26 research outputs found

    Ice-water dynamics over a land-terminating sector of western Greenland

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    In this dissertation I investigate the dynamics of a land-terminating reach of the west Greenland ice sheet through three projects utilizing unique field data and modeling experiments. In Chapter 1 I use in-situ water pressure data and numerical modeling to elucidate the conceptual model of subglacial hydrologic drainage beneath Greenland. Measurements in boreholes drilled to the ice sheet bed along a transect in the ablation zone reveal water pressures that question the stability of water-draining conduits. I apply numerical techniques to model transient evolution of subglacial conduits and show that seasonal growth of such features is unsupported in the ice sheet interior. Low potential gradients that drive energy availability to melt channel walls limit conduit growth. This elucidates the importance of other processes in facilitating seasonal development of the subglacial hydrologic system in the interior setting. In Chapter 2 I investigate the effect of thermal boundary conditions on the thermo-mechanical state of western Greenland. I propose new boundary fields from measurements of temperature near the surface and basal heat flux beneath the ice sheet. Comparison of these observation-based fields with model-driven datasets suggests that model-derived basal heat flux is too high, and surface temperatures too low in the study area. By applying different boundary conditions to a thermo-mechanically coupled ice sheet model I show that thermal conditions at the ice/bedrock interface critically depend on the boundary conditions at both the surface and bed. Unrealistically cold conditions are induced if basal heat flux alone is driven by observations. Warmer surface conditions consistent with observations are sufficient to reintroduce melted conditions at the bed, elucidating the importance of the surface boundary in thermo-mechanical model exploration. In Chapter 3 I address the processes responsible for inducing a region of anomalously low driving stress that is evident in west-southwest Greenland. I show that the feature corresponds to a consistent reduction in surface slope rather than a strong bedrock topographic expression. Kinematic wave experiments show that the diffusive nature of the ice sheet renders the development of such a feature infeasible from surface mass balance perturbations. Low driving stress necessitates a change in dynamics and I surmise it is this variation in basal sliding that is an important factor in inducing changes in the surface slope, and thus the driving stress

    Borehole Measurements of Dynamic Basal Drainage Adjustments During Sliding Accelerations: Bench Glacier, Alaska

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    Artificial perturbations of borehole water levels, or slug tests, are a commonly employed means of characterizing the glacier hydrologic system. Results documenting the influence of slug testing on a field of boreholes and its change with time, however, are scarce. Slug tests were performed on Bench Glacier, AK in 21 boreholes over three field seasons during an annual late spring glacier speed up event. Fifty four slug tests were conducted, with water level monitoring in up to five boreholes adjacent to the slugged borehole. Seven of the slug tests were performed in conjunction with dye dispersion tests to identify water pathways within the slugged borehole following perturbation. Underdamped and overdamped slug test responses show a high degree of connectivity among boreholes connected via the glacier bed. The nature and degree of connectivity is temporally variable, suggesting that the drainage network at the bed is highly dynamic on time and space scales of hours and 10’s of meters, respectively. The changes we document in slug test responses over time and space can be used to constrain explanations for the cause of the underdamped response. Examination of the underdamped response necessitates an understanding of the process(es) acting as the spring to produce the oscillatory water level behavior. We propose that coherent air packages are a likely means of producing the compliance needed to generate the underdamped slug test response, and that these air packages may exist within the glacier at the tips of subglacially propagated fractures. Synthesis of slug testing with other methods of study, such as video observation and dye tracing, helps lend insight into the governing processes at the glacier bed

    Compressional and EM Wave Velocity Anisotropy in a Temperate Glacier Due to Basal Crevasses, and Implications for Water Content Estimation

