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Giant, flat-topped icebergs the size of the city of Cambridge drifted off the coast of Britain during the last ice age, according to a study that has uncovered evidence of their existence for the first time.
A series of distinctive, comb-like grooves found preserved in sediment near Aberdeen in Scotland 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, the researchers said.
The finding, published in Nature Communications, could provide clues as to how the climate emergency might affect Antarctica.
Scientists from the British Antarctic Survey (BAS) found the evidence in seismic survey data that was used to locate sites for drilling platforms in the Witch Ground basin, between Scotland and Norway.
When the grooves were made, an ice sheet covering much of Britain and Ireland was retreating due to a warmer climate.
“We’re talking about enormous flat-topped, or tabular, icebergs,” said Dr James Kirkham, a marine geophysicist from the BAS and lead author of the paper. “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.”
The size of the parallel grooves allowed the scientists to estimate the icebergs’ size.
Single grooves, made by the narrow keels of smaller icebergs, have been seen before but the broad tramlines of the Witch Ground basin are the first clear evidence that larger icebergs floated off the North Sea.
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.
“We can actually document the catastrophic collapse of these ice shelves at the end of the last ice age using our data,” said co-author Dr Kelly Hogan, a marine geophysicist at BAS.
This is because about 18,000 years ago there was a shift in the type of iceberg plough-mark 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.
Few recorded examples of this transition behaviour exist in Antarctica, but the most dramatic is the Larsen B ice shelf – at 3,250 sq kilometres, or 1,250 sq miles – which collapsed in the space of a month in 2002.
A series of warm Antarctic summers produced meltwater at the shelf’s surface which then trickled down, shattering it into small icebergs. After the shelf collapsed, the release of the glaciers held back behind it sped up by several times, accelerating their contribution, albeit tiny, to sea level rise.
This phenomenon appeared to happen in the North Sea during between 18,0000 and 20,0000 years ago when the British and Irish ice sheet was shrinking rapidly by 200-300 metres a year at its edges.
“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?”
The answer could provide clues as to how ice shelves influence the modern day Antarctic ice sheet, they said. Better dating of the sediments might provide an answer, said Larter.
