# How long were alignments of heating stones hearths used during the Early Iron Age in Western Europe? Evidence of a long chronology by archaeomagnetic dating at Saint-Maximin-la-Sainte-Baume (Southern France)

G. Hervé, M. Marmara, Annick Chauvin, C. Vaschalde, E. Martin-Kobierzyki, B. Brossier, F. Demory

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## To cite this version:

G. Hervé, M. Marmara, Annick Chauvin, C. Vaschalde, E. Martin-Kobierzyki, et al., How long were alignments of heating stones hearths used during the Early Iron Age in Western Europe? Evidence of a long chronology by archaeomagnetic dating at Saint-Maximin-la-Sainte-Baume (Southern France). Journal of Archaeological Science: Reports, 2025, 66, pp.105241. 10.1016/j.jasrep.2025.105241. hal-05108794

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# How long were alignments of heating stones hearths used during the Early Iron Age in Western Europe? Evidence of a long chronology by archaeomagnetic dating at Saint-Maximin-la-Sainte-Baume (Southern France)

G. Hervé<sup>a, *</sup>, M. Marmara<sup>b</sup>, A. Chauvin<sup>c</sup>, C. Vaschalde<sup>d</sup>, E. Martin-Kobierzyki<sup>e</sup>, B. Brossier<sup>f</sup>,
F. Demory<sup>g</sup>

## ARTICLE INFO

**Keywords:**

Archaeomagnetic dating

Heating stones hearths

Secular variation of the geomagnetic field

Early Iron Age

Western Europe

## ABSTRACT

Alignments of heating stones hearths are common in Southeast France and in Switzerland at the Late Bronze and Early Iron Ages. Because of the plateau effect on the radiocarbon calibration curve, the hypothesis that the structures were in use almost synchronously within an alignment is still debated. For archaeomagnetic dating purposes, we studied eight heating stones hearths, six of them organized in two parallel alignments, from the site Clos de Roques - Route de Barjols in Saint-Maximin-la-Sainte-Baume in Southern France. Archaeodirections were determined after thermal and alternating field demagnetizations and archaeointensities using the Thellier-Thellier protocol with corrections for the anisotropy and cooling rate effects. The large difference between average archaeodirections (more than 20° in declination) clearly demonstrate that the last use of the structures was not contemporaneous, bearing in mind that no archaeological evidence of multiple uses was found. The archaeomagnetic dates were obtained with the local prediction of declination, inclination and intensity by the European geomagnetic model SCHA.DIF.4k. They range between the end of the 8th century BCE and the end of the 5th century BCE with a ~ 150 years precision at 95 % of confidence for most structures. These results confirm the occupation of the site at the Early Iron Age given by other dating methods but imply a longer use than the one around 625-575 BCE suggested by the typochronology of ceramic artefacts. The different archaeomagnetic dates obtained within the alignments show that the alignments of hearths were not established over a short period as previously thought but over a couple centuries, highlighting the important and still poorly understood role of these fired structures in the area.

## 1. Introduction

In Western Europe, heating stones hearths are common features on prehistoric and protohistoric archaeological sites since the Mesolithic (e. g., Frère-Sautot, 2003). These (sub)rectangular pits, mostly of a few m²

size, are defined by burnt walls and by a filling of charcoals and stones that bear signs of heating inside the structure. A culinary function of these features was first proposed by comparison with modern structures in Oceania (hangi stones) (see Orliac, 2003) and by the way, this type of fired structures was called "Polynesian kiln" in the French

<footnote><sup>a</sup> Laboratoire des Sciences du Climat et de l'Environnement/IPSL, UMR 8212, CEA, CNRS, UVSQ, 91191 Gif-sur-Yvette, France</footnote>

<footnote><sup>b</sup> Univ. Lyon, ArAr UMR5138 Archéologie et Archéométrie/Archéorient UMR5133 Environnements et sociétés de l'Orient ancien, F-69365 Lyon, France</footnote>

<footnote><sup>c</sup> Univ Rennes, CNRS, Geosciences Rennes - UMR 6118, 35000 Rennes, France</footnote>

<footnote><sup>d</sup> Mosaiques Archéologie, 34660 Cournonterral, France - Membre associé, Aix Marseille Univ, CNRS, LA3M, Aix-en-Provence, France</footnote>

<footnote><sup>e</sup> ConcepTruelle, 81350 Sérénac, France</footnote>

<footnote><sup>f</sup> ISEM, Univ Montpellier, CNRS, IRD, Montpellier, France</footnote>

<footnote><sup>g</sup> Aix-Marseille Univ., CNRS, IRD, Coll. de France, CEREGE, Aix en Provence, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail addresses: gwenael.herve@lsce.ipsl.fr (G. Hervé), marylise.marmara@mom.fr (M. Marmara), annick.chauvin@univ-rennes1.fr (A. Chauvin), c.vaschalde@mosaiquesarcheologie.com (C. Vaschalde), elodie_mk@yahoo.fr (E. Martin-Kobierzyki), benoit.brossier@umontpellier.fr (B. Brossier), demory@cerege.fr (F. Demory).</footnote>
archaeological literature before the 2000 s. Recent studies have not questioned this ethnological analogy of the use of such type of archaeological features: organic analyses on several sites showed the presence of animal lipids on the burnt stones (e.g., Dubesset et al., 2014; Ropiot et al., 2015), while anthracology highlighted a prevailing use of fresh wood favouring a slow combustion (e.g., Henry et al., 2021). It is now widely admitted that these structures were used for food stewing (e.g., Pranyiès et al., 2012; Dubesset, 2021).

In Western Europe, the occurrence of the heating stones hearths varied in space and time. They were especially frequent at the Late Bronze (XIII<sup>th</sup>-IX<sup>th</sup> c. BCE) and Early Iron Age (VIII<sup>th</sup>-V<sup>th</sup> c. BCE) in Switzerland and in the eastern half of France as for example in Provence, Auvergne, Lyon area and Alsace (e.g., Frère-Sautot, 2003; Pranyiès et al., 2012; Hart, 2017; Dubesset, 2021). This abundance would indicate that they are an important marker of the social transformations observed at the gradual transition from the Bronze to the Iron Age in the north-western and western periphery of the Alps.

The heating stones hearths were found in similar contexts over this area, either isolated nearby a settlement or in alignments ("batteries" in the French literature) including up to several dozens of structures (e.g., Beausoleil et al., 2007). These concentrations were apparently isolated from settlements and, for most of them, arranged in one or several parallel alignments. These characteristics suggest that the alignments of heating stones hearths would have played a major role in the territorial organization. Their use does not appear related to ordinary domestic activities but rather could happen during collective events as banquets and ceremonies, perhaps shared by several settlements (e.g., Vital, 1993). This last hypothesis implies a synchronous use of the structures or at least during a short period of time (e.g., a few years) (Beausoleil et al., 2007; Pranyies et al., 2012).

