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Geologic evidence that volcanic lightning promotes life on Earth

Geologic evidence that volcanic lightning promotes life on Earth

13 February 2024

Large quantities of fixed nitrogen found in ash and pyroclastic deposits confirm suspicions about a likely source of life’s earliest building blocks.

Every protein molecule in every cell in your body contains nitrogen, but the abundant N2 molecules in the atmosphere are useless to you. The triple bonds that hold them together are just too strong. Atmospheric nitrogen is made available to organisms by breaking apart that N2 and bonding the free nitrogen with oxygen or hydrogen—a process known as fixation. Nitrogen fixing can be accomplished in a few ways, including biotic processing by bacteria and algae, high-energy industrial-fertilizer production, and lightning strikes.

Lightning seen in the ash plume during a volcanic eruption.
Volcanic lightning, like that seen in the ash plume of an eruption in Iceland in 2010, may be a significant source of abiotically fixed nitrogen. Credit: Terje Sørgjerd/CC BY-SA 3.0 DEED

Without fertilizer factories, algae, or bacteria around to provide the fixed nitrogen for life on Earth, its emergence would have required abiotic nitrogen fixation. Lightning has been the leading candidate for the source of that process. Lab experiments and theoretical models have suggested that volcanic lightning in particular could have played a vital role because ash and gas plumes promote the highest rates of lightning strike. Yet, until now, no significant quantities of abiotically fixed nitrogen have been found in the geologic record or from present-day eruptions.

Adeline Aroskay and Erwan Martin of Sorbonne University and their colleagues were looking in volcanic deposits for sulfates, which contribute to eruption-related climate change. They turned to rocks in arid environments in Turkey and Peru, where the soluble sulfate molecules would be preserved and not flushed away by water over time. Alongside the sulfur and chlorine compounds that they expected to find, they discovered a surprisingly high concentration of nitrates, a fixed form of nitrogen.

Looking at the stable isotopes of oxygen in the nitrates, the researchers found that they contained high concentrations of oxygen-17—as much as 17 parts per thousand—that would require contributions from ozone, which is rich in 17O. The researchers concluded that the nitrates in the volcanic deposits must have been formed in the atmosphere. “We found natural samples in which we have the end product of this process,” says Martin.

Using the nitrate concentrations in their samples and estimates of the volcanic deposit volume, they calculate that as much as 282 × 109 kg of nitrogen were deposited in one eruption. That’s on the same order of magnitude as what is produced for industrial fertilizer in a year. But volcanic events large and explosive enough to fix so much nitrogen are rare—occurring perhaps once every 100 000 years—as are the arid conditions necessary to preserve the highly soluble nitrates. So it’s not surprising that they hadn’t been found in volcanic deposits before. (A. Aroskay et al., Proc. Natl. Acad. Sci. USA 121, e2309131121, 2024.)

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