Modeling p N ₂ through Geological Time: Implications for Planetary Climates and Atmospheric Biosignatures

Nitrogen is a major nutrient for all life on Earth and could plausibly play a similar role in extraterrestrial biospheres. The major reservoir of nitrogen at Earth's surface is atmospheric N 2 , but recent studies have proposed that the size of this reservoir may have fluctuated significantly over the course of Earth's history with particularly low levels in the Neoarchean—presumably as a result of biological activity. We used a biogeochemical box model to test which conditions are necessary to cause large swings in atmospheric N 2 pressure. Parameters for our model are constrained by observations of modern Earth and reconstructions of biomass burial and oxidative weathering in deep time. A 1-D climate model was used to model potential effects on atmospheric climate. In a second set of tests, we perturbed our box model to investigate which parameters have the greatest impact on the evolution of atmospheric p N 2 and consider possible implications for nitrogen cycling on other planets. Our results suggest that (a) a high rate of biomass burial would have been needed in the Archean to draw down atmospheric p N 2 to less than half modern levels, (b) the resulting effect on temperature could probably have been compensated by increasing solar luminosity and a mild increase in p CO 2 , and (c) atmospheric oxygenation could have initiated a stepwise p N 2 rebound through oxidative weathering. In general, life appears to be necessary for significant atmospheric p N 2 swings on Earth-like planets. Our results further support the idea that an exoplanetary atmosphere rich in both N 2 and O 2 is a signature of an oxygen-producing biosphere. Key Words: Biosignatures—Early Earth—Planetary atmospheres. Astrobiology 16, 949–963.