How flowering plants may have regulated atmospheric oxygen

Ultimately, the source of free oxygen in the Earth System is photosynthesis, but that is the result of a chemical balance in the biosphere and hydrosphere that operates at the surface and just beneath it in sediments. Burial of dead organic carbon in sedimentary rocks allows free oxygen to accumulate whereas weathering and oxidation of that carbon, largely to CO2, tends to counteract oxygen build-up. The balance is reflected in the current proportion of 21% oxygen in the atmosphere. Yet in the past oxygen levels have been much higher. During the Carboniferous and Permian periods it rose dramatically to an all-time high of 35% in the late Permian (about 250 Ma ago). This is famously reflected in fossils of giant dragonflies and other insects from the later part of the Palaeozoic Era.  Insects breathe passively by tiny tubes (trachea) through whose walls oxygen diffuses, unlike active-breathing quadrupeds that drive air into lung alveoli to dissolve O2 directly in blood. Insect size is thus limited by the oxygen content of air; to grow wing spans of up to 2 metres a modern dragon fly’s body would consist only of trachea with no room for gut; it would starve.

Woman holding a reconstructed Late Carboniferous dragonfly (Namurotypus sippeli)

During the early Mesozoic oxygen fell rapidly to around 15% during the Triassic then rose through the Jurassic and Cretaceous Periods to about 30%, only to fall again to present levels during the Cenozoic Era. Incidentally, the mass extinction at the end of the Cretaceous (the K-Pg boundary event) was marked in the marine sedimentary record by unusually high amounts of charcoal. That is evidence for the Chixculub impact being accompanied by global wild fires that a high-oxygen atmosphere would have encouraged. The high oxygen levels of the Cretaceous marked the emergence of modern flowering plants – the angiosperms. Six British geoscientists have analysed the possible influence on the Earth System of this new and eventually dominant component of the terrestrial biosphere. (Belcher, C.M. et al. The rise of angiosperms strengthened fire feedbacks and improved the regulation of atmospheric oxygenNature Communications, v. 12, article 503; DOI 10.1038/s41467-020-20772-2)

The episodic occurrence of charcoal in sedimentary rocks bears witness to wildfires having affected terrestrial ecosystems since the decisive colonisation of the land by plants at the start of the Devonian 420 Ma ago. Fire and vegetation have since gone hand in hand, and the evolution of land plants has partly been through adaptations to burning. For instance the cones of some conifer species open only during wildfires to shed seeds following burning. Some angiosperm seeds, such as those of eucalyptus, germinate only after being subject to fire . The nature of wildfires varies according to particular ecosystems: needle-like foliage burns differently from angiosperm leaves; grassland fires differ from those in forests and so on. Massive fires on the Earth’s surface are not inevitable, however. Evidence for wildfires is absent during those times when the atmosphere’s oxygen content has dipped below an estimated 16%. The current oxygen level encourages fires in dry forest during drought, as those of Victoria in Australia and California in the US during 2020 amply demonstrated. It is possible that with oxygen above 25% dry forest would not regenerate without burning in the next dry season. Wet forest, as in Brazil and Indonesia, can burn under present conditions but only if set alight deliberately. Evidence of a global firestorm after the K-Pg extinction implies that tropical rain forest burns easily when oxygen is above 30%. So, how come the dominant flora of Earth’s huge tropical forests – the flowering angiosperms – evolved and hung on when conditions were ripe for them to burn on a massive scale?

Early angiosperms had small leaves suggesting small stature and growth in stands of open woodland [perhaps shrubberies] that favoured the fire protection of wetlands. ‘Weedy’ plants regenerate and reach maturity more quickly than do those species that are destined to produce tall trees. With endemic wildfires, tree-sized plants – e.g. the gymnosperms of the Mesozoic – cannot attain maturity by growing above the height of flames. Diminutive early angiosperms in a forest understory would probably outcompete their more ancient companions.  Yet to become the mighty trees of later rain forests angiosperms must somehow have regulated atmospheric oxygen so that it declined well below the level where wet forest is ravaged by natural wild fires. The oldest evidence for angiosperm rain forest dates to 59 Ma, when perhaps more primitive tropical trees had been almost wiped-out by wildfires. Did angiosperms also encourage wildfires, that consumed oxygen on a massive scale, as well as evolving to resist their affects on plant growth? Claire Belcher et al. suggest that they did, through series of evolutionary steps. Key to their stabilising oxygen levels at around 21%, the authors allege, was angiosperms’ suppression of weathering of phosphorus from rocks and/or transfer of that major nutrient from the land to the oceans. On land nitrogen is the most important nutrient for biomass, whereas phosphorus is the limiting factor in the ocean. Its reduction by angiosperm dominance on land thereby reduces carbon burial in ocean sediments. In a very roundabout way, therefore, angiosperms control the key factor in allowing atmospheric build-up of oxygen; by encouraging mass burning and suppressing carbon burial.  Today, about 84 percent of wildfires are started by anthropogenic activities. As yet we have little, if any, idea of how such disruption of the natural flora-fire system is going to affect future ecosystems. The ‘Pyrocene’ may be an outcome of the ‘Anthropocene’ …

