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Archaeomagnetic investigation of bricks from the VIIIth–VIIth century BC Greek–indigenous site of Incoronata (Metaponto, Italy)
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Physics and Chemistry of the Earth 33 (2008) 523–533 www.elsevier.com/locate/pce
Archaeomagnetic investigation of bricks from the VIIIth–VIIth century BC Greek–indigenous site of Incoronata (Metaponto, Italy) Mimi J. Hill a,*, Philippe Lanos a,1, Mario Denti b, Philippe Dufresne a,1 a
Civilisations Atlantiques et Arche´osciences, CNRS, UMR 6566, Universite´ de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France b University of Haute-Bretagne Rennes 2, Place Recteur H. Le Moal, CS 24307, 35043 Rennes Cedex, France Available online 20 February 2008
Abstract An archaeomagnetic investigation of two sets of brick fragments (in total 39) along with a radiocarbon date from one of the most important Greco–indigenous archaeological sites in the Central Mediterranean, the VIIIth–VIIth Century BC site of Incoronata (Metaponto, Italy) has been carried out in order to aid archaeological understanding of the site as well as to produce high quality archaeomagnetic data. A full suite of rock magnetic experiments have been carried out in addition to the classical Thellier method experiments with correction for anisotropy of TRM and cooling rate. The results indicate that the two sets of bricks are magnetically identical and have the same heating history and thus it is inferred the same origin. It seems that the bricks had been reused in two different contexts: (1) mixed with stone and ceramics in deposit pits and (2) used to consolidate an artificial plateau. The brick samples all contain a single component of remanence and thus, importantly, this study has shown that the hypothesis of destruction by fire is no longer tenable to explain the deposit pits previously interpreted as being storage houses or dwellings (oikoi), but is consistent with the suggestion that the pits are ritual deposits. Whilst it did not prove possible to obtain an estimate of the inclination of the geomagnetic field (since the bricks did not gain their remanence whilst on one of their flat surfaces), the archaeointensity experiments (with anisotropy of TRM and cooling rate correction) give a mean intensity of 85 ± 5 lT for the field at Incoronata during the VIIIth–VIIth century BC. This is almost twice the present day field strength and thus provides further evidence that the field was strong over at least a 30° longitude area of the globe during this time. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Archaeomagnetism; Archaeointensity; Incoronata; Greek Archaic
1. Introduction Direct measurements of the Earth’s magnetic field only go back a few hundred years. In order to extend the record further back in time the remanent magnetisation held by archaeological material, lava flows and sediments must be * Corresponding author. Present address: Geomagnetism Laboratory, University of Liverpool, Oliver Lodge Building, Oxford Street, Liverpool L69 7ZE, UK. Tel.: +44 151 794 3460. E-mail address: [email protected] (M.J. Hill). 1 Present affiliation: CNRS, Institut de Recherche sur les Arche´omate´riaux (IRAMAT, UMR 5060), Centre de Recherche en Physique Applique´e a` l’Arche´ologie (CRP2A), Universite´ Bordeaux 3 and Ge´osciences-Rennes (UMR 6118), Universite´ Rennes 1, France. Present address: Equipe Arche´omagne´tisme Campus scientifique de Beaulieu, CS 74205 35042 RENNES Cedex, France.
