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Ultraviolet light can split atmospheric water vapor, and hydrogen escaping to space can leave oxygen behind. But making oxygen is not the same as building an oxygen-rich atmosphere: early Earth’s volcanic gases, crust and oceans could consume it faster than it formed. This photosynthesis-free pathway helps explain possible traces of oxygen, not by itself the later, lasting rise in atmospheric oxygen.
How can oxygen form without photosynthesis?
In the proposed abiotic pathway, ultraviolet radiation breaks water molecules into fragments in the atmosphere. Some hydrogen can reach the upper atmosphere and escape into space. The oxygen-bearing material left behind can contribute to oxidation of the planet’s surface reservoirs.
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James F. Kasting’s 1979 photochemical model examined oxygen production from water photodissociation followed by hydrogen escape. It treated the result as a redox balance, not an automatic buildup: “Steady state solutions for the amount of O_{2} in the atmosphere are possible only when the combined loss rate from all three processes can balance the production of oxygen from photodissociation of H_{2}O, followed by escape of hydrogen to space.” (Kasting, 1979)
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Atmospheric oxygen persists only when its supply exceeds the processes that remove it. Kasting’s model compared photochemical production with losses from volcanic hydrogen and carbon monoxide outgassing, as well as oxidation of the crust. Ocean chemistry also forms part of the broader early-Earth oxygen budget.
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Under the assumptions of Kasting’s 1979 model, ground-level oxygen was about 10-12 of the present atmospheric level (PAL) or lower. The paper also modeled much higher oxygen at altitude; that upper-atmosphere profile should not be mistaken for the oxygen concentration near the ground. The estimate is conditional on the modeled atmosphere and its assumptions, not a direct measurement of ancient air.
What do geological clues say about early oxygen?
Ancient rocks preserve indirect evidence of redox conditions. Molybdenum distributions and isotope observations in Archean sedimentary rocks have been used to set model-based lower limits, but these are not direct measurements of the atmosphere.
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Reinhard and colleagues’ 2021 study considered two end-member interpretations of that evidence:
| Interpretation | Model-based constraint | What it means |
|---|---|---|
| Oxygen globally mixed in the atmosphere | PO2 above 10-6.9 PAL for substantial intervals | A lower limit for the global-mixing scenario, not an observed atmospheric concentration. |
| Oxygen spatially restricted near producers | Production above 0.01 Tmol O2 per year | A lower-limit production flux for the localized-oxygen scenario, not a global atmospheric abundance. |
The authors concluded that calculated Archean oxygen was very low but substantially above their predicted abiotic Earth-system values. Some proxy signatures indicate surface redox cycling, however, and do not necessarily show oxidation by O2 alone. The findings therefore support careful comparison of multiple proxy lines rather than treating any one signature as a direct oxygen reading. (Reinhard et al., 2021)
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Why did local oxygen and atmospheric oxygen rise at different times?
A localized oxygenated environment, oxygen in the hydrosphere and a lasting rise in atmospheric oxygen are distinct milestones. A 2026 review places free oxygen in the hydrosphere by about 3.0 billion years ago, while the initial lasting atmospheric rise—the Great Oxidation Event—occurred later, around 2.5–2.3 billion years ago. The review describes the delay as the outcome of interacting geodynamic, magmatic and biological controls on oxygen sources and sinks. (Vulpius et al., 2026)
One proposed explanation for strong early oxygen sinks concerns the composition of exposed continental crust. Smit and Mezger compiled chromium-to-uranium ratios in terrigenous sediments and inferred that early exposed crust was predominantly mafic before shifting toward a more modern andesitic composition over 500–700 million years. They propose that hydration of magnesium-rich mafic minerals formed serpentine and released oxygen-scavenging gases, including hydrogen, hydrogen sulfide and methane. They associate the decline of that mafic crust with the first oxygen accumulation in oceans and later in the atmosphere; this is a proposed interpretation, not settled consensus. (Smit and Mezger, 2017)
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How does the abiotic pathway relate to biological oxygen?
Water photolysis and hydrogen escape provide a possible abiotic source, while oxygenic photosynthesis is a biological source. Neither label alone tells how much oxygen accumulated: the key question is whether production outpaced sinks, and whether oxygen remained localized or mixed more widely.
A 2021 Nature Communications paper proposes a specific abiotic pathway that could produce hydrogen peroxide and oxygen in the Archean before oxygenic photosynthesis. That proposal broadens the possible chemistry, but it is not evidence that a photosynthesis-free process created a substantial, persistent atmospheric oxygen reservoir. (Nature Communications, 2021)
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What remains uncertain?
The photolysis-and-escape mechanism is a possible oxygen-producing process in atmospheric photochemical models. The amount that could accumulate depends on factors including atmospheric composition, water available in the upper atmosphere, hydrogen escape, volcanic outgassing and surface sinks. Geological proxies constrain early oxygen indirectly, and some signatures admit more than one interpretation. Quantifying how oxygen sources and sinks changed through time remains an important challenge, as the 2026 review notes.
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