Browsing by Subject "Subglacial hydrology"

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The impact of subglacial hydrology on force balance for a physically modeled ice stream
(2012-05) Wagman, Benjamin Moore; Catania, Ginny A.; Mohrig, David; Buttles, James
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.
Spatial and temporal evolution of the glacial hydrologic system of the western Greenland ice sheet : observational and remote sensing results
(2015-12) Andrews, Lauren Cristy; Catania, Ginny A.; Cardenas, Meinhard Bayani; Hoffman, Matthew J.; Jackson, Charles S.; Mohrig, David
The Greenland Ice Sheet is losing mass at an accelerating rate due to a combination of increased surface melting and changes in dynamical behavior, both of which are associated with changing climate. In the ablation zone, seasonal melting results in a dynamic ice-sheet response as supraglacial meltwater reaches the ice–bed interface via moulins and crevasses. Meltwater delivery to the bed increases subglacial water pressure and decreases basal traction, leading to regional ice acceleration. However, these processes and their future evolution are poorly constrained. An improved understanding of the complex relationship between the glacial hydrologic system and ice velocity will ultimately improve predictions of ice-sheet mass change. In this dissertation, I use a suite of techniques to quantify the response of the glacial hydrologic system to changes in melt supply on daily to inter-decadal timescales. Moulins represent the primary englacial connection between the ice surface and its bed. As such, they play a critical role in determining the location of subglacial channels in the ablation zone. I observe inter-decadal persistence in moulin locations, which can result in positive feedbacks that allow for rapid growth at the onset of the melt season and encourage persistence of subglacial channels. These observations suggest that inter-decadal variability in the relationship between supraglacial melt production and ice velocity is caused by altering the rate at which efficient subglacial drainage pathways develop. Further, my observations indicate that daily changes in ice velocity are mirrored by moulin water levels, but this pattern does not hold at seasonal timescales. This relationship suggests that the channelized portion of the subglacial hydrologic system adjusts rapidly to the available meltwater; therefore, long-term trends in ice velocity are the result of increasing hydrologic connectivity of poorly connected regions of the bed, lowering regional subglacial water pressure. Finally, the subglacial hydrologic system experiences variability on multiple timescales, some of which are not accounted for in existing models of this system. By modeling the mechanisms causing both diurnal to seasonal and changes in moulin water level, I further constrain the physical processes impacting mass change in land-terminating regions of the Greenland Ice Sheet.