Some semifusinite in coal may form during diagenesis, not wildfires

Semifusinite can be an important component of coals and has previously been interpreted to represent partially charred material produced in palaeowildfires. Low reflecting semifusinite is well known for its chemically reactive properties, good burning behaviour, and for its reflectance to increase with coal rank. Yet, when inertinite in coal and sediments is used to interpret palaeowildfire history, the diagenetic history of the basin is rarely considered. Here, we sought to explore whether the low temperature heating that would occur during diagenesis over geological timescales is sufficient to produce semifusinite. Considering the interpretation that semifusinite represents partially charred material, we reheated thermally degraded, but not charred, wood at 100 °C and 200 °C for 24 h. Through prolonged heating at low temperatures, the previously thermally altered but non-charred wood increased in reflectance to such an extent that it would be classified as semifusinite. This finding suggests that not all semifusinite necessarily has a wildfire origin and calls into question its utility in interpreting palaeowildfire activity.