Geological Record of Atmospheric Nitrogen Fixation During Explosive Supereruptions: The Case of Tecopa Basin, California

Nitrate in dry sedimentary records has long been associated with long‐term atmospheric deposition and biological activity. Recent discoveries of nitrate with an atmospheric origin in volcanic deposits from large eruptions have been tentatively linked to volcanic lightning. Lightning can break the chemical bonds of stable molecules like dinitrogen, releasing NO, which is rapidly oxidized by ozone, forming atmospheric nitrate with a high 17 O‐anomaly. Another possible source of nitrate in volcanic deposits is atmospheric long‐term deposition. To confirm and quantify the origins of nitrate in volcanic deposits, we compare for the first time the multi‐isotopic (O, N) signatures of nitrate in sediments and volcanic deposits from supereruptions. Samples were taken in the Tecopa Basin (California), where the dry conditions have been favorable to sample preservation. The mean O and N isotopic compositions in nitrate from sediments (Δ 17 O = 12 ± 1‰ and δ 15 N = 2 ± 2‰ (2 σ )) and volcanic ash layers (Δ 17 O = 19 ± 5‰ and δ 15 N = −5 ± 3‰ (2 σ )) are substantially different. Only a massive and fast production of atmospheric nitrate during supereruptions can explain the higher Δ 17 O values in volcanic ash layers. The most plausible N‐fixation mechanism of this atmospheric nitrate is volcanic lightning. Our results indicate that volcanic lightning from supereruptions such as Lava Creek Tuff (0.64 Ma, Yellowstone) could fix up to ∼100 Tg of N. Alteration and erosion of nitrate‐rich volcanic deposits are expected to free large quantities of nitrate into the environment, which living organisms assimilate and could play a significant role in developing the local biosphere.