Kurt Gödel's Incompleteness Theorem says (i) all closed systems are unprovable within themselves and, that (ii) all open systems are rightly understood as incomplete. - R.E. Slater

The God among us is the God who refuses to be God without us, so great is God's Love. – Tripp Fuller

According to some Christian outlooks we were made for another world. Perhaps, rather, we were made for this world to recreate, reclaim, redeem, and renew unto God's future aspiration by the power of His Spirit. - R.E. Slater

Our eschatological ethos is to love. To stand with those who are oppressed. To stand against those who are oppressing. It is that simple. Love is our only calling and Christian Hope. - R.E. Slater

Secularization theory has been massively falsified. We don't live in an age of secularity. We live in an age of explosive, pervasive religiosity... an age of religious pluralism. - Peter L. Berger

Exploring the edge of life and faith in a post-everything world. - Todd Littleton

I don't need another reason to believe, your love is all around for me to see. – Anon

Thou art our need; and in giving us more of thyself thou givest us all. - Khalil Gibran, Prayer XXIII

Be careful what you pretend to be. You become what you pretend to be. - Kurt Vonnegut

Religious beliefs, far from being primary, are often shaped and adjusted by our social goals. - Jim Forest

People, even more than things, need to be restored, renewed, revived, reclaimed, and redeemed; never throw out anyone. – Anon

Certainly, God's love has made fools of us all. - R.E. Slater

An apocalyptic Christian faith doesn't wait for Jesus to come, but for Jesus to become in our midst. - R.E. Slater

Christian belief in God begins with the cross and resurrection of Jesus, not with rational apologetics. - Eberhard Jüngel, Jürgen Moltmann

Our knowledge of God is through the 'I-Thou' encounter, not in finding God at the end of a syllogism or argument. There is a grave danger in any Christian treatment of God as an object. The God of Jesus Christ and Scripture is irreducibly subject and never made as an object, a force, a power, or a principle that can be manipulated. - Emil Brunner

“Ehyeh Asher Ehyeh” means "I will be that who I have yet to become." - God (Ex 3.14) or, conversely, “I AM who I AM Becoming.”

Our job is to love others without stopping to inquire whether or not they are worthy. - Thomas Merton

The church is God's world-changing social experiment of bringing unlikes and differents to the Eucharist/Communion table to share life with one another as a new kind of family. When this happens, we show to the world what love, justice, peace, reconciliation, and life together is designed by God to be. The church is God's show-and-tell for the world to see how God wants us to live as a blended, global, polypluralistic family united with one will, by one Lord, and baptized by one Spirit. – Anon

The cross that is planted at the heart of the history of the world cannot be uprooted. - Jacques Ellul

The Unity in whose loving presence the universe unfolds is inside each person as a call to welcome the stranger, protect animals and the earth, respect the dignity of each person, think new thoughts, and help bring about ecological civilizations. - John Cobb & Farhan A. Shah

If you board the wrong train it is of no use running along the corridors of the train in the other direction. - Dietrich Bonhoeffer

God's justice is restorative rather than punitive; His discipline is merciful rather than punishing; His power is made perfect in weakness; and His grace is sufficient for all. – Anon

Our little [biblical] systems have their day; they have their day and cease to be. They are but broken lights of Thee, and Thou, O God art more than they. - Alfred Lord Tennyson

We can’t control God; God is uncontrollable. God can’t control us; God’s love is uncontrolling! - Thomas Jay Oord

Life in perspective but always in process... as we are relational beings in process to one another, so life events are in process in relation to each event... as God is to Self, is to world, is to us... like Father, like sons and daughters, like events... life in process yet always in perspective. - R.E. Slater

To promote societal transition to sustainable ways of living and a global society founded on a shared ethical framework which includes respect and care for the community of life, ecological integrity, universal human rights, respect for diversity, economic justice, democracy, and a culture of peace. - The Earth Charter Mission Statement

Christian humanism is the belief that human freedom, individual conscience, and unencumbered rational inquiry are compatible with the practice of Christianity or even intrinsic in its doctrine. It represents a philosophical union of Christian faith and classical humanist principles. - Scott Postma

