Late formation of the Earth’s inner core

The layered structure of the Earth was discovered using the varying arrival times of seismic waves from major earthquakes, which pass through the Earth, at seismometer stations located across the planet’s surface. Analysis of these arrival times indicates the wavepaths taken through the planet, involving reflections and refractions at boundaries of materials with distinctly different physical properties. S-waves from an earthquake do not arrive in a wide ‘shadow zone’ around its antipode. Since that kind of wave depends on shearing and cannot pass through liquid the shadow reveals the presence of an outer core made of very dense liquid iron and nickel. P-waves that travel in a manner akin to sound waves also show a shadow but it is annular in form around the antipode because of refraction at the core-mantle boundary, but they do penetrate to reach the antipode. However, their arrival times there show faster speeds than expected from an entirely liquid core, and so reveal a central mass, the inner core, which is a ball of solid iron-nickel alloy about 70% of the Moon’s size.

The Earth’s internal structure as revealed by seismic waves (Credit: Smithsonian Institute)

Movements of liquid Fe-Ni in the outer core generate Earth’s magnetic field in the manner of a self-exciting dynamo. Motion in the outer core results from convection of heat from below – probably mainly heat generated by planetary accretion – coupled with the Earth’s rotation and the Coriolis Effect.  The present style of motion is in a thick molten layer trapped between the solid mantle and the inner core. Its circulation results in a magnetic field with two distinct poles close to the geographic ones. The field is crudely similar to that of a bar magnet, with lesser deviations spread around the planet. However, it is not particularly stable, as shown by periodic flips or reversals of polarity through geological time (see: How the core controls Earth’s magnetic field reversals; April 2005).

Few geoscientists doubt that the core formed early in Earth’s history from excess iron, nickel and sulfur, plus other siderophile elements such as gold, that cannot be accommodated by the dominant silicates of the mantle. This could not have been achieved other than by iron-rich melts sinking in some way because of their density. Gradual loss of original heat of accretion and declining radiogenic heat from rare isotopes (e.g. 40K) in the melt suggests an original, totally molten core that at some time began to crystallise under stupendous pressure in its lowest parts. A fully molten core would have been turbulent and therefore able to generate a magnetic field, and Archaean rocks still retain remanent magnetisation. The form that the field took can only be modelled. At times it may have been dipolar – paleomagnetic pole positions match geological evidence for early supercontinents –  and it may have undergone reversals. When the inner core formed has long remained disputed, yet thanks to advances in palaeomagnetic analysis it may now have been resolved  (Zhou, T. and 11 others 2022. Early Cambrian renewal of the geodynamo and the origin of inner core structure. Nature Communications, v. 13, article 4161; DOI:10.1038/s41467-022-31677-7).

Tinghong Zhou of the University of Rochester, USA, and colleagues from other US, Chinese and British institutions have assiduously measured the original magnetic intensities locked in tiny iron- and iron-titanium oxide needles trapped in feldspars that dominate plutonic igneous rocks, known as anorthosites, of late Precambrian age. They found that, by about 565 Ma ago during the Ediacaran Period, the Earth’s magnetic field strength had fallen to almost a sixth of its value in the early Archaean: about 15 times less than it is today. Within a mere 30 Ma it had risen to become 5 times its lowest value , as recorded by a Cambrian anorthosite, and then rose steadily through the Phanerozoic Eon to its present strength. Modelling of the rapid rebound suggests that the inner core had begun to crystallise by about 550 Ma to reach half its present radius by the end of the Ordovician Period (~450 Ma).

That event may also have been a milestone for the continuation of biological evolution on Earth. While Mars once probably had a molten core and magnetic field, it vanished 4 billion years ago, probably when its core became solid. Early Mars had an ocean in its northern hemisphere up to about 3.8 Ga, and there is plenty of evidence for erosion by water on its higher surfaces. For liquid water to have existed there for hundreds of million years demands a thick, warm atmosphere able to initiate a greenhouse effect. With low atmospheric pressure water could have existed only as ice or water vapour. Now its atmosphere is very thin and except at its poles there is no sign of surface water, even as ice (it is possible that significant amounts of water ice remain protected beneath the surface of Mars). One hypothesis is that when Mars lost its magnetic field it also lost protection from the stream of energetic particles known as the solar wind, which can strip water vapour and carbon dioxide – and thus their ability to retain atmospheric heat – from the top of the atmosphere. Earth is currently protected from the solar wind by its strong magnetic field and magnetosphere that deflects high-speed, charged particles. During the Ediacaran Period it almost lost that protection, but was spared by the self-exciting dynamo being regenerated.

See also: How did Earth avoid a Mars-like fate? Ancient rocks hold clues. Science Daily, 25 July 2022

The dangers of rolling boulders

Field work in lonely and spectacular places is a privilege. Though it can be great, boredom sometimes sets in, which is hard for the lone geologist. Today, I guess a cell phone would help, especially in high places where the signal is good. That means of communication and entertainment only emerged in the 1980s and did not reach wild places until well into the 90s. Pre-cellnet boredom could be relieved by what remains a dark secret: lone geologists once rolled large boulders down mountains and valley sides, shouting ‘Below!’ as a warning to others. Their excuse to themselves for this unique thrill (bounding boulders reach speeds of up to 40 m s-1) was vaguely scientific: sooner or later a precarious rock would fall anyway. This week it emerged that Andrin Caviezel of the Institute for Snow and Avalanche Research in Davos, Switzerland, an Alpine geoscientist, rolls boulders for a living (Caviezel, A. 2022. The gravity of rockfalls. Where I work, Nature, v. 607, p. 838; DOI: 10.1038/d41586-022-02044-9). He finds that ‘…flinging giant objects down a mountain is still super fun’. The serious part of his job attempts to model how rockfalls actually move downslope, as an aid to risk assessment (Caviezel, A. and 23 others 2021. The relevance of rock shape over mass – implications for rockfall hazard assessments. Nature Communications, v. 12, article 5546; DOI: 10.1038/s41467-021-25794-y)

Caviezel’s team (@teamcaviezel) don’t use actual rocks but garishly painted, symmetrical blocks of reinforced concrete weighing up to 3 tonnes, which are more durable than most outcropping rock and can be re-used. A Super Puma helicopter shifts a block to the top of a slope, from which it is levered over the edge (watch video). The team deploys two types of block, one equant and resembling a giant garnet crystal, the other wheel-shaped with facets. The first represents boulders of rock types with uniform properties throughout, such as granite. The wheel type mimics boulders formed from rocks that are bedded or foliated, which are usually plate-like or spindly.

