21 January 2022

PhD defense by Julien Westhoff

PhD defense:

Congratulations to Julien Westhoff at Physics of Ice, Climate and Earth who successfully defended his PhD thesis on 21 January 2022, and obtained the degree of Doctor of Philosophy.

Julien Westhoff presenting his PhD defense on Zoom
Julien Westhoff defending his PhD thesis on Zoom

Title: Visual Stratigraphy of the EastGRIP Ice Core - Of the Lost Ice Core Orientation, Deformation Structures, Extreme Warm Events, and Trapped Ancient Air

Abstract: The polar regions are the ideal place to observe the effects of climate change. Ice sheets accumulate annual layers of snow, and later ice, over millennia and thereby create a valuable climate archive. To date, most ice cores have been drilled to unravel the secrets of the past climate, i.e. to reconstruct atmospheric conditions and temperatures.

With the ongoing EastGRIP ice core drilling, the main focus is to increase our understanding of the flow of ice. We can then make better predictions of how much, and especially how fast, the Greenland ice sheet will contribute to sea level rise. To learn about ice flow, the core is drilled through the fast moving Northeast Greenland ice stream (NEGIS).

The main focus of my work lays on understanding ice flow on the centimeter scale. To investigate small-scale features in the ice cores, I use high resolution line scan images, which cover the entire 2120 m of the ice core drilled so far.

To date, all images-data were analyzed without knowledge about the orientation of the sample. For an accurate description and analysis of deformation structures in ice, knowledge of the orientation of the sample is crucial. Thus, I have developed a method to reconstruct the orientation of physical properties image samples.

I use this newly won knowledge to analyze and interpret small-scale deformation structures found in the line scan images. I find sudden changes in the tilt of layers and interpret these as duplex structures, where stacks of layers imbricate between shear zones. Understanding the small-scale structures will help advance our knowledge on ice flow and how ice internally deforms. Furthermore, it can help unfold disturbed and folded stratigraphy in the bottom sections of ice cores, and thus extend climate archives even further into the past.

I also analyze the line scan images to investigate prominent, millimeter-thick melt layers in the ice stratigraphy. These melt layers are caused by melting on the surface of the ice sheet and tell us about extreme warm events. I provide a reconstruction of these melt events on the Greenland ice sheet over the past 10,000 years.

As snow falls, and is buried over the years, the air between the single grains is trapped and enclosed into the ice matrix. Ancient air is preserved and enables us to reconstruct atmospheric conditions of the past. The depth at which the air is sealed into the matrix, the lock-in depth, is crucial to determine the age of the air. By analyzing reflections caused by rounded air-ice interfaces, I have developed an optical method to find the lock-in depth.

My findings presented in this thesis can be considered a large boost in ice coring knowledge.

Not only have I further extended the application range of the line scan device, I have also solved the problem of determining ice core orientation, which had persisted for decades.

I have applied the newly won knowledge to interpret deformation structures. Furthermore, I have created a valuable melt layer archive and found a low cost solution to determine the lock-in depth of air into the ice matrix.

Supervisors
Dorthe Dahl-Jensen (supervisor), Ilka Weikusat (AWI, co-supervisor)

Assessment of Committee
Professor Mary Albert, Thayer School of Engineering, Dartmouth College
Professor Nobuhiko Azuma, Nagaoka University of Technology, Department of Mechanical Engineering
Professor Jørgen Peder Steffensen, Ice, Climate and Earth, Niels Bohr Institute, University of Copenhagen