Can freeze–thaw cycles erase traces of fire use? An experimental approach to understanding combustion residues preservation

The use of fire by past hominins is widely considered an ubiquitous and essential technology for survival in cold conditions. However, recent research on fire use shows a surprising scarcity of combustion features during cold periods in Middle and Late Pleistocene Europe. One possible explanation is that cryoturbation processes may degrade combustion remains. Yet, the effects of repetitive freeze–thaw cycles on the integrity and visibility of combustion residues remains unexplored. Here, we present the results of a controlled experiment in which we simulated repetitive freeze–thaw cycles on standardised flat layers of ash and charcoal, systematically varying: the position of the layers, moisture content of the sediments, and the number and duration of freeze–thaw cycles. Our macroscopic analysis detected no significant effect of repeated cycles on the thickness or increased irregularity of surface charcoal or buried residues. The only significant deformation was visible on the surface ash layers. We then used micromorphological and µXRF elemental analysis, which revealed progressive development of freeze–thaw related structures in the experimental substrate as well as a weak but progressive mixing of ash into the underlying charcoal layers. Our overall results indicate that combustion residues are relatively robust and resistant to freeze–thaw at least under experimental conditions. Although other site formation processes and topography may further disrupt preservation or reduce the visibility of combustion features at archaeological sites, freeze–thaw alone cannot fully account for their absence. These results carry important implications for interpretations of the scarcity of combustion features in inhabited glacial and periglacial environments.