However, this strong assumption of contemporaneity relies more on the closeness and the organization in alignments of the structures than on robust chronological data (Pranyies et al., 2012). Accurately estimating the duration of the use of an alignment of heating stones hearths is limited by the number and the nature of available chronological data. Defining a relative chronology is generally impossible because structures do not overlap. Ceramic artefacts are rare or absent in the filling layers of the structures and they usually belong to common imprecisely dated types. In this context, physical dating techniques appear the most useful to establish the chronology of the sites with grouped heating stones hearths. The widely predominant technique is radiocarbon dating on charcoals from the filling of the structures (see some examples in Frère-Sautot, 2003). For different reasons (no appropriate charcoals, funding, time...), only a couple of structures are often dated within the alignments, which is not sufficient to conclude about the site lifespan. Moreover, when a more extensive dating program is performed, the interpretation of radiocarbon dates is limited by the successive plateau effects on the calibration curve between ~800-400 BCE and ~400-200 BCE resulting in wide and multimodal probability density functions (e. g., Dubesset et al., 2014). At the Iron Age, other methods are essential to obtain a better knowledge of the temporality of alignments of heating stones hearths.

An alternative to radiocarbon is dendrochronology on large charcoals found in the filling of the structures (anthraco-chronology, Brossier and Poirier, 2018; Brossier et al., 2020). Another possible method is archaeomagnetic dating. The recent acquisition of a large number of reference data in Western Europe and the resulting improvements in local secular variation curves of the geomagnetic field (e.g., Hervé et al., 2013, 2017; Molina Cardin et al., 2018; Schnep et al., 2020a; Rivero-Montero et al., 2021) and in the European geomagnetic model (Pavón-Carrasco et al., 2021) extended the applicability of the method to the first millennium BCE in this area. The fast secular variation observed at this period, both in direction (declination and inclination) and intensity, results in a high precision of the dating technique (Hervé and Lanos, 2018; Gómez-Paccard et al., 2019; García-Redondo et al., 2021). Archaeomagnetic results can also be discussed with a perspective of

relative chronology to investigate site temporalities (e.g. Gallet and Butterlin, 2014; Herrejón-Lagunilla et al., 2024) and synchronize sites at the regional scale (e.g., Kostadinova-Avramova et al., 2014).

In comparison to other types of structures (pottery kilns, domestic kilns...), heating stones hearths are not commonly studied in archaeomagnetism. Hedley (2003) obtained successful results on the structure walls but not on the stones because these ones slightly moved since the last use of the structure. Several others studies in Central France (Moutmir, 1995, see also Gallet et al., 2002, Surmely, 2003) and Switzerland (von Burg et al., 2003; Kovacheva et al., 2009a) also showed that the walls of the heating stones hearths generally carry a stable thermoremanent magnetization (TRM). Beyond Western Europe, the method was applied in New Zealand to similar fired structures named hangi (Turner et al., 2020).

Here, we present the archaeomagnetic results obtained on eight heating stones hearths from the archaeological site named Clos de Roques - Route de Barjols in Saint-Maximin-la-Sainte-Baume in Southern France. In addition to date each structure, the objective was to discuss whether the fired structures were almost synchronously used or the activity happened over a longer period of time. This study highlights further the relevance of archaeomagnetic dating to precise the chronology of sites during the radiocarbon plateau of the first millennium BCE.

## 2. Archaeological context

Saint-Maximin-la-Sainte-Baume is a small city in Provence around 40 km northeast of Marseille (Fig. 1). It is located in a Pleistocene polje (i.e., a karst field) between the limestone massifs of Sainte-Baume and Sainte-Victoire. The city is rapidly urbanizing and several rescue excavations took place in the past 20 years especially north of the city (Sillano and Thomas, 2003; Cockin et al., 2009; Remicourt et al., 2012; Marmara et al., 2018). The numerous discovered settlements, dispersed over the area, highlighted a long, almost continuous, occupation between the Middle Neolithic (beginning of the 5th millennium BCE) and the Late Iron Age (second half of the first millennium BCE).

### 2.1. *The site Clos de Roques – Route de Barjols*

The site Clos de Roques - Route de Barjols (Lat.: 43.4582°N; Long.: 5.8634°E) was excavated over 8500 m² between March and June 2016 by Mosaïques Archéologie and Paléotime companies (Marmara et al., 2018). Nearly 800 anthropic structures were unearthed, all dug in the silt substrate. No horizon layers or buildings were preserved. The remains can be grouped in three main periods: first, the Middle and Late Neolithic, secondly the Iron Age, both with structures associated to settlements, and finally two networks of planting holes, with a morphology characteristic of vine culture, established at the earliest during the 3rd-1st centuries BCE.

This study focuses on the Iron Age structures (Fig. 1). Among the about 60 features, the most remarkable ones were 10 rectangular heating stones hearths, all having similar characteristics. Eight were located in the SE corner of the excavated area and organized in two parallel SW-NE alignments. The longest alignment (around 30 m), named here western alignment, included 6 heating stones hearths (FY1569, FY1570, FY1571, FY1572, FY1573 and FY1574) and a seventh pit (FS1568) with the same morphology but without traces of firing. The second alignment, the eastern alignment, presented only two heating stones hearths (FY1575 and FY1576). No stratigraphical relationships were observed between the structures. It is unlikely that the alignments continued outside the limits of the excavation area but we cannot fully rule out this hypothesis.

Two other isolated heating stones hearths were found on the site: one in the central area (FY1754) and the other one close to the western limit (FY1662) (Fig. 1). Both had a similar NW-SE orientation.

The main other Iron Age structures were related to storage such as
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Location of Saint-Maximin-la-Sainte-Baume in SE France and map of the Iron Age structures at Le Clos de Roques - Route de Barjols archaeological site. The heating stones hearths are plotted in red. The eight structures studied by archaeomagnetism are labelled with bold characters. The map is in Lambert III coordinates. (From Marmara et al., 2018).</figcaption></figure>

silos and a possible elevated granary (Fig. 1). Several large pits, 3 to 6 m long and up to 2.5 m deep, were also identified. One was likely an extraction pit, while the function of others remained unknown. Eight smaller 1–2 m pits were filled by a partly reddened sediment with inclusions of charcoals and ashes, interpreted as discharges from combustion structures. They were located all over the site, generally not close to the heating stones hearths and had probably no relationship with them. These structures and the characteristics of the associated artefacts, especially ceramics, suggest the presence of a nearby settlement.

## 2.2. Description of the heating stones hearths

Most of the 10 heating stones hearths were preserved over a 30–50 cm depth, except FY1574 and FY1574 over 15 cm (Table. 1). All the structures, including the associated unburnt pit FS1568 of the western alignment, had similar characteristics. Their shape was rectangular with rounded corners and vertical walls showing marks of tools (picks). Except the narrower structure FY1662, the dimensions were rather homogeneous with a 190–275 cm length and 80–95 cm width. The thickness of the reddened part of the walls varied between structures

**Table 1**

Characteristics of heating stones hearths. Location of the structure in the site; Name of the structure; Archaeomagnetic (AM) laboratory code; Length, width and preserved depth of the structure; Thickness of the reddened part of the walls; Interval of date provided by typochronology of ceramic artefacts; Number of sherds found in the filling of the structure; Minimal Number of Individuals of ceramics; Interval of radiocarbon date at 95% of confidence.

