oxygenation of the Earth
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The Great Oxidation Event (GOE), also called the Great Oxygenation Event, the Oxygen Catastrophe, the Oxygen Revolution, the Oxygen Crisis, or the Oxygen Holocaust, was a time interval during the
Paleoproterozoic The Paleoproterozoic Era (;, also spelled Palaeoproterozoic), spanning the time period from (2.5–1.6  Ga), is the first of the three sub-divisions ( eras) of the Proterozoic Eon. The Paleoproterozoic is also the longest era of the Earth's ...
era when the
Earth's atmosphere The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing fo ...
and the shallow ocean first experienced a rise in the amount of oxygen. This began approximately 2.460–2.426 Ga (billion years) ago, during the Siderian period, and ended approximately 2.060 Ga, during the
Rhyacian The Rhyacian Period (; grc, ῥύαξ, translit=rhýax, meaning "stream of lava") is the second geologic period in the Paleoproterozoic Era and lasted from Mya to Mya (million years ago). Instead of being based on stratigraphy, these dates are ...
. Geological, isotopic, and chemical evidence suggests that
biologically Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary i ...
-produced molecular oxygen (
dioxygen There are several known allotropes of oxygen. The most familiar is molecular oxygen (O2), present at significant levels in Earth's atmosphere and also known as dioxygen or triplet oxygen. Another is the highly reactive ozone (O3). Others are: * ...
, O2) started to accumulate in Earth's atmosphere and changed it from a weakly
reducing atmosphere A reducing atmosphere is an Atmosphere, atmospheric condition in which oxidation is prevented by removal of oxygen and other oxidizing gases or vapours, and which may contain actively Reducing agent, reducing gases such as hydrogen, carbon monoxi ...
practically free of oxygen into an oxidizing atmosphere containing abundant oxygen, with oxygen levels being as high as 10% of their present atmospheric level by the end of the GOE. The sudden injection of toxic oxygen into an anaerobic biosphere may have caused the extinction of many existing anaerobic species on Earth. Although the event is inferred to have constituted a
mass extinction An extinction event (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the biodiversity on Earth. Such an event is identified by a sharp change in the diversity and abundance of multicellular organisms. I ...
, due in part to the great difficulty in surveying microscopic species' abundances, and in part to the extreme age of fossil remains from that time, the Great Oxidation Event is typically not counted among conventional lists of "great extinctions", which are implicitly limited to the
Phanerozoic eon The Phanerozoic Eon is the current geologic eon in the geologic time scale, and the one during which abundant animal and plant life has existed. It covers 538.8 million years to the present, and it began with the Cambrian Period, when anima ...
. In any case, isotope geochemical data from
sulfate minerals The sulfate minerals are a class of minerals that include the sulfate ion () within their structure. The sulfate minerals occur commonly in primary evaporite depositional environments, as gangue minerals in hydrothermal veins and as secondary mine ...
have been interpreted to indicate a decrease in the size of the
biosphere The biosphere (from Greek βίος ''bíos'' "life" and σφαῖρα ''sphaira'' "sphere"), also known as the ecosphere (from Greek οἶκος ''oîkos'' "environment" and σφαῖρα), is the worldwide sum of all ecosystems. It can also ...
of >80% associated with changes in nutrient supplies at the end of the GOE. The GOE is inferred to have been caused by cyanobacteria producing the oxygen, which may have enabled the subsequent development of
multicellular A multicellular organism is an organism that consists of more than one cell, in contrast to unicellular organism. All species of animals, land plants and most fungi are multicellular, as are many algae, whereas a few organisms are partially un ...
life-forms.


