Oct. 20, 2000 Some 34 million years ago, almost 90 percent of the tiny, shell-bearing sea creatures living along the U.S. Gulf Coast were wiped out and replaced by completely new species of shellfish or mollusks. The same pattern was occurring worldwide, marking the largest global mass extinction since the dinosaurs disappeared. Until now, the cause behind the mass extinction event was a mystery.
Using a new technique that is revolutionizing the way scientists study climate and temperature change, researchers at Syracuse University and the University of Michigan, Ann Arbor, showed that colder winters along the Gulf Coast resulted in the mass extinction at what is known in geological time as the Eocene/Oligocene boundary. Their research will be published in the Oct. 19 issue of Nature.
Their findings are based on an analysis of the chemical composition of fossilized "ear stones," called otoliths, from a group of fish that survived the mass extinction event. The technique enabled the researchers to determine the temperature of the water during winters and summers, which had not been done before.
"We found that while the summer temperatures remained the same, winter temperatures dropped 4 degrees Celsius," says Linda C. Ivany, a visiting assistant professor of earth sciences at Syracuse University. "Paleontologists had always suspected that temperature changes had something to do with the extinction, but we couldn't prove it. Mean annual temperature records for the Gulf Coast that exist for that time period don't show any change. But they don't tell you about seasonal variations in temperature."
Ivany collaborated on the project with Kyger C. Lohmann of the Department of Geological Sciences at the University of Michigan, Ann Arbor; and with William Patterson, assistant professor of earth sciences in SU's College of Arts and Sciences. Ivany worked on the project at SU and at the University of Michigan, where she spent three years (fall 1997 to August 2000) as a fellow of the Michigan Society and assistant professor of geological sciences. She began the project in 1998.
Lohmann developed a method to collect tiny microsamples of calcium carbonate that could then be chemically analyzed for their stable isotopic composition. Patterson, working with Lohmann, refined the technique and applied it to fossilized otoliths, which are made of calcium carbonate, to determine seasonal climate conditions during a fish's lifetime. Because the chemical composition of otoliths changes with the temperature of the water in which the fish live, researchers can use the ratio of the stable isotopes of oxygen atoms (oxygen-16 and oxygen-18) found in the calcium carbonate to determine water temperature.
New material forms on the otolith in a series of growth rings much like those found in trees. When the water is cool, the material accumulates relatively more of the heavier oxygen-18 isotope. By analyzing samples of calcium carbonate taken from individual otolith rings, the researchers can, among other things, recover seasonal temperature records for periods of time where none now exist.
It was Ivany's idea to use this technique to study seasonal variations in an attempt to determine the cause of the mass extinction events at the Eocene/Oligocene boundary. "While scientists often develop methods of resolving Earth history at time scales of days to weeks or months, it takes the insight of researchers like Dr. Ivany to apply existing analytical techniques to questions that address global scale problems," Lohmann says.
In order to obtain the samples, Patterson and Lohmann created precision instruments and computer programs that allow them to take slices of calcium carbonate that are a fraction of the size of a grain of salt. The researchers used three such instruments for this study--one at SU, which Patterson developed in 1997, and two at the University of Michigan, one of which Patterson and Lohmann used to develop otolith-sampling techniques.
"This is the first time anyone has looked at seasonality as a variable for an extinction event across a geological time boundary," Patterson says. "We proved that winter temperatures caused the extinction. Existing records weren't able to resolve the change because the records are based on summertime growth. The fish survived the drop in winter temperatures and left a permanent record, while the mollusks didn't make it to the other side of the boundary."
Ivany plans to continue her work on Eocene/Oligocene climate and how it relates to extinction events. She will extend the study, applying the same techniques on both otoliths and mollusk shells, to determine seasonal variations in temperature in the North Sea with colleagues from Belgium and on Seymour Island in Antarctica with Lohmann and other colleagues. From that research, they hope to understand how climate change affected evolution and extinction during that time interval. She and Patterson will also continue their work on Gulf Coast fossils in an effort to reconstruct the ecology of ancient species.
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