Volcanoes trigger greenhouse effect
Earth's repeated flip-flopping between greenhouse and icehouse states over the past 500 million years may have been caused by volcanoes at particular spots where enormous amounts of carbon dioxide are released into the atmosphere.
"We found that Earth's continents serve as enormous 'carbonate capacitors'," says Rice University's Cin-Ty Lee. "Continents store massive amounts of carbon dioxide in sedimentary carbonates like limestone and marble, and it appears that these reservoirs are tapped from time to time by volcanoes, which release large amounts of carbon dioxide into the atmosphere."
The Earth's greenhouse and icehouse states each last somewhere between 10 million and 100 million years. We've now been in an icehouse state for the past 50 million years; the last greenhouse period spanned the late Cretaceous.
Lee and his team now believe that the oscillations between the two are a natural consequence of plate tectonics. It appears that tectonic activity drives an episodic flare-up of volcanoes along continental arcs, particularly during periods when oceans are forming and continents are breaking apart.
The continental arc volcanoes that arise during these periods are located on the edges of continents, and the magma that rises through the volcanoes releases enormous quantities of carbon dioxide as it passes through layers of carbonates in the continental crust.
"The standard view of the greenhouse state is that you draw carbon dioxide from the deep Earth interior by a combination of more activity along the mid-ocean ridges - where tectonic plates spread -- and massive breakouts of lava called 'large igneous provinces'," says Lee.
"Though both of these would produce more carbon dioxide, it is not clear if these processes alone could sustain the atmospheric carbon dioxide that we find in the fossil record during past greenhouses."
Lee's theory, he says, can explain how the same basic set of geophysical conditions could produce and sustain a greenhouse or an icehouse for many millions of years.
"The length of subduction zones and the number of arc volcanoes globally don't have to change," he says. "But the nature of the arcs themselves, whether they are continental or oceanic, does change. It is in the continental-arc stage that CO2 is released from an ever-growing reservoir of carbonates within the continents."