Oxygen Catastrophe

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O₂ build-up in earth's atmosphere: 1) (3.85–2.45Gyr ago (Ga)) no O₂ produced, 2) (2.45–1.85Ga) O₂ produced, but absorbed in oceans & seabed rock, 3) (1.85–0.85Ga) O₂ starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer, 4) (0.85–0.54Ga) and 5) (0.54Ga–present) O₂ sinks filled and the gas accumulates

The Oxygen Catastrophe was a massive environmental change believed to have happened during the Siderian period at the beginning of the Paleoproterozoic era of the Precambrian, about 2.4 billion years ago. It is also called the Oxygen Crisis, Oxygen Revolution, or The Great Oxidation.

When evolving lifeforms developed oxyphotosynthesis about 2.7 billion years ago, molecular oxygen was produced in large quantities. This plentiful oxygen eventually caused an ecological crisis to the biodiversity of the time, as oxygen was toxic to the microscopic anaerobic organisms dominant then.

However, this transforming change also provided a new opportunity for biological diversification, as well as tremendous changes in the nature of chemical interactions between rocks, sand, clay, and other geological substrates and the earth's air, oceans, and other surface waters. Despite natural recycling of organic matter, life had remained energetically limited until the widespread availability of oxygen. This breakthrough in metabolic evolution greatly increased the free energy supply to living organisms, having a truly global environmental impact.

Time lag

There was a lag of about 300 million years between the time oxygen production from photosynthetic organisms started, and the time of the Oxygen Catastrophe's geologically rapid increase in atmospheric oxygen.

One phenomenon that explains this lag is that the oxygen increase had to await tectonically driven changes in the earth's 'anatomy,' including the appearance of shelf seas where reduced organic carbon could reach the sediments and be buried.^[1] Also, the newly produced oxygen was first consumed in various chemical reactions in the oceans, primarily with iron. Evidence for this phenomenon is found in older rocks that contain massive banded iron formations that were apparently laid down as this iron and oxygen first combined; most of the planet's commercial iron ore deposits are in these deposits. But these chemical phenomena do not seem to account for the lag completely.

Photosynthetic organisms were also a source of methane, which was also a big trap for molecular oxygen, because methane oxidizes readily to carbon dioxide (CO₂) in the presence of UV radiation.

A 2006 (bistability) theory to explain the 300-million-year lag comes from a mathematical model of the atmosphere which recognizes that UV shielding decreases the rate of methane oxidation once oxygen levels are sufficient to formation of an ozone layer. This explanation proposes a system with two steady states, one with lower (0.02%) atmospheric oxygen content, and the other with higher (21% or more) oxygen content. The Great Oxidation can then be understood as a switch between lower and upper stable steady states.^[1]

Another factor in the delay in atmospheric oxygen enrichment may have been photosynthetic production of molecular hydrogen which, as it formed, got into the atmosphere and was slowly lost to space.

See also

• Banded iron formation

References

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