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. 

Hunting down the Tully Monster

The word ‘monster’ has its origin in the Latin monere ‘to warn’ but has broadened out in its usage.  It has even reverted to its origins as a verb: a highly critical, verbal attack. But I prefer ‘something about which one needs to be warned’, and the Tully Monster encapsulates that meaning. It once lived in Illinois, specifically at just a single location, Mazon Creek, where thousands of them have been seen. But should you be especially fearful of Tullimonstrum gregarium? Well, at first sight, no; it’s only about 10 cm long and apparently has no proper bones and it’s dead. The first was spotted in a coal-mine waste heap by Francis Tully in 1958, a pipefitter with an interest in Carboniferous fossils. Two years after his death in 1987, he and his monster were honoured by a bill that the Illinois State Legislature passed to make it the official State Fossil.

Artist's impression of the Carboniferous Tully Monster (
Artist’s impression of the Carboniferous Tully Monster (Tullimonstrum gregarium) (credit: Sean McMahon, Yale University)

It seems to have become a ‘monster’ by stumping all previous attempts to categorise it; so much so that it long served as a warning to eager palaeontologists not to tangle with its taxonomy. That’s not surprising, because as well as bearing a passing resemblance to Captain Nemo’s submarine in Jules Verne’s 20 000 leagues Under the Sea, it has some truly astonishing features.  Portholes down its sides are not the weirdest – actually they are gill openings. It has a biting apparatus at the end of an absurdly lengthy forward protuberance, that would not be unexpected if it were one of those fish from the Amazon that, you know, men really ought to be warned about. Most of us would not share a bath with it if we had been. And then, there are the eyes on the ends of a dorsal bar which would give Tullimonstrum gregarium superb stereoscopic vision to guide it unerringly to its target, lashing its efficient-looking caudal fin. The fact that it has only a single nostril is merely puzzling by comparison.

Six decades on, Victoria McCoy of Yale University (now at Leicester University, UK) and 15 undeterred colleagues have pored over more than 1200 Tully Monster fossils and seem to have cracked its affinities (McCoy, V.E. et al. 2016. The ‘Tully monster’ is a vertebrate. Nature, v. 532, p. 496-499). In fact, it’s surprising that it has remained an enigma for so long, because McCoy and colleagues have documented almost every aspect of its anatomy, available from a huge number of superbly preserved specimens – teeth, fin, muscle traces, gills, nostril, notochord, gut and so on. As well as being a vertebrate, its dreadful proboscis is very like that of the Cambrian oddity Opabinia from the Burgess Shale. A  separate study by four British palaeontologists and a Texan concentrated on the eyes using electron microscopy and found ‘ultrastructural details’, including pigment cells (Clements, T. et al. 2016. The eyes of Tullimonstrum reveal a vertebrate affinity. Nature, v. 532, p. 500-503) which unequivocally confirm that it is a vertebrate. It has all the hallmarks of being related to lampreys and hagfishs. They devour rotting, drowned corpses.

Very persistent cycles

Carboniferous shale
Carboniferous shale (Photo credit: tehsma)

The last of five written papers in my 1967 final-year exams was, as always, set by the ‘Prof’.  One question was ‘Rock and rhythm: discuss’ – it was the 60s. Cyclicity has been central to observational geology, especially to stratigraphy, the difference from that era being that rhythms have been quantified and the rock sequences they repeat have been linked to processes, in many cases global ones. The most familiar cyclicity to geologists brought up in Carboniferous coalfields, or indeed any area that preserves Carboniferous marine and terrestrial rocks, is the cyclothem of, roughly, seat-earth – coal – marine shale – fluviatile sandstone – seat-earth and so on. Matched to the duration of Carboniferous to Permian glaciations of the then southern hemisphere, and with the relatively  new realisation that global sea level goes down  and up as ice caps wax and wane, the likeliest explanation is eustatic regression and transgression of marine conditions in coastal areas in response to global climate change. Statistical analysis of cyclothemic sequences unearths frequency patterns that match well those of astronomical climate forcing proved for Pleistocene glacial-interglacial cycles.

