Fire in deep time
Studied by reflected light microscopy, the organic constituents of coal are subdivisible into three maceral groups: vitrinite, liptinite and inertinite. The inertinite group of macerals are synonymous with charcoal (Scott and Glasspool, 2007) and almost all inertinite is a by-product of fire, though some may be generated by geothermal activity such as pyroclastic flows (Scott and Glasspool, 2005).
Within sub-anthracitic rank coals, petrographic analyses of inertinites reveal a variety of morphologies that are united in having reflectances elevated above the other organic constituents of the coal. An improved ability to understand the morphogenesis of these inertinites will allow a better understanding of coal taphonomy and paleoecology, the paleoclimatological controls on coal accumulation and potentially variations in coal quality. The data generated should allow major re-assessments of tools such as the Gelification Index and Tissue Preservation Index (e.g. Diessel, 1986), which have been widely applied to model and predict environments of coal accumulation. Such data are more than purely scientific and have direct applicability to industries wanting to model variations in coal quality.
By studying the formation of charcoal/inertinite using experimental charring protocols combined with organic petrology it has been possible to examine the impact of ancient fires on the studies of paleoecology, paleoclimatology, coal quality, Earth system processes and in particular oxygen. Oxygen has been described as “one of the “master variables” of the Earth system” (Lenton, 2003) and biotically has been linked to the evolution of metazoans (Lenton, 2003), to gigantism in Late Carboniferous insects (Dudley, 1998), to the diversification of tetrapods, to the origins of flight (Graham et al., 1995), to the evolution of large placental mammals (Falkowski, et al., 2005) and to the mass extinction at the end of the Permian (Kerr, 2005). Preliminary research, arising out of the experimental charring and coal petrographic studies outlined above, has established linkages between charcoal abundance and distribution and fluctuations in atmospheric oxygen concentration (pO2) (Scott and Glasspool, 2006; Glasspool and Scott, 2010). The overarching significance of the pO2 data this tool can generate has made its refinement a research priority. It is hoped to gather new data from key intervals throughout the Phanerozoic, in particular the Middle Devonian “charcoal gap”, the Late Paleozoic leading up to the Permo-Triassic mass extinction event, the Early-Middle Triassic and the earliest Cretaceous.
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