The Great Oxygenation Event
Old sedimentary rocks record the history of oxygen in the form of redox-sensitive chemical species such as iron, uranium or cerium ions, and mass-independent fractionation of sulphur isotopes. These proxies show that oxygen became a stable component of the atmosphere around 2.4 GYA, a transition dubbed as the “Great Oxygenation Event” (GOE). Oxygenic photosynthesis is by far the main source of oxygen on Earth. Evidence for transient “oxygen oases” suggests that oxygenic photosynthesis appeared long before the GOE. Methane photolysis in the atmosphere was probably the main oxygen sink preventing stable oxygen accumulation before 2.4 GYA. Around this date, a change in planetary geochemistry permitted average oxygen concentration to rise above a threshold level of about 0.001%; the consequent formation of a thin ozone layer reduced methane photolysis and triggered the transition to an oxic atmosphere. The GOE was coeval with Huronian global glaciations, but the causal link between the two events is uncertain. Oxygen atmospheric concentration stabilized at a low level during most of the Proterozoic. A second rise in oxygen concentration, probably reflecting an increase in global productivity and organic carbon sequestration, started around 800 MYA. The oxygen level was at least 3% 570 MYA and probably exceeded 10% at the beginning of Phanerozoic, thus supporting the evolution of complex life. Water in the ocean depth remained largely anoxic until 600 MYA and accumulated sulphide from biogenic sulphate reduction (euxinic oceans). The GOE profoundly affected biochemistry by promoting the evolution of high energy-yielding aerobic respiration, aerobic lytotrophy and novel biosynthetic pathways involving P450 cytochromes.
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