MSc Thesis Defense by Jon Arrizabalaga-Iriarte and Lide Lejonagoitia-Garmendia

Title: Firn densification under arbitrary stresses

Abstract:
Firn densification refers to the transformation of snow into ice, a process that directly impacts the interpretation of past climatic records from ice cores and mass-loss estimates from altimetry data, among others. This compaction is driven by a complex combination of mechanisms yet to be understood, which compels the use of empirical models that usually assume the absence of horizontal stresses. However, Gagliardini and Meyssonnier (1997) developed a compressible porous law that describes how firn deforms when exposed to arbitrary stress configurations. In this project, we have recalibrated this semi-empirical model by extending the analysis to in-situ depth-density measurements from six Greenlandic sites. The results of this calibration process are compatible with a universal set of model parameters but suggest that the widely used original values can be improved by a more extensive study like this. After the calibration, we have found that the densification model can only reproduce the shallower firn columns found around the shear margins of the Northeast Greenland Ice Stream if the horizontal stresses are considered. This result shows that the stress-induced strain softening affects the densification rates and, thus, firn densification models should be forced not only by the accumulation rate and temperature but by the horizontal stresses as well. Finally, we propose a method that can give insights into potential structural improvements that maximize the durability of future firn tunnels. We have found that, consistent with field observations, this optimization scheme favors narrower tunnels with an arch-like ceiling, and supports the idea of filling additional supporting walls around.

Supervisors:  Aslak Grinsted, Christine S. Hvidberg, Nicholas M. Rathmann
External censor: Sebastian B. Simonsen