UVM Theses and Dissertations
Format:
Online
Author:
Rumrill, Julie A.
Dept./Program:
Geology
Year:
2009
Degree:
MS
Abstract:
In this thesis, I present results from a two-year study of strain-rate variations along a flow line on the western margin of the Greenland ice sheet. I used baseline network solutions to investigate variations in longitudinal strain rates over the 2006 and 2007 melt seasons. Analyses revealed high-magnitude, short-duration events of increased longitudinal strain early in the melt season coincident with a high melt year, suggesting a link between melt production and its effects on seasonal ice flow.
Results from 2006 data show that longitudinal strain rates became variable shortly after the onset of melt (day 186) changing up to ~ 15 x 10⁻⁴ a⁻¹ within 24 hours. The onset of melting occurred earlier in 2007 (day 153) and was also followed closely by strain-rate deviation from background rates calculated prior to melting. The data revealed rapid (hours to days), high-magnitude (two to ten times greater than background rates) changes in longitudinal strain rates (hereafter referred to as 'high-strain' events) that occurred both on the small-scale (affecting 1-4 baselines) and on the large-scale (affecting 5 or more baselines). Large-scale high-strain events were infrequent, on the order of two events per season. Events were likely caused by drainage of supraglacial meltwater that penetrated to the bed of the glacier raising the basal water pressure. The increase in pressure reduced the basal resistive stress, and allowed rapid local acceleration. The basal stress reduction was transmitted to areas of higher stress which resulted in longitudinal compression of the ice down glacier and longitudinal extension up glacier. The evolution of high-strain events altered longitudinal strain rates more than 15 km along flow from the site of initiation. I estimated the origin and spatial extent of highstrain events by assessing the magnitude of the strain-rate variations in various baselines, and observing whether the altered strain regime was extensive or compressive. Magnitude and timing of changes in strain suggest that high-strain events originated in the ablation zone, the equilibrium zone, and inland of the equilibrium zone, and indicate that short-term altered stress conditions are not confined to the ablation zone.
The background strain-rate for 2007 (~ -7 x 10⁻⁴a⁻¹ for a 37 km longitudinal baseline) was similar to the 2006 longitudinal background rate. When extrapolating the 2006 background rate over the melt season, the expected change in baseline length (~ 11 m) was similar to the observed change (~ 9 m). In contrast, when extrapolating the 2007 background rate over the melt season, the expected shortening was ~ 6 m, but the observed shortening was less than 1 m. This result suggests that seasonal high-strain events have the ability to alter longitudinal baseline length, allowing a greater ice flux to lower elevations where melting occurs for a larger portion of the year. However, the cumulative seasonal effects of both large-scale and small-scale strain events are modest, and indicate that seasonal changes in strain rates have a minor effect on the overall stability of the ice sheet. Nevertheless, it is possible that over much longer timescales these seasonal changes may become more important with increasing temperatures and available melt. Results from this study may also be useful in making broader inferences regarding the response of grounded portions of the ice sheet to seasonal changes in basal resistive stress.
Results from 2006 data show that longitudinal strain rates became variable shortly after the onset of melt (day 186) changing up to ~ 15 x 10⁻⁴ a⁻¹ within 24 hours. The onset of melting occurred earlier in 2007 (day 153) and was also followed closely by strain-rate deviation from background rates calculated prior to melting. The data revealed rapid (hours to days), high-magnitude (two to ten times greater than background rates) changes in longitudinal strain rates (hereafter referred to as 'high-strain' events) that occurred both on the small-scale (affecting 1-4 baselines) and on the large-scale (affecting 5 or more baselines). Large-scale high-strain events were infrequent, on the order of two events per season. Events were likely caused by drainage of supraglacial meltwater that penetrated to the bed of the glacier raising the basal water pressure. The increase in pressure reduced the basal resistive stress, and allowed rapid local acceleration. The basal stress reduction was transmitted to areas of higher stress which resulted in longitudinal compression of the ice down glacier and longitudinal extension up glacier. The evolution of high-strain events altered longitudinal strain rates more than 15 km along flow from the site of initiation. I estimated the origin and spatial extent of highstrain events by assessing the magnitude of the strain-rate variations in various baselines, and observing whether the altered strain regime was extensive or compressive. Magnitude and timing of changes in strain suggest that high-strain events originated in the ablation zone, the equilibrium zone, and inland of the equilibrium zone, and indicate that short-term altered stress conditions are not confined to the ablation zone.
The background strain-rate for 2007 (~ -7 x 10⁻⁴a⁻¹ for a 37 km longitudinal baseline) was similar to the 2006 longitudinal background rate. When extrapolating the 2006 background rate over the melt season, the expected change in baseline length (~ 11 m) was similar to the observed change (~ 9 m). In contrast, when extrapolating the 2007 background rate over the melt season, the expected shortening was ~ 6 m, but the observed shortening was less than 1 m. This result suggests that seasonal high-strain events have the ability to alter longitudinal baseline length, allowing a greater ice flux to lower elevations where melting occurs for a larger portion of the year. However, the cumulative seasonal effects of both large-scale and small-scale strain events are modest, and indicate that seasonal changes in strain rates have a minor effect on the overall stability of the ice sheet. Nevertheless, it is possible that over much longer timescales these seasonal changes may become more important with increasing temperatures and available melt. Results from this study may also be useful in making broader inferences regarding the response of grounded portions of the ice sheet to seasonal changes in basal resistive stress.