A Genevey
email: agnes.genevey@upmc.fr

D Kondopoulou

P Pétridis

E Aidona

A Muller

F Blondé

J S Gros

New constraints on geomagnetic field intensity Variations in the Balkans during the Early Byzantine period from ceramics unearthed at Thasos and Delphi, Greece

Keywords: Archeomagnetism, Archeointensity, Secular variation, Greece, Early Byzantine period, Pottery

increase observed in the Balkans from 4th to 7th C.

Introduction

Archeomagnetic data constitute a valuable source of information on the Earth's magnetic field variations during the last few millennia. Data compilations from all over Europe, where the discipline has flourished over the last few decades, have resulted in the construction of robust databases and the computation of secular variation curves for several regions including France [START_REF] Gallet | Three millennia of directional variation of the Earth's magnetic field in Western Europe as revealed by archaeological artefacts[END_REF]Hervé et al. 2013a,b;[START_REF] Genevey | New Archeointensity Data from French Early Medieval Pottery Production (6th-10th century AD). Tracing 1500 years of Geomagnetic Field Intensity Variations in Western Europe[END_REF], Germany [START_REF] Schnepp | Archaeomagnetic secular variation in Germany during the past 2500 years[END_REF], Italy [START_REF] Tema | Archaeomagnetism in Italy: a compilation of data including newresults and a preliminary Italian secular variation curve[END_REF][START_REF] Tema | New archaeointensity data from Italy and geomagnetic field intensity variation in the Italian Peninsula[END_REF] and the UK [START_REF] Zananiri | Archaeomagnetic secular variation in the UK during the past 4000 years and its application to archaeomagnetic dating[END_REF] among others. There are, however, usually more directional than intensity data available, a feature related to the time-consuming and complicated protocols required to determine the archeointensity, together with the frequently high rate of failure. Numerous archeomagnetic investigations conducted within the last forty years in the Balkans have resulted in a particularly well-documented secular variation of the Earth's magnetic field, with a complete, though not continuous, recovery of the geomagnetic behavior (i.e. both in direction and in intensity) over the past eight millennia. This largely relies on datasets obtained in Bulgaria, where the compilation of archeomagnetic results has recently been updated [START_REF] Kovacheva | Extended and revised archaeomagnetic database and secular variation curves from Bulgaria for the last eight millennia[END_REF], and includes full vector data, obtained exclusively from in-situ burnt structures.

Archeomagnetic studies were initiated in Greece, in the 80's [START_REF] Thomas | Archaeomagnetism of Greek pottery and Cretan kilns[END_REF][START_REF] Walton | Re-evaluation of Greek archaeomagnitudes[END_REF][START_REF] Aitken | Geomagnetic intensity measurements using bricks from Greek churches of the first and second millennia A[END_REF] and in contrast to the general overview in Europe, there is more archeointensity than directional data. Nevertheless, the quality of several Greek archeointensity data has often been criticized (for a review see De Marco et al., 2008a) while several newly acquired results [START_REF] Spatharas | New magnetic mineralogy and archaeointensity results from Greek kilns and baked clays[END_REF][START_REF] Tema | Intensity of the Earth's magnetic field in Greece during the last five millennia: New data from Greek pottery[END_REF][START_REF] Fanjat | Archaeointensities in Greece during the Neolithic period: new insights into material selection and secular variation curve[END_REF][START_REF] De Marco | An updated catalogue of Greek archaeomagnetic data for the last 4500 years and a directional secular variation curve[END_REF] have supplemented the database with reliable input. As a result, the main trends of the geomagnetic field secular variation in the Balkans can now be used for the dating of baked clay structures of uncertain age. Several examples exist in the literature where archeomagnetic dating results are successfully compared with thermoluminescence or radiocarbon data and archeological constraints [START_REF] Aidona | First archaeomagnetic results and dating of neolithic structures in Northern Greece[END_REF][START_REF] Tema | Dating of ancient kilns: A combined archaeomagnetic and thermoluminescence analysis applied to a brick workshop at Kato Achaia, Greece[END_REF][START_REF] Kondopoulou | Archaeomagnetic study and thermoluminescence dating of protobyzantine kilns (Megali Kypsa, North Greece)[END_REF].

In spite of its great potential, the precision of archeomagnetic dating in the Balkans is still limited by the dispersion seen in the data, which suggests that some of the published results are biased and/or their uncertainties underestimated (e.g. [START_REF] Pavón-Carrasco | Intensity of the geomagnetic field in Europe for the last 3 ka: influence of data quality on geomagnetic field modeling[END_REF].

As a consequence, the detailed recovery of the geomagnetic fluctuations in this region, although an achievable target given the richness of its archeological and historical heritage, still requires the acquisition of new, high-quality, data with a precise control of age. The present study is part of this effort.

