COLUMBUS, Ohio -- A team of scientists has reconstructed a detailed climate record for the last 130,000 years from a thousand-foot-long ice core they drilled into a glacier on the Tibetan Plateau in 1992. Analyses suggest that the record in the core actually may go back more than 500,000 years
The ice core draws a vivid picture of a climate that changed frequently and dramatically in that region throughout the last glacial sequence -- an interval that reached back 110,000 years through the last glacial period and into the warm stage, the Eemian, that preceded it. Their report was carried in this week's issue of the journal Science.
Ellen Mosley-Thompson, professor of geography at Ohio State University, said that the extreme age of the ice at the bottom of the core isn't the most important discovery coming from the analysis. "A record of this length from the sub-tropics is truly unprecedented," Mosley-Thompson said. "It's good that we've got very old ice at the bottom but the age of the ice is almost secondary to the amount of detail the core provides."
With support from the National Science Foundation, the National Geographic Society, Ohio State and the Chinese National Science Foundation, an international expedition retrieved the core five years ago from the Guliya Ice Cap, a 77-square-mile glacier sitting 22,014 feet high in the Kunlun Shan Mountains of western China. Although Guliya is in the sub-tropics the ice is very cold, making it a valuable reservoir of ancient climate records.
The team, headed by Lonnie Thompson, professor of geological sciences at Ohio State, used mechanical and then thermal drills to remove a 1,012-foot (308.6-meters) core from the ice cap. The core, which reached through the ice to bedrock, was split and divided between Chinese and American researchers.
Ohio State researchers cut their half of the core into 34,800 individual samples that were then tested for oxygen isotope ratios, dust, pollen, and nitrate, chloride and sulfate ions. Each of these give clues to the climate in the area when the ice was originally formed. After five years of analysis, the researchers discovered the following:
The detailed record dates back 130,000 years making it the oldest recovered from a tropical or sub-tropical ice cap. Ice at the bottom of the core may be even older. Before now, a core Thompson drilled from the Dunde ice cap in China was the oldest with an age of 40,000 years.
The record also shows that during the last glacial sequence, there were three or four periods called interstadials when the temperature warmed to more like those today. These warm events occurred when methane, a greenhouse gas, was more abundant in the Earth's atmosphere.
These warmer interstadials, along with carbon dioxide and methane increases, were first identified in cores taken from polar ice caps but they appeared as only modest changes. The changes in the Guliya core are quite substantial.
And within parts of the core representing 15,000 to 33,000 years ago, researchers found evidence of about 100 "abrupt climate changes" as inferred from the oxygen isotope record. During this time, the oscillations occurred about every two centuries.
To gauge temperature changes, the researchers measure the ratio of oxygen-18 to oxygen-16 in the ice. A reduction in the proportion of oxygen 18 molecules generally indicates a drop in atmospheric temperature as well.
"The isotope ratio changes seemed to indicate a temperature shift of up to 30 degrees C," explained Keith Henderson, a graduate fellow at the Byrd Polar Research Center. "But we know that would be ludicrous. We need to come up with a much better explanation for these data."
Thompson and his colleagues have spent 20 years seeking out stable ice sheets from the tropics and subtropics, and retrieving the climate records they hold. Polar ice caps such as those in Greenland and the Antarctic are so large that they can control their own weather. But the much smaller non-polar ice caps respond more directly to changes in their climate, making them excellent research tools for studying past climate variations of shorter duration.
"The tropics and subtropics cover half of the Earth's surface and house more than 75 percent of the human population," Thompson said. Changes in this region can have profound impacts. For years, researchers have assumed that the climate in the tropics and sub-tropics has been fairly stable, Mosley-Thompson said. But the new core from Guliya, along with their other low-latitude ice core records, suggests that the tropics and sub-tropics may have experienced considerable climate variability during the last 100,000 years.
Along with Thompson, Mosley-Thompson and Henderson, the research team included John Bolzan, senior research associate; M.E. Davis, P.-N. Lin and J. Cole-Dai, all research associates from Ohio State's Byrd Polar Research Center; T. Yao from the Lanzhou Institute of Glaciology and Geocryology, Lanzhou, China; J. Beer, Swiss Federal Institute for Environmental Science and Technology, Dubendorf, Switzerland; and H.-A. Synal, Paul Scherrer Institute, Zurich, Switzerland.