The early atmosphere

The composition of the Earth's earliest atmosphere is not known with certainty. However, the bulk was likely
nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
, , and
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
, , which are also the predominant nitrogen- and carbon-bearing gases produced by
volcanism Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics, and volcanic gases erupt through a break in the surface called a ...
today. These are relatively inert gases. Oxygen, , meanwhile, was present in the atmosphere at just 0.001% of its present atmospheric level. The Sun shone at about 70% of its current brightness 4 billion years ago, but there is strong evidence that liquid water existed on Earth at the time. A warm Earth, in spite of a faint Sun, is known as the
faint young Sun paradox The faint young Sun paradox or faint young Sun problem describes the apparent contradiction between observations of liquid water early in Earth's history and the astrophysical expectation that the Sun's output would be only 70 percent as intense ...
. Either carbon dioxide levels were much higher at the time, providing enough of a
greenhouse effect The greenhouse effect is a process that occurs when energy from a planet's host star goes through the planet's atmosphere and heats the planet's surface, but greenhouse gases in the atmosphere prevent some of the heat from returning directly ...
to warm the Earth, or other greenhouse gases were present. The most likely such gas is
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Ea ...
, , which is a powerful greenhouse gas and was produced by early forms of life known as
methanogens Methanogens are microorganisms that produce methane as a metabolic byproduct in hypoxic conditions. They are prokaryotic and belong to the domain Archaea. All known methanogens are members of the archaeal phylum Euryarchaeota. Methanogens are co ...
. Scientists continue to research how the Earth was warmed before life arose. An atmosphere of and with trace amounts of , ,
carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...
(), and
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
(), is described as a weakly
reducing atmosphere A reducing atmosphere is an Atmosphere, atmospheric condition in which oxidation is prevented by removal of oxygen and other oxidizing gases or vapours, and which may contain actively Reducing agent, reducing gases such as hydrogen, carbon monoxi ...
. Such an atmosphere contains practically no oxygen. The modern atmosphere contains abundant oxygen, making it an oxidizing atmosphere. The rise in oxygen is attributed to
photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...
by cyanobacteria, which are thought to have evolved as early as 3.5 billion years ago. The current scientific understanding of when and how the Earth's atmosphere changed from a weakly reducing to a strongly oxidizing atmosphere largely began with the work of the American geologist
Preston Cloud Preston Ercelle Cloud, Jr. (September 26, 1912 – January 16, 1991) was an American earth scientist, biogeologist, cosmologist, and paleontologist. He served in the United States Navy (in which he was a bantamweight boxing champion), and led s ...
in the 1970s. Cloud observed that detrital sediments older than about 2 billion years ago contained grains of
pyrite The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic luster and pale brass-yellow hue giv ...
,
uraninite Uraninite, formerly pitchblende, is a radioactive, uranium-rich mineral and ore with a chemical composition that is largely UO2 but because of oxidation typically contains variable proportions of U3O8. Radioactive decay of the uranium causes ...
, and
siderite Siderite is a mineral composed of iron(II) carbonate (FeCO3). It takes its name from the Greek word σίδηρος ''sideros,'' "iron". It is a valuable iron mineral, since it is 48% iron and contains no sulfur or phosphorus. Zinc, magnesium and ...
, all minerals containing reduced forms of iron or uranium that are not found in younger sediments because they are rapidly oxidized in an oxidizing atmosphere. He further observed that continental red beds, which get their color from the oxidized (
ferric In chemistry, iron(III) refers to the element iron in its +3 oxidation state. In ionic compounds (salts), such an atom may occur as a separate cation (positive ion) denoted by Fe3+. The adjective ferric or the prefix ferri- is often used to sp ...
) mineral hematite, began to appear in the geological record at about this time.
Banded iron formation Banded iron formations (also known as banded ironstone formations or BIFs) are distinctive units of sedimentary rock consisting of alternating layers of iron oxides and iron-poor chert. They can be up to several hundred meters in thickness ...
largely disappears from the geological record at 1.85 billion years ago, after peaking at about 2.5 billion years ago. Banded iron formation can form only when abundant dissolved
ferrous In chemistry, the adjective Ferrous indicates a compound that contains iron(II), meaning iron in its +2 oxidation state, possibly as the divalent cation Fe2+. It is opposed to " ferric" or iron(III), meaning iron in its +3 oxidation state, suc ...
iron is transported into depositional basins, and an oxygenated ocean blocks such transport by oxidizing the iron to form insoluble ferric iron compounds. The end of the deposition of banded iron formation at 1.85 billion years ago is therefore interpreted as marking the oxygenation of the deep ocean. Heinrich Holland further elaborated these ideas through the 1980s, placing the main time interval of oxygenation between 2.2 and 1.9 billion years ago, and they continue to shape the current scientific understanding. Constraining the onset of atmospheric oxygenation has proven particularly challenging for geologists and geochemists. While there is a widespread consensus that initial oxygenation of the atmosphere happened sometime during the first half of the
Paleoproterozoic The Paleoproterozoic Era (;, also spelled Palaeoproterozoic), spanning the time period from (2.5–1.6  Ga), is the first of the three sub-divisions ( eras) of the Proterozoic Eon. The Paleoproterozoic is also the longest era of the Earth's ...
, there is disagreement on the exact timing of this event. Scientific publications between 2016 to 2022 have differed in the inferred timing of the onset of atmospheric oxygenation by approximately 500 million years, with estimates ranging from as early as 2.7 Ga to as late as 2.225 Ga. This is in large part due to an incomplete sedimentary record for the Paleoproterozoic (e.g., because of subduction and
metamorphism Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of ch ...
), uncertainties in depositional ages for many ancient sedimentary units, and uncertainties related to the interpretation of different geological/geochemical proxies. While the effects of an incomplete geological record have been discussed and quantified in the field of
paleontology Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of fossi ...
for several decades, particularly with respect to the evolution and extinction of organisms ( the Signor-Lipps Effect), this is rarely quantified when considering geochemical records, and may therefore lead to uncertainties for scientists studying the timing of atmospheric oxygenation.


Geological evidence

Evidence for the Great Oxidation Event is provided by a variety of petrological and geochemical markers that define this
geological event A geological event is a temporally and spatially heterogeneous and dynamic (diachronous) happening in Earth history that contributes to the transformation of Earth system and the formation of geological strata. Event stratigraphy was first propo ...
.


