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The Great Oxidation of the ancient atmosphere explained

Atmospheric oxygen rose to significant levels in the Earth's atmosphere 2.4 billion years ago in the so-called Great Oxidation, probably the biggest chemical transition in Earth history.

Yet oxygenic photosynthesis, the source of atmospheric oxygen, is thought to have evolved at least 300 million years earlier and the cause of this time lag has puzzled scientists

Researchers in the School of Environmental Sciences may have solved this conundrum with a new conceptual model of oxygen evolution, published in the scientific journal Nature. Colin Goldblatt, Tim Lenton and Andy Watson show that, once oxygenic photosynthesis had evolved, there were two stable states for atmospheric oxygen. The Great Oxidation marked the switch between these low- and high-oxygen states. 

The result arises from coupled modelling of the biosphere and atmospheric chemistry. On the early Earth, there was a source of both methane and oxygen from the biosphere. These were then destroyed in a reaction mediated by ultraviolet light. When there was low oxygen and no ozone layer, this reaction was fast. But when oxygen exceeded 0.001% of present levels the ozone layer started to form, which shielded the atmosphere from ultraviolet radiation. This slowed the reaction of methane and oxygen allowing atmospheric oxygen levels to rocket. 

This work may have implications for the search for life on other planets. It has been thought that a planet which is host to photosynthesis should have an oxygen rich atmosphere and an ozone layer, which would be detectable remotely. These new results indicate this is not the case and photosynthesis can be accompanied by low oxygen levels without the presence of an ozone layer.

Steady states of atmospheric oxygen: solid lines are stable states and dotted lines unstable states. The model is of the redox balance of the surface Earth system; oxygen levels (measured in fraction of present atmospheric level, PAL) depend on the balance of input of oxidant from photosynthesis and reduced material from volcanic outgassing. (a) With respect to reductant input at present marine net primary productivity (NPP) and (b) with respect to net primary productivity at present reductant input.

Note how stable states correspond to oxygen constraints before and after the Great Oxidation.

The full manuscript for this work can be found in the journal Nature at: 

C. Goldblatt, T.M. Lenton, A.J. Watson, “Bistability of atmospheric oxygen and the Great Oxidation”, Nature, Volume 443, Pages 683-686, (2006).