16 December 2001
Antarctic Mud Reveals Global Climate Change
by Kate Melville
Scientists concerned about global warming are especially troubled by dramatic signs of climate change in Antarctica - from rapidly melting glaciers to unexplained declines in penguin populations. Records show that average winter temperatures are 10 degrees higher in parts of Antarctica today than they were 50 years ago. If that warming trend continues, say many climate experts, the vast Antarctic ice sheets could melt, causing catastrophic coastal flooding as the world`s oceans rise.
Ironically, say researchers, the most pristine continent on Earth is heating up primarily because of increased greenhouse gas emissions from cars, power plants and other human endeavors elsewhere on the planet.
But new geologic evidence unearthed from deep-sea mud deposits strongly suggests that Antarctica experienced periods of extreme warming and cooling long before the invention of the automobile.
"We`ve got a sedimentary record that reveals very significant changes in water temperature and ice melt during the past 7,000 years," says Robert Dunbar, professor of geological and environmental sciences at Stanford. "The cause of these highly variable climate changes is still a mystery."
Dunbar and Boston University collaborators Richard W. Murray and Kelly A. Kryc will present their findings at the annual meeting of the American Geophysical Union (AGU) in San Francisco on Dec. 14, during a session titled, "Antarctic Glacial Evolution: the Marine Geologic Record II."
The researchers based their study on a bioigeochemical analysis of sediments obtained during a recent cruise of the JOIDES Resolution, a research vessel operated by the Ocean Drilling Program (ODP) - an international project dedicated to exploring the geological history and evolution of the Earth. ODP is principally funded by the National Science Foundation with additional support from institutions representing nearly two dozen other countries, including Germany, Japan and Australia.
In 1998, ODP scientists extracted a 150-foot-long sediment core from the muddy bottom of the Palmer Deep - a submerged section of the continental shelf along the west Antarctic Peninsula about 3,000 feet below sea level. The sediment sample was loaded with the shells of microscopic creatures called diatoms dating back some 10,000 years to the beginning of the Holocene - the most recent geologic epoch.
"The Antarctic Peninsula is an ideal region to investigate climate change at decadal to millennial time scales due to its location in one of the Earth`s most dynamic climate systems," notes Dunbar. "The ODP sample gives us the first continuous, high-resolution Holocene sediment record from the Antarctic continental margin."
The sediment sample revealed higher concentrations of diatom shells during the mid-Holocene, roughly 5,500 to 7,000 years ago, which indicates that the waters surrounding the Antarctic Peninsula were more biologically productive then.
According to Dunbar, higher productivity suggests that sea ice was less abundant during the mid-Holocene - a further indication that temperatures were higher.
"We think it was quite a bit warmer then," he observes, noting that geochemical analysis of the sediment also revealed higher levels of nitrogen during the mid-Holocene.
"Warmer temperatures may have produced freshwater streams that fed nitrogen and other nutrients into coastal waters," he explains.
Further analysis revealed other surprises. According to the researchers, Western Antarctica appears to have undergone periods of warming and cooling during the mid-Holocene - regular cycles lasting 400, 200, 140 and 70 years.
"We believe these cycles of warming and cooling may have been caused by variations in the amount of energy emitted by the Sun," says Dunbar, noting that solar activity routinely increases and decreases on a predictable 11-year cycle.
There may be other explanations for these ancient periods of cooling and warming, he adds, but one fact is certain: They were not caused by people.
However, Dunbar is careful to point out that, while increased solar activity may be influencing climate change today, it is a separate phenomenon from the greenhouse effect, which is largely attributed to human-induced CO2 emissions.
The Palmer Deep findings mirror Dunbar`s recent studies at Lake Titicaca, which is located more than two miles above sea level on the border of Peru and Bolivia. Those studies revealed that, during the mid-Holocene, water levels in the high-altitude lake rose and fell as much as 250 feet, as Titicaca experienced drought and increased rainfall.
"These results, combined with the Antarctica findings, indicate that something major happened in the Pacific in the past few thousand years," says Dunbar. "Traditionally this was considered a region with a very stable climate, but clearly there are forces operating in the Pacific and perhaps globally that we need to figure out. If there is a theme for all of these studies it`s that we really don`t understand the climate in the Southern Hemisphere.
The AGU session also will feature a presentation by Stanford graduate student Kevin Theissen on the Lambert Glacier-Amery Ice Shelf system, which is part of the East Antarctic Ice Sheet - the largest ice mass on Earth. Theissen will describe how the ice sheet periodically advanced and retreated during the Pleistocene Epoch - between 780,000 and 1.3 million years ago.
The study is based on core samples drilled in the Prydz Bay region of eastern Antarctica during an ODP cruise in early 2000.
Based on the geochemical record from the early Pleistocene, says Theissen, it appears that there was a brief interval of warmer conditions accompanied by a reduction of the ice sheet.
"How warm it was relative to the present, we don`t know," he observes. "Afterwards, the record indicates that conditions became gradually cooler in the Prydz Bay region."
Theissen notes that there only have been a few significant advances of the Amery Ice Shelf system in Prydz Bay in the last 780,000 years, suggesting that maximum ice volumes in the interior of the East Antarctic Ice Sheet - the source of the Lambert Glacier-Amery Ice Shelf system - have decreased since that time.
"A greater understanding of the history of the East Antarctic Ice Sheet is important because of significant questions about its future behavior and stability," he says.
Some climate experts predict that, if the ice sheet melts, the world`s oceans could rise some 170 feet, submerging many low-lying countries as well as the entire state of Florida.
"As we continue to work on the record from Prydz Bay, we hope to make correlations with other areas of eastern Antarctica, which should help us understand how and why the ice sheet changed during the Pleistocene," Theissen observes.