The Second Largest Mass Extinction
About 450 million years ago, Earth suffered the second-largest mass extinction in its history-the Late Ordovician mass extinction, during which more than 75 percent of marine species died. Exactly what caused this tremendous loss
in biodiversity remains a mystery, but now a team led by researchers at the California Institute of Technology
(Caltech) has discovered new details supporting the idea that the mass extinction was linked to a cooling climate.
"While it's been known for a long time that the mass extinction is intimately tied to climate change, the precise
mechanism is unclear," says Seth Finnegan, a postdoctoral researcher at Caltech and the first author of the paper
published online in Science on January 27, 2011. The mass extinction coincided with a glacial period, during which global
temperatures cooled and the planet saw a marked increase in glaciers. At this time, North America was on the equator,
while most of the other continents formed a supercontinent known as Gondwana that stretched from the equator to the South Pole.
The Impact of Glaciation on Ocean Temperatures
By using a new method to measure ancient temperatures, the researchers have uncovered clues about the timing and
magnitude of the glaciation and how it affected ocean temperatures near the equator. "Our observations imply a
climate system distinct from anything we know about over the last 100 million years," says Woodward Fischer,
assistant professor of geobiology at Caltech and a coauthor.
Challenges of Temperature and Ice Sheets
The fact that the extinction struck during a glacial period, when huge ice sheets covered much of what's now Africa
and South America, makes it especially difficult to evaluate the role of climate. "One of the biggest sources of
uncertainty in studying the paleoclimate record is that it's very hard to differentiate between changes in
temperature and changes in the size of continental ice sheets," Finnegan says.
Ordovician: Oldest Soft-Bodied Fossils
Both factors could have played a
role in causing the mass extinction: with more water frozen in ice sheets, the world's sea levels would have been
lower, reducing the availability of shallow water as a marine habitat. But differentiating between the two effects
is a challenge because until now, the best method for measuring ancient temperatures has also been affected by the
size of ice sheets.
Investigating Oxygen Isotopes
The conventional method for determining ancient temperature requires measuring the ratios of oxygen isotopes in
minerals precipitated from seawater. The ratios depend on both temperature and the concentration of isotopes in
the ocean, so the ratios reveal the temperature only if the isotopic concentration of seawater is known. But ice
sheets preferentially lock up one isotope, which reduces its concentration in the ocean.
Since no one knows how
big the ice sheets were, and these ancient oceans are no longer available for scientists to analyze, it's hard to
determine this isotopic concentration. As a result of this "ice-volume effect," there hasn't been a reliable way
to know exactly how warm or cold it was during these glacial periods.
A Paleothermometer and Marine Fossils
But by using a new type of paleothermometer developed in the laboratory of John Eiler, Sharp Professor of Geology
and professor of geochemistry at Caltech, the researchers have determined the average temperatures during the Late
Ordovician-marking the first time scientists have been able to overcome the ice-volume effect for a glacial episode
that happened hundreds of millions of years ago. To make their measurements, the researchers analyzed the chemistry
of fossilized marine animal shells collected from Quebec, Canada, and from the midwestern United States.
The Eiler lab's method, which does not rely on the isotopic concentration of the oceans, measures temperature by looking
at the "clumpiness" of heavy isotopes found in fossils. Higher temperatures cause the isotopes to bond in a more random
fashion, while low temperatures lead to more clumping.
Establishing the Size of Continental Ice Sheets
"By providing independent information on ocean temperature, this new method allows us to know the isotopic composition
of 450-million-year-old seawater," Finnegan says. "Using that information, we can estimate the size of continental ice
sheets through this glaciation." And with a clearer idea of how much ice there was, the researchers can learn more about
what Ordovician climate was like-and how it might have stressed marine ecosystems and led to the extinction.
"We have found that elevated rates of climate change coincided with the mass extinction," says Aradhna Tripati, a
coauthor from UCLA and visiting researcher in geochemistry at Caltech.
A Glaciation Peak During the Extinction Intervals
The team discovered that even though tropical ocean temperatures were higher than they are now, moderately sized glaciers
still existed near the poles before and after the mass extinction. But during the extinction intervals, glaciation peaked.
Tropical surface waters cooled by five degrees, and the ice sheets on Gondwana grew to be as large as 150 million cubic
kilometers-bigger than the glaciers that covered Antarctica and most of the Northern Hemisphere during the modern era's
last ice age 20,000 years ago.
"Our study strengthens the case for a direct link between climate change and extinction," Finnegan says. "Although polar
glaciers existed for several million years, they only caused cooling of the tropical oceans during the short interval that
coincides with the main pulse of mass extinction."
The Research Team and Support
In addition to Finnegan, Eiler, Tripati, and Fischer, the other authors on the Science paper, "The magnitude and duration
of Late Ordovician-Early Silurian glaciation magnitude," are Kristin Bergmann, a graduate student at Caltech; David Jones
of Amherst College; David Fike of Washington University in St. Louis; Ian Eisenman, a postdoctoral scholar at Caltech and
the University of Washington; and Nigel Hughes of the University of California, Riverside.
This research was funded by the Agouron Institute and the National Science Foundation.
Coal Fires: Smoking Gun of the Permian Extinction
|
 |
| The researchers analyzed the chemistry of fossils to determine the temperature during the Late Ordovician period. Shown are fossilized shells, brachiopods, trilobites, and gastropods, with a hammer for scale. Photo by Woody Fischer / Caltech. |
| Rock strata on Anticosti Island, Quebec, Canada, one of the sites from which the researchers collected fossils. Photo by Woody Fischer / Caltech. |
| Climate Change in the News |
|
