PhD Thesis Defense: Ilyse Horlings

"Understanding firn dynamics: modeling and microstructure from East Antarctica"

Abstract

Glacier ice is formed from the accumulation of snow and its compaction through a transitional material called firn. Firn dynamics are fundamentally influenced by climatic factors, such as temperature and snow accumulation rate, and so are crucial for understanding a number of cryospheric applications. For example, ice-sheet mass loss contributions to sea-level rise from repeat satellite-altimetry observations depend on calculating firn density and its evolution. Past atmospheric gases in ice core bubbles are often younger than the surrounding ice, and their exact age depends on how firn closes off interconnected pores to become impermeable. Models often estimate bulk properties, such as density, to predict firn characteristics for these applications; however, recent research efforts have suggested that firn microstructure is integral for their construction.

My thesis examines and integrates new and emerging modeling and experimental methods to study firn dynamics and microstructure, and specifically centers on two East Antarctic sites of different depositional histories: South Pole and Allan Hills. I used micro-computed tomography to investigate firn microstructure, and uniaxial compression to explore its evolution in a range of temperature and overburden conditions. I integrated these methods to build and explore the relevance of two-phase modeling and pore-network modeling, which are new model frameworks for firn dynamics.

My major findings include: the large difference in material properties between air and ice in firn will reduce the effect of air diffusivity on compaction, negating the strengths of two-phase modeling (Chapter 1); uniaxial compression is an analog for natural compaction at South Pole, where we observe enhanced grain-boundary sliding in shallow samples within the transition region of the firn layer, and where warmer temperatures and higher strain promote more rapid evolution in parameters (e.g., specific surface area) (Chapter 2); spatial variability in wind speed causes distinct microstructural differences at Allan Hills, such as existence of depth hoar (Chapter 3); pore-network modeling is an efficient way to simulate firn behavior and estimate bulk properties, such as permeability, that are reflections of microstructure (Chapter 4). These results offer insight into firn’s material response to microstructure, and overall contribute to better understanding its role in the cryosphere.

Thesis Committee

• Zoe Courville (Chair)
• Kaitlin Keegan
• Mary Albert
• Hélène Seroussi

Contact

For more information, contact Thayer Registrar at thayer.registrar@dartmouth.edu.