An “excuse me” might be nice. Researchers have found that Earth belches a potent greenhouse gas known as tetrafluoromethane (CF4) during earthquakes and other tectonic events. The emissions likely aren’t making a significant contribution to global warming, but the findings could change the way scientists model future climate scenarios. They also complicate the use of CF4 as a way to measure how the continents and climate have changed over millennia.
CF4 is among the most persistent greenhouse gasses. It lingers in the atmosphere for an estimated 50,000 to 100,000 years, says Karl Haase, a U.S. Geological Survey (USGS) research chemist in Reston, Virginia, who reviewed the new paper but was not affiliated with the study. “Once it’s in the air, it’s there.” CF4’s long life span makes it interesting to scientists studying the history of Earth’s climate over the eons, but until now not much was understood about how it gets into the atmosphere, he says.
Scientists did know that the slow weathering of granite and other metamorphic rocks by rainfall created CF4. But when USGS hydrologist Daniel Deeds in San Diego, California, and colleagues observed elevated levels of the gas in groundwater samples near an active fault in the Mojave Desert, they began to suspect tectonic activity might also play a role.
For the new study, they tested 14 groundwater samples from aquifers along the “Big Bend” area of the San Andreas fault northwest of Los Angeles, California. All but one of the samples showed levels of CF4 that were higher than waters that had been in contact with air, indicating that the chemical was being absorbed from stone deep underground.
After accounting for the CF4 that could be attributed to weathering, the team found that water samples closer to the fault contained excess CF4 levels that were significantly higher than those from farther away. And ground waters that were within 200 meters of the San Andreas fault contained excess levels of CF4 that were “orders of magnitude” higher than elsewhere in the region, according to the paper. “You get these slower emissions from weathering, and then you get these burps of CF4 from tectonic activity,” Deeds says.
It’s still not clear whether the CF4 is created by the fracturing of Earth during an earthquake or by chemical changes in the high pressures of deep fault zones—geological stresses that would lead to an earthquake eventually. Either way, Deeds notes, the emissions can take thousands of years to reach Earth’s surface, which means scientists couldn’t use CF4 release to predict earthquakes.
Human activity like aluminum and semiconductor production is a much bigger source of CF4 in the atmosphere, but understanding where weaker natural emissions come from is still important because of its extraordinarily long life, says Jochen Harnisch, an atmospheric scientist with the environment and climate policy division of KfW Development Bank in Frankfurt, Germany, who was not affiliated with the study.
A potentially more significant result of the study, published online before print in Earth and Planetary Science Letters, could be on how CF4 is used in studying changes in the planet’s carbon balance over time, Deeds notes. Understanding how CF4 is released could be useful to scientists modeling future climate scenarios, helping them understand how long elevated levels of the gas would linger in the atmosphere even if human emissions were curbed. “Small natural sources may not seem important, but if they accumulate in the atmosphere over thousands to tens of thousands of years they can be significant,” Deeds says.
Because of the chemical’s long life span, scientists have proposed using CF4 found in ice core samples as a tracer to study changes in Earth’s climate over geologic time. Fluctuations in CF4 were seen as indicators of how fast the granite material in Earth’s crust was weathering, which in turn could be used to estimate changes in rainfall, temperature, and glacier activity. But now, Deeds says, those fluctuations can’t be attributed exclusively to weathering. “There may be other processes at play, specifically tectonic activity.”