By Sean Henahan, Access Excellence

The core was obtained by a international team of researchers five years ago at the Guliya Ice Cap, a 77-square-mile glacier sitting 22,014 feet high in the Kunlun Shan Mountains.The team, headed by Ohio State professor of geological sciences Lonnie Thompson  used mechanical and thermal drills to remove a 1,012-foot core from the ice cap.

Five years later, the OSU team has reconstructed a climate history going back 130,000 years. The researchers cut the core into 34,800 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 at the time the ice originally formed. The researchers believe the oldest ice in the core to be as old as 500,000 years.

The ice record 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.

The researchers also found evidence of about 100 "abrupt climate changes" in parts of the core representing a period 15,000 to 33,000 years ago. During this time, the oscillations occurred about every two centuries.

The researchers measure the ratio of oxygen-18 to oxygen-16 in the ice to determine temperature changes over time. A reduction in the proportion of oxygen 18 molecules generally indicates a drop in atmospheric temperature.

"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 has spent the past 20 years evaluating climate history through samples derived from stable ice sheets from the tropics and subtropics. 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, he notes.

"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. But thenew 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.

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."

The research appears in the June 19, 1997 issue of  the journal Science.

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