Ancient Glacial Melting Sent Building-Sized Icebergs To Florida

Imagine icebergs as tall as the Eiffel Tower silently drifting by Florida's balmy beaches.

Add a few braying walruses, and mammoths grazing nearby on a broad coastal plain, and there's Ice Age Florida for you. And believe it or not, the icebergs are among the most indisputable parts of this picture.

During the last Ice Age, massive chunks of ice plowed deep grooves and furrows into the Atlantic seafloor from South Carolina to southern Florida. Named keel scours, after the V-shaped structures on boats, the features record the passage of massive glacial floods unleashed from Canada, according to a study published today (Oct. 12) in the journal Nature Geoscience. The far-traveled floodwaters suggest future ice sheet melting may be more complex than previously thought, the researchers said.

"We can't simply make the assumption that all of the cold, fresh water from ice sheet melting stays in the North Atlantic. Our results show that smaller, coastal currents can be very effective at redistributing this fresh water and impacting a much larger area," said lead study author Jenna Hill, a geologist at Coastal Carolina University in Conway, South Carolina.

The icebergs likely came from one of the huge glacial lakes, such as Lake Agassiz, that flooded northern North America during the ice ages. Now vanished, Lake Agassiz was once as big as the Caspian Sea. Several large lakes existat any one time during the period when ice sheets were at their peak, between 22,000 and 12,000 years ago. Ice dams walled off these glacial lakes from the ocean. When the dams collapsed, catastrophic floods of frigid freshwater spilled into the North Atlantic via the Gulf of St. Lawrence or the Hudson Bay, carrying an enormous armada of icebergs.

When the huge floodwater pulse reached the North Atlantic, the cold, fresh water sat on top of hotter, salty water, forcing the latter to sink, scientists think. This shut down the ocean's natural currents, which keep the northern hemisphere warm. The rapid shift triggered a long cold snap.

Climate and currents

But the discovery of icebergs in Florida suggests not all of the ice age floodwaters went east. Some of the icy overflow headed south, beating back the warm Gulf Stream and insulating the icebergs on their journey southward.

"Previous research would have suggested the meltwater would have gone much further north, so people weren't expecting the subtropics to become fresher," said study co-author Alan Condron, an oceanographer at the University of Massachusetts, Amherst. "This actually has enormous implications for that model and for what triggers climate change."

Condron's detailed models of ocean currents off the shores of North America suggest the glacial floods could have carried icebergs from Canada south to Florida in as little as four months. This southward flow may have temporarily shut down the Gulf Stream, the current that warms the East Coast and Europe.

The iceberg scours match up with the modeling: The southwest-trending furrows indicates the icebergs drifted opposite the direction of the Gulf Stream current. The size and shape of the features also rule out other causes, such as river channels or trawling by fishing boats.

Hill discovered the scours off the coast of South Carolina, about 660 miles (1,060 kilometers) south of North America's giant Ice Age glaciers. The marks were found as far south as the tip of Florida, more than 3,100 miles (5,000 km) away from Hudson Bay. The furrows are beneath 500 to 660 feet (152 meters to 201 m) of water, because sea level is much higher today than it was during the glacial period.

The scour marks are each as long as a football field and as wide as a city bus is long, measuring about 360 feet (109 m) long and 30 feet (9 m) wide. They are up to 30 feet (9 m) deep. Some sets of marks can be traced for more than 18 miles (30 km). The marks' huge size implies that the icebergs were 100 feet (30 m) tall by the time they sailed past Florida, the researchers said.

As the icebergs neared the end of their journey, some hit shallow ground and got stuck, leaving behind a series of circular pits. The hunks of ice would melt, float forward, get stuck again and then repeat the cycle, like a giant, gleaming-white pogo stick.

Hill said the icebergs may also have had a more local effect on Earth's landscape. Many of the places where she found seafloor iceberg scours are now rich habitats for deepwater corals, fish and other marine organisms. "I have wondered if these corals were living there when the icebergs came through or if the iceberg scour grooves have helped shape the habitat in some way," she told Live Science.