1474-7065/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.pce.2008.02.026
investigated. Global geomagnetic models using archaeomagnetic data, such as the model of Korte and Constable (2005), can offer important insights into geomagnetic field evolution and core mantle boundary processes. However, it is necessary to have many data with good spatial and temporal coverage in order to provide a complete field description. Due mainly to experimental difficulties there are many more directional data than intensity data, and whilst data coverage is reasonable for some locations (e.g. Bulgaria (Kovacheva, 1997) and France (Gallet et al., 2002, 2005; Chauvin et al., 2000; Genevey and Gallet, 2002)) many more high quality data are still needed (Korte et al., 2005). As well as furthering understanding of the geomagnetic field, archaeomagnetic studies can be an extremely useful archaeological tool, not only for dating purposes, but also
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for providing information on provenance and firing conditions (e.g. Lanos et al., 1999). This study presents results from an archaeomagnetic study of two sets of brick fragments (in total 39) found at two different locations at the VIIIth–VIIth century BC Greek–indigenous site of Incoronata, Metaponto, Italy. The aims of the study are twofold; firstly, to further archaeological understanding of the site and secondly, to produce high quality geomagnetic data for a time period and location for which there are currently few high quality data. 2. Archaeological context The site of Incoronata (40°220 N, 16°490 E) is located on a plateau overlooking the river Basento, about 7 km from the town of Metaponto, in the Basilicata region of Italy. It is one of the oldest Greek settlements along the Ionian coast of Southern Italy and is one of the most important Greek sites of the more ancient stage of the colonisation of the Central Mediterranean. Incoronata is at the heart of the Hellenic Mediterranean world within the future Achaean colony of Metaponto: the Metapontan, a rich area which along with Siritide is a key place for understanding the first relationships between Iron Age native communities (the Oenotres) and the Greek people who arrived at the end of the VIII century BC – beginning of the VII century BC (Centre Jean Berard and Fondazione Paestum, 1998; Denti, 2007). Excavations at Incoronata started in the 1970s under the direction of Professor Piero Orlandini of the University of Milan (see Orlandini and Castoldi, 1991, 1992, 1995, 1997, 2000 and Orlandini, 1986 for excavation reports). According to the traditional interpretation (Orlandini, 1995, 1999), the history of the site is characterised by two different phases: an Iron Age native village at the top of the plateau dating from the end of the IX century BC followed at the beginning of the VII century BC, by a settlement of Greek people coming from the Eastern Aegean which was occupied until the third quarter of that century. The nature of this ancient site is the subject of important scientific debate (Stea, 1999).
Incoronata contains one of the biggest known deposits of VII century BC orientalizing ceramics in the Mediterranean. Within pits huge quantities of broken pieces of large ceramic vessels have been found mixed with stones and in some places brick fragments. The ceramics (of so-called ‘‘colonial” type) are manufactured locally or have been imported from various Aegean centres. There are large vases, storage jars (pithoi), amphorae and wine drinking vessels; many decorated, and often painted with mythical subjects (Orlandini, 1988; Denti, 2000, 2002). Additionally, a number of locally produced large ritual vases decorated in relief (perirrhanteria) have also been found (Denti, 2005). The pits excavated at Incoronata have been interpreted as oikoi, which are dwellings or storage rooms for prestigious artefacts assigned for sale or exchange with inland native people. They would have been built with stone foundations, adobe bricks (unbaked clay bricks dried in the sun) and incannucciata (cane structures). This type of site should have been destroyed by the arrival of the new Achaean colonists that founded Metaponto city at the end of the VII century BC. One argument supporting this hypothesis is the presence of brick fragments in some of the deposits (Fig. 1a and photo on p. 54 of Orlandini, 1986), which were interpreted by Orlandini as adobe bricks that had become baked due to a (destructive) fire. However, on the basis of recent archaeological observations made by the team from the University of Rennes 2 (directed by M. Denti) it is suggested that the pits are in fact a series of ritual deposits and that the site was not destroyed by fire. By investigating the magnetic remanence properties of the brick fragments excavated by Orlandini in 1981 from trench N, this study aims to determine their heating history and see if there is any evidence for a fire. In 2003–2004, the University of Rennes 2 team discovered a large artificial plateau built with compacted earth mixed with brick fragments. This is interpreted as terracing to regularise and consolidate the narrowing of the natural plateau which occurs at this point. A large trench at the south of the artificial plateau (9 7 m) was excavated in
Fig. 1. Brick fragments found: (a) in the 1981 Orlandini University of Milan excavation from trench N and (b) within the artificial plateau from the University of Rennes 2, 2003 excavation.
