Rock-magnetic analyses as a tool to investigate archaeological fired sediments: a case study of Mirador cave (Sierra de Atapuerca, Spain)

Abstract

SUMMARY Here we report a detailed mineral magnetic study of Neolithic burnt levels in the Mirador Cave (Sierra de Atapuerca, Burgos, Spain) to reconstruct the burning history and to investigate their suitability for archaeomagnetic purposes. As a consequence of the ancient burning, a characteristic facies sequence was developed along the Holocene stratigraphy. From top to bottom it includes: (i) 2–10 cm ash layers, (ii) ∼2 cm underlying rubefied layers and (iii) clay, mainly unburnt and of variable thickness. In some cases a thermally altered facies (2–6 cm) with a heterogeneous texture was identified (facies TF), usually between rubefactions and the unburnt levels. 126 oriented samples from 4 units (MIR12, 15, 18 and 21) and a 2 m section, all comprised between units MIR21 (6380 ± 40 14C BP) and MIR9 (5090 ± 40 14C BP) were analysed with rock magnetic methods. In addition, bulk sediment from each facies that comprise the Neolithic sequence was investigated. Measurements included: stepwise alternating field and thermal demagnetization of natural remanent magnetization (NRM), viscosity experiments, determination of the anisotropy of the magnetic susceptibility (AMS), the susceptibility frequency dependence at room temperature and determination of the temperature dependence of the susceptibility. Additional experiments consisted in the determination of the behaviour of anhysteretic and isothermal (IRM) remanences, magnetic hysteresis loops, first-order-reversal-curve diagrams, and thermal demagnetization of three-axial IRM. It appeared that the facies all show a fairly similar magnetic mineralogy and grain size dominated by low-titanium magnetite that is often partially maghaemitized. Main differences constitute the amount of superparamagnetic particles that is higher in unburnt strata concurring with a less well-defined NRM behaviour. The magnetic mineral concentration is notably higher in ashes. This homogeneity strongly suggests that similar sources and burial conditions prevailed during Neolithic times. Agreeing with archaeological observations and favoured by rapid burial conditions, very limited alterations have been deduced. AMS data revealed the absence of fluid flow in the ash lenses sampled. Demagnetization revealed a stable single NRM component in ashes, a single or two-component NRM in rubefactions and less stable multicomponent behaviour in clays. In ashes, magnetic minerals are likely secondary in origin formed by low-temperature oxidization soon after burning. Although this thermochemical nature of the NRM invalidates the use of these sediments for palaeointensity studies, archaeomagnetic (directional) data can be successfully obtained because the burning and oxidation are closely confined in time.