The Cambrian Explosion: a broader view

The base of the Cambrian has long been defined as the level where abundant shelly fossils and most phyla first occur in the stratigraphic record. That increase in diversity led to the nickname ‘Cambrian Explosion’, despite the fact that sheer numbers and diversity of lesser taxa took a long time to rise to ‘revolutionary’ levels. Yet a great deal of animal evolution was going on during the preceding Proterozoic Era that was revealed once palaeobiological research blossomed in rocks of that age range. Today, the earliest occurrences, or at least hints, of quite a few phyla can be traced to the last 100 Ma of the Precambrian. Clearly, the Cambrian Explosion needs a fresh look now that so many data are in. Any palaeontologist would benefit from reading a Perspective article in the latest issue of Nature Ecology & Evolution (Wood, R. and 8 others 2019. Integrated records of environmental change and evolution challenge the Cambrian Explosion. Nature Ecology & Evolution, v. 3, online publication; DOI: 10.1038/s41559-019-0821-6)

Rachel Wood of Edinburgh University and co-authors working elsewhere in Britain, Canada, Japan and Finland sift the growing wealth of fossil and trace-fossil evidence that predate the start of the Cambrian. They also consider the geochemical events that stand out in the Ediacaran Period that succeeds the Snowball Earth events of the Cryogenian. Their account recognises that the geochemical changes – principally a series of carbon-isotope (δ13C) excursions – may have resulted from tectonic changes. The carbon-isotope data mark a series of short-lived penetrations of oxygen-rich conditions deep into the ocean water column and longer periods of oxygen-starved deep water. Such perturbations in oceanic redox conditions ‘speed-up’ thorough the late-Ediacaran into the Cambrian: a profound and protracted transition from the Neoproterozoic world to that of the Phanerozoic. Over the same time span there is a ‘progressive addition of biological novelty’ in the form and function of the evolving biota, so that  each successive assemblage builds on the earlier advances.

The fossil evidence suggests that the earliest Ediacaran fauna was metazoan but with no sign of bilaterian affinities (i.e. having ‘heads’ and ‘tails’). The rise of bilaterians of which most animal phyla are members occupied the later Ediacaran , with the first evidence of locomotion – and almost by definition animals with ‘fore’ and ‘aft’ – being around 560 Ma. Each discrete shift from more to less oxic conditions in the oceans seems to have knocked-back animal life, the reverse being accompanied by diversification of survivors. Oxygenation at the very start of the Cambrian marked the beginnings of a diversification clearly manifested by animals capable of biomineralisation and the secretion of hard parts with clear patterns. Such ‘shelly faunas’ are present in the latest Ediacaran sediments but with a multiplicity of seemingly arbitrary forms, although trace fossils suggest soft-bodied animals did have definite morphological pattern.

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Diorama of the Lower Cambrian Qingjiang fauna (Credit: Fu et al. 2019; Fig 4)

Adding yet more information to early metazoan history is the recently discovered Cambrian Qingjiang lagerstätte of Hubei Province in southern China dated at 518 Ma; similar in its exquisite preservation to the Burgess (508 Ma) and Chengjiang (518 Ma) biotas (Fu, D. and 14 others 2019. The Qingjiang biota—A Burgess Shale-type fossil Lagerstätte from the early Cambrian of South China. Science, v. 363, p. 1338-1342; DOI: 10.1126/science.aau8800). The two previously discovered Cambrian lagerstättes are notable for their very diverse arthropod and sponge faunas. That at Qingjiang adds an abundance of cnidarians, jellyfish, sea anemones, corals and comb jellies, rare in the other two biotas, plus kinorhynchs or mud dragons – moulting invertebrates known only from Cambrian and modern sediments. The fossils at Qingjiang include only about 8% of the taxa of the same age found at Chengjiang, suggesting different environments

The idea of a sudden, discrete explosive event in the history of life, which coincided with the start of the Cambrian, now seems difficult to support. This should not damage the status of 541 Ma as the start of the Phanerozoic because stratigraphy basically gives form to the passage of time and has done since its emergence in the 19th century, so keeping the names of the divisions is essential to continuity.