<table><thead><tr><th>Location</th><th>Structure</th><th>AM Lab. Code</th><th>Length (cm)</th><th>Width (cm)</th><th>Depth (cm)</th><th>Wall reddened thickness (cm)</th><th>Ceramic dating (BCE)</th><th>Sherds number</th><th>MNI</th><th><sup>14</sup>C dating at 95% (Cal yrs BCE)</th></tr></thead><tbody><tr><td rowspan="2">Isolated structures</td><td>FY1662</td><td>SMBA</td><td>190</td><td>56</td><td>48</td><td>3-7.5</td><td>Early Iron Age</td><td>40</td><td>2</td><td rowspan="2">[791; 544]</td></tr><tr><td>FY1754</td><td>–</td><td>225</td><td>95</td><td>15</td><td>2-3</td><td>Iron Age</td><td>0</td><td>0</td></tr><tr><td rowspan="6">SE corner, western alignment</td><td>FY1569</td><td>SMBB</td><td>252</td><td>81</td><td>46</td><td>2-4</td><td>Transition 7-6th c.</td><td>22</td><td>2</td><td rowspan="6">[543; 396]</td></tr><tr><td>FY1570</td><td>SMBC</td><td>273</td><td>82</td><td>40</td><td>5</td><td>7th - mid 6th c.</td><td>136</td><td>11</td></tr><tr><td>FY1571</td><td>SMBD</td><td>265</td><td>90</td><td>36</td><td>4</td><td>Iron Age</td><td>26</td><td>0</td></tr><tr><td>FY1572</td><td>SMBE</td><td>250</td><td>90</td><td>40</td><td>4</td><td>Late 7th - early 6th c.</td><td>237</td><td>32</td></tr><tr><td>FY1573</td><td>SMBF</td><td>210</td><td>90</td><td>35</td><td>4</td><td>7th c.</td><td>135</td><td>13</td></tr><tr><td>FY1574</td><td>–</td><td>230</td><td>82</td><td>17</td><td>0-7</td><td>Iron Age</td><td>11</td><td>1</td><td rowspan="2">[751; 406]</td></tr><tr><td rowspan="4">SE corner, eastern alignment</td><td>FY1568</td><td>–</td><td>246</td><td>80</td><td>32</td><td>0</td><td>7th - 6th c.</td><td>55</td><td>7</td></tr><tr><td>FY1575</td><td>SMBG</td><td>275</td><td>80</td><td>40</td><td>3-4</td><td>Late 7th - early 6th c.</td><td>100</td><td>14</td></tr><tr><td>FY1576</td><td>SMBH</td><td>265</td><td>85</td><td>35</td><td>4</td><td>7th c.</td><td>82</td><td>7</td></tr></tbody></table>
from a few millimeters to a couple centimeters. The upper parts of the walls were the most baked. This morphology appeared characteristic of this type of structure during the Iron Age in Provence (e.g., Dubesset, 2021) as well as in other regions (e.g., Pranyiès et al., 2012).

The heating stones hearths presented the same succession of filling layers with, from the top downwards, one or several brown silty-clay layers with occasionally sediment from the natural ground, an irregular level of slightly heated reddish silty-clay sediment (Fig. 2a) above a level of burnt limestone blocks (Fig. 2b) and finally one dark-colored layer with charcoals and fragments of burnt logs (Fig. 2c). The bottom of the structures appeared discontinuously heated with sparse patches of reddened sediment (Fig. 2d). Nevertheless, the thickness of reddening was not lower than for the walls and even reached 10 cm in the case of the structure FY1572. No archaeological evidence of multiple uses was found.

The study of organic residues on the surface of the limestone blocks revealed markers of animal and plant lipids, which confirms the cooking and/or stewing use of these heating stones hearths. Faunal remains found in the filling layers belonged to several domestic animals (cattle, ovine, caprine, suidae). 44 % of them were burnt but it is difficult to assess whether they were cooked in the structure or rejected inside once abandoned.

## 2.3. Ceramic typology

Except for FY1574, the filling of all heating stones hearths contained ceramic artefacts, most of them coming from the upper layers, corresponding to the end of use of structures. The number of sherds varied from a few to more than 200 and the Minimal Number of Individuals (MNI) between 0 and 32 (Table 1). Once discarded a couple of intrusive Neolithic sherds in FY1571, FY1573 and FY1574 structures, the pottery set appeared relatively homogeneous in the different heating stones hearths as well as in other Iron Age structures. The set was largely dominated by local greyish handmade (i.e., not wheel-made) potteries with 94 % of sherds. Cups are the most common type, followed by urns and bowls. The preferential use of cups could be related to the culinary function of the heating stones hearths. Among the handmade potteries, individuals showing affinities with the Suspendien facies (Coste et al., 1976), mainly urns, were found in six fired structures, FY1569, FY1570, FY1572, FY1573, FY1575 and FY1576. This facies was typical of Eastern Languedoc. Links between this region and Provence were highlighted by

Lachenal et al. (2016) on sites dated in the late 7th and early 6th centuries BCE, questioning the existence of a Provençal Suspendi.

The presence of Suspendien sherds associated with the near absence of Marseille Greek ceramic (only one sherd in FY1570) assigned the archaeological artefacts of the six aforementioned heating stones hearths and of the unburnt pit FS1568 to the Early Iron Age and more precisely between the 7th and the mid 6th centuries BCE (Table 1). The dating could be centred on the end of the 7th and the beginning of the 6th centuries BCE, suggesting a fairly short duration of use. No chronological trend appeared between and within the two alignments of structures. Without Suspendien related ceramics, the other fired structures (FY1662, FY1754, FY1571 and FY1574) cannot be dated more precisely than on the whole Iron Age, because the chronology of local hand-shaped ceramics has not yet been finely established. These uncertainties leave open the questions of the contemporaneity of the structures and of the duration of the occupation.

## 2.4. Radiocarbon dating and dendrometry

Before radiocarbon dating, an anthracological study was performed on the 1513 charcoal fragments found in Iron Age structures, whose 845 carbon from the heating stones hearths (Battentier et al., 2019). The coupling of evergreen and deciduous oak is typical of the mixed Mediterranean forest and revealed a local wood supply. Most charcoals presented shrinkage slits, suggesting the use of green wood. Four charcoals were selected for dating. Three samples came from the basal layer of fired structures FY1571, FY1574 and FY1575. The fourth charcoal were found in one of the pits with rejected pieces of burnt structures, the structure FS1155 in the western part of the site.

Measurements were performed at Poznan Laboratory. Conventional ages were calibrated with IntCal20 curve (Reimer et al., 2020) using Chronomodel 2.0 software (Lanos and Dufresne, 2019). The four samples were dated in the plateau of the calibration curve, resulting in several intervals of date between the 8th and the 4th centuries BCE (Tab. S1 in supplementary material). These results are consistent with the typochronological 700–550 BCE estimations. However, one can note that a slightly more recent age cannot be excluded, as the probability density function of the charcoal from FY1571 clearly favors the second half of the 6th and the 5th centuries BCE.

In order to expect having more precise dates, a dendrometry study was also performed at ISEM Montpellier on the burnt logs found in the

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Views of the heating stones hearth FY1576 (laboratory code SMBH) throughout the excavation. (a) Layer of slightly heated reddish sediment. (b) Layer of heated limestone blocks. (c) Carbonaceous layer with burnt logs. (d) Bottom after excavation. (© F. Chateauneuf, J. Terrom).</figcaption></figure>
fired structures using the method of Brossier and Poirier (2018). Unfortunately, used species revealed not appropriate to such investigation (see Brossier et al., 2020).

