Evidence of marine ice-cliff instability in Pine Island Bay from iceberg-keel plough marks

Paper published in Nature: Matthew G. Wise, Dowdeswell, J.A., Jakobsson, M., and Larter R.D. ‘Evidence of marine ice-cliff instability in Pine Island Bay from iceberg-keel plough marks.’ Nature (2017). DOI: doi:10.1038/nature24458

Thousands of marks on the Antarctic seafloor, caused by icebergs which broke free from glaciers more than ten thousand years ago, show how part of the Antarctic Ice Sheet retreated rapidly at the end of the last ice age as its margins balanced precariously on sloping ground and became unstable. Today, as the global climate continues to warm, rapid and sustained retreat may be close to happening again, and could trigger runaway ice retreat into the interior of the continent.

Ploughmarks in Pine Island Bay Icebraker Oden close to an Iceberg. Photo: Björn Eriksson

The study shows as seas warmed at the end of the last ice age, Pine Island Glacier retreated to a point where its grounding line – the points where it enters the ocean and starts to float – was perched precariously at the end of a seaward-shallowing submarine slope. It has long been thought that glaciers in this configuration are unstable. Break up of a floating ‘ice shelf’ in front of the glacier left tall ice ‘cliffs’ at its edge. The height of these cliffs made them unstable, triggering the release of thousands of icebergs into Pine Island Bay, and causing the glacier to retreat rapidly until its grounding line reached a restabilising point in shallower water.

Today, as warming waters caused by climate change flow underneath the floating ice shelves in Pine Island Bay, the Antarctic Ice Sheet is once again at risk of losing mass from rapidly retreating glaciers. Significantly, if ice retreat is triggered, there are no relatively shallow points in the ice sheet bed along the course of Pine Island and Thwaites glaciers to prevent possible runaway ice retreat into the interior of West Antarctica. Pine Island Glacier and the neighbouring Thwaites Glacier are responsible for nearly a third of total ice loss from the West Antarctic Ice Sheet, and this contribution has increased greatly over the past 25 years. In addition to basal melt, the two glaciers also lose ice by breaking off, or calving, icebergs into Pine Island Bay.

Today, the icebergs that break off from Pine Island and Thwaites glaciers are mostly large table-like blocks, which cause characteristic ‘comb-like’ ploughmarks as these large multi-keeled icebergs grind along the sea floor. By contrast, during the last ice age, hundreds of comparatively smaller icebergs broke free of the Antarctic Ice Sheet and drifted into Pine Island Bay. These smaller icebergs had a v-shaped structure like the keel of a ship, and left long and deep single scars in the sea floor. The multibeam bathymetric data used to investigate the shape and distribution of ploughmarks on the sea floor in Pine Island Bay, made it possible to analyse the relative size and drift direction of icebergs in the past. The analysis showed that these smaller icebergs were released due to a process called marine ice-cliff instability (MICI). More than 12,000 years ago, Pine Island and Thwaites glaciers were grounded on top of a large wedge of sediment, and were buttressed by a floating ice shelf, making them relatively stable even though they rested below sea level.

Eventually, the floating ice shelf in front of the glaciers ‘broke up’, which caused them to retreat onto land sloping downward from the grounding lines to the interior of the ice sheet. This exposed tall ice ‘cliffs’ at their margin with an unstable height, and resulted in rapid retreat of the glaciers from marine ice cliff instability between 12,000 and 11,000 years ago. This occurred under climate conditions that were relatively similar to those of today.

The data used in this study were collected by Swedish icebreaker Oden and the British research vessel James Clarke Ross.

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