We report on five new archeointensity data obtained from groups of ceramics unearthed in Greece on the islands of Thasos and Delphi, two historical excavation sites of the French School at Athens [START_REF] Péntazos | La redécouverte de Delphes[END_REF][START_REF] Jacquemin | Delphes. Cent ans après la grande fouille. Essai de bilan[END_REF][START_REF] Mulliez | Histoire des fouilles de l'École française d'Athènes à Thasos[END_REF][START_REF] Muller | Cent ans de fouilles françaises à Thasos / 1911-2011 / Εκατό χρόνια γαλλικές ανασκαφές στην Θάσο[END_REF]. These ceramics are precisely dated to the Early Byzantine period, between the 4 th and the 7 th centuries CE. We will see that this time interval proves to be particularly interesting for both geomagnetic and archeological dating purposes as it was characterized by significant and rapid geomagnetic field intensity fluctuations.

Archeological collection

Our archeological collection is composed of six groups of ceramic fragments, three of them collected at Thasos (40.7°N, 24.6°E) and the other three at Delphi (38.5°N, 22.5°E; Fig. 1a). We underline that these groups were selected among the large amount of ceramic material unearthed from these two major archeological sites. The accuracy and reliability of the dating (an uncertainty of 100 years was fixed as a limit) and the precise knowledge of the location of the ceramic production site were a pre-requisite for our sampling in addition to considerations regarding the clay paste and the atmosphere of heating/cooling. At Thasos, a joint project of the French School at Athens (represented by the University of Lille -UMR Halma -and the National and Kapodistrian University of Athens) and the Greek Ministry of Culture (represented by the Ephorate of antiquities of Kavala-Thasos) has brought to light a Late Roman/Early Byzantine urban villa called DOM5 (for the latest excavation report see [START_REF] Blondé | Thasos. Les abords Nord de l'Artémision (THANAR). Campagnes 2012-2013 -Collaboration XVIII e EPKA -12 e EBA -EFA[END_REF]. This villa was built at the end of the 4 th / beginning of the 5 th century CE and was destroyed no later than 619/620 CE. This date is attested to by numerous coins dated to 619 CE found in the destruction layers of the villa, whereas no later coins have been discovered. From the 5 th century to its abandonment, the status of the villa had evolved from a luxurious domus, with the construction of private baths during the 5 th or 6 th century, to a more modest use after ca. 570 CE that saw a rearrangement of the house.

The three studied groups of pottery fragments were discovered in various occupation or dump layers of the villa. Each group is not associated to a single stratigraphic unit or context but in fact comes from different parts of the villa. The temporal homogeneity and the dating of the groups are, however, ensured by the typo-morphology of the fragments. Two of them belong to well-known and broadly distributed types of a famous Asia Minor production of Late Roman Terra Sigillata or Phocaean Red Slip ware (LI03 and LI04 groups, Fig. 1b; Hayes, 1972, LRC ware) with, for each group, one well-known shape selected (Table 1). The typo-chronology for the LRC ware was derived from the identification and classification of ten specific forms (and their variants) of this production and from their discovery in a number of archeological locations of the Eastern Mediterranean, which provided closed and welldated (particularly from coins) contexts [START_REF] Hayes | Late Roman Pottery, British School at Rome[END_REF][START_REF] Hayes | A supplement to Late Roman Pottery, British School at Rome[END_REF]. This type of production was widely distributed in the Eastern Mediterranean region from the 4 th to the 7 th century CE and its provenance was identified at the region of Phocaea (38.7°N,26.8°E) in Asia Minor (for the workshops see [START_REF] Hayes | A supplement to Late Roman Pottery, British School at Rome[END_REF][START_REF] Empereur | A propos d'un nouvel atelier de Late Roman C[END_REF][START_REF] Mayet | Une sigillée phocéenne tardive (« Late Roman C ware ») et sa diffusion en Occident[END_REF]. We underline that its recognition on archeological sites is largely used as a dating marker together with other imported fine wares such as the African Red Slip Ware [START_REF] Hayes | Late Roman Pottery, British School at Rome[END_REF][START_REF] Bonifay | Études sur la céramique romaine tardive d'Afrique[END_REF] or Late Roman D Ware [START_REF] Hayes | Late Roman Pottery, British School at Rome[END_REF]. We note that group LI03 includes also a few potsherds retrieved from the southern sector of the Agora of Thasos, identified as the Macellum [START_REF] Marc | Thasos. Abords Sud de l'Agora[END_REF] Excavations carried out by the French School at Athens). These fragments display the characteristic shape of the LI03 group, i.e. the 3C form dated to the second half of the 5 th century.

