Science & Environment

Monster iceberg's pivot and turn

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Media captionStrong wind has blown the southern end of the berg out to sea

The monster Antarctic iceberg A-68 looks finally to be on the move.

For 13 months after breaking away from the White Continent's long peninsula, the trillion-tonne block did little more than shuffle back and forth on the spot.

But now its southern end has swung round almost 90 degrees, indicating the berg has been caught in ocean currents.

The approaching southern summer should only assist its anticipated slow drift northwards, experts say.

"After more than a year of moving to and fro near its parent ice shelf, iceberg A-68, which calved from the Larsen C Ice Shelf on 12 July 2017, has finally escaped," commented Prof Adrian Luckman from Swansea University, UK.

"Until recently, the iceberg was hemmed in by dense sea-ice in the east and shallow waters in the north.

"Now, a strong foehn wind blowing eastwards off the ice shelf in early September has pushed the southerly end of the iceberg out into the Weddell Gyre. This persistent clockwise drift of ocean waters and floating sea-ice flowing north past the Larsen Ice Shelf has rotated A-68 out into the Weddell Sea.

"Here, it is much more free to begin moving away and be carried further north into warmer waters."

A-68's pivot-and-spin behaviour is common in large tabular icebergs. Its contact with the seabed will be leaving big gouge marks in the sediment.

These troughs should be evident to the sonar surveys that will be conducted by international teams at the turn of the year.

Germany's Alfred Wegener Institute will lead one of these scientific cruises, using the Research Vessel Polarstern; the UK's Scott Polar Research Institute will lead the other, run off the icebreaker SA Agulhas II.

A-68 has managed to knock off some of its sharp edges over the past year, but its scale remains much the same - roughly 150km long and some 55km wide.

Two largish chunks have detached, one of them sufficiently big to get its own designation (A-68b) in the list of giant bergs kept by the US National Ice Center.

The American agency has officially now put A-68 at number six in its all-time size ranking.

Past history suggests the largest blocks will all eventually be exported on one of the four major "iceberg highways" that lead beyond Antarctica.

In this instance, the fragments should slot into the great circumpolar current that surrounds the continent and head on an eastward arc towards the South Atlantic.

The penguins and seals on the British Overseas Territory of South Georgia may well get to see these remnants as they pass by in a few years' time.

Image copyright ESA
Image caption The drift paths (red lines) of countless bergs have been tracked around the Antarctic continent (black). This collective history strongly suggests A-68 will head for the South Atlantic

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos

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