Tag: Oxidation

Detecting oxygenic photosynthesis in the Archaean Earth System

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For life on Earth, one of the most fundamental shifts in ecosystems was the Great Oxygenation Event 2.5 to 2.3 billion years (Ga) ago. The first evidence for its occurrence was from the sedimentary record, particularly ancient soils (palaeosols) that mark exposure of the continental surface above sea level and rock weathering. Palaeosols older than 2.4 Ga have low iron contents that suggest iron was soluble in surface waters, i.e. in its reduced bivalent form Fe2+. Sediments formed by flowing water also contain rounded grains of minerals that in today’s oxygen-rich environments are soon broken down and dissolved through oxidising reactions, for instance pyrite (FeS2) and uraninite (UO2). After 2.4 Ga palaeosols are reddish to yellowish brown in colour and contain insoluble oxides and hydroxides of Fe3+ principally hematite (Fe2O3) and goethite (FeO.OH). After this time sediments deposited by wind action and rivers are similar in colour: so-called ‘redbeds’. Following the GOE the atmosphere initially contained only traces of free oxygen, but sufficient to make the surface environment oxidising. In fact such an atmosphere defies Le Chatelier’s Principle: free oxygen should react rapidly with the rest of the environment through oxidation. That it doesn’t shows that it is continually generated as a result of oxygenic photosynthesis. The CO2 + H2O = carbohydrate + oxygen equilibrium does not reach a balance because of continual burial of dead organic material.

Free oxygen is a prerequisite for all multicelled eukaryotes, and it is probably no coincidence that fossils of the earliest known ones occur in sediments in Gabon dated at 2.1 Ga: 300 Ma after the Great Oxygenation Event. However, the GOE relates to surface environments of that time. From 2.8 Ga – in the Mesoarchaean Era – to the late Palaeoproterozoic around 1.9 Ga, vast quantities of Fe3+ were locked in iron oxide-rich banded iron formations (BIFs): roughly 105 billion tons in the richest deposits alone (see: Banded iron formations (BIFs) reviewed; December 2017). Indeed, similar ironstones occur in Archaean sedimentary sequences as far back as 3.7 Ga, albeit in uneconomic amounts. Paradoxically, enormous amounts of oxygen must have been generated by marine photosynthesis to oxidise Fe2+ dissolved in the early oceans by hydrothermal alteration of basalt lava upwelling from the Archaean mantle. But none of that free oxygen made it into the atmosphere. Almost as soon as it was released it oxidised dissolved Fe2+ to be dumped as iron oxide on the ocean floor. Before the GOE that aspect of geochemistry did obey Le Chatelier!

A limestone made of stromatolites

The only likely means of generating oxygen on such a gargantuan scale from the earliest Archaean onwards is through teeming prokaryote organisms capable of oxygenic photosynthesis. Because modern cyanobacteria do that, the burden of the BIFs has fallen on them. One reason for that hypothesis stems from cyanobacteria in a variety of modern environments building dome-shaped bacterial mats. Their forms closely resemble those of Archaean stromatolites found as far back as 3.7 Ga. But these are merely peculiar carbonate bodies that could have been produced by bacterial mats which deploy a wide variety of metabolic chemistry. Laureline Patry of the Université de Bretagne Occidentale, Plouzané, France, and colleagues from France, the US, Canada and the UK have developed a novel way of addressing the opaque mechanism of Archaean oxygen production (Patry, L.A. and 12 others. Dating the evolution of oxygenic photosynthesis using La-Ce geochronology. Nature, v. 642, p. 99-104; DOI: 10.1038/s41586-025-09009-8).

They turned to the basic geochemistry of rare earth elements (REE) in Archaean stromatolitic limestones from the Superior Craton of northern Canada. Of the 17 REEs only cerium (Ce) is capable of being oxidised in the presence of oxygen. As a result Ce can be depleted relative to its neighbouring REEs in the Periodic Table, as it is in many Phanerozoic limestones. Five samples of the limestones show consistent depletion of Ce relative to all other REE. It is also possible to date when such fractionation occurred using 138La– 138Ce geochronology.  The samples were dated at 2.87 to 2.78 Ga (Mesoarchaean), making them the oldest limestones that show Ce anomalies and thus oxygenated seawater in which the microbial mats thrived. But that is only 300 Ma earlier than the start of the GOE. Stromatolites are abundant in the Archaean record as far back as 3.4 Ga, so it should be possible to chart the link between microbial carbonate mats and oxygenated seawater to a billion years before the GOE, although that does not tell us about the kind of microbes that were making stromatolites.

