The Geologic Nitrogen Cycle and its Relationship to Oxygenation of the Early Earth

Abstract:

There is no evidence that the partial pressure of atmospheric nitrogen (pN₂) changed greatly in the Phanerozoic but the Precambrian is different. Some suggest that Archean pN₂ was higher because it would pressure broaden absorption lines of greenhouse gases and counteract a fainter young Sun [1]. However, analysis of raindrop imprints and fluid inclusions indicate that pN₂ was no more than ~0.5-1.2 bar [2, 3] while basalt vesicles show pN₂ < 0.5 bar at 2.7 Ga [4]. Low pN₂ suggests that the Archean N cycle operated differently than today, which is unsurprising given the absence of O₂.

The coupling of the N cycle to oxygenation can be understood by comparing modern and ancient fluxes. Today, the long-term N source is from volcanism and metamorphism as well as oxidative weathering of organics. The geologic sink is the burial of organic matter, with minor subduction. But in the Archean, ammonium would have been the dominant N species in seawater. NH₄⁺ substitutes for K⁺ in seafloor phyllosilicates. NH₄⁺ in silicates can be stable under igneous and metamorphic conditions. Thus, the subduction sink should have been larger. Moreover, the N source from oxidative weathering was absent. With a more efficient geologic sink than today and smaller relative degassing, the steady-state pN₂ would be lower.

Nitrogen levels can be modeled and with plausible fluxes, Archean pN₂ is lower. Once O₂ becomes available, nitrifying chemoautotrophs make nitrate and the sink via ammonium declines. A speculative possibility is that oxidized sediments after the Great Oxidation raised the redox state in subduction zones. Higher oxygen fugacity would lead to more N₂ degassing [5]. In any case, pN₂ changes need not have been monotonic. [1] Goldblatt C. et al. (2009) Nat Geosci 2, 891-896. [2] Som S. M. et al. (2012) Nature 484, 359-362. [3] Marty B. et al. (2013) Science 342, 101-104. [4] Som S. M. et al. (2015), submitted. [5] Mikhail S., Sverjensky D. A. (2014) Nat Geosci 7, 816-819.