During earthquakes, spider webs of faults open up below ground, allowing gases deep within our planet to percolate upward. Researchers have now compiled the first long-term record that shows a relationship between earthquakes and the release of carbon dioxide gas.
While the amount of carbon dioxide released by tectonic activity is a pittance compared with the billions of tons that human activity pumps into the atmosphere each year, the research published Wednesday in Science Advances sheds light on the planet’s climate-controlling carbon cycle.
“It’s modulating Earth’s climate on geological time scales,” said James Muirhead, a geologist at the University of Auckland in New Zealand, not involved in the research who praised the data set a team of Italian scientists had collected.
The results could also potentially pave the way toward forecasting seismic activity.
The region around Italy’s central Apennine Mountains, roughly an hour east of Rome, is riddled with faults. Devastating earthquakes have repeatedly struck the area, including the L’Aquila earthquake in 2009. That temblor, which killed hundreds of people, made headlines again in 2012 when a judge ruled that seven Italian earthquake experts were guilty of manslaughter because they had failed to warn nearby residents of the potential risk. The area’s seismic activity has been linked to escaping carbon dioxide.
Giovanni Chiodini, a geochemist at the National Institute of Geophysics and Volcanology in Bologna, and his colleagues analyzed the carbon content of groundwater in the Apennines. From April 2009 through December 2018, the researchers gathered hundreds of water samples from 36 different springs. They calculated the carbon dioxide concentration in each sample after subtracting contributions from rainwater and soil.
The researchers estimated that about 1.7 million tons of carbon dioxide were discharged by tectonic activity in the study area over a decade. That’s roughly equivalent to the carbon dioxide emitted by one volcanic eruption.
The real surprise came when Dr. Chiodini and his collaborators compared their data with the records of roughly 17,000 earthquakes that had occurred nearby. The researchers found that seismic activity and carbon dioxide degassing clearly tracked one another in time — periods of high earthquake activity lined up with peaks in gas release. For example, carbon dioxide concentrations measured in the months following the L’Aquila earthquake were roughly twice as high as they were in 2013, a period of low earthquake activity, the team showed. High concentrations were again measured in September and November 2016, just a few months after several large earthquakes rocked the region.
This link makes sense, the scientists propose, based on what’s going on miles beneath the surface. The central Apennines sit on top of a subduction zone where slabs of carbon-rich rock are continuously diving downward. As those rocks sink, they’re exposed to hotter and hotter conditions until they melt, which releases gases, Dr. Chiodini said. “One hundred kilometers below the Apennines, you have a huge source of carbon dioxide,” he said. As pressure builds underground, the crust eventually fractures, resulting in earthquakes.
Earthquakes themselves might also trigger more carbon dioxide degassing, the scientists suggest. That’s because ground movement might cause bubbles of gas to form deep underground, akin to shaking a bottle of champagne, Dr. Chiodini said. This feedback loop might help explain aftershocks in the Apennines, the researchers propose.
The big question is what happens first, the earthquake or the carbon dioxide degassing.
“If the carbon dioxide discharges are leading the large earthquakes in time, then perhaps these methods could be used alongside other tools as earthquake indicators,” Dr. Muirhead said.
But a lot more observations — with measurements spaced closer together in time — would be needed, he said, and even then it would still be a fraught endeavor to forecast seismic activity. “The uncertainties are still too high and the consequences too huge,” he said.