Continental indicators

Paleosol In the geosciences, paleosol (''palaeosol'' in Great Britain and Australia) is an ancient soil that formed in the past. The precise definition of the term in geology and paleontology is slightly different from its use in soil science. In geolo ...
s,
detrital Detritus (; adj. ''detrital'' ) is particles of rock derived from pre-existing rock through weathering and erosion.Essentials of Geology, 3rd Ed, Stephen Marshak, p G-7 A fragment of detritus is called a clast.Essentials of Geology, 3rd Ed, Stephe ...
grains, and red beds are evidence of low-level oxygen. Paleosols (fossil soils) older than 2.4 billion years old have low iron concentrations that suggest anoxic weathering.
Detrital Detritus (; adj. ''detrital'' ) is particles of rock derived from pre-existing rock through weathering and erosion.Essentials of Geology, 3rd Ed, Stephen Marshak, p G-7 A fragment of detritus is called a clast.Essentials of Geology, 3rd Ed, Stephe ...
grains composed of
pyrite The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic luster and pale brass-yellow hue giv ...
,
siderite Siderite is a mineral composed of iron(II) carbonate (FeCO3). It takes its name from the Greek word σίδηρος ''sideros,'' "iron". It is a valuable iron mineral, since it is 48% iron and contains no sulfur or phosphorus. Zinc, magnesium and ...
, and
uraninite Uraninite, formerly pitchblende, is a radioactive, uranium-rich mineral and ore with a chemical composition that is largely UO2 but because of oxidation typically contains variable proportions of U3O8. Radioactive decay of the uranium causes ...
(redox-sensitive detrital minerals) are found in sediments older than ca. 2.4 Ga. These minerals are only stable under low oxygen conditions, and so their occurrence as detrital minerals in fluvial and
deltaic A river delta is a landform shaped like a triangle, created by deposition of sediment that is carried by a river and enters slower-moving or stagnant water. This occurs where a river enters an ocean, sea, estuary, lake, reservoir, or (more rarel ...
sediments are widely interpreted as evidence of an anoxic atmosphere. In contrast to redox-sensitive detrital minerals are red beds, red-colored
sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates ...
s that are coated with hematite. The occurrence of red beds indicates that there was sufficient oxygen to oxidize iron to its ferric state, and represent a marked contrast to sandstones deposited under anoxic conditions which are often beige, white, grey, or green.


Banded iron formation (BIF)

Banded iron formations are composed of thin alternating layers of
chert Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a ...
(a fine-grained form of
silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...
) and iron oxides,
magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With th ...
and hematite. Extensive deposits of this rock type are found around the world, almost all of which are more than 1.85 billion years old and most of which were deposited around 2.5 billion years ago. The iron in banded iron formation is partially oxidized, with roughly equal amounts of ferrous and ferric iron. Deposition of banded iron formation requires both an anoxic deep ocean capable of transporting iron in soluble ferrous form, and an oxidized shallow ocean where the ferrous iron is oxidized to insoluble ferric iron and precipitates onto the ocean floor. The deposition of banded iron formation before 1.8 billion years ago suggests the ocean was in a persistent ferruginous state, but deposition was episodic and there may have been significant intervals of
euxinia Euxinia or euxinic conditions occur when water is both anoxic and sulfidic. This means that there is no oxygen (O2) and a raised level of free hydrogen sulfide (H2S). Euxinic bodies of water are frequently strongly stratified, have an oxic, highly ...
.


Iron speciation

Black
laminated Lamination is the technique/process of manufacturing a material in multiple layers, so that the composite material achieves improved strength, stability, sound insulation, appearance, or other properties from the use of the differing materia ...
shales, rich in organic matter, are often regarded as a marker for
anoxic The term anoxia means a total depletion in the level of oxygen, an extreme form of hypoxia or "low oxygen". The terms anoxia and hypoxia are used in various contexts: * Anoxic waters, sea water, fresh water or groundwater that are depleted of diss ...
conditions. However, the deposition of abundant organic matter is not a sure indication of anoxia, and burrowing organisms that destroy lamination had not yet evolved during the time frame of the Great Oxygenation Event. Thus laminated black shale by itself is a poor indicator of oxygen levels. Scientists must look instead for geochemical evidence of anoxic conditions. These include
ferruginous The adjective ferruginous may mean: * Containing iron, applied to water, oil, and other non-metals * Having rust on the surface * With the rust (color) See also * Ferrous, containing iron (for metals and alloys) or iron(II) cations * Ferric, cont ...
anoxia, in which dissolved ferrous iron is abundant, and euxinia, in which hydrogen sulfide is present in the water. Examples of such indicators of anoxic conditions include the degree of pyritization (DOP), which is the ratio of iron present as pyrite to the total reactive iron. Reactive iron, in turn, is defined as iron found in oxides and oxyhydroxides, carbonates, and reduced sulfur minerals such as pyrites, in contrast with iron tightly bound in silicate minerals. A DOP near zero indicates oxidizing conditions, while a DOP near 1 indicates euxinic conditions. Values of 0.3 to 0.5 are transitional, suggesting anoxic bottom mud under an oxygenated ocean. Studies of the
Black Sea The Black Sea is a marginal mediterranean sea of the Atlantic Ocean lying between Europe and Asia, east of the Balkans, south of the East European Plain, west of the Caucasus, and north of Anatolia. It is bounded by Bulgaria, Georgia, Rom ...
, which is considered a modern model for ancient anoxic ocean basins, indicate that high DOP, a high ratio of reactive iron to total iron, and a high ratio of total iron to aluminum are all indicators of transport of iron into a euxinic environment. Ferruginous anoxic conditions can be distinguished from euxenic conditions by a DOP less than about 0.7. The currently available evidence suggests that the deep ocean remained anoxic and ferruginous as late as 580 million years ago, well after the Great Oxygenation Event, remaining just short of euxenic during much of this interval of time. Deposition of banded iron formation ceased when conditions of local euxenia on continental platforms and shelves began precipitating iron out of upwelling ferruginous water as pyrite.


