Some mud stashed away in the back of
a refrigerator in the geology department may begin to answer a number of questions
scientists are asking about El Niño.
Donald T. Rodbell, assistant professor of geology, 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 '97, 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 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.
“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.”
Science is on the Web at: www.sciencemag.org.