Wildfires and the formation of sugar-loaf hills

One iconic feature of Rio de Janeiro is Corcovado Mountain, topped by the huge Cristo Redentor (Christ the Redeemer) statue. Another is the Sugar Loaf (Pão de Açúcar) that broods over Botafogo Bay. Each is an inselberg: a loan word from the German for ‘island mountain’. Elsewhere they are known as kopjes (southern Africa), monadnocks (North America) or bornhardts after the German explorer who first described them. But, being on the coast, the Brazilian examples are not typical. Most rise up spectacularly from almost featureless plains, a well-known case being Uluru (Ayers Rock) almost at the centre of Australia. Arid and semi-arid plains of Africa and the Indian subcontinent are liberally dotted with them. So scenically dominant and spectacularly stark, inselbergs are often revered by local people, and have been so for millennia. The only thing that I remember from a desperately boring, but compulsory, first-year course on geomorphology in 1965 is their connection with the ‘cosmogonic egg’: a mythological motif that spans Eurasia, Australia and Africa, signifying that from which the universe hatched. It is perhaps no coincidence that hills in England that suddenly rise from flat land, such as the Wrekin in Shropshire and Malvern Hill in Worcestershire, still host the sport of rolling hard-boiled eggs to celebrate the pagan festival of Eostre (now Easter) that marks the spring rebirth of the land.    

Vista of Rio de Janeiro and its inselbergs (Credit: Leonardo Ferreira Mendes, Creative Commons)

How inselbergs and their surrounding plains formed has long been a hot topic in tropical geomorphology. One theory is that they are especially resistant rocks around which eroding rivers meandered during the formation of peneplains, a variant being that they were surrounded by lines of weakness, such as faults or major joint systems. Another is that they formed by erosion into a deeply but irregularly weathered surface. Then there is L.C. Kings theory of escarpment retreat and, of course, a mixture of processes in different stages, or a unique origin for each inselberg. In effect, there has been no final, widely agreed explanation. But that that may be about to change.

A common element to most inselbergs is their very steep and sometimes vertical flanks. Some even display overhangs at their base. Such potential shelters encouraged local people to camp there and, in response to the awe inspired by the sheer majesty of the looming inselberg, to use them for sacred rites and decoration. That is especially true of Australia, so it is fitting that what may be a breakthrough in understanding inselberg formation should have arisen there. (Buckman, S. et al. 2021. Fire-induced rock spalling as a mechanism of weathering responsible for flared slope and inselberg developmentNature Communications, v. 12, article 2150; DOI: 10.1038/s41467-021-22451-2). Breaking rock by deliberate use of fire has been done for millennia. For instance, Hannibal is said to have used fire to break down huge fallen boulders that blocked passage for his war elephants as his army advanced on Rome. Fire setting is still used by villagers in South India to spall large flakes of rock from outcrops. It is done with such skill that thin slabs up to 3-4 metres across can be lifted, and then split into thin posts for fencing or training vines: an essential alternative to wooden posts that termites would otherwise devour in a matter of months.

Solomon Buckman and colleagues from the University of Wollongong, Australia were drawn to a new hypothesis for inselberg formation by observations around low rock faces and boulders after the 2019-20 “Black Summer” wildfires in eastern Australia. Where burned trees had fallen against rock faces up to hundreds of kg of spalled flakes lay at the base of each face, which also bore freshly formed scars: clear signs of fire action. Thermal expansion and contraction of rock caused by air temperatures of hundreds of degrees close to wildfires is clearly a powerful means of rapid erosion. If the rock is damp – most likely at the base of a rockface as all rainfall on the outcrop drains in its direction – the mechanism is enhanced: Hannibal’s engineers poured vinegar onto the boulders heated by fire, to great effect. Buckman et al. estimate the rate of lateral erosion by fire at slope bases in Australia to be around ten thousand times faster than those operating on horizontal rock surfaces, which are not exposed to fire as no vegetation grows on them. Over time, slopes steepen aided by the formation of flared surfaces at the base. If spalled debris is carried away quickly the developing inselberg evolves to its classical sugarloaf shape. In more arid conditions the debris builds around the outcrop to steadily smother inselberg development, leaving tors and kopjes. The paper came to press remarkably quickly relative to the authors’ field work and analyses. This is a work-in-progress to be followed up by cosmogenic-isotope and other means of surface dating of the tops and flanks of suitably accessible inselbergs and simiar features such as Western Australia’s famous Wave Rock (a flared escarpment).