# 3. Archaeomagnetic methods

## 3.1. Sampling

Archaeomagnetic sampling focused on the 8 best preserved heating stones hearths (Fig. S1). The two other structures FY1754 and FY1574 were too levelled down and truncated by more recent features for a reliable sampling. Between 9 and 18 blocks were collected per structure for a total of 115 blocks. They were prepared in the upper, more reddened, part of the walls. In the case of FY1572 (archaeomagnetic code, SMBE), one block was also sampled in the floor of the structure. In order to better detect possible movements of the walls during the burial, samples were distributed as much as possible over the structure, generally on three sides.

Blocks were sampled using the plaster cap technique. They were levelled horizontal with a bubble and then oriented using both magnetic and sun compasses. For all structures, the average angle between the two orientations agrees within 1° with the local declination predicted at the date of the sampling by the International Geomagnetic Reference Field model (IGRF13, Alken et al., 2021). At the laboratory, blocks were cut in 2 × 2 × 2 cm specimens after consolidation in sodium silicate. Between 2 and 7 specimens were obtained per block for a total of 488 specimens. They were prepared at different distances from the surface of the walls, the first row of cubes corresponding to ~ 0–3 cm, the second to ~ 2–5 cm and the third to ~ 4–7 cm.

## 3.2. Magnetic mineralogy

Rock magnetic experiments were performed to identify the ferromagnetic grains carrying the TRM and investigate their thermal stability. Hysteresis curves up to 1 Tesla and backfield curves of isothermal remanent magnetization (IRM) were performed on chips from three blocks per structure using a 8600 Lakeshore vibrating sample magnetometer (VSM) at LSCE. On powders from the same blocks, continuous thermomagnetic curves using a 200 mT field were measured on a Petersen Instruments variable field translation balance (VFTB), while heating and cooling in air from room temperature first to 450 °C and next to 620 °C.

## 3.3. Directional study

In order to retrieve the direction of the TRM acquired during the last cooling of the structures, one specimen per block was demagnetized either thermally (41 specimens) or by alternating field (AF, 74 specimens) at CEREGE laboratory. Stepwise thermal demagnetization was done in an MMTD80A oven with 9–11 steps from 120 °C to 520–565 °C and stepwise AF demagnetization using a device in line with the 2G cryogenic magnetometer with 14 steps between 4 and 90 mT. In both cases, the direction of the TRM was estimated by principal component analysis and average direction per structure was calculated with Fisher statistics.

Because of anisotropy effect, the TRM direction could be deviated relative to the local geomagnetic field at the time of the last cooling. The correction of this effect is generally considered in archaeointensity protocols and rarely investigated in directional studies. Actually, this correction has a low impact on TRM directions in most archaeological kilns and fireplaces (e.g., Kovacheva et al., 2009b). However, it can also modify the direction by 10-15° with a systematic effect such as an inclination flattening (e.g., Palencia-Ortas et al., 2021). In order to check the anisotropy degree of our structures and the reliability of their TRM direction, we determined the TRM tensor of each demagnetized specimen, except the most brittle ones as those from FY1573 – SMBF

structure. Using the protocol of Chauvin et al. (2000), the specimens were heated at 420 °C or 490 °C in six successive positions (+z, -z, +x, -x, +y and -y) with a laboratory field of 40 µT. As specimens could present mineralogical changes or material loss throughout the heating, the procedure ended by a stability check with a further heating at the same temperature and with the field along +z. The anisotropy correction was applied only when the difference between the two partial TRMs (% alt in Tab. S2 in supplementary material) was below 10 %.

## 3.4. Archaeointensity experiments

The classical Thellier-Thellier protocol (Thellier and Thellier, 1959) was performed on 85 cubic specimens at Géosciences Rennes laboratory. Specimens were heated and cooled twice at each temperature in a laboratory field applied in opposite directions. A partial thermoremanent magnetization (pTRM) check was performed every two steps to monitor potential mineralogical changes throughout the protocol. We selected preferentially specimens with an intensity of Natural Remanent Magnetization (NRM) higher than 0.5–1 A/m that let expect less predisposition to alteration.

Measurements were carried out in two successive batches of specimens. For the first batch (71 specimens), heating-cooling cycles were performed with 12 steps from 120 to 560 °C in a 70 µT laboratory field applied along the z axis of the specimens. For the second batch (14 specimens), the sequence of temperature steps was slightly different (13 steps from 150 to 575 °C) and the field was fixed to 65 µT and applied along the y axis of the specimen. In this configuration, the higher angle between the NRM and laboratory field directions enhanced the detection of non-ideal behaviors (e.g., Biggin, 2006; Hervé et al., 2011).

The corrections for the TRM anisotropy and cooling rate effects were performed at 490 °C and 460 °C steps for the first and second batch respectively. The TRM tensor was determined using the same protocol as for the directional study. The cooling through the Thellier-Thellier protocol and the anisotropy correction lasted around one hour, which is likely less than the archaeological cooling and can result in a difference, generally an underestimate, of the TRM intensity (e.g., Genevey et al., 2008; Gómez-Paccard et al., 2006; Hervé et al., 2019). For taking account of this potential bias, the stability check of the anisotropy protocol was followed by a slow cooling along ~ 24 h and a further fast cooling to verify the absence of mineralogical changes during this step. The cooling rate correction was applied when the correction factor (% corrCR in Tab. S3) was higher than the alteration factor (%altCR in Tab. S3).

# 4. Results

## 4.1. Magnetic mineralogy

The magnetic properties of the eight heating stones hearths appeared rather homogeneous (Fig. 3). Hysteresis and IRM measurements showed a low contribution of high coercivity phases to the remanence, as backfield curves were saturated or close to saturation at 300 mT. With a Curie temperature between 550 and 580 °C, the main low coercivity phase highlighted on thermomagnetic curves was a Ti-poor titanomagnetite. This phase had a good stability with temperature with reversible curves up to 450 °C in almost all specimens. The reversibility after heating to 620 °C was better for specimens with higher NRM intensities. A few specimens (e.g., SMBA-8 and SMBA-1 in minor proportion, Fig. 3a) also presented Ti-richer titanomagnetite grains with a Curie temperature around 200 °C. This phase seemed more prone to mineralogical alteration than the Ti-poor titanomagnetite with irreversible curves after heating to 450 °C.

The NRM intensity varied between ~ 0.01 and ~ 6 A/m. The structures FY1569 – SMBB, FY1571 – SMBD and FY1572 – SMEB had overall lower NRM intensities than other structures with median values around 0.1 A/m for specimens from the first 3 cm of the walls, while
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** (a-c) Normalized thermomagnetic and IRM backfield curves of three representative structures. The thick curves are the variation of the magnetization during the heating and the thin curves during the cooling from 450 °C (generally reversible with the heating curve) and 620 °C. Specimens in green (blue) provided accepted (rejected) archaeointensity results. Specimens in red had a lower NRM intensity (below 0.5–1 A/m) and were not selected for the Thellier-Thellier protocol. (d) Box plots of the NRM intensity of specimens closest to the surface wall (~0–3 cm, first row in Fig. S2). The thick lines mark the median values, the box the interquartile range and the error bar the values at ± 1.5 times this interquartile range.</figcaption></figure>

FY1162 – SMBA, FY1570 – SMBC and FY1573 – SMBF had median values close to 1 A/m (Fig. 3d). In the absence of significant differences in the composition of the ferromagnetic mineralogy, we interpreted this variation of NRM intensities between structures by the heating degree with a higher temperature resulting in a higher proportion of strongly magnetic grains (e.g., Jordanova et al., 2019). The gradient of NRM intensity with temperature is here clearly evidenced at the block level with an around 10-fold decrease of the median value beyond 3–4 cm from the surface of the walls (Fig. S2). At the structure level, higher NRM intensities were also observed in the central part, closer to the fire, than in the extremities.

