Dec. 4, 1998 COLUMBUS, Ohio -- An analysis of ice cores drilled from a glacier atop a Bolivian volcano is painting a vivid picture of climate conditions in the tropics over the past 25,000 years. The ice at the bottom of the cores was formed during the last glacial maximum -- the coldest part of the last ice age -- making it the oldest core recovered from the tropics.
In a paper in the journal Science, the research team describes a climate in the tropics that was different from what many researchers have thought. The findings are the latest result from a 20-year effort to build a global climate record that reaches from the North to the South Pole.
Lonnie Thompson, professor of geological sciences and research scientist with the Byrd Polar Research Center at Ohio State University, said that the new cores are the best evidence yet that the tropics were much cooler during the last glacial stage.
Until very recently, most researchers believed that only the polar and mid-latitude regions experienced drastic cooling during that period and that the tropics were largely unaffected.
The new cores also showed that the in the region was eight times less dusty during the height of that last ice age than it is now. A decrease in dust indicates that the regional climate was the much wetter at the time, he said.
Thompson’s team included researchers from Ohio State, Pennsylvania State University, the Russian Academy of Sciences and ORSTOM in Ecuador. In 1997, the team scaled Sajama, a 21,463-foot (6,542-meter) extinct volcano towering over the Bolivian Altiplano, a vast plateau that covers 79,151 square miles (205,000 square kilometers) of Bolivian desert.
They used a solar-powered drill to bore through the ice cap at the summit and retrieve two cores -- 434.3 feet (132.4 meters) and 435.6 feet (132.8 meters) -- that reached to bedrock. Both cores contain a record of ancient climate trapped in the ice that reaches back to the formation of the ice cap. The team also retrieved two additional shorter cores as well.
An analysis of the cores showed the following:
* The ice at the bottom of both cores dates back 25,000 years into the coldest period of the last ice age -- the last glacial maximum (LGM). The abrupt beginning and end of a Younger-Dryas-type event suggest that atmospheric processes as the probable driving mechanisms. It also indicates that the natural climate system is capable of large-magnitude shifts on a global scale. (The Younger-Dryas was a severe cooling period that began around 12,700 years ago -- right in the middle of the major warming that followed the last glacial stage. It lasted only 1,300 years and was thought only to have occurred in the Northern Hemisphere. But in the Sajama core, the event actually lasted 2,500 years but ended at the same point.)
* Trapped within the cores were insects and paper-thin bark fragments from the polylepis trees that populate the lower flanks of Sajama.
* This was the first time material retrieved from a tropical ice core has been carbon-dated, offering a new, additional technique for dating old ice. The carbon-dating was done at Lawrence Livermore National Laboratory and at Woods Hole Oceanographic Institute and the organic material at less than 4.9 feet (1.5 meters) from the bottom of the cores was dated back 24,000 years.
* The period of the last glacial stage as recorded in polar ice cores and cores from Huascuran (Peru) was thought to be cold and dry but the eight-fold drop in dust suggests much wetter conditions in the vicinity of Sajama.
Previously, Thompson has led expeditions to ice caps in the Andes at Huascuran and Quelccaya, Peru, to retrieve other ice cores. Taken together, the information from Huascuran and Sajama cores clearly points to a cooler tropics during the late glacial stage.
“We’re seeing about a 5- to 6-degree Celsius cooling in the region,” Thompson said. “That’s the same magnitude of response recorded in the ice core records from Huascuran 1,000 miles north.” Huascuran is located at 9 degrees south latitude and Sajama sits at 18 degrees south. Similar cooling was found in ice cores drilled from Western Tibet, Antarctica and the Greenland ice cap.
Evidence from the Sajama cores showing that the region was wetter during the last glacial stage reinforces researchers’ understanding of the history of the South American Altiplano. Periodically, as the regional climate changed from dry to wet, huge lakes formed on the plateau.
“These were massive lakes. One of them -- a freshwater lake -- covered more than 38,610 square miles (100,000 square kilometers) and was up to 984 feet (300 meters) deep.” Thompson said the lakes filled during the wet periods and later emptied -- in some cases forming salt flats -- during the dry periods. Some researchers believe the lakes may have been refilled several times during the last 60,000 years.
“Historically, this is a very dynamic place,” he said, “although you wouldn’t think that when you look at it today.”
While it still isn’t confirmed, Thompson and colleagues suggest that the mechanism controlling the onset and end of these cold, glacial periods may be tied to the changes in the tropical hydrological system.
“It’s very important to better understand the processes of this system since they may be relevant to what is happening today. There may be mechanisms in the tropical hydrological system capable of intensifying the impact that humans could be making on the current climate,” he said.
Expedition members included Thompson, Bruce Koci of the University of Alaska; V.S. Zagorodnov and Ping-Nan Lin from Ohio State; V.N. Mikhalenko from the Russian Academy of Sciences; T.A. Sowers from Pennsylvania State University; and B. Francou from ORSTOM in Ecuador.
In addition, Byrd Polar Research Center team members included Ellen Mosley-Thompson, professor of geography; John Bolzan, senior research associate; Mary E. Davis and Jihong Cole-Dai, both research associates. Keith Henderson of Ohio State and R.K. Campen of Penn State, both Ph.D students, were part of the research team.
The project was supported by the National Oceanic and Atmospheric Administration and the National Science Foundation.
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