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Geological history of oxygen

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O
2 build-up in Earth's atmosphere: 1) no O
2 produced; 2) O
2 produced, but absorbed in oceans & seabed rock; 3) O
2 starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer; 4–5) O
2 sinks filled and the gas accumulates

Free oxygen gas was almost nonexistent in Earth's atmosphere before photosynthetic archaea and bacteria evolved. Free oxygen first appeared in significant quantities during the Paleoproterozoic era (between 2.5 and 1.6 billion years ago). At first, the oxygen combined with dissolved iron in the oceans to form banded iron formations. Free oxygen started to gas out of the oceans 2.7 billion years ago, reaching 10% of its present level around 1.7 billion years ago.[1]: 522–23 

The presence of large amounts of dissolved and free oxygen in the oceans and atmosphere may have driven most of the anaerobic organisms then living to extinction during the oxygen catastrophe about 2.4 billion years ago. However, cellular respiration using O
2 enables aerobic organisms to produce much more ATP than anaerobic organisms, helping the former to dominate Earth's biosphere.[2]: 214, 586  Photosynthesis and cellular respiration of O
2 allowed for the evolution of eukaryotic cells and ultimately complex multicellular organisms such as plants and animals.

Since the beginning of the Cambrian era 540 million years ago, O
2 levels have fluctuated between 15% and 30% by volume.[3] Towards the end of the Carboniferous era (about 300 million years ago) atmospheric O
2 levels reached a maximum of 35% by volume,[3] which may have contributed to the large size of insects and amphibians at this time.[4] Human activities, including the burning of 7 billion tonnes of fossil fuels each year have had very little effect on the amount of free oxygen in the atmosphere.[5] At the current rate of photosynthesis it would take about 2,000 years to regenerate the entire O
2 in the present atmosphere.[6]


O2 build-up in earth's atmosphere: 1) no O2 produced, 2) O2 produced, but absorbed in oceans & seabed rock, 3) O2 starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer, 4-5) O2 sinks filled and the gas accumulates

Oxygen was almost nonexistent in Earth's atmosphere before the evolution of water oxidation in photosynthetic bacteria. Free oxygen first appeared in significant quantities during the Paleoproterozoic era (between 2.5 billion years ago and 1.6 billion years ago) as a product of the photosynthetic action of early anaerobes (archaea and bacteria). These organisms, fossil evidence for which occurs in the form of stromatolites and oncolites, developed the mechanism of oxygen evolution in the Archean era, between 3.5 and 2.7 billion years ago. At first, the produced oxygen dissolved in the oceans, where it was reduced by dissolved iron compounds, precipitating iron oxide (Fe2O3) and creating banded iron formations that are now a valuable resource of iron ore, hematite. Oxygen started to gas out of the oxygen-saturated waters from about 2.7 billion years ago, as is evident from the rusting of iron-rich terrestrial rocks starting around that time. The amount of oxygen in the atmosphere increased gradually at first and then more rapidly around 2.2 to 1.7 billion years ago to about 10% of its present level, as available reducing agents in the oceans and crustal rocks became oxidized.[1]

Fluctuations of oxygen levels in the atmosphere over the past 500+ million years, with accompanying events:
  1. Radiation of animal phyla (Cambrian explosion)
  2. First land plants
  3. Ordovician-Silurian extinction events
  4. Huge forests form on land, first land animals and seed plants
  5. Coal formation, first conifers, insect and amphibian giantism
  6. Low ocean levels, supercontinent Pangaea forms
  7. Permian-Triassic extinction event
  8. First primitive flowering plants and dinosaurs
  9. Triassic-Jurassic extinction event
  10. Age of dinosaurs
  11. Radiation of flowering plants
  12. Cretaceous-Tertiary extinction event
  13. Radiation of mammals

The development of an oxygen-rich atmosphere was one of the most important events in the history of life on Earth. The presence of large amounts of dissolved and free oxygen in the oceans and atmosphere may have driven most of the anaerobic organisms then living to extinction during the oxygen catastrophe about 2.4 billion years ago. However, the high electronegativity of O2 creates a large potential energy drop for cellular respiration, thus enabling organisms using aerobic respiration to produce much more ATP than anaerobic organisms. This makes them so efficient that they have come to dominate Earth's biosphere.[2] Photosynthesis and cellular respiration of oxygen allowed for the evolution of eukaryotic cells and ultimately complex multicellular organisms such as plants and animals.

