magnetic investigations of buried palaeohearths inside a palaeolithic cave (lazaret, nice, france)

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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 red structures in similar geological contexts. Using magnetic eld and susceptibility mapping, excavated and potentially still-buried palaeohearths of the cave are investigated. Our study reveals some difculties in conducting a magnetic eld survey to detect combustion features in a cave due to noise and ambiguities in anomaly assignment. To overcome these difculties, discrete measurements and a specic post-processing methodology were applied to remove the magnetic noise generated by surrounding articial 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 replaces. We conrm that a short-term replace 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 eld 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 res 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 modies 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 replaces has been the sub- ject of numerous studies utilizing eld and laboratory analyses. Gibson (1986) focused on short-term replaces made by nomadic people of the Palaeolithic era. The combination of low-resolution open-air eld prospection with reconstruction of a campre allowed him to dene a weak magnetic anomaly for such features (about 8 nT for 30 cm distance between eld probe and soil surface). Barbetti (1986) combined eld magnetic prospection with a laboratory examination of the magnetic fabric to detect evidence of re. He suggested an experimental recon- struction to determine the impact of heating. Bellomo (1993) developed a methodological approach to deter- mine evidence of anthropogenic res. 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: [email protected] Copyright © 2013 John Wiley & Sons, Ltd. Received 24 March 2013 Accepted 11 October 2013 Archaeological Prospection Archaeol. Prospect. 21, 87101 (2014) Published online 8 November 2013 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/arp.1469

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Page 1: Magnetic Investigations of Buried Palaeohearths Inside a Palaeolithic Cave (Lazaret, Nice, France)

* Correspondence to: A. JrCEREGE UM34, Aix-en-Pro

Copyright © 2013 John

Archaeological ProspectionArchaeol. Prospect. 21, 87–101 (2014)Published online 8 November 2013 in Wiley Online Library(wileyonlinelibrary.com) DOI: 10.1002/arp.1469

Magnetic Investigations of BuriedPalaeohearths Inside a Palaeolithic Cave(Lazaret, Nice, France)

ABIR JRAD1,2*, YOANN QUESNEL1, PIERRE ROCHETTE1,CHOKRI JALLOULI2,4, SAMIR KHATIB3, HANANE BOUKBIDA1

AND FRANCOIS DEMORY1

1 Aix-Marseille Universite, CNRS, IRD, CEREGE UM34, Aix-en-Provence, France2 Departement de Geologie, FST, Tunis El-Manar Universite, 2092 Manar 2, Tunis, Tunisia3 Laboratoire Departemental de Prehistoire du Lazaret, Nice France4 Geological Department, College of Science, King Saud University, P.O. Box. 2455, Riyadh 11451, KSA

ABSTRACT We present a magnetic study of palaeohea

rths within Lazaret cave (Nice, France) that demonstrates how to recognizefired structures in similar geological contexts. Using magnetic field and susceptibility mapping, excavated andpotentially still-buried palaeohearths of the cave are investigated. Our study reveals some difficulties in conductinga 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 toremove the magnetic noise generated by surrounding artificial sources. In addition, experimental and numericalmodelling constrained by laboratory examinations of the magnetic mineralogy were performed to better identify themagnetic 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 ofpseudo-single-domain (PSD) size. Such a burnt soil layer is the main source of the ca. 50 nT amplitude magnetic fieldanomaly at a sensor height of 15 cm observed over the excavated palaeohearth, as well as over an experimentalhearth. Copyright © 2013 John Wiley & Sons, Ltd.

Key words: Lazaret cave; magnetic prospection; palaeohearths; magnetic mineralogy

Introduction

In Palaeolithic times, nomadic people temporarilyoccupied caves and used camp fires for light, cookingand heating. On archaeological sites these palaeohearthsare often confused with environments enriched inorganic matter. To better identify this type of archaeo-logical vestige, non-destructive geophysical methodssuch 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 tobe distinguished.

ad, Aix-Marseille Universite, CNRS, IRD,vence, France. E-mail: [email protected]

Wiley & Sons, Ltd.

Magnetic study of ancient fireplaces has been the sub-ject of numerous studies utilizing field and laboratoryanalyses. Gibson (1986) focused on short-term fireplacesmade by nomadic people of the Palaeolithic era. Thecombination of low-resolution open-air field prospectionwith reconstruction of a campfire allowed him to define aweakmagnetic anomaly for such features (about 8 nT for30 cm distance between field probe and soil surface).Barbetti (1986) combinedfieldmagnetic prospectionwitha laboratory examination of the magnetic fabric to detectevidence 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, Morinagaet al. (1999) studied the impact of heating on themineralogical transformations of different soil types anddeveloped a laboratory magnetic method to detectheated soils in ancient sites. Linford and Canti (2001)