The third group from Thasos is composed of potsherds identified as Central Greek Painted Ware (LI05 group; [START_REF] Hayes | Late Roman Pottery, British School at Rome[END_REF][START_REF] Pétridis | Delphes dans l'Antiquité tardive : première approche topographique et céramologique[END_REF][START_REF] Pétridis | Les productions protobyzantines de céramique peinte en Grèce Continentale et dans les îles[END_REF]. The classification and dating of this production rely also on cross-data obtained on different archeological dated contexts from consumption sites among which we may cite Argos, Athens, Delphi, Thessaloniki or even more distant sites such as Abu Mena in Egypt and Constantinople. Its production site was identified at Nea Anchialos (39.3°N, 22.8°E) in Thessaly (Fig. 1a, [START_REF] Pétridis | Delphes dans l'Antiquité tardive : première approche topographique et céramologique[END_REF][START_REF] Pétridis | Les productions protobyzantines de céramique peinte en Grèce Continentale et dans les îles[END_REF] Late Roman/Early Byzantine remains were unearthed at Delphi as early as the end of the 19 th century but they had not received the attention they deserved [START_REF] Pétridis | Delphes dans l'Antiquité tardive : première approche topographique et céramologique[END_REF]. The restarting of excavations of Late Roman/Early Byzantine layers at Delphi in the 1990's by the French School at Athens allowed the uncovering of two artisanal districts with evidence of pottery activity. The first, dated to the second half of the 4 th century is located close to the Gymnasium of ancient Delphi, while the second, that was precisely dated by coins and imported pottery between 590 and 620 CE, is located South East of the Peribolos, in a place once occupied by the so-called South-Eastern Villa [START_REF] Déroche | Fouilles de Delphes II. Topographie et Architecture[END_REF]. These excavations revealed a ceramic production of good quality, with a large corpus of shapes, covering the needs of a local and regional clientele [START_REF] Pétridis | Fouilles de Delphes V, Monuments figurés 4: La céramique protobyzantine de Delphes: une production et son contexte[END_REF]. The two younger groups, referred to as DEL01 and DEL02, are associated with the South-Eastern Villa area. They were found in pottery deposits located close to the kilns and were identified as ceramic wasters (Fig. 1c).

The dating interval is first constrained by stratigraphical arguments. The workshop indeed settled over a villa whose abandonment as a habitation unit is dated at about 580 CE by coins and imported ceramics. A half follis of Maurice struck in 588/589 CE, which was discovered in one of the structures of the workshop further helps to date the beginning of its activity. This activity stopped some thirty years later as evidenced by coins all dated before 620 CE and imported vessels (in particular by African Red Slip Wares), none of which can be dated to after 620 CE. Finally, the dating also relies on the hypothesis of the capacity and lifetime of the deposit [START_REF] Pétridis | Fouilles de Delphes V, Monuments figurés 4: La céramique protobyzantine de Delphes: une production et son contexte[END_REF]. We note that the two groups were named differently to distinguish between fragments of pottery bases and fragments of pottery lips. They however come from exactly the same context and share the same age. Therefore their intensity results will be discussed together.

The last group from Delphi (DEL03) is from a large ensemble of ceramic fragments discovered in a shop, possibly a workshop, in the North-Eastern corner of the Roman Agora.

The fragments were in fact used to fill-in the space after its partial destruction, probably due to the earthquake of July 21, 365 CE (Petridis, 2010). This devastating seism was described by the historian Ammianus Marcellinus (330-395) in his Res gestae and was felt throughout the eastern Mediterranean region [START_REF] Guidoboni | Catalogue of Ancient Earthquakes in the Mediterranean area Up to the 10th Century: Rome[END_REF]. The dating of the DEL03 ensemble is constrained by imported terra sigillata wares and reinforced by the study of the coins found in this context (most of them belonging to the Constantinian dynasty). All the fragments examined belong to the local production, which has already been an object of both a chemical and a petrographic analysis (see Kouzéli in [START_REF] Pétridis | Fouilles de Delphes V, Monuments figurés 4: La céramique protobyzantine de Delphes: une production et son contexte[END_REF]. We note that these combined experiments of XRF, XRD, and thin section observations allowed the distinction of two claypaste groups with as a common feature their high content in iron oxides (Fe 2 O 3 ) with a percentage of 7.90%.

Our analyses aim at recovering the intensity of the ancient geomagnetic field recorded by ceramics during their manufacture, more precisely during their cooling in the kilns. This information is recorded through the acquisition of a Thermo-Remanent Magnetization (TRM) whose direction is parallel to that of the ambient Earth's magnetic field and whose moment is proportional to its intensity. When ceramics are removed from the kiln, the information on the geomagnetic direction is lost, while that of the intensity is retained and may be retrieved.

