Mega-iceberg A74: German ship squeezes through narrow ice channel

Jonathan Amos
Science correspondent
@BBCAmoson Twitter

Published
image captionA weekend satellite image with a zoom-in on the Polarstern

The German Research Vessel Polarstern has made a remarkable circumnavigation of Antarctica's latest mega-iceberg.

The ship sailed a complete circuit of the 1,290-sq-km (500 sq miles) frozen block, known as A74, at the weekend.

To do so, RV Polarstern had to navigate the very narrow channel that separates A74 from the Brunt Ice Shelf - the frozen floating platform from which the berg broke two weeks ago.

The EU's Sentinel-2 satellite managed to image the ship in the process.

image copyrightRalph Timmermann / AWI
image captionRV Polarstern at the eastern edge of A74 at the weekend

It was an opportunity too good to miss for the research icebreaker, which is operated by the Alfred Wegener Institute (AWI) in Bremerhaven.

The vessel was already working nearby on a pre-planned expedition, so it was easy enough to divert and conduct some serendipitous science.

To put A74 in context, it covers an area approaching that of Greater London in the UK. A similarly sized chunk of ice has not broken away from this particular sector of Antarctica since 1971.

It means the uncovered seafloor and the water column above it are now being exposed to the influence of sunlight, wind and temperature fluctuations in a way they haven't been for 50 years.

Polarstern sailed behind A74 through a channel in the ice that in places was less than 500m wide, in a bid to sample conditions before changes inevitably set in.

image copyrightCopernicus Data/Sentinel-2/EO Browser
image captionThere are locations where the gap is still less than 500m

"I would want photos and video of the seafloor to see what was living there," commented Dr Huw Griffiths from the British Antarctic Survey (BAS).

"The iceberg isn't that wide so the distance under the ice shelf isn't that huge (about 30km from daylight maximum) but how much food gets there will depend on the currents.

"The geologists and glaciologists would want sediment cores to look at the history of the area. The oceanographers are bound to want current readings and water samples," he told BBC News.

Dr Griffiths recently published research detailing the extraordinary ability of seafloor organisms to flourish under ice cover some 250km from the open ocean.

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Ordinarily, it might have fallen to BAS scientists to conduct the sampling undertaken by AWI at the weekend, given that the British have a base just 23km from the fracture zone that created A74. But this research station, known as Halley, is currently closed.

In part, that's because of Covid; very little Antarctic science is being practised at present. But it's also because BAS has been waiting to see how the Brunt Ice Shelf would behave when bergs started to calve from the platform. A74 is one of two major breakaways that had been forecast. The second, if it happens, would be closer still to Halley.

The image at the top of this page was acquired by the Sentinel-2 satellite on Sunday and shows (inset) the position of the RV Polarstern at the time of the overflight. The picture was prepared by Dr Bert Wouters from Utrecht University and the Delft University of Technology in the Netherlands.

The ship is now back in the main part of the Weddell Sea.

AWI said it would release details of the Polarstern's observations as the week progressed. Keep an eye on the institute's German and English twitter accounts, and also on its website.

image copyrightBig Ocean Data
image captionSatellite tracking by Big Ocean Data records Polarstern's circumnavigation

Where is this in Antarctica?

A74 broke away from the Brunt Ice Shelf, which is the floating protrusion of glaciers that have flowed off the land into the Weddell Sea. On a map, the Weddell Sea is that sector of Antarctica directly to the south of the Atlantic Ocean. The Brunt is on the eastern side of the sea. Like all ice shelves, it will periodically calve icebergs.

Just how big is the new berg?

Satellite measurement puts it at around 1,290 sq km. Greater London is roughly 1,500 sq km; the Welsh county of Monmouthshire is about 1,300 sq km. That's big by any measure, although not as large as the monster A68 berg which calved in July 2017 from the Larsen C Ice Shelf on the western side of the Weddell Sea. That was originally some 5,800 sq km but has since shattered into many small pieces.