M.J. Hill et al. / Physics and Chemistry of the Earth 33 (2008) 523–533
2004, approximately 50 m away from trench N. The brick fragments found within the artificial plateau look extremely similar to those recovered in the Orlandini excavations (Fig. 1). In order to test if both sets of bricks could have the same origin the magnetic mineralogy and remanence characteristics have been investigated.
a
3. Age of the bricks
b
The bricks found in trench N by Orlandini from the supposed oikoi will be contemporaneous in age or earlier than the archaeological artefacts with which they were found. These are Oenotre (native) or Greek ceramics from the VIIIth to the third quarter of the VIIth century BC. An AMS radiocarbon date has been obtained from charcoal fragments found during the 2004 excavation of the artificial plateau (sector 1, trench 3). The uncalibrated 14 C age obtained by the radiocarbon dating centre in Lyon (Lyon-3120(Poz)) is 2520 ± 35 BP. Using atmospheric data from Stuiver et al. (1998) OxCal v3.5 (available from http://www.rlaha.ox.ac.uk/oxcal/oxcal_h Ramsey, 1995, 1998) gives a calibrated age of 800–510 BC at the 95% confidence level confirming that the site is Archaic (Fig. 2). This gives an independent age estimate in addition to the archaeological dating evidence for the artificial plateau. 4. Archaeomagnetic sampling Thirty nine brick fragments from the two different locations were sampled for archaeomagnetic investigation (Fig. 3). As the brick fragments were not in situ (they had all been displaced) it was not possible to orient them to geographic coordinates. However, it is sometimes possible to determine the past inclination of the geomagnetic field if the bricks all acquired their remanence whilst on a flat surface (e.g. Lanos et al., 1999). The first sample set, named INCOA, consists of 15 bricks fragments from the artificial plateau. Eleven were found during the 2004 exca-
Radiocarbon determination
2800BP
Lyon-3120 (Poz) : 2520±35BP
2700BP
Calibration Curve OxCal v3.5
2600BP 2500BP
68.2% probability 790BC (13.9%) 750BC 690BC (10.0%) 660BC 650BC (44.3%) 540BC 95.4% probability 800BC (95.4%) 510BC
525
Drill core
Brick
Fig. 3. Sampling: (a) cartoon showing how the samples were drilled with orientation to a straight edge where possible and (b) photograph showing some of the brick samples with drill holes.
vation in sector 1, trench 1C US1. The other four (samples A14–A17) were found lying along the edge of a nearby trench dug in 2003. The second set of 24 brick fragments, named INCOB, are from the supposed oikoi excavated in 1981 from trench N by Orlandini. Twelve bricks came from the collection stored at Metaponto museum and 12 bricks were collected on site in 2004. The bricks collected on site were from the 1981 Orlandini excavation which had been discarded in a pile as they were not as well preserved as the ones kept for storage in the museum. Samples were drilled using an electric water-cooled rock drill. The samples were drilled perpendicular to the flat surface, interpreted as the top or bottom of the brick. Where possible, samples were oriented to a straight edge as shown in Fig. 3. It was possible to orient 23 of the 39 bricks (8 from INCOA and 15 from INCOB). In the laboratory the cores were cut to produce a sample for remanence measurements and another for rock magnetic experiments (apart from sample B12 where there was only enough material for remanence measurement). Twenty of the 39 samples were extremely friable, so the cores for remanence measurements were coated in water glass prior to any experimentation.
2400BP 2300BP
5. Experimental methods
2200BP
Probability Density
1000CalBC
800CalBC
600CalBC
400CalBC
Calibrated date
Fig. 2. Calibrated radiocarbon age determination for the charcoal sample from the 2004 excavation, sector 1, trench 3. The terminus post quem for the use of the wood is 800 BC at the 95% confidence level.
