Abstract
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.
Highlights
Improved constraints of atmospheric oxygen levels over Earth history are important for an enriched understanding of how life and Earth have co-evolved
We explicitly demonstrate that some classic results are unstable equilibrium solutions (Section 4.3) and argue that many intermediate oxygen concentrations are photochemically unstable
Having determined from Cases 4-6 that H2 and CO flux boundary conditions produce rather different model atmospheres to mixing ratio lower boundary conditions (LBCs), but that the magnitude of the fluxes and the inclusion of negative feedback fluxes do not affect the primary result of pO2 bistability, we focus our discussion on Cases 1-3
Summary
Improved constraints of atmospheric oxygen levels over Earth history are important for an enriched understanding of how life and Earth have co-evolved. In a brief summary overview, these previous studies (reproduced below) found that the ozone column density (the number of ozone molecules in an atmospheric column with a surface area of one square centimetre) initially increases as a power-law with pO2, before saturating at higher O2 concentrations (Fig. 1a) The results of these 1-D photochemical modelling studies have been simplified, parametrised and incorporated into a number of Earth system evolution models, which include shorter-term biological and atmospheric feedbacks with longer-term planetary redox fluxes, such as volcanic degassing and hydrogen escape. These models have predicted bimodal behaviour with respect to oxygen concentrations.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