It is never wise to have a self-appointed religious institution determine a nation's moral code. The opportunities for moral compromise and failure are high; the moral codes and creeds assuredly racist, discriminatory, or subjectively and religiously defined; and the pronouncement of inhumanitarian political objectives quite predictable. - R.E. Slater

God's love must both center and define the Christian faith and all religious or human faiths seeking human and ecological balance in worlds of subtraction, harm, tragedy, and evil. - R.E. Slater

Tuesday, October 26, 2021

Earth's Evolutionary History - The Great Oxidation Event


Artist's impression of asteroid bombardment on early Earth.
 (SwRI/Dan Durda, Simone Marchi)


The Great Oxidation Event (GOE), also called the Great Oxygenation Event, was a time period when the Earth's atmosphere and the shallow ocean first experienced a rise in oxygen, approximately 2.4–2.0 Ga (billion years ago) during the Paleoproterozoic era. Geological, isotopic, and chemical evidence suggests that biologically-produced molecular oxygen (dioxygen, O2) started to accumulate in Earth's atmosphere and changed it from a weakly reducing atmosphere practically free of oxygen into an oxidizing atmosphere containing abundant oxygen, causing many existing anaerobic species on Earth to die out. The event is inferred to have been caused by cyanobacteria producing the oxygen, which stored enough chemical energy to enable the subsequent development of multicellular life forms.

Asteroids May Have Stolen The Oxygen
From Earth's Ancient Atmosphere

by Michelle Starr
October 21, 2021

For a period of Earth's history, between roughly 2.5 to 4 billion years ago, our planet was a punching bag for asteroids.

During this time, Earth was absolutely pelted with large space rocks, compared to the relative quiet of our existence today. This activity would have produced significant alterations to the chemistry of the planet's atmosphere – but the scale and shape of those alterations, especially the effect on oxygen levels, has been difficult to quantify.

Now, a study of tiny, once-molten particles in Earth's crust has revealed that these asteroid impacts were far more numerous than we had thought, which may have delayed the oxygenation of Earth's atmosphere.

These particles are called impact spherules, and they're created when an asteroid slams into Earth, generating such intense heat that the crust melts and sprays into the air. When the material settles, cools, and hardens, it forms a layer of spherules in the planet's crust.

In recent years, far more of these spherules have been unearthed in drill cores and excavations, which means, in turn, that the asteroid collision rate may be 10 times higher than previous analyses suggested. This would have had a much more significant effect on Earth's oxygen levels than previous models.

"Current bombardment models underestimate the number of late Archean spherule layers," says planetary geologist Simone Marchi of the Southwest Research Institute. "[This suggests] the impactor flux at that time was up to 10 times higher than previously thought."

All of this extra rock from space generates chemistry that results in a lot more oxygen being held back from the atmosphere.

How, when, and why Earth's atmosphere became rich with oxygen is deeply important to our understanding of planet habitability. Most multicellular organisms on Earth can't live without oxygen; without it, we probably wouldn't be here.

For reasons not fully understood, however, oxygen levels didn't start to significantly rise in what we call the Great Oxidation Event until the emergence of photosynthesizing cyanobacteria on the scene 2.4 billion years ago.

Asteroid bombardment, the team's new analysis reveals, could have been one of the mechanisms at play preventing oxygen levels from rising. As space rocks repeatedly slammed into Earth, their impact vapors would have removed the limited amounts of oxygen present in the early atmosphere.

"Late Archean bombardment by objects over 6 miles in diameter would have produced enough reactive gases to completely consume low levels of atmospheric oxygen," said astronomer and geologist Laura Schaefer of Stanford University.

"This pattern was consistent with evidence for so-called 'whiffs' of oxygen, relatively steep but transient increases in atmospheric oxygen that occurred around 2.5 billion years ago.

"We think that the whiffs were broken up by impacts that removed the oxygen from the atmosphere. This is consistent with large impacts recorded by spherule layers in Australia's Bee Gorge and Dales Gorge."

