My research focus on the flow dynamics of ice sheets and in particular the role of ice shelves – the floating areas of ice at the coast of the continental ice sheets. They are crucial for providing resistance to the flow of grounded ice.
I use a number of tools to help improve understanding: ice-sheet modelling; idealized mathematical models; laboratory-scale fluid-mechanical experiments; and analysis of geophysical data, such as ice-surface velocities and ice-penetrating radar.
Ice-Shelf Basal Melt Channels Stabilized by Secondary Flow
Ice-shelf basal melt channels form due to concentrated ocean melting and have been observed on many Antarctica ice shelves from satellite remote sensing. Basal channels can reduce ice-shelf structural integrity, potentially destabilizing ice shelves by full-depth incision. We use the full-Stokes ice flow model ELMER/Ice to investigate the viscous ice response to the presence of a basal channel.
Secondary flow increases as a basal channel is incised by concentrated ocean melting, with flow directed towards the channel centre.
Antarctic Ice Shelf Vulnerability to Meltwater Fracturing
Henry Ice Rise:
The Henry Ice Rise is a grounded area of ice found within the floating Ronne Ice Shelf, in the Weddell Sea sector of West Antarctica. Ice rises act as pinning points for ice shelves, providing resistance to flow and buttressing the ice upstream of them. This reduces the rate at which grounded ice is discharged into the ocean and therefore the contribution of the ice sheets to sea-level rise.
Ground-based ice-penetrating radar data has revealed a collection of relic crevasses at the base of the Henry Ice Rise, which could not have formed in the current flow configuration. Their presence suggests that the ice rise has grown from a previously smaller configuration and crevasses that formed at former grounding lines have since been buried and deformed.
The details of this process are report in an article in the Journal of Geophysical Research: Earth Surface:
Wearing, M. G., & Kingslake, J. (2019). Holocene Formation of Henry Ice Rise, West Antarctica, Inferred From Ice‐Penetrating Radar. Journal of Geophysical Research: Earth Surface, 2018JF004988.
An overview of the work can be seen below in a poster presented at the AGU 2017 Fall Meeting in New Orleans.
This radar data also provides crucial evidence for the Holocene evolution of the West Antarctic Ice Sheet and suggests that the grounding line may have retreated upstream of it’s current position following the Last Glacial Maximum (LGM). Further evidence of grounding line retreat was found in subglacial sediments in the Ross Sea Sector 200 km upstream of the current grounding line, which showed exposure to a marine environment within the past 40,000 yrs. Ice-sheet model simulations suggest that the re-advance of the ice sheet was in part controlled by Glacial Isostatic Adjust (GIA) and ice-shelf grounding on bedrock outcrops to form pinning points.
Kingslake, J., Scherer, R P., Albrecht, T., Coenen, J., Powell, R D., Reese, R., Stansell, N D., Tulaczyk, S., Wearing, M. G. and Whitehouse, P L. (2018). Extensive retreat and re-advance of the West Antarctic Ice Sheet during the Holocene. Nature, 558(7710), 430–434. https://doi.org/10.1038/s41586-018-0208-x
Analysis of Ice Surface Velocities:
Insights into the flow and dynamics of ice shelves can be sought through analysis of the surface velocity field. Using ice surface velocities, derived from interferometric synthetic aperture radar (InSAR) (reference: MEaSURES: Rignot, E., Mouginot, J., & Scheuchl, B. (2011). Ice Flow of the Antarctic Ice Sheet. Science) strain rate fields can be generated and analysed. Figure (1) shows plots for the Fimbul Ice Shelf, Antarctica. Ice flows out from a narrow glacier at the lower boundary of the images and spreads out into a wide ice shelf. The most prominent areas of deformation are in the margins of this tongue of fast flow, with large strain and shear rates. These areas correspond to the largest principal strain rates, which are orientated at approximate 45 degrees to the main flow. From the visual MODIS images (Haran, T., Bohlander, J., Scambos, T., Painter, T., & Fahnestock, M. (2005). MODIS Mosaic of Antarctica 2003-2004 (MOA2004) Image Map.) it is clear that these areas also contain extensive crevassing.
Idealized mathematical models:
A large part of my PhD work consisted of using mathematical models to improve understanding of the flow and buttressing of ice shelves. Ice shelves are the floating extensions of ice sheets, which flow out over the ocean. They are often confined within embayments and provide resistance to the flow of grounded ice upstream, which is referred to as ice-shelf buttressing.
I considered the flow of ice shelves in parallel and linearly diverging channels. In these cases the flow of the ice shelf is mainly controlled by the shear stress between the lateral side walls, until a region near the ice front (the end of the channel), where both extensional and shear stresses control the dynamics.
Figure (2) below shows the analytical solution for a steady-state ice shelf under the assumption that flow is controlled by lateral shear. This can be compared to a numerical solution for an ice shelf with both extension and shear dynamics in Figure (3).
I have also used geophysical data in the form of ice surface velocities, strain rates, shelf widths and ice thickness to confirm an analytical scaling relationship for the flow of ice at a laterally confined calving front, where the ice shelf contains ice of a uniform rheology. This scaling relationship takes the form:
u∝ w(εH)^(3/4), where u is the centreline velocity, w is the calving front width, ε is the along flow strain rate and H is the ice thickness, all evaluated at the calving front.
Figure (4a) shows a linear regression in log-log space, confirming the relationship determined analytically. Data points represented by purple crosses all lie above the regression line and correspond to ice shelves that are not laterally confined, or that have weak margins leading to greater than expected velocities at the calving front.
For more details see:
Lab-scale fluid-mechanical experiments:
During my PhD at the Department of Applied Mathematics and Theoretical Physics at the University of Cambridge I undertook a series of fluid-mechanical experiments designed to simulate ice shelves confined within an idealized embayment. In these experiments parameters such as, channel width, length, fluid rheology and flux rate could be varied and directly compared with numerical models. Schematics of the experimental set-up are shown in Figures (5a) & (5b).
Figure (6) shows a side view of the floating current, flowing from right to left. The surface of the viscous current is marked by a dashed orange line, while the ocean surface is marked with a red dashed line. White tracer particles are distributed on the surface of the current and are used to infer the surface velocity field from images captured from a plan-view camera.
Below (Fig. (7)) are some results from a Newtonian model ice shelf, where the viscous current is made of Golden Syrup. The red dashed line corresponds to the location of the grounding line, with the current flowing from top to bottom. A clear shear profile can be seen across the channel with no-slip at the side walls and maximum speed in the centre of the channel. In the upper two thirds of the channel the dynamics are dominated by shear and the along-flow strain rate is low. In the final section there is an increase in extensional strain, as predicted by theoretical models. This can be seen in both the plan view strain rate and centreline velocity.