Received 24 March 2013Accepted 11 October 2013

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88 A. Jrad et al.

characterized the magnetic anomaly of experimentalcampfires. They revealed that short-term campfires canproduce significant magnetic anomalies mainly due tothe magnetically enhanced ash layer. Maki et al. (2006)investigated prehistoric temporary base camps in orderto characterize the associated magnetic field gradientanomaly. They found that poor correlation between themagnetic gradient and the hearth may be produced bythe decrease of magnetic susceptibility under high tem-perature, which creates a negative inducedmagnetic fieldgradient anomaly. Herries (2009) examined the magneticmineralogy of purported prehistoric hearths within cavesin order to discuss the anthropogenic origin of burning.He also studied the palaeomagnetic behaviour of anexperimental hearth in order to differentiate burnt andunburnt zones. Later, Herries and Fisher (2010) investi-gated the magnetic mineralogy of a prehistoric hearthwithin a cave and its relation with the localization of thefireplace in order to reconstruct the organization of lifein the Stone Age. More recently, Carrancho and Villalaín(2011) investigated an experimental fire to follow thevariation of the magnetic parameters under fire. Theybrought to light the spatial variation of the magneticproperties over the fireplace. Two mechanisms wereidentified for the magnetic remanence acquisition underheating: thermoremanent magnetization (TRM) in thecentre of the fireplace, and thermo-chemical remanentmagnetization (TCRM) in the periphery.From this research it appears clear that a simple

magnetic field survey is not sufficient to identify buriedhearths, and that it is necessary to combine differentmethods in order to understand the related magneticanomaly. In this study we present a magnetic methodol-ogy that can identify a short-term hearth within a con-fined space in the case of the Lazaret Palaeolithic cave.Indeed excavations within this cave revealed severalpalaeohearths in different stratigraphic units. Furtherhearths are expected to be buried. In this work we firstintroduce the Lazaret cave and its archaeological context,and thenwedescribe themethods used to investigate thepotential buried palaeohearths in this cave. The resultingmagnetic maps are analysed using constraints frommagnetic propertymeasurements on samples. Addition-ally, results from experimental and numerical modellingof palaeohearths are shown and compared with thoseobtained in the cave. The last section compares theseresults with those from other studies.

Figure 1. Stratigraphic section of the excavated well in Lazaretcave. The top right panel indicates the geographical position ofthe cave in France.

Archaeological context

The Lazaret cave is located at the bottom of the westernslopes of the Mount Boron in Nice (France), within a

Copyright © 2013 John Wiley & Sons, Ltd.

cliff of Jurassic limestone. Its entrance faces thesouthwest direction at 26m above the present levelof the Mediterranean Sea. During glacial periods itwas situated 140m above the sea level (Valensiet al., 2007). This cave was occupied by Homo erectusbetween 130 and 190 ka (de Lumley et al., 2004). Thecave is about 40m long and 15m wide, with a thickPleistocene sediment section. A well dug under theentrance allows the observation of a 7-m-thick sequenceof continental sediments enriched in archaeologicalmaterial (Figure 1; Valensi et al., 2007). Systematicexcavation of the cave proceeded by excavating thinhorizontal layers that represent archaeological strati-graphic units (UA) and these were then correlated withdata from the well.

Archaeol. Prospect. 21, 87–101 (2014)DOI: 10.1002/arp

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Figure 2. Location of the magnetic field and susceptibility surveys in Lazaret cave, with the UA27 archaeological findings as background. Squaresare 1m2. This figure is available in colour online at wileyonlinelibrary.com/journal/arp

89Magnetic Study of Lazaret Palaeohearths

Archaeologists identified several fired structurescharacterized by a dark colour and charcoal fragments(de Lumley et al., 2004). Five palaeohearths were exca-vated in UA27. This was the last unit totally removedbefore our prospection. This UA represents a dark

Figure 3. Correlation between the frequency dependence parameters χf d meafrom this study. A linear correlation (green line) produces the relation χfd

(MS2) = 0.45bounds), but we preferred to assume zero intercept (black line). This figure is a

Copyright © 2013 John Wiley & Sons, Ltd.

brown iron-rich clay level of the upper Acheuleanperiod (about 170 ka). The surface of the level is litteredwith stone tools, bones and limestone blocks ofvariable dimensions (Figure 2). On this surface apalaeohearth is evident on the southwest side of the

sured with the MFK1 and MS2 instruments on the same set of samples� χfd

(MFK) + 4.16, withR2 = 0.53 (the red dashed lines delimit the confidencevailable in colour online at wileyonlinelibrary.com/journal/arp

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90 A. Jrad et al.

Q11 square. During our surveys, archaeologists wereexcavatingUA28 (no archaeologicalmaps are available),stratigraphically below UA27, and corresponding to theupper C II stratigraphical complex dating from thelast Middle Pleistocene (OIS 6 – 130 to 160 ka) glacialperiod (Figure 1; Valensi et al., 2007).