Vertical aerial photograph of a uniform, south-facing slope in the Swiss Alps used to roll concrete ‘boulders’. The red X marks the release point; the blue symbols show the points of rest of equant ‘boulders, the sizes of which are shown in the inset, the wheel-shaped ones are magenta. Coloured circles with crosses show the mean rest position of each category (the lighter the colour the smaller the set of ‘boulders’). The coloured ellipses indicate the standard deviation for each category. (Credit: Caviezel et al., Fig 2)

Unlike other gravity-driven hazards, such as avalanches and mudflows, the directions that rockfalls may follow by are impossible to predict. Rather than hugging the surface, boulders interact with it, bouncing and being deflected, and they spin rapidly. To follow each experiment’s trajectory a block contains a motion sensor, measuring speed and acceleration, and a gyroscope that shows rotation, wobbling and motion direction, while filming records jump heights – up to 11 m in the experiments. Despite the similarity of the blocks, the same release point for each roll and a uniform mountainside slope, with one cliff line, the final resting places are widely spread. That hazard zone of rockfalls is distinctly wider than that of snow avalanches; observing a boulder once it starts to move gives a potential victim little means of knowing a safe place to shelter.

The most important conclusion from the experiments is that the widest spread of tumbling ‘boulders’ is shown by the wheel-shaped ones. So, slopes made from bedded or foliated sedimentary and metamorphic rocks may pose wider hazards from rockfalls than do those underpinned by uniform rocks. However, plate-like or spindly boulders are more stable at rest than are equant ones. Yet boulders rarely fall as a result of being pushed (except in avalanches). On moderate slopes they are undermined by erosion, and on steep slopes or cliffs winter ice wedges open joints allowing blocks to fall during a thaw.

Rare meteorite gives clues to the early history of Mars

Apart from the ages and geochemistry of a few hundred zircon grains we have no direct evidence of what the earliest crust of the Earth was like. The vast bulk of the present crust is younger than about 4 billion years. The oldest tangible crustal rocks occur in the 4.2 billion year (Ga) old Nuvvuagittuq greenstone belt on Hudson Bay. The oldest zircon grains have compositions that suggest that they formed during the crystallisation of andesitic magmas about 4.4 Ga ago about 140 Ma after the Earth accreted. But, according to an idea that emerged decades ago, that does not necessarily represent the earliest geology. Geochemists have shown that the bulk compositions of the Earth and Moon are so similar that they almost certainly share an early history. Rocks from the lunar highlands – the light areas that surround the dark basaltic maria – collected during the Apollo missions are significantly older (up to 4.51 Ga). They are made mainly of calcium-rich feldspars. These anorthosites have a lower density that basaltic magma. So it is likely that the feldspars crystallised from an all-enveloping ‘magma ocean’ and floated to form an upper crust on the moon. Such a liquid outer layer could only have formed by a staggering input of energy. It is believed that what became the Moon was flung from the Earth following collision with another planetary body as vapour, which then collapsed under gravity and condensed to a molten state (see: Moon formed from vapour cloud; January 2008). Crystallisation of the bulk of anorthosites has been dated to between 4.42 to 4.35 Ga (see: Moon-forming impact dated; March 2009). The Earth would likely have had a similar magma ocean produced by the impact (a much fuller discussion can be found here), but no tangible trace has been discovered, though there is subtle geochemical evidence.

The surface geology of Mars has been mapped in great detail from orbiting satellites and various surface Rovers have examined sedimentary rocks – one of them is currently collecting samples for eventual return to Earth. Currently, the only materials with a probable Martian origin are rare meteorites; there are 224 of them out of 61 thousand meteorites in collections. They are deemed to have been flung from its surface by powerful impacts to land fortuitously on Earth. It is possible to estimate when they were ejected from the effects of cosmic-ray bombardment to which they were exposed after ejection, which produces radioactive isotopes of a variety of elements that can be used in dating. So far, those analysed were flung into space no more than 20 Ma ago. Meteorites with isotopic ‘signatures’ and mineral contents so different from others and from terrestrial igneous rocks are deemed to have a Martian origin by a process of elimination. They also contain proportions of noble gases (H, Ne, Ar, Kr and Xe) that resemble that of the present atmosphere of Mars. Almost all of them are mafic to ultramafic igneous rocks in two groups: about 25 % that have been dated at between 1.4 to 1.3 Ga; the rest are much younger at about 180 Ma. But one that was recovered from the desert surface in West Sahara, NW Africa (NWA 7034, nicknamed ‘Black Beauty’) is unique. It is a breccia mainly made of materials derived from a sodium-rich basaltic andesite source, and contains much more water than all other Martian meteorites.

The ‘Black Beauty’ meteorite from Mars (NWA 7035) with a polished surface and a 2 mm wide microscope view of a thin section: the pale clasts are fragments of pyroxenes and plagioclase feldspars; the rounded dark grey clast is a fine-grained basaltic andesite. (Credits: NASA; Andrew Tindall)

If you would like to study the make-up of NWA 7035 in detail you can explore it and other Martian meteorites by visiting the Virtual Microsope devised by Dr Andrew Tindall and Kevin Quick of the British Open University.

The initial dating of NWA 7034 by a variety of methods yielded ages between 1.5 to 1.0 Ga, but these turned out to represent radiometric ‘resetting’ by a high-energy impact event around 1.5 Ga ago. Its present texture of broken clasts set in a fine-grained matrix suggests that the breccia formed from older crustal rock smashed and ejected during that impact to form a debris ‘blanket’ around the crater. Cosmogenic dating of the meteorite indicates that the debris was again flung from the surface of Mars at some time in the last 10 Ma to launch NWA 7034 beyond Mars’s gravitational field eventually to land in northwest Africa. But that is not the end of the story, because increasingly intricate radiometric dating has been conducted more recently.

‘Black Beauty’ contains rock and mineral fragments that have yielded dates as old as 4.48 Ga. So the breccia seems to have formed from fragments of the early crust of Mars. Indeed it represents the oldest planetary rock that has ever come to light. Some meteorites (carbonaceous chondrites) date back to the origin of the Solar System at around 4.56 Ga ago, and were a major contributor to the bulk composition of the rocky planets. However, the material in NWA 7034 could only have evolved from such primordial materials through processes taking place within the mantle of Mars. That was very early in the planet’s history: less than 80 Ma after it first began to accrete. It could therefore be a key to the early history of all the rocky planets, including the Earth.

There are several scenarios that might account for the composition of NWA 7034. The magma from which its components originated may have been produced by direct partial melting of the planet’s mantle shortly after accretion. However, experimental partial melting of ultramafic mantle suggests that andesitic magmas would be unlikely to form by such a primary process. But other kinds of compositional differentiation, perhaps in an original magma ocean, remain to be explored. Unlike the Earth-Moon system, there is no evidence for anorthosites exposed at the Martian surface that would have floated to become crust once such a vast amount of melt began to cool. Some scientists, however, have suggested that to be a possibility for early Mars. Another hypothesis, by analogy with what is known about the earliest Archaean processes on Earth, is secondary melting of a primordial basaltic crust, akin to the formation of Earth’s early continental crust.

