CHICAGO By studying the chemical composition of
ancient marine fossils, researchers have found new evidence
that greenhouse gases such as carbon dioxide have a big impact
on climate, they said on Wednesday.
The U.S. and Canadian researchers used a new method of
studying marine fossils to learn temperatures millions of years
ago, turning each ancient shell or piece of coral into a
"All of the information needed to study the surface
temperature at the time the animal lived is stored in the
fossil itself," said Rosemarie Came of the California Institute
of Technology, whose study appears in the journal Nature.
This new method relies on a study of rare clumps of oxygen
and carbon isotopes -- charged particles -- that bond with each
other. This bonding varies depending on surface temperature,
with more formed at low temperatures and fewer formed at higher
By knowing the age of the marine fossil and measuring the
concentration of these isotopes, the scientists can tell the
temperature of the seawater in which they lived.
"The shell is the thermometer," said John Eiler, a Caltech
professor of geochemistry who worked on the study.
He said the researchers studied the concentration of these
isotopes, which are made up of oxygen-18 and carbon-13 atoms.
"We measure how many are stuck to each other rather than
being randomly scattered. That tells us the growth temperature
of that fossil," Eiler said in a telephone interview.
The method differs from the current approach that involves
a study of both the fossil's carbon and oxygen content and the
knowledge about the chemistry of ancient seawater.
Using this new method, researchers studied fossil growth
temperatures from two ancient time periods to see if changes in
atmospheric carbon dioxide are actually linked with temperature
change, an important part of understanding climate change.
They studied fossils from the Silurian period about 400
million years ago, during which carbon dioxide levels are
believed to have been 10 times higher than at present.
They compared these with fossil temperature readings in the
Carboniferous period, roughly 300 million years ago, during
which atmospheric carbon dioxide concentrations are thought to
have resembled current levels.
"Our study found that in fact there was a huge temperature
change -- that the time of high carbon dioxide was a time of
quite high temperature in tropical oceans, about 34 degrees
centigrade (93 F)," Eiler said. "If you got in it, you would
think it was a pretty warm bath."
Based on this finding, the researchers believe they have
fossil evidence that changes in atmospheric carbon dioxide are
associated with changes in temperature.
"It shows that carbon dioxide has been a powerful driver of
climate change in the Earth's past," Eiler said.