See also: Tracing oxygenic photosynthesis via La-Ce geochronology. Bioengineer.org, 29 May 2025; Allen, J.F. 2016. A proposal for formation of Archaean stromatolites before the advent of oxygenic photosynthesis. Frontiers in Microbiology, v. 7; DOI: 10.3389/fmicb.2016.01784.

Arsenic: an agent of evolutionary change?

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The molecules that make up all living matter are almost entirely (~98 %) made from the elements Carbon, Hydrogen, Oxygen, Nitrogen and Phosphorus (CHONP) in order of their biological importance. All have low atomic numbers, respectively 6th, 1st, 8th, 7th and 15th in the Periodic Table. Of the 98 elements found in nature, about 7 occur only because they form in the decay schemes of radioactive isotopes. Only the first 83 (up to Bismuth) are likely to be around ‘for ever’; the fifteen heavier than that are made up exclusively of unstable isotopes that will eventually disappear, albeit billions of years from now. There are other oddities that mean that the 92 widely accepted  to be naturally occurring is not strictly correct. That CHONP are so biologically important stems partly from their abundances in the inorganic world and also because of the ease with which they chemically combine together. But they are not the only ones that are essential.

About 20 to 25% of the other elements are also literally vital, even though many are rare. Most of the rest are inessential except in vanishingly small amounts that do no damage, and may or may not be beneficial. However some are highly toxic. Any element can produce negative biological outcomes if above certain levels. Likewise, deficiencies can result in ill thrift and event death. For the majority of elements, biologists have established concentrations that define deficiency and toxic excess. The World Health Organisation has charted the maximum safe levels of elements in drinking water in milligrams per litre. In this regard, the lowest safe level is for thallium (Tl) and mercury (Hg) at 0.002 mg l-1.Other highly toxic elements are cadmium (Cd) (0.003 mg l-1), then arsenic (As) and lead (Pb) (0.01 mg l-1) that ‘everyone knows’ are elements to avoid like the plague. In nature lead is very rarely at levels that are unsafe because it is insoluble, but arsenic is soluble under reducing conditions and is currently responsible for a pandemic of related ailments, especially in the Gangetic plains of India and Bangladesh and similar environments worldwide.

Biological evolution has been influenced since life appeared by the availability, generally in water, of both essential and toxic elements. In 2020 Earth-logs summarised a paper about modern oxygen-free springs in Chile in which photosynthetic purple sulfur bacteria form thick microbial mats. The springs contain levels of arsenic that vary from high in winter to low in summer. This phenomenon can only be explained by some process that removes arsenic from solution in summer but not in winter. The purple-bacteria’s photosynthesis uses electrons donated by sulfur, iron-2 and hydrogen – the spring water is highly reducing so they thrive in it. In such a simple environment this suggested a reasonable explanation: the bacteria use arsenic too. In fact they contain a gene (aio) that encodes for such an eventuality. The authors suggested that purple sulfur bacteria may well have evolved before the Great Oxygenation Event (GOE). They reasoned that in an oxygen-free world arsenic, as well as Fe2+ would be readily available in water that was in a reducing state, whereas oxidising conditions after the GOE would suppress both: iron-2 would be precipitated as insoluble iron-3 oxides that in turn efficiently absorb arsenic (see: Arsenic hazard on a global scale, May 2020).

Colour photograph and CT scans of Palaeoproterozoic discoidal fossils from the Francevillian Series in Gabon. (Credit: El Albani et al. 2010; Fig. 4).

A group of geoscientists from France, the UK, Switzerland and Austria have investigated the paradox of probably high arsenic levels before the GOE and the origin and evolution of life during the Archaean  (El Khoury et al. 2025. A battle against arsenic toxicity by Earth’s earliest complex life forms. Nature Communications, v. 16, article 4388; DOI: 10.1038/s41467-025-59760-9). Note that the main, direct evidence for Archaean life are fossilized microbial mats known as stromatolites, some palaeobiologists reckoning they were formed by oxygenic photosynthesising cyanobacteria others favouring the purple sulfur bacteria (above). The purple sulfur bacteria in Chile and other living prokaryotes that tolerate and even use arsenic in their metabolism clearly evolved that potential plus necessary chemical defence mechanisms, probably when arsenic was more available in the anoxic period before the GOE. Anna El Khoury and her colleagues sought to establish whether or not eukaryotes evolved similar defences by investigating the earliest-known examples; the 2.1 Ma old Francevillian biota of Gabon that post-dates the GOE. They are found in black shales, look like tiny fried eggs and are associated with clear signs of burrowing. The shales contain steranes that are breakdown products of steroids, which are unique to eukaryotes.