The main principle of the archeointensity measurements is to replace the Natural Remanent Magnetization (NRM) recorded by the fragments by a new TRM acquired in known field conditions (direction and intensity). In this study the archeological artifacts were analyzed using a tailored experimental protocol designed for the Triaxe magnetometer (Le [START_REF] Goff | A new three-axis vibrating sample magnetometer for continuous high-temperature magnetization measurements: applications to paleo-and archeo-intensity determinations[END_REF]. The Triaxe protocol derives from the method of [START_REF] Thellier | Sur l'intensité du champ magnétique terrestre dans le passé historique et géologique[END_REF]. Its originality relies on the fact that the magnetization measurements are performed continuously at high temperatures. This protocol was described in detail in previous publications (e.g. Le Goff and[START_REF] Goff | A new three-axis vibrating sample magnetometer for continuous high-temperature magnetization measurements: applications to paleo-and archeo-intensity determinations[END_REF][START_REF] Gallet | High-temperature archeointensity measurements from Mesopotamia[END_REF][START_REF] Gallet | Archaeomagnetism at Ebla (Tell Mardikh, Syria). New data on geomagnetic field intensity variations in the Near East during the Bronze Age[END_REF] and only a concise overview is given below.

The entire sequence of measurements is achieved within a little more than 2 hours and is performed on one specimen of 1 cm in length and with a thickness/diameter depending on the shape of the potsherd. It involves 4 successive steps:

• Step 1: Almost complete demagnetization of the NRM up to a high temperature referred to as T2, here comprised between 450 °C and 530 °C depending on the fragments.

• Step 2: The specimen is cooled to a low temperature referred to as T1 (set to 150 °C) and again heated to T2 in order to measure the temperature dependence of the spontaneous magnetization of the small fraction of the NRM remaining above T2.

• Step 3: Acquisition of a TRM by cooling the specimen from T2 to T1. Its direction is precisely set parallel to that of the NRM. The intensity of the applied field is also chosen to be close to the expected one.

• Step 4: Demagnetization of the newly acquired laboratory TRM between T1 and T2.

For each temperature Ti running from T1 to T2 by steps of ~5 °C, the R'(Ti) parameter is computed (Le [START_REF] Goff | A new three-axis vibrating sample magnetometer for continuous high-temperature magnetization measurements: applications to paleo-and archeo-intensity determinations[END_REF]. This parameter is given by the ratio between the NRM and TRM fractions demagnetized between T1 and Ti multiplied by the intensity of the laboratory field. A mean intensity value is finally estimated at the specimen level from the averaging of all R'(Ti) values. When a secondary magnetization component partly overprints the original TRM, the R'(Ti) values are then computed between a higher temperature T1' and T2, i.e. the temperature range where the original TRM is reliably isolated.

In the Triaxe protocol, it is worth stressing that the laboratory field is applied so that the direction of the new TRM is parallel to that of the NRM. This allows us to overcome the TRM anisotropy effect, which originates from the stretching of the clay during the manufacture of the object (e.g. [START_REF] Aitken | Archaeomagnetic determination of the past geomagnetic intensity using ancient ceramics: allowance for anisotropy[END_REF]. Furthermore, it has been experimentally and repeatedly observed on archeological artifacts of different types and origins that the intensity data obtained using the R'(Ti) parameters took into account the cooling rate effect (Le [START_REF] Goff | A new three-axis vibrating sample magnetometer for continuous high-temperature magnetization measurements: applications to paleo-and archeo-intensity determinations[END_REF][START_REF] Genevey | Evidence for rapid geomagnetic field intensity variations in Western Europe over the past 800 years from new French archeointensity data[END_REF][START_REF] Hartmann | Archeointensity in Northeast Brazil over the past five centuries[END_REF][START_REF] Genevey | New Archeointensity Data from French Early Medieval Pottery Production (6th-10th century AD). Tracing 1500 years of Geomagnetic Field Intensity Variations in Western Europe[END_REF].

We briefly recall that this effect is related to the dependence of the TRM acquisition on the cooling rate (e.g. [START_REF] Fox | Cooling rate dependance of thermoremanent magnetization[END_REF] and to the fact that there is generally a large difference between the original cooling rate, during which the original TRM/NRM is recorded in the kiln and the cooling rate applied for the laboratory TRM acquisition (e.g. [START_REF] Genevey | ArcheoInt: an upgraded compilation of geomagnetic field intensity data for the past ten millennia and its application to the recovery of the past dipole moment[END_REF].

Possible alteration due to heating was first investigated through susceptibility measurements performed from 50 °C up to ~500-525 °C, i.e. on the temperature range used for intensity determinations. Only pottery fragments showing reversible heating and cooling susceptibility curves were retained for further analyses (Fig. 2a,b,c,d). For these selected fragments, a second series of susceptibility measurements were carried out up to ~680 °C. For the Terra Sigillata produced at Phocaea (LI03 and LI05) and for the Central Greek Painted Ware production (LI04), the heating at higher temperature induced a strong alteration of the magnetic mineralogy, with higher susceptibilities observed on the cooling curves, most probably originating from the formation of new magnetite (Fig. 2f,g). These results highlight the advantage of investigating the magnetic mineral stability on the same temperature range as the one used for the intensity determinations rather than on a wider temperature interval. In the latter case, this would have led to the rejection of these three groups of pottery fragments.