## 4.2. Directional study

Except one (SMBC11A22), all specimens presented a clear characteristic remanent magnetization (ChRM) after removal of a secondary component generally below 10–12 mT or 200–250 °C (Fig. 4a-b). ChRM directions are listed per specimen in Table S2 in supplementary material. They were well-defined with maximum angular deviation (MAD) values no more than 3.4° and below 1° for half specimens. Most specimens had a corrected TRM anisotropy degree P' (Jelinek, 1981) between 1.05 and 1.15. No trend was observed in the shape of the TRM tensor with T factor ranging between –1 and 1.

Directions were well-grouped at the structure level (Fig. 4c) with kappa parameter of the Fisher statistics ranging between 179 and 857

(Table 2). A few specimens had an outlier direction and were discarded from the calculation of the average directions. This behaviour was likely explained by a slight local movement of the structure walls since the last cooling and/or by cutting uncertainties at the laboratory. The anisotropy correction did not significantly modify the mean direction, as uncorrected and corrected values are within 1°. The correction did not also change the grouping of the ChRM directions, except in the case of FY1569 – SMBB and FY1570 – SMBC for which kappa increased by 45 % and 37 % respectively. For this reason and regarding the rather high P' factors, we considered the corrected mean directions for archaeomagnetic dating.

## 4.3. Archaeointensity results

The success rate of archaeointensity experiments was ~ 62 % (53/85 specimens). Accepted specimens presented linear NRM/TRM and Zijderveld plots (Fig. 5a-b), while other specimens showed a concave-up NRM-TRM diagram associated to a deviation of the NRM direction towards the direction of the laboratory field (Fig. 5c-d). This deviation was much less noticeable when the field was applied on the z-axis of the specimen (Fig. 5c) than on the y-axis (Fig. 5d) almost perpendicular to the NRM direction.

Individual accepted values are presented in Table S3 in supplementary material with results of corrections for TRM anisotropy and cooling rate effects. We did not strictly apply selection criteria on quality
<figure><img src="image_7.png" /><figcaption>Fig. 4. Directional results with representative Zijderveld plots of thermal (a) and AF (b) demagnetizations. The open and full circles are the projections upon vertical and horizontal planes respectively. Plots are in geographical coordinates. (c) Equal area plots per structure of directions of ChRMs after anisotropy correction. Mean directions (red star) are plotted with their 95% confidence circle. Specimens in grey are not included in the calculation of the mean directions.</figcaption></figure>

Table 2

Mean results per heating stones hearth. Structure: archaeological name of the structure; Lab. code: laboratory name of the structure; N<sub>dir</sub>/n<sub>dir</sub>: number of blocks used in the mean direction over number of studied blocks; Dec, Inc: mean declination and inclination; kappa: Fisher precision parameter; α<sub>95</sub>: radius of the Fisher 95 % confidence circle; N<sub>F</sub>/n<sub>F</sub>: number of specimens with an accepted archaeointensity over number of studied specimens; F<sub>raw</sub> ± SD: mean archaeointensity and standard deviation without corrections; F<sub>ATRM</sub> ± SD: mean archaeointensity and standard deviation corrected for TRM anisotropy effect; F<sub>CR</sub> ± SD: mean archaeointensity and standard deviation corrected both for TRM anisotropy and cooling rate effects.

<table><thead><tr><th rowspan="2">Structure</th><th rowspan="2">Lab. code</th><th rowspan="2">N<sub>dir</sub>/n<sub>dir</sub></th><th colspan="4">Directions without ATRM correction</th><th colspan="4">with ATRM correction</th><th colspan="3">Intensities</th></tr><tr><th>Dec (°)</th><th>Inc (°)</th><th>kappa</th><th>α<sub>95</sub> (°)</th><th>Dec (°)</th><th>Inc (°)</th><th>kappa</th><th>α<sub>95</sub> (°)</th><th>N<sub>F</sub>/n<sub>F</sub></th><th>F<sub>raw</sub> ± SD (μT)</th><th>F<sub>ATRM</sub> ± SD (μT)</th><th>F<sub>CR</sub> ± SD (μT)</th></tr></thead><tbody><tr><td>FY1662</td><td>SMBA</td><td>9/9</td><td>3.3</td><td>67.7</td><td>240</td><td>3.3</td><td>4.2</td><td>67.2</td><td>255</td><td>3.2</td><td>5/11</td><td>85.3 ± 2.6</td><td>85.8 ± 1.9</td><td>84.2 ± 4.4</td></tr><tr><td>FY1569</td><td>SMBB</td><td>14/14</td><td>0.4</td><td>64.6</td><td>407</td><td>2.0</td><td>0.6</td><td>64.9</td><td>592</td><td>1.6</td><td>6/10</td><td>76.5 ± 4.8</td><td>75.5 ± 3.3</td><td>71.1 ± 2.3</td></tr><tr><td>FY1570</td><td>SMBC</td><td>13/14</td><td>13.1</td><td>66.1</td><td>417</td><td>2.0</td><td>11.4</td><td>66.0</td><td>571</td><td>1.7</td><td>8/14</td><td>76.4 ± 6.5</td><td>76.3 ± 6.2</td><td>71.0 ± 6.8</td></tr><tr><td>FY1571</td><td>SMBD</td><td>11/12</td><td>355.7</td><td>63.4</td><td>179</td><td>3.4</td><td>358.4</td><td>63.1</td><td>189</td><td>3.3</td><td>3/6</td><td>76.2 ± 8.0</td><td>73.8 ± 6.7</td><td>68.1 ± 6.9</td></tr><tr><td>FY1572</td><td>SMBE</td><td>13/15</td><td>359.6</td><td>65.8</td><td>857</td><td>1.4</td><td>0.8</td><td>65.9</td><td>860</td><td>1.4</td><td>3/7</td><td>74.2 ± 0.6</td><td>74.2 ± 0.7</td><td>70.6 ± 3.5</td></tr><tr><td>FY1573</td><td>SMBF</td><td>16/18</td><td>20.3</td><td>68.4</td><td>349</td><td>2.0</td><td></td><td></td><td></td><td></td><td>12/15</td><td>75.1 ± 5.4</td><td>75.2 ± 5.0</td><td>68.8 ± 4.4</td></tr><tr><td>FY1575</td><td>SMBG</td><td>14/15</td><td>7.5</td><td>66.0</td><td>412</td><td>2.0</td><td>8.4</td><td>66.6</td><td>443</td><td>1.9</td><td>7/12</td><td>76.2 ± 5.8</td><td>76.1 ± 5.5</td><td>71.0 ± 4.6</td></tr><tr><td>FY1576</td><td>SMBH</td><td>17/18</td><td>16.5</td><td>68.0</td><td>646</td><td>1.4</td><td>18.8</td><td>67.8</td><td>649</td><td>1.4</td><td>10/10</td><td>76.7 ± 5.4</td><td>76.9 ± 5.4</td><td>73.3 ± 4.9</td></tr></tbody></table>
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Archaeointensity results of accepted (a-b) and rejected (c-d) specimens. Solid circles on NRM-TRM diagrams depict temperature steps used in the intensity determination. Zijderveld plots are in specimen coordinates with open (full) circles corresponding to the projection on the vertical (horizontal) planes.</figcaption></figure>

