Assessing post-depositional processes in archaeological cave fires through the analysis of archaeomagnetic vectors

Abstract

This paper presents a methodological application of archaeomagnetism and rock-magnetism as a tool to evaluate post-depositional mechanical alterations in archaeological cave fires. Most taphonomic and post-depositional studies on anthropogenic sediments have mainly focused on the diagenetic alterations that these contexts undergo from a geochemical point of view. However, physical alterations are still largely assessed from mere macroscopic observations of the burnt facies comprising these fires. This paper compares the archaeomagnetic directions recorded in the ashes of two Holocene fires from the Mirador Cave (Sierra de Atapuerca, Burgos, Spain), one apparently well-preserved and the other clearly bioturbated. Vector analyses of archaeomagnetic directions, together with the study of magnetic properties combined with field (macroscopic) observations, can provide a powerful tool to assess when a fire is actually in situ. The anisotropy of magnetic susceptibility (AMS) in both fires exhibits a dominant sedimentary fabric produced by compaction, and also exhibits differences between the areas mechanically disturbed within these fires from those which are not. The following set of magnetic features was identified when an archaeological fire preserves its primary position: i) univectoral and high intensity NRM orthogonal demagnetisation diagrams in ashes, ii) Koenigsberger (Qn) ratio values higher than unity indicative of an undisturbed thermo-remanence (TRM) or a partial-thermoremanence (p-TRM) and iii) a good clustering of characteristic directions defining an statistically representative mean archaeomagnetic direction. The concurrence of these observations can be used as criterion to determine when an archaeological cave fire is physically in situ, thus allowing a correct interpretation of the archaeological record.