Related articles: Daley, A.C. 2019. A treasure trove of Cambrian fossils. Science, v. 363, p. 1284-1285; DOI: 10.1126/science.aaw8644. Switek, B. 2019. Fossil Treasure Trove of Ancient Animals Unearthed in China (Smithsonian.com)

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Oxygen, magnetic reversals and mass extinctions

In April 2005 EPN reported evidence for a late Permian fall in atmospheric oxygen concentration to about 16% from its all-time high of 30% in the Carboniferous and earlier Permian.. This would have reduced the highest elevation on land where animals could live to about 2.7 km above sea level, compared with 4 to 5 today. Such an event would have placed a great deal of stress on terrestrial animal families. Moreover, it implies anoxic conditions in the oceans that would stress marine animals too. At the time, it seemed unlikely that declining oxygen was the main trigger for the end-Permian mass extinction as the decline would probably have been gradual; for instance by oxygen being locked into iron-3 compounds that give Permian and Triassic terrestrial sediments their unrelenting red coloration. By most accounts the greatest mass extinction of the Phanerozoic was extremely swift.

The possibility of extinctions being brought on by loss of oxygen from the air and ocean water has reappeared, though with suggestion of a very different means of achieving it (Wei, Y. and 10 others 2014. Oxygen escape from the Earth during geomagnetic reversals: Implications to mass extinction. Earth and Planetary Science Letters, v. 394, p. 94-98). The nub of the issue proposed by the Chinese-German authors is the dissociation and ionization by solar radiation of O2 molecules into O+ ions. If exposed to the solar wind, such ions could literally be ‘blown away’ into interplanetary space; an explanation for the lack of much in the way of any atmosphere on Mars today. Mars is prone to such ionic ablation because it now has a very weak magnetic field and may have been in that state for 3 billion years or more. Earth’s much larger magnetic field diverts the solar wind by acting as an electromagnetic buffer against much loss of gases, except free hydrogen and to a certain extent helium. But the geomagnetic field undergoes reversals, and while they are in progress, the field drops to very low levels exposing Earth to loss of oxygen as well as to dangerous levels of ionising radiation through unprotected exposure of the surface to the solar wind.

Artist's rendition of Earth's magnetosphere.
Artist’s rendition of Earth’s magnetosphere deflecting the solar wind. (credit: Wikipedia)

Field reversals and, presumably, short periods of very low geomagnetic field associated with them, varied in their frequency through time. For the past 80 Ma the reversal rate has been between 1 and 5 per million years. For much of the Cretaceous Period there were hardly any during a magnetic quiet episode or superchron. Earlier Mesozoic times were magnetically hectic, when reversals rose to rates as high as 7 per million years in the early Jurassic. This was preceded by another superchron that spanned the Permian and Late Carboniferous. Earlier geomagnetic data are haphazardly distributed through the stratigraphic column, so little can be said in the context of reversal-oxygen-extinction connections.

Geomagnetic polarity over the past 169 Ma, tra...
Geomagnetic polarity over the past 169 Ma (credit: Wikipedia)

Wei et al. focus on the end-Triassic mass extinction which does indeed coincide, albeit roughly, with low geochemically modelled atmospheric oxygen levels (~15%). This anoxic episode extended almost to the end of the Jurassic, although that was a period of rapid faunal diversification following the extinction event. Yet it does fall in the longest period of rapid reversals of the Mesozoic. However, this is the only clear reversal-oxygen-extinction correlation, the Cenozoic bucking the prediction. In order to present a seemingly persuasive case for their idea, the authors assign mass extinctions not to very rapid events – of the order of hundreds of thousand years at most – which is well supported by both fossils and stratigraphy, but to ‘blocks’ of time of the order of tens of million years.

My own view is that quite possibly magnetic reversals can have adverse consequences for life, but as a once widely considered causal mechanism for mass extinction they have faded from the scene; unlikely to be resurrected by this study. There are plenty of more plausible and better supported mechanisms, such as impacts and flood-basalt outpourings. Yet several large igneous provinces do coincide with the end of geomagnetic superchrons, although that correlation may well be due to the associated mantle plumes marking drastic changes around the core-mantle boundary. According to Wei et al., the supposed 6th mass extinction of the Neogene has a link to the general speeding up of geomagnetic reversals through the Cenozoic: not much has happened to either oxygen levels or biodiversity during the Neogene, and the predicted 6th mass extinction has more to do with human activity than the solar wind.

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