parameters but their values highlight the good quality of our determinations. The archaeointensity values were calculated over a NRM fraction (f factor) between 0.47 and 0.92 with 5–12 temperature steps. The quality factors (q) were between 4.9 and 91.9 and the maximum angular deviation (MAD) and deviation angle (DANG) parameters were below 5° except for 6 specimens. Almost all specimens would therefore largely pass main sets of quality criteria as PICRIT-03 (Kissel and Laj, 2004), SELCRIT2 (Biggin et al., 2007) and the stricter TTA (Leonhardt et al., 2004).

Accepted results were obtained on the eight heating stones hearths. The structures with high NRM intensities (e.g., FY1573 – SMBF and FY1576 – SMBH) tended to provide better results, which is in line with an NRM intensity proportional to the heating degree. The average archaeointensity values were calculated using 3 to 12 specimens. On most structures, the standard deviation decreased after TRM anisotropy and cooling rate corrections (Table. 2).

# 5. Discussion

## 5.1. *Archaeomagnetic dating*

The eight heating stones hearths provided significantly different

average directions, especially in declination with values distributed between 358° and 19° (Table. 2, Fig. 6a). In relative chronology, this result unambiguously proves that the last heating-cooling of the structures was not contemporaneous. Average archaeointensity values are well grouped between 68 and 73 µT, except the one of FY1662 – SMBA around 84 µT (Table. 2, Fig. 6b).

Archaeomagnetic dating requires the prior knowledge of the secular variation at the site location. Two types of reconstruction of the secular variation are generally used for dating purposes, both obtained from reference data well dated by other techniques. First, regional secular variation curves, traditionally the most used in Europe, are defined at a reference location to which the average direction and intensity have to be relocated using Virtual Geomagnetic Pole (VGP) and Virtual Dipole Moment (VDM) assumptions respectively. For direction, the reference location of the Italian curve (Tema and Lanos, 2021), Viterbo, is the closest from Saint-Maximin-la-Sainte-Baume (around 500 km). However, this curve has a low resolution at the first millennium BCE and the one of Hervé and Lanos (2018), valid between 1300 and 0 BCE and defined at Paris around 650 km far from our site, appears more appropriate in our study. For intensity, Hervé et al. (2017) proposed a curve defined at 46°N – 7°E, i.e. around 300 km far from Saint-Maximin-la-Sainte-Baume, from French, German, Italian and Swiss data available at this date.

An alternative to these regional secular variation curves is geomagnetic modelling at the European scale by inversion of archaeomagnetic data using spherical cap harmonic analysis (SCHA) (Pavón-Carrasco et al., 2009). By predicting the secular variation of the geomagnetic components at the site, this approach has the advantage to avoid the relocation process which induces an error that can reach a few degrees and microTeslas (Casas and Incoronato, 2007). The recently updated version of this European model, named SCHA.DIF.4k (Pavón-Carrasco et al., 2021), benefited from the large increase of the database in the BCE period for ten years.

In Fig. 6, the prediction of SCHA.DIF.4k at Saint-Maximin-la-Sainte-Baume between 1300 and 0 BCE is compared to the relocated regional secular variation curves of Hervé and Lanos (2018) and Hervé et al. (2017). While the agreement between both approaches is relatively good in intensity (to a lesser extent between 900 and 500 BCE), significant differences are observed in direction from 1300 to 400 BCE. The French curve of Hervé and Lanos (2018) exhibits a clockwise loop with a high declination maximum ~ 25° circa 800 BCE followed by a sharp and regular decrease to ~ 0° up to circa 400 BCE and with an inclination maximum ~ 70° circa 600 BCE. Conversely, SCHA.DIF.4k model predicts a more complex secular variation path, especially in inclination, as the increase and decrease of the declination before and after 800 BCE are associated to a fluctuating variation of the inclination around 65°. After 400 BCE, the two reconstructions are very close and show a slow variation with a roughly stable declination within -5/0° and a slight

<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Comparison of Saint-Maximin-la-Sainte-Baume mean directions (a) and intensities (b) in red with the prediction at the site of European SCHA.DIF.4k geomagnetic model in orange (Pavón-Carrasco et al., 2021) and with the regional secular variation curves relocated at the site in blue (Hervé and Lanos, 2018; Hervé et al., 2017). Color changes within each curve indicate century timescales. Average directions are plotted with their $\alpha_{95}$ confidence circles.</figcaption></figure>
inclination decrease.

These differences raised the question of the choice of the reconstruction for archaeomagnetic dating. We preferred to use SCHA.DIF.4k model rather than the regional secular variation curves because it includes more recent datasets from Austria, Germany, Italy and the Iberian Peninsula (Schnepf et al., 2020a, 2020b; Rivero-Montero et al., 2021; Molina Cardin et al., 2018). The fact that the European model does not require a relocation of the data is an additional argument to favour it. The relocation error is indeed time-varying with higher values during periods with a high non-dipole field such as the beginning and the middle of the first millennium BCE (e.g., Shaar et al., 2016; Rivero-Montero et al., 2021).

The dating was processed using the ArchaeoPy Dating tool developed at the Universidad Complutense Madrid (http://pc213fis.fis.ucm.es:8080/). The prior period was fixed between 1300 and 0 BCE, a time period centred around the Early Iron Age. The comparison of the declination, inclination and intensity values, corrected for anisotropy and cooling rate effects, with their SCHA.DIF.4k prediction provides three probability density functions (PDFs) (Figs. S3-S10). The oscillatory behaviour of the secular variation resulted in multimodal PDFs and several intervals of date at 95 per cent of confidence. For all structures, the three PDFs largely overlapped at one or two periods, making meaningful their combination. The PDF of the combination with the three parameters provided intervals of date at the Early Iron Age or at the beginning of the Late Iron Age, in the period expected by the archaeological context (Fig. 7, Table. 3).

The width of the interval of date at 95 per cent of confidence was generally between 140 and 200 years and reached several centuries for two structures. The precision mainly depended on the secular variation

<caption>**Table 3**</caption>

Archaeomagnetic intervals of date at 95 per cent of confidence defined on the combination of PDFs of declination (Dec) and inclination (Inc) and on the combination of PDFs of Dec, Inc and intensity (Int). Results were obtained with the prediction at the site of SCHA.DIF.4k European model (Pavón-Carrasco et al., 2021). When several intervals of date are possible, the most likely interval regarding the archaeological context is indicated in bold characters.

