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. 

An early oxygenated atmosphere

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.