Soil heating under wildfires and prescribed burns and their relevance to archaeological investigations

Fires can alter the properties of soil and other material via heat transfer. The identification of soil heating effects in hearths, for example, has long been a cornerstone in archaeological investigations. However, wildfires can also alter soils, and there is a surprising level of uncertainty into what degree soils are heated and to which depth this occurs in wildfires. This can lead to erroneous assumptions regarding the potential impact of wildfires when attributing heat induced changes in the soil, especially when laboratory heating results are extrapolated to field conditions.

To address this research gap, we compiled and examined new and published field data on maximum temperatures and heating durations for mineral soils during wildfires and prescribed burns in forests, shrublands and grasslands around the globe; and compared these to data obtained from laboratory heating experiments.

Most fires heated only the uppermost centimetres of the mineral soil, rarely exceeding 300 °C below 1 cm depth. Their heat pulses were shorter (<500 s) than those often applied in laboratory studies (1800-3600 s). The highest near-surface temperatures occurred in shrubland wildfires, whereas the longest heating durations in forests with deep organic layers and high fuel loads.

While it is clear that smouldering logs, tree trunks and root systems, or slash pile burns can impart intense heating to substantial depths akin to that under hearths, most landscape-scale fires generate short and shallow heat pulses that are unlikely to lead to detectable lasting changes in the mineral soil.