Three archaeomagnetic applications of archaeological interest to the study of burnt anthropogenic cave sediments

Abstract

Recent archaeomagnetic studies carried out on Mid-to Late Holocene burnt anthropogenic cave sediments have shown that under certain conditions, these materials are suitable geomagnetic field recorders. Archaeomagnetic analyses carried out on these contexts constitute a rich source of information not only for geophysical purposes -in terms of reconstructing the variation of Earth's magnetic field in the past- but also from the archaeological point of view, for example by archaeomagnetic dating. Here, we report three different archaeomagnetic applications to the study of burnt cave sediments: (i) archaeomagnetic dating; (ii) determining palaeotemperatures and (iii) assessing post-depositional processes. The first case study is a dating attempt carried out on a Late Holocene (Bronze Age) burnt level from El Mirador Cave (Burgos, Spain). Using the directional European secular variation curve, several dating intervals were obtained for the last burning of this combustion feature. Considering the archaeological evidence and the independent radiometric (14C) dating available the possible ages obtained are discussed. This is the first archaeomagnetic dating obtained in these contexts so far. The second case study is an application of the method to determine the last heating temperatures reached by the carbonaceous facies of these fires. Stepwise thermal demagnetization of oriented samples can be used to quantitatively estimate heating temperatures. An intermediate normal polarity component interpreted as a partial thermo-remanence (pTRM) with maximum unblocking temperatures of 400 e450 C was systematically identified, revealing the last heating temperatures experienced by this facies. These temperatures were confirmed with partial thermomagnetic curve experiments. Finally, archaeomagnetic analyses on a partially bioturbated burning event were performed in order to evaluate until what spatial extent the burnt sediments were affected by post-depositional mechanical alteration processes. For each case study, the archaeological implications are discussed highlighting the potential of archaeomagnetic methods to retrieve archaeological information.