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Grad student discovers way of plotting the appearances of El Nino through history
STANFORD -- A Stanford University graduate student in the School of Earth Sciences has found a way to pinpoint El Nino years in past centuries by studying the muck at the bottom of the ocean, a technique that should reveal more about how human activity has affected global climate.
The student, geochemist Julie Kennedy, plans to use the technique to study sediment layers deposited centuries ago, before anyone kept records of El Ninos. By identifying El Nino years before and after industrial activity began to change the earth's atmosphere, she hopes to learn if human activity may have changed their frequency.
El Nino is an occasional disruption of currents in the Pacific Ocean. It gets its name - a Spanish term referring to the Christ child - because the disruption often begins near Christmastime. In El Nino years, the sea surface in the eastern Pacific warms by several degrees. This contributes to profound changes in the world's weather, including increased rainfall in the American Southwest, mild winters in the northern Midwest, drought in Africa, and the failure of Asian monsoons.
Kennedy studied the molecular remains of single-celled algae in ocean floor deposits that settle in annual layers like the growth rings of trees. She was able to trace sea-surface temperature changes year by year through the 20th century, correctly spotting each recorded El Nino year.
"Of the major El Ninos that have occurred in the 20th century, I've been able to pick every one up very cleanly," she said.
Kennedy, together with her graduate adviser Simon Brassell - now at Indiana University - published her results in the May 7 issue of Nature.
Now, she plans to use her technique to examine layers from years in which no El Nino records exist.
Kennedy studied molecules produced by Emiliania huxleyi, a golden-brown alga that drifts in the plankton near the ocean's surface. Though each single-celled individual is less than a thousandth of an inch long, E. huxleyi is so abundant that collectively it outweighs almost any other species on earth, Kennedy said.
"Not only is it extremely important, but it's one of the most accommodating organisms on the face of the earth," making its home everywhere from near-freezing Arctic waters to warm tropical oceans, she said.
During the 1980s, Brassell - then based in Bristol, England - suggested that E. huxleyi adjusts to cold water by changing certain chemical bonds in a class of its molecules called alkenones. The changes help cold-water alkenones stay fluid at temperatures where warm-water alkenones would stiffen up, in much the same way that vegetable oil stays liquid at room temperature while butter solidifies.
Brassell found he could tell at what water temperature the algae lived by measuring the relative proportion of cold-water and warm- water alkenones produced by the algae. Work led by Fred Prahl of Oregon State University let Brassell identify what temperature corresponded to each mix of alkenones.
In her research, Kennedy applied Brassell's technique to alkenones found in annual layers of dead algae that settle on the ocean floor.
At Kennedy's study site in the Santa Barbara Basin off the California coast, distinct annual bands accumulate undisturbed because a layer of stagnant, oxygen-depleted water lies along the ocean bottom. Since bottom-dwelling animals can't survive without oxygen, their tunnels and burrows don't mix up the sediment layers.
Kennedy collected sediment cores from the basin, painstakingly scraping off each year's layer - about a tenth of an inch thick - to analyze the structure of its alkenones.
She found that in every year in this century when oceanographers recorded a strong El Nino warming, the sediment record reflected an increase in sea surface temperature. In other words, Kennedy's work so far has shown that she can use alkenones to spot El Nino events that oceanographers already knew about.
"Basically, it was like a calibration. Does this technique work on a year-by-year basis, and can you use it to identify El Ninos?" she said.
Armed with this technique, Kennedy will begin sampling deeper, older sediments from the Santa Barbara Basin and the Guaymas Basin in the Gulf of California this summer.
"Laminated sediments go back tens of thousands of years in the Santa Barbara Basin," she said, so this El Nino record may extend back to the last ice age.
Similar layers occur in some sedimentary rocks millions of years old, raising hopes that scientists may eventually trace ocean temperature changes even further into the past, Kennedy said.
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