Only a new robotic or crewed mission to the area from which NWA 7034  was ‘launched’ can take ideas much further. But where on Mars did ‘Black Beauty’ originate? A team from Australia, France, Cote d’ Ivoire, and the US have used a range of Martian data sets to narrow down the geographic possibilities (Lagain, A., and 13 others 2022. Early crustal processes revealed by the ejection site of the oldest martian meteorite. Nature Communications, v. 13, article 3782; DOI 10.1038/s41467-022-31444-8). The meteorite contains a substantially higher content of the elements thorium and potassium than do other Martian meteorites. Long-lived radioactive isotopes of K, Th and U generate gamma-ray emissions with distinctly different wavelengths and energy levels. Those for each element have been mapped from orbit. NWA 7034 also has very distinct magnetic properties, and detailed data on variations on the magnetic field intensity of Mars have also been acquired by remote sensing. Images from orbit allow relative ages of the surface to be roughly mapped from the varying density of impact craters: the older the surface, the more times it has been struck by projectiles of all sizes. These data also detect of craters large enough to have massively disrupted Martian crustal materials to form large blankets of impact breccias like NWA 7034. That is, ‘targets’ for the much later impact that sent the meteorite Earthwards. Using a supercomputer, Lagain et al. have cut the possibilities down to 19 likely locations. Their favoured source is the relatively young Karratha crater in the Southern Hemisphere to the west of the Tharsis Bulge. It formed on a large ejecta blanket associated with the ancient (~1.5 Ga) 40 km wide Khujirt crater.

Interesting, but sufficiently so to warrant an awesome bet in the form of a mission budget?

Ancient deep groundwater

Worldwide, billions of people depend on groundwater for their water needs from wells, deep boreholes and natural springs. Even surface water in rivers and lakes is directly connected to that moving sluggishly below the surface. In fact the surface water level marks where the water table coincides with the land surface. From season to season the water table rises and falls and so too do river and lake levels, depending on fluctuations in rainfall, snow melt, evaporation and extraction. Where it is present, vegetation plays a role in the hydrological cycle, through transpiration from roots through stems and leaves, from which it is exhaled by minute pores or stomata; effectively plants are able to pump water through their tissues to a height of up to a hundred metres.  Groundwater, like that at the surface, moves under gravity roughly parallel to the slope of the land surface from the place where precipitation infiltrates soil and rock. But the deeper it is the slower the flow and the less it is in direct contact with surface processes to be replenished by infiltration. Wells and boreholes rarely penetrate deeper than a few hundred metres, so that the vast bulk of groundwater is never used. Indeed most deep groundwater would not be drinkable or suitable for irrigation since over millennia or longer it dissolves material from the rock that contains it to become saline. In some deep sedimentary aquifers it may actually be composed of seawater trapped at the time of sedimentation.

Damp conditions in the Mponeng gold mine near Johannesburg, South Africa, the world’s deepest at 3.8 km below the surface with planned expansion to 4.3 km (Credit: AngloGold Ashanti)

The pore spaces in sandstones and fractures in limestones, the most common aquifers, are not the only conduits for groundwater. Crystalline igneous and metamorphic rocks are generally full of minute fractures resulting from their tectonic history. The deepest mines in crystalline basement, such as the gold mines of the Johannesburg area in South Africa, penetrate almost 4 km below the surface, yet are by no means dry and have to be pumped to stave off flooding. The water is a brine containing sodium and calcium chloride with high concentrations of dissolved, reduced gases such as hydrogen, methane and ethane (C2H6). Studies of the proportions of oxygen isotopes in the water reveal that the water in the fractures is very different from that in modern rainwater: this fluid is completely isolated from the modern hydrological cycle and is very old indeed. Just how old has now been determined (Warr, O. et al. 2022. 86Kr excess and other noble gases identify a billion-year-old radiogenically-enriched groundwater system. Nature Communications v. 13, Article number 3768; DOI: 10.1038/s41467-022-31412-2).

Brine extracted from a borehole in the floor of the Moab Khotsong gold/uranium mine also contains the noble gases helium, neon, argon, krypton and xenon. Noble gases are present in today’s atmosphere, so conceivably they may have originally entered the brine in rain water that seeped along fractures. However, when their isotopes are measured their proportions are very different from those in air. There are excesses of 4He, 21Ne, 22Ne, 40Ar, 86Kr and several isotopes of Xe. These isotopes are emitted during the radioactive decay of uranium, thorium and 40K, the main heat producing isotopes in the crust and mantle. Oliver Warr of the University of Toronto Canada and geochemists from Oxford University UK, Princeton University and the New Mexico Institute of Mining and Technology US, and the Sorbonne France show that originally atmospheric noble gases have been enriched in these radiogenic isotopes. Their present isotopic proportions therefore give clues to the time when air dissolved in groundwater was trapped in the host rock more than a billion years ago. A complicating factor is that the host rocks themselves are dated at about three times that age. They suggest that the fractures systems were initiated by the Vredfort asteroid impact at 2.0 Ga to form aquifers, but they became isolated from hydrological circulation around 1.2 Ga and now now contain the world’s oldest groundwater.

One of the implications of the study is that such trapped water may be present at depth in the crust of Mars, despite its current aridity. Another is that, because the fluid contains hydrogen, sulfate ions and hydrocarbon gases, it can potentially support organisms that use them to power their metabolism and reproduce. In 2008 microbes were found living in similar ancient groundwater 2.4 km below the surface in the Kidd Creek Mine, Canada, at a level of around 5 thousand cells per millilitre (50 times less than in surface water). They are powered by reduction of sulfate ions to sulfide. In 2008 another peculiar discovery in the deep biosphere emerged from the Mponeng gold mine near Johannesburg, South African (the world’s deepest) in the form of a living sulfate reducing bacterium Desulforudis audaxviator. DNA  analysis of the ancient water revealed that it was the sole inhabitant, a biological mystery confirmed by later deep-biosphere studies in Death Valley, USA, and Siberia.

See also: Researchers uncover life’s power generators in the Earth’s oldest groundwaters, EurekaAlert, 5 July 2022; Mantle link with biosphere, July 2009

New dating questions previous ideas about early hominins

The Sterkfontein cave 40 km northwest of Johannesburg in South Africa first sprang to the attention of scientists in 1936, with the discovery there of an adult hominin skull. This showed clear affinities with the discovery 400 km to the SW in 1924 of the fossil skull of a juvenile primate, which Raymond Dart claimed to be ancestral to modern humans, naming it Australopithecus africanus. Sterkfontein has since yielded more than 500 hominin fossils, many of which are Au. africanus.