The team's new analysis on spherule layers challenges previous impact models and scales up the intensity of collisions, finding that an asteroid larger than 10 kilometers (6 miles) across would have hit Earth once every 15 million years or so.

That may seem infrequent, but geologically speaking, that's a lot of big asteroids – and 10 times more frequent than we had thought.

Modelling then revealed the cumulative oxygen sink effect these impacts would have had. Only once bombardment slowed did oxygen levels start to rise, changing Earth's surface chemistry and transforming the planet into a habitable world. This, the researchers now believe, is no coincidence.

"Impact vapors caused episodic low oxygen levels for large spans of time preceding the Great Oxidation Event," Marchi said.

"As time went on, collisions became progressively less frequent and too small to be able to significantly alter post-Great Oxidation Event oxygen levels. The Earth was on its course to become the current planet."

The research has been published in Nature Geoscience



* * * * * * * * 


Great Oxidation Event

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O2 build-up in the Earth's atmosphere. Red and green lines represent the range of the estimates while time is measured in billions of years ago (Ga).
  • Stage 1 (3.85–2.45 Ga): Practically no O2 in the atmosphere. The oceans were also largely anoxic with the possible exception of O2 in the shallow oceans.
  • Stage 2 (2.45–1.85 Ga): O2 produced, rising to values of 0.02 and 0.04 atm, but absorbed in oceans and seabed rock.
  • Stage 3 (1.85–0.85 Ga): O2 starts to gas out of the oceans, but is absorbed by land surfaces. No significant change in oxygen level.
  • Stages 4 and 5 (0.85 Ga – present): Other O2 reservoirs filled; gas accumulates in atmosphere.[1]

The Great Oxidation Event (GOE), also called the Great Oxygenation Event, was a time period when the Earth's atmosphere and the shallow ocean first experienced a rise in oxygen, approximately 2.4–2.0 Ga (billion years ago) during the Paleoproterozoic era.[2] Geological, isotopic, and chemical evidence suggests that biologically-produced molecular oxygen (dioxygen, O2) started to accumulate in Earth's atmosphere and changed it from a weakly reducing atmosphere practically free of oxygen into an oxidizing atmosphere containing abundant oxygen,[3] causing many existing anaerobic species on Earth to die out.[4] The event is inferred to have been caused by cyanobacteria producing the oxygen, which stored enough chemical energy[5] to enable the subsequent development of multicellular life forms.[6]

The early atmosphere

The composition of the Earth's earliest atmosphere is not known with certainty. However, the bulk was likely dinitrogenN
2
, and carbon dioxideCO
2
, which are also the predominant carbon- and nitrogen-bearing gases produced by volcanism today. These are relatively inert gases. 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.[7] Either carbon dioxide levels were much higher at the time, providing enough of a greenhouse effect to warm the Earth, or other greenhouse gases were present. The most likely such gas is methaneCH
4
, which is a powerful greenhouse gas and was produced by early forms of life known as methanogens. Scientists continue to research how the Earth was warmed before life arose.[8]

An atmosphere of N
2
 and CO
2
 with trace amounts of H
2
O
CH
4
carbon monoxide (CO), and hydrogen (H
2
), is described as a weakly reducing atmosphere. Such an atmosphere contains practically no oxygen. The modern atmosphere contains abundant oxygen, making it an oxidizing atmosphere.[9] The rise in oxygen is attributed to photosynthesis by cyanobacteria, which are thought to have evolved as early as 3.5 billion years ago.[10]

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, in the 1970s.[7] Cloud observed that detrital sediments older than about 2 billion years ago contained grains of pyriteuraninite,[7] and siderite,[9] 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 redbeds, which get their color from the oxidized (ferric) mineral hematite, began to appear in the geological record at about this time. Banded iron formation largely disappears from the geological record at 1.85 billion years ago, after peaking at about 2.5 billion years ago.[11] Banded iron formation can form only when abundant dissolved ferrous iron is transported into depositional basins, and an oxygenated ocean blocks such transport by oxidizing the iron to form insoluble ferric iron compounds.[12] 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.[7] 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.[8]

Geological evidence

Evidence for the Great Oxidation Event is provided by a variety of petrological and geochemical markers.

