Sediments vs. Historical narratives: The use of soil magnetic properties to evaluate the existence of a historical fire in an 18th century Spanish fort (Patagonia, Argentina)

Abstract

The present work aims to evaluate if sediments from the Spanish fort Fuerte San José (1779–1810 CE) located in Península Valdés, Province of Chubut, Argentina, show signs that could be attributed to the documented fire caused by native populations in 1810 CE. Three different magnetic signatures indicative of thermoalteration were assessed in raw and experimental heated samples: the presence of superparamagnetic minerals, the existence of reversibility on thermomagnetic curves and further relative changes concerning ferri-, anti-ferro- and paramagnetic minerals. Results show that only samples corresponding to an adobe-wall foundation recorded magnetic changes caused by thermoalteration processes, below 400 °C, likely related to the historical fire. Relatively higher organic matter values contained in the adobe-walls, thatched roofs and/or any combustible content placed inside the structure could have been the factors which promoted such magnetic thermoalterations circumscribed to only one sector of the site. A low-magnitude fire may also explain the lack of a magnetic-soil thermoalteration in other sectors of the Fuerte San José. Thus, data do not support a massive fire, as narratives repeated by historians described. The methodological design presented here could be a fruitful approach to discuss natural or anthropogenic paleo-fires in arid/ semi-arid environments.