No description available.

This thesis examines the hydrological characteristics of West Washmawapta Glacier, an overdeepened cirque glacier located in the Vermillion Range, B.C., Canada. Fieldwork involved drilling nine boreholes, which were surveyed with a borehole camera and instrumented with combinations of pressure transducers, thermistors and electrical conductivity sensors. Results show the cirque hydrology consisted of a predominantly subglacial, distributed drainage system. Hydraulic jacking occurred within the overdeepened region in both the summers of 2007 and 2008. Hydrological shut-down occurred very late, potentially due to the effect of the riegel on basal drainage, preventing flow out of the overdeepening at lower water pressures. Basal water temperatures were observed to fluctuate diurnally (up to 0.8^0C) above the local pressure melting point, likely due to influx of geothermally-heated groundwater and insulation of water within a sediment aquifer. Varying basal water pressures and temperatures suggest that hydraulic potential and supercooling effects are often over-simplified in glacier studies.

We use a physical model to investigate how changes in the distribution of subglacial hydrology affect ice motion of Antarctic ice streams. Ice streams are modeled using silicone polymer placed over a thin water layer to mimic ice flow dominated by basal sliding. Dynamic similarity between modeled and natural ice streams is achieved through direct comparison of the model force balance and the observed force balance of Whillans Ice Stream (WIS). The WIS force balance has evolved over time due to increased basal resistance. We test two hypotheses: 1) the subglacial water distribution influences the ice flow speed and thus the force balance and; 2) shear margins are locations where transitions in water layer thickness occur. The velocity and force balance are sensitive to pulsed water discharge events and changes in lubrication associated with sticky spots, and model shear margins tend to overlie water lubrication boundaries. Local changes in basal lubrication near margins (possibly as a result of the presence of sticky spots or subglacial lakes) influences the stability of the margin position and may be responsible for large and rapid shifts in margin location. / text

No description available.

No description available.

Records of subglacial drainage features of previous ice sheets, such as tunnel channels (TCs), provide insight into drainage mechanisms of modern ice sheets. More than 60 tunnel channels were identified in the formerly glaciated landscape of Wisconsin, USA, in the footprint of the Green Bay Lobe of the Laurentide Ice Sheet. I used a combination of a reflection-seismic survey and a hydraulic-potential model to reveal the geometry of a TC and attempt to further understand the dynamics of TC formation. The seismic cross section along the Plainfield tunnel channel (PTC), 16 km up-ice from the terminal margin, shows an absence of a channelized feature in the subsurface. Therefore, the Plainfield tunnel channel likely initiates between 7-16 km up-ice from its terminus in a subglacial setting where the water-flow velocity may have increased, as dictated by the regional bed slope. Relatively fast subglacial water flow would have caused erosion at the ice-bed interface, whereas slower velocities produced little to no erosion of the underlying sediments. The hydraulic-potential model suggests a total area of ~107 km2 for potential subglacial lakes up-ice from the PTC, although this estimate is not sufficient to account for the volume of water needed to form the channel. However, when water does pool in subglacial lakes, permafrost in the area helps to seal in large amounts of water at the ice-bed interface. Additionally, an increase in traction at the bed after a single discharge event likely facilitates supraglacial lake formation. The supraglacial water sources subsequently drain into the subglacial system, which can cause successive subglacial drainage events to occur. The results from the seismic cross-section and the hydraulic-potential model, along with inferences from previous studies, indicate that the PTC is likely formed over several drainage events and had a substantial influence from moulin drainage of supraglacial water in addition to the water stored in subglacial lakes. The Laurentide Ice Sheet was able to store large amounts of subglacial water in this region due to the combination of an adverse bed slope, as well as expansive permafrost during the time of TC formation. This likely means that modern ice sheets, such as along the Western edge of the Greenland Ice Sheet or Thwaites Glacier in West Antarctica, could exhibit similar subglacial and supraglacial drainage behavior in areas with adverse bed slopes similar to this portion of the Laurentide Ice Sheet. / Geology

No description available.

