Volcanic eruptions were responsible for a deadly ice age 450 million years ago, as well as — in an ironic twist — a period of global warming that preceded it, a new study finds.

The finding underscores the importance of carbon in Earth's climate today, said study researcher Matthew Saltzman of Ohio State University.

The ancient ice age featured glaciers that covered the South Pole on top of the supercontinent of Gondwana (which would eventually break apart to form the present-day continents of the southern hemisphere). Two-thirds of all species perished in the frigid climate.

Previously, Saltzman and his team linked this same ice age, which took place in the Ordovician period, to the rise of the Appalachian Mountains. As the exposed rock weathered, chemical reactions pulled carbon from Earth's atmosphere, causing the deadly global cooling.

With models, the researchers have now pieced together 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 said. "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."

To figure out this geologic history, Saltzman and his colleagues used computer models to draw together measurements of isotopes of chemical elements from rocks in Nevada, Virginia and Pennsylvania with measurements of volcanic ash beds in the same locations. They also factored in temperature models developed by other scientists.

The ash deposits demonstrated when the volcanoes stopped erupting, and the isotope measurements pinpointed the Appalachians as the source of the volcanic rock.

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 to levels 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 research, partly supported by the National Science Foundation, is detailed in an online edition of the journal Geology and will also appear in a future print edition.