Figure 5. Interpolated magnetic field anomaly map (lower probe) overUA28 with the same grid scale as in Figure 2. Distances are indicatedin metres. This figure is available in colour online at wileyonlinelibrary.com/journal/arp

Methods

Methods applied to the cave palaeohearths

A magnetic field survey was performed over UA28(Figure 2) using an optically pumped caesium vapourGeometrics G858 magnetometer in a vertical gradientconfiguration. The heights of the lower and upperprobes above the soil surface were 15 and 65 cm, respec-tively. Some difficulties were encountered in thecave: the variable height of the roof, the presenceof large limestone blocks and many archaeologicalremains such as bones and surface palaeohearthsprevented us from performing a high-resolutioncontinuous survey. Therefore discrete measurementswith spacing of about 20 cm were used. The Lazaretcave is equipped with a net of safety cables, stair-cases and metallic bars as well as electric wires, allof which considerably pollute the magnetic signal.The lateral variation of the surface magnetic suscep-tibility (MS) over the Q11 square was also measuredusing a ZH Instruments SM30 (Figure 2; Lecoanetet al., 1999; Jordanova et al., 2003).Twenty soil samples from palaeohearths of UA27 and

UA28 were analysed using aMFK1 Agico device to mea-sure the bulkMS at different frequencies (F1=976Hz andF3=15 616Hz) and to calculate the frequency depen-dence, χfd, in order to estimate the proportion ofsuperparamagnetic (SP) grains (Dearing et al., 1996)

χf d ¼ χl f � χh fð Þ=χl f � 100 (1)

where χlf and χhf are, respectively, the susceptibilitiesmeasured at low and high frequencies. Despite the fact

Figure 4. Photographs of the different stages during the experiment: (a) beforthe SM30 susceptibility meter (20� 15cm) is shown for scale. In (c), the red anand the SM30 susceptibility meter, respectively. This figure is available in colo

Copyright © 2013 John Wiley & Sons, Ltd.

that the MFK instrument is, with respect to the well-known Bartington MS2 bridge, a more sensitive andprecise device for measuring the frequency depen-dence parameter (Hrouda and Pokorný, 2011;Chlupáèová et al., 2011), the χfd values presented inthis paper will be calibrated empirically to the MS2values in order to be comparable with the literature.Indeed the MS2 use a significantly different set offrequencies: 476Hz and 4760Hz.

e the first fire, (b) during the second fire and (c) after the second fire. In (a),d blue squares show the surveyed areas using the G858 magnetometerur online at wileyonlinelibrary.com/journal/arp

Archaeol. Prospect. 21, 87–101 (2014)DOI: 10.1002/arp

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91Magnetic Study of Lazaret Palaeohearths

The χf d parameter depends on both the frequencyrange and spread, and it was defined originallywith the MS2 frequencies. To obtain such calibra-tion we measured the same set of samples and av-eraged the ratio [ χfd(MS2)/χfd(MFK)], leading to thefollowing relationship:

χf dcal MS2ð Þ ¼ 0:66� χf d

MFK1ð Þ (2)

Figure 3 shows that the two χfd estimates are rea-sonably correlated, with the standard deviation onthe ratio being ±0.06.The combination of the MFK1 device with a CS3

furnace was used to perform thermomagnetic analysison these samples, with temperature ranging from roomtemperature up to 700 °C. Three steps were used: 250,450 and 700 °C. The data were analysed using Cureval.8(Hrouda, 2003). Using a PMC MicroMagTM VibratingSample Magnetometer, Model 3900 (μVSM) with a

Figure 6. Resulting magnetic field anomaly map (lower probe) afterlocal 1m2 processing. The red squares show anomalies potentiallydue to palaeohearths. Distances are indicated in metres. This figureis available in colour online at wileyonlinelibrary.com/journal/arp

Copyright © 2013 John Wiley & Sons, Ltd.

sensitivity of 5� 10�9 A m�2, hysteresis loops and iso-thermal remanence magnetization (IRM) measurementswere also performed. The natural remanent magnetiza-tion (NRM) was measured using a supraconductingrock magnetometer (2G Enterprises, model 760R DCSQUID, with a sensitivity of 2� 10�11 A m�2), and alter-nating field demagnetization was performed using 2GEnterprises’ online coil system. Fifteen levels of demag-netization up to 100 mTwere applied. The results weresubsequently processed using the Paleomac software(Cogné, 2003).All the laboratory measurements were performed in

CEREGE (Aix en Provence, France).

