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Release: Oct. 18, 2002

Scientists discover 30-year history of El Niño in cave

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Scientists have discovered a history of the past 30 years of El Niño events recorded in Central American stalagmites, according to two University of Iowa researchers and their University of New Hampshire colleagues.

Their findings, published in the Oct. 18, 2002 issue of the journal Science, indicate that variations in a stalagmite found in a cave in the Central American country of Belize reflect changes in the carbon cycle of the overlying rain forest that are related to El Niño.

Luis Gonzalez, director of the Paul H. Nelson Stable Isotope Laboratory, associate professor of geoscience in the UI College of Liberal Arts and Sciences, and researcher at the Center for Global and Regional Environmental Research (CGRER), and Scott Carpenter, CGRER research scientist, adjunct associate professor of geoscience and manager of the Paul H. Nelson Stable Isotope Laboratory, say that this is the first time that a correlation between El Niño and stable isotopes in stalagmites has been documented.

Gonzalez notes that the Belize site has no apparent weather effects correlated with El Niño. However, variations in carbon isotope values recorded in the stalagmite analyzed by the researchers result from changes in the carbon budget of the overlying rainforest ecosystem. He says that those changes, in turn, are sensitive to subtle weather modifications related to El Niño. Gonzalez, an internationally recognized expert on stalagmites and paleoclimate, says he was surprised by the strong response of the stalagmite to El Niño.

"The Belize results are very encouraging because there are no other terrestrial records in tropical regions with the apparent sensitivity of this stalagmite," Gonzalez says. "I expect that my students working in Jamaica and Venezuela will find similar or stronger responses."

The stalagmite, collected in January 2001 from the Actun Tunichil Muknal cave (aka "Cave of the stone tomb") in central Belize, contains visible growth increments associated with the onset of the dry season each year in March. The age of these bands has been confirmed by radiometric dating using lead. Using a high-resolution micro-sampling device designed and constructed by Carpenter, his co-workers were able to mill exceptionally small samples of the stalagmite's calcium carbonate. About 1,300 micro-samples were taken at increments of 20 microns over a 30-year period of stalagmite growth.

"The stable carbon isotope ratio of each micro-sample was measured at the Nelson Laboratory and has resulted in nearly weekly resolution in the stalagmite," Carpenter says. "These analyses are the highest resolution sampling ever conducted on a stalagmite."

The data published by the Iowa researchers and Amy Frappier and Dork Sahagian, of the University of New Hampshire, show carbon isotope levels peaking in 1974, 1983, 1988 and 1999, roughly the dates of major El Niño events. In their Science paper the researchers write that their work suggests the impact of El Niño "extends over a much larger area than just where fluctuations in weather patterns are discernable" and that "stalagmites may record important information about weather and carbon fluxes in tropical rainforest ecosystems on interannual and shorter time scales."