At the start of the so-called Ordovician ice age, about 450 million years ago, the planet went from a state of greenhouse warmth to one of glacial cold, culminating in mass extinctions of ocean life. This period has always posed a problem for climate modelers, notes geologist Matthew Saltzman of Ohio State University. "The models for CO2 that span that interval have always shown levels that are much too high to have an ice age," he explains. "That was a real paradox." Researchers believe that the last ice age, which began 40 million years ago, was kicked off by the rise of the Himalayas during the collision of tectonic plates and a corresponding plunge in atmospheric carbon dioxide. Ocean deposits of calcium carbonate, or limestone, indicate that CO2-rich rainwater stripped calcium and strontium from the Himalayan rock; these elements fused with the carbon dioxide and spilled into the sea, effectively pulling carbon from the atmosphere.
The same chemical weathering commenced before the Ordovician ice age, Saltzman and his student Seth Young reported today at a meeting of the Geological Society of America. The pair analyzed the ratio of strontium isotopes in rocks from Nevada and Europe that date to the Ordovician climate reversal. Right before the ice age begins the ratio becomes low in strontium 87, which accumulates as rock ages, suggesting weathering of relatively young rock. And indeed at that time the Appalachians would have been forming from a Japan-like arc of islands smashing onto what is now North America. "We have pretty good evidence that in fact there was a weathering event that had to involve a significant removal of carbon dioxide," Saltzman says. "If you include the strontium data [in CO2 models] then you can very easily force the drop in CO2 that hadn't been there."
The result helps clear up a "particularly enigmatic" ice age, says geoscientist Lee Kump of Pennsylvania State University. "Solving the enigma is an important step forward" in building confidence in the relationship between CO2 and climate change, he says.