Methods to model the cave palaeohearths

Experimental modelling

Using excavated soil from UA27, a 9-cm-thick, 25-cm-radius vertical cylinder containing a mixture ofcompacted brown clay was built over a wood platform(Figure 4a): it is called ‘the experimental hearth’ in thefollowing. All the material used comes from the wholeUA27, part of which had been fired in the Palaeolithic.Therefore this material is a mixture of previously firedand unfired material, and although the proportion isunknown the fired component is likely to be minor.The final volume was about 18 L with a mass of about32 kg. The cylinder was confined within limestoneblocks. To simulate the Palaeolithic conditions ofcombustion, the ventilation of the fire was manuallycontrolled, except for a few occasions where a blower

Figure 7. Resulting magnetic field anomaly map (lower probe) overR12–R13 and S12–R13 after local 4m2 processing. This figure isavailable in colour online at wileyonlinelibrary.com/journal/arp

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Figure 8. Magnetic susceptibility map centred on Q11 in Lazaret cave.Distances are indicated in metres. This figure is available in colouronline at wileyonlinelibrary.com/journal/arp

92 A. Jrad et al.

was used to keep the fire alive. In a first experiment,dried seaweed was used as fuel, after initiating the firewith dry pine branches. Indeed Valensi et al. (2007)have found evidence for the use of such fuel in theLazaret cave. The fire lasted 4 h 42min. Several weekslater, a second experiment was performed with a fuelcomposed of seaweed and dry pine branches(Figure 4b). This allowed the combustion to occur ata higher temperature. The fire lasted 6 h 08min. Inboth experiments, the soil temperature was measuredby a K-type thermocouple buried at 2 cm beneath thecentre of the experiment. Maximum soil temperaturesof 69 °C (after 5 h) and 141 °C (after 6 h) werereached during the first and second fires, respec-tively. The second fire was more intense due to theadded fuel and to the soil dryness after the first fire

Figure 9. Plot of calibrated percentage frequency dependent magnetic suscesamples. The samples with weak susceptibility (< 2� 10�7m3 kg�1) are notare not plotted because their frequency dependence is poorly defined.

Copyright © 2013 John Wiley & Sons, Ltd.

(Morinaga et al., 1999). Once the fire stopped, wegently cleaned unburnt fuel and unstable ashes toobtain a measurable surface. Magnetic field mea-surements were performed northward on a woodenruler placed at 8 cm above the hearth’s surface. Thesurvey was discrete with a 10 cm grid (Fig 4c).Surface MS measurements were also acquired usinga 5 cm grid. All measurements were performed inopen air, away from artificial sources.To better understand the mineralogy of these sam-

ples, thermomagnetic measurements (in air) were alsoperformed on samples taken from the experimentalhearth before the fires (BF samples) and from the stableash layer after the fires (AL sample; see the grey sur-face in Figure 4c). Alternating field demagnetizationof NRM was also applied with 15 steps up to 100 mT.Such analysis ensures that the measured remanenceis the natural one rather than a secondary spuriousremagnetization. Hysteresis curves of BF, AL andanother sample from the final state of the experimentalhearth (depth of 0.7 cm; AF0.7) were used to assess theeffect of the fires on the soil’s viscosity. We compare thecurves of the latter samples with those of somesamples (A2, B9, B6’, Q13) from the Lazaret cave.Finally, the variation of the soil’s magnetic propertieswith depth was investigated by sampling (2mmresolution) the experimental hearth after the secondfire down a vertical profile beneath the centre. TheKoenigsberger ratio (Q: remanent versus inducedmagnetization intensity) was calculated for this verti-cal profile (Dunlop and Ozdemir, 1997). However, itwas not possible to apply this sampling on the hearthbefore the fires, due to its potential effect on thestability of the experiment.

ptibility (χfdcal) versus magnetic susceptibility of the cave’s palaeohearth

plotted. The samples B3 and B4 with weak susceptibility (see Fig.11)

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Figure 10. Thermomagnetic curves of the sample taken on the palaeohearth in the S14 square. This figure is available in colour online atwileyonlinelibrary.com/journal/arp

93Magnetic Study of Lazaret Palaeohearths

Numerical modelling

Using magnetic field anomaly observations over theexperimental hearth, the magnetic properties andgeometry of the hearth’s layers before and after the fireswere constrained by numerical modelling. The GM-SYSmodule of the GEOSOFT Oasis Montaj software wasused. The initial magnetic properties (susceptibilitiesand NRM) were set according to the measurements onthe BF and AL samples. An uncertainty up to 20%(combination of laboratory measurement error, soilheterogeneity and uncertainty on soil density) was con-sidered for the magnetic parameters during the model-ling. The ambient magnetic field parameters werethose observed for the experiment day and location(F=46286 nT, I=59.3°, and D=0.6°; Finlay et al., 2010),while the soil density was approximated to about1700 kgm�3 (Bevan, 1999). Practically the approachconsists in building a reasonable starting model underGM-SYS (forward modelling: manual adjustment ofgeometry and parameters constrained by the experi-mental hearth observations). Then an automaticadjustment (inversion) requiring iterations is performed,

Figure 11. The NRM compared with the bulk MS of the cave’s samples.

