The rise of fire: Fossil charcoal in late Devonian marine shales as an indicator of expanding terrestrial ecosystems, fire, and atmospheric change

Abstract

Fossil charcoal provides direct evidence for fire events that, in turn, have implications for the evolution of both terrestrial ecosystems and the atmosphere. Most of the ancient charcoal record is known from terrestrial or nearshore environments and indicates the earliest occurrences of fire in the Late Silurian. However, despite the rise in available fuel through the Devonian as vascular land plants became larger and trees and forests evolved, charcoal occurrences are very sparse until the Early Mississippian where extensive charcoal suggests well-established fire systems. We present data from the latest Devonian and Early Mississippian of North America from terrestrial and marine rocks indicating that fire became more widespread and significant at this time. This increase may be a function of rising O 2 levels and the occurrence of fire itself may have contributed to this rise through positive feedback. Recent atmospheric modeling suggests an O 2 low during the Middle Devonian (around 17.5%), with O 2 rising steadily through the Late Devonian and Early Mississippian (to 21-22%) that allowed for widespread burning for the first time. In Devonian-Mississippian marine black shales, fossil charcoal (inertinite) steadily increases upsection suggesting the rise of widespread fire systems. There is a concomitant increase in the amount of vitrinite (preserved woody and other plant tissues) that also suggests increased sources of terrestrial organic matter. Even as end Devonian glaciation was experienced, fossil charcoal continued to be a source of organic matter being introduced into the Devonian oceans. Scanning electron and reflectance microscopy of charcoal from Late Devonian terrestrial sites indicate that the fires were moderately hot (typically 500 -600 o C) and burnt mainly surface vegetation dominated by herbaceous zygopterid ferns and lycopsids, rather than being produced by forest crown fires. The occurrence and relative abundance of fossil charcoal in marine black shales are significant in that these shales may provide a more continuous record of fire than is preserved in terrestrial environments. Our data support the idea that major fires are not seen in the fossil record until there is both sufficient and connected fuel and a high enough atmospheric O 2 content for it to burn.

Earliest land plants created modern levels of atmospheric oxygen

“…The early charcoal record (Table S1) was compiled from the literature (11,12,28, using existing compilations (12,(44)(45)(46)(47) and checking them where possible against the original sources. This process involved some reconciling of disparate results between existing compilations and revision of some erroneous quoted values.…”

Charcoal Evidence That Rising Atmospheric Oxygen Terminated Early Jurassic Ocean Anoxia

“…A few studies have looked at the fossil-charcoal record within Devonian marine black shales and related them to changes in pO 2 and fire-feedbacks 18,19 . However, fossil charcoal has not been studied across events such as OAEs in order to test the hypothesis that fire-feedbacks to pO 2 may have acted together to rebalance the Earth system during these events 9,12 .…”

The interaction of fire and mankind: Introduction

“…Our understanding of fire in deep time comes mainly from the fossil record of charcoal [3], and in some cases its botanical identity can reveal information on the vegetation being burned. More recently, attempts to understand ancient fires have included data from charcoal reflectance that provides some information regarding pyrolysis intensity [3,23].…”

“…The evidence of fire on the Earth goes back over 400 Myr [1,2] and has been a significant part of the Earth system for 350 Myr [3]. The occurrence of fire from the study of fossil charcoal has allowed our understanding of the role fire plays on the Earth to develop rapidly over the past 30 years [4].…”