Archaeomagnetic and rock magnetic study of six kilns from North Africa (Tunisia and Morocco)

Abstract

International audienceNew full-vector archaeomagnetic data for North Africa recovered from the study of six kilns, five from Tunisia and one from Morocco, are presented. Archaeological and historical considerations, along with three radiocarbon dates, indicate that the age of the kilns ranges between the 9th and 15th centuries AD. Rock magnetic analyses showed that the principal magnetic carriers are magnetite and low Ti titanomagnetite, along with variable contributions of thermally stable maghemite and a high coercivity phase with low unblocking temperatures. The magnetic mineralogy of the studied material is thermally stable and behaves ideally during archaeointensity experiments. Stepwise alternating field demagnetization isolated a single, stable, characteristic remanence component with very well defined directions at both specimen and structure levels. Mean archaeointensities have been obtained from successful classical Thellier experiments conducted on between five and eight independent samples per kiln. Thermoremanent magnetization (TRM) anisotropy and cooling rate effects upon TRM intensity have been investigated. The results showed that these effects are low for four of the six studied kilns, with differences between the uncorrected and corrected means of less than 3 per cent. For the other two structures differences between the uncorrected and corrected mean site intensities are 4.4 per cent and 5.8 per cent. These results highlight the necessity for TRM anisotropy and cooling rate corrections in archaeomagnetic studies if accurate archaeointensities are to be obtained. The new results suggest that high intensities occurred in Northwest Africa during the 9th century. Although more data are clearly needed to define this period of high intensity, the results are in agreement with the available European archaeointensity data. Acomparison between the newdata, other available archaeomagnetic determinations in nearby locations, and palaeosecular variation (PSV) curves derived from the regional SCHA.DIF.3k and global ARCH3K.1 geomagnetic field models shows good agreement between the new data and directional results derived from the models. However, some differences are observed between geomagnetic field models intensity results and available archaeointensity data for the studied regions. This highlights the need for new data for unexplored regions such as North Africa. The new data presented here better constrains the evolution of the geomagnetic field during historical times in this region. They represent a new step towards the construction of a reference PSV curve for Northwest Africa. Once established, this curve will represent a new dating method for this region

On the ‘centre of gravity’ method for measuring the composition of magnetite/maghemite mixtures, or the stoichiometry of magnetite-maghemite solid solutions, via57Fe Mössbauer spectroscopy

“…Initial refinements including variable site occupancies were determined to be unreliable given the possible confounding effects of texture in the samples, so that phasespecific model site occupancies were used thereafter. Rietveld refinement of the DTU100 sample was consistent with the cubic Fd-3m crystal structure for magnetite [46], with no other phases being evident within the estimated ±2 vol.% uncertainty of the measurement 9 .…”

“…If it is known that the sample in question is a magnetite/maghemite mixture, then you may choose to convert the measured α ± dα into a weight percentage w ± dw using equation (9). If it is a magnetite-maghemite solid solution you may prefer to report the stoichiometry x ± dx using equation (5).…”

“…Indeed, magnetite is the most magnetic of all the naturally occurring oxides on Earth, having a room temperature saturation magnetisation by mass of M s = 92 Am 2 kg −1 , while maghemite is also strongly magnetic, with M s = 76 Am 2 kg −1 at room temperature [2]. These phases play a significant role in geomagnetism and palaeomagnetism [3][4][5][6], and are also important markers in archaeomagnetism, largely due to their presence in clays and clay products such as pottery [7][8][9]. Maghemite is an oxidation product of magnetite, and as such the interplay and balance between the two phases is often informative, both in centuries-old samples [10] and in more contemporary materials, such as the corrosion products of carbon steels [11][12][13].…”

Investigating the archaeointensity determination success of prehistoric ceramics through a multidisciplinary approach: new and re-evaluated data from Greek collections

“…The experimental procedures followed are the same as in previous studies (e.g. Gómez-Paccard et al 2006, 2012b. In order to detect possible alteration in the magnetic mineralogy of the samples, regular partial thermoremanent magnetization (pTRM) checks were performed every two temperature steps.…”

The multimethod palaeointensity approach applied to volcanics from Terceira: full-vector geomagnetic data for the past 50 kyr

“…If more than one interval was obtained, a horizontal thin grey line connects them and the thickness of the bars is equivalent to their relative probability. The VADM literature data (d) from São Miguel are yellow squares ; the Canary Islands are diamonds in blue and pink (Ferk et al 2011); West-Africa are triangles (pointing up) in red (Mitra et al (2013) and green (Gómez-Paccard et al 2012b); Portugal are triangles (pointing down) in purple (Hartmann et al 2009) and green (Nachasova & Burakov 2009); Spain are circles in pink (Catanzariti et al 2012), light-blue (Gómez-Paccard et al 2006, dark blue (Gómez-Paccard et al 2008), green (Gómez-Paccard et al 2012a) and orange (Gómez-Paccard et al 2013). Symbols for the data obtained in this study are slightly larger, have error bars, and use the same symbols as in (c).…”