Continental indicators

Paleosolsdetrital grains, and redbeds are evidence of low-level oxygen.[13] Paleosols (fossil soils) older than 2.4 billion years old have low iron concentrations that suggest anoxic weathering.[14] Detrital grains found in sediments older than 2.4 billion years old contain minerals that are stable only under low oxygen conditions.[15] Redbeds are red-colored sandstones that are coated with hematite, which indicates that there was enough oxygen to oxidize iron to its ferric state.[16]

Banded iron formation (BIF)

Banded iron formations are composed of thin alternating layers of chert (a fine-grained form of silica) and iron oxides, magnetite 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.[17] 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.[12] 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.[18]

Iron speciation

Black laminated shales, rich in organic matter, are often regarded as a marker for anoxic 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 anoxia, in which dissolved ferrous iron is abundant, and euxinia, in which hydrogen sulfide is present in the water.[19]

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.[20] A DOP near zero indicates oxidizing conditions, while a DOP near 1 indicates euxenic conditions. Values of 0.3 to 0.5 are transitional, suggesting anoxic bottom mud under an oxygenated ocean. Studies of the Black Sea, 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.[19]

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.[18][13][19]

Isotopes

Some of the most persuasive evidence for the Great Oxidation Event is provided by the mass-independent fractionation (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.[21] The presence of this signature all but eliminates the possibility of an oxygenated atmosphere.[9]

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.[9][13]

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.[22][23] It is also possible that oxygen was present earlier only in localized "oxygen oases".[24] 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.[25]

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.[13]

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.[26] Structures that are claimed to be fossils of cyanobacteria exist in rock as old as 3.5 billion years old[27] These include microfossils of supposedly cyanobacterial cells and macrofossils called stromatolites, 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.[27] However, it has increasingly been inferred that at least some of these Archaean fossils were generated abiotically or produced by non-cyanobacterial phototrophic bacteria.[28]

Additionally, Archaean sedimentary rocks were once found to contain biomarkers, 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 Pilbara, Western Australia.[29] 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.[30]

Other indicators

Some elements in marine sediments are sensitive to different levels of oxygen in the environment such asthe transition metals molybdenum[19] and rhenium.[31] Non-metal elements such as selenium and iodine are also indicators of oxygen levels.[32]

https://en.wikipedia.org/wiki/Timeline_of_the_evolutionary_history_of_life

Hypotheses

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 iron (Fe2+) is a primary factor in oxygen accumulation.[39] 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 15.8±3.3 Tmol (1 Tmol = 1012 moles) of O2 per year. This creates a net O2 flux from the global oxygen sources.The ability to generate oxygen via photosynthesis likely first appeared in the ancestors of cyanobacteria.[33] These organisms evolved at least 2.45–2.32 billion years ago,[34][35] and probably as early as 2.7 billion years ago or earlier.[7][36][2][37][38] However, oxygen remained scarce in the atmosphere until around 2.0 billion years ago,[8] and banded iron formation continued to be deposited until around 1.85 billion years ago.[7] 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.[8]

The rate of change of oxygen can be calculated from the difference between global sources and sinks.[13] The oxygen sinks include reduced gases and minerals from volcanoes, metamorphism and weathering.[13] 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.[40] For the weathering mechanisms, 12.0±3.3 Tmol of O2 per year today goes to the sinks composed of reduced minerals and gases from volcanoes, metamorphism, percolating seawater and heat vents from the seafloor.[13] On the other hand, 5.7±1.2 Tmol of O2 per year today oxidizes reduced gases in the atmosphere through photochemical reaction.[13] On the early Earth, there was visibly very little oxidative weathering of continents (e.g., a lack of redbeds) 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 (Fe2+Fe3+
2
O
4
) that is insoluble in water, and sank to the bottom of the shallow seas to create banded iron formations such as the ones found in Minnesota and Pilbara, Western Australia.[40] It took 50 million years or longer to deplete the oxygen sinks.[41] 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.[42] Today, the average time that an O2 molecule spends in the air before it is consumed by geological sinks is about 2 million years.[43] That residence time is relatively short in geologic time - so in the Phanerozoic there must have been feedback processes that kept the atmospheric O2 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.[44][45] 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.[44][45] 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.[11]

