Volcanoes belched the gases that formed Earth’s atmosphere. Those volcanoes could have also spurred lightning that fixed nitrogen, freeing it from nitrogen gas to form nitrogen-containing molecules that living things can use. And scientists have now found proof in the geological record.
On Earth, nitrogen primarily comes in the form of two atoms bonded together, or N2. “It’s very strongly bonded,” said Erwan Martin, a geochemist and volcanologist at Sorbonne University in Paris. Some organisms can break this bond and fix nitrogen. But nitrogen fixation was needed to form the building blocks for life itself: molecules such as the amino acids in proteins. Breaking the N2 bond abiotically (without life) often requires a high-energy process—such as volcanic lightning.
“Nobody ever found any geological and natural archive of this kind of process. We were able touch it for the first time.”
Volcanoes produce lightning when the ash and other particles in their plumes rub together, forming electrical charges in a way similar to the colliding of ice particles in clouds. Laboratory experiments have suggested that volcanic lightning could fix nitrogen. “But nobody ever found any geological and natural archive of this kind of process,” Martin said. “We were able touch it for the first time.”
Martin’s team happened upon the fixed nitrogen when searching for volcanic deposits of sulfate to study their formation and role in climate processes, such as cooling the atmosphere. Because sulfates are very soluble, the team scoured desert environments, where sulfates were unlikely to have been washed away. That led the team to central Türkiye and southern Peru. In samples from nine eruptions at these locales, the researchers were surprised by the levels of nitrate, a fixed form of nitrogen that is bonded to three oxygen atoms.
The oxygen in the nitrates showed an isotopic signature revealing that it came from ozone in the atmosphere and had not been fixed by organisms. On the basis of the concentration of nitrate in the samples, the team found that some 60 teragrams (about the mass of 10 Pyramids of Giza) of nitrogen can be fixed in a very explosive eruption.
Such high local concentrations of nitrogen show that volcanoes could have helped the formation of essential molecules for life, the researchers reported.
Explosive Origins of Life
The bigger the eruption is, the greater the potential to create electrical charges and lightning is, said Corrado Cimarelli, a volcanologist at Ludwig Maximilians University Munich in Germany who wasn’t part of the study. The massive January 2022 eruption of Hunga Tonga–Hunga Ha‘apai, for example, produced record lightning—some 400,000 strikes in a 6-hour period.
Some of the deposits analyzed in the new study are “peculiar,” Cimarelli said. They’re large and from a highly explosive event—“some type of eruption that we never really experienced in our lifetime.” This event could have produced intense lightning.
The work is “really promising.”
The work is “really promising,” Cimarelli said. Researchers may be able to spot other eruptions that have fixed nitrogen, especially in places where deposits are well preserved or recent. In the laboratory, researchers such as Cimarelli have been recreating the chemical processes that occur during lightning strikes.
The ages of samples from the new study range from 2 million to 50,000 years old—far more recent than the time when life emerged more than 3 billion years ago. At that time, Earth’s atmosphere would have been oxygen poor. But volcanic lightning could still have fixed nitrogen, likely forming compounds such as ammonia that are used to form biological molecules, Martin said.
It’s plausible that volcanic lightning was common on early Earth, Cimarelli said, “probably even earlier than the thunderstorm lightning, because we didn’t have the atmospheric circulation as we know it today.”
But the volcanoes of early Earth may have been far less explosive than the ones analyzed, Cimarelli said. Volcanoes that have magma with higher silica content tend to be more explosive because the thick, viscous magma traps more gas. But basaltic volcanoes with thinner magmas dominated the landscape of early Earth, he said. These volcanoes likely lacked the gas to propel big explosions and generate lots of lightning. Further research could investigate the limits on conditions at which volcanoes make lightning.
Fixing nitrogen isn’t the only way that volcanic lightning may have helped to spur life. Such lightning can spark many chemical processes, Cimarelli said. For instance, a volcanic lightning zap to certain rock types can help convert the phosphorus in them to forms that life can use.
For researchers investigating how life could have emerged from rocks and lightning, the challenge is figuring out how the chemicals required could have been concentrated enough to potentially kick-start life. Now with lab work and fieldwork, Cimarelli said, “all these things are basically coming together.”
—Carolyn Wilke (@CarolynMWilke), Science Writer