THE ORIGIN OF WIDESPREAD FIRE SYSTEMS AND THEIR WIDER IMPACT

Relatively frequent charcoal deposits from the Late Silurian and Early Devonian provide the earliest evidence of wildfire events. Following this, evidence of fires is rare until the late Famennian. During this interval, terrestrial vegetation diversified into a range of new ecological niches and biomass increased greatly (by the Mid-Late Devonian the first forests are recorded). Despite these parametric changes that would favor fire, there is currently scant evidence of charcoal during this interval. Fires will not spread when atmospheric oxygen levels are below 13% and are predicted to be uncommon below 17%. Therefore, low levels of atmospheric oxygen may account for the absence of charcoal during the Mid-Late Devonian. However, by the late Famennian, charcoal evidence from terrestrial and marine sequences in North America and Europe indicates a rapid increase in fire frequency and fire system diversification. By the Devonian/Carboniferous boundary and into the early Mississippian coals and organic-rich sediments contain abundant charcoal and indicate extensive wildfire activity that suggests levels of atmospheric oxygen were equivalent to or greater than those at Present. The nature of the charcoal indicates that these were largely surface fires that burned an herbaceous lycopsid/fern vegetation and were not forest crown fires. Frequent fires may have maintained an herbaceous vegetation in some areas and fostered the development of fire-prone vegetation. The rise of these fire systems in the Late Devonian and Early Carboniferous would have resulted in extensive charcoal production and burial, which would in turn would have affected both atmospheric composition and climate change, leading to positive feedback resulting in increased fire frequency. From this time fires have remained an important element in the Earth system.