Copyright © 2013 John Wiley & Sons, Ltd.

constrained by the initial model and by the parameter(including geometry) uncertainties. The quality of thefinal model is checked by the root mean square (RMS)error value. Several initial geometries were testedfor the model representing the last state of theexperimental hearth.

Results on palaeohearths of the cavemagneticfield survey over UA28

The resulting magnetic field anomaly map is shownin Figure 5 for the low probe. High dynamics areobserved (250 nT of maximum amplitude), espe-cially near the edges of the area surveyed, which causedby the surrounding metallic artefacts. A negativemagnetic anomaly signal characterizes the southernpart around squares R13 and Q10 that may repre-sent areas not influenced by surrounding magneticperturbations. To remove this noise from the mag-netic observations, we first applied standard filterssuch as removing a Gaussian or a polynomial

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Figure 12. Magnetic anomaly maps over the experimental hearth at the lower probe height, for the following three stages: (1) before the first fire, (2)after the first fire and (3) after the second fire. Distances are indicated in metres. This figure is available in colour online at wileyonlinelibrary.com/journal/arp

94 A. Jrad et al.

surface. It was successful to isolate an anomaly overthe Q11 square, but large signals are still observedon most of the map. Therefore specific localized pro-cessing was applied to better highlight potentialmagnetic anomalies due to palaeohearths. The trendof each prospection line within every 1m2 of thearea surveyed was removed (Figure 6). Indeed thesize of the Lazaret cave palaeohearths is generallyless than 1m2, so we expected by this method toisolate anomalies possibly linked to palaeohearths.For instance, in the Q11 square, a palaeohearth visi-ble on the ground of UA28 is clearly associated witha dipolar magnetic anomaly with a wavelength ofabout 0.5m and a coherent amplitude of about 50nT (Figure 6). Three other anomalies in Q12, R13and R12 squares possess comparable properties tothe Q11 anomaly (Figure 6). The R12 anomaly hasthe largest wavelength and intensity, possibly dueto a large buried hearth not entirely revealed byour 1m2 localized processing. To better characterizethis anomaly and to minimize the edge effect seenon Figure 6, an area of 4m2 (e.g. red square onFigure 6) was processed by removing the trend ofeach profile in the 1m2 squares included. Theresulting anomaly was then fully characterized asshown in Figure 7. This presumed palaeohearthproduces a large wavelength anomaly (0.85m) and

Figure 13. Magnetic susceptibility maps obtained on the hearth’s surface: (1Distances are indicated in metres. This figure is available in colour online a

Copyright © 2013 John Wiley & Sons, Ltd.

should be found buried at a maximum depth of1m. Thus it may correspond to the largest fireplaceever found in the Lazaret cave.

Surface magnetic susceptibility survey around Q11

The largest susceptibility values (about 3� 10�3 SI)are located over the palaeohearth Q11 (Figure 8).On the northern side of Q12, the area with highsusceptibility values corresponds to a significantmagnetic field anomaly (Figure 6). The variabilityof these surface susceptibility measurements alsoappears to be controlled by the presence of (diamagnetic)limestone blocks.

Magnetic properties of Lazaret’s palaeohearths

Figure 9 compares the frequency dependence with theMS for the samples collected on palaeohearths. Itshows that no distinction can be made betweensamples from UA27 and those from UA28: a clusteraround the mean MS of 3.5� 10–6m3 kg�1 is observed.Only sample Q13b possesses a significantly higher MS:it was taken at the edges of the palaeohearth Q13 fromUA27. On the other hand, limestone samples B3, B4and B6p have weak MSs. The frequency dependence ofhearth samples is generally higher than 15%. These

) before the first fire, (2) after the first fire and (3) after the second fire.t wileyonlinelibrary.com/journal/arp

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Figure 14. Thermomagnetic curves of samples taken on the experimental hearth before and after the fires. This figure is available in colour online atwileyonlinelibrary.com/journal/arp

95Magnetic Study of Lazaret Palaeohearths

results confirm that the heating of the ground pro-duces SP grains (Le Borgne, 1960), and that the soilburnt in hearths has a strong MS and a small grainsize evidenced by a strong frequency dependence.Figure 10 shows one of the thermomagnetic analy-ses on the hearth samples. It clearly reveals that acommon Curie point of 580 °C is observed for all

Figure 15. Demagnetization of the NRM of the experimental hearth’ssoil sampled before the fires (BF) and after the fire in the ash layer(AL). This figure is available in colour online at wileyonlinelibrary.com/journal/arp