Wave Rock in the interior of Western Australia is 15 m high and 100 m long and revered by the local Ballardong people as a creation of the Rainbow Serpent

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’ …

Younger Dryas impact trigger: evidence from Chile

A sudden collapse of global climate around 12.8 ka and equally brusque warming 11.5 ka ago is called the Younger Dryas. It brought the last ice age to an end. Because significant warming preceded this dramatic event palaeoclimatologists have pondered its cause since it came to their attention in the early 20th century as a stark signal in the pollen content of lake cores – Dyas octopetala, a tundra wild flower, then shed more pollen than before or afterwards; hence the name. A century on, two theories dominate: North Atlantic surface water was freshened by a glacial outburst flood that shut down the Gulf Stream [June 2006]; a large impact event shed sufficient dust to lower global temperatures [July 2007]. An oceanographic event remains the explanation of choice for many, whereas the evidence for an extraterrestrial cause – also suggested to have triggered megafaunal extinctions in North America – has its supporters and detractors. The first general reaction to the idea of an impact cause was the implausibility of the evidence [November 2010], yet the discovery by radar of a major impact crater beneath the Greenland ice cap [November 2018] resurrected the ‘outlandish’ notion. A recent paper in Nature: Scientific Reports further sharpens the focus.

Temperature fluctuations over the Greenland ice cap during the past 17,000 years, showing the abrupt cooling during the Younger Dryas. (credit: Don Easterbrook)

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Greening the Earth, Devonian forest fires and a mass extinction

Land plants begin to appear in the fossil record as early as the late Ordovician (~450 Ma), show signs of diversification during the Silurian and by the end of the Devonian Period most of the basic features of plants are apparent. During the Carboniferous Period terrestrial biomass became so high as to cause a fall in atmospheric carbon dioxide, triggering the longest period of glaciation of the Phanerozoic, and such a boost to oxygen in the air (to over 30%) that insects, huge by modern standards, were able to thrive and the risk of conflagration was perhaps at its highest in Earth’s history. Yet surprisingly, the first signs of massive forest fires appear in the Devonian when vegetation was nowhere near so widespread and luxuriant as it became in the Carboniferous (Kaiho, K. et al. 2013. A forest fire and soil erosion event during the Late Devonian mass extinction. Palaeogeography, Palaeoclimatology, Palaeoecology, v. 392, p. 272-280). Moreover, Devonian oxygen levels were well below those of the present atmosphere and CO2 was more than 10 times even the post-industrial concentration (387 parts per million in 2013). Such atmospheric chemistry would probably have suppressed burning.

Kunio Kaiho of Tohoku University in Japan and colleagues from Japan, the US and Belgium analysed organic molecules in Belgian marine sediments from the time of the late-Devonian mass extinction (around the Frasnian-Famennian boundary at 372 Ma). A range of compounds produced by hydrocarbon combustion show marked ‘spikes’ at the F-F boundary. The thin bed that marks the extinction boundary also shows sudden increase then decrease in δ13C and total organic carbon, indicative of increase burial of organic material and a likely increase in atmospheric oxygen levels. Another biomarker that is a proxy for soil erosion follows the other biogeochemical markers, perhaps signifying less of a binding effect on soil by plant colonisation: a likely consequence of large widlfires. Unlike the biomarkers, magnetic susceptibility of the boundary sediments is lower than in earlier and later sediments. This is ascribed to a decreased supply of detrital sediment to the Belgian marine Devonian basin, probably as a result of markedly decreased rainfall around the time of the late-Devonian mass extinction. But the magnetic data from 3 metres either side of the boundary also reveal the influence of the 20, 40, 100 and 405 ka Milankovich cycles.

Juan Ricardo Cortes , a placoderm from the Dev...
Dunkleosteus, a giant (10 m long) placoderm fish from the Devonian, which became extinct in the late Devonian along with all other placoderms (credit: Wikipedia)

This set of environmentally-related data encourages the authors to suggest a novel, if not entirely plausible, mechanism for mass extinction related to astronomically modulated dry-moist climate changes that repeatedly killed off vegetation so that dry woody matter could accumulate en masse during the Frasnian while atmospheric oxygen levels were too low for combustion. A mass burial of organic carbon at the end of that Age then boosted oxygen levels above the burning threshold to create widespread conflagration once the wood pile was set ablaze. Makes a change from continental flood basalts and extraterrestrial impacts… Yet it was about this time that vertebrates took it upon themselves to avail themselves of the new ecological niche provided by vegetation to haul themselves onto land.