Biogeochemical modelling of the rise in atmospheric oxygen

Abstract

Understanding the evolution of atmospheric molecular oxygen levels is a fundamental unsolved problem in Earth's history. We develop a quantitative biogeochemical model that simulates the Palaeoproterozoic transition of the Earth's atmosphere from a weakly reducing state to an O2‐rich state. The purpose is to gain an insight into factors that plausibly control the timing and rapidity of the oxic transition. The model uses a simplified atmospheric chemistry (parameterized from complex photochemical models) and evolving redox fluxes in the Earth system. We consider time‐dependent fluxes that include organic carbon burial and associated oxygen production, reducing gases from metamorphic and volcanic sources, oxidative weathering, and the escape of hydrogen to space. We find that the oxic transition occurs in a geologically short time when the O2‐consuming flux of reducing gases falls below the flux of organic carbon burial that produces O2. A short timescale for the oxic transition is enhanced by a positive feedback due to decreasing destruction of O2 as stratospheric ozone forms, which is captured in our atmospheric chemistry parameterization. We show that one numerically self‐consistent solution for the rise of O2 involves a decline in flux of reducing gases driven by irreversible secular oxidation of the crust caused by time‐integrated hydrogen escape to space in the preoxic atmosphere, and that this is compatible with constraints from the geological record. In this model, the timing of the oxic transition is strongly affected by buffers of reduced materials, particularly iron, in the continental crust. An alternative version of the model, where greater fluxes of reduced hydrothermal cations from the Archean seafloor consume O2, produces a similar history of O2 and CH4. When climate and biosphere feedbacks are included in our model of the oxic transition, we find that multiple ‘Snowball Earth’ events are simulated under certain circumstances, as methane collapses and rises repeatedly before reaching a new steady‐state.

Benthic perspective on Earth’s oldest evidence for oxygenic photosynthesis

“…2B). Shown for comparison are modern global primary production (close to 10 3 times the carbon buried annually), of which ∼50% is terrestrial (55), estimated Archean global primary production (∼1/10 the modern value), the modern methane flux, and an upper estimate for the Archean methane flux (see Table S2 for values and data sources)-the latter two contributing to global redox by their positive effect on hydrogen escape (48)(49)(50)56). Using the median areal benthic net O 2 production rate from our compilation (Fig.…”

“…Crucially, profound rearrangement of O 2 sources and sinks, for example O 2 production upon the evolution of oxygenic photosynthesis accompanied by immediate oxidative continental weathering and rereduction of the reaction products (see below), can be achieved with a net zero change in gross redox balance. Thus, such changes can operate on temporal and quantitative scales that should be considered independent of A B Table S2 for data compilation and sources; see also Claire et al (48), Kasting (49), and Catling (50) for a detailed treatment. Only small degrees of benthic photosynthetic coverage are required to account for the earliest signals of oxidative weathering, highlighting a strong but previously unrecognized potential sensitivity in these signals to the evolution of terrestrial oxygenic photosynthesis.…”

Molecular biogeochemistry of modern and ancient marine microbes

“…The results of this model, shown in Figure 4, indicate that microaerobic ocean surface environments capable of supporting steroid biosynthesis are entirely consistent witheven required by -models of Archean atmospheric evolution. To provide the oxygen outgassing fluxes called for in the models of Pavlov and Kasting (2002) and Zahnle et al (2006), that the average supersaturation in ocean regions contributing to sea-to-air oxygen outgassing would be in the range of 0.1-1µM. This is 10-100 times our experimental determination of the concentration required for sterol biosynthesis.…”

“…The sink or loss term, on the other hand, is rather complicated: a wide variety of geochemical pathways exist for oxygen reduction, involving many atmospheric and mineral reaction partners. Fortunately, the loss of molecular oxygen from the Archean atmosphere has been the subject of several recent, detailed modeling studies (Pavlov and Kasting, 2002;Claire et al, 2006;Goldblatt et al, 2006;Zahnle et al, 2006), which provide valuable constraints on how the composition of the atmosphere might evolve given a biogenic O 2 flux.…”

Microaerobic steroid biosynthesis and the molecular fossil record of Archean life

“…This geologic record has given rise to divergent interpretations of the relative timing of the origin of oxygenic photosynthesis and the oxygenation of the atmosphere: Either (i) the GOE records both the evolutionary origin of oxygenic photosynthesis and resultant rapid atmospheric oxygenation, and apparent indicators of earlier O 2 production have been erroneously interpreted (15,16), or (ii) the origin of oxygenic photosynthesis predates the GOE by at least several hundred million years, with the time gap between the two events reflecting the variety of geochemical sinks and buffers that had to be overcome before O 2 could accumulate in the atmosphere (17,18). Models of Archean atmospheric evolution have shown how a biogenic O 2 flux to the atmosphere could have persisted for hundreds of millions of years without causing oxygenation to an extent that would "trip" the geologic and geochemical proxies whose signals appear at or near the GOE (19)(20)(21)(22). So long as the O 2 is accompanied by sufficient inputs of appropriate reductants (such as methane), atmospheric consumption of O 2 is rapid and there is no particular threshold value of the biogenic O 2 flux that forces the atmosphere to become oxygenated.…”