The fossils have been preserved by precipitation of pyrite (Fe2S) granules under highly reducing conditions. Curiously, the cores of the pyrite granules in the fossils are rich in arsenic, yet pyrite grains in the host sediments have much lower As concentrations. The latter suggest that seawater 2.1 Ma ago held little dissolved arsenic as a result of its containing oxygen. The authors interpret the apparently biogenic pyrite’s arsenic cores as evidence of the organism having sequestered As into specialized compartments in their bodies: their ancestors must have evolved this efficient means of coping with significant arsenic stress before the GOE. It served them well in the highly reducing conditions of black shale sedimentation. Seemingly, some modern eukaryotes retain an analogue of a prokaryote As detoxification gene.

Did Precambrian BIFs ‘fall’ into the mantle to trigger mantle plumes?

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How the Earth has been shaped has depended to a large extent on a very simple variable among rocks: their density. Contrasts in density between vast rock masses are expressed when gravity attempts to maintain a balance of forces. The abrupt difference in elevation of the solid surface at the boundaries of oceans and continents – the Earth’s hypsometry – stems from the contrasted densities of continental and oceanic crust: the one dominated by granitic rocks (~2.8 t m-3) the other by those of basaltic composition (~ 3.0 t m-3). Astronomers have estimated that Earth’s overall density is about 5.5 t m-3 – it is the densest planet in the Solar System. The underlying mantle makes up 68% of Earth’s mass, with a density that increases with depth from 3.3 to 5.4 t m-3 in a stepwise fashion, at a number of discontinuities, because mantle minerals undergo changes induced by pressure. The remaining one third of Earth’s mass resides in the iron-nickel core at densities between 9.5 to 14.5 t m-3. Such density layering is by no means completely stable. Locally increased temperatures in mantle rocks reduce their density sufficiently for masses to rise convectively to be replaced by cooler ones, albeit slowly. By far the most important form of convection affecting the lithosphere involves the resorption of oceanic lithosphere plates at destructive margins, which results in subduction. This is thought to be due to old, cold oceanic basalts undergoing metamorphism as pressure increases during subduction. They are transformed at depth to a mineral assemblage (eclogite) that is denser (3.4 to 3.5 t m-3) than the enveloping upper mantle. That density contrast is sufficient for gravity to pull slabs of oceanic lithosphere downwards. This slab-pull force is transmitted through oceanic lithosphere that remains at the surface to become the dominant driver of modern plate tectonics. As a result, extension of the surface oceanic lithosphere at constructive margins draws mantle upwards to partially melt at reduced pressure, thus adding new basaltic crust at mid-ocean rift systems to maintain a form of mantle convection. Seismic tomography shows that active subducted slabs become ductile about 660 km beneath the surface and below that no earthquakes are detected. Quite possibly, the density of the reconstituted lithospheric slab becomes less than that of the mantle below the 660 km discontinuity. So the subducted slab continues by moving sideways and buckling in response to the ‘push’ from its rigid upper parts above. But it has been suggested that some subducted slabs do finally sink to the core-mantle boundary, but that is somewhat conjectural.

Typical banded iron formation

There are sedimentary rocks whose density at the surface exceeds that of the upper mantle: banded iron formations (BIFs) that contain up to 60% iron oxides (mainly Fe2O3) and have an average density at the surface of around 3.5 t m-3. BIFs formed mainly in the late Archaean and early Proterozoic Eons  (3.2 to 1.0 Ga) and none are known from the last 400 Ma. They formed when soluble iron-2 (Fe2+) – being added to ocean water by submarine hydrothermal activity –was precipitated as Fe3+ in the form of iron oxide (Fe2O3) where oxygen was present in ocean water. With little doubt this happened only in shallow marine basins where cyanobacteria that appeared about 3.5 Ga ago had sufficient sunlight to photosynthesise. Until about 2.4 Ga the atmosphere and thus the bulk of ocean water contained very little oxygen so the oceans were pervaded by soluble iron so that BIFs were able to form wherever such biological activity was going on. Conceivably (but not proven), that BIF-forming biochemical reaction may even have operated far from land in ocean surface water, slowly to deposit Fe2O3 on the deep ocean floor. After 2.4 Ga oxygen began to build in the atmosphere after the Great Oxidation Event had begon. That time was also when the greatest production of BIFs took place. Strangely, the amount of BIF in the geological record fell during the next 600 Ma to rise again to a very high peak at 1.8 Ga. Since there must have been sufficient soluble iron and an increasing amount of available oxygen for BIFs to form throughout that ‘lean’ period the drop in BIF formation is paradoxical. After 1.0 Ga BIFs more or less disappear. By then so much oxygen was present in the atmosphere and from top to bottom in ocean water that soluble iron was mostly precipitated at its hydrothermal source on the ocean floor. Incidentally, modern ocean surface water far from land contains so little dissolved iron that little microbiological activity goes on there: iron is an essential nutrient so the surface waters of remote oceans are effectively ‘wet deserts’.

