An experimental approach to the preservation potential of magnetic signatures in anthropogenic fires

Abstract

Archaeomagnetic and rock-magnetic methods are of great value in the identification of archaeological fire, especially in Palaeolithic sites where evidence is usually scarce, ambiguous or poorly preserved. Although taphonomic processes can significantly modify Palaeolithic combustion structures, the extent to which such processes affect the magnetic record remains unknown. Here we report the results of an archaeomagnetic study involving five, two-to-five-year-old experimental combustion structures in open-air and cave settings. Some of these combustion structures involved post-combustion human actions such as trampling and relighting. Our results show pseudo-single domain (PSD) magnetite as the main magnetic carrier. Wood ash layers of combustion structures are the most magnetic facies followed by thermally altered sediments constituting the combustion substrates. A decreasing magnetic concentration pattern in depth was observed as a function of temperature. Positive correlation was found between good-quality directional data and macroscopically well-preserved combustion structures. Partial thermoremanent magnetization (pTRM) was the main magnetization mechanism identified in the combustion substrate facies. These data coupled with partial thermomagnetic curve experiments show the potential of these methods to estimate maximum temperatures of the last combustion event. Relightings show very good directional results, but they cannot be identified because the time between them is not enough to statistically distinguish directional variations of the local Earth´s magnetic field. The substrate sediment of an intensively trampled combustion structure yielded reliable archaeomagnetic directions. The results are discussed in terms of magnetization preservation potential and the effects of taphonomic processes on the archaeomagnetic record.

Stratigraphy and Chronology of Sodicho Rockshelter – A New Sedimentological Record of Past Environmental Changes and Human Settlement Phases in Southwestern Ethiopia

“…The high values of the magnetic susceptibility most certainly derived from thermally alteration of the sedimentary substrate, due to burning activity. Chemical changes of magnetic minerals induce the development of ferrimagnetic mineral, such as maghemite that increases the magnetizability of the sediment (Le Borgne, 1955;Herrejón Lagunilla et al, 2019). Furthermore, manganese shows paramagnetic properties, which contributes to the relatively high ratios of magnetic susceptibility in the cultural layers.…”

Applying thermal demagnetization to archaeological materials: A tool for detecting burnt clay and estimating its firing temperature

“…This method has been used to reconstruct site formation processes of structures destroyed by conflagration [ 25 – 27 ]. Thermal demagnetization has also been used in order to examine the stability of the natural remanent magnetization (NRM) [ 18 , 21 , 22 ]. The use of thermal demagnetization for the reconstruction of firing temperatures was suggested by Goulpeau [ 28 ], who used it to distinguish between magnetic signals recorded when building materials were pre-fired and signals recorded when they were reheated to lower temperatures during the use of a Roman hypocaust.…”

Contribution of small mammal taphonomy to the last Neanderthal occupations at the El Salt site (Alcoi, southeastern Spain)

“…It is a silty, microgranular and calcitic unit, with abundant earthworm channels and tufa fragments. This unit has yielded 61 combustion features (Sistiaga et al, 2011; Mallol et al, 2013; Galván et al, 2014a, b; Herrejón Lagunilla, 2019; Leierer et al, 2019). The combustion area, which is near the travertine wall, contains abundant faunal and flint remains and anthropogenically modified cobbles, forming a dense palimpsest of recurrent human occupations.…”