Since at least the 1970s, scientists have noticed that areas with frequent earthquakes also tend to have springs that bubble with carbon dioxide. They reasoned that slipping tectonic plates opened small rifts in the crust, like stretching an accordion, which lets deep carbon dioxide seep into aquifers and up to the surface. But tracking and measuring the gas that escapes from these faults, called Earth degassing, is an enormous task. Researchers have only performed more detailed studies in a few locations: the Himalayas, central Italy, and the East African rift.
To see if the relationship between shifting tectonics and carbon dioxide release holds true worldwide, DCO Reservoirs and Fluxes Community members Giancarlo Tamburello and Giovanni Chiodini (both at Istituto Nazionale di Geofisica e Vulcanologia, Italy) with INGV colleagues Silvia Pondrelli and Dmitri Rouwet, analyzed global geological databases of earthquakes, volcanic eruptions, and carbon dioxide-rich springs. In a new paper in Nature Communications , they report a correlation between the locations of carbon dioxide leaks and active faults, especially those that create openings in the crust. These findings could inform future estimates of how Earth degassing has impacted atmospheric carbon dioxide levels and global temperature in the past.
Tamburello painstakingly compiled and digitized published reports of carbon dioxide-rich springs from around the world. Using statistical analyses, the researchers compared the location of the springs to the sites of active faults, taking into account the type of fault, and whether it creates openings in the crust (extensional) or squeezes the crust together (compressional). "With the statistical analysis we show that there is a correlation between extensional tectonics and the release of crustal carbon dioxide," said Tamburello.
Many springs occurred near volcanoes, suggesting that the carbon dioxide may come from melted magma. But the analysis also identified three regions with leaks far from recent eruptions, which points to a non-volcanic carbon source. The Pannonian Basin, a large region in Central Europe that used to be a shallow sea, the Cordillera Blanca mountain range in Peru, and the Himalayas mountain range are regions where researchers have established that non-volcanic carbon dioxide escapes from the mantle or through crustal metamorphism. The findings confirm that the current study’s approach is useful for identifying carbon dioxide leaks independent of volcanic activity.
By combining these data, researchers can also predict where tectonic forces are opening rifts for deep carbon to escape. These findings suggest that the current database of global tectonic degassing is fairly complete, and that additional exploration likely would not affect global degassing estimates. Except possibly in Melanesia, the analyses did not identify any regions where springs are predicted, but unknown.
Now, Tamburello wants to use this discovery to look back in time at carbon dioxide leaks. "This correlation may help us refine our model of Earth’s degassing over time and how that degassing may have influenced global temperature in the past,” said Tamburello. Carbon dioxide release through Earth degassing represents a sizable part of the total deep carbon that enters the atmosphere each year from natural sources.
The researchers plan to work with the EarthByte group (University of Sydney, Australia) to use the software program GPlates  to model the movement of tectonic plates over hundreds of millions of years. Brune et al. did a similar analysis in 2017  by correlating the length of rifts over time with the total outflux of carbon dioxide from the subsurface. By including the results from the new study, researchers aim to achieve deeper insights into the mechanisms that controlled the amount of carbon dioxide that accumulated in the atmosphere in the past.