In contrast, the ceramics from Delphi appear more stable upon heating (Fig. 2e,h). Whatever the production, the heating susceptibility curves usually exhibit two inflexion points, at low (200-300 °C) and medium-high temperatures (450-550 °C).

The magnetic mineralogy of this collection was further investigated through the thermal demagnetization of three-axis Isothermal Remanent Magnetization (2 T, 0.4 T, and 0.2 T) acquired in three perpendicular directions [START_REF] Lowrie | Identification of ferromagnetic minerals in a rock by coercivity and unblocking temperatures properties[END_REF]. These experiments were carried on two to four fragments per group among those that provided reliable intensity results. In most fragments, they show the predominance of a magnetic phase, likely from the magnetite family (with possible impurities), of low coercivity (<0.2 T) and with unblocking temperatures below 550 °C (Fig. 3). Another phase characterized by low unblocking temperatures (200-250 °C) and high coercivity is also systematically observed, but in various proportions (Fig. 3). The very same phase has been widely documented in archeomagnetic studies, while its clear identification remains an open question [START_REF] Mcintosh | Widespread occurrence of a novel high coercivity, thermally stable, low unblocking temperature magnetic phase in heated archeological material[END_REF][START_REF] Mcintosh | High coercivity remanence in baked clay materials used in archeomagnetism[END_REF].

Finally, a small fraction of hematite is also observed, which, we note, was not detected from the susceptibility measurements (Figs. 2,3). In summary, the magnetic mineralogy hence appears as a combination of magnetite (in majority), a mineral of low-unblocking temperatures (200-250°C) and high coercivity (>2T), and a small fraction of hematite. Such a combination is in fact rather common in baked clay archeological artifacts (e.g. [START_REF] Hartmann | Archeointensity in Northeast Brazil over the past five centuries[END_REF][START_REF] Genevey | New Archeointensity Data from French Early Medieval Pottery Production (6th-10th century AD). Tracing 1500 years of Geomagnetic Field Intensity Variations in Western Europe[END_REF].

Intensity Results

In addition to the required stability of the magnetic mineralogy constrained by susceptibility measurements, a set of selection criteria was applied on the archeointensity data in order to only retain the most reliable results. These criteria are the same as those used in our previous investigations using the Triaxe protocol (e.g. [START_REF] Genevey | Evidence for rapid geomagnetic field intensity variations in Western Europe over the past 800 years from new French archeointensity data[END_REF][START_REF] Genevey | Geomagnetic field intensity variations in Western Europe over the past 1100 years[END_REF][START_REF] Hartmann | New historical archeointensity data from Brazil: evidence for a large regional non-dipole field contribution over the past few centuries[END_REF][START_REF] Gallet | Archaeomagnetism at Ebla (Tell Mardikh, Syria). New data on geomagnetic field intensity variations in the Near East during the Bronze Age[END_REF]. These criteria aim to test the quality of the intensity determinations at the specimen level, with analyses strictly performed on the same temperature range where the primary TRM was reliably isolated. Over this temperature interval, the slope of the R'(Ti) values must be of less than 10% and must concern more than 50% of the magnetization fraction with an unblocking temperature larger than T1 (or T1', K% parameter; Supplementary Table S1; [START_REF] Gallet | High-temperature archeointensity measurements from Mesopotamia[END_REF]. At the fragment level, two to three specimens were analyzed and the results must agree to within 5%. Finally, at least three fragments are required to define a mean value at the group level and its standard deviation must be of less than 10% and 5 µT.

A total of 63 specimens from 23 fragments were successfully analyzed and used to derive five new archeointensity values encompassing the Early Byzantine period in Greece (Table 1 andsupplementary Table S1). All retained intensity determinations were reported at the specimen level in Fig. 4, with one panel per group. For each group, an example of thermal demagnetization is also presented in Fig. 4. These diagrams illustrate the different behaviors observed; that is, for part of the fragments, two components of magnetization observed above 150°C (Fig. 4b,f,j) and an intensity determination performed on the temperature range where the primary component was isolated (Fig. 4a,e,i). The thermal demagnetization of the other fragments revealed only one component of magnetization above 150 °C (Fig. 4d,h) and the intensity determination therefore concerned the entire temperature range above T1 (Fig. 4c,g).

The percentage of success is relatively low, between 33% and 50%, on the understanding that this percentage relates to the number of fragments whose magnetic moment was strong enough to be measured on the Triaxe (supplementary Table S1). For these fragments, the main causes of rejection were either related to their unstable magnetic mineralogy upon heating, or because the slope of the R'(Ti) values was too strong over the temperature range where the TRM was isolated, or because it was not possible to reliably isolate this primary magnetization component (i.e. the magnetization acquired during the manufacturing of the artifacts).