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    We have conducted a series of experiments designed to investigate elastic and electromagnetic (EM) velocity anisotropy associated with a preferentially aligned fracture system on a temperate valley glacier in south-central Alaska, USA. Measurements include a three-dimensional compressional wave (P-wave) seismic reflection survey conducted over a 300m × 300m survey patch, with uniform source grid and static checkerboard receiver pattern. Additionally, we acquired a multi-azimuth, multi-offset, polarimetric ground-penetrating radar (GPR) reflection experiment in a wagon-wheel geometry with 94° of azimuthal coverage. Results show azimuthal variation in the P-wave normal-moveout velocity of \u3e3% (3765 and 3630ms-1 in the fast and slow directions respectively) and difference of nearly 5% between the fast (0.164mns-1) and slow (0.156mns-1) EM velocities. Fracture orientations estimated from the GPR and seismic velocity data are consistent and indicate a preferred fracture orientation that is 30–45° oblique to glacier flow; these measurements agree with borehole observations. Anisotropic analysis of the polarimetric data gives a single volumetric water content estimate of 0.73 ± 0.11%. We conclude that meaningful estimates of physical properties in glaciers based on EM or seismic velocity measurements require collecting data such that the presence of anisotropy can be evaluated and an anisotropic analysis employed when necessary

    Sensitivity of the Frozen/Melted Basal Boundary to Perturbations of Basal Traction and Geothermal Heat Flux : Isunnguata Sermia, Western Greenland

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    A full-stress, thermomechanically coupled, numerical model is used to explore the interaction between basal thermal conditions and motion of a terrestrially terminating section of the west Greenland ice sheet. The model domain is a two-dimensional flowline profile extending from the ice divide to the margin. We use data-assimilation techniques based on the adjoint model in order to optimize the basal traction field, minimizing the difference between modeled and observed surface velocities. We monitor the sensitivity of the frozen/melted boundary (FMB) to changes in prescribed geothermal heat flux and sliding speed by applying perturbations to each of these parameters. The FMB shows sensitivity to the prescribed geothermal heat flux below an upper threshold where a maximum portion of the bed is already melted. The position of the FMB is insensitive to perturbations applied to the basal traction field. This insensitivity is due to the short distances over which longitudinal stresses act in an ice sheet

    Air compression as a mechanism for the underdamped slug test response in fractured glacier ice

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    Artificial perturbations of borehole water levels, known as slug tests, are a useful means of characterizing the glacier hydrologic system. Slug tests were performed on Bench Glacier, Alaska, in 21 boreholes over three field seasons during the transition from a winter to a summer drainage mode. Fifty-four slug tests were conducted, with water level monitoring in up to five boreholes adjacent to the slugged borehole. Seven of the slug tests were performed in conjunction with dye dispersion tests to identify water pathways within the slugged borehole following perturbation. Nearly 60% of monitored adjacent boreholes showed a hydraulic connection to the slugged borehole via the glacier bed. The nature and degree of connectivity was temporally variable, suggesting that the drainage network at the bed was highly dynamic on a daily timescale and spatial scale of tens of meters. The variability of slug test responses over time and space limit the feasibility of six alternative explanations for the oscillatory water level behavior characteristic of the underdamped response. We propose a seventh, that is, that coherent air packages are a reasonable means of producing the compliance needed to generate the underdamped slug test responses on Bench Glacier, and that these air packages may exist within the glacier at the tips of subglacially propagated fractures

    Thermal boundary conditions on western Greenland: Observational constraints and impacts on the modeled thermomechanical state

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    The surface and basal boundary conditions exert an important control on the thermodynamic state of the Greenland Ice Sheet, but their representation in numerical ice sheet models is poorly constrained due to the lack of observations. Here we investigate a land-terminating sector of western Greenland and (1) quantify differences between new observations and commonly used boundary condition data sets and (2) demonstrate the impact of improved boundary conditions on simulated thermodynamics in a higher-order numerical flow model. We constrain near-surface temperature with measurements from two 20mboreholes in the ablation zone and 10m firn temperature from the percolation zone. We constrain basal heat flux using in situ measurement in a deep bedrock hole at the study area margin and other existing assessments. To assess boundary condition influences on simulated thermal-mechanical processes, we compare model output to multiple full-thickness temperature profiles collected in the ablation zone.Our observation-constrained basal heat flux is 30mW m2 less than commonly used representations. In contract, measured near-surface temperatures are warmed than common surface warmer than common surface temperature data sets by up to 15 degrees C. Application of lower basal heat flux increases a model cold bias compared to the measured temperature profiles and causes frozen basal conditions across the ablation zone. Temperate basal conditions are reestablished by our warmer surface boundary. Warmer surface ice and firn can introduce several times more energy to the modeled ice mass than what is lost at the bed from reduced basal heat flux, indicating that the thermomechanical state of the ice sheet is highly sensitive to near-surface effects

    Greenland and Canadian Arctic ice temperature profiles database

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    Here, we present a compilation of 95 ice temperature profiles from 85 boreholes from the Greenland ice sheet and peripheral ice caps, as well as local ice caps in the Canadian Arctic. Profiles from only 31 boreholes (36 %) were previously available in open-access data repositories. The remaining 54 borehole profiles (64 %) are being made digitally available here for the first time. These newly available profiles, which are associated with pre-2010 boreholes, have been submitted by community members or digitized from published graphics and/or data tables. All 95 profiles are now made available in both absolute (meters) and normalized (0 to 1 ice thickness) depth scales and are accompanied by extensive metadata. These metadata include a transparent description of data provenance. The ice temperature profiles span 70 years, with the earliest profile being from 1950 at Camp VI, West Greenland. To highlight the value of this database in evaluating ice flow simulations, we compare the ice temperature profiles from the Greenland ice sheet with an ice flow simulation by the Parallel Ice Sheet Model (PISM). We find a cold bias in modeled near-surface ice temperatures within the ablation area, a warm bias in modeled basal ice temperatures at inland cold-bedded sites, and an apparent underestimation of deformational heating in high-strain settings. These biases provide process level insight on simulated ice temperatures

    Amplified melt and flow of the Greenland ice sheet driven by late-summer cyclonic rainfall

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    Intense rainfall events significantly affect Alpine and Alaskan glaciers through enhanced melting, ice-flow acceleration and subglacial sediment erosion, yet their impact on the Greenland ice sheet has not been assessed. Here we present measurements of ice velocity, subglacial water pressure and meteorological variables from the western margin of the Greenland ice sheet during a week of warm, wet cyclonic weather in late August and early September 2011. We find that extreme surface runoff from melt and rainfall led to a widespread acceleration in ice flow that extended 140 km into the ice-sheet interior. We suggest that the late-season timing was critical in promoting rapid runoff across an extensive bare ice surface that overwhelmed a subglacial hydrological system in transition to a less-efficient winter mode. Reanalysis data reveal that similar cyclonic weather conditions prevailed across southern and western Greenland during this time, and we observe a corresponding ice-flow response at all land- and marine-terminating glaciers in these regions for which data are available. Given that the advection of warm, moist air masses and rainfall over Greenland is expected to become more frequent in the coming decades, our findings portend a previously unforeseen vulnerability of the Greenland ice sheet to climate change

    Force balance along Isunnguata Sermia, west Greenland

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    Ice flows when gravity acts on gradients in surface elevation, producing driving stresses. In the Isunnguata Sermia and Russel Glacier catchments of western Greenland, a 50% decline in driving stress along a flow line is juxtaposed with increasing surface flow speed. Here, these circumstances are investigated using modern observational data sources and an analysis of the balance of forces. Stress gradients in the ice mass and basal drag which resist the local driving stress are computed in order to investigate the underlying processes influencing the velocity and stress regimes. Our results show that the largest resistive stress gradients along the flowline result from increasing surface velocity. However, the longitudinal coupling stresses fail to exceed 15 kPa, or 20% of the local driving stress. Consequently, computed basal drag declines in proportion to the driving stress. In the absence of significant resistive stress gradients, other mechanisms are therefore necessary to explain the observed velocity increase despite declining driving stress. In the study area, the observed velocity - driving stress feature occurs at the long-term mean position of the equilibrium line of surface mass balance. We hypothesize that this position approximates the inland limit where surface meltwater penetrates the bed, and that the increased surface velocity reflects enhanced basal motion associated with seasonal meltwater perturbations
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