Giant Antarctic iceberg A68a is not done yet

It could have suffered a major breakup this week, but the A68a iceberg still carries considerable mass.
The latest satellite analysis shows that this Antarctic colossus is of a thickness that could still be trapped in the waters around the South Atlantic island of South Georgia.
In this case, worries about the mountain's impact on the territory's wildlife will resurface.
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Penguins and seals could be hampered when searching for fish and krill.
And these predators need to feed not only themselves, but also their young. South Georgia enters the main breeding season.
The Royal Air Force flew another A68a mission on Monday to assess the situation. This page contains videos and still images of this reconnaissance mission.
The giant iceberg A68a breaks into large fragments
Giant iceberg A68a prangs seabed and loses corner
Mission to investigate the giant iceberg A68a
The military flight took place shortly after a new block, A68d, broke out of the main block - but before the major fragmentation event on Tuesday.
This means that A68a has been divided into three main segments.
What had looked like "a pointing hand" snapped its "index finger and knuckle".
The fractures occurred along predictable weaknesses that have been evident since the mountain fist was calved from Antarctica in 2017.
Staff at the Nerc Center for Polar Observation and Modeling (CPOM) at the University of Leeds, UK, have studied the changing shape of A68a over its three and a half year history.
They used four separate satellite systems to study not only the developing area of ​​the frozen block but also its thickness.
A68a: The "knuckles" and the "index finger" broke off on Tuesday
The area was tracked with the European Sentinel-1 satellite and the American Modis imager.
What began as a giant with a size of 5,664 km² (about a quarter of the area of ​​Wales) is now only 2,606 km² (about the size of the English county of Durham).
Profiles of the height of the iceberg above the sea surface were also recorded by Europe's CryoSat-2 and America's IceSat-2 spacecraft. When scientists know a mountain's freeboard, they can calculate its draft - the hidden part below the waterline.
When A68 first calved from Antarctica, it was an average of 232 m thick, with its thickest section measuring 285 m.
Today, the rest of the A68a is generally 32 m thinner, although there are places where the thickness has been reduced by over 50 m.
In combination, the change in area and thickness means a 64% reduction in iceberg volume - from the original 1,467 cubic kilometers to today's 526 cubic kilometers.
It has now been three and a half years since A68 calved from Antarctica
The vulnerabilities in A68a are gradually opening
The big question, of course, is whether the greatly reduced iceberg still has the ability to ground itself in South Georgia.
With a submerged keel of 206m at its thickest point still in place, this remains a real possibility.
At the moment, all of the mountain's many fragments are being carried away in a fast-moving water current known as the Southern Antarctic Circumpolar Current Front. This will likely sweep the chunks around South Georgia and then throw them north.
Satellites will monitor their progress as they bypass the flat continental shelf. There are a number of places where large blocks of ice could get caught and anchor in place.
Interestingly, the measured thickness of the two main fragments that freed themselves from A68a on Tuesday - called A68e and A68f - are about 50 m thinner. This would potentially allow them to get closer to the island's coast.
The submerged bank is known as the "iceberg foot". It introduces buoyancy stress that helps break up
December 23: The Modis imager on Nasa's Aqua satellite shows the "broken finger"
Much has been said about the difficulty a large offshore object would have in finding marine predators. However, there are other notable effects as well, including the introduction of a body of freshwater into the ecosystem as a stationary mountain that has melted over several months.
The CPOM team even tried to calculate this input.
The group says the A68's average melt rate was 2.5 cm per day and the mountain is now releasing 767 cubic meters of fresh water per second into the surrounding ocean - that's 12 times the runoff of the English Thames.
For those who have followed the A68a on its long journey from Antarctica to the South Atlantic, Tuesday was a long time coming for the major fragmentation event.
RAF cameras capture the largest iceberg in the world
The largest iceberg in the world is fraying at its edges
Commentators were surprised at the mountain's longevity
Many observers had predicted that this would happen months ago.
And we might face a much more spectacular resolution of the remaining elements of A68.
As the blocks move in increasingly warmer air conditions, abundant meltwater is created on the surface of the ice. These waters then penetrate down through cracks to open the ice even further.
There may come a point where the remains of A68a let go at some catastrophic moment.
The CPOM analysis was carried out by Anne Braakmann-Folgmann and Jamie Izzard.
Caught in the moment of another cliff fall
Why A68a has little to do with climate change
The iceberg came from a part of Antarctica where it is still very cold - the Larsen C Ice Shelf. This is a mass of floating ice formed by glaciers that flowed into the ocean from the east side of the Antarctic Peninsula. Upon entering the water, the floating fronts of the glaciers rise and merge into a single protrusion. The calving of Bergs on the front edge of this shelf is a very natural behavior. The shelf maintains a balance, and the throwing out of bergs is one way to offset the accumulation of mass from snowfall and the entry of more ice from the eating glaciers ashore. Larsen C calves large icebergs like A68 on decadal time scales.
The mountain and its bits move in a fast flowing stream called the SACCF
Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos

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