Fires in the Cenozoic: a late flowering of flammable ecosystems

Abstract

Modern flammable ecosystems include tropical and subtropical savannas, steppe grasslands, boreal forests, and temperate sclerophyll shrublands. Despite the apparent fiery nature of much contemporary vegetation, terrestrial fossil evidence would suggest we live in a time of low fire activity relative to the deep past. The inertinite content of coal, fossil charcoal, is strikingly low from the Eocene to the Pleistocene and no charcoalified mesofossils have been reported for the Cenozoic. Marine cores have been analyzed for charcoal in the North Pacific, the north and south Atlantic off Africa, and the south China sea. These tell a different story with the oldest records indicating low levels of fire activity from the Eocene but a surge of fire from the late Miocene (~7 Ma). Phylogenetic studies of woody plants adapted to frequent savanna fires show them beginning to appear from the Late Miocene with peak origins in the late Pliocene in both South American and African lineages. Phylogenetic studies indicate ancient origins (60 Ma+) for clades characteristic of flammable sclerophyll vegetation from Australia and the Cape region of South Africa. However, as for savannas, there was a surge of speciation from the Late Miocene associated with the retreat of closed fire-intolerant forests. The wide geographic spread of increased fire activity in the last few million years suggests a global cause. However, none of the potential global factors (oxygen, rainfall seasonality, CO2, novel flammable growth forms) provides an adequate explanation as yet. The global patterns and processes of fire and flammable vegetation in the Cenozoic, especially since the Late Miocene, deserve much more attention to better understand fire in the earth system.

From Cosmic Explosions to Terrestrial Fires?

“…The evidence for the increase in fires after impact events can be found in soot and other carbon-related sediments (Wolbach et al, 1985(Wolbach et al, , 2018. Such evidence might be found here and is in fact cited in Zhou et al (2014) and Bond (2015). While impactrelated fires would show a sudden spike, we would expect the SN-CR wildfires to take place over a much more extended period of time, 10 4 -10 5 y for each supernova.…”

The deforestation story: testing for anthropogenic origins of Africa's flammable grassy biomes

“…These show a sharp increase in charcoal fluxes also from the Late Miocene [34,35]. Increased fire activity within the last approximately 10 Myr, is a worldwide phenomenon with evidence for a surge in charcoal fluxes in the North Pacific, the North and South Atlantic off Africa, and terrestrial records in Australia [36]. In Africa, dated molecular phylogenies indicate that growth forms characteristic of fire-maintained savannas developed at least 6-8 Ma in the tropics spreading to South Africa by 1-2 Ma [37].…”

Terrestrial biome distribution in the Late Neogene inferred from a black carbon record in the northeastern equatorial Pacific

“…Thus, the role of fire after the establishment of open-canopy environment seems to maintain C 4 -dominated environment, rather than causing a major event of C 4 vegetation. Also, low coefficient of correlation between carbon isotope value and BC content suggests not only fire but also other environmental factors, such as CO 2 , temperature, seasonality, precipitation84748, or edaphic ghettos49, would have played roles in controlling the C 3 –C 4 ratio of the ecosystem, as shown from the case of modern vegetation50.…”