Date of Award

2011

Level of Access Assigned by Author

Open-Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

Advisor

Gordon S. Hamilton

Second Committee Member

C.T. Hess

Third Committee Member

James Fastook

Abstract

When snow falls on glaciers or ice sheets, it persists for many tens, hundreds and sometimes thousands of years before becoming ice. The granular material in between fresh snow and glacial ice is known as firn and is generally 50 to 100 m thick over polar ice sheets. The compaction mechanism of firn into ice (called densification) has important glaciological ramifications in determination of ice sheet stability and related sea level rise effects via remote sensing altimetry. Firn densification is also important for correctly interpreting ice core paleoclimate records, especially those analyzing gases trapped in air bubbles within the glacial ice. Densification is thought to depend strongly on microstructure: the sizes, shapes, orientations and inter-particle bonds of the ice grains that make up polar firn. Microstructure-dependent densification is poorly understood and occurs in the region where two-thirds of the overall densification takes place. This work focuses on developing non-destructive methods for simultaneously evaluating changes in both the bulk density and microstructure of polar firn to better understand structuredependent densification processes. The first method is an automated density gauge which uses gamma-ray transmission methods to non-destructively produce high resolution (3.3 mm) and high precision (±4 kg m-3) density profiles of firn and ice cores. This instrument was used to collect a density profile for the first 160 m of the West Antarctic Ice Sheet Divide WDC06A deep ice core. The second method involves optical scattering measurements on firn and ice cores to determine the important microstructural parameters of ice grain and air bubble size and air-ice interface surface area. These measurements are modeled using both Monte Carlo radiative transfer and ray-tracing geometric optics methods, and are then tested against experiment using digital photography of the WDC06A core. Combining the results of both bulk density and optical scattering measurements for the same core reveals that microstructure-dependent densification did occur at this site and is readily detectable by purely photonic methods. This work lays the theoretical and experimental foundations for a novel, non-destructive and field deployable instrument for further study of structure-dependent firn densification.

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