The atmospheric abundance of free oxygen in later geological epochs and its gradual increase up to the present has been largely due to synthesis by photosynthetic organisms. Over the past 500 million years, oxygen levels fluctuated between 15% and 30% per volume.[3] Towards the end of the Carboniferous era (coal age) about 300 million years ago, atmospheric oxygen levels reached a maximum of 35% by volume.[3] Today, oxygen is the second-most-common component of the earth's atmosphere (about 21% by volume), the most-common being nitrogen. Human activities, including the burning of 7 billion tonnes of fossil fuels each year have had very little effect on the amount of free oxygen in the atmosphere.[5] It was estimated that, at the current rate of photosynthesis, it would take about 2,000 years to regenerate the entire oxygen in the present atmosphere.[7]

Effects on life

The concentration of atmospheric oxygen is often cited as a possible contributor to large-scale evolutionary phenomena, such as the origin of large, complex organisms, the Cambrian explosion, trends in animal body size, and other extinction and diversification events.[4]

The large size of insects and amphibians in the Carboniferous period, where oxygen reached 35% of the atmospheric concentration, has been attributed to the limiting role of diffusion in these organisms' metabolism. However, the biological basis for this correlation is not firm, and many lines of evidence show that oxygen concentration is not size-limiting in modern insects.[4] Interestingly, there is no significant correlation between atmospheric oxygen and maximum body size elsewhere in the geological record.[4] Ecological constraints can better explain the diminuitive size of post-Carboniferous dragonflies - for instance, the appearance of flying competitors such as birds and bats.[4]

Rising oxygen concentrations have been cited as a driver for evolutionary diversification, although the physiological arguments behind such arguments are questionable, and a consistent pattern between oxygen levels and the rate of evolution is not clearly evident.[4] The most celebrated link between oxygen and evolution occurs at the end of the last of the Snowball glaciations, where complex multicellular life is first found in the fossil record. Under low oxygen levels, regular 'nitrogen crises' could render the ocean inhospitable to life.[4] More fundamentally, an oxygen concentration of at least 40% of present atmospheric levels is necessary for metazoans to produce biochemicals, such as collagen, that are essential to their existance.[4] Models based on uniformitarian principles (i.e. extrapolating present-day ocean dynamics into deep time) suggest that such a level was only reached immediately prior to the first appearance of metazoa in the fossil record.[4] Further, anoxic or otherwise chemically 'nasty' oceanic conditions that resemble those supposed to inhibit macroscopic life re-occur at intervals through the early Cambrian, and also in the late Cretaceous – with no apparent impact on lifeforms at these times.[4] This might suggest that the geochemical signatures found in ocean sediments reflect the atmosphere in a different way before the Cambrian - perhaps as a result of the fundamentally different mode of nutrient cycling in the absence of planktivory.[8][4]

References

  1. ^ a b Campbell, Neil A. (2005). Biology (7th ed.). San Francisco: Pearson - Benjamin Cummings. ISBN 0-8053-7171-0. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help) Cite error: The named reference "Campbell" was defined multiple times with different content (see the help page).
  2. ^ a b Freeman, Scott (2005). Biological Science, 2nd. Upper Saddle River, NJ: Pearson – Prentice Hall. pp. 214, 586. ISBN 0-13-140941-7. Cite error: The named reference "Freeman" was defined multiple times with different content (see the help page).
  3. ^ a b c d Attention: This template ({{cite pmid}}) is deprecated. To cite the publication identified by PMID 10500106, please use {{cite journal}} with |pmid=10500106 instead.
  4. ^ a b c d e f g h i j k Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1111/j.1472-4669.2009.00188.x, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1111/j.1472-4669.2009.00188.x instead. Cite error: The named reference "Butterfield2009" was defined multiple times with different content (see the help page).
  5. ^ a b Emsley, John (2001). "Oxygen". Nature's Building Blocks: An A-Z Guide to the Elements. Oxford, England, UK: Oxford University Press. pp. 297–304. ISBN 0198503407.
  6. ^ Dole 1965, 5–27
  7. ^ Attention: This template ({{cite pmid}}) is deprecated. To cite the publication identified by PMID 5859927, please use {{cite journal}} with |pmid=5859927 instead.
  8. ^ Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1111/j.1475-4983.2006.00613.x, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1111/j.1475-4983.2006.00613.x instead.


Cook, Gerhard A. (1968). "Oxygen". In Clifford A. Hampel (ed.). The Encyclopedia of the Chemical Elements. New York: Reinhold Book Corporation. pp. 499–512. LCCN 68-29938. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help) Stwertka, Albert (1998). Guide to the Elements (Revised ed.). Oxford University Press. ISBN 0-19-508083-1.

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