Plate tectonics, responsible for deadly earthquakes and volcanoes around the globe, may also be a key factor in the success of life as we know it on the third planet, a new study suggests.
As the rigid blocks of crust have ground over, under and past one another, they have helped produce earth’s unique nitrogen-rich atmosphere, researchers reported Sunday in Nature Geoscience online. The findings may also help explain why Venus and Mars, which formed with similar amounts of nitrogen as the Earth, evolved to have very different atmospheres.
Using a mathematical model to predict what happens to nitrogen during plate tectonics, scientists determined that it is subduction that led to an accumulation of the gas that makes up nearly 80 percent of our current atmosphere.
“Our data lead us to conclude that the composition of the atmosphere is buffered by subduction zones, and it is certainly possible that without subduction zones our atmosphere would be radically different in composition,” said Sami Mikhail, the study’s lead author and a postdoctoral scientist at Bristol University in England. “Maybe life would have survived for billions of years without subduction zones, but without subduction zones the atmosphere would have been drastically different chemically, and therefore so would life.”
As oceanic plates dive underneath the continents, sediments and water are pulled down with them into a wedge-shaped area between the two plates. As the materials in the wedge heat up, Mikhail said, ammonium, which is made up of nitrogen and hydrogen, reacts with oxygen to produce nitrogen and water, both of which spew out of the volcanoes lining the subduction zone.
While some nitrogen is released at mid-ocean ridges, such as the one running through Iceland, and hot spots like the Hawaiian chain of islands, this contribution is nowhere near the amount that is pumped out at the chains of volcanoes rising above the subducted slab on the continental plate, Mikhail said.
“Because subduction only occurs on Earth, this has not happened on Venus and Mars,” Mikhail said. “Thus the atmospheric composition of the three planets diverged once plate tectonics got going on earth. The atmospheres, and potentially habitabilities of Venus and Mars have evolved along different paths through geologic time because they lack plate tectonics.”
The new study shows how important plate tectonics is in making a habitable planet, said David Hilton, a professor of geochemistry at the University of California, San Diego, who is unaffiliated with the new study.
“Earth is this special planet in the so-called Goldilocks zone,” Hilton said, meaning that everything has to be just right for life as we know it to flourish.
Data on the content of gases spewing from volcanoes supports Mikhail’s work, Hilton said.
“It corresponds to what we see in the sediments being put back into the mantle,” he explained. “We see the isotopic signature of the sediments in volcanoes.”
Models like Mikhail’s may help scientists understand the interplay between nitrogen and greenhouse gases, said Norman Sleep, a geophysicist and planetary scientist at Stanford University who was not affiliated with the new study.
Current climate change models assume that nitrogen has been stable over time, but these findings suggest that it has been increasing over the last 2.45 million years, Sleep said.