Limestone cave deposits are difficult to date precisely, unlike sediments that are interbedded with volcanic rocks, the most amenable material being that deposited by water flowing through the cave to form flowstone or speleothem. Using the U-Pb method of radiometric dating yielded an age of between 2.1 to 2.6 Ma for flowstone that cements the breccia in which the Au. africanus fossils occur. Clearly, the flowstone formed after burial so that was a minimum age for them, awaiting the use of a different chronological tool to suggest when burial of the bones took place

The face of an Australopithecus africanus: ‘Mrs Ples’. (Credit University of Zurich)

An almost complete skeleton of another australopithecine found in another part of the Sterkfontein cave system was dated in 2015 by a different approach. This used the decay of 10Be and 26Al isotopes that high-energy cosmic rays produce in quartz grains while they are exposed at the surface. Burial of irradiated sedimentary grains protects them from such bombardment, and the two isotopes  then steadily decay at a known rate. Quartz grains associated with this specimen (fondly known as ‘Little Foot’) turned out to be far older than the flowstone U-Pb age, with a cosmogenic burial age of about 3.7 Ma. Its much greater antiquity prompted scientists to regard ‘Little Foot’ as a different species – Au. prometheus – despite being similar to Au. africanus.

Since that success, much the same team from South Africa, the US and France has been working on sedimentary grains buried with the abundant Au. africanus specimens from Sterkfontein (Granger D.E. et al. 2022. Cosmogenic nuclide dating of Australopithecus at Sterkfontein, South AfricaProceedings of the National Academy of Sciences, v. 119, article e2123516119; DOI: 10.1073/pnas.2123516119). Their newly published efforts show that “Little Foot’s” burial took place between 3.41 and 3.49 Ma, more than a million years earlier than suggested by the flowstone U-Pb dating and just ~200 ka younger than the ‘Little Foot’ skeleton. More surprising is that Au. africanus lived during the same period (3.4 to 3.7 Ma) as did Au. afarensis – the species to which ‘Lucy’ belonged – 3500 km to the north in Ethiopia.

So it is no longer justifiable  to suggest that the first known human species (Homo habilis ~2.3 to 1.65 M) is either a more ‘advanced’ australopithecine or a direct descendant from that genus, for the new dating opens a million-year gap in the history of human evolution. That age range does contain stone tools but no plausible candidates for an australopithecine-human evolutionary connection. One of the most recently suggested link is Au. sediba (see: Another candidate for earliest, direct human ancestor, October 2011; and Australopithecus sediba: is she or is she not a human ancestor? April 2013). The snag with that candidate is that the well-established age (2.0 Ma) of known specimens falls in the middle of the range for H. habilis. The two may have been cohabiters of Africa but are very different.

The million years that separated Au. africanus together with afarensis from H. habilis is the period when the defining character of humans, tool making, evolved. So the hunt is on for hominins associated with stone tools in that huge stratigraphic gap. One of the drawbacks with famous sites, such as the ‘Cradle of Humankind’ that includes Sterkfontein, is that they almost become clichés so that scientists return to them again and again, while the key that they seek may well lie elsewhere.

Climate out of control after the Permian-Triassic mass extinction

The snuffing out of up to 90 percent of all terrestrial and marine species at the end of the Permian (252 Ma) was the outcome of lethal climatic warming. It probably stemmed from a stupendous episode of flood basalt volcanism and intrusions in what is now Siberia that burned vast amounts of peat or coal in the basin that the flows filled (see: Coal and the end-Permian mass extinction; March 2011). The carbon dioxide so released created planetary hyperthermia and toxic acid rain. For at least five million years Earth was an almost sterile world, a notable absence being dense vegetation on the land surface – the Early Triassic is devoid of coal, whereas there is plenty of Late Permian age. Much the same slow recovery of life is found in meagre collections of land and marine animal fossils of that age. Yet, other mass extinctions were followed by recovery and species diversification at a much faster pace.

One conceivable explanation could be the near absence of vegetation whose photosynthesis and burial would otherwise draw down CO2 and the same goes for its marine equivalent phytoplankton. But there is a powerful inorganic means of carbon sequestration: silicate weathering. The chemistry depends on carbon dioxide dissolved in water. For simple silicates it can be expressed as:

2CO2 + H2O + CaSiO3 → Ca2+ + 2HCO3 + SiO2.

The higher the ambient temperature, the faster such reactions proceed. Most silicates are more complex and many common ones, such as feldspars, include aluminium, so that another product of weathering is insoluble, fine-grained clay minerals. So various soluble metal ions (Ca, Mg, K, Na etc), dissolved bicarbonate ions, silica in various guises and clays eventually end up in the sea. Once there, it is possible for them to recombine, as for instance calcium and bicarbonate ions:

Ca2+ + 2HCO3→ CaCO3 + CO2 + H2O

Despite some CO2 gas being released, this reaction results in a net sequestration of carbon in calcium carbonate. Incidentally, the same kind of chemical reaction occurs in the soils produced by weathering. The carbonate may cement soils to form a hard crust of caliche or ‘calcrete’. Chemical weathering enhanced by a hot climate, it might seem, should reduce the greenhouse effect quickly: a feedback mechanism that normally stabilises climate. But that did not happen after the P-Tr extinction event, thereby stressing all remaining life forms. A group of scientists at the University of Waikato in New Zealand have developed a possible explanation for this potentially fatal hazard for life on Earth (Isson, T.T. et al. 2022. Marine siliceous ecosystem decline led to sustained anomalous Early Triassic warmth. Nature Communications, v. 13, article 3509; DOI: 10.1038/s41467-022-31128-3). It focuses on the silica (SiO2) released by chemical weathering, which enters the ocean in the form of a colloid: Si(OH)4, a form of silicic acid known as ‘reactive silica’. Under ‘normal’ conditions, this is removed by organisms, such as diatoms and radiolaria, and is constantly recycled on a time scale of about 400 years, some contributing to deep-ocean oozes in the form of chert. But, like all other marine organisms, they too were victims of the P-Tr mass extinction.