Isotopes

Some of the most persuasive evidence for the Great Oxidation Event is provided by the
mass-independent fractionation Mass-independent isotope fractionation or Non-mass-dependent fractionation (NMD), refers to any chemical or physical process that acts to separate isotopes, where the amount of separation does not scale in proportion with the difference in the mas ...
(MIF) of sulfur. The chemical signature of the MIF of sulfur is found prior to 2.4–2.3 billion years ago but disappears thereafter. The presence of this signature all but eliminates the possibility of an oxygenated atmosphere. Different isotopes of a chemical element have slightly different atomic masses. Most of the differences in geochemistry between isotopes of the same element scale with this mass difference. These include small differences in molecular velocities and diffusion rates, which are described as mass-dependent fractionation processes. By contrast, mass-independent fractionation describes processes that are not proportional to the difference in mass between isotopes. The only such process likely to be significant in the geochemistry of sulfur is photodissociation. This is the process in which a molecule containing sulfur is broken up by solar ultraviolet (UV) radiation. The presence of a clear MIF signature for sulfur prior to 2.4 billion years ago shows that UV radiation was penetrating deep into the Earth's atmosphere. This in turn rules out an atmosphere containing more than traces of oxygen, which would have produced an ozone layer that shielded the lower atmosphere from UV radiation. The disappearance of the MIF signature for sulfur indicates the formation of such an ozone shield as oxygen began to accumulate in the atmosphere. Mass-dependent fractionation of sulphur also indicates the presence of oxygen in that oxygen is required to facilitate repeated redox cycling of sulphur. Mass-dependent fractionation also provides clues to the Great Oxygenation Event. For example, oxidation of manganese in surface rocks by atmospheric oxygen leads to further reactions that oxidize chromium. The heavier 53Cr is oxidized preferentially over the lighter 52Cr, and the soluble oxidized chromium carried into the ocean shows this enhancement of the heavier isotope. The chromium isotope ratio in banded iron formation suggests small but significant quantities of oxygen in the atmosphere before the Great Oxidation Event, and a brief return to low oxygen abundance 500 million years after the Great Oxidation Event. However, the chromium data may conflict with the sulfur isotope data, which calls the reliability of the chromium data into question. It is also possible that oxygen was present earlier only in localized "oxygen oases". Since chromium is not easily dissolved, its release from rocks requires the presence of a powerful acid such as sulfuric acid (H2SO4) which may have formed through bacterial oxidation of pyrite. This could provide some of the earliest evidence of oxygen-breathing life on land surfaces. Other elements whose mass-dependent fractionation may provide clues to the Great Oxidation Event include carbon, nitrogen, transitional metals such as molybdenum and iron, and non-metal elements such as
selenium Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium, ...
.


Fossils and biomarkers (chemical fossils)

While the Great Oxidation Event is generally thought to be a result of oxygenic photosynthesis by ancestral cyanobacteria, the presence of cyanobacteria in the Archaean before the Great Oxidation Event is a highly controversial topic. Structures that are claimed to be fossils of cyanobacteria exist in rock as old as 3.5 billion years old These include microfossils of supposedly cyanobacterial cells and macrofossils called
stromatolite Stromatolites () or stromatoliths () are layered sedimentary formations ( microbialite) that are created mainly by photosynthetic microorganisms such as cyanobacteria, sulfate-reducing bacteria, and Pseudomonadota (formerly proteobacteria). T ...
s, which are interpreted as colonies of microbes, including cyanobacteria, with characteristic layered structures. Modern stromatolites, which can only be seen in harsh environments such as Shark Bay in Western Australia, are associated with cyanobacteria and thus fossil stromatolites had long been interpreted as the evidence for cyanobacteria. However, it has increasingly been inferred that at least some of these Archaean fossils were generated abiotically or produced by non-cyanobacterial phototrophic bacteria. Additionally, Archaean sedimentary rocks were once found to contain
biomarker In biomedical contexts, a biomarker, or biological marker, is a measurable indicator of some biological state or condition. Biomarkers are often measured and evaluated using blood, urine, or soft tissues to examine normal biological processes, p ...
s, also known as chemical fossils, interpreted as fossilized membrane lipids from cyanobacteria and eukaryotes. For example, traces of 2α-methylhopanes and steranes that are thought to be derived from cyanobacteria and eukaryotes, respectively, were found in the
Pilbara The Pilbara () is a large, dry, thinly populated region in the north of Western Australia. It is known for its Aboriginal peoples; its ancient landscapes; the red earth; and its vast mineral deposits, in particular iron ore. It is also a g ...
of Western Australia. Steranes are diagenetic products of sterols, which are biosynthesized utilizing molecular oxygen. Thus, steranes can additionally serve as an indicator of oxygen in the atmosphere. However, these biomarker samples have since been shown to have been contaminated and so the results are no longer accepted.


