Assessing post-depositional processes in archaeological cave fires through the analysis of archaeomagnetic vectors Á. Carrancho a, b, * , J.J. Villalaín a , J.M. Vergès c , J. Vallverdú c a Dpto. Física, Universidad de Burgos, Escuela Politécnica Superior, Avda. Cantabria S/N, 09006 Burgos, Spain b National Research Center on Human Evolution (CENIEH), Avenida Sierra de Atapuerca, s/n, 09002 Burgos, Spain c Institut Català de Paleoecologia Humana i Evolució Social (IPHES) e Àrea de Prehistòria, Universitat Rovira i Virgili, Pl. Imperial Tarraco,1, 43005 Tarragona, Spain article info Article history: Available online xxx 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 (Q n ) 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. Ó 2012 Elsevier Ltd and INQUA. All rights reserved. 1. Introduction Determine whether an archaeological feature is in situ or not is a key aspect in the interpretation of the occupation of archaeological sites and cave deposits. The correct interpretation of the type, nature and duration of human occupations depends largely on the proper preservation of the (geo)archaeological record. Once deposited, both the archaeological remains and the surrounding sediments can undergo multiple syndepositional and post-depositional processes related to anthropogenic, geogenic and biogenic activity (Goldberg and Sherwood, 2006). These secondary processes include physical alterations that can lead to mechanical reorganization of the sedi- ments (e.g., trampling, digging, burrowing) and/or chemical alterations (e.g., diagenesis and mineralogical transformations). Responsible agents may be human/biological or natural in origin and depending on the degree of impact, the integrity of the archaeological record can be severely affected and therefore its interpretation. The study of site formation and post-depositional processes at archaeological sites have been traditionally addressed with stan- dard sedimentological techniques to bulk samples including grain size and compositional analysis, pH, calcium carbonate or organic matter contents, among others (e.g., Bonifay, 1956; Farrand, 1975). More recently, others analytical techniques such as soil micro- morphology, scanning electron microscopy (SEM), X-ray analyses (e.g., microprobe and X-ray diffraction), and Fourier Transform Infrared spectrometry (FTIR) have been extensively employed in studies of mineral diagenesis, bone preservation or evolution of the sediments at many sites worldwide (e.g., Weiner et al., 1993, 2002, 2007; Shahack-Gross et al., 2003; Karkanas, 2010). The potential of these techniques with regard to post-depositional processes at archaeological sites is indisputable and have expanded over time as new analytical techniques have become available. Magnetic methods represent one of the most versatile tech- niques applied to archaeology, increasingly used in recent years to * Corresponding author. Dpto. Física, Universidad de Burgos, Escuela Politécnica Superior, Avda. Cantabria S/N, 09006 Burgos, Spain. E-mail address: acarrancho@ubu.es (Á. Carrancho). Contents lists available at SciVerse ScienceDirect Quaternary International journal homepage: www.elsevier.com/locate/quaint 1040-6182/$ e see front matter Ó 2012 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2012.01.010 Quaternary International xxx (2012) 1e9 Please cite this article in press as: Carrancho, Á., et al., Assessing post-depositional processes in archaeological cave fires through the analysis of archaeomagnetic vectors, Quaternary International (2012), doi:10.1016/j.quaint.2012.01.010