Britain’s icebergs the size of Cambridge

Scientists from the British Antarctic Survey (BAS) have found the first evidence that giant, flat-topped icebergs the size of the city of Cambridge drifted off the coast of Britain during the last ice age. These massive icebergs, like the ones we see in Antarctica today, were once drifting less than 90 miles off the U.K. coastline.

The evidence was uncovered in seismic survey data that was used to locate sites for drilling platforms in the Witch Ground basin, between Scotland and Norway. A series of distinctive, comb-like grooves were found preserved in sediment near Aberdeen in Scotland, which researchers say were left behind by the underside of huge “tabular” icebergs that dragged across the North Sea floor between 18,000 and 20,000 years ago. From the size of the parallel grooves, researchers can estimate the dimensions of the icebergs responsible.

What does this discovery reveal about the past?

Dr Kelly Hogan, the study’s co-author and a marine geophysicist at BAS says the data allows researchers to “actually document the catastrophic collapse of these ice shelves at the end of the last ice age”.

While single grooves made by the narrow keels of small bergs have been observed before, the broad Witch Ground tramlines are the first clear evidence that monster blocks of ice were also roaming across the North Sea.

There was a marked shift in the type of iceberg plough-mark made about 18,000 years ago and recorded in seafloor sediments. These changed from deep, comb-like grooves left by giant tabular icebergs – produced by the normal calving life cycle of ice shelves – to single grooves made by the keels of smaller icebergs as the ice shelves disintegrated. 

What does this discovery herald for the future?

When the grooves were made, an ice sheet covering much of Britain and Ireland was retreating due to a warmer climate. The findings could therefore provide important clues about to how ice shelves influence the modern-day Antarctic ice sheet, and significantly how the climate emergency might affect Antarctica.

In Antarctica, tabular icebergs calve off from ice shelves, which are the floating fronts of glaciers that have flowed from land into the ocean. These structures, which buttress and hold back glacier ice which would otherwise flow into the ocean, are vital for the stability of the ice sheets. 

“There’s this transition from having ice shelves and producing multi-keel icebergs, and then suddenly they’re gone” said Dr Rob Larter, co-author of the paper. “The question is a chicken and egg one: “Did the ice shelves just disappear because of changes already in progress or did the disappearance have consequences for the ice retreat?”. Better dating of the sediments might provide the answer.

There are currently very few examples of this transition behaviour in Antarctica. This phenomenon seems to have occurred on a much larger scale in the North Sea during a period when the British and Irish Ice Sheet was shrinking rapidly by 200–300 meters per year at its edges. “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” BAS co-author Dr. Rob Larter said.