Fire ecology of the Carboniferous tropical zone

Abstract

Fossil charcoal (fusain), indicating the occurrence of wildfire, is common in the Carboniferous rocks of North America and Europe. During the Carboniferous these regions lay on the southern margins of the Laurasian continent and within the tropical zone. This study examines the facies distribution and botanical identity of charcoal in order to investigate the influence of fire on Carboniferous tropical ecology. New data from charcoal-bearing units in eastern Canada, Poland and the British Isles are presented and synthesised with previously published data of charcoal occurrence. Tournaisian–Viséan charcoal-bearing strata were deposited in volcanic, lacustrine, alluvial plain and coastal marine environments. In these deposits, charred floras are dominated by (pro?)gymnospermous plants, whilst uncharred (compressed or permineralised) floras are dominated by lycopsids. Uncharred lycopsid remains were probably derived from wetland communities growing close to the depocentres, whilst charred gymnospermous remains may have been sourced from well-drained extrabasinal settings. Based on the vertical distribution of charcoal layers in annually(?) laminated lacustrine deposits, it is estimated that fires occurred with high frequency, approximately every 3–35 years. Palaeoclimatic analysis indicates that southern Laurasia at this time was subject to a seasonal (monsoonal) tropical climate. Such environments today are characterised by tropical savannas with high fire frequencies, and Early Carboniferous fire-prone gymnospermous plant communities may have represented a comparable vegetation biome. Namurian–Stephanian charcoal-bearing strata were deposited in alluvial plain, coastal plain and coastal marine environments. Channelised sandstone units, interpreted as river channel deposits, contain charcoal derived from cordaitaleans and conifers. This assemblage records fires in extrabasinal coniferopsid forests. Mudstone units containing coals are interpreted as the deposits of floodbasins and peat mires. Hollow uncharred lepidodendrid stumps occur in this facies and contain lepidodendrid charcoal in the stump bases and lepidodendrid, pteridosperm, coniferopsid and sphenopsid charcoal outside the stumps. This assemblage records fires in wetland lepidodendrid-dominated forests. Lepidodendrid trunks, comprising of a spongy parenchymatous core surrounded by dense periderm, became hollowed out during fire events. An estimate of fire frequency based on the vertical spacing of charcoal layers in coals implies that fires occurred every 105–1085 years in the peat mires. Palaeoclimatic analysis indicates that southern Laurasia was subject to a humid tropical climate during the Late Carboniferous. Such environments today are characterised by tropical rainforests where fires occur rarely, and Late Carboniferous communities may have represented a comparable biome.