for 283 shallow lakes in the survey area having surface areas greater than about 0.8 ha. These lakes together are estimated to contain 20,343,051 m3 (5,374,064,326 gal) of water, of which 724,813 m3 (191,475,524 gal) is found in 83 lakes with depths greater than 1.5 m (5 ft) and 116,753 m3 (30,842,854 gal) is found in 38 lakes with depths greater than 2.1 m (7 ft).Bureau of Economic Geolog