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The Oxygen Catastrophe was a massive environmental change believed to have happened during the Siderian period at the beginning of the Paleoproterozoic era, about 2.4 billion years ago. It was described as a catastrophe because there was an increase in oxygen which was toxic to the organisms living at the time (see below). It is also called the Oxygen Crisis or The Great Oxidation.

When evolving life forms developed oxyphotosynthesis about 2.7 billion years ago, molecular oxygen was produced in large quantities. This means there was a time lag of about 300 million years between the time oxygen production from photosynthetic organisms started, and the Oxygen Catastrophe.

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

Photosynthetic organisms were also a source of methane. This was also a big trap for molecular oxygen, because methane oxidizes readily in the presence of UV radiation. A new idea to explain the 300 million year lag comes from a mathematical model of the atmosphere. The result is a system with two steady states with lower (0.02%) and higher (21% or more) content of oxygen. The Great Oxidation can be understood as a switch between lower and upper stable steady states. This bistability was caused by UV shielding decreasing the rate of methane oxidation once oxygen levels were sufficient to form an ozone layer.^[2]

Another part of the effect may have been photosynthetic production of molecular hydrogen which as it formed got into the atmosphere and was slowly lost to space.

The plentiful oxygen eventually caused an ecological crisis. From the point of the view of the anaerobic organisms already present on earth and in the oceans, the environment was poisoned with oxygen, and the oceans were scrubbed free of important nutrients. It is from their point of view, and because of the drastic change that the oxygen caused to the air and oceans, that the Oxygen Catastrophe gets its name.

However, it also provided a new opportunity. Despite recycling, 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.

• Banded iron formation

1. ^ Lenton, T. M.; H. J. Schellnhuber, E. Szathmáry (2004). "Climbing the co-evolution ladder". Nature 431: 913. 
2. ^ Goldblatt, C.; T.M. Lenton, A.J. Watson (2006). "The Great Oxidation at 2.4 Ga as a bistability in atmospheric oxygen due to UV shielding by ozone". Geophysical Research Abstracts 8: 00770.