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Theses Doctoral

Geophysical perspectives of subglacial settings and their influence on glacial dynamics

Block, Adrienne

The subglacial settings of glaciers and continental ice sheets serve as a fundamental, long term control on the dynamics of the ice but are also obscured by the great thickness of the ice. Integrated analysis of geophysical datasets is a fundamental tool for understanding a glacier's underpinnings. Using a combination of gravity, magnetic, radar and surface elevation data, we examine the relationship between the ice and its subglacial setting in three chapters that address the question on varying scales. The first explores the Gamburtsev Subglacial Mountains, a region of high topography in the center of East Antarctica. This range is thought to be the nucleation site of the East Antarctic Ice Sheet despite the lack of evidence to constrain its age. The geophysical data suggest a crustal architecture which, when compared with global mountain ranges, provides insight to the age and origin of the Gamburtsev Subglacial Mountains. We conclude that the mountains are over crust that was thickened in the Precambrian but have been been reactivated, creating their high modern relief. In the second chapter, we explore the Recovery Ice Stream in East Antarctica. Here the onset of fast flow is more than 500 km in the ice sheet interior and is thought to be linked to periodic drainages from four large Recovery Lakes. Using new aerogeophysical data in the region, we uncover two tectonic boundaries that each play a key role in the dynamics of the ice stream. The inland boundary limits the catchment of the lakes and their potential for frequent flood events. The second boundary is a transition from rugged bedrock to smooth, low-lying sediments. We conclude that the basal sediments facilitate fast flow independent of water input from the large Recovery Lakes. In the third and final chapter, we examine Jakobshavn Isbrae, West Greenland, the fastest moving non-surge glacier. Since the loss of its floating ice tongue beginning in 1998, this glacier has accelerated and now maintains fast flow speeds throughout the year. The glacier has a known subglacial trough and high driving stresses but even these optimal conditions cannot explain the late summer velocities. We find that the geophysical signals in the Jakobshavn Isbrae region require more geophysical contrasts than the coastal outcrops suggest. Specifically, we interpret a trough-centered gravity low as evidence of sediments underlying the radar-detected trough. These sediments facilitate fast flow by reducing basal resistance. Though Jakobshavn Isbrae is a glaciological endmember, its fate is linked to the underlying geology and best demonstrates the need for continued aerogeophysical survey to constrain subglacial settings. Understanding conditions at the bed is fundamental to understanding the state and fate of the planet's ice.

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More About This Work

Academic Units
Earth and Environmental Sciences
Thesis Advisors
Bell, Robin E.
Degree
Ph.D., Columbia University
Published Here
July 21, 2015
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