Earth Without Life: A Systems Model of a Global Abiotic Nitrogen Cycle

Abstract

Nitrogen is the major component of Earth's atmosphere and plays important roles in biochemistry. Biological systems have evolved a variety of mechanisms for fixing and recycling environmental nitrogen sources, which links them tightly with terrestrial nitrogen reservoirs. However, prior to the emergence of biology, all nitrogen cycling was abiological, and this cycling may have set the stage for the origin of life. It is of interest to understand how nitrogen cycling would proceed on terrestrial planets with comparable geodynamic activity to Earth, but on which life does not arise. We constructed a kinetic mass-flux model of nitrogen cycling in its various major chemical forms (e.g., N₂, reduced (NH_x) and oxidized (NO_x) species) between major planetary reservoirs (the atmosphere, oceans, crust, and mantle) and included inputs from space. The total amount of nitrogen species that can be accommodated in each reservoir, and the ways in which fluxes and reservoir sizes may have changed over time in the absence of biology, are explored. Given a partition of volcanism between arc and hotspot types similar to the modern ones, our global nitrogen cycling model predicts a significant increase in oceanic nitrogen content over time, mostly as NH_x, while atmospheric N₂ content could be lower than today. The transport timescales between reservoirs are fast compared to the evolution of the environment; thus atmospheric composition is tightly linked to surface and interior processes. Key Words: Nitrogen cycle-Abiotic-Planetology-Astrobiology. Astrobiology 18, 897-914.

FIG. 1.

Model topology used in this study. Circles represent nitrogen reservoirs and correspond, clockwise, to atmosphere (atm), lower (lma) and upper (uma) mantle, marine sediments (sed), continental crust (ccr), and oceans (oce). Arrows represent fluxes between reservoirs, with arrow style indicating the nitrogen speciation of that flux.

FIG. 2.

Model time dependence of the fluxes. (a) Net nitrogen addition to the Earth system from extraterrestrial sources. (b) Atmospheric N₂ fixation by impacts. This flux is scaled to 1 PAL N₂. (c) Ocean volume fraction circulated through hydrothermal vents per 10 Ma. (d) Mantle fraction circulated per 10 Ma. This constrains the rate at which the upper and lower mantle equilibrate. The last two functions act as a prefactor to the overall mass flux calculation.

FIG. 3.

Nitrogen content as a function of time in the main reservoirs for a high abiotic atmospheric fixation rate. (a) Atmospheric N₂, (b) upper and lower mantle (NH_x), dissolved oceanic NO_x (c) and NH_x (d). Dashed lines represent present-day values. For oceanic NH_x this value is 3 × 10⁻⁴ mM and therefore does not appear on the figure. The randomly seeded range of initial conditions all collapse on the same evolutionary trend during the first ∼500 Ma (see also Fig. 5).

FIG. 4.

Nitrogen content as a function of time in the main reservoirs for a low abiotic fixation rate. (a) Atmospheric N₂, (b) upper and lower mantle (NH_x), dissolved oceanic NO_x (c) and NH_x (d). Dashed lines represent present-day values. For oceanic NH_x this value is 3 × 10⁻⁴ mM and therefore does not appear on the figure. The randomly seeded range of initial conditions take several billion years to collapse on the same evolutionary trend (see also Fig. 5).

FIG. 5.

Example of evolutionary curves in the atmospheric N₂ (x axis)/upper mantle NH_x (y axis) plane for the model presented in Fig. 3. Circles denote initial conditions and the star the common final state. Every simulation joins the common evolutionary track in less than 500 Ma.

FIG. 6.

Atmospheric nitrogen content after 4.5 Ga as a function of (a) mantle mixing rate and (b) ocean hydrothermal circulation rate for different values of α_m in the high abiotic atmospheric fixation rate case. The parameter α_m controls the partitioning between different volcanic degassing styles; see the text for more details.

FIG. 7.

(a) Atmospheric nitrogen content and (b) volcanic degassing rate as a function of mantle redox state α_m after 4.5 Ga for the high abiotic atmospheric fixation rate case. The redox state controls the relative contribution of arc-like (high values, degassed as N₂ to the atmosphere) to MORB-like (low values, degassed as NH_x, which rains out instantaneously to the oceans) volcanic degassing. The vertical dashed line represents the value estimated for present-day Earth.

FIG. 8.

Atmospheric nitrogen content after 4.5 Ga as a function of erosion rate for different values of (a) continental accretion efficiency ε and (b) oceanic crust subduction timescale (parameter D from Table 3).

FIG. B1.

Nitrogen content as a function of time in the main model reservoirs for different average oceanic crust subduction rates. (a) Atmospheric N₂, (b) lower mantle (NH_x), oceanic NO_x (c) and NH_x (d). Dashed lines represent present-day values. For oceanic NO_x and NH_x these values are 10⁻² and 3 × 10⁻⁴ mM, respectively, and therefore do not appear on the figures.

FIG. B2.

Nitrogen content as a function of time in the main model reservoirs for different ocean volume evolution scenarios (values are given in percent change since formation). (a) Atmospheric N₂, (b) lower mantle (NH_x), oceanic NO_x (c) and NH_x (d). Dashed lines represent present-day values. For oceanic NO_x and NH_x, these values are 10⁻² and 3 × 10⁻⁴ mM, respectively, and therefore do not appear on the figures.

FIG. B3.

Nitrogen content as a function of time in the main model reservoirs. (a) Atmospheric N₂, (b) lower mantle (NH_x), oceanic NO_x (c) and NH_x (d). Dashed lines represent present-day values. For upper mantle and oceanic NH_x, these values are 3.5 ppm and 3 × 10⁻⁴ mM, respectively, and therefore do not appear on the figures.

FIG. B4.

Nitrogen content as a function of time in the main model reservoirs. (a) Atmospheric N₂, (b) lower mantle (NH_x), oceanic NO_x (c) and NH_x (d). Dashed lines represent present-day values. For upper mantle and oceanic NH_x, these values are 3.5 ppm and 3 × 10⁻⁴ mM, respectively, and therefore do not appear on the figures.