Rock magnetic experiments were carried out using the Variable Field Translation Balance (MMVFTB) at the University of Liverpool. Isothermal remanent (IRM) acquisition and back field coercivity experiments were carried out as well as hysteresis loops and Curie curves in order to characterise mineralogy. The classical Thellier method (Thellier and Thellier, 1959) with partial thermal remanent magnetisation (pTRM) checks and correction for anisotropy of TRM
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a
ments were carried out when approximately 30% of the NRM remained (at temperatures 450, 510 or 540 °C). Remanence acquisition is influenced by the rate of cooling (e.g. Fox and Aitken, 1980; Dodson and McClellandBrown, 1980). Cooling times used in the Thellier experiments (1.5 h) will be quicker than the cooling rate when the sample acquired its natural remanence. In order to determine the influence, and correct for cooling rate, experiments were carried out on each sample after the Thellier experiment had finished. The samples were heated and cooled four times in a field of 60 lT in custom built ovens each time imparting a full laboratory TRM. The samples were cooled twice with a quick cooling rate (1.5 h) and twice were slowly cooled in order to compare the slow and fast cooled TRM and check for alteration during the experiment (see Go´mez-Paccard et al., 2006 for methodological details). A linear cooling time of around 24 h was used to approximate the natural slow cooling rate. Whilst it is realised that this is an approximation of the natural cooling time, it has been demonstrated that the choice of
Normalised Magnetisation cv
and cooling rate has been used to determine both the directions and the archaeointensity. Samples were heated and cooled in a MMTD oven and remanence was measured with a Molspin spinner magnetometer at the Universite´ Rennes 1. The heating steps used were 100, 150, 200 and then 240 up to 570 °C in 30 °C step (or until less than 10% of the original remanence remained). Samples were heated twice at each temperature step in a laboratory field of 60 lT, the first time with the field on in the +Z sample direction (along the long axis of the sample), and the second time the samples were rotated by 180° (field in the Z direction). PTRM checks were carried out at least after every other temperature step. The manufacturing process of fired archaeological material often results in magnetic anisotropy (Rogers et al., 1979). It is therefore necessary to determine and correct for this effect. Anisotropy of TRM was determined by heating the sample in six perpendicular orientations, inducing a remanence in the ±X, ±Y, and ±Z sample directions (Veitch et al., 1984; Chauvin et al., 2000). These experi-
90
B08 40
-80
-60
-40
-10 -20 0
20
40
60
80
-60
IRM 10-3 Am2/kg
Magnetisation 10-3 Am2/kg
140 10 8 6 4 2 0
-110
0
20
40
60
1 0.8 0.6 0.4 0.2 0 0
80
100 200 300 400 500 600 700 Temperature °C
Field mT
-160 Field mT
B01
20
-80
-60
-40
0 -20 0 -20
20
40
60
80
Normalised Magnetisation cv
Magnetisation 10-3 Am2/kg
40
IRM 10-3 Am2/kg
60
b
4 3 2 1 0 0
-40
20
40
60
1 0.8 0.6 0.4 0.2
80
0 0
100 200 300 400 500 600 700
Field mT
Temperature °C
-60 Field mT
c
200
A03
100
-80
-60
-40
0 -20 0 -100 -200 -300 -400
20
40
60
80
Normalised Magnetisation df
300 IRM 10-3 Am2/kg
Magnetisation 10-3 Am2/kg
400 30 25 20 15 10 5 0 0
20
40 Field mT
60
80
1 0.8 0.6 0.4 0.2 0 0
100 200 300 400 500 600 700 Temperature °C
Field mT
Fig. 4. Examples of the three types of rock magnetic behaviour. From left to right hysteresis loop, IRM acquisition and Curie curve (solid (dashed) line heating (cooling) curve).