Copyright © 2013 John Wiley & Sons, Ltd.

curves, indicating that magnetite is the main magne-tization carrier of the palaeohearth’s samples. Theincrease upon heating to 450 °C is not large, show-ing that it previously underwent heating of the soilduring prehistory. At high temperature, the MS in-creases during the cooling. This is probably due tothe neoformation of magnetite through the reduc-tion of Fe3+ present in the samples.To illustrate the loss of NRM by alternating field

demagnetization, NRM and mass MS for the 0 mTand 10 mT levels are shown in Figure 11. Thehomogeneity of samples from the cave is clear,except for the limestone samples (B3, B4 and B6p).The mean MS of the Q11 hearth’s samples is about3.6� 10�6m3 kg�1, corresponding to a Q value ofabout 0.5. This indicates that the observed magneticanomalies are due mainly to induced magnetizationbut with a significant additional contribution of aremanence nearly parallel to induced magnetiza-tion. Indeed the characteristic remanence measure-ments of oriented samples (B5, B6, B6p, B7, B8, B9and B10) from UA28 record a mean declinationand inclination of 14 ± 8.4 and 67 ± 3.3 (withk= 335.6 and α95 = 3.3), respectively. These nearpresent-day directions (I = 59.6, D = 1.1) are consis-tent with the age of the layer and previouspalaeomagnetic investigations of the Lazaret wholesection that revealed only normal polarity (de Lumleyet al., 2004).

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Figure 16. Normalized viscous decay of saturation remanence (IRM) versus log of time. This figure is available in colour online at wileyonlinelibrary.com/journal/arp

96 A. Jrad et al.

Results of palaeohearth modelling

Experimental modelling

Figure 12 shows the magnetic anomaly maps over theexperimental hearth before, between and after the twoconsecutive fires. It appears that the original unburntsoil (composed of brown clays) itself is sufficiently mag-netic to produce a 40-nT-amplitude dipolar magneticanomaly approximately centred at the experiment cen-tre, typical of magnetization parallel to ambient field.Moderate vertical gradients are located in the northeastcorner and probably correspond to the effect of the ther-mocouple and its fixture. The weak SW–NE gradientvisible on the upper probe before heating is difficult toexplain, but probably originates outside the experiment.The magnetic anomaly amplitude is about 90 nT at thelower-sensor altitude after the second fire. This is con-sistent with the increase observed over experimentalfires performed by Linford and Canti (2001). Figure 13shows that the first fire had a limited impact on the sur-face MS, whereas the second fire significantly increasedit at the centre of the fireplace. However, these previoussurface MS measurements concern only the firstcentimetres of the experimental hearth (Lecoanet et al.,

Table 1. Mineral magnetic data of palaeohearth and experimental hear

Sample χ χfdcal Hcr H

(m3 kg�1� 10�6) (%) (mT) (m

BF 2.5E + 00 11 17 6AL 2.0E + 01 11 22 7AL0.7 2.7E + 00 11 18 6A2 3.2E + 00 10 16 6B6p 1.6E + 00 11 16 5Q13 3.5E + 00 10 17 5

Copyright © 2013 John Wiley & Sons, Ltd.

1999; Kapièka et al., 2011). The susceptibility values atlow frequency for the sample taken from the hearth be-fore fires (denoted BF) and from the upper millimetresafter burning (AL) are 2.5� 10�6m3kg�1 with χfdcal =11.4% and 20.4� 10�6m3 kg�1 for χfdcal = 11.1%, respec-tively. The high values of χfd underline the abundanceof ultrafine grains.All thermomagnetic curves on the same samples

show reversibility upon heating to 250 °C (Figure 14),indicating no alteration of the ferromagnetic fraction.For the 450 °C and the 700 °C steps, a variation of thebehaviours of the BF and AL samples is observed.For the latter, the largest increase in susceptibility isobserved after heating at 450 °C. The onset of magneticmineral neoformation occurs after 300 °C. At highertemperature, the neoformed minerals are partly des-troyed and the final MS becomes the same as the initialone. The maximum Curie point of 580 °C is typical ofpure magnetite, although more substituted phases arepresent. For the BF sample, the thermomagnetic curvesshow little irreversible changes with the heating up to450 °C, with a maximum shift of the cooling curve atlower MS values after 270 °C. With 700 °C heating,the same Curie points are observed but here thesusceptibility has increased after heating. This latter

th samples.

c Ms Mrs Mrs/Ms Hcr/Hc

T) (μAm2) (μAm2)

95 14 0.15 2.8862 129.4 0.15 3.1106 16 0.15 3107 17 0.16 2.788 14 0.16 3.2

130 16 0.12 3.4

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Figure 17. Day plot of hysteresis parameters.

