Ice Motion Across Deep Fjords

Abstract

The behaviour of ice crossing deep subglacial valleys has not been studied in detail, and this has important consequences for ice sheet modelling. In Western Norway striation markings left by the Scandinavian Ice Sheet indicate perpendicular ice motion crossing deep fjords (Mangerud et al., 2019).

Using a numerical model, this thesis presents a detailed picture of ice motion for deep and narrow fjords with perpendicular ice flow. For a section of the Scandinavian Ice Sheet, ice motion is modelled for Veafjorden, a fjord in Western Norway with a relief of ~1300 meters. Perpendicular ice motion across deep fjords show large, spiralling Moffat eddies that transport ice laterally along the fjord. At the base of the fjord, flow is reversed, contrary to assumptions made by known basal sliding relationships. The steep fjord topography reduce the surface velocity above the fjord. Motion of ice inside the fjord itself is slowed down to roughly an eighth of the average surface velocity.

Numerous locations in western Norway share similar traits to Veafjorden. Potential locations are also found in Greenland that resemble in subglacial valley orientation and flow direction. In general, current ice sheet models use a spatial resolution that is too coarse to capture the details of Veafjorden and similar areas. An additional problem is the fidelity of bedrock elevation maps in Greenland. The widely used subglacial elevation model "BedMachine" has very low fidelity, especially in the interior of Greenland.

The model results of Veafjorden are compared to a smoothed control domain, which represents the basal topography of BedMachine. BedMachine control simulations do not show flow reversal, lateral transport, or Moffat eddies. Results show a great discrepancy between actual and low fidelity topography. However, after adjustments for driving stress (slope), velocity patterns resemble each other and basal motion reflects similarly on surface velocity.

Several ice streams in Greenland also show similar changes in surface velocity, reflecting the findings at Veafjorden. These findings indicate that subglacial valleys with perpendicularly flow could be widespread in previous and current ice sheets. The flow patterns identified for Veafjorden points to challenges in defining basal sliding relationship for ice sheets, which is an important task for future ice sheet and sea level predictions.