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 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.[46]

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.[8]

Nickel famine

Early chemosynthetic organisms likely produced methane, an important trap for molecular oxygen, since methane readily oxidizes 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 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.[47] From 2.7 to 2.4 billion years ago, the rate of deposition of nickel declined steadily from a level 400 times today's.[48]

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.[49] Several model results show possibilities of long-term increase of carbon burial,[50] but the conclusions are indecisive.[51]

Decreasing sink

In contrast to the increasing flux hypothesis, there are also several hypotheses that attempt to use decrease of sinks to explain GOE. One theory suggests that the composition of the volatiles from volcanic gases was more oxidized.[39] Another theory suggests that the decrease of metamorphic gases and serpentinization 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.[52] 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.[52] 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.[24]

Tectonic trigger

2.1-billion-year-old rock showing banded iron formation

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.[53][54] The newly produced oxygen was first consumed in various chemical reactions in the oceans, primarily with iron. Evidence is found in older rocks that contain massive banded iron formations 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.[55] 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.[56]

Bistability

Another hypothesis posits a model of the atmosphere that exhibits bistability: two steady states 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.[57][58]

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.[59][60]

Consequences of oxygenation

Eventually, oxygen started to accumulate in the atmosphere, with two major consequences.

  • Oxygen likely oxidized atmospheric methane (a strong greenhouse gas) to carbon dioxide (a weaker one) and water. This weakened the greenhouse effect 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, bracketing an age range of 2.45–2.22 billion years ago.[61][62][63]
  • The increased oxygen concentrations provided a new opportunity for biological diversification, as well as tremendous changes in the nature of chemical interactions between rockssandclay, 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. Due to its relatively weak double bond, oxygen is a high-energy molecule[5] and produced a breakthrough in metabolic evolution that 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.[64]

Role in mineral diversification

The Great Oxygenation Event triggered an explosive growth in the diversity of minerals, with many elements occurring in one or more oxidized forms near the Earth's surface.[65] 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 and oxidized forms due to dynamic mantle and crust processes.[66]

In field studies done in Lake FryxellAntarctica, 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.[67] The evolution of such oxygen-dependent organisms eventually established an equilibrium in the availability of oxygen, which became a major constituent of the atmosphere.[67]

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.[68] Oxidative stress involving production of reactive oxygen species (ROS) might have acted in synergy with other environmental stresses (such as ultraviolet radiation and/or desiccation) to drive selection in an early archaeal lineage towards eukaryosis. This archaeal ancestor may already have had DNA repair mechanisms based on DNA pairing and recombination and possibly some kind of cell fusion mechanism.[69][70] The detrimental effects of internal ROS (produced by endosymbiont proto-mitochondria) on the archaeal genome could have promoted the evolution of meiotic sex from these humble beginnings.[69] 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.[68] Thus the evolution of eukaryotic sex and eukaryogenesis were likely inseparable processes that evolved in large part to facilitate DNA repair.[68]

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,[71] (named for a district of Southern Rhodesia) and the time period has been termed Jatulian. 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 shales were deposited. The negative excursion is called the Shunga-Francevillian event. Evidence for the Lomagundi-Jatuli event has been found globally: in Fennoscandia and northern Russia, Scotland, Ukraine, China, the Wyoming craton in North America, Brazil, South Africa, India and Australia. Oceans seem to have been oxygenated for some time even after the termination of the isotope excursion itself.[72][73]

It has been hypothesized that eukaryotes first evolved during the LJE.[72] The Lomagundi-Jatuli event coincides with the appearance, and subsequent disappearance, of curious fossils found in Gabon, termed Francevillian biota, 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