The five new intensity results are reported in Fig. 5, together with a selection of intensity results available inside a region of 700 km around Thessaloniki (see below). These new data are well defined with standard deviations between 1% and 5% of the corresponding means, the latter ranging from 52.0 µT to 61.5 µT after reduction to Thessaloniki (Table 1).

They clearly show an increase of the geomagnetic field intensities in the Balkans between the middle of the 4 th century and the beginning of the 7 th century CE.

Discussion

Archeomagnetism has largely been developed in the Balkan Peninsula, in particular through the numerous archeomagnetic studies carried out in Bulgaria since the late seventies (Kovacheva et al., 2014 and references therein) and with the development of this discipline in Greece over the past fifteen years [START_REF] Spatharas | Archaeointensity results from two ceramic kilns from N. Greece[END_REF]De Marco et al, 2008a,b;[START_REF] Aidona | First archaeomagnetic results and dating of neolithic structures in Northern Greece[END_REF]De Marco et al., 2014 among others). Regarding intensity, the available dataset allows a description of the geomagnetic variations over nearly the past eight millennia, making it, together with the one from the Middle East [START_REF] Gallet | New Late Neolithic (c. 7000-5000 BC) archeointensity data from Syria. Reconstructing 9000 years of archeomagnetic field intensity variations in the Middle East[END_REF], one of the longest archeomagnetic records.

The Balkan archeointensity dataset for the past three millennia was evaluated by [START_REF] Pavón-Carrasco | Intensity of the geomagnetic field in Europe for the last 3 ka: influence of data quality on geomagnetic field modeling[END_REF], who underlined the rather small percentage of results satisfying modern quality criteria (see also the discussion in [START_REF] Tema | Secular variation of the Earth's magnetic field in the Balkan region during the last eight millennia based on archaeomagnetic data[END_REF]. Taking advantage of the recent update of the Bulgarian archeomagnetic database [START_REF] Kovacheva | Extended and revised archaeomagnetic database and secular variation curves from Bulgaria for the last eight millennia[END_REF], we explore here again the Balkan results available for the first millennium CE, which is the period of interest of our study. The quality criteria considered below are those used in our previous studies conducted in Western Europe (e.g. [START_REF] Genevey | New Archeointensity Data from French Early Medieval Pottery Production (6th-10th century AD). Tracing 1500 years of Geomagnetic Field Intensity Variations in Western Europe[END_REF] and can be summarized as follows. We only retain the data obtained using the original and derived versions of the method of [START_REF] Thellier | Sur l'intensité du champ magnétique terrestre dans le passé historique et géologique[END_REF] or the original method of [START_REF] Shaw | A new method of determining the magnitude of the palaeomagnetic field: Application to five historic lavas and five archaeological samples[END_REF].

The mean intensity must be defined by at least three independent values and the associated error must ≤ 15% of the corresponding mean. The TRM anisotropy effect must be taken into account for objects generally regarded as particularly anisotropic, such as pottery and tiles.

The data respecting these criteria are reported in Fig. 5a. In this figure, a distinction was also made between the data whose reliability was further constrained by partial-TRM checks (pTRM-checks; solid symbols), allowing to better control the magnetic mineralogy stability on heating of the studied fragments, and the data with no such stability test (empty symbols).

As a whole, Fig. 5a shows a rather large variability of the archeointensity results.

When the data are close in age, this scatter may, in some cases, raise questions about their accuracy, either regarding their experimental determinations or their dating. This is particularly the case for the data belonging to the first four centuries CE (see below).

However, taken as a whole, the data support the occurrence of rapid and large intensity variations during the first millennium CE. For the Early Byzantine period, all results converge to indicate a significant increase in intensity from the 4 th century CE until about the first half of the 7 th century. Our new data are in good agreement with this evolution (Fig. 5a). A relative minimum in intensity is then observed around the transition between the 7 th and 8 th century CE, the intensity increasing again afterwards. It is interesting to note that a similar evolution was recently documented in Western Europe [START_REF] Gómez-Paccard | Improving our knowledge of rapid geomagnetic field intensity changes observed in Europe between 200 and 1400 AD[END_REF][START_REF] Genevey | New Archeointensity Data from French Early Medieval Pottery Production (6th-10th century AD). Tracing 1500 years of Geomagnetic Field Intensity Variations in Western Europe[END_REF], even though the fluctuations appear to be more intense in Eastern Europe.