M.J. Hill et al. / Physics and Chemistry of the Earth 33 (2008) 523–533
527
seen in both the hysteresis and IRM curves is likely to be hematite. There is no evidence for a high coercivity component in the rest of the samples with the hysteresis and IRM acquisition curves saturating by 20 mT (Fig. 4b and c). Hysteresis loops for these samples indicate low coercivity (less than 10 mT) and a ratio of saturation remanence to saturation magnetisation (Mrs/Ms) ranging from 0.06 to 0.18, which is well below the expected value for a single population of single domain grains of magnetite (Day et al., 1977). Maximum Curie points for all samples are between 500 and 600 °C. Generally the Curie curves show an almost linear decrease in magnetisation with increasing temperature (Fig. 4a and b) indicating a distributed range of Curie temperatures. Seven samples have a discernable low
slow cooling rate is not critical (Genevey et al., 2003; Go´mez-Paccard et al., 2006). 6. Rock magnetism A total of 38 samples (all samples bar B12) were investigated. Fig. 4 shows examples of the different types of rock magnetic behaviour found. Ten samples (4 from INCOA and 6 from INCOB) exhibit wasp waisted (restricted) hysteresis loops indicating the presence of two populations of grains (Tauxe et al., 1996) (Fig. 4a). The IRM acquisition curves for these samples also indicate the presence of two grain populations with magnetisation increasing rapidly at low fields and then increasing more slowly up to the maximum field of 80 mT. The high coercivity component
Table 1 Location
Sample
NRM A/m
MAD
DANG
t1
t2
N
f
g
q
b
F
bF
Fa
INCOA
01 02 03 04 05 06 07 08 11 12 13 14 15 16 17
3.0 6.1 18.0 2.5 2.9 9.7 1.7 4.2 11.0 4.3 13.0 13.0 1.8 4.6 8.5
1.7 2.3 3.2 3.2 3.4 3.2 2.3 3.8 2.7 2.8 2.8 4.6 3.2 2.1 2.5
1.0 1.4 0.8 1.3 2.3 0.3 1.4 1.4 3.0 0.6 1.2 1.6 2.5 1.2 0.6
150 200 240 150 100 100 150 150 240 150 150 150 150 100 100
480 540 570 540 540 540 570 480 570 570 570 570 570 480 540
11 12 12 13 14 14 14 11 12 14 14 13 14 12 14
0.80 0.82 0.87 0.79 0.89 0.85 0.85 0.85 0.84 0.85 0.87 0.77 0.80 0.86 0.89
0.88 0.91 0.73 0.91 0.92 0.87 0.92 0.88 0.85 0.92 0.85 0.84 0.92 0.88 0.85
53.8 118.0 19.8 55.0 65.5 52.2 77.6 27.7 68.1 123.0 21.2 29.2 62.4 63.4 28.4
0.013 0.006 0.032 0.013 0.012 0.014 0.010 0.027 0.010 0.006 0.035 0.022 0.012 0.012 0.027
94.7 103.4 94.4 101.2 93.9 100.5 90.9 89.8 98.7 106.6 79.3 93.7 91.9 92.3 91.2
1.3 0.7 3.0 1.3 1.1 1.4 0.9 2.4 1.0 0.7 2.8 2.1 1.1 1.1 2.5
92.3 1.1 91.3 0.6 91.1 2.9 102.9 1.3 95.8 1.1 97.5 1.4 92.8 1.0 No result 97.4 1.0 91.0 0.7 86.6 3.1 105.9 2.7 90.7 1.0 91.7 1.1 92.1 2.5
82.7 87.1 85.0 92.9 88.9 87.3 82.5
01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
2.2 2.4 2.0 3.8 3.4 2.9 2.1 5.8 1.8 4.7 3.0
3.9 2.8 2.9 2.3 3.2 2.0 3.3 2.2 3.0 2.6 1.6 7.1 3.4 2.5 3.0 2.8 2.5 2.5 2.3 2.8 2.6 2.6 2.4 1.4
2.1 1.5 2.2 1.1 0.4 0.3 1.7 1.5 0.6 1.2 0.3 2.4 1.9 0.9 0.9 0.1 3.5 0.6 0.6 0.2 1.0 0.8 1.5 1.2
100 100 100 100 100 100 100 100 150 150 100 150 150 150 100 100 100 150 100 100 100 150 100 100
540 510 540 480 570 540 540 510 540 540 480 540 540 510 540 540 480 570 540 540 480 540 570 480
14 13 14 12 14 14 14 13 13 13 12 13 13 12 14 14 12 14 14 14 12 13 15 12
