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O'Neel, Shad, 2006

Understanding the mechanics of tidewater glacier retreats: Observations and analyses at Columbia Glacier, Alaska

Bibliographic Reference

O'Neel, Shad, 2006, Understanding the mechanics of tidewater glacier retreats: Observations and analyses at Columbia Glacier, Alaska: University of Colorado, Boulder, Ph.D. dissertation, 232 p.

Abstract

Columbia Glacier is a large, temperate tidewater glacier located in south-central Alaska, which has been in dynamic retreat since the early 1980's. Dynamic retreat is driven by dynamics rather than by mass imbalances. At Columbia Glacier, rapid flow over the past 30 years has transferred ice to the ocean at rates as high as 7 km 3 yr-1 , while calving of icebergs at a slightly higher rate has caused shortening of the glacier by ?16 km. Dynamic instabilities are common to marine-terminating glaciers, and allow very rapid land-to-sea mass transfer, such that these glaciers constitute one of the largest components of global eustatic sea-level rise, and by a wide margin the fastest acting component. Recent onsets of dynamic retreat in Greenland highlight the importance of understanding this poorly constrained process. This dissertation aims to improve our knowledge of flow and calving mechanics during marine-terminating glacier retreat. I first present results of time series analyses of glacier geometry, velocity, strain-rate and the associated forces controlling ice flow during the period 1977-2001, spanning the entire retreat. I then present results from a passive seismic array deployed at the glacier during 2004-05. Seismic analysis focused on detection and characterization of calving processes, and our results describe the unique nature of seismic energy produced during calving events. New methods were developed to directly detect the occurrence and estimate the magnitude of individual events. We analyzed temporal and energy distributions of calving events, and demonstrate a power law distribution for calving energy produced. Combined with analysis of the origin of the waveforms, this research suggests that calving results from complex interactions between small, unconnected fractures formed as the ice is transported to the terminus. Our research suggests that glacier geometry is critical in controlling secular variations in flow and calving while tides, water inputs, velocity gradients and internally regulated processes govern short-lived perturbations to both processes, but in different ways.

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