Plots of probability of LIPs and BIFs forming at the Earth’s surface during Precambrian times, based on actual occurrences (Credit: Keller, et al., modified Fig 1A)

Spurred by the fact that if a sea-floor slab dominated by BIFs was subducted it wouldn’t need eclogite formation to sink into the mantle, Duncan Keller of Rice University in Texas and other US and Canadian colleagues have published a ‘thought experiment’ using time-series data on LIPs and BIFs compiled by other geoscientists (Keller, D.S. et al. 2023. Links between large igneous province volcanism and subducted iron formations. Nature Geoscience, v. 16, article; DOI: 10.1038/s41561-023-01188-1.). Their approach involves comparing the occurrences of 54 BIFs through time with signs of activity in the mantle during the Palaeo- and Mesoproterozoic Eras, as marked by large igneous provinces (LIPs) during that time span. To do this they calculated the degree of correlation in time between BIFs and LIPs. The authors chose a minimum area for LIPs of 400 thousand km2 – giving a total of 66 well-dated examples. Because the bulk of Precambrian flood-basalt provinces, such as occurred during the Phanerozoic, have been eroded away, most of their examples are huge, well-dated dyke swarms that almost certainly fed such plateau basalts. Rather than a direct time-correlation, what emerged was a match-up that covered 74% of the LIPs with BIFs that had formed about 241 Ma earlier. They also found a less precise correlation between LIPs associated with 241 Ma older BIFs and protracted periods of stable geomagnetic field, known as ‘superchrons’. These are thought by geophysicists to be influenced by heat flow through the core-mantle boundary (CMB).

The high bulk density of BIFs at the surface would be likely to remain about 15 % greater than that of peridotite as pressure increased with depth in the mantle. Such slabs could therefore penetrate the 660 mantle discontinuity. Their subduction would probably result in their eventually ‘piling up’ in the vicinity of the CMB. The high iron content of BIFs may also have changed the way that the core loses heat, thereby triggering mantle plumes. Certainly, there is a complex zone of ultra-low seismic velocities (ULVZ) that signifies hot, ductile material extending above the CMB. Because BIFs’ high iron-content makes them thermally highly conductive compared with basalts and other sediments, they may be responsible. Clearly, Keller et al’s hypothesis is likely to be controversial and they hope that other geoscientists will test it with new or re-analysed geophysical data. But the possibility of BIFs falling to the base of the mantle spectacularly extends the influence of surface biological processes to the entire planet. And, indeed, it may have shaped the later part of its tectonic history having changed the composition of the deep mantle. The interconnectedness of the Earth system also demands that the consequences – plumes and large igneous provinces – would have fed back to the Precambrian biosphere. See also: Iron-rich rocks unlock new insights into Earth’s planetary history, Science Daily, 2 June 2023

Hydrogen and how the Earth formed

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A third piece with hydrogen as its focus in a couple of months? Well, from a galactic perspective there’s a lot of it about. Modern cosmology suggests that only 4.6% of the energy in the universe consists of elemental atoms made of protons, neutrons and electrons, dwarfed by dark energy and dark matter that are something of mystery. But of the more familiar energy equivalent, tangible matter (as in E=mc2), 74% of the universe is hydrogen, 24% is helium and the other 92 elements amount to just 2%. That tiny proportion of heavier elements was created by nucleosynthesis within stars from the two products of the Big Bang (H and He). Nuclear fusion reactions formed those with atomic numbers (protons in their nuclei) up to that of iron (26), whereas the heavier elements were created through neutron- and proton capture when the largest stars destroyed themselves cataclysmically as supernovae. Yet the planet whose surface we inhabit contains only minute amounts of helium and elemental hydrogen. Of course water at and beneath the surface, in the form of atmospheric vapour and locked within minerals retains some of the cosmically available hydrogen. But current estimates suggest that hydrogen accounts for a mere 0.03% of Earth’s mass. Despite the fact that some forms of radioactive decay generate alpha particles that become helium it forms a vanishingly small proportion of terrestrial mass.