0.85 0.83 0.83 0.83 0.86 0.84 0.85 0.84 0.78 0.82 0.85 0.86 0.81 0.78 0.85 0.83 0.81 0.83 0.86 0.88 0.88 0.84 0.88 0.84
0.92 0.90 0.92 0.90 0.92 0.92 0.92 0.90 0.91 0.91 0.90 0.87 0.91 0.90 0.91 0.92 0.90 0.92 0.92 0.92 0.89 0.90 0.93 0.89
62.5 59.0 23.3 42.3 38.1 35.7 42.8 41.3 54.2 65.8 57.1 28.6 28.4 44.9 57.2 51.3 38.8 38.3 113.2 45.4 54.9 54.5 72.3 45.4
0.013 0.013 0.033 0.018 0.020 0.022 0.018 0.018 0.013 0.011 0.013 0.026 0.026 0.016 0.014 0.015 0.019 0.020 0.007 0.018 0.014 0.014 0.011 0.017
99.7 89.7 95.2 84.8 93.9 96.5 88.4 104.2 95.6 101.2 92.2 101.1 107.1 96.7 97.7 93.7 90.3 96.4 92.8 95.1 79.7 96.5 95.9 86.0
1.3 1.1 3.1 1.5 1.9 2.1 1.6 1.9 1.3 1.1 1.2 2.6 2.8 1.5 1.3 1.4 1.7 1.9 0.7 1.7 1.1 1.3 1.1 1.4
94.7 90.9 93.1 84.6 94.0 94.2 89.2 97.0 93.0 86.3 89.1 100.8 103.3 92.1 97.4 91.5 85.2 94.4 90.2 97.0 78.8 95.5 91.5 86.4
85.8 86.6 83.8 82.5 84.9 85.0 81.2 91.7 87.3 81.3 84.2 87.4 93.5 87.0 87.8 76.6 81.0 83.0 79.5 89.3 76.8 88.1 83.6 83.6
INCOB
4.7 3.2 3.2 2.4 2.6 3.3 3.9 5.0 3.6 4.7 2.9 3.8
bFa
1.3 1.1 3.1 1.5 1.9 2.1 1.7 1.8 1.3 1.0 1.3 2.7 2.7 1.4 1.3 1.4 1.6 1.9 0.6 1.7 1.1 1.3 1.1 1.4
Fa+crc
88.4 82.4 76.8 96.8 78.5 86.4 83.9
Sample results giving location; brick sample number; natural remanent magnetisation NRM; maximum angular deviation MAD and deviation angle DANG in degrees; t1 and t2 the minimum and maximum temperatures (°C) defining the primary component of remanence and the archaoeointensity estimate; N the number of data points; f, g, and q the archaeointensity quality parameters of Coe et al. (1978) and b is the ratio of the error in the slope to the slope of the best straight line in the NRM/TRM plot; F is the archaeointensity estimate and bF the error in F; Fa is the anisotropy corrected archaeointensity estimate and Fa+crc is the anisotropy and cooling rate corrected archaeointensity estimate, all in lT.
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M.J. Hill et al. / Physics and Chemistry of the Earth 33 (2008) 523–533
temperature Curie point (189–315 °C) as shown in Fig. 4a. Four samples from INCOA have natural remanent magnetisation (NRM) greater than the other samples (>10 A/ m as opposed to a mean of 3.7 A/m for the other samples) (see first column of Table 1 and Fig. 4c). These samples have a more pronounced high temperature Curie point and likely contain a greater proportion of this magnetic grain population compared to the other samples. The heating and cooling curves for all samples are very similar, indicating that the magnetic minerals are stable to heating up to 700 °C. The rock magnetic behaviour of the Incoronata bricks is similar to that seen in other studies of bricks, tiles and pottery (e.g. Chauvin et al., 2000; Genevey and Gallet, 2002). Titanomagnetites, with various amounts of titanium, aluminium substituted magnetite or stable substituted maghaemite are likely to be the main magnetic minerals plus a high coercivity mineral such as haematite, is certainly present in type Fig. 4a samples. The bricks from the two locations have very similar rock magnetic properties; however, samples with the strong NRM and type Fig. 4c behaviour were only found amongst the INCOA bricks.
