A Flammable Biosphere

  • Andrew Y. GliksonEmail author
Part of the SpringerBriefs in Earth Sciences book series (BRIEFSEARTH)


The advent of plants on land surfaces since about 420 million years-ago created an interface between carbon-rich organic layers and an oxygen-rich atmosphere, leading to recurrent fires triggered by lightning, volcanic eruptions, high-temperature combustion of peat and, finally, ignition by humans, constituting the blueprint for the Anthropocene. For a species to be able to control ignition and energy output, leading to increase in entropy in nature higher by orders of magnitude than its own physical energy outputs, the species would need to be perfectly wise and responsible. No species can achieve such levels.


Atmospheric Oxygen Geological Timescale Soil Carbon Storage Optical Refractive Index Extensive Fire 
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The emergence of land plants in the late Silurian ~420 Ma, the earliest being vascular plants (Cooksonia, Baragwanathia) and later Cycads and Ginkgo in the Permian (299–251 Ma) (Figs.  1.2, 5.1), combined with the rise in photosynthetic oxygen above 13 % (Fig.  2.2), with consequent juxtaposition of carbon-rich land surfaces and rising atmospheric oxygen, have set the stage for extensive fires. Fires became an integral part of the atmosphere/land carbon and oxygen cycles and have led to development of fire-adapted (pyrophyte) plants, enhancing the distribution of seeds and control of parasites. With the exception of anaerobic chemo-bacteria which metabolize sulphur, carbon and metals, photosynthesis has become the basis for the land-based food chain. oxidation reactions through fire and by plant-consuming organisms enhance degradation and entropy. The decay of plants coupled with charcoal from fires coated many parts of the land with carbon, located in cellulose of trees and grasses, in soils and marshes, in methane hydrate and methane clathrate deposits in bogs, sediments and permafrost.
Fig. 5.1

Fossil Palaeozoic plants. a Baragwanathia longifolia—Silurian; b Baragwanathia longifolia—Silurian, Yea in Victoria; c Rhacopteris ovate—Carboniferous; d Rhacopterid with partly divided leaves—Carboniferous. (From ‘Greening of Gondwana’ 1986, Reed Books, French’s Forest; photographs by Jim Frazier; Courtesy Mary White)

Charcoal, a proxy for fire, occurs in the fossil record from the Late Silurian ~420 Ma. Scott and Glasspool (2006) document Silurian through to end-Permian charcoal deposits with reference to the frequency of Paleozoic fires in relation to atmospheric oxygen concentrations. As atmospheric oxygen levels rose from ~13 % in the Late Devonian to ~30 % in the Late Permian, fires progressively occur in an increasing diversity of ecosystems. Late Silurian to early Devonian charcoal indicates the burning of diminutive rhyniophytoid vegetation. There is an apparent paucity of charcoal in the Middle to Late Devonian coinciding with low atmospheric oxygen (Fig.  2.2). Fires become widespread during the Early Mississippian (Lower Carboniferous) and in the Middle Mississippian. During the Pennsylvanian (Upper Carboniferous) oxygen rose toward levels above 30 %. Charcoal is recorded in upland settings and is important in many Permian mire settings, suggesting the burning of even moist vegetation. The decline of oxygen levels through much of the Mesozoic (250–65 Ma) to below 15 % (Fig.  2.2) and its gradual resurgence through the late Mesozoic and Cenozoic limited the effect of fire.

Atmospheric CO2 levels are buffered by the oceans (~37,000 GtC), which contain about x46 times the atmospheric CO2 inventory (~800 GtC). The solubility of CO2 in water decreases with higher temperature and salinity and the transformation of the CO 3 [−2] ion to carbonic acid (HCO 3 [−1] ) retards the growth of calcifying organisms, including corals and plankton. Plants and animals work in opposite directions of the entropy scale, where plants synthesize complex organic compounds from CO2 and water, producing oxygen, whereas animals burn oxygen and expel CO2. Disturbances in the carbon and oxygen balance occur when changes occur in the extent of photosynthetic processes, CO2 solubility in the oceans, burial of carbon in carbonate and organic remains of plants and oxidation of carbon through fire and combustion.

Prior to the mastery of fire by Hominins, wildfires were ignited by lightning, incandescent fallout from volcanic eruptions, meteorite impacts and combustion of peat. Consuming vast quantities of biomass, fires have an essential role in terrestrial biogeochemical cycles (Belcher et al. 2010), including consequences for the oxygen cycle and the evolution of biodiversity over geological timescales. Subsequent burial of carbon in sediments stored the fuel over geological periods, affecting the carbon and oxygen cycle in favor of oxygen. The role of extensive fires during the Paleozoic and Mesozoic (Fig. 5.2) is represented by charcoal remains whose pyrogenetic origin is identified by high optical refractive indices. Experiments and models by Belcher et al. (2010) suggest fire is suppressed below 18.5 % O2, switched off below 16 % O2 and enhanced between 19 and 22 % O2. According to Belcher et al. (2010) fires were important during ~350–300 Ma and 145–65 Ma, intermediate effects during the 299–251, 285–201 and 201–145 Ma and low effects between 250 and 240 Ma. During the Carboniferous-Permian period, when atmospheric oxygen levels reached ~31 % or higher (Beerling and Berner 2000; Berner et al. 2007), instantaneous combustion affected even moist vegetation (Scott and Glasspool 2006; Bowman et al. 2009). Thus, Permian (299–251 Ma) coals may contain charcoal concentrations as high as 70 % (Bowman et al. 2009).
Fig. 5.2

Qualitative scheme of global fire activity through time, based on pre-Quaternary distribution of charcoal, Quaternary and Holocene charcoal records, and modern satellite observations, in relation to the percentage of atmospheric O2 content, CO2 (in parts per million), appearance of vegetation types, and presence of hominins. Dotted lines indicate periods of uncertainty. a—Mesozoic and Palaeozoic flammability peaks; b—historic and pre-historic flammability period, in part related to human-lit fires; c—Anthropocene fuel combustion; (Bowman et al. 2009, Fig. 1; American association for advancement of science, by permission)

Above a certain level of atmospheric oxygen fires would constrain the development of forests, constituting strong negative feedback against excessive rise of atmospheric oxygen (Watson et al. 1978). Conversely a decline of oxygen reduces the frequency and intensity of fires. The association of fossil charcoal with fossil trees suggests O2 levels continued to be replenished, whereas the upper oxygen limit of Phanerozoic atmospheres is uncertain. Robinson (1989) pointed to Paleobotanical evidence for a higher frequency of fire-resistant plants during the Permo-Carboniferous, supporting distinctly higher O2 levels at that time. Model calculations of the interaction between terrestrial ecosystems and the atmosphere by Beerling and Berner (2000) suggest the rise from 21 to 35 % O2 during the Carboniferous resulted in a decline in organic productivity of about 20 % and a loss of more than 200 GtC (billion ton carbon) in vegetation and soil carbon storage, due to burning in an atmosphere of ~300 ppm CO2. However, in a CO2-rich atmosphere of ~600 ppm carbon fertilization of the soil productivity increases lead to the net sequestration of 117 GtC. In both cases these effects resulted from strong interaction between O2, CO2 and climate in the tropics.


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© The Author(s) 2014

Authors and Affiliations

  1. 1.School of Archaeology and AnthropologyAustralian National UniversityCanberraAustralia

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