Historical evidence that colossal icebergs like the ones seen in Antarctica today were once drifting less than 90 miles off the UK coastline has offered scientists new clues as to how climate change could affect Earth’s southern polar region.

For the first time, scientists have discovered the distinctive plough marks left behind by “spectacular giants of ice” as their undersides dragged across the floor of the North Sea some 18,000 to 20,000 years ago.

This was during the last ice age – a period when an ice sheet covering much of the British and Irish Isles was retreating due to a warming climate.

According to research published this week in the scientific journal, Nature Communications, deep, comb-like grooves – some hundreds of metres wide – have been preserved in the sediments buried beneath the present-day seafloor.

They are visible in seismic survey data that was used to locate sites for drilling platforms in the Witch Ground Basin, a swathe of the North Sea situated between Scotland and Norway.

“We’re talking about enormous flat-topped, or ‘tabular’, icebergs,” said marine geophysicist, Dr James Kirkham from the British Antarctic Survey. “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.”

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 ‘monster blocks of ice’ were also roaming across the North Sea.

In Antarctica, tabular bergs are discharged from ice shelves – the floating fronts of glaciers that have flowed off the land into the ocean. Most of the continent is surrounded by these buoyant platforms. 

The findings published this week recognises that not only did large flat-topped bergs exist in the North Sea up to 20,000 years ago, but that the British and Irish Ice Sheet had such ice shelves too.

These structures, the British Antarctic Survey has explained, are important for ice sheet stability and their presence can buttress and hold back glacial ice, which would otherwise drain much faster into the ocean. 

The regular breakaway of tabular bergs at the leading edge of shelves, occurring sometimes only every few decades, helps to maintain the glaciers to their rear in equilibrium. 

It’s uncertain – as the world gets ever warmer – as to how this process could change in Antarctica. However, researchers suggest the ancient North Sea plough marks could offer some valuable insights. 

The study’s co-author, Dr Kelly Hogan, a marine geophysicist at British Antarctic Survey, said that the “catastrophic collapse” of these ice sheets at the end of the last ice age can now actually be documented using the newly found data.

“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 sheets – to much more numerous and smaller icebergs as the ice shelves disintegrated,” said Dr Hogan.

While there are currently very few examples of this transition behaviour in Antarctica, the best is perhaps what happened to the Larsen B ice shelf.

In 2002, warming produced abundant ponds of meltwater at its surface which then trickled down through the platform, shattering the ice into countless small bergs over the course of just one week. After its collapse, the glaciers previously held back behind the now vanished ice shelf sped up, accelerating their contribution to sea level rise. 

It’s this same phenomenon that seems to have occurred – on a much larger scale – in the North Sea those 18,000 years ago, when the British and Irish Ice Sheet was shrinking rapidly by almost 300 metres per year.

Researchers are unable to yet say whether this speedy withdrawal was triggered by disintegration of its ice shelves or if the fragmentation was simply a symptom of enhanced ice sheet mass losses that were already underway.

“It’s an interesting question that goes to the heart of how ice shelves influence the modern Antarctic Ice Sheet,” said British Antarctic Survey co-author, Dr Rob Larter.

“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.”

The study – Change in iceberg calving behaviour preceded North Sea ice shelf disintegration during the last deglaciation is published in Nature Communications.

Click here for more from the Oceanographic Newsroom.