Photochemical modelling of atmospheric oxygen levels confirms two stable states
Open accessAbstract
Various proxies and numerical models have been used to constrain O2 levels over geological time, but considerable uncertainty remains. Previous investigations using 1-D photochemical models have predicted how O3 concentrations vary with assumed ground-level O2 concentrations, and indicate how the ozone layer might have developed over Earth history. These classic models have utilised the numerical simplification of fixed mixing ratio boundary conditions. Critically, this modelling assumption requires verification that predicted fluxes of biogenic and volcanic gases are realistic, but also that the resulting steady states are in fact stable equilibrium solutions against trivial changes in flux. Here, we use a 1-D photochemical model with fixed flux boundary conditions to simulate the effects on O3 and O2 concentrations as O2 (and CH4) fluxes are systematically varied. Our results suggest that stable equilibrium solutions exist for trace- and high-O2/O3 cases, separated by a region of instability. In particular, the model produces few stable solutions with ground O2 mixing ratios between 6×10−7 and 2×10−3 (3×10−6 and 1% of present atmospheric levels). A fully UV-shielding ozone layer only exists in the high-O2 states. Our atmospheric modelling supports prior work suggesting a rapid bimodal transition between reducing and oxidising conditions and proposes Proterozoic oxygen levels higher than some recent proxies suggest. We show that the boundary conditions of photochemical models matter, and should be chosen and explained with care.
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Atmospheric O2 during the mid-Proterozoic was well below that of the modern Earth.
“Although the precise range of atmospheric pO 2 values during the mid-Proterozoic is not yet clear (Gregory et al. 2021, Liu et al. 2021, Planavsky et al. 2018a), it is widely accepted that atmospheric O 2 was well below that of the modern Earth.”
From The History of Ocean Oxygenation (2022) · cites this paper as Gregory et al. 2021
The precise range of atmospheric pO2 values during the mid-Proterozoic is not yet clear.
“Although the precise range of atmospheric pO 2 values during the mid-Proterozoic is not yet clear (Gregory et al. 2021, Liu et al. 2021, Planavsky et al. 2018a)”
From The History of Ocean Oxygenation (2022) · cites this paper as Gregory et al. 2021
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- Authors
- Gregory, B. S., Claire, M. W., & Rugheimer, S.
- Affiliations
- University of St Andrews (GB)
- University of Oxford (GB)
- Corresponding author
- Gregory, Bethan S.
- Venue
- Earth and Planetary Science Letters
- Publisher
- Elsevier BV
- Published
- 2021-05
- Volume
- 561
- Pages
- 116818
- ISSN
- 0012-821X (ISSN-L)
- OpenAlex
- W3129988418
- Cited by
- 50
- References
- 53
- Citations by year
- 2026: 1 · 2025: 9 · 2024: 8 · 2023: 11 · 2022: 14 · 2021: 7
- FWCI
- 7.51
- Type
- journal article
- Language
- en
- Open access
- hybrid
- doi.org
- Licence
- Locations
- Earth and Planetary Science Letters (publishedVersion, cc-by)
- St Andrews Research Repository (St Andrews Research Repository) (submittedVersion, cc-by)
- Funders
- Horizon 2020 (678812)
- European Research Council
- Topics
- Physical Sciences › Earth and Planetary Sciences › Paleontology › Paleontology and Stratigraphy of Fossils
- Physical Sciences › Earth and Planetary Sciences › Atmospheric Science › Earth Systems and Cosmic Evolution
- Physical Sciences › Earth and Planetary Sciences › Atmospheric Science › Atmospheric Ozone and Climate
- Keywords
- photochemical modeling, ozone, stable equilibrium, methane, boundary conditions, UV protection, volcanic gas flux
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