Ferns and Fires: Experimental Charring of Ferns Compared to Wood and Implications for Paleobiology, Paleoecology, Coal Petrology, and Isotope Geochemistry

Abstract

We report the effects of charring on the ferns Osmunda, Pteridium, and Matteucia with coniferous wood (Sequoia) for comparison. Like charred wood, charred ferns shrink, become black and brittle with a silky sheen, and retain three-dimensional cellular structure. Ferns yield recognizable charcoal (up to 800؇C) that could potentially survive in the fossil record enabling reconstruction of ancient fire-prone vegetation containing ferns. Charred fossils of herbaceous ferns would indicate surface fires. Like charred wood, cell-wall layers of charred ferns homogenize, and their reflectance values increase with rising temperature. Charcoalified fragments of thick-walled cells from conifer wood or fern tissues are indistinguishable and so cannot be used to infer the nature of source vegetation. Charred conifer wood and charred fern tissues show a relationship between mean random reflectance and temperature of formation and can be used to determine minimum ancient fire temperatures. Both charred conifer wood and charred fern tissues show some tendency toward increasingly lighter ␦ 13 C values up to charring temperatures of 600؇C, which should be taken into account in analyses of ␦ 13 C in charcoals. Charred fern tissues consistently have significantly more depleted ␦ 13 C values (Յ4‰) than charred wood. Therefore, if an analysis of ␦ 13 C through time included fern charcoal among a succession of wood charcoals, any related shifts in ␦ 13 C could be misinterpreted as atmospheric changes or misused as isotope stratigraphic markers. Thus, charcoals of comparable botanical origin and temperatures of formation should be used in order to avoid misinterpretations of shifts in ␦ 13 C values.

The use of reflectance values for the interpretation of natural and anthropogenic charcoal assemblages

“…The connection between increasing temperature of formation and increasing reflectance value of charcoal from wood has long been recognised Scott andJones 1991, 1994;Scott 2000;Scott andGlasspool 2005, 2007;Braadbaart and Poole 2008). This relationship has also been observed in the reflectance values of ferns and bracket fungi (McParland et al 2007;Scott and Glasspool 2007). The reflectance/temperature proxy can be applied to charcoals from many contexts including those formed by inclusion in volcanic deposits (Scott and Glasspool 2005;Scott et al 2008), charcoals formed as a result of natural wildfire both in the present day and through time (Scott and Jones 1994;Scott et al 2000) and potentially charcoals associated with anthropologic activity.…”

“…The experimental charring method and equipment used follows that of Scott andGlasspool (2005, 2007) and McParland et al (2007). Pieces of the oak were pre-milled to fit steel containers, which were specially designed to exclude oxygen from entering the experiment, thus allowing pyrolysis and preventing complete combustion, but also permitting the release of volatiles from the heated wood.…”

Geochemical evidence for combustion of hydrocarbons during the K-T impact event

“…If there was intense conflagration, then it is probable that the proportion of microscopic charcoal (6) (usually transported in the smoke plume) would be greater; if the macroscopic charcoal (6) was formed at Ͼ800°C it would be more fragile and likely to break in to smaller size fractions and become part of the microscopic fraction (34,35,36). Moreover, the reflectance of the K-T charcoals is moderate and certainly does not indicate extremely high temperatures of formation (35).…”

Changes of wood anatomical characters of selected species of Araucaria- during artificial charring - implications for palaeontology

“…Thus, it seems problematic to use charcoal for delimitation between different species of closely related taxa, as such taxa often differ only slightly in the dimensions of anatomical features (e.g. Jones et al 1991;McParland et al 2007).…”