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FERNS AND FIRES: EXPERIMENTAL CHARRING OF FERNS COMPARED TO WOOD AND IMPLICATIONS FOR PALEOBIOLOGY, PALEOECOLOGY, COAL PETROLOGY, AND ISOTOPE GEOCHEMISTRY

¹ Geology Department, Royal Holloway University of London, Egham, Surrey, TW20 0EX, UK m.collinson{at}gl.rhul.ac.uk

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 ¹³C values up to charring temperatures of 600°C, which should be taken into account in analyses of ¹³C in charcoals. Charred fern tissues consistently have significantly more depleted ¹³C values (4) than charred wood. Therefore, if an analysis of ¹³C through time included fern charcoal among a succession of wood charcoals, any related shifts in ¹³C 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 ¹³C values.