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

The Great Oxygenation Event (GOE, also referred to as the oxygen catastrophe or oxygen crisis) was a major environmental change around 2,400 million years ago associated with the appearance of free oxygen (O₂) in the atmosphere.

Prior to this time, oxygen produced as a waste product of photosynthesis quickly reacted with rocks, and could not accumulate in the atmosphere. At the GOE, readily-oxidised minerals were no longer available, and atmospheric oxygenation began to accumulate - although its levels continued to fluctuate to the present day.^[1]

[edit] Timing

The most widely accepted chronology of the Great Oxygenation Event suggests that oxygen began to be produced by photosynthesis by organisms (prokaryotic, then eukaryotic) that emitted oxygen as a waste product. These organisms lived long before the GOE,^[2] perhaps as early as 3,500 million years ago. The oxygen they produced would have almost instantly been removed from the atmosphere by the weathering of reduced minerals, most notably iron. This 'mass rusting' led to the deposition of banded iron formations. Oxygen only began to persist in the atmosphere in small quantities shortly (~50 million years) before the start of the GOE.^[3] Without a draw-down, oxygen can accumulate very rapidly: at today's rates of photosynthesis (which are admittedly much greater than those in the plant-free Precambrian), modern atmospheric O2 levels could be produced in around 2,000 years.^[4]

Another theory is that there is another interpretation of the supposed oxygen indicator, Mass-independent fractionation of sulfur isotopes, used in previous studies, and that oxygen producers did not evolve until right before the major rise in atmospheric oxygen concentration.^[5] This theory would eliminate the need to explain a lag in time between the evolution of oxyphotosynthetic microbes and the rise in oxygen.

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.

[edit] Time lag theory

The lag (which may have been as long as 900 million years) was between the time oxygen production from photosynthetic organisms started and the time of the oxygen catastrophe's geologically rapid increase in atmospheric oxygen (about 2.5 - 2.4 billion years ago). There are a number of hypotheses to explain this time lag:

[edit] Tectonic trigger

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.^[6] 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 is in these deposits. But these chemical phenomena do not seem to account for the lag completely.

[edit] Nickel famine

Photosynthetic organisms were a source of methane, which was also a big trap for molecular oxygen, because oxygen readily oxidizes methane to carbon dioxide (CO₂) and water in the presence of UV radiation. Modern methanogens require nickel as an enzyme cofactor. As the Earth's crust cooled, the supply of nickel from volcanoes was reduced and less methane was produced allowing oxygen to dominate the atmosphere. From 2.7 to 2.4 billion years ago, the levels of nickel deposited declined steadily; it was originally 400 times today's levels.^[7]

[edit] Bistability

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 support the 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.^[8]

[edit] Hydrogen leakage

This subsection may be confusing or unclear to readers. Please help clarify the article; suggestions may be found on the talk page. (April 2009)

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.

[edit] Late evolution of oxyphotosynthesis theory

There is a possibility that the oxygen indicator was misinterpreted. During the proposed time of the lag in the previous theory, there was change from mass-independently fractionated (MIF) sulfur to mass-dependently (MDF) fractionated sulfur in sediments. This was assumed to be a result of the appearance of oxygen in the atmosphere (since oxygen would have prevented the photolysis of sulfur dioxide, which causes MIF). This change from MIF to MDF of sulfur isotopes also may have been caused by an increase in glacial weathering, or the homogenization of the marine sulfur pool as a result of an increased thermal gradient during a glacial period.^[5]

[edit] See also

• Banded iron formation
• Iodide
• Medea hypothesis
• Pasteur point
• Huronian glaciation

[edit] References

1. ^ Frei, R.; Gaucher; Poulton; Canfield, D. (2009). "Fluctuations in Precambrian atmospheric oxygenation recorded by chromium isotopes". Nature 461 (7261): 250. doi:10.1038/nature08266. PMID 19741707. Lay summary.  edit
2. ^ Dutkiewicz, A.; Volk, H.; George, S. C.; Ridley, J.; Buick, R. (2006). "Biomarkers from Huronian oil-bearing fluid inclusions: an uncontaminated record of life before the Great Oxidation Event". Geology 34: 437. doi:10.1130/G22360.1.  edit
3. ^ Anbar, A.; Duan, Y.; Lyons, T.; Arnold, G.; Kendall, B.; Creaser, R.; Kaufman, A.; Gordon, G. et al. (2007). "A whiff of oxygen before the great oxidation event?". Science (New York, N.Y.) 317 (5846): 1903–1906. doi:10.1126/science.1140325. PMID 17901330.  edit
4. ^ Dole, M. (1965). "The Natural History of Oxygen". The Journal of General Physiology 49: 5. doi:10.1085/jgp.49.1.5.  edit
5. ^ ^{a b} Kopp, R.; Kirschvink, J.; Hilburn, I.; Nash, C. (2005). "The Paleoproterozoic snowball Earth: a climate disaster triggered by the evolution of oxygenic photosynthesis". Proceedings of the National Academy of Sciences of the United States of America 102 (32): 11131–11136. doi:10.1073/pnas.0504878102. PMID 16061801.  edit
6. ^ Lenton, T. M.; H. J. Schellnhuber, E. Szathmáry (2004). "Climbing the co-evolution ladder". Nature 431: 913. doi:10.1038/431913a. 
7. ^ Kurt O. Konhauser, et al.. "Oceanic nickel depletion and a methanogen famine before the Great Oxidation Event". Nature 458: 750-753. doi:10.1038/nature07858. 
8. ^ 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. http://www.cosis.net/abstracts/EGU06/00770/EGU06-J-00770.pdf.