7. Anisotropy of TRM Alteration checks carried out as part of the anisotropy experiment indicate that little or no alteration occurred. The check differs from the first remanence acquisition step by less than 5% for all but seven samples, with the biggest difference being 15% for sample A03. One of the magnetometer measurements for sample A08 was found to be anomalous (experimental error), hence this sample has been excluded from further consideration. The degree of anisotropy defined as the ratio of the maximum and minimum axes of the anisotropy tensor varies from 4% to 46%, but generally it is less than 20% (Fig. 5). There is no preferred direction of anisotropy with samples showing both lineation and foliation. There is no way to distinguish the two sets of brick fragments using anisotropy of TRM. 8. Remanence results All brick samples (from both locations) are dominated by a stable component of remanence heading towards the origin with a viscous component that is removed by 150 °C (Fig. 6). In three cases the characteristic direction
INCOA
INCOB
1.4
INCOA + INCOB
Lineation
1.3
INCOA INCOB
1.2
1.1
1 1
1.1
1.2
1.3
1.4
Foliation
Fig. 5. Histograms showing the degree of anisotropy for INCOA, INCOB and INCOA + INCOB and a Flinn diagram of lineation against foliation.
M.J. Hill et al. / Physics and Chemistry of the Earth 33 (2008) 523–533
has been defined from 200 or 240 °C as the samples do not start to demagnetise until higher temperatures and the low temperature data is a little noisy (see Fig. 6 sample A03 and Table 1). The shape of the demagnetisation curves are similar to the shape of the Curie curves indicating that the magnetic grains dominating the thermomagnetic signature hold a stable remanence (Figs. 4 and 6). The maximum angular deviation (MAD) and the deviation angle (DANG) are very similar for both sample sets (see Table 1) with the mean MAD 2.9° and mean DANG 1.3°. The MAD for sample 12 is the largest at 7° due to the fact that the sample is small and has an anomalous shape and was therefore not easy to orient in the magnetometer. The directional results corrected for anisotropy of TRM are plotted on a stereoplot in Fig. 7. It can be seen that the directions do not consistently fall into one of the areas expected if the bricks had been placed on one of their flat surfaces (flat, on edge or upright) when acquiring their remanence. Analysing the results assuming that the bricks did in fact acquire their remanence whilst on any of the three flat surfaces, whichever position maximises the inclination (e.g. see Lanos et al., 1999), does not reduce the scatter in the data. Restricting the analysis to only those samples which were oriented to a straight edge of the brick
X
Y 75˚ 55˚
Fig. 7. Directional results plotted on a stereonet. INCOA (INCOB) are depicted by black (grey) symbols. Solid (open) symbols indicate a positive (negative) vertical component. If the bricks gained their remanence whilst flat, it is expected that the directions would fall within the band defined by inclination of 55–75°. Similarly the bands close to the X and Y axes indicate areas that the directions are expected to be if the bricks gained their remanence whilst on edge or upright.
1
1
0.8
0.6
+X
0.4 0.2
100
200
300
400
500
0
1
600
Temperature C
8
0.6
100
200
300
400
500
600
+X 3
Temperature C
0.8
-8
0.4
0
+Y
NRM
0.8
NRM
0.2
4 0
0.6 0.4
A03
0.2
0.4
1.2
0
1
0.6
NRM
NRM
0.8
1.2
529
A02
0.2
-3
0 0
0.1
0.2
0.3
0.4
0.5
0.6
+Y +Z
0
+Z
0.7
0
0.1
0.2
0.3
TRM
0.4
0.5
0.6
0.7
0.8
TRM
1
1 0.8
0.8
1.2
1.2
0.4 0.2
1
NRM
NRM
+X 0.6
0
0
0
100
200
300
400
500
0.8
600
NRM
NRM
0.8
Temperature C
0.6 1
0.4
B19
0.2
0.4 0.2
1
2
+X
0.6
+Y
1 0
100
200
300
400
500
600
Temperature C
0.6
1
-1
+Y
0.4 -1
B11
0.2 -1 +Z
0 0
0.1
0.2
0.3
0.4
TRM
0.5
0.6
0.7
0.8
+Z
0 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
TRM
Fig. 6. Examples of results from the Thellier experiment. NRM/TRM plots with pTRM checks and demagnetisation of NRM with temperature, NRM and TRM normalised to the initial NRM. Orthoganal vector plots (OVP) in sample coordinates, solid (open) symbols represent the vertical (horizontal) component of remanence, scale is in A/m.
