El arqueomagnetismo como técnica para diferenciar ocupaciones neandertales en palimpsestos paleolíticos

Résumé

Neanderthal occupations from peninsular Middle Palaeolithic sites (ca. 250- 40 ky BP) often contain dense accumulations of lithic and faunal remains around combustion structures or hearths. These accumulations are known as palimpsests in the archaeological literature. They are so densely stratified that it is very difficult to distinguish whether they correspond to one or more occupations and even, if they are temporally different. Recent geoarchaeological researches focus on isolating and temporalize different occupations considering hearths as single analytical units. We report preliminary results from archaeomagnetic analyses applied to some hearths from El Salt unit X (Alcoy, Alicante) [~55 ky BP]. Although Archaeomagnetism is a discipline with long tradition in Earth Sciences, its application to the Palaeolithic Archaeology is barely explored. The main objective is to show the potential and limits of the method for temporal palimpsests´ dissection through the study of the magnetic record of prehistoric hearths. The basic premises of the discipline are: (i) the Earth´s magnetic field (EMF) undergoes subtle directional changes (± 20° from present-day North) at regional scale which under certain conditions may be recorded by archaeological materials; (iii) Any material burnt at high temperature may record the EMF direction at the time of cooling through a thermoremanence (TRM) Therefore, a prehistoric hearth may have recorded an isolated (snapshot) direction of EMT; (iii) to obtain this EMF direction in the past, materials must be preserved in situ. Our hypothesis is based on whether archaeomagnetic directions obtained from several hearths from the same archaeological paleosurface are different or statistically distinguishable, most likely these hearths are diachronic. We explain the process of sampling, magnetic orientation and subsampling, in which accuracy is critical. Knowing the type of ferromagnetic mineral in the materials, its concentration and grain-size is important to assess the stability of palaeomagnetic signal. Preliminary results show that remanence is mainly carried by magnetite, indicating that the hearths were performed in an essentially reducing atmosphere. Most demagnetization diagrams of the natural remanent magnetization from the three studied hearths display a high-intensity, stable and normal polarity palaeomagnetic component. This suggests that these materials are suitable recorders of the EMF in the past. Three different mean directions were obtained. Currently, additional samples are processed to refine the statistic. Although preliminary, these results show great potential to identify diachronies among prehistoric occupations associated to combustion structures.