Modeling pN2 Through Geological Time: Implications for Planetary Climates and Atmospheric Biosignatures E.E. Stüeken1,2,3,4*, M.A. Kipp1,4, M.C. Koehler1,4, E.W. Schwieterman2,4,5, B. Johnson6, R. Buick1,4 1. Dept. of Earth & Space Sciences and Astrobiology Program, University of Washington, Seattle, WA 98195, USA 2. Dept. of Earth Sciences, University of California, Riverside, CA 92521, USA 3. Department of Earth & Environmental Sciences, University of St Andrews, St Andrews KY16 9AL, Scotland, UK 4. NASA Astrobiology Institute’s Virtual Planetary Laboratory, Seattle, WA 981195, USA 5. Dept. of Astronomy and Astrobiology Program, University of Washington, Seattle, WA 98195, USA 6. School of Earth & Ocean Sciences, University of Victoria, Victoria, BC V8P 5C2, Canada * corresponding author ([email protected]) Astrobiology, Volume 16, Number 12, doi: 10.1089/ast.2016.1537 Abstract 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 N2, 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 N2 pressure. Parameters for our model are constrained by observations of the 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 pN2 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 pN2 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 pCO2, and (c) atmospheric oxygenation could have initiated a stepwise pN2 rebound through oxidative weathering. In general, life appears to be necessary for significant atmospheric pN2 swings on Earth-like planets. Our results further support the idea that an exoplanetary atmosphere rich in both N2 and O2 is a signature of an oxygen- producing biosphere. 1. Introduction strong triple bond in N≡N. Lightning can break this bond and Life as we know it is implausible without nitrogen. It combine nitrogen with other atmospheric species, but on is an essential major nutrient for all living Earth this process has been relatively inefficient (Borucki and things because it is a key component of the nitrogenous bases Chameides, 1984; Navarro-Gonzalez et al., 2001). Much more in the nucleic acids that store, transcribe, and translate genetic significant for removing N2 from Earth’s atmosphere is information, a necessary ingredient of the amino acids microbial N2 fixation to ammonium, a reaction catalyzed by the constituting the proteins responsible for most cellular catalysis enzyme nitrogenase. Today, most of this fixed nitrogen is and at the core of the ATP molecule that is the principal energy returned to the atmosphere via the biogeochemical nitrogen transfer agent for biological metabolism. As nitrogen’s cosmic cycle, a series of microbially modulated redox reactions that abundance is only slightly less than that of carbon and oxygen ultimately transform organic nitrogen back to gaseous N2. and because it condenses at moderate distances in circum- Thus, life’s demand for nitrogen regulates its atmospheric stellar disks, it should have been available to other potential abundance. exoplanetary biospheres. However, N2 gas, the most likely Though N2 is not a greenhouse gas itself, its nitrogen species near planetary surfaces in the habitable zone, atmospheric partial pressure affects planetary environmental is nearly inert at standard conditions because of the very conditions. Higher N2 pressure can enhance greenhouse 1 warming by pressure broadening the absorption bands of such Pierrehumbert and Gaidos, 2011; Seager, 2013). After gases as water vapor, CO2, CH4, and N2O (Goldblatt et al., exploring a range of variables, we concluded that some 2009). Hence, the partial pressure of N2 indirectly influences combinations of N2 abundance with other gases could act as surface temperature and thus habitability. Moreover, a low extraterrestrial biosignatures, others could be “false total atmospheric pressure of all gases combined weakens the positives,” and yet others may indicate that a planet is cold-trap for water vapor at the tropopause (Wordsworth and uninhabited. Overall, our results suggest that an anaerobic Pierrehumbert, 2014). Where N2 is a major atmospheric biosphere can greatly facilitate the removal of large amounts constituent, a drop in pN2 can make the tropopause cold-trap of N2 from a planetary atmosphere. leaky (Zahnle and Buick, 2016), allowing water vapor into the upper layers of the atmosphere where it can either remain as 