M.J. Hill et al. / Physics and Chemistry of the Earth 33 (2008) 523–533
also does not improve matters. The directions are in fact completely scattered leading to the conclusion that the bricks did not acquire their remanence whilst on a flat surface and it is therefore not possible to determine the inclination of the geomagnetic field at the time of remanence acquisition. The brick samples from the two locations behave in a similar manner and it is not possible to distinguish the results from the two locations. 9. Archaeointensity results High quality archaeointensity results have been obtained from all samples with little evidence for alteration occurring during the experiment as shown in the NRM/ TRM diagrams (Fig. 6). All results are listed in Table 1 along with the Coe et al. (1978) quality parameters. The fraction of NRM used (f) ranges between 0.77 and 0.89 and the quality parameter q ranges from 20 up to 123 (mean value 53). The archaeointensity estimates for INCOA and INCOB are very similar and range from 79 to 106 lT. The mean archaeointensity per sample set is 95 lT for both INCOA and INCOB (Table 2). After correction for anisotropy of TRM (Tables 1 and 2) the mean archaeointensity reduces by a few micro Tesla and the scatter about the mean reduces slightly. The results of the cooling rate experiment are shown in Fig. 8. All samples gave a positive cooling rate correction (between 2% and 16%) as expected for single domain acting grains (e.g. Dodson and McClelland-Brown, 1980). The alteration check varies between 7% and +1%. A negative check indicates that the evaluated cooling rate correction is likely to be a minimum estimate. All samples have been corrected for cooling rate and the corrected archaeointensities are listed in Table 1. The mean archaeointensity (Table 2) is reduced by 9% to 85 lT, a value almost twice that of the present day field (calculated using IGRF 2005). 10. Discussion The only difference seen in the magnetic behaviour of the brick samples from the two locations is that some of Table 2 Mean archaeointensity results per location (INCOA and INCOB) and combined mean results Samples
N
F lT
SD
Raw data
INCOA INCOB ALL
15 24 39
95 95 95
7 6 6
Anisotropy corrected
INCOA INCOB ALL
14 24 38
94 92 93
5 5 5
Anisotropy and cooling rate corrected
INCOA INCOB ALL
14 24 38
86 85 85
5 4 5
Where N is the number of brick samples and F and SD are the archaeointensity and standard deviation respectively.
18 16
INCOA 14
INCOB Cooling Rate Correction %
530
12 10 8 6 4 2 0
-10
-8
-6
-4 -2 Alteration Check %
0
2
Fig. 8. Percentage cooling rate correction versus percentage alteration.
the bricks from INCOA have a higher NRM. This is most probably due to natural variation in the material used to manufacture the bricks. There is therefore nothing to magnetically distinguish the two sets of bricks strongly suggesting that all the bricks have the same origin. It is certainly the case that all the bricks gained their remanence in the same field (presumably at the same time) as indicated by the similarity in the archaeointensity results. This along with the fact that no complete bricks, only fragments, have been found suggests that the bricks have been discovered in a secondary context. The remanent magnetisation is single component (with a small viscous component) therefore the last heating event was to 570 °C (maximum heating step used) or higher. This remanence was most likely acquired during deliberate heating such as in a kiln during their manufacture or if the bricks themselves formed part of a kiln or fire place. The directional results indicate that the bricks were not lying on a flat edge (all on the same one or in any combination) when they acquired their remanence, contrary to what has been found in other studies (e.g. Lanos, 1987; Lanos et al., 1999). Instead, the bricks may have been stacked at an angle inside the kiln or fire place. The bricks were baked at high temperature, homogeneously, without any reheating to lower temperatures as would be expected if they had been in a fire. It can thus be inferred that the bricks found with the ceramic deposits in trench N are not adobe bricks from an oikoi that had been destroyed by fire as suggested by Orlandini (1986). Instead it seems likely that the brick fragments were mixed with stone and systematically broken ceramics in the deposit pits. A possible reason is that the pits are ritual