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.
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 oxygen. Nature 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’ …