The use of the pTRM-checks as a selection criterion has a strong impact for the time interval covering the first four centuries CE (compare Fig. 5a andb). For instance, there is at present no result constrained by pTRM-checks available for the period between the end of the second century CE and the end of the third century CE (Fig. 5b). Using only the most reliable archeointensity data obtained over Europe, [START_REF] Pavón-Carrasco | Intensity of the geomagnetic field in Europe for the last 3 ka: influence of data quality on geomagnetic field modeling[END_REF] developed an archeomagnetic field model which predicts for the Balkans a decrease in intensity during the first three centuries followed, as previously mentioned, by an increase during the Early Byzantine period (Fig. 5b). Our new result dated to the second half of the 4 th century (DEL03) is in good agreement with this trend, and so it is with two precisely dated results obtained in southern Italy [START_REF] Tema | New archaeointensity data from Italy and geomagnetic field intensity variation in the Italian Peninsula[END_REF]. We note however that a few recent data [START_REF] Spatharas | New magnetic mineralogy and archaeointensity results from Greek kilns and baked clays[END_REF][START_REF] Kondopoulou | Archaeomagnetic study and thermoluminescence dating of protobyzantine kilns (Megali Kypsa, North Greece)[END_REF], whose dating remains poorly defined, and which were not retained to derive the archeomagnetic field model A of [START_REF] Pavón-Carrasco | Intensity of the geomagnetic field in Europe for the last 3 ka: influence of data quality on geomagnetic field modeling[END_REF], could also support the occurrence of rapid variations during the 4 th century.

Although our new results appear to agree with the trend in intensity fluctuations, they seem to be systematically lower than the other mentioned data, in particular for the 5 th and 6 th centuries (Fig 5b). It is noteworthy that the Bulgarian data are not corrected for the cooling rate effect, which, when taken into account, usually tends to decrease the intensity values. This issue was discussed in [START_REF] Kovacheva | Extended and revised archaeomagnetic database and secular variation curves from Bulgaria for the last eight millennia[END_REF] who acknowledged that this effect might indeed affect intensity values, at least for the data obtained on bricks. In Fig. 5b, the Bulgarian data are reported with different symbols depending on the type of analyzed objects (i.e. with a distinction between bricks, kilns, baked clays and baked soils). A cooling rate correction of a 5% decrease was applied on the Bulgarian data obtained from bricks, which represent the majority of the dataset for the 5 th and 6 th centuries (Fig. 5c), the other data remaining unchanged. Such a correction is a reasonable educated guess (e.g. [START_REF] Genevey | ArcheoInt: an upgraded compilation of geomagnetic field intensity data for the past ten millennia and its application to the recovery of the past dipole moment[END_REF], although we recognize that the cooling rate effect may in fact vary from one object to another.

Clearly a 5% decrease of the Bulgarian intensity values would improve their agreement with our new results (Fig. 5c). At this stage, however, the question of the exact cooling rate correction factors required for the Bulgarian data remains unresolved and any further discussion on the significance of possible shifts between different datasets would require a systematic evaluation of the cooling rate effect for all data.

In spite of the complexity described above, we further selected the data with age uncertainties of less than 100 years. This smaller dataset, that includes our five new archeointensity data, was used to derive a mean archeointensity variation curve spanning the first millennium using a sliding windows of 75 years shifted every 10 years (Fig. 5c). We emphasize the fact that this curve, presented with its 2-s envelope, provides a likely accurate picture of the present knowledge on the secular geomagnetic field intensity variations in the Balkans during the first millennium CE, as constrained by using only the most reliable regional data.

This study reports on 5 new archeointensity data obtained from Greek pottery fragments precisely dated to the Early Byzantine period. These data allow to better constrain a significant geomagnetic field intensity increase in the Balkans between the fourth and seventh centuries CE. Such a feature appears particularly well established, even though the Balkan archeointensity dataset is severely penalized by a large scatter in the available results. It is worth pointing out that our study highlights the great potential of precisely dated potsherds for archeomagnetism, which have been, surprisingly, rarely exploited until now in this broad area.

Beside its implication for the regional geomagnetic field behavior, i.e. with the occurrence of rapid, centennial-scale intensity variations, the Early Byzantine intensity increase may also have interesting implications for archeological purposes. Archeointensity analyses may indeed help in deciphering the duration of certain ceramic productions in the Balkans such as the production of the Late Roman Phocaean Terra Sigillata, which may have persisted during the 7 th century. Such issues would clearly strengthen effective collaboration between archeologists and archeomagnetists. Comparison of our new data (pink crosses) with three successive selections of available archeointensity data obtained in the Balkans [START_REF] Nachasova | Geomagnetic Field intensity in the Eastern Mediterranean region in the second half of the 1st millennium BC and the beginning of our era[END_REF]De Marco et al., 2008a;[START_REF] Tema | Archaeointensity determinations from Italy: new data and the Earth's magnetic field strength variation over the past three millennia[END_REF][START_REF] Tema | Intensity of the Earth's magnetic field in Greece during the last five millennia: New data from Greek pottery[END_REF][START_REF] Tema | New archaeointensity data from Italy and geomagnetic field intensity variation in the Italian Peninsula[END_REF][START_REF] Spassov | Rock magnetic property and paleointensity determination on historical Santorini lava flows[END_REF][START_REF] Spatharas | New magnetic mineralogy and archaeointensity results from Greek kilns and baked clays[END_REF]Tema and

Figure Captions

Fig. 1 .

