INVITED: THE DIVERSIFICATION OF FIRE SYSTEMS IN THE LATE PALEOZOIC: VEGETATION, CLIMATE OR ATMOSPHERE?

Fire is a widespread and important phenomenon in a range of modern terrestrial ecosystems. Fire ignition depends on the availability of oxygen and a source of fuel and heat. Fire spread requires widespread fuel but, is also dependent on weather and topography. In the fossil record, the earliest documented fire, late Silurian in age, occurs only once terrestrial vegetation had become established and was available to act as fuel. The occurrence of Silurian fire establishes that the atmospheric oxygen concentration must have been at least 13%, or the fire could not have propagated. However, as Silurian and earliest Devonian plants were small, insufficient fuel was available to permit extensive and widespread fires. By the late Devonian, forests had evolved in lowland habitats. Despite this, charcoal occurrences of this age are rare and scattered. Extensive charcoal beds are only first found in the earliest Carboniferous in Spitzbergen and by the mid-late Mississippian there is evidence of extensive wildfire activity in a diversity of vegetation types and a range of environments. Atmospheric models indicate a rapid rise in atmospheric oxygen levels at the end of the Devonian and early Carboniferous that may have been responsible for increasing the frequency of major widespread wildfires. However, changes in fuel load and climate may also have played a role. Extensive peat formation in wetland mire systems first occurs in the mid-late Mississippian and becomes widespread in the Euramerican tropics in the Pennsylvanian. These peat systems record evidence of frequent and extensive wildfires. The widespread nature of fire systems across a range of vegetation types in both lowland and upland settings may support the suggestion of elevated oxygen levels. However, as yet, the data is equivocal. We conclude that whilst charcoal occurrences may relate to the changing atmospheric oxygen levels in the late Paleozoic, plant evolution, habitat diversification and climate change may be equally responsible. Until we increase our knowledge of ancient fire systems and link field and laboratory observations with climate and atmospheric models, the impact of high atmospheric oxygen on fire systems will remain theoretical.