Some first observations on ant-nest morphology and micromorphology, the effects of wildfires, and their implications for the understanding of archaeological features

Ants are among the soil mesofauna that cause significant bioturbation at the location of their nests. They can have significant impact on the preservation of soil features and on post-depositional artefact distribution. Moreover, there is discussion on the natural or anthropogenic nature of so-called ‘pit-hearth’ features dating to the Mesolithic. Such features are common in parts of the Netherlands, NW Belgium, and NW Germany, and form an important body of evidence in the study of hunter-gatherer landscape use. However, it has been hypothesized that these features represent ant nests burnt due to climate-related wildfires, instead of anthropogenic pit hearths, and are therefore of little archaeological value, other than as a potential proxy for climate conditions. Considering the lack of direct evidence on underground characteristics of ant nests, we investigated the characteristics of two wood ant nests: Oone abandoned and one accidently burnt in a wildfire c. 10 years earlier. We trenched through the ant nest remains and used micromorphology to study the characteristics of the ant-influenced soil profiles. The surface domes of the ant nests had disappeared, leaving behind a bowl-shaped depression. In the burnt ant nest, a thin band of charred litter-like organic material was all that was left over from the burned superstructure. These depressions seem not to reach deeper than the top of cemented podzol Bhs-horizons. Surrounding and underneath the depression, extensive networks of tunnels and chambers were found in the B- and C-horizons. If archaeological artefacts had been present on the site, they would have become buried and moved to the top of the B-horizon. Any soil features would have been destroyed. Moreover, the extensive tunnel and chamber network extending from the depression would potentially affect the botanical record and the OSL signal of the deposits.

Comparison of our results with Mesolithic pit-hearth features shows substantial differences between the two types of remains. Ant nest depressions do not extend into (cemented) podzol B horizons, whereas Mesolithic pit hearths typically have their base in the C horizons. The extensive tunnelling in B- and C-horizons of large ant nests has not been observed in Mesolithic pit-hearth features. On the other hand, the large amount of charred humus and charcoal fragments in Mesolithic pit hearths are lacking in the burned ant nest we studied. We therefore conclude that Mesolithic pit-hearth features are not the result of the burning down of ant nests, but should be regarded as anthropogenic features.