PhD defence by Alexandra Messerli

Surface velocities and hydrology at Engabreen: observations from feature tracking and hydro-meteorological measurements

Recent studies have likened the seasonal observations of ice flow at the marginal regions of the Greenland Ice Sheet (GrIS) to those found on smaller alpine and valley counterparts. These similarities highlight the need for further small scale studies of seasonal evolution in the hydrological and dynamic structure of valley glaciers, to aid interpretation of observations from the margins of the GrIS. This thesis aims to collate a large suit of glacio-hydrological data from the outlet glacier Engabreen, Norway, in order to better understand the role the subglacial drainage configuration has on surface velocities recorded at the site.

The Svartisen Subglacial Laboratory (SSL) under Engabreen, augmented by additional subglacial pressure and hydrological measurements, provides invaluable observations for detailed process-oriented studies. However, the lack of complementary surface velocity data complicates comparisons with other surface-oriented glacio-hydrological studies. One major aim of this thesis is to provide a longer record of surface velocity, enabling a more complete understanding of the glacial hydro-mechanical relationship at Engabreen. In order to extend the velocity dataset here, a time-lapse camera based study was carried out, providing seasonal velocity maps over a large portion of an inaccessible region of the glacier. The processing and feature tracking of terrestrially based imagery, in order to obtain quantitative velocity measurements, is challenging. Whilst optical feature tracking has a relatively long history in glaciology, the availability of adaptable terrestrial georectifiaction and feature tracking tools is somewhat limited. A key achievement of this thesis is the development of a new, comprehensive, georectification and feature tracking toolbox, ImGRAFT. This adaptable tool performs both rectification of the time-lapse camera imagery, and feature tracking. The resulting velocity estimates are able to capture seasonal changes in velocity structure at Engabreen, as well as short-term speed-up events, in response to hydro-meteorological forcings.

GPS velocity data from the lower tongue of Engabreen are analysed in detail alongside the hydro-meteorological time-series. The higher temporal resolution of the GPS allows the effect of short-term hydrological forcings on ice flow to be assessed. Two key events: the spring-speed up event (P1a) and a short-term rain induced event (P1b), provide the focus for the analysis, where causal relationships between the changes in the surface velocities are discussed within the context of changes in the subglacial hydrological system. Engabreen exhibits a classic spring-speed up event, but appears insensitive to subsequent pulse inputs of meltwater.

Interpretation of these data are aided by examining geomorphologial evidence at Engabreen. The recent deglaciation at the margin of the glacier provides insight into the detailed structure of the former, and current, subglacial drainage system. The unique geomorphology observed on the bedrock at Engabreen, characterised by deeply incised perpendicular cutting canyons, suggests a mixed Nye-channel linked-cavity drainage configuration. These prominent features are likely to explain the lower sensitivity of Engabreen to pulse inputs of meltwater.

Academic advisors:
Aslak Grinsted
Dorthe Dahl-Jensen
Nanna Karlsson
Centre for Ice and Climate, Niels Bohr Institute, University of Copenhagen, Denmark
Miriam Jackson
Norwegian Water Resources and Energy Directorate (NVE), Oslo, Norway

Chair of defense committee
Thomas Blunier, Centre for Ice and Climate

Assessment Committee:
Bryn Hubbard & Peter Nienow, UK

If interested please contact Alexandra Messerli for a copy of the thesis