The record of Triassic charcoal and other evidence for palaeo-wildfires: Signal for atmospheric oxygen levels, taphonomic biases or lack of fuel?

As wildfires are today important sources of disturbance in many terrestrial ecosystems, it is of great interest to understand how different environmental parameters and fire-activity interacted during past periods of the Earth history. Fossil charcoal, inertinites, and pyrogenic polycyclic aromatic hydrocarbons (PAHs) represent the only direct evidence for the occurrence of such palaeo-wildfires. In the present study, a review of published data, together with new data on the occurrence of fossil charcoal for the Permian and the Triassic is presented. For a long time, it has been speculated, that an assumed lack of evidence for palaeo-wildfires during the Triassic should be explained by a large drop in atmospheric oxygen concentration following or during the end-Permian mass extinction event, preventing the occurrence of wildfires. However, evidence for palaeo-wildfires is relatively common in many middle and late Triassic strata, whereas such evidence is almost totally lacking from early Triassic sediments. The interpretation of this “charcoal gap” or depression is difficult, as many factors (e.g. atmospheric oxygen concentration, taphonomical biases, lack of sediments suitable for the preservation of macroscopic charcoal, lack of fuel, and “ignorance” of scientists) may have influenced not only the production, but also the preservation and recovery of evidence for palaeo-wildfires during this period. Thus, it is not clear whether this Early Triassic “charcoal gap” can also be seen as evidence for an assumed “wildfire gap” or not. Without any doubt further investigations on the early Triassic record of charcoal and other evidence for palaeo-wildfires will be necessary before this problem can be solved. In fact, it can be expected that the number of published records of (early) Triassic evidence for palaeo-wildfires will increase in the future as more and more scientist working on sediments of this age may become aware of the interest in fires from this time. This will certainly make it possible to give a much better picture of the temporal and regional distribution of wildfires during this period in the future.