About 20,000 years ago, a giant slab of floating ice covered Baffin Bay, the stretch of ocean between Greenland and Canada. It was more than a mile thick. Scientists now say this ice shelf helped hold three of the largest ice sheets in the north firmly in place. When it finally disappeared, those ice sheets lost their support. More ice slid off the land and into the sea. Over time, the oceans rose by hundreds of feet. That rise shaped the coastlines we know today.
The finding comes from a new study in the journal Nature Geoscience, published on October 21. An international team carried out the work. It included scientists from the Lamont-Doherty Earth Observatory at Columbia University, the University of Florida, the University of North Carolina Wilmington, and four other institutions. Until now, no one knew that an ice shelf this large had ever existed in that part of the world.
To understand why this discovery matters, picture a great stone cathedral. Robert Hatfield, a geologist at the University of Florida and a co-author of the study, uses that image. The ice sheet on land is the tall cathedral. The floating ice shelves are like the arched supports along their sides, which keep the walls from falling outward. Ice shelves do the same job. They push back against the ice on land and slow its slide into the ocean. Take the supports away, and the walls begin to move. This is a top concern in Antarctica today. Scientists want to know how quickly land ice will flow into the sea once its shelves are gone.
The team did not set out to find a giant ice shelf. They took a five-week research trip to Baffin Bay to collect cores of seafloor mud. Their goal was to learn what weakened the Greenland Ice Sheet at the end of the last ice age. While they mapped the seabed with sonar, they saw something they did not expect. The seafloor held clear signs that one huge ice shelf had once filled the whole bay. It had pressed against the Greenland Ice Sheet to the east, the Innuitian Ice Sheet to the north, and the Laurentide Ice Sheet to the west.
The clues were deep grooves in the seabed. Brendan Reilly, a marine geologist at Lamont, explains that the shelf grew so heavy that its underside scraped the slope of the ocean floor and left long scars. Scientists can still trace these marks around much of the basin. The scars show how thick the ice became. They point to the role of ice flowing out of the islands of the Canadian Arctic. That ice helped build the shelf, and the shelf helped keep the Greenland ice steady during the last ice age.
What stunned the team most was the depth. The marks sat more than a mile below the surface. That makes this shelf at least half a mile thicker than any other known in the Northern Hemisphere. Hatfield notes that ice naturally wants to float, not sink. To reach that deep, the ice had to be extraordinarily thick.
Sediment cores from across the bay suggest the shelf was about as big as some major Antarctic shelves today. They also suggest it vanished within about 500 to 1,000 years, though the exact timing is still unclear. As it faded, the glaciers behind it could speed up, melt, and push ocean levels higher.
Hatfield found that the most unsettling part. A shelf of that size can vanish in a few centuries. Modellers now have fresh data to work with, which should sharpen their forecasts for Antarctic glaciers. It is a warning from the past.
