Icebergs the size of Cambridge once floated off east coast of UK

Giant icebergs the size of Cambridge once roamed off the coast of the UK, according to British Antarctic Survey researchers.

They have found distinctive grooves carved in sediments below the seafloor that were left by the icebergs during the last ice age, 18,000 to 20,000 years ago.

It is the first evidence of large, tabular icebergs off the coast of the UK and suggests the British and Irish Ice Sheet had ice shelves.

The findings are significant because they could give clues as to how climate change may impact Antarctica.

The plough-marks were left by the massive icebergs as their undersides dragged across the floor of the North Sea, off the east coast of the UK.

At the time, an ice sheet covering much of the British and Irish Isles was retreating due to a warming climate.

"We're talking about enormous flat-topped, or 'tabular', icebergs," explained marine geophysicist Dr James Kirkham from the Madingley-based British Antarctic Survey (BAS).

"Conservatively, they measured five to perhaps a few tens of kilometres in width — comparable to the area of a medium-sized UK city such as Cambridge or Norwich — and could be a couple of hundred metres thick."

In the new study, published in the journal Nature Communications, the researchers explain how the deep, comb-like grooves, hundreds of metres wide, have been preserved in the sediments buried beneath the present-day seafloor.

They are visible in seismic survey data used to locate sites for drilling platforms in the Witch Ground Basin, between Scotland and Norway.

Single grooves made by the narrow keels of small bergs have been observed before.

But the broad Witch Ground tramlines are the first clear evidence that enormous blocks of ice were also roaming across the North Sea.

In Antarctica, such tabular icebergs are discharged from ice shelves, which are the floating fronts of glaciers that have flowed off the land into the ocean. Three-quarters of Antarctica is surrounded by them.

They are important for ice sheet stability as their presence buttresses and holds back glacial ice, which would otherwise drain much faster into the ocean.

Scientists know that the regular breakaway of tabular icebergs at the leading edge of shelves, which may only occur every few decades, helps to maintain the glaciers to their rear in a steady-state, or equilibrium.

It is not clear how this could change as the world’s climate warms.

But the new findings may offer a fresh insight.

Study co-author Dr Kelly Hogan, a marine geophysicist at BAS, said: "We can actually document the catastrophic collapse of these ice shelves at the end of the last ice age using our data, because around 18,000 years ago we detect a shift in the type of iceberg plough-mark recorded in seafloor sediments, from giant tabular bergs - produced by the normal calving lifecycle of ice shelves - to much more numerous and smaller icebergs as the ice shelves disintegrated."

There are very few examples of this transition behaviour in Antarctica currently, but the Larsen B ice shelf is one.

In 2002, warming produced abundant ponds of meltwater at its surface. This trickled down through the platform, shattering the ice into countless small icebergs over the course of just a week.

After the ice shelf collapsed, the glaciers previously held back behind it sped up to several times their former speed, which accelerated their contribution to a rise in sea levels.

It seems this phenomenon happened on a much larger scale in the North Sea during the period when the British and Irish Ice Sheet was shrinking rapidly by 200-300 metres per year at its edges.

The researchers do not know if this rapid retreat was caused by the disintegration of its ice shelves or if the fragmentation was a symptom of enhanced ice sheet mass losses already under way.

They hope that better dating of the sediments could provide the answer.

BAS co-author Dr Rob Larter said: "It's an interesting question that goes to the heart of how ice shelves influence the modern Antarctic Ice Sheet. If we observe a similar transition from large tabular icebergs to smaller icebergs, it could indicate the continent is about to experience significant and rapid mass loss.”