 1 Fig. 1. a) Location map of the Thasos and Delphi archeological excavations sites, where the

Fig. 2 .

 2 Fig. 2. K-T curves for four representative fragments among those providing reliable intensity

Fig. 3 .

 3 Fig. 3. Thermal demagnetization of orthogonal 3-axis composite IRM for six representative

Fig. 4 .

 4 Fig. 4. Archeointensity determination for the different studied pottery groups and examples of

Fig. 5 .

 5 Fig. 5. Geomagnetic intensity variations in the Balkans area for the first millennium CE.

  Figure 1.

  Figure 3.

  Figure 4.

Table 1 .

 1

	#Group	Age (CE)	Production site Lat (°N) Long (°E)	Archeological context/description	N Frag. (n Spec.)	F±σF (µT)	F Thessaloniki (µT)
	DEL03	[340-400]	Delphi	38.5	22.5	Delphi -North-Eastern corner of the Roman Agora -(Units: AG90; AG91; AG92) -Local production	N=4 (n=11)	50.7±0.5	52.0

Acknowledgments

The excavations of the French School at Athens (EfA) established at Thasos and Delphi were indicated to us for their potential by Prof. D. Mulliez, Director of EfA in 2010, who included our emerging project as part of the 2012-2016 five-year contract. Prof. A. Farnoux, the current Director of the School has continued the support of our study, together with J.

Fournier, the previous Director of Studies. We address our deep acknowledgements to all.

The visits to the storage rooms of both Museums and the permissions to obtain the selected materials for study were provided by the Ephorates of Kavala-Thasos and Fokis. It is our pleasure to thank their Directors, Ms. S. Dadaki and Ms. A. Psalti respectively. We also thank Dr. D. Malamidou and Dr. N. Petrocheilos for their help in accessing the material in Thasos and Delphi.

We are grateful to Yves Gallet for his helpful comments on the manuscript, to Maxime Le Goff for his support regarding the Triaxe measurements, and to Ruven Pillay for his corrections to the manuscript's English. Finally we thank two anonymous reviewers for a careful reading that has helped to improve the manuscript.

This study was partly financed by the INSU-CNRS program "PNP".

 Kondopoulou, 2011;[START_REF] Kovacheva | Extended and revised archaeomagnetic database and secular variation curves from Bulgaria for the last eight millennia[END_REF][START_REF] Kondopoulou | An Archaeometric and Archaeological Approach to Hellenistic-Early Roman Ceramic Workshops in Greece: Contribution to Dating[END_REF][START_REF] Kondopoulou | Archaeomagnetic study and thermoluminescence dating of protobyzantine kilns (Megali Kypsa, North Greece)[END_REF]. All data were first reduced at the latitude of Thessaloniki (40.6°N). a) Archeointensity results satisfying a set of quality criteria as described in the main text. Solid symbols correspond to the data obtained using pTRM-checks while open symbols refer to those obtained without such magnetic mineralogy test. b) Archeointensity obtained using pTRM-checks, with results from Bulgaria reported with different symbols depending of the artifacts analyzed. The red curve (with yellow error band at 95% of confidence) corresponds to the prediction of the archeomagnetic model A at Thessaloniki [START_REF] Pavón-Carrasco | Intensity of the geomagnetic field in Europe for the last 3 ka: influence of data quality on geomagnetic field modeling[END_REF]. c) Archeointensity data obtained using pTRM-checks and whose age error is < 100 years. A 5% decrease accounting for the cooling rate effect was applied for the Bulgarian data obtained on bricks.

The mean curve of the intensity variations, with its 2-σ envelope (blue curves) was computed using a sliding window of 75 years shifted every 10 years.

Supplementary material:

Supplementary Table 1. Intensity results obtained at the specimen level.

"Tmin-Tmax" indicates the temperature interval used for the intensity determination. "F Lab " is the intensity of the laboratory field used during the experiments. "K T1 (T1')" gives the percentage of the magnetization fraction with unblocking temperature larger than T1 (or T1') involved for the intensity determination while the "slope R' " is the slope of the R'(Ti) values over the Tmin-Tmax interval. F Triaxe is the intensity value at the specimen level. At the fragment level, the mean intensity value is given with its standard error (resp. standard deviation) when computed from two (resp. three) values. (N1/N2/N3)* indicates the number of collected fragments (N1), the number of fragments whose magnetization was strong enough to be measured on the Triaxe (N2) and the number of fragment retained for the computation of the mean intensity value (N3).