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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