How a massive ice shelf buttressed the last ice age’s glaciers — and what it could mean for Antarctica today

About 20,000 years ago, an enormous, floating ice shelf more than a mile thick helped lock in three of the largest ice sheets in northern Canada and Greenland. When that ice shelf disappeared, it helped unlock part of these ice sheets, which contributed to hundreds of feet of sea level rise, establishing modern coastlines as we know them.

Those are the findings of a new study that uncovered evidence of an ancient ice shelf in Baffin Bay, west of Greenland. The size and scale of such a floating fortress of ice was not previously known, and it provides clues to the future of today’s massive ice sheets in Antarctica and the sea-level rise that could follow if they melt.

“These ice sheets are like a gothic cathedral. You have this big, tall cathedral of ice and these flying buttresses of floating ice shelves that support them. They help stop the ice on land from flowing into the ocean and contributing to sea level rise,” said Robert Hatfield, Ph.D., an associate professor of geology at the University of Florida and co-author of the new study. “One of the biggest concerns in Antarctica right now is how fast will that ice that’s on land flow into the ocean when we lose that buttressing effect.”

Hatfield and his collaborators at the University of North Carolina Wilmington, Columbia University and four other institutions published their findings (DATE) in Nature Geoscience.

The scientists did not set out to discover a mile-thick ice shelf. Instead, they embarked on a five-week-long research cruise in Baffin Bay intent on collecting sediment cores to reconstruct what destabilized the Greenland Ice Sheet at the end of the last ice age.

Yet with sonar mapping the seafloor, they uncovered clear evidence of a large ice shelf occupying the entire bay and holding back not only the margin of the Greenland Ice Sheet to the east but the Innuitian Ice Sheet to the north and the Laurentide Ice Sheet to the west. 

“The thing that really surprised us was this evidence was over a mile deep into the ocean. That’s at least half a mile thicker than any previously known ice shelf in the Northern Hemisphere.” Hatfield said. “Ice doesn’t want to flow down underwater, it wants to be afloat. It just speaks to how thick that ice shelf became.”

Sediment cores from across the bay suggest that this floating ice shelf — which was roughly the size of some of the key ice shelves in Antarctica today — disappeared over about 500-1000 years, allowing the large glaciers on land to flow faster into the sea, melt and raise the ocean levels.

“That’s one of the scary takeaways we didn’t expect, that an ice shelf the scale of the ones in Antarctica could completely disappear in a matter of centuries,” Hatfield said.

With these new findings in hand about the end of the last ice age, scientists who model past and present ice shelves and glaciers have new data to work with, Hatfield said. That could lead to more accurate predictions about how quickly Antarctic glaciers might melt and raise sea levels.

And the study reveals how following unexpected evidence can reveal entirely new pictures of the planet’s past.  

“We didn’t go out to look for an ice shelf this big,” Hatfield said. “That’s science, right? You go out to look for one thing and all sorts of other questions fall into your lap.”