Geological Sciences Theses and Dissertations

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Browsing Geological Sciences Theses and Dissertations by Subject "Antarctica"

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    From the Subglacial Environment to the Coastal Ocean: Exploring Feedbacks Between Glacial Meltwater and Tidewater Glacier Dynamics.
    (University of Oregon, 2023-03-24) Hager, Alexander; Sutherland, David
    Mass loss from the Antarctic and Greenland ice sheets has accelerated in recent decades and is predicted to contribute < 40 cm of mean sea level rise in the 21st Century. However, there is significant uncertainty in projections of ice sheet mass balance arising from unknowns in the dynamic response of tidewater glaciers to ocean forcing. At both the ice-ocean and ice-bed boundaries, glacial meltwater plays a vital role in governing the dynamics of tidewater glaciers, yet many meltwater processes are difficult to observe and are subsequently parameterized with unvalidated approximations in ice sheet models. Here, I employ a suite of numerical modeling experiments and observations to investigate how glacial meltwater at the bed and in the ocean affects the susceptibility of tidewater glaciers in Antarctica, Alaska, and Greenland to enhanced ocean forcing. It has historically been assumed that the formation of channelized subglacial drainage beneath Antarctic ice sheets is not possible, leading to the use of simplifying parameterizations of subglacial drainage under Antarctic ice sheets. However, recent observations have suggested subglacial channels exist beneath some Antarctic tidewater glaciers and could have a substantial impact on ice shelf ablation and glacier dynamics. In Chapter II, I pair numerical modeling experiments with observed radar specularity content from Thwaites Glacier, West Antarctica, to demonstrate that enough basal meltwater exists to form subglacial channels, which increase frontal ablation and basal friction beneath Thwaites Glacier. In Chapter III and IV, I transition to investigating the impact of glacial meltwater on glacial fjord dynamics. Leveraging numerical modeling with hydrographic observations from LeConte Bay, Alaska, I show that the sill-driven mixing and buoyancy forcing of subglacial discharge drives strong seasonal circulation regimes in LeConte Bay and may impede ice sheet models from accurately parameterizing ocean thermal forcing of tidewater glaciers. I then run further modeling experiments to test the accuracy ocean thermal forcing parameterizations in Greenland ice sheet models. By identifying the dominant local controls on local water transformation, I develop simple improvements to existing thermal forcing parameterizations that decrease parameterization error by < 89%. This dissertation includes previously published and co-authored material.
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    ItemOpen Access
    Quantifying the Effects of Lateral Advection on Shear Margin Thermal Structure and Meltwater Production
    (University of Oregon, 2020-12-08) Hunter, Pierce; Rempel, Alan
    Ice streams transport the majority of the ice lost to the ocean from the Antarctic Ice Sheet. In many cases these fast-flowing streams are bordered by nearly stagnant ice ridges that supply cold ice through lateral advection. The interplay between concentrated viscous heating in shear margins and advective cooling from ridge influx and surface accumulation determines margin ice temperatures, which determine ice viscosity, hence exerting a primary control on peak strain rates and ice stream velocity. A significant meltwater supply, which has potential to lubricate the ice stream bed, comes from basal friction and also from viscous dissipation within temperate zones that are predicted to develop within some shear margins. The presence of temperate ice in the shear margin also alters the melt distribution to the bed, and has been invoked as a control on ice stream widening. Here, we present a quasi-three-dimensional, steady-state ice stream model that includes both temperature-dependent ice rheology, and lateral and vertical advection. Through the study of an idealized ice stream we identify key parameters, and examine how varying conditions over realistic values alters ice stream behavior. We then consider a specific, natural system, Bindschadler Ice Stream, testing our model against measured data and previous studies. We also utilize predicted future climate conditions from CMIP5 and find a warming polar region could increase shear melting within Bindschadler, if steady-state is reached, by up to 750%, corresponding to a near tripling in total meltwater supply to the subglacial system. The expanded temperate ice zone at Bindschadler is also predicted to result in an increase to centerline velocity of up to 200%.