Ephemeral grounding of ice-keels on Pine Island Glacier Ice Shelf: observations, modelling, and dynamic impacts.

In Antarctica, changes in ice dynamics dominate the ice sheet’s contribution to global sea-level rise. Changes to the continent’s ice shelves play a key role in this dynamic evolution through the buttressing that they provide to the upstream grounded ice. One striking and well observed example of the importance of ice shelves is Pine Island Glacier Ice Shelf, which has undergone dramatic changes in the modern observational period. This includes major periods of grounding line retreat and ice-shelf thinning due to influxes of warm ocean water, combined with large calving events and the disintegration of its southern shear margin. Overall, these changes have reduced the buttressing support provided by the ice shelf, leading to increased ice discharge and dynamic thinning on the inland portion of the glacier.

Concurrent with the erosion of Pine Island Glacier Ice Shelf, ice-shelf thickness anomalies originating at the glacier grounding line, known as ice keels, have regularly bumped along the bedrock underneath the ice shelf. This has caused small regions ephemeral grounding, which occur irregularly in the central shelf. While known, these events remain largely unstudied, and the effects of this ephemeral grounding on stresses within the ice shelf and the evolution of ice-shelf dynamics remain poorly understood.

Here we use a combination of satellite observations and ice sheet modelling to study the movement of a prominent ice keel over a bathymetric ridge during the period 2014-2021 and analyse the effects this had on the dynamics of the ice shelf.

To observe the grounding of the ice keel, we use the differential range offset tracking technique applied to synthetic-aperture radar (SAR) data from the European Space Agency and European Commission Copernicus' Sentinel-1 satellites to produce a dense timeseries of ice keel grounding without the need for interferometric coherence. With this dataset we track the motion of the ice keel in the last decade, showing that at times up to 10 km² of the central ice shelf was grounded. Alongside these observations, we use the BISICLES ice sheet model to analyse the impacts of this ephemeral grounding on the dynamics and stress regime of the ice shelf. Finally, we discuss our results in terms of re-grounding as a mechanism which may stabilise the retreat of marine ice sheets.