Over the last 500 million years, the Earth’s climate has cycled between greenhouse and icehouse states. These climactic fluctuations lasted from tens to hundreds of millions of years, and were marked by changes in the amount of carbon dioxide in the atmosphere as well as changes in glacial activity.
The main driver of this change in climate was the amount of carbon dioxide in the atmosphere, which was largely governed by volcanic outgassing. But a question that has baffled researchers until now: what caused the dramatic and long-lived shifts in the rate of volcanic carbon dioxide outgassing?
A new study co-authored by Senior Fellow Mark Jellinek (UBC) in GeoSphere suggests that the Earth’s oscillations between greenhouse and icehouse states are a natural consequence of the changing nature of volcanic carbon dioxide outgassing during the formation and subsequent breakup of the supercontinent Pangea between 300 and 140 million years ago. In the study, the team found that Pangean continental crust was a storehouse of tremendous amounts of carbon dioxide in sedimentary carbonates such as limestone and marble. During the breakup of Pangaea, volcanoes formed where tectonic plates shifted, and the magma from these volcanoes interacted with the carbonates in the Earth’s crust, purging carbon dioxide from the crust into the atmosphere. This input of carbon dioxide into the atmosphere was 500 per cent more than what is observed today, driving a tropical climate between 100 to 40 million years ago.
“Climate wise, about a third to a half of the carbon dioxide in the world today is caused by the outgassing of these arc volcanoes,” says Dr. Jellenik. “For instance, Mount Etna in Italy is a volcano that sits on a continental arc, and it is responsible for 10 to 20 per cent of the world’s total carbon dioxide. Given that there are thousands of volcanoes in existence today, this is a significant amount.”
The results of the study differ from the conventional understanding of greenhouse and icehouse states.
“The traditional view of the ‘super-greenhouse period’ of 100 to 40 million years ago, for example, is that it was driven by massive eruptions of basalt that ultimately evolved to form islands like Hawaii. The problem with this view is that chemical weathering of silicate materials actually draws carbon dioxide out of the atmosphere. With this theory, we would need to observe a volcanic eruption every million years to maintain the super-greenhouse state, and we know that this did not happen. Our proposed mechanism, on the other hand, is consistent with the observed geological record.”
This study was funded by CIFAR, the Natural Sciences and Engineering Research Council and the National Science Foundation.