Ancient rock structure discovered under the ice of East and West Antarctica is controlling the flow of ocean water under the Ross Ice Shelf and may affect rate of future melting

An ancient divide in the rocks beneath East and West Antarctica is controlling the flow of water under the Ross Ice Shelf and the rates at which parts of the ice melt. 

The boundary was discovered using a new, plane-mounted detector which can gather magnetic and gravitational data on the ice shelf as it flies around.

In addition, the researchers also discovered that a patch of open waters in the Ross Sea that cools the ocean in the winter can also lead to significant warming and local ice melt in the summertime.

Together, the findings highlight the importance of local ocean currents in the future retreat of the Antarctic ice shelf.

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The Ross Ice Shelf is a massive expanse of floating ice that slows the release of about 20 per cent of grounded Antarctic ice out in the ocean, making understanding its evolution vital to predicting future sea level rise.

To better determine how the Ross Ice Shelf interacts with the ocean, atmosphere and underlying geology, an international team led by researchers from Columbia University set about conducting a comprehensive survey of the shelf.

This is no mean feat, as the Ross Ice Shelf is the size of Spain, and contains many areas in which the ice is more than a thousand feet thick, preventing traditional, ship-based, surveys of the sea floor. 

Their solution was to develop IcePod, a unique, cargo-plane-mounted system designed specifically to collect high resolution data from the polar regions.

IcePod can measure both the height, thickness and internal structure of the ice shelf, as well as the magnetic and gravity-based signals from the underlying rock. 

As they flew back-and-forth across the ice shelf, researchers noticed that IcePod‘s magnetometer — which measures the strength of the Earth‘s magnetic field — kept recording unusual signals as they crossed the middle of the shelf.

These magnetic anomalies, researchers concluded, were a reflection of a transition across a previously-unmapped segment of the geological boundary between the rocks of East and West Antarctica.

On the west side lies young sedimentary rocks, volcanic materials and stretched-out continental blocks, whereas the east side is characterised by ancient continental material and the remains of former mountain ranges.

Next, the team modelled the shape of the sea floor beneath the ice shelf using measurements of the Earth‘s gravitational field. 

‘We could see that the geological boundary was making the seafloor on the East Antarctic side much deeper than the West,‘ said lead researcher Kirsty Tinto, a polar geophysicist at Columbia University‘s Lamont–Doherty Earth Observatory.

‘That affects the way the ocean water circulates under the ice shelf,‘ she added. 

To find out how, the researchers used their newly-made map of the seabird to model the circulation under the Ross Ice Shelf and determine its effect on the shelf itself.

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They found that little warm water reaches it in comparison with the Amundsen Sea in the east, where warm flows cross the shallow waters resulting in rapid melting of the overlying ice shelves.

Instead, any water flowing under the ice shelf from the warmer deep ocean is first cooled in the Ross Sea by the cold winter atmosphere when it passes through a region of open water known as the Ross Shelf Polynya.

This cold water may melt deeper portions of east Antarctic glaciers, the researchers‘ model revealed, but such flows are steered away from the west Antarctic side by the depth change encountered at the ancient tectonic boundary.

The team were surprised to see, however, that the polynya might also lead to intense melting of the ice shelf‘s leading edge in the summer, with the open patch of water allowing for heating of the upper ocean. 

They confirmed these findings by looking at the internal structure of the ice shelf as seen through radar images.

‘We found that the ice loss from the Ross Ice Shelf and flow of the adjoining grounded ice are sensitive to changes in processes along the ice front,‘ said co-author and Polar oceanographer Laurie Padman of the Seattle-based Earth and Space Research institute.

Such changes, he added, can include ‘increased summer warming if sea ice or clouds decrease.‘ 

Together, the findings indicate the importance of considering local-scale ocean currents near the ice front in models used to predict future Antarctic ice loss, rather than just large-scale changes in the circulation of warm, deep water. 

‘We found out that it‘s these local processes we need to understand to make sound predictions,‘ said Dr Tinto. 

The full findings of the study were published in the journal .

HOW COULD A WARMING ANTARCTICA IMPACT SEA LEVELS?

Antarctica holds a huge amount of water.

The three ice sheets that cover the continent contain around 70 per cent of our planet’s fresh water – and these are all to warming air and oceans. 

If all the ice sheets were to melt due to global warming, Antarctica would raise global sea levels by at least 183ft (56m).

Given their size, even small losses in the ice sheets could have global consequences. 

In addition to rising sea levels, meltwater would slow down the world’s ocean circulation, while changing wind belts may affect the climate in the southern hemisphere. 

In February 2018, Nasa revealed El Niño events cause the Antarctic ice shelf to melt by up to ten inches (25 centimetres) every year.

El Niño and La Niña are separate events that alter the water temperature of the Pacific ocean.

The ocean periodically oscillates between warmer than average during El Niños and cooler than average during La Niñas.

Using Nasa satellite imaging, researchers found that the oceanic phenomena cause Antarctic ice shelves to melt while also increasing snowfall. 

In March 2018, it was revealed that more of a giant France-sized glacier in Antarctica is floating on the ocean than previously thought.

This has raised fears it could melt faster as the climate warms and have a dramatic impact on rising sea-levels.

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