2. Model setup vapor, perhaps increasing overall greenhouse warming (Rind, 2.1. Biogeochemical nitrogen box model 1998; Solomon et al., 2010; Dessler et al., 2013) (but for a We used the Isee Stella software to construct a box contrary view see Huang et al., 2016), or freeze as high-altitude model of the global biogeochemical nitrogen cycle (fig. 1), ice clouds in polar regions, warming the high latitudes and tracking total nitrogen. Boxes included the atmosphere, making planetary climate more equable (Sloan and Pollard, pelagic marine sediments deposited on oceanic crust, 1998). Thus, planetary habitability is dependent, at least in continental marine sediments deposited on continental part, on atmospheric nitrogen levels. shelves and in epeiric seas, continental crust, and the mantle. Though we know next to nothing about the evolution Continental marine sediments were defined to become part of of the biogeochemical nitrogen cycles on other planets, we the continental crust after 100 million years and from there now have a better resolved picture of the behavior of nitrogen onwards were subjected to metamorphism, which returns through Earth’s history (Ader et al., 2016; Stüeken et al., 2016; nitrogen to the atmosphere. Similarly, we let pelagic marine Weiss et al., 2016). It seems that (1) microbial nitrogen fixation sediments accumulate for 100 million years before they were evolved very early in Earth’s history such that a nitrogen crisis subjected to subduction, metamorphism, and volcanism. for the primordial biosphere was averted (Stüeken et al., These timescales were based on the observation that C/N 2015a; Weiss et al., 2016), (2) the partial pressure of ratios in sedimentary rocks older than about 100 Myr become atmospheric N2 has fluctuated through time to a greater more variable with higher average values (Algeo et al., 2014). degree than previously anticipated (Som et al., 2016), (3) an The two sinks of nitrogen from the atmosphere were burial in aerobic nitrogen cycle arose before the Great Oxidation Event pelagic sediments and burial in continental sediments with at ~2.35 Ga (Garvin et al., 2009; Godfrey and falkowski, 2009), proportions of ~1:25 (Berner, 1982). The sources of (4) during the mid-Proterozoic aerobic and anaerobic nitrogen atmospheric nitrogen were volcanic and metamorphic cycling was spatially separated under low oxygen conditions degassing of subducted pelagic sediments, metamorphism of (Stüeken, 2013; Koehler et al., in press), and (5) a modern continental crust, oxidative weathering of continents, and nitrogen cycle with widespread aerobic activity did not arise mantle outgassing. Denitrification was not explicitly included until the late Neoproterozoic (Ader et al., 2014). The main as a source, because we did not track the marine nitrogen constraints on these developments were evidently biological reservoir. evolution and redox changes in Earth’s surface environments, The model was run in 1 million-year time-steps from principally the oxygenation state of the atmosphere and ocean 4.5 Ga to present. Differential equations listed in the appendix (Stüeken et al., 2016). Other Earth-like planets may have were solved with the Euler method. The chosen step size is evolved along somewhat similar pathways with respect to much lower than the residence time of nitrogen in our nitrogen cycling, provided that they also originated Earth-like modeled reservoirs (>100 Myr), which eradicates life. If so, then atmospheric swings in pN2 may be a common computational artifacts that can result in mass imbalances. feature of terrestrial inhabited planets. The initial abundances of nitrogen in the mantle, continental In the present study, we investigated the diversity of crust, and sediments were set such that the concentrations terrestrial planetary nitrogen cycles by modeling the evolution were the same, assuming that any disequilibrium in of Earth’s atmospheric N2 reservoir. We then perturbed the concentrations observed today is due to biogeochemical model to examine several ahistorical extreme scenarios that overprinting. The rock masses and modern nitrogen could arise on Earth-like exoplanets, defined here as planets inventories were taken from the work of Johnson & Goldblatt with a silicate rock mantle and iron core (empirically < 1.6 (2015).