The solar system formed around 4.6 billion years ago by a complex gravitational accretion of the gas and dust of an interstellar cloud: mainly H and He. Its dynamic collapse resulted in gravitational potential energy being transformed into heat: in the case of the Sun, sufficient to set off self-sustaining nuclear fusion. As a body grows in this way so does its gravity and thus the speed needed for matter to escape from its pull (escape velocity). As temperature increases so does the speed at which atoms of each element vibrate; the lower the atomic mass the faster the vibration and the greater the chance of escape. So the ‘blend’ of elements that an astronomical body retains during its early evolution depends on its gravity and its surface temperature. The Sun is so massive that very little has escaped its pull, despite a surface temperature of about 5 to 6 thousand degrees Celsius. Its composition is thus close to the cosmic average. Those of the giant planets Jupiter, Saturn, Uranus and Neptune are not far short because of their large gravities and low surface temperatures. Even today, the smaller Inner Planets are unable to cling on to elemental hydrogen and helium and nearly all that is left of the matter from which they formed is the 2% of heavier cosmic elements locked into solids, liquids and gases.

Processes in the early solar system were far more complicated than they are today. In the mainly gaseous disc, from which the solar system evolved, gravity dragged matter towards its centre. That eventually ignited nuclear fusion of hydrogen to form our star. More remote from its gravitational pull vortices aggregated dust into bodies known as planetesimals that in turn accreted to larger protoplanets. Solar gravity dragged gas from the inner solar system leaving rocky protoplanets, whereas gas was able to be attracted to the surface of what became the gas giants where their gravity outweighed that of the far-off Sun. This was complicated by a sort of Milankovich Effect on steroids in which protoplanets continuously changed their orbits and underwent collisions. The best known of these was between the protoEarth and a Mars-sized body that formed the Earth-Moon system, both bodies having deep magma oceans as a result of the huge energy focussed on them by the collision. What may have happened to the protoplanet that became Earth before the Moon-forming collision has been addressed by three geoscientists at the University of California Los Angeles and the Carnegie Institution for Science Washington DC, USA (Young, E.D. et al. 2023. Earth shaped by primordial H2 atmospheres. Nature, v. 616, p. 306–311; DOI: 10.1038/s41586-023-05823-0 [PDF request to: eyoung@epss.ucla.edu]).

A thick hydrogen-rich atmosphere’s interacting chemically with a protoplanet (left). A possible later stage (right) where iron oxide in the magma ocean of the Early Hadean after Moon formation oxidises a hydrogen atmosphere to form surface water (Credit: Sean Raymond 2023, Fig 1)

The focus of the work of Edward Young, Anat Shahar and Hilke Schlichting is directed at the possibility that the Earth-forming protoplanets originally retained thick hydrogen atmospheres. They use thermodynamic modelling of the equilibrium between hydrogen and silicate magma oceans that had resulted from the energy of their accretion. The authors’ main assumption is that insufficient time had elapsed during accretion for the protoplanets to cool and crystallise: a distinct possibility because loss of accretionary heat by thermal radiation would have been ‘blanketed’ by actively accreting dust and gas in orbit around the growing protoplanets. Effectively, the equilibrium would have been chemical in nature: reactions between highly reducing hydrogen and oxidised silicate melts or even vaporised rock evaporated from the very hot surface. The authors suggest that protoplanets bigger than Mars (0.2 to 0.3 times that of Earth) could retain a hydrogen-rich atmosphere long enough for the chemical reactions to come to a balance, despite high temperatures. There would have been no shortage of hydrogen at this early stage in solar system evolution: perhaps as much as 0.2% percent the mass of the Earth surrounding a protoplanet about half its present size.

Two outcomes may have emerged. Reaction between hydrogen and anhydrous silicates could produce H2O in amounts up to three times that currently in the Earth’s oceans, some locked in the magma ocean, some in the dense atmosphere. A by-product would have been iron oxide, giving the current mantle its oxidising properties known from the geochemistry of basaltic magmas.  Hydrogen might also have dissolved in molten iron alloys, thereby contributing to the nascent core. That second outcome would help explain why the modern core is less dense than expected for iron-nickel alloy, both solid and liquid. In fact densities calculated by geophysicists from the speeds of seismic waves that have travelled through the core are 5 to 10% percent lower than expected for the alloy. So the core must contain substantial amounts of elements with low atomic numbers.