<table><thead><tr><th>Structure</th><th>Lab. code</th><th>(Dec, Inc)</th><th>(Dec, Inc, Int)</th></tr></thead><tbody><tr><td>FY1662</td><td>SMBA</td><td>[-1300; -1125], [-1040; -960], [-605; -450], [-375; -185]</td><td>[-575; -435]</td></tr><tr><td>FY1569</td><td>SMBB</td><td>[-1300; -1140], [-1030; -1010], [-575; -110]</td><td>[-570; -420], [-410; -300]</td></tr><tr><td>FY1570</td><td>SMBC</td><td>[-1300; -1140], [-1035; -935], [-750; -495]</td><td>[-1215; -1160], [-1035; -930], [-765; -490]</td></tr><tr><td>FY1571</td><td>SMBD</td><td>[-1160; -1130], [-1095; -1035], [-535; 0]</td><td>[-530; 0]</td></tr><tr><td>FY1572</td><td>SMBE</td><td>[-1300; -1145], [-1015; -1005], [-575; -440], [-385; -170]</td><td>[-580; -440], [-390; -275]</td></tr><tr><td>FY1573</td><td>SMBF</td><td>[-965; -660]</td><td>[-975; -910], [-840; -655]</td></tr><tr><td>FY1575</td><td>SMBG</td><td>[-1300; -1130], [-1035; -950], [-720; -675], [-620; -480]</td><td>[-1030; -950], [-745; -660], [-635; -475]</td></tr><tr><td>FY1576</td><td>SMBH</td><td>[-975; -655]</td><td>[-980; -930], [-810; -650]</td></tr></tbody></table>

rate of the direction, higher at the Early Iron Age and lower after 450 BCE (Fig. 6a). The intensity had less impact on the precision because the model shows a slower secular variation during the Early Iron Age. Nevertheless, the fired structures of Saint-Maximin-la-Sainte-Baume highlight the interest of using intensity in archaeomagnetic dating.

<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Comparison of chronological data on heating stones hearths. Red curves are the PDFs of archaeomagnetic dating (combination of inclination, declination and intensity PDFs) obtained with SCHA.DIF.4k model. Orange curves are the PDFs of radiocarbon dating and the blue bars indicate the date intervals given by ceramic typochronology.</figcaption></figure>
The PDF of the full vector combination presented a simpler shape than the one obtained using only declination and inclination, strengthening archaeomagnetic dates in the Early Iron Age (Figs. S3-S10). The addition of intensity removed or lessened the modes on the PDFs at other times, which generally resulted in more precise date intervals (Table. 3). Intensity especially played a role for results at the Late Iron Age, characterized by a slow directional secular variation in Western Europe (e.g., for FY1662 – SMBA and FY1569 – SMBB, Figs. S3 and S4). The average archaeointensities of FY1571 – SMBD and FY1572 – SMBE, calculated from only three specimens, should be considered of lower quality than the data obtained on other structures. However, this concern does not question the final dating results because intensity did not significantly modify the retained interval of date (Table. 3, Figs. S6 and S7).

The accuracy on the calendar scale of the intervals of date at 95% of confidence is difficult to ascertain nowadays. The comparison of the archaeodirections with the prediction of SCHA.DIF.4k model (Fig. 6a) shows that our data clearly recorded the fast decrease of the declination during the Early Iron Age. In inclination, they also follow the secular variation trend of the model, as values of FY1570 – SMBC and FY1575 – SMBG are consistent with the slight inclination minimum around 600 BCE. However, the inclination values of Saint-Maximin-la-Sainte-Baume fired structures tend to be systematically in the upper part of the error envelope of SCHA.DIF.4k prediction. More archaeomagnetic reference data, preferentially from well-dated stratified sites, are required to check if this behaviour could be related to a local field effect or to a slight inaccuracy of the model in Southern France at the Early Iron Age. Considering this moderate uncertainty, one cannot exclude that the bounds of the intervals of date slightly change (by a few years or a few decades?) in the next years with future improvements of the archaeomagnetic database. Nevertheless, the chronological attribution to the Early Iron Age is ensured, the declination values providing a relative chronology between the eight heating stones hearths. The structures FY1573 – SMBF and FY1576 – SMBH having the highest declination values are the oldest, while FY1571 – SMBD with the lowest value is the youngest. Regarding archaeointensity, the obtained high values are in agreement with published Early Iron Age data of the literature (e.g., Hervé et al., 2017, 2021; Molina Cardin et al., 2018; Osete et al., 2020). The small differences between the structures suggest that the geomagnetic field strength, contrary to the direction, was relatively stable during the occupation of the site.

## 5.2. Comparison with radiocarbon and typochronology

The comparison of archaeomagnetic dates with radiocarbon is limited by the Early Iron Age plateau effect on IntCal20 calibration curve (Tab. S1 in supplementary material). This effect makes unclear the chronological relationships between our results and the two fired structures FY1574 and FY1754 that could not be dated by archaeomagnetism. In the case of FY1571 – SMBD structure, radiocarbon favours the earlier part of the SCHA.DIF.4k archaeomagnetic interval at the end of the 6th or during the 5th century BCE (Fig. 7). It mitigates the imprecision of archaeomagnetism in the second half of the first millennium BCE due to the low secular variation of the geomagnetic field, which emphasizes the interest of the complementarity between the two methods at the transition between the Early and Late Iron Ages.

Regarding the comparison with dates given by pottery artefacts, the combination of radiocarbon and archaeomagnetism confirm the archaeological hypothesis that FY1571 – SMBD structure, imprecisely dated by ceramics in the whole Iron Age, was in use at the Early Iron Age (Fig. 7). Archaeomagnetism also precises the chronology of FY1662 – SMBA structure. On other heating stones hearths, the presence of Suspendien related artefacts and the near absence of Marseille Greek ceramic supported a dating at the end of the 7th or at the beginning of the 6th century BCE. Archaeomagnetic and typochronological dates are in full agreement only for the structure FY1570 – SMBC. The overlap between the two estimations is partial for two structures

FY1575 – SMBG and FY1576 – SMBH. Archaeomagnetism is finally inconsistent with the typochronological date for the three other heating stones hearths FY1573 – SMBF, FY1572 – SMBE and FY1569 – SMBB. The fact that the two methods date different moments could explain this inconsistency: archaeomagnetism refers to the last cooling from high temperature of the heating stones hearth, providing a *terminus ante quem* to its use, while ceramic artefacts date the subsequent filling of the structure. However, this hypothesis is unlikely because the stratigraphy of the structures suggests that their filling occurred close to their use. Moreover, the time shift between the two methods is not systematic: the archaeomagnetic date is earlier than the typochronological one for FY1573 – SMBF and younger for FY1572 – SMBE and FY1569 – SMBB.