Examples of marine radiolaria (top)

Reactive silica colloids in seawater also participate in inorganic chemical reactions, combining with dissolved metal ions to form complex hydrated aluminosilicates, i.e. more clay minerals. The reactions change the alkalinity of seawater. As a result dissolved HCO3ions transform to CO2 gas and water. Despite the complexity of the chemistry that interweaves the carbon and silicon cycles, there is a simple conclusion. If the abundance of silica-secreting marine organisms falls drastically while continental weathering continues to deliver silica, clay-mineral formation on the ocean floor results in release of CO2 that reverses the effect of enhanced weathering and thus maintains hyperthermal conditions. The other outcome is that less chert and flint granules form Terry Isson and colleagues examined the varying proportion of chert in cores through Lower Triassic marine sediments. A ‘chert gap’characterises the 4 to 6 Ma following the P-Tr boundary event. This can be explained in part by extinction of silica-secreting organisms and by inorganic reactions converting the reactive silica that enhanced weathering delivered to the oceans to clay minerals. This supports the idea that the inorganic part of the silica cycle maintained greenhouse conditions in the absence of organic ‘competition’ for reactive silica. Many other biogeochemical cycles link biological and chemical processes that combine to affect climate: involving phosphorus, nitrogen and iron, to name but three.

A new twist to Pleistocene climate cycles

The combined gravitational pulls of the sun and moon modulate variations in local tidal range. High spring tides occur when the two bodies are opposed at full moon or in roughly the same direction at new Moon. When the positions of sun and moon are at right angles (1st quarter and 3rd quarter) their gravitational pulls partly cancel each other to give neap tides. Consequently, there are two tidal cycles every lunar month.  In a similar way, the varying gravitational pulls of the planets during their orbital cycles impart a repetitive harmony to Earths astronomical behaviour. But their combined effects are on the order of tens of thousand years. Milutin Milankovich (1879-1958), a Serbian engineer, pondered on the possible causes of Earth’s climatic variations, particularly the repetition of ice ages. He was inspired by 19th century astronomers’ suggestion that maybe the gravitational effects of other planets might be a fruitful line of research. Milankovich focussed on how the shape of Earth’s orbit, the tilt of its rotational axis and the way the axis wobbles like that of a spinning top affect the amount of solar heating at all points on the surface: the effects of varying eccentricity, obliquity and precession, respectively.

 Earlier astronomers had calculated cycles of gravitational effects on Earth of the orbits of Jupiter and Saturn of the three attributes of Earth’s astronomical behaviour and found periods of about 100, 41 and 23 thousand years (ka) respectively. The other 3 inner planets and the much more distant giants Uranus and Neptune also have gravitational effects on Earth, but they are negligible compared with those of the two nearest giant planets, because gravitation force varies with mass and inversely with the square of distance. Sadly, Milankovich was long dead when his hypothesis of astronomical climate forcing was verified in 1976 by frequency analysis of the record of oxygen isotopes in foraminifera found in two ocean sediment core from the Southern Indian Ocean. It revealed that all three periods interfered in complex ways during the Late Pleistocene, to dominate variations in sea-surface temperatures and the fluctuating volume of continental ice sheets for which δ18O is a proxy (see: Odds and ends about Milankovich and climate change; February 2017).

Precession of the axis of a spinning top and that of the Earth. At present the northern end of Earth’s axis points to what we now call the Pole Star. Around 11.5 ka from now it will point to the star Vega

This was as revolutionary for climatology as plate tectonics was for geology. We now know that in the early Pleistocene glacial-interglacial cycles were in lockstep with the 41 ka period of axial obliquity, and since 700 ka followed closely – but not perfectly – the 100 ka orbital eccentricity forcing. The transitional period between 1.25 and 0.7 Ma (the Mid-Pleistocene Transition or MPT) suggested neither one nor the other. Milankovich established that axial tilt variations have the greatest influence on solar heating, so the early 41 ka cycles were no surprise. But the dominance of orbital eccentricity on the last 700 ka certainly presented a puzzle, for it has by far the weakest influence on solar heating: 10 times less than those of axial obliquity and precession. The other oddity concerns the actual effect of axial precession on climate change. There are no obvious 23 ka cycles in the climate record, despite the precession signal being clear in frequency analysis and its effect on solar heating being almost as powerful as obliquity and ten times greater than that of orbital eccentricity. Precessional wobbling of the axis controls the time of year when one hemisphere or the other is closest to the Sun. At one extreme it will be the Northern and 11.5 ka later it will be the Southern. The times of solstices and equinoxes also change relative to the calendar that we use today.

There is an important, if obvious, point about astronomical forcing of climate. It is always there, with much the same complicated interactions between the factors: human activities have absolutely no bearing on them. Climatic ‘surprises’ are likely to continue!

Changes in ice-rafted debris (IRD) since 1.7 Ma in a sediment core from the North Atlantic (orange fill) compared with its oxygen-isotope (δ18O) record of changes in continental ice cover (blue fill). At the top are the modelled variations in 23 ka axial precession (lilac) and 41 ka obliquity (green). The red circles mark major interglacial episodes, blue diamonds show the onset of significant ice rafting and orange diamonds are terminations of ice-rafting (TIR). (Credit: Barker et al., Fig. 2)

Sea temperature and ice-sheet volume are not the only things that changed during the Pleistocene. Another kind of record from oceanic sediments concerns the varying proportion in the muddy layers of abnormally coarse sand grains and even small pebbles that have been carried by icebergs; they are known as ice-rafted debris (IRD). The North Atlantic Ocean floor has plenty of evidence for them appearing and disappearing on a layer-by-layer basis. They were first recognised in 1988 by an oceanographer called Helmut Heinrich, who proposed that six major layers rich in IRD in North Atlantic cores bear witness to iceberg ‘armadas’ launched by collapse, or ablation, at the front of surging ice sheets on Scandinavia, Greenland and eastern Canada. Heinrich events, along with Dansgaard-Oeschger events (rapid climatic warming followed by slower cooling) in the progression to the last glacial maximum have been ascribed to a variety of processes  operating on a ‘millennial’ scale. However, ocean-floor sediment cores are full of lesser fluctuations in IRD, back to at least 1.7 Ma ago. That record offers a better chance of explaining fluctuations in ice-sheet ablation. A joint European-US group has investigated their potential over the last decade or so (Barker, S. et al. 2022. Persistent influence of precession on northern ice sheet variability since the early Pleistocene. Science, v. 376, p. 961-967; DOI: 10.1126/science.abm4033). The authors noted that in each glacial cycle since 1.7 Ma the start of ice rafting consistently occurred during a time of decreasing axial obliquity. Yet the largest ablation events were linked to minima in the precession cycles. In the last 700 ka, such extreme events are associated with the terminations of each ice age.

