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Contact: Dane Konop, Charles Casey
For the first time, a team of government and university scientists has found a high- resolution, 15,000-year record of rain-induced erosion in sediment layers of an Ecuadorian lake that indicates El Niño-like climate fluctuations became more common about 5,000 years ago.
Writing in the current issue of Science, the researchers found that a core sample of layers of sediment deposited during severe storms in Lake Pallacacocha in southwestern Ecuador closely correlates with El Niños that are known to have occurred over the past 200 years.
"The full sediment record indicates that 15,000 years ago severe El Niño-like storms occurred at least about every 15 years, and that they have since occurred with progressively increasing frequency. Over the past 5,000 years, storms from El Niño-like climate fluctuations have occurred about every two to eight and one-half years, possibly due to enhanced trade winds," said the study's lead author, Donald T. Rodbell of Union College, Schenectady, N.Y.
The authors point out that there are proxy records of prehistoric El Niños in a variety of natural archives, including corals, ice cores, tree rings, flood deposits, beach ridges, archeological middens and soils. But high-resolution records in corals and ice cores are limited to the past 2,000 years, while longer records are not continuous.
Sea surface temperatures near this part of Ecuador are among the first to warm in the region during the onset of an El Niño, when rainfall greatly increases. Since extreme El Niño-driven storms are known to deposit organic and inorganic debris in coastal basins, the scientists analyzed a 9.2-meter-long core of sedimentary rock obtained in June 1993 from Lake Pallacacocha, which is about 75 kilometers from the Pacific Ocean. These layers of sediment, known as clastic laminae, are made up of fragments of vegetation that were washed into the lake from the surrounding landscape during torrential rain storms.
The sediment record from 1800 to 1976 A.D. reveals a close match between the layers of clastic laminae and moderate to severe El Niños. Of the seventeen El Niños that occurred in this time period, eleven correlate within two years of major layers of clastic laminae, and one is within three years. The other five severe El Niños during this period occurred within two years of relatively minor layers of clastic sediment. The eight severe El Niños of the past 100 years correlate precisely with clastic laminae in the core.
In addition to Rodbell, authors of the Science paper are J. H. Newman, Department of Geology, Union College, Schenectady, N.Y.; G. O. Seltzer, Department of Earth Sciences, Syracuse University, Syracuse, N.Y.; D. M. Anderson, National Environmental Satellite, Data and Information Service, NOAA, Boulder, Colo.; M. B. Abbot, Department of Geosciences, University of Massachusetts, Amherst, Mass.; and D. B. Enfield, Atlantic Oceanographic and Meteorological Laboratory, NOAA, Miami, Fla.