At the site scale, our archaeomagnetic dates are against the short chronology over one generation between the end of the 7th and the beginning of the 6th centuries suggested by Suspendien ceramic artefacts. Reconciliating archaeomagnetism and typochronology would imply a very high secular variation rate of ~ 25° in 25 years, i.e. about 1°/year, which is not supported by none contemporaneous data in Europe or elsewhere. Such extremely high rate is also very unlikely because it was never observed apart periods of reversals of the geomagnetic field (Sagnotti et al., 2014) and even very controversially (Evans and Muxworthy, 2018). Therefore, it must be admitted that the chronology given by ceramic artefacts is too tightened and that the heating stones hearths were in use over a longer period than thought before the archaeomagnetic analyses. Several hypotheses could explain this result. The structures may have been filled in by surrounding sediments containing older artifacts. It is also possible that Suspendien related ceramics were used for a longer period in Provence, or even that a certain type of ceramic could have been used during the firing events. A better understanding of the inconsistency between the typochronological and archaeomagnetic results would imply a thorough study of the ceramic assemblages at the regional scale, which is beyond the scope of this article. Finally, it should be borne in mind that the ceramic study could provide a precise date around the transition of the 7th and 6th centuries BCE only for a few structures (FY1569 – SMBB, FY1572 – SMBE and FY1575 – SMBG). This estimation was generalized to the whole occupation considering the alignment of the heating stones hearths. Such spatial organization lead archaeologists to the assumption, a priori reasonable, that structures were used over a short period of time within an alignment (Dubesset, 2021).

Despite its limits, the typochronological analysis improves the precision of archaeomagnetic dates with bimodal PDFs of FY1569 – SMBB and FY1572 – SMBE structures (Fig. 7), because the ceramic assemblages exclude that the site was occupied beyond the Early Iron Age. The younger mode of the PDFs in the 4-3th centuries BCE can reasonably be discarded and the archaeomagnetic dates can be restricted to the intervals [-570; -420] for FY1569 – SMBB and [-575; -435] for FY1572 – SMBE (Table. 3). The combination of the three dating methods, archaeomagnetism, radiocarbon and typochronology of ceramics, therefore indicates the use of the eight heating stones hearths between the end of the 9th century BCE and the end of the 5th century BCE, that is over all the Early Iron Age.

## 5.3. Archaeological implications

Our archaeomagnetic results significantly benefit to the knowledge of Clos de Roques – Route de Barjols archaeological site and of its alignments of heating stones hearths. They undoubtedly demonstrate that the fired structures were not used at the same time but that their establishment should be considered over a couple centuries. No chronological trend is observed between the two alignments or within them along the SW-NE direction. The dates of the two isolated structures, FY1662 – SMBA and FY1754 (undated in archaeomagnetism) also do not differ from those of aligned structures.

The hypothesis of an area shared by several communities during one event, as a banquet regarding the culinary function of the heating stones
hearths (e.g., Vital, 1993), is impossible. However, the long chronology of the structures does not contradict the fact that this site was a strong territorial marker for the social groups (Dubesset, 2021). The coherent spatial organization of the heating stones hearths over a long period suggests a specificity of these recurrent culinary activities, the nature of which still escapes us.

On a regional scale, our study is one of the first to clearly demonstrate the non-contemporaneity of heating stones hearths within an alignment at the Late Bronze and Early Iron Ages. At Issoire Le Brot in Auvergne, archaeomagnetism reaches the same conclusion but it was based on only two fired structures (Surmely and Moutmir, 1995). On most other sites, the investigation of the duration of the occupation is limited by the sole use of radiocarbon (see the regional syntheses of Pranyies et al., 2012; Dubesset, 2021): because of the plateau effect on the calibration curve, their chronology is generally too imprecise to know if the alignment was used over one generation or over a long period as at Saint-Maximin-la-Sainte-Baume. The study of Clos de Roques - Route de Barjols site encourages the use of archaeomagnetism to better understand the temporality of alignments of heating stones hearths, a major cultural characteristic at the Late Bronze and Early Iron Ages in Southeast France and in Switzerland. In the near future, this potential of archaeomagnetic dating will even likely be enhanced by the acquisition of new archaeomagnetic reference data in Western Europe and by the subsequent improvements in the resolution of geomagnetic models. The fired structures of Clos de Roques - Route de Barjols are not suitable to complete the reference archaeomagnetic database because they cannot be dated with precision and accuracy by another method.

## 6. Conclusion

Eight heating stones hearths from Clos de Roques - Route de Barjols archaeological site were studied for archaeomagnetic dating purposes. An average archaeodirection and archaeointensity could be determined on all structures. The large differences in the average declination clearly demonstrate that the use of the eight structures was not contemporaneous. Archaeomagnetic dates, obtained using SCHA.DIF.4k European geomagnetic model, range between the end of the 9th century BCE and the end of the 5th century BCE with a ~ 150 years precision at 95 % of confidence for most structures. These dates confirm the occupation of the site at the Early Iron Age given by radiocarbon dates and the typochronology of ceramic artefacts. However, they also indicate that the typochronological estimation centred on the end of the 7th and the beginning of the 6th centuries BCE is likely too tightened and that the heating stones hearths were in use over a couple centuries, a much longer period than previously thought. The temporal persistence of their spatial organization in alignments likely highlights the important cultural role of the associated culinary activities at the Early Iron Age. The temporality of the alignment complex of heating stones hearths of Clos de Roques - Route de Barjols is one of the best known among the large number of such sites at the Early Iron Age in SE France. The success of our study encourages to develop the use of archaeomagnetism to know if such other alignments have also a long chronology or if they worked during a short period. Even it is time consuming, our study shows the interest to date exhaustively the fired structures, in order to obtain more relevant conclusions on the life of the site.

From a methodological point of view, the good precision of archaeomagnetic dating is related to the fast secular variation of the geomagnetic field during the first millennium BCE. The discrepancies between different geomagnetic reconstructions highlight the need of a better knowledge of the secular variation in Europe at this period. The acquisition of new archaeomagnetic reference data, if possible from well-dated stratified sites, is required in the continuity of the large improvements in the archaeomagnetic database for a few years (e.g., Molina Cardin et al., 2018; Rivero-Montero et al., 2021; Shaar et al., 2018; Tema et al., 2021). Meanwhile, the accuracy of the bounds of the archaeomagnetic intervals of date on Clos de Roques - Route de Barjols

structures cannot yet be fully ascertained within 10 years. Nevertheless, estimating the duration of an alignment of heating stones hearths does not necessarily require a strict accuracy of the secular variation reconstructions. With a perspective of relative chronology, investigating through archaeomagnetism whether fired structures are contemporaneous or not within a site can be considered in periods and/or regions without a precise knowledge of the secular variation, such as the Neolithic in Western Europe.

## CRediT authorship contribution statement

G. Hervé: Writing – original draft, Methodology, Investigation, Formal analysis, Conceptualization. M. Marmara: Writing – original draft, Methodology, Investigation, Formal analysis, Conceptualization. A. Chauvin: Writing – review & editing, Methodology, Investigation, Formal analysis. C. Vaschalde: Writing – review & editing, Methodology, Investigation, Conceptualization. E. Martin-Kobierzyki: Writing – review & editing, Methodology, Investigation, Formal analysis. B. Brossier: Writing – review & editing, Methodology, Investigation. F. Demory: Writing – review & editing, Methodology, Investigation.

## Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

We warmly acknowledge the excavation team of Clos de Roques - Route de Barjols site. For this study, GH benefited from the funding of ANR-CONACYT SVPIntMex project ANR-15-CE31-0011-01. The 8600 Lakeshore VSM used for hysteresis and IRM measurements was acquired with the support of CNRS-INSU and of DIM-MAP program from Ile-de-France region (MeV project). We acknowledge two anonymous reviewers for their comments and Dr. Danielle Macdonald for editorial handling.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jasrep.2025.105241.

## Data availability

Data will be made available on request.

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