Several other possibilities have been suggested to account for Earth’s abundance of water. Two popular ideas are comets arriving in the ‘settled’ times of the Hadean or by original accretion of hydrous chondrite meteorites, whose hydrogen isotope proportions match those of ocean water. Hydrogen as the light element needed in the core is but one possibility along with oxygen, sulfur and other ‘light’ elements. Also, the oxidising potential of the modern mantle may have resulted from several billion years of wet lithosphere being subducted. To paraphrase Sean Raymond (below), ‘other hypotheses are available’!

See also: Raymond, S.N. 2023. Earth’s molten youth had long-lasting consequences. Nature (News & Views), v. 616, p. 251-252; DOI: 10.1038/d41586-023-00979-1 [PDF request to: rayray.sean@gmail.com]

An early oxygenated atmosphere

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The Earth’s earliest atmosphere undoubtedly had a chemistry dominated by carbon dioxide and nitrogen, together with transient water vapour, outgassed from volcanoes giving pervasive reducing conditions at the surface and in the oceans. Until the last couple of decades the only clear evidence of a switch to oxidising conditions and presumably significant atmospheric oxygen was direct, mineralogical evidence. The most obvious signs are ancient, reddened soils formed when soluble Fe2+ lost electrons to molecular oxygen to form the distinct red, orange and brown oxides and hydroxides of insoluble Fe3+ that impart a deep staining in even small quantities. Others include the disappearance from river-transported sediments of clearly transported grains of metal sulfides and uranium oxide that remain stable under reducing conditions but quickly break down in the presence of oxygen.

Widespread observations in Precambrian sediments, eventually linked with reliable radiometric ages, strongly suggested a fundamental environmental change at around 2.3 billion years ago: the Great Oxidation Event. A few such signs emerge from somewhat older rocks back to 2.7 Ga, but only the 2.3 Ga event created a permanent feature of our home world; at first toxic to many of the prokaryote life forms of earlier times but eventually a prime condition for the rise of the Eukarya and eventually metazoan animals. Isotopic analysis of sulfur from Precambrian sediments also gave hints of a more complex but much debated transition because of the way S-isotopes fractionate under different environmental conditions. Now other  indirect, isotopic approaches to redox conditions have become feasible, with a surprising result: powerful evidence that about 3 billion years ago there was appreciable atmospheric oxygen (Crowe, S.A. et al. 2013. Atmospheric oxygenation three billion years ago. Nature, v. 501, p. 535-538).

The Danish-South African-German-Canadian group relied on a fractionation process among the isotopes of chromium, which can exist in several oxidation states. When minerals that contain Cr3+  are weathered under oxidising conditions to release soluble Cr6+ the loss in solution preferentially removes the 53Cr isotope from residual soil. If the isotope enters groundwater with reducing conditions to precipitate some Cr3+ -rich material yet more 53Cr remains in solution. Eventually such enriched water may enter the oceans, where along with iron and other transition-group metal ions chromium can end up in banded iron formations (BIFs) to preserve isotopic evidence for oxidising conditions along it route from land to sea.

This image shows a 2.1 billion years old rock ...
Banded iron formation (BIF) from the Precambrian of North America belonging to the National Museum of Mineralogy and Geology in Dresden, Germany. (credit: Wikipedia)

The team analysed both a palaeosol and a BIF unit from a stratigraphic sequence in the Achaean of NE South Africa that is between 2980 and 2924 Ma old. A substantial proportion of the palaeosol is depleted in 53Cr whereas the lower part of the slightly younger BIF is significantly enriched. Changes in the concentration of redox sensitive elements, such as chromium itself, uranium and iron, in the two lithologies helps confirm the isotopic evidence for a major ~3 Ga oxidation event. It is possible to use the data to estimate what the atmospheric oxygen content might have been at that time: not enough to breathe, but significant at between 6 x 10­-5 to 3 x 10-3 the atmospheric level at present. Oxygen can be produced abiogenically through irradiation of water vapour in the atmosphere as well as by organic photosynthesis. However, the first route seems incapable of yield more than a billionth of present atmospheric concentrations, so the spotlight inevitably falls on a ‘much deep history’ of the action of blue-green bacteria (cyanobacteria) than hitherto suspected.

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