The Milankovich signals of the Carboniferous are now part of the geological canon, but rocks of that age more finely layered than sediments of the tropical continental margins do occur. Among them are rhythmic sequences interpreted as lake deposits from high latitudes, akin to varves formed in such environments nowadays. Those from south-western Brazil present spectacular evidence of climate change in the Late Carboniferous and Early Permian (Franco, D.R. et al. 2012. Millennial-scale climate cycles in Permian-Carboniferous rhythmites: Permanent feature throughout geological time. Geology, v. 40, p. 19-22). They comprise couplets of fine-grained grey quartz sandstones from 1-10 cm thick interleaved with black mudstones on a scale of millimetres, which together build up around 45 m of sediment. Their remanent magnetism and magnetic susceptibility vary systematically with the two components. Frequency analysis of plots of both against depth in the sequence show clear signs of regular repetitions. Low-frequency peaks reveal the now well-known influence of astronomical forcing of Upper Palaeozoic climate, but it is in the lower amplitude, higher frequency part of the magnetic spectrum that surprises emerge from a variety of peaks. They are reminiscent of the Dansgaard-Oeschger events of the last Pleistocene glacial, marked by sudden warming and slow cooling while world climate cooled towards the last glacial maximum (~1.5 ka cyclicity) and Heinrich events, the ‘iceberg armadas’ that occurred on a less regular 3 to 8 ka basis. There are also signs of the 2.4 ka solar cycle. The relatively brief cycles would have been due to events in a very different continental configuration from today’s – that of the supercontinent Pangaea – and their very presence suggests a more general global influence over short-term climate shifts that has been around for 300 Ma or more.

OSTM/Jason-2's predecessor TOPEX/Poseidon caug...
El Niño effect on sea -surface temperatures in the eastern Pacific Ocean. Image via Wikipedia

Closer to us in time, and on a much finer time scale are almost 100 m of finely laminated shales from the marine Late Cretaceous of California’s Great Valley (Davies, A. et al. 2012. El Niño-Southern Oscillation variability from the late Cretaceous Marca Shale of California. Geology, v. 40, p. 15-18). The laminations contain fossil diatoms: organisms that are highly sensitive to environmental conditions and whose species are easily distinguished from each other. It emerges from studies of the diatoms in each lamination set that they record an annual cycle of seasonal change related to marine upwellings and their varying strengths, with repeated evidence for influx of fine sediment derived from land above sea level and for varying degrees of bioturbation that suggests periods of oxygenation. Spectral analysis of the intensity of bioturbation, which assumes the lamina are annual, and other fluctuating features reveals peaks that are remarkably close to those of the ENSO cyclicity that operates at present, at 2.1-2.8 and 4.1-6.3 a, as well as repetitions with a decadal frequency.

The annual cycles bear similar hallmarks to those imposed by the monsoonal conditions familiar from modern California, which fluctuated in the Late Cretaceous in much the same way as it does now – roughly speaking, alternating El Niño and La Niña conditions. That is not so surprising, as the relationship between California and the Pacific Ocean in the Cretaceous would not have been dissimilar from that now. The real importance of the study is that it concerns a period in Earth’s climate history characterised by greenhouse conditions, that some predict would create a permanent El Niño – an abnormal warming of surface ocean waters in the eastern tropical Pacific that prevents the cold Humboldt Current along the Andean coast of South America from supplying nutrient to tropical waters. The very cyclicity recorded by the Marca Shale strongly suggests that the ENSO is a stable feature of the western Americas. Recent clear implications of ENSO having teleconnections that affect global climate, on this evidence, may not break down with anthropogenic global warming. This confirms similar studies from the Palaeogene and Neogene Periods.