97Magnetic Study of Lazaret Palaeohearths

result indicates the formation of magnetite, instead ofdestruction, for the AL sample. The final MS valuebecomes similar to the sample collected after the fires.Figure 15 shows that the NRMof the soil was 76 times

stronger after the experiment (20.5� 10�4 A m�2 kg�1)than before (0.27� 10�4 A m�2 kg�1). An easydemagnetization at the early stages is observed. TheBF sample loses 80% of its magnetization at 20mT. Only6% of the AL sample magnetization is lost at 10 mTand40% at 20 mT. This indicates that the fire increased thecoercivity of the soil, as Morinaga et al. (1999) and

Figure 18. Variation of magnetic properties of the experimental hearth with

Copyright © 2013 John Wiley & Sons, Ltd.

Linford and Canti (2001) observed. These authors alsorevealed that the NRM intensity variations due toheating are much larger than theMS variations, and thatthe magnetic stability of the remanent magnetizationincreases with temperature. Figure 16 shows the satura-tion remanence decay curves of BF, AL and anothersample from the experimental hearth after fires, as wellas of samples from Lazaret cave (Table 1). Burnt soilsamples cannot be distinguished from unburnt ones.Thus, the fires seem to have no effect on the magneticgranulometry of the soil, but should enhance the concen-tration of magnetic minerals. The Day plot of hysteresisparameters of all samples confirms the finemineralogicalcomposition of Lazaret cave’s palaeohearths (Figure 17).The samples fall in the PSD (pseudo-single domain)region of the Day Plot (Dunlop, 2002), with a deviationtoward larger Hcr/Hc, typical of mixing with SPgrain. The importance of SP grains in the Lazaretsoil samples has been further demonstrated by mea-surement of viscous decay of saturation remanence(IRM) on the scale of several hundreds of seconds,using the VSM. Large relative decreases are observed,with similar behaviour being observed for all thevarious samples (Figure 16).The vertical profiles in Figure 18 show that the

heating affected the first millimetres of the experimen-tal hearth for the induced magnetization and the firstcentimetre for the NRM. Within the top 3 cm NRM islarger than the induced magnetization, so it must betaken into account to explain the magnetic field

depth.

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Table 2. Resulting parameters of the model corresponding to theexperimental hearth.

Layer K (SI) Magnetization (A m�1)

Ash 0.04 4.5

98 A. Jrad et al.

anomaly observed over the experiment after the sec-ond fire. This situation is explained by a combinationof the effect of neoformation of magnetic grains and ac-quisition of a TRM, a magnetization process muchmore efficient that the pre-existing NRM of depositionor viscous origin. We can conclude for our case that theTRM affects 3 cm at the top of the experimental hearth.The neoformation of magnetic minerals, which enhancedthe magnetic susceptibility, is mostly restricted to the firstfew millimetres (at least when heated to 300 °C), whilemore moderate heat has remagnetized the pre-existingNRM of the soil without creating new magneticminerals. This effect is illustrated by the variationof the Koenigsberger ratio from about 3 above 5mm toabout 0.5 below 4 cm.

Figure 19. Resulting numerical models of the experimental hearth(a) before and (b) after the heating by fire. This figure is availablein colour online at wileyonlinelibrary.com/journal/arp

Copyright © 2013 John Wiley & Sons, Ltd.

Numerical modelling

The final properties and geometry of the modelledlayers before and after fires are shown in Table 2and Figure 19, respectively. The observed magneticfield anomaly is well predicted: RMS values of 1.9and 2.6 nT were obtained for the models beforeand after the fires, respectively. The resultingmagnetic properties and the geometry of each layer

Base 0.0059 0.055

Figure 20. Resulting numerical models of (a) a known palaeohearth ofLazaret cave, beneath the magnetic field anomaly observed in Q11and (b) a potential palaeohearth of Lazaret cave, beneath the mag-netic field anomaly observed in Q12. This figure is available in colouronline at wileyonlinelibrary.com/journal/arp

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Table 3. Resulting parameters of the models corresponding tothe Q11 and Q12 palaeohearths.

Layer K (SI) Magnetization (A m�1)

Ash 0.039 4.5Base 0.0059 0.055Soil 0.002 0.04Limestone pebbles 0 0

99Magnetic Study of Lazaret Palaeohearths

are close to those observed in the experiment. Thisconstrained modelling method can thus serve todetect the thickness (and the magnetic properties)of burnt soil layers associated with palaeohearthsin Lazaret cave.These results have been compared with the anomaly

observed over Q12. For the model the ambient mag-netic field parameters observed on the day of thesurvey and location (F= 46473 nT, I = 59.6°, andD= 1.1°) were used (Finlay et al., 2010). The same struc-ture as used for the experimental hearth after firingwas applied to model the buried palaeohearth. It isconfined with limestone blocks and composed of athin ash layer overlying a base layer with weakermagnetic properties (but stronger than the non-firedsoil). The resulting model predicts very well theobserved magnetic anomaly (Figure 20a; RMS of1.8 nT). The magnetic properties of the resultingmodel (Table 3) are similar to those measured onthe Q11 and experimental hearth samples. Despitethe non-uniqueness of the resulting model, we hopeto have applied the necessary constraints (using theprevious results of the experimental hearth and thesamples from the cave) to converge to a reasonablepalaeohearth model at the origin of the Q12 anomaly(Figure 20b; RMS of 0.8 nT).

