The chaos of lightning crackling through the sky has been speculated to play a key role in freeing elements like phosphorus, making it more accessible to burgeoning life on ancient Earth.
Now, scientists have found geological evidence that the lightning discharges associated with volcanic events may have also played a role in nitrogen fixation, making it available for biological processes too.
Since Earth is thought to have once been significantly more volcanically volatile in its younger years, the discovery suggests that rampant volcanism may be crucial to the emergence of life as we know it.
"For the first time, significant amounts of nitrate have been discovered in volcanic deposits from very large explosive eruptions," writes a team led by geoscientist Adeline Aroskay of Sorbonne University.
"These findings provide geological support for a unique role played by subaerial explosive eruptions in high energy-demanding processes, which were essential in supplying building blocks for life during its emergence on Earth."
Somehow, back when Earth was wild and young, processes on our planet transformed a mixture of ingredients into the stuff needed to kickstart life. One of the things life needs is nitrogen, an essential nutrient for building things like proteins, amino acids, and nucleic acids.
Earth has scads of nitrogen; some 78 percent of the atmosphere is made up of the stuff. But biology can't access it in its atmospheric form of molecular nitrogen, or N2; the nitrogen atoms need to be split apart and fixed to other atoms to form more reactive compounds, such as nitrates or ammonia.
Now that life exists, biological processes such as those in microbes living in the roots of plants can speed up the process. Human industrial techniques can also pump in nitrogen and pump out compounds like ammonia by the tank load.
But before there was life, a non-biological process was needed to kickstart nitrogen fixation. And here's where electricity – lightning – comes into play. Electrical discharges can fix nitrogen, as first established in 1784.
And, because lightning is so ubiquitous during volcanic eruptions, triggered by the vast billowing clouds of ash, scientists thought that volcanic lightning might have played a role in triggering the nitrogen cycle before life had wriggled its way into the world.
Experimental studies showed that this was possible – but geological evidence for fixed nitrogen as a result of this process has been thin on the ground.
Aroskay and her team have found that evidence, in the form of nitrates embedded in ancient volcanic deposits. They collected samples from a number of volcanic deposits from explosive eruptions in Turkey and Peru, which took place between 1.6 and 20 million years ago, and looked specifically for nitrates, which are the end product of the oxidation of nitrogen.
They found significant concentrations of nitrates in all of the deposits they sampled, with features that indicated most of it was formed during volcanic eruptions. This, the researchers said, was remarkable.
They also sampled younger volcanic deposits, approximately between 75,000 and 55,000 years old, from less explosive eruptions; there, too, they found nitrates.
There was no correlation between the age of the deposit and the concentration of nitrates, which means, the researchers believe, that the deposition of the compounds is not a result of gradual processes that occur over long timescales. The nitrates are dumped into the volcanic rock in one fell swoop.
The concentrations of sulfur and chlorine in the deposits were also consistent with a volcanic origin. Taken together, this suggests that volcanic lightning can fix nitrogen in amounts significant enough to play a role in life's emergence.
And it could give us a tool to interpret other nitrate deposits around the world. In the Atacama desert, for example, the presence of nitrates was attributed to atmospheric deposition, because it was thought that the ratios of oxygen isotopes weren't possible with volcanic deposition. The team's findings show that these bonds are possible in large explosive events.
"The discovery of nitrate in volcanic deposits makes them the first field evidence and geological archive of N-fixation by volcanic lightning, and this can have important implications," the researchers write.
"We estimate that, on average, about 60 teragrams of nitrogen can be fixed during a large explosive event. Our findings hint at a unique role potentially played by subaerial explosive eruptions in supplying essential ingredients for the emergence of life on Earth."
The research has been published in the Proceedings of the National Academy of Sciences.