In the earlier part of the record, the 41 ka obliquity ‘signal’ was sufficient to drive glacial-interglacial cycles, hence their much greater regularity and symmetry than those that followed the Mid-Pleistocene Transition. The earlier ice sheets in the Northern Hemisphere also had consistently smaller extents than those after the MPT. Although the records show a role for precession in pre-MPT times in the form of ice-rafting events, the lesser effect of precession on summer warming at higher latitudes, compared with that of axial obliquity, gave it no decisive influence. After 700 ka the northern ice sheets extended much further south – as far as 40°N in North America – where summer warming would always have been commensurately greater than at high northern latitudes. So they were more susceptible to melting during the increased summer warming driven by the precession cycles. When maximum summer heating induced by axial precession in the Northern Hemisphere coincided with that of obliquity the ice sheets as a whole would have become prone to catastrophic collapse.

It is hard to say whether these revelations have a bearing on future climate. Of course, astronomical forcing will continue relentlessly, irrespective of anthropogenic greenhouse gas emissions. Earth has been in an interglacial for the last 11.5 ka, since the Younger Dryas; i.e. about half a precession cycle ago. The combination of obliquity- and precession-driven influences suggest that climate should be cooling and has been since 6,000 years ago, until the Industrial Revolution intervened. Can the gravitational pull of the giant planets prevent a runaway greenhouse effect, or will human effects defy astronomical forces that continually distort Earth’s astronomical behaviour?

Climate and tectonics since 250 Ma

A central feature of the Earth’s climate system is the way that carbon bound in two gases – carbon dioxide (CO2) and methane (CH4) – controls the amount of incoming solar energy that is retained by the atmosphere. Indeed, without one or the other our home world would have been locked in frigidity since shortly after its formation: a sterile, ice-covered planet. The ‘greenhouse effect’ has been ever-present because the material from which the Earth accreted contained carbon as well as every other chemical element from hydrogen to uranium. Naturally reactive, it readily combines with hydrogen and oxygen to form methane and carbon dioxide, which would have escaped the inner Earth as gases to enter the earliest atmosphere as a ‘comfort blanket’, along with water vapour, another greenhouse gas.  Their combined effects have remained crudely balanced so that neither inescapable frigidity nor surface temperatures high enough to boil-off the oceans have ever occurred in the last 4.5 billion years. Earth has remained like the wee bear’s porridge in the Goldilocks story! Even so, global climate has fluctuated again and again from that akin to a steamy greenhouse, through long periods of moderation to extensive glacial conditions, including three that extended from pole-to-pole – ‘Snowball’ Earths –  during in the Precambrian. During the Phanerozoic the Earth has entered three long periods of generally low global temperatures, in the Ordovician, the Carboniferous and during the last 2.5 Ma  that allowed polar ice caps and sea-ice to extend a third of the way to the Equator. These were forced back and forth repeatedly by cyclical influences apparently triggered by astronomically controlled changes to Earth’s orbital and rotational parameters – the Milankovich Effect. Anthropogenic emissions of greenhouse gases in vast and increasing amounts now threaten to disrupt natural climate variation, effectively overthrowing the gravitational influences of distant giant planets that have controlled climate changes that shaped our own evolution since the genus Homo first emerged.

Bubbles of air trapped in cores through the ice sheets of Antarctica and Greenland record decreased volumes of land ice as CO2 content increased and the opposite during glacial episodes. Somehow in step with the astronomical forcing the Earth released greenhouse gas to warm the climate and drew it down to bring on cooling. Since all life forms are built from carbon-rich compounds and some extract it from the environment to build carbonate hard parts, climate and life on land and in the oceans are interlinked. In fact life and death are involved, because once dead organisms and their hard parts are buried before being oxidised in sediments on land, as in peat and ultimately coal, and on the ocean floors as limestones or carbonaceous mudstones, atmospheric carbon is sequestered. Exposed to acid water containing dissolved CO2 from the atmosphere or to oxygen, respectively, the two forms of carbon in solid form are released as greenhouse gas once more. Both take place when sedimentary deposits are exhumed as a result of erosion and tectonics. Another factor is the abundance of available nutrients, themselves released and distributed by erosion and agents of transportation. At present surface waters of the most distant parts of the oceans contains plenty of such nutrients, except for a vital one, dissolved iron. So they are wet ‘deserts’. It seems that during the much dustier times of glacial episodes iron in fine form reached far out into the world’s oceans so that phytoplankton at the base of the food chain ‘bloomed ‘and so did planktonic animals. Dead organisms ‘rained’ to the ocean floor so drawing down CO2 from the atmosphere and decreasing the greenhouse effect. The surface parts of the carbon and rock cycles are extremely complex and climatologists have yet to come to grips with modelling its future climates convincingly. Yet the carbon cycle and much deeper parts of the rock cycle are interwoven too.

Carbon in sedimentary rock can be heated by burial, and some can be subducted to great depths at destructive plate margins together. The same applies to in ocean-floor basalts that have been permeated by circulating sea water through hydrothermal circulation to form carbonates in the altered volcanic rock. In both cases carbon stored for hundreds of million years can be released by metamorphism in orogenic belts at zones of continental collision and deep below island arcs. Carbon from mantle depths that has never ‘seen the light of day’ is also added to the atmosphere when magmas form below oceanic constructive margins, hot spots and subduction zones, and where magmas flood the continental surface. Consequently, plate tectonics and deep mantle convection have surely played a long-term role in the evolution of our planet’s climate system. Geoscientists based in Australia and the UK have used geochemical data to reconstruct the stores of carbon in oceanic plates and thermodynamic modelling to track what may have happened to it and the climate through the last 250 Ma (Müller, R.D. et al. 2022. Evolution of Earth’s tectonic carbon conveyor belt. Nature, v. 605, p. 629-639; DOI: 10.1038/s41586-022-04420-x). Their review is an important step in understanding what underpins climate on a geological time scale, onto which much shorter-term surface influences are superimposed.

The amount of carbon being outgassed as CO2 each year along plate boundaries in the early Jurassic (185 Ma) shown in dark purple (low) to yellow (high). Also shown in shades of blue is the accumulation of carbon stored in each square metre of the ocean plates. Plate motions are shown as grey arrows (credit: Müller, R.D. et al. Clip from video in Supplementary Information)

At mid-ocean ridges basaltic magma wells up from mantle depths and loses much of its content of dissolved CO2. The annual outgassing at ridges, which depends on the global rate of plate formation, has varied from 13 to 30 million tonnes of carbon  (MtC yr-1) since the start of the Mesozoic Era 250 Ma ago. Similarly, there is greenhouse-gas escape from volcanic arcs above subduction zones, estimated to have ranged from 0 to 18 MtC yr-1. As an oceanic plate moves away from its source various processes sequester CO2 into the oceanic crust and upper mantle through accumulation of deep-sea sediments and hydrothermal alteration of basaltic crust and peridotite mantle (ranging from 30 to 311 MtC yr-1). Of this influx of carbon into oceanic plates between 36 to 103 MtC yr-1 has gone down subduction zones in descending slabs. Between 0 to 49 MtC yr-1 of that has been outgassed by arc volcanic activity or absorbed into the overriding plate. The rest continues down into the deep mantle, perhaps to form diamonds. Overall, when the rate at which oceanic plates grow is rapid and plate motion speeds up, outgassing should be high. When plate growth slows, so does the rate of CO2 release. Variations in plate growth can be estimated from the magnetic reversal stripes above the ocean floors.  The authors have released an animation of the break-up of Pangaea (well worth watching at full screen – you can skip the ad at the start), with the rate of carbon emission at ridges and volcanic arcs being colour-coded. Also shown is the storage of carbon within oceanic plats plates as time passes.

