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Ancient Iceberg Scars Reveal Insights into Ice Sheet Retreat
Enormous, city-sized icebergs formerly drifted off the coast of Britain during a period of rapid ice sheet melting in Northern Europe, roughly 18,000 to 20,000 years ago. A recent investigation into these glacial phenomena provides valuable data for understanding and contextualizing current events in Antarctica.
James Kirkham, from the British Antarctic Survey, along with his team, unearthed preserved markings left by these massive icebergs as their underbellies scraped across the ocean floor. These elongated, comb-like features, now buried beneath mud in the North Sea, remain detectable in seismic survey data initially gathered for oil and gas exploration.
Size Estimation of Ancient Icebergs
Kirkham stated, “Based on the breadth of the scouring marks and existing knowledge of past sea levels, we estimate these icebergs ranged from five to several tens of kilometers in width and potentially reached a thickness around a couple of hundred meters – putting them on par with a mid-sized British city.”
Tabular Icebergs in Antarctica
In Antarctica, tabular icebergs present a spectacular sight. Certain notable examples, such as the recent A23a and A68a, rival even some smaller US states in terms of surface area. These icebergs break away, or ‘calve,’ from ice shelves—extensive, floating extensions of glaciers flowing from land into the ocean.
Implications for Antarctic Ice Shelf Decline
The discovery that tabular icebergs once existed in the North Sea strongly implies that the seaward edges of the British and Irish ice sheet also possessed ice shelves. According to Kirkham, this proposes possible implications for future Antarctic decline.
The straight, tramline-like grooves left by the vast icebergs in the North Sea are superimposed by the winding troughs created by the narrow keels of much smaller ice fragments. This signifies a “regime shift,” where substantial icebergs are replaced by numerous smaller ones as ice shelves disintegrate due to rising temperatures, explained Kirkham.
Dating the Shift
Radiocarbon dating of the sediments indicates that this transition occurred over a period of 20,000 to 18,000 years ago.
Mega-bergs Do Not Necessarily Indicate Ice Shelf Collapse
These observations challenge the assumption that the calving of mega-bergs, like A23a and A68a, might signal the widespread collapse of Antarctic ice shelves.
Emma MacKie from the University of Florida, who has been monitoring tabular iceberg size using satellite data since the mid-1970s, has found that the trend is largely consistent.
MacKie mentions, “James’s research reinforces my findings, highlighting that significant calving occurrences do not necessarily indicate instability or reasons for alarm. The breakdown of ice shelves instead occurs through disintegration by “a thousand cuts.” We should be concerned when we stop seeing major calving events.”