Discussion and conclusions

Several critical aspects of this study should be consi-dered. First, the observed magnetic anomalies overthe Lazaret cave palaeohearths and over the experi-mental hearth seem to have similar amplitudes (about50 nT at 15 cm altitude). However, larger anomalieswere observed by Linford and Canti (2001). This ismainly explained either by the longer duration of theirfires, or by the different types of soil material theyused, or by the altitude of their magnetic field mea-surements, or by a combination of these factors. Indeedthe maximum temperatures observed in their studyare greater than the maximum temperature measuredin our experimental hearth. Our temperatures are

Copyright © 2013 John Wiley & Sons, Ltd.

lower than those observed by Maki et al. (2006) at2 cm depth because of their use of longer burning time(several days), and also than those observed byLinford and Canti (2001) using a more detailed net ofthermocouples. Other factors such as the size of theexperiment, the water and organic content in the soilas well as the calorific values of the fuel, control the soiltemperature evolution during a fire (Cromer andVines, 1966; Bellomo, 1993; Canti and Linford, 2000,Maki et al., 2006; Carrancho and Villalaín, 2011). Alsoof importance is the eventual repetition of the campfires as well as post-fire reworking during subsequentoccupations and depositions. The latter parametercan significantly affect the thickness of the burnt layerremaining from the hearth.A second important issue concerns the expected

induced and remanent magnetization enhancementof the soil’s particles affected by the heat of the fire(Le Borgne, 1960; Marmet et al., 1999; Carrancho andVillalaín, 2011). Our experiment reveals that only thefirst centimetre of the ground presents such enhance-ment. This corresponds to a thin crust of charcoalmixed with strongly heated soil (see above). This ashlayer has a variable thickness with a maximumbeneath the centre of the fire. Morinaga et al. (1999),Linford and Canti (2001), Maki et al. (2006) andCarrancho and Villalaín (2011) also observed this mag-netic ash layer with a large magnetization dependingon the temperature reached by the fire as well as onthe soil type. In our case, the main remanence carrierin this ash layer is magnetite, as indicated by the ther-momagnetic curves, as Carrancho and Villalaín (2011)observed in their experimental fire. The measurementsof MS and the calculation of χcalfd indicate that heatingenhances the proportions of magnetic minerals andfine grains. On the other hand, hysteresis measure-ments indicate that fire does not affect the magneticgranulometry, because a mixture of PSD and SP grainsdominates the UA27 and UA28 samples measured.Another original finding from our study is the feasibil-ity of magnetic survey in a cave environment. Indeedthe magnetic field observations can be influenced bythe proximity of the roof in the case of Lazaret cave,which corresponds to limestone with veins enrichedin brown clays. However, the geological magneticnoise from the roof on the measurements is generallyfar less than the artificial noise (see Figure 5) due tothe electrical cables and associated metallic materialnecessary to light up the cave. Specific survey(discrete measurements with one sensor close to theground) and post-processing (filtering) were appliedhere, but different configurations can be used forother caves.

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100 A. Jrad et al.

Future investigations on palaeohearth detection inLazaret cave may require the application of other geo-physical methods (Scollar et al., 1990). For instance, anelectromagnetic device in the temporal and frequencydomains could also be used to reveal subsurfaceviscosity, susceptibility and electrical resistivity con-trasts. As in the case of magnetic field mapping, hetero-geneity of the ground (limestone pebbles, bones, soil,etc.), electromagnetic noise and possible weak contrastsfor electrical resistivity may lead to bad results withthese methods. Ground penetrating radar survey mayalso be performed to test its ability to reveal the subsur-face in this environment. However, it may be hamperedby the rugosity of the ground and by potential radarreflections from the roof and the walls. Finally, it is notcertain that there are large dielectric contrasts betweena burnt soil layer and a normal soil layer.Our future objective is to apply the magnetic meth-

odology developed in this study to the prospection ofother caves with palaeohearths, in different geologicaland environmental contexts. This would allow us tocharacterize the magnetic signature of palaeohearths.

Acknowledgements

We thank the Utique Hubert Curien program forfinancing the stays in France of Abir Jrad during herjoint PhD in Aix-Marseille and Tunis Universities.

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