Length of mid-ocean ridges (orange) and subduction zones (blue) through the last 250 Ma (top). The areas of oceanic crust produced at ridges and consumed by subduction (bottom) (credit: Müller, R.D. et al., Figs 1a, 1c)

Before Pangaea began to break up at the end of the Triassic (200 Ma) the total length of mid-ocean ridges was at a minimum of about 40 thousand km. Through the Jurassic it never exceeded 50,000 km, but rose to a maximum of 80,000 km during the Cretaceous then declined slowly to the current length of 60,000 km. Throughout the last 250 Ma the length of subduction zones stayed roughly the same at about 65 thousand km – not always in the same places – although the overall rate of subduction changed in line with the rate of oceanic plate growth  (the volume that is added must be balanced roughly by the amount that returns to the mantle).  Between the end of the Jurassic and the mid-Cretaceous crustal production and destruction doubled, shown by the bottom plot in the figure above. The very fast  movement of plates and an increase in the global length of ridges during Jurassic to mid-Cretaceous times led to a dramatic increase in CO2 outgassing from ridges so that its content in the atmosphere rose as high as 1200 ppm – more than four times that before the Industrial Revolution. That level resulted in global ‘hothouse’ conditions during the Cretaceous. Another factor behind the Cretaceous climate was a decrease in the global complement of mountains. That led to decreases in erosion and the weathering of silicates by acid rain, thus reducing natural sequestration of carbon.

During the Cenozoic (after 65 Ma) declining ridge outgassing was actually outpaced by that associated with subduction, according to the modelling. That is strange, for by around 35 Ma glaciation had begun  on Antarctica as the Earth was cooling, which implies a major, unexpected sink for excess CO2. The most likely way this might have arisen is through increased erosion and silicate weathering on the exposed continents that consumed CO2 faster than tectonics was releasing the gas. The length of continental arcs shows no sign of a major increase during the Cenozoic, which might have accelerated that kind of sequestration, but a variety of proxies for signs of weathering definitely suggests that there was an upsurge. Also there was increased storage of carbon on the deep ocean floor, shown by the video. Increased calcium released by weathering to enter ocean water in solution would allow more planktonic organisms to secrete calcite (CaCO3) skeletons that would then fall to the ocean floor when they died.

There may be more to be discovered in this hugely complex interplay between tectonics and climate. For instance, when the bottom waters of the oceans are oxygenated by deep currents of cold dense seawater sinking from polar regions, carbon in tissues of sunken dead organism is oxidised to release CO2. If bottom waters are anoxic, this organic carbon is preserved in sediments. The authors mention this as something to be considered in their future work on  the ‘tectonic carbon conveyor belt’.

Wider traces of the elusive Denisovans

We know that when anatomically modern humans (AMH) arrived in Asia they shared the landscape with ‘archaic’ humans that had a much longer pedigree. In 2010 an individual’s little-finger bone dated to around 30 to 49 ka old was found in the Denisova Cave in central Siberia (at 50°N). It yielded a full genome that was distinctly different from those of AMH and Neanderthals (see: Other rich hominin pickings; May 2010). Four other fossils found subsequently in the Denisova Cave contained similar DNA. Checking the DNA of living humans and fossil Neanderthal remains revealed that the newly discovered human group had interbred with both. In the case of AMH, segments of Denisovan DNA are found in the genomes of indigenous people living in East and South Asia, Australia, the Pacific Islands and the Americas, at levels of 0.2%, rising to 6% in Melanesian people of Papua-New Guinea. But such introgressions have not been found in Europeans (but see below), suggesting that the Denisovans were restricted to Asia.

There have been suggestions that at least some of the ‘archaic’ human remains found widely and abundantly in China may have been Denisovans; although they might equally be of Homo erectus. But none of the Chinese fossils have been subjected to gene sequencing – those found in caves outside tropical and sub-tropical climates might retain DNA just as well as Neanderthal and even older remains in temperate Europe. Yet a partial lower jaw discovered in a cave on the Tibetan Plateau (at 35°N) did yield proteins that had close affinities to those recovered from Siberian Denisovans. Now similar analyses have been performed on an abnormally large molar found in a cave in Northern Laos, showing that it too is most likely to be from a young (as suggested by its being little worn), possibly female (it lacks male-specific peptides), Denisovan. The locality lies at about 20°N, far to the south of the other two Denisovan sites (Demeter, F. et al. A Middle Pleistocene Denisovan molar from the Annamite Chain of northern Laos. Nature Communications, v. 13, article 2557; DOI: 10.1038/s41467-022-29923-z). Sparse as the evidence is, Denisovans were able to tolerate climate differences across 30 degrees of latitude.

A probable Denisovan molar from 164 to 131 ka old cave sediments in northern Laos. (credit: Demeter, et al.; Fig. 2)

The Wikipedia entry for Denisovans is a mine of additional information. For instance, detailed analysis of the roughly 5% of their genome that indigenous people of New Guinea carry suggests that the two groups may have interbred there as late as 30 ka. Since Both New Guinea and Australia were until 8 thousand years ago part of the Sahul landmass when sea level was low during the last ice age, these inferences add tropical occupancy to the Denisovan range. Does this suggest that Papuans and indigenous Australians migrated with Denisovans, or had the latter crossed the sea from Timor earlier and independently, after moving from Asia by ‘hopping’ from island to island through eastern Indonesia? There is a possibility that Denisovans could even have survived in Sahul until as late as 14.5 ka. Even more odd, modern Icelandic people are unique among Europeans in having detectable traces of Denisovan DNA. However, rather than having been directly shared between Denisovans and ancestral Scandinavians – a possibility – it may have been carried by Neanderthal-Denisovan hybrids migrating westwards from Siberia with whom the Icelanders’ ancestors interbred. There are other interesting points in the Wikipedia entry. One is that the consistently lower Denisovan ancestry in living East Asians compared with people of Oceania, may indicate two separate waves of eastward migration by AMH. The latter may have arrived first, had greater contact with Denisovans and then moved on across seaways to remain isolated from the later migrants.

