Geological evidence of extensive N-fixation by volcanic lightning during very large explosive eruptions

Significance

Nitrogen (N) fixation is an essential process for life as it converts atmospheric dinitrogen into biologically assimilable forms. Experimental and theoretical studies have proposed that volcanic lightning could have contributed to N-fixation in early prebiotic environments on Earth but geological evidence are still lacking. For the first time, significant amount of nitrate have been discovered in volcanic deposits from very large explosive eruptions. Geochemical analyzes, including oxygen multi-isotopes, indicate that this nitrate is the oxidation end-product of atmospheric N-fixation by volcanic lightning. 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.

Abstract

Most of the nitrogen (N) accessible for life is trapped in dinitrogen (N ₂), the most stable atmospheric molecule. In order to be metabolized by living organisms, N ₂ has to be converted into biologically assimilable forms, so-called fixed N. Nowadays, nearly all the N-fixation is achieved through biological and anthropogenic processes. However, in early prebiotic environments of the Earth, N-fixation must have occurred via natural abiotic processes. One of the most invoked processes is electrical discharges, including from thunderstorms and lightning associated with volcanic eruptions. Despite the frequent occurrence of volcanic lightning during explosive eruptions and convincing laboratory experimentation, no evidence of substantial N-fixation has been found in any geological archive. Here, we report on the discovery of a significant amount of nitrate in volcanic deposits from Neogene caldera-forming eruptions, which are well correlated with the concentrations of species directly emitted by volcanoes (sulfur, chlorine). The multi-isotopic composition (δ ¹⁸O, Δ ¹⁷O) of the nitrates reveals that they originate from the atmospheric oxidation of nitrogen oxides formed by volcanic lightning. According to these first geological volcanic nitrate archive, we estimate that, on average, about 60 Tg of N 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.