SAN FRANCISCO, CALIFORNIA--A million years ago, a large meteorite smashed into what is now northern Quebec and created a crater that may become an unprecedented repository of data with which to study long-term climate change, researchers reported here this week at a meeting of the American Geophysical Union.
Canada and the northern United States are dotted with tens of thousands of lakes, most of them formed by meltwater at the end of the last ice age about 12,000 years ago. Sediments at the bottom of those lakes hold chemical and biological evidence of how the planet's climate has varied for even longer periods, over many ice ages and interglacial cycles, and how those variations have affected the local ecosystems. But almost all of these sediments have been bulldozed repeatedly by glaciers as the giant rivers of ice have advanced and retreated over the last 2 million years, scrambling the geological record.
Pingualuit Crater in northern Quebec seems to have escaped this fate. Its 3.7-kilometer-wide, nearly circular lake not only is deep enough--nearly 270 meters--to have avoided the glacial pummeling but also has remained sequestered from any other body of water during its entire history. So the sediment that has collected on the lake's bottom has preserved a pristine record of the climate and biological activity in the lake for more than a million years--much longer than any similar climate data source currently available. Until recently, however, the technology necessary to retrieve samples from the bottom, without disturbing the samples or contaminating the lake's ultraclear water, did not exist.
So, paleolimnologist Sonja Hausmann of the University of Arkansas, Fayetteville, and colleagues employed a new type of coring rig devised specifically for obtaining delicate samples. Last May, after trekking across the then-frozen lake on foot without the aid of potentially polluting snowmobiles or sled dogs, the team deployed the bottom-dwelling rig, which is suspended from a Kevlar cable, and spent 2 weeks carefully extracting cores from the top 10 meters of Pingualuit's estimated 150 meters of sediment. Those samples are beginning to provide a treasure trove of data going back at least 250,000 years.
"We think the samples span at least two [interglacial] cycles," Hausmann says. They include diatoms--microscopic algae whose silicate shells can provide exquisite historical portraits of the lake's water quality and climatic conditions--as well as trace metals and pollen that have fallen from the atmosphere. Other records can be compiled from sources such as the ice cores in Greenland and the sea beds, she says, but the Pingualuit cores are the only ones available from the North American land that contain the skeletal remains of climate-sensitive algae.
"Collecting this core was no small endeavor," says paleolimnologist John Smol of Queen's University in Kingston, Canada. "Many of us had previously assumed that the last Ice Age had obliterated older sediment records," he says, but Pingualuit's cores show that "there is a remarkable history book still present."