COLUMBUS, Ohio -- Researchers here have discovered the pivotal
role that volcanoes played in a deadly ice age 450 million years
ago.
Perhaps ironically, these volcanoes first caused global warming
-- by releasing massive amounts of carbon dioxide into the
atmosphere.
When they stopped erupting, Earth's climate was thrown off
balance, and the ice age began.
The discovery underscores the importance of carbon in Earth's
climate today, said Matthew Saltzman, associate professor of earth
sciences at Ohio State University.
The results will appear in the journal Geology, in a
paper now available online.
Previously, Saltzman and his team linked this same ice age to
the rise of the Appalachian Mountains. As the exposed rock
weathered, chemical reactions pulled carbon from Earth's
atmosphere, causing a global cooling which ultimately killed
two-thirds of all species on the planet.
Now the researchers have discovered the other half of the story:
giant volcanoes that formed during the closing of the
proto-Atlantic Ocean -- known as the Iapetus Ocean -- set the stage
for the rise of the Appalachians and the ice age that followed.
"Our model shows that these Atlantic volcanoes were spewing
carbon into the atmosphere at the same time the Appalachians were
removing it," Saltzman explained. "For nearly 10 million years, the
climate was at a stalemate. Then the eruptions abruptly stopped,
and atmospheric carbon levels fell well below what they were in the
time before volcanism. That kicked off the ice age," he said.
This is the first evidence that a decrease in carbon from
volcanic degassing -- combined with continued weathering of the
Appalachians -- caused the long-enigmatic glaciation and extinction
in the Ordovician period.
Here is the picture the researchers have assembled: 460 million
years ago, during the Ordovician, volcanoes along the margin of
what is now the Atlantic Ocean spewed massive amounts carbon
dioxide into the atmosphere, turning the world into a hothouse.
Lava from those volcanoes eventually collided with North America to
form the Appalachian Mountains.
Acid rain -- rich in carbon dioxide -- pelted the newly exposed
Appalachian rock and wore it away. Chemical reactions trapped the
carbon in the resulting sediment, which formed reefs in the vast
seas that covered North America.
For about 10 million years, the volcanoes continued to add
carbon to the atmosphere as the Appalachians removed it, so the
hothouse conditions remained stable. Life flourished in the warm
oceans, including abundant species of trilobites and
brachiopods.
Then, 450 million years ago, the eruptions stopped. But the
Appalachians continued weathering, and atmospheric carbon levels
plummeted. The Earth swung from a hothouse to an icehouse.
By 445 million years ago, glaciers had covered the south pole on
top of the supercontinent of Gondwana (which would eventually break
apart to form the continents of the southern hemisphere).
Two-thirds of all species had perished.
When they started this research, Saltzman and his team knew that
Earth's climate must have changed drastically at the end of the
Ordovician. But they didn't know for certain that volcanoes were
the driving force, explained Seth Young, who did this research for
his doctoral degree at Ohio State. He is now a postdoctoral
researcher at Indiana University.
"This was not necessarily what we expected when we started
investigating, but as we combined our data sources, the story began
to fall into place," Young said.
Using a computer model, they drew together measurements of
isotopes of chemical elements -- including strontium from rocks in
Nevada and neodymium from rocks in Virginia and Pennsylvania --
with measurements of volcanic ash beds in the same locations. Then
they factored in temperature models developed by other
researchers.
The ash deposits demonstrated when the volcanoes stopped
erupting; the strontium levels indicated that large amounts of
volcanic rock were being eroded and the sediment was flooding
Earth's oceans during this time; and the neodymium levels
pinpointed the Appalachians as the source of the sediment.
The new findings mesh well with what scientists know about these
ancient proto-Atlantic volcanoes, which are thought to have
produced the largest eruptions in Earth's history. They issued
enough lava to form the Appalachians, enough ash to cover the far
ends of the earth, and enough carbon to heat the globe. Atmospheric
carbon levels grew 20 times higher than they are today.
This study shows that when those volcanoes stopped erupting,
carbon levels dropped, and the climate swung dramatically back to
cold. The timing coincides with today's best estimates of
temperature fluctuations in the Ordovician.
"The ash beds start building up at the same time the Appalachian
weathering begins, but then the record of volcanism ends, and the
temperature drops," Saltzman said. "Knowing these details can help
us understand how carbon in the atmosphere is changing Earth's
climate today."
Next, the researchers will examine the role of the ancient
volcanic ash more closely. While the ash was in the atmosphere --
before it settled around the globe -- it might have blotted out the
sun, and cooled the earth somewhat. Saltzman and his team want to
make some estimate of this short-term cooling effect to refine
their computer model.
Meanwhile, Young is just starting to re-analyze the same rock
samples, this time looking for a different isotope -- sulfur. This,
he hopes, will offer clues to how much oxygen was in the oceans,
and how that oxygen may have affected life in the Ordovician.
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