Other indicators

Some elements in marine sediments are sensitive to different levels of oxygen in the environment such as the
transition metal In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that ca ...
s molybdenum and
rhenium Rhenium is a chemical element with the symbol Re and atomic number 75. It is a silvery-gray, heavy, third-row transition metal in group 7 of the periodic table. With an estimated average concentration of 1 part per billion (ppb), rhenium is one ...
. Non-metal elements such as
selenium Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium, ...
and iodine are also indicators of oxygen levels.


Hypotheses

The ability to generate oxygen via photosynthesis likely first appeared in the ancestors of cyanobacteria. These organisms evolved at least 2.45–2.32 billion years ago, and probably as early as 2.7 billion years ago or earlier. However, oxygen remained scarce in the atmosphere until around 2.0 billion years ago, and banded iron formation continued to be deposited until around 1.85 billion years ago. Given the rapid multiplication rate of cyanobacteria under ideal conditions, an explanation is needed for the delay of at least 400 million years between the evolution of oxygen-producing photosynthesis and the appearance of significant oxygen in the atmosphere. Hypotheses to explain this gap must take into consideration the balance between oxygen sources and oxygen sinks. Oxygenic photosynthesis produces organic carbon that must be segregated from oxygen to allow oxygen accumulation in the surface environment, otherwise the oxygen back-reacts with the organic carbon and does not accumulate. The burial of organic carbon, sulfide, and minerals containing
ferrous In chemistry, the adjective Ferrous indicates a compound that contains iron(II), meaning iron in its +2 oxidation state, possibly as the divalent cation Fe2+. It is opposed to " ferric" or iron(III), meaning iron in its +3 oxidation state, suc ...
iron (Fe) is a primary factor in oxygen accumulation. When organic carbon is buried without being oxidized, the oxygen is left in the atmosphere. In total, the burial of organic carbon and pyrite today creates of O per year. This creates a net O flux from the global oxygen sources. The rate of change of oxygen can be calculated from the difference between global sources and sinks. The oxygen sinks include reduced gases and minerals from volcanoes, metamorphism and weathering. The GOE started after these oxygen-sink fluxes and reduced-gas fluxes were exceeded by the flux of O2 associated with the burial of reductants, such as organic carbon. For the weathering mechanisms, of O per year today goes to the sinks composed of reduced minerals and gases from volcanoes, metamorphism, percolating seawater and heat vents from the seafloor. On the other hand, of O per year today oxidizes reduced gases in the atmosphere through photochemical reaction. On the early Earth, there was visibly very little oxidative weathering of continents (e.g., a lack of red beds) and so the weathering sink on oxygen would have been negligible compared to that from reduced gases and dissolved iron in oceans. Dissolved iron in oceans exemplifies O2 sinks. Free oxygen produced during this time was chemically captured by dissolved iron, converting iron Fe and Fe2+ to
magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With th ...
() that is insoluble in water, and sank to the bottom of the shallow seas to create
banded iron formation Banded iron formations (also known as banded ironstone formations or BIFs) are distinctive units of sedimentary rock consisting of alternating layers of iron oxides and iron-poor chert. They can be up to several hundred meters in thickness ...
s such as the ones found in Minnesota and the Pilbara of Western Australia. It took 50 million years or longer to deplete the oxygen sinks. The rate of photosynthesis and associated rate of organic burial also affect the rate of oxygen accumulation. When land plants spread over the continents in the Devonian, more organic carbon was buried and likely allowed higher O2 levels to occur. Today, the average time that an O molecule spends in the air before it is consumed by geological sinks is about 2 million years. That residence time is relatively short in geologic time - so in the Phanerozoic there must have been feedback processes that kept the atmospheric O level within bounds suitable for animal life.


Evolution by stages

Preston Cloud originally proposed that the first cyanobacteria had evolved the capacity to carry out oxygen-producing photosynthesis, but had not yet evolved enzymes (such as superoxide dismutase) for living in an oxygenated environment. These cyanobacteria would have been protected from their own poisonous oxygen waste through its rapid removal via the high levels of reduced ferrous iron, Fe(II), in the early ocean. Cloud suggested that the oxygen released by photosynthesis oxidized the Fe(II) to ferric iron, Fe(III), which precipitated out of the sea water to form banded iron formation. Cloud interpreted the great peak in deposition of banded iron formation at the end of the Archean as the signature for the evolution of mechanisms for living with oxygen. This ended self-poisoning and produced a population explosion in the cyanobacteria that rapidly oxygenated the ocean and ended banded iron formation deposition. However, improved dating of Precambrian strata showed that the late Archean peak of deposition was spread out over tens of millions of years, rather than taking place in a very short interval of time following the evolution of oxygen-coping mechanisms. This made Cloud's hypothesis untenable. Most modern interpretations describe the GOE as a long, protracted process that took place over hundreds of millions of years rather than a single abrupt event, with the quantity of atmospheric oxygen yo-yoing in relation to the capacity of oxygen sinks and the productivity of oxygenic photosynthesisers over the course of the GOE. More recently, families of bacteria have been discovered that show no indication of ever having had photosynthetic capability, but which otherwise closely resemble cyanobacteria. These may be descended from the earliest ancestors of cyanobacteria, which only later acquired photosynthetic ability by lateral gene transfer. Based on
molecular clock The molecular clock is a figurative term for a technique that uses the mutation rate of biomolecules to deduce the time in prehistory when two or more life forms diverged. The biomolecular data used for such calculations are usually nucleo ...
data, the evolution of oxygen-producing photosynthesis may have occurred much later than previously thought, at around 2.5 billion years ago. This reduces the gap between the evolution of oxygen photosynthesis and the appearance of significant atmospheric oxygen.


