French geologists analyzed the composition of nitrates in volcanic sediments from the Neogene period and discovered that lightning in ash clouds could fix atmospheric nitrogen. Researchers found several lines of evidence showing that nitrates in sediments are formed from atmospheric nitrogen.

The formation of proteins and nucleic acids is the most important process in the origin of life. But this is not possible without nitrogen, one of the most common elements on Earth. Nitrogen occupies almost three-quarters of the volume of atmospheric air, but it is in a form that living organisms cannot absorb. Conversion of nitrogen into a bioavailable form is called nitrogen fixation. Nowadays this is done by microorganisms or nitrogenous fertilizers, but before the emergence of life, the role of nitrogen fixer was hypothetically played by various phenomena – especially storms and volcanic lightning.

The possibility of such nitrogen fixation has been proven in laboratory experiments. During explosions, large amounts of sulfur and halogens (chlorine, fluorine, etc.) are released into the atmosphere, and lightning called dirty storms occur in the ash clouds. During discharges, nitrogen in the air is oxidized and the resulting oxides fall to the ground with volcanic emissions. As a result, nitrates, which are fixed nitrogen, had to remain in the soil. However, geological confirmation of this has not yet been found.

A group of French geologists decided to find nitrate deposits in volcanic deposits after major Neogene eruptions (1.65-23.5 million years ago) in arid regions. Researchers collected samples of volcanic tephra and pyroclastic flows in Peru and Turkey and found high levels of nitrate as well as sulfur and chlorine in these sediments, indicating their volcanic origin. An article on this subject was published in the magazine. Proceedings of the National Academy of Sciences.

As scientists note, the similar elemental composition in the sediments suggests that most of the nitrates were formed precisely as a result of volcanic eruptions, and not in the process of further infiltration of precipitation. This was also proven by additional analysis of other samples from earlier eruptions that occurred on the Italian island of Ischia about 75 thousand years ago. According to the authors, the finest sediments were washed away by rainwater more than the thicker ones.

By studying the multiisotopic composition of nitrates, researchers were able to trace the oxidation pathway of nitrogen and sulfur. The abnormally high content of the oxygen-17 isotope in the samples indicated its atmospheric origin, as it was inherited from ozone, a product of lightning discharge.

The researchers also estimated the amount of nitrogen fixed after volcanic lightning. Taking into account the density and volume of each sediment layer as well as the average nitrate concentration, they calculated that an average of nine volcanic events could produce about 60 teragrams of nitrogen (60 million tonnes). It is worth recalling here that before the emergence of life on Earth, today the main ways of fixing nitrogen (with the help of microorganisms) were excluded. By the way, nitrogen is absolutely necessary for the origin of our form of life. So the role of volcanic lightning may be key to this.

The study’s authors believe that large volcanic eruptions and lightning strikes may have made a major local contribution to the origin of life on early Earth. Additionally, according to them, volcanic lightning was potentially associated with the conversion of mineral phosphorus into biologically useful forms.