Studying an exotic form of sea ice, known as platelet ice, has enabled an international research team led by New Zealand scientists to construct a century-long record of the condition of Antarctica’s Ross Ice Shelf. The first-of-its-kind dataset indicates that over the past century this major ice shelf has remained largely stable in contrast with shelves in other parts of Antarctica.

The Ross Ice Shelf is the largest ice shelf in the world. It is fed by several glaciers that discard their ice masses into the Ross Sea, thus forming this massive ice shelf floating on Antarctic waters.
The Ross Ice Shelf is the largest ice shelf in the world. It is fed by several glaciers that discard their ice masses into the Ross Sea, thus forming this massive ice shelf floating on Antarctic waters.

In recent decades several ice shelves in the Antarctic have been thinning from below and collapsing, likely due in part to warmer water temperatures. An example is the Amundsen Sea Embayment in West Antarctica. Here warm water is driven by wind and ocean circulation under the ice shelf and melts it from below.

The question was if similar processes occur in the Ross Sea. An international researcher team, led by Professor Pat Langhorne from Otago University in New Zealand, has recently been able to show that over the past century the Ross Ice Shelf remained largely stable.

Instead of directly measuring the ice shelves, the team studied a signature in sea ice cores taken from the frozen ocean that surrounds the continent in winter. This signature revolves around the presence or absence of platelet ice, which grows at the base of sea ice when super-cooled seawater flows out from under the shelves and into the surrounding ocean.

Sketch of the Antarctic coast with glaciological and oceanographic processes. Warm water can penetrate under the ice shelf and melt it from below. Super-cooled water from the bottom of the ice shelf can form platelet ice when the pressure decreases and settle on the underside of sea-ice. Graphic by Hannes Grobe, AWI, Bremerhaven
Sketch of the Antarctic coast with glaciological and oceanographic processes. Warm water can penetrate under the ice shelf and melt it from below. Super-cooled water from the bottom of the ice shelf can form platelet ice when the pressure decreases and settle on the underside of sea-ice. Graphic by Hannes Grobe, AWI, Bremerhaven

Associate Professor Langhorne says melting at depth and refreezing in the shallower parts of an ice shelf is a natural process that has taken place over millennia. “In many places, this very cold seawater— which is on the point of refreezing—flows out from under the ice shelf to the bottom of coastal sea ice. The platelet ice this forms reflects lower temperatures at the base of the shelf which prevent its thinning.”

The colouring of ice is given by the thickness of the ice as such and the amount of air trapped in between the ice crystals. Photo: Katja Riedel
The colouring of ice is given by the thickness of the ice as such and the amount of air trapped in between the ice crystals. Photo: Katja Riedel

With their method they establish a continent-wide baseline for melt-refreeze processes at the bases of Antarctic ice shelves so that future change may be measured. Being able to monitor the ‘health’ of the bases of ice shelves is vital because the shelves act as ‘corks’ that keep the ice on continental Antarctica from flowing into the ocean and catastrophically raising global sea levels, she says.

“Once ice shelves collapse, there can be a dramatic increase in the rate at which ice flows into the ocean. The need for better understanding and monitoring of ice shelf basal processes is urgent and our work is helping to provide this.”

Their methods and data have been published recently in the international journal Geophysical Research Letters.

Source: University of Otago, New Zealand