Nutrient famines

A second possibility is that early cyanobacteria were starved for vital nutrients, and this checked their growth. However, a lack of the scarcest nutrients, iron, nitrogen, and phosphorus, could have slowed, but not prevented, a cyanobacteria population explosion and rapid oxygenation. The explanation for the delay in the oxygenation of the atmosphere following the evolution of oxygen-producing photosynthesis likely lies in the presence of various oxygen sinks on the young Earth.


Nickel famine

Early chemosynthetic organisms likely produced
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Ea ...
, an important trap for molecular oxygen, since methane readily oxidizes to
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
(CO2) and water in the presence of
UV radiation Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30  PHz) to 400 nm (750  THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...
. Modern
methanogens Methanogens are microorganisms that produce methane as a metabolic byproduct in hypoxic conditions. They are prokaryotic and belong to the domain Archaea. All known methanogens are members of the archaeal phylum Euryarchaeota. Methanogens are co ...
require
nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow ...
as an enzyme cofactor. As the Earth's crust cooled and the supply of volcanic nickel dwindled, oxygen-producing algae began to out-perform methane producers, and the oxygen percentage of the atmosphere steadily increased. From 2.7 to 2.4 billion years ago, the rate of deposition of nickel declined steadily from a level 400 times today's. This nickel famine was somewhat buffered by an uptick in sulphide weathering at the start of the GOE that brought some nickel to the oceans, without which methanogenic organisms would have declined in abundance more precipitously, plunging Earth into even more severe and long-lasting icehouse conditions than those seen during the Huronian glaciation.


Increasing flux

Some people suggest that GOE is caused by the increase of the source of oxygen. One hypothesis argues that GOE was the immediate result of photosynthesis, although the majority of scientists suggest that a long-term increase of oxygen is more likely. Several model results show possibilities of long-term increase of carbon burial, but the conclusions are indecisive.


Decreasing sink

In contrast to the increasing flux hypothesis, there are also several hypotheses that attempt to use decrease of sinks to explain the GOE. One theory suggests that the composition of the volatiles from volcanic gases was more oxidized. Another theory suggests that the decrease of metamorphic gases and
serpentinization Serpentinization is a hydration and metamorphic transformation of ferromagnesian minerals, such as olivine and pyroxene, in mafic and ultramafic rock to produce serpentinite. Minerals formed by serpentinization include the serpentine group mine ...
is the main key of GOE. Hydrogen and methane released from metamorphic processes are also lost from Earth's atmosphere over time and leave the crust oxidized. Scientists realized that hydrogen would escape into space through a process called methane photolysis, in which methane decomposes under the action of ultraviolet light in the upper atmosphere and releases its hydrogen. The escape of hydrogen from the Earth into space must have oxidized the Earth because the process of hydrogen loss is chemical oxidation. This process of hydrogen escape required the generation of methane by methanogens, so that methanogens actually helped create the conditions necessary for the oxidation of the atmosphere.


Tectonic trigger

One hypothesis suggests that the oxygen increase had to await tectonically driven changes in the Earth, including the appearance of shelf seas, where reduced organic carbon could reach the sediments and be buried. The newly produced oxygen was first consumed in various chemical reactions in the oceans, primarily with
iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...
. Evidence is found in older rocks that contain massive
banded iron formation Banded iron formations (also known as banded ironstone formations or BIFs) are distinctive units of sedimentary rock consisting of alternating layers of iron oxides and iron-poor chert. They can be up to several hundred meters in thickness ...
s apparently laid down as this iron and oxygen first combined; most present-day iron ore lies in these deposits. It was assumed oxygen released from cyanobacteria resulted in the chemical reactions that created rust, but it appears the iron formations were caused by anoxygenic phototrophic iron-oxidizing bacteria, which does not require oxygen. Evidence suggests oxygen levels spiked each time smaller land masses collided to form a super-continent. Tectonic pressure thrust up mountain chains, which eroded releasing nutrients into the ocean that fed photosynthetic cyanobacteria.


Bistability

Another hypothesis posits a model of the atmosphere that exhibits bistability: two
steady state In systems theory, a system or a process is in a steady state if the variables (called state variables) which define the behavior of the system or the process are unchanging in time. In continuous time, this means that for those properties ''p' ...
s of oxygen concentration. The state of stable low oxygen concentration (0.02%) experiences a high rate of methane oxidation. If some event raises oxygen levels beyond a moderate threshold, the formation of an ozone layer shields UV rays and decreases methane oxidation, raising oxygen further to a stable state of 21% or more. The Great Oxygenation Event can then be understood as a transition from the lower to the upper steady states.


