Epic Flood Triggered Ancient "Big Chill," Study Says

John Roach
for National Geographic News

December 6, 2007

An epic gush of fresh water into the North Atlantic slowed a deep ocean current and triggered a century-long chill in Europe and North America some 8,200 years ago, according to a new study.

The finding confirms scenarios suggested by previous models of the ancient climate and should raise confidence in predictions made about how the oceans will respond to Greenland's rapidly melting glaciers, an outside expert said.

Some scientists are concerned that Greenland's fast-melting ice could again slow the deep current, sparking changes in weather around the world ranging from reduced rainfall to a new mini ice age.

The current, called the North Atlantic Deep Water, helps keep Europe's climate mild. It shuttles cold, dense waters from the northern seas to the tropics, allowing the warm surface waters of the Gulf Stream to flow north. (Learn more about weather patterns.)

Since fresh water is less dense than cold salt water, climate models suggest a flood of fresh water into the North Atlantic should force the current to slow or shut down.

Scientists suspect the sudden draining of North America's ancient glacial lake Agassiz—which was seven times larger than all of the Great Lakes combined—caused a well-studied cold snap about 8,200 years ago. But evidence that the deep-water current slowed was lacking.

Now, a team of European scientists has found the evidence in the contents of a 39-foot (12-meter) plug of seabed mud pulled from some 11,200 feet (3,400 meters) deep in the northwest Atlantic.

"We show that there's a sudden disruption in the deep circulation which takes place just at the time of the flood outburst," said Helga Kleiven, a paleoclimate expert at the Bjerknes Center for Climate Research in Bergen, Norway.

She and her colleagues report the finding in tomorrow's issue of the journal Science.

Evidence Found in Ancient, Tiny Bugs

Kleiven and her colleagues drilled the core off the southern tip of Greenland, where sediment-rich deep waters slow down and deposit their loads.

"The sedimentation rate is 10 to 15 times higher at these drift sites than they are in the rest of the North Atlantic," Kleiven said.

The researchers identified a section of the core that corresponds to a hundred-year period around 8,200 years ago. The chemistry of the sediment there is unlike that from any other time over the past 10,000 years, Kleiven said.

The sediment grains in this section are also much smaller, suggesting the larger, heavier grains had already fallen out of slower-moving waters or were never picked up.

In addition, oxygen isotopes in the shells of microscopic bugs found in this section suggest the surface water temperature was markedly colder.

"Basically, we have this deepwater response which we see in both chemical and sedimentological properties fitting right in time with the [draining of] glacial lake Agassiz," Kleiven said.

"And also on top of that, by looking at … these little bugs, we strengthen the connection between deep ocean change and the climate anomaly."

Richard Alley is a glaciologist at Pennsylvania State University who originally proposed in 1997 that changes in ocean circulation could have triggered the cooling that occurred 8,200 years ago.

He said the core preserves the short-term record beautifully and corresponds well with the climate record detected in ice cores pulled from Greenland.

"The data are just so clean," he said.

No Day After Tomorrow?

The new findings suggest that the changes in the ocean circulation pattern and cooling of the ocean surface happened over the course of a few decades at most, Kleiven noted.

"The response we see in these deep-ocean changes [is that] they occur on timescales which are rapid enough [that] they could impact human societies," she said.

While no immediate freshwater supply the size of lake Agassiz exists today, Greenland's rapidly melting ice sheets could potentially slow the deepwater current and affect global weather patterns.

A slowing could thrust large portions of Europe and North America into a mini ice age and weaken the monsoon rains in Africa and Asia.

"That's the rain that a couple billion people rely on for crops," Alley said.

This fear has even spilled over to Hollywood, where it inspired the 2004 eco-disaster film The Day After Tomorrow. (Read: "'Day After Tomorrow' Ice Age 'Impossible,' Researcher Says" [May 27, 2004].)

To study the possibility of future freshwater-induced disasters, scientists build computer models based on their understanding of past events like the cooling 8,200 years ago.

The new sediment core findings, Alley noted, suggest that these climate models are accurate.

And this, he added, is good news. When scientists plug the melting rates of Greenland's ice sheets into these models, they indicate catastrophe will most likely be avoided.

As an analogy, he equated potential disasters like a shutdown of the North Atlantic Deep Water to drunk drivers on a dark road.

"We now have confidence that there are fewer drunk drivers out there than we thought there were," he said. "But they're not gone."

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