Magnetic Investigations of Buried Palaeohearths Inside a Palaeolithic Cave (Lazaret, Nice, France) ABIR JRAD 1,2 * , YOANN QUESNEL 1 , PIERRE ROCHETTE 1 , CHOKRI JALLOULI 2,4 , SAMIR KHATIB 3 , HANANE BOUKBIDA 1 AND FRAN¸ COIS DEMORY 1 1 Aix-Marseille Universit´e, CNRS, IRD, CEREGE UM34, Aix-en-Provence, France 2 D´epartement de G´eologie, FST,Tunis El-ManarUniversit´e,2092 Manar 2, Tunis, Tunisia 3 Laboratoire D´epartemental de Pr´ehistoire du Lazaret, Nice France 4 Geological Department, College of Science, King Saud University, P.O. Box. 2455, Riyadh 11451, KSA ABSTRACT We present a magnetic study of palaeohearths within Lazaret cave (Nice, France) that demonstrates how to recognize fired structures in similar geological contexts. Using magnetic field and susceptibility mapping, excavated and potentially still-buried palaeohearths of the cave are investigated. Our study reveals some difficulties in conducting a magnetic field survey to detect combustion features in a cave due to noise and ambiguities in anomaly assignment. To overcome these difficulties, discrete measurements and a specific post-processing methodology were applied to remove the magnetic noise generated by surrounding artificial sources. In addition, experimental and numerical modelling constrained by laboratory examinations of the magnetic mineralogy were performed to better identify the magnetic imprint of such fireplaces. We confirm that a short-term fireplace produces a thin ash-bearing layer charac- terized by a high magnetic susceptibility and a high frequency dependence due to a large proportion of grains of pseudo-single-domain (PSD) size. Such a burnt soil layer is the main source of the ca. 50 nT amplitude magnetic field anomaly at a sensor height of 15 cm observed over the excavated palaeohearth, as well as over an experimental hearth. Copyright © 2013 John Wiley & Sons, Ltd. Key words: Lazaret cave; magnetic prospection; palaeohearths; magnetic mineralogy Introduction In Palaeolithic times, nomadic people temporarily occupied caves and used camp fires for light, cooking and heating. On archaeological sites these palaeohearths are often confused with environments enriched in organic matter. To better identify this type of archaeo- logical vestige, non-destructive geophysical methods such as magnetometry can be used (Scollar et al., 1990). Indeed, heating modifies the magnetic signa- ture of soil (Le Borgne, 1960; Maki et al., 2006; Carrancho and Villalaín, 2011; Brodard et al., 2012), allowing palaeohearths and normal organic soil to be distinguished. Magnetic study of ancient fireplaces has been the sub- ject of numerous studies utilizing field and laboratory analyses. Gibson (1986) focused on short-term fireplaces made by nomadic people of the Palaeolithic era. The combination of low-resolution open-air field prospection with reconstruction of a campfire allowed him to define a weak magnetic anomaly for such features (about 8 nT for 30 cm distance between field probe and soil surface). Barbetti (1986) combined field magnetic prospection with a laboratory examination of the magnetic fabric to detect evidence of fire. He suggested an experimental recon- struction to determine the impact of heating. Bellomo (1993) developed a methodological approach to deter- mine evidence of anthropogenic fires. Later, Morinaga et al. (1999) studied the impact of heating on the mineralogical transformations of different soil types and developed a laboratory magnetic method to detect heated soils in ancient sites. Linford and Canti (2001) * Correspondence to: A. Jrad, Aix-Marseille Universit´e, CNRS, IRD, CEREGE UM34, Aix-en-Provence, France. E-mail: abirgeo@gmail.com Copyright © 2013 John Wiley & Sons, Ltd. Received 24 March 2013 Accepted 11 October 2013 Archaeological Prospection Archaeol. Prospect. 21, 87–101 (2014) Published online 8 November 2013 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/arp.1469