Increasing photoperiod

Cyanobacteria tend to consume nearly as much oxygen at night as they produce during the day. However, experiments demonstrate that cyanobacterial mats produce a greater excess of oxygen with longer photoperiods. The rotational period of the Earth was only about six hours shortly after its formation, 4.5 billion years ago, but increased to 21 hours by 2.4 billion years ago, in the Paleoproterozoic. The rotational period increased again, starting 700 million years ago, to its present value of 24 hours. It is possible that each increase in rotational period increased the net oxygen production by cyanobacterial mats, which in turn increased the atmospheric abundance of oxygen.


Consequences of oxygenation

Eventually, oxygen started to accumulate in the atmosphere, with two major consequences. * Oxygen likely oxidized
atmospheric methane Atmospheric methane is the methane present in Earth's atmosphere. Atmospheric methane concentrations are of interest because it is one of the most potent greenhouse gases in Earth's atmosphere. Atmospheric methane is rising. The 20-year globa ...
(a strong greenhouse gas) to carbon dioxide (a weaker one) and water. This weakened the
greenhouse effect The greenhouse effect is a process that occurs when energy from a planet's host star goes through the planet's atmosphere and heats the planet's surface, but greenhouse gases in the atmosphere prevent some of the heat from returning directly ...
of the Earth's atmosphere, causing planetary cooling, which has been proposed to have triggered a series of ice ages known as the
Huronian glaciation The Huronian glaciation (or Makganyene glaciation) was a period where several ice ages occurred during the deposition of the Huronian Supergroup, rather than a single continuous event as it is commonly misrepresented to be. The deposition of th ...
, bracketing an age range of 2.45–2.22 billion years ago. * The increased oxygen concentrations provided a new opportunity for biological diversification, as well as tremendous changes in the nature of chemical interactions between rocks,
sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class o ...
,
clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...
, and other geological substrates and the Earth's air, oceans, and other surface waters. Despite the natural recycling of organic matter, life had remained energetically limited until the widespread availability of oxygen. The availability of oxygen greatly increased the free energy available to living organisms, with global environmental impacts. For example, mitochondria evolved after the GOE, giving organisms the energy to exploit new, more complex morphologies interacting in increasingly complex ecosystems, although these did not appear until the late Proterozoic and Cambrian.


Role in mineral diversification

The Great Oxygenation Event triggered an explosive growth in the diversity of
mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...
s, with many elements occurring in one or more oxidized forms near the Earth's surface. It is estimated that the GOE was directly responsible for more than 2,500 of the total of about 4,500 minerals found on Earth today. Most of these new minerals were formed as
hydrated Drinking is the act of ingesting water or other liquids into the body through the mouth, proboscis, or elsewhere. Humans drink by swallowing, completed by peristalsis in the esophagus. The physiological processes of drinking vary widely among o ...
and
oxidized Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...
forms due to dynamic mantle and crust processes.


Role in cyanobacteria evolution

In field studies done in
Lake Fryxell Lake Fryxell is a frozen lake long, between Canada Glacier and Commonwealth Glaciers at the lower end of Taylor Valley in Victoria Land, Antarctica. It was mapped in the early 1900s and named during Operation Deep Freeze in the 1950s. There are s ...
,
Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest cont ...
, scientists found that mats of oxygen-producing cyanobacteria produced a thin layer, one to two millimeters thick, of oxygenated water in an otherwise anoxic environment, even under thick ice. By inference, these organisms could have adapted to oxygen even before oxygen accumulated in the atmosphere. * The evolution of such oxygen-dependent organisms eventually established an equilibrium in the availability of oxygen, which became a major constituent of the atmosphere.


Origin of eukaryotes

It has been proposed that a local rise in oxygen levels due to cyanobacterial photosynthesis in ancient microenvironments was highly toxic to the surrounding biota, and that this selective pressure drove the evolutionary transformation of an archaeal lineage into the first eukaryotes.
Oxidative stress Oxidative stress reflects an imbalance between the systemic manifestation of reactive oxygen species and a biological system's ability to readily detoxify the reactive intermediates or to repair the resulting damage. Disturbances in the normal ...
involving production of
reactive oxygen species In chemistry, reactive oxygen species (ROS) are highly reactive chemicals formed from diatomic oxygen (). Examples of ROS include peroxides, superoxide, hydroxyl radical, singlet oxygen, and alpha-oxygen. The reduction of molecular oxygen () p ...
(ROS) might have acted in synergy with other environmental stresses (such as
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30  PHz) to 400 nm (750  THz), shorter than that of visible light, but longer than X-rays. UV radiation ...
radiation and/or desiccation) to drive selection in an early archaeal lineage towards eukaryosis. This archaeal ancestor may already have had
DNA repair DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as radiation can cause DNA da ...
mechanisms based on DNA pairing and recombination and possibly some kind of cell fusion mechanism. The detrimental effects of internal ROS (produced by
endosymbiont An ''endosymbiont'' or ''endobiont'' is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship. (The term endosymbiosis is from the Greek: ἔνδον ''endon'' "within ...
proto- mitochondria) on the archaeal
genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding g ...
could have promoted the evolution of meiotic sex from these humble beginnings. Selective pressure for efficient DNA repair of oxidative DNA damage may have driven the evolution of eukaryotic sex involving such features as cell-cell fusions, cytoskeleton-mediated chromosome movements and emergence of the
nuclear membrane The nuclear envelope, also known as the nuclear membrane, is made up of two lipid bilayer membranes that in eukaryotic cells surround the nucleus, which encloses the genetic material. The nuclear envelope consists of two lipid bilayer membra ...
. Thus the evolution of eukaryotic sex and eukaryogenesis were likely inseparable processes that evolved in large part to facilitate DNA repair.


