|January 18, 1999|
Core sample is natural archive of ancient El Niño, Union geologist reports in Science
Schenectady, N.Y. (Jan. 18, 1999) Some mud stashed away in the back of a refrigerator at Union College may begin to answer to a number of questions scientists are asking about El Niño.
Donald T. Rodbell, assistant professor of geology at Union, reports in the Jan. 22 issue of Science on a 30-foot-long column of sediment he and a fellow researcher obtained in 1993 from a high-elevation lake in the southern Ecuadoran Andes.
Noting that more work needs to be done to duplicate the findings, Rodbell writes that this sample suggests that during the past 5,000 years, El Niño occurred every two to eight years, the same frequency we see in modern times. The sample also suggests that El Niño was weak or non-existent between 5,000 and 12,000 years ago.
Rodbell and his student, Jeremy Newman, in 1996 began to take a serious look at the sample that had been sitting in their lab cooler for several years. What they found in the core is a data-rich natural archive, the first continuous record of El Niño events dating back more than 5,000 years.
"We didn't know exactly what we had," says Rodbell. "When we took this core, we thought the light-colored striations from landscape and flood events were very pronounced and unusual. We called them 'zebra stripes.' At the time, we were more interested in climate change associated with the last ice age, but in the back of my mind I wondered if these patterns were somehow connected to El Niño.
Rodbell is lead author of the Science article. His co-authors are Newman; Geoffrey Seltzer of Syracuse University; David Anderson of the National Oceanic and Atmospheric Administration; Mark Abbott of the University of Massachusetts, Amherst; and David Enfield of the Atlantic Oceanographic and Meteorological Laboratory.
Their study, while preliminary, has generated excitement among scientists worldwide who are trying to unlock the mysteries of El Niño. "Based on this work, we realized that this one lake, while interesting because it may have provided the longest El Niño record ever found, is only one sample," Rodbell says. "We want to go in and compare it with samples from other lakes in the area."
Rodbell was recently awarded a two-year grant of $90,000 from the National Science Foundation to investigate the climatic record preserved in sediment cores from other high-elevation lakes in southern Ecuador. The initial core suggests that lakes in this area may provide a natural archive of El Niño events covering the last 15,000 years, the longest continuous record of El Niño activity ever discovered.
Rodbell and Seltzer obtained the sediment core sample from Lake Pallcacocha in southern Ecuador. After initial analysis, the core went back to the refrigerator, where it stayed for three years until the scientists pulled it out for a closer look. Newman and Rodbell adapted medical imaging software to quantify shifts in the patterns of layers.
The core sample contains hundreds of layers of sediment deposited over a period of about 15,000 years. About midway through the 30-foot core (representing a period of time beginning about 5,000 years ago) there are a series of light-colored sediment bands that occur approximately every 10 years or less. The bands contain the type of debris -- mostly inorganic material washed from the slopes of nearby mountains -- which would flow into the lake only during periods of heavy rainfall and flooding -- conditions likely triggered by ancient El Niño events, Rodbell says.
Characterized by warm ocean currents that begin off of the western South American coast, El Niño can affect weather systems across both North and South America -- from tumultuous rainfall in northern Peru and southern Ecuador to unusually warm and dry conditions in the northeastern United States and western tropical Pacific. Until now, scientists did not have a clear understanding of when these events first began. Written records and anecdotal observations of El Niño events go back only several centuries.
"The question many scientists are asking is, 'What will happen to El Niño as the global climate gets warmer,'" Rodbell says. "The computer models are good, but they are limited by our understanding of how El Niño works. El Niño may be driven by a number of factors other than global climate."
Donald T. Rodbell, Department of Geology, Union College (518) 388-6034 firstname.lastname@example.org
Charlie Casey, director of the news bureau, Union College (518) 388-6090 email@example.com