Finally, something that puzzles me as a non-geneticist. If both Denisovans and Neanderthals died out as genetically distinct groups tens of millennia ago how could the genetic traces of interbreeding with AMH have been retained at such high levels until the present; i.e. through thousands of generations? Each of us carries a 50% deal of genes from our parents. Then with each subsequent generation the proportion is diluted, so that we inherit 25% from grandparents, 12.5 % from great-grandparents and so on. Yet Papuans still have 5 to 6 percent of Denisovan DNA: much the same holds for Europeans’ Neanderthal heritage. Does such a high level of retention of this ancestry suggest that a large proportion of the earliest migrating AMH individuals stemmed from generation to generation interbreeding on a massive scale? Did the ‘newcomers’ and ‘locals’ band eventually together almost completely to merge genetically, or am I missing something … ? Probably

The end of the Carboniferous ‘icehouse’ world

From about 340 to 290 Ma the Earth experienced the longest episode of repeated ice ages of the Phanerozoic. The climate then was similar in many ways to that of the Pleistocene. The South Polar region was then within the Pangaea supercontinent and thus isolated from any warming effect from the surrounding ocean: much the same as modern Antarctica but on a much larger scale. Glaciation extended as far across what became the southern continents and India as did the continental ice sheets of the Northern Hemisphere during Pleistocene glacial maxima. Tropical sedimentary rocks of the time, display evidence for repeated alternations of high and low sea levels that mark cycles of glacial maxima and interglacial episodes akin to those of the Pleistocene. In fact they probably reflect the influence of changes in the Earth’s orbit and geometry of its axis of rotation very similar to those predicted by Milankovich from astronomical factors to explain Pleistocene climatic cycles. At the end of the Carboniferous what was an ‘ice-house’ world changed suddenly to its opposite – ‘greenhouse’ conditions – that persisted through the Mesozoic Era until the later part of the Cenozoic, when Antarctica developed is ice cap and global climate slowly cooled to become extremely cyclical once again.

Sedimentary evidence for global climates 320 Ma ago. As well as the large tracts of glaciogenic sediments, smaller occurrences and examples of polished rock surfaces over which ice had passed show the probable full extent (blue line) of ice sheets across the southern, Gondwana sector of Pangaea (Credit: after Fig 7.3, S104, Earth and Space, ©Open University 2007)

The end of the Carboniferous witnessed the collapse of the vast Equatorial rainforests, which formed the coal deposits that put ‘Carbon’ into the name of the Period. By its end this ecosystem had vanished to result in a minor mass extinction of both flora and fauna. Temperatures rose and aridity set in, to the extent that the latest Carboniferous in the British coalfields is marked by redbeds that presage the spread of desert conditions across the Equatorial parts of Pangaea during the succeeding Permian. A team of researchers based at the University of California at Davis have been studying data pertaining to this sudden change have now published their findings (Chan J. and 17 others 2022. Marine anoxia linked to abrupt global warming during Earth’s penultimate icehouse. Proceedings of the National Academy of Sciences, v. 119, article e2115231119; DOI: 10.1073/pnas.2115231119). They used carbon-, oxygen- and uranium isotopes, together with proxies for changes in atmospheric CO2 concentrations, to model changes in the carbon cycle in the Late Carboniferous of China.

Changes in uranium isotopes within marine carbonates are useful indicators of the amount of oxygen available in ocean water at the sea floor. Between 304 and 303.5 Ma ago oxygen content declined by around 30%, the peak of this anoxia being at 303.7 Ma. This occurred about 100 ka after atmospheric CO2 had risen to ~700 parts per million (ppm) from around 350 ppm in the preceding 300 ka, as marked by several proxies.  The authors suggest that the lower ‘baseline’ for the main greenhouse gas marked an extreme glacial maximum. Changes in the proportions of 18O relative to ‘lighter’ 16O in fossil shells suggest that sea-surface temperatures increased in step with the doubling of the greenhouse effect. At the same time there was a major marine transgression as sea level rose. This would have been accompanied by a massive increase in low density freshwater in surface ocean water derived from melting of Pangaea’s ice cap. The team suggests that the freshened surface layer could not sink to carry oxygen to deeper levels, thereby creating anoxic conditions across an estimated 23% of the global seafloor, and thus toxic ‘death zones’ for marine organisms.

One possibility for this sudden rise of atmospheric CO2 is a massive episode of volcanism, perhaps a large igneous province, but there is scanty evidence for that at the end of the Carboniferous. A coinciding sharp decrease in δ13C  in carbonate shells suggests that the excess carbon dioxide probably had an organic origin. So a more plausible hypothesis is massive burning on the continental surface. In the tropics, the huge coals swamps would have contained vast amounts of peat-like decayed vegetable matter as well as living green vegetation. How might that have caught fire? The peat precursor to Carboniferous coal deposits derived from photosynthesis on an unprecedented, and never repeated, scale during tens of million years of thriving tropical rain forest during that Period. This built up atmospheric oxygen levels to about 35%, compared with about 21% today. Insects, whose maximum size is governed by their ability to take in oxygen through spiracles in their bodies, and by the atmospheric concentration of oxygen, became truly huge during the earlier Carboniferous. The more oxygen in the air, the greater the chance that organic matter will catch fire. In fact wet vegetation can burn if oxygen levels rise above 25%. At the levels reached in the Carboniferous huge wildfires in forests and peatlands would have been inevitable. Evidence that huge fires did occur comes from the amount of charcoal found in Carboniferous coal seams, which reach 70% compared with the 4 to 8 % in more recent coals. They may have been ignited by lightning strikes or even spontaneous combustion if decay of vegetation generated sufficient heat, as sometimes happens today in wet haystacks or garden compost heaps.  But how in a short period around 304 Ma could 9 trillion tons of carbon dioxide be released in this way. The preceding  glacial super-maximum, like glacial maxima of the Pleistocene, may have been accompanied by decreased atmospheric humidity: this would dry out the vast surface peat deposits.

The succeeding Permian is famous for its extensive continental redbeds, and so too those of the Triassic. They are red because sediment grains are coated in the iron oxide hematite (Fe2O3). As on Mars, the redbeds are a vast repository for oxygen sequestered from the atmosphere by the oxidation of dissolved Fe2+ to insoluble Fe3+. This had been going on throughout the Permian, the nett result being that by 250 Ma atmospheric oxygen content has slumped to 16% and remained so low for another 50 million years. Photosynthesis failed to resupply oxygen against this inorganic depletion, and there are few coal deposits of Permian or Triassic age: for about 100 Ma Earth ceased to have green continents.

See also: Carbon, climate change and ocean anoxia in an ancient icehouse world. Science Daily, 2 May 2022.