Lomagundi-Jatuli event

The rise in oxygen content was not linear: instead, there was a rise in oxygen content around 2.3 Ga ago, followed by a drop around 2.1 Ga ago. The positive excursion, or more precisely, the carbon isotopic excursion evidencing it, is called the Lomagundi-Jatuli event (LJE) or Lomagundi event, (named for a district of
Southern Rhodesia Southern Rhodesia was a landlocked self-governing British Crown colony in southern Africa, established in 1923 and consisting of British South Africa Company (BSAC) territories lying south of the Zambezi River. The region was informally kno ...
) and the time period has been termed Jatulian; it is believed to be part of the
Rhyacian The Rhyacian Period (; grc, ῥύαξ, translit=rhýax, meaning "stream of lava") is the second geologic period in the Paleoproterozoic Era and lasted from Mya to Mya (million years ago). Instead of being based on stratigraphy, these dates are ...
period. In the Lomagundi-Jatuli event, oxygen content reached as high as modern levels, followed by a fall to very low levels during the following stage where
black shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especia ...
s were deposited. The negative excursion is called the . Evidence for the Lomagundi-Jatuli event has been found globally: in Fennoscandia and northern
Russia Russia (, , ), or the Russian Federation, is a transcontinental country spanning Eastern Europe and Northern Asia. It is the largest country in the world, with its internationally recognised territory covering , and encompassing one-eig ...
,
Scotland Scotland (, ) is a Countries of the United Kingdom, country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a Anglo-Scottish border, border with England to the southeast ...
,
Ukraine Ukraine ( uk, Україна, Ukraïna, ) is a country in Eastern Europe. It is the second-largest European country after Russia, which it borders to the east and northeast. Ukraine covers approximately . Prior to the ongoing Russian inv ...
, China, the
Wyoming Craton The Wyoming Craton is a craton in the west-central United States and western Canada – more specifically, in Montana, Wyoming, southern Alberta, southern Saskatchewan, and parts of northern Utah. Also called the Wyoming Province, it is the init ...
in North America,
Brazil Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...
,
Uruguay Uruguay (; ), officially the Oriental Republic of Uruguay ( es, República Oriental del Uruguay), is a country in South America. It shares borders with Argentina to its west and southwest and Brazil to its north and northeast; while bordering ...
,
Gabon Gabon (; ; snq, Ngabu), officially the Gabonese Republic (french: République gabonaise), is a country on the west coast of Central Africa. Located on the equator, it is bordered by Equatorial Guinea to the northwest, Cameroon to the nort ...
,
Zimbabwe Zimbabwe (), officially the Republic of Zimbabwe, is a landlocked country located in Southeast Africa, between the Zambezi and Limpopo Rivers, bordered by South Africa to the south, Botswana to the south-west, Zambia to the north, and ...
,
South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. It is bounded to the south by of coastline that stretch along the Atlantic Ocean, South Atlantic and Indian Oceans; to the ...
,
India India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the so ...
, and Australia. Oceans seem to have been oxygenated for some time even after the termination of the isotope excursion itself. It has been hypothesized that eukaryotes first evolved during the LJE. The Lomagundi-Jatuli event coincides with the appearance and subsequent disappearance of curious fossils found in Gabon that have been termed the
Francevillian biota The Francevillian biota (also known as Gabon macrofossils or Gabonionta) is a group of 2.1-billion-year-old Palaeoproterozoic, macroscopic organisms known from fossils found in Gabon in the Palaeoproterozoic Francevillian B Formation, a black shal ...
, which seem to have been multicellular. This appears to represent a "false start" of multicellular life. The organisms apparently went extinct when the LJE ended, because they are absent in the layers of shale deposited after the LJE.


See also

*
Boring Billion The Boring Billion, otherwise known as the Mid Proterozoic and Earth's Middle Ages, is the time period between 1.8 and 0.8 billion years ago (Ga) spanning the middle Proterozoic eon, characterized by more or less tectonic stability, climatic st ...
– Earth history between 1.8 and 0.8 billion years ago, characterized by tectonic stability, climatic stasis, and a slow biological evolution with very low oxygen levels and no evidence of glaciation *
Geological history of oxygen Before photosynthesis evolved, Earth's atmosphere had no free oxygen (O2). Small quantities of oxygen were released by geological and biological processes, but did not build up in the atmosphere due to reactions with reducing minerals. Oxygen ...
– Timeline of the development of free oxygen in the Earth's oceans and atmosphere * * * Rare Earth hypothesis – Hypothesis that complex extraterrestrial life is an extremely rare phenomenon * *


References


External links

* {{Extinction Paleoproterozoic Origin of life Oxygen Events in the geological history of Earth Evolution of the biosphere Extinction events Mass extinction timelines Meteorological hypotheses