You never forget the first time you see an iceberg. The horizon of a ship at sea is a two dimensional space and to see a three dimensional piece of ice appear in the ocean is quite something. But, in truth, the first iceberg you see is likely to be small. Most icebergs that make it far enough north from Antarctica to where they are danger to shipping are sometimes many years old and at the end of their lives. They are small fragments of what once left the continent.
Once in a while, however, a monster breaks free from the edge of Antarctica and drifts away. Tens of kilometres long these bergs can tower perhaps 100 metres above the sea and reach several hundred more below the surface. These are called tabular icebergs – and while it is rare for humans to see something on such a scale they are part of the normal cycle of glacial ice in Antarctica.
On reaching the sea, these glaciers fracture and release icebergs or form large regions of floating ice known as ice shelves. In a few special places glaciers can extend tens of kilometres into the ocean – giant fingers of ice several hundred metres thick, pointing out into the sea.
Just like a wall they shield what is in their lee, and rather than the ocean being covered by drifting sea ice it can remain open throughout the year to form what is called a polynya. The ocean still freezes, but the ice is constantly pushed away by the prevailing winds. Open water throughout winter helps seals and penguins survive, and stimulates phytoplankton production.
Tracking the mega icebergs
A new research article in the journal Nature Communications by a French team working in Antarctica has looked at the history of the polynya in the lee of the Mertz Glacier going back 250 years. This glacier forms one of these fingers of ice reaching out from the continent and the polynya in its lee can be up to 6,000 square kilometres.
The proxies tell us which species of plankton dominated the region in a particular period: if the sediment is dominated by species which live in open water then they can infer that the polynya existed and so the Mertz Glacier had a long tongue extending north. If the sediment is dominated by species which live in the sea ice, then the polynya and the glacier tongue were absent. It is quite an elegant way to investigate glacier flow.
These days we can see this happen in almost real time through the amazing access we have to satellite imagery and in February 2010 an iceberg containing almost 900 billion tonnes of fresh water broke free.What they found is that every 70 or so years the Mertz polynya is absent for tens of years. Given that the glacier is advancing about 1 km per year this means a super-iceberg tens of kilometres in length has regularly formed in this region.