Concerns regarding the issue of climate change and, in particular, the rapid retreat of glaciers around the world, have placed great importance on glacial monitoring. Some of the methods most commonly used to observe glacial change--direct mass balance measurements and remote sensing--provide valuable information about glacier change. However, these methods do not address the englacial and subglacial environments. Surface meltwater that enters englacial and subglacial hydrological networks can contribute to acceleration of ice flow, increased calving on marine-terminating glaciers, surges or outburst floods, and greater overall ablation rates. Because subsurface drainage systems often freeze during the winter and re-form each summer, examining the seasonal evolution of these networks is crucial for assessing the impact that internal drainage may have on the behavior of a glacier each year. The goal of this study is to determine the role englacial and subglacial drainage system evolution plays in influencing summer ablation and discharge at the terminus of Exit Glacier, a small valley glacier located in South-central Alaska. During the summers of 2010 and 2011, we used ground-penetrating radar (GPR) to locate internal drainage features on the lower 100 meters of the glacier. GPR surveys were conducted in June and August of each year in an effort to observe the evolution of the drainage systems over the course of an ablation season. Three antenna frequencies--250, 500, and 800 MHz--were used on a dual frequency GPR so that various resolutions and depths in the ice could be viewed simultaneously. Stream monitoring was conducted to document discharge in the proglacial stream throughout the 2011 season. These data were compared with weather records to differentiate noticeable meltwater releases from precipitation events. Additionally, morphological changes in the glacier were observed through photographic documentation. Throughout the observation period, significant subglacial tunnels appeared, followed by the collapse of terminal ice above the tunnels. This phenomenon was most noticeable in 2011. These observations indicate that the internal drainage systems near the terminus of Exit Glacier became very well-developed each summer, and contributed approximately 75 meters of ice loss between June, 2010 and August, 2011.

Thesis (Ph. D.)--University of Texas at Austin, 2008. / Vita. Includes bibliographical references.

Iceland possesses numerous subglacial volcanoes (tuyas and tindars) that act as geological records of the thick ice sheets that once spanned across the country. Flat topped volcanic summits, which were able to penetrate through the covering ice, provide an estimate of the ice sheet's minimum thickness at the time the volcano was in its final eruptive stage. Iceland not only provides a large number of potential subglacial edifices to sample but is geologically interesting as it sits above a magmatic hotspot and spreading ridge, providing a long term record of volcanism during considerable climate variability. However, it is unknown exactly in which glacial cycle these volcanic edifices formed, due to the lack of accurate and reliable eruption ages. Helium isotopes are valuable tracers of the mantle source for the origin of volcanic rocks. Helium isotopes are comprised of a mixture of magmatic helium trapped whilst in the mantle, cosmogenic helium built up by interaction with cosmic rays during surface exposure and radiogenic helium produced from the in situ decay of U and Th. These helium components yield invaluable information on the mantle source, time of eruption and length of exposure respectively. The principle aim of this study is to determine whether reliable and accurate eruption ages of basaltic subglacial volcanoes in Iceland are possible, by applying a relatively new geochronological tool: U-Th/He dating. These eruption ages combined with cosmogenic derived 3He surface exposure ages of the summits helps to establish ice surface elevation and thickness fluctuations over time. Olivine bearing basalt samples were acquired from 14 tuyas, 4 tindars and 1 shield volcano across the Western and Northern Volcanic Zones in Iceland (WVZ and NVZ). Samples were collected from the base of each edifice for U-Th/He dating and the summits of 3 tuyas for cosmogenic 3He surface exposure dating. Olivine phenocrysts were hand-picked and analysed on two noble gas mass spectrometers (VG5400 and Helix-MC instruments). As part of the analytical procedure for these techniques, magmatic helium trapped in the olivine grains is also analysed. The mantle helium isotope ratios indicate that nearly all edifices sampled obtain a MORB-like source (1-18RA) except for an OIB-like (24RA) centrally located tuya, Bláfell. The helium ratios acquired in the NVZ are lower (4-11RA) than those in the WVZ (8-20RA). There are no significant differences seen in the mantle components between the different types of edifice sampled. Mantle helium isotope ratios confirm the heterogeneity of the mantle beneath Iceland. The cosmogenic 3He exposure ages of Hlöđufell and Bláfell in the WVZ and Gaesafjöll in the NVZ are determined as 0.5±7.4ka, 67±0.7ka and 108±46ka respectively. This study presents the first 3He cosmogenic exposure age determined to be over 20ka in Iceland. The low concentrations of U and Th observed in Icelandic olivines (0.002-0.04 nmol/g and 0.01-0.1 nmol/g respectively) prevent any reliable disequilibrium ages to be established. Minimum secular equilibrium U-Th/He ages are determined and provide a range of erroneously large eruption ages (0.4 - 80.4 Ma), due to potential helium implantation and under-detection of 3He released during sample analysis. Progress can be made towards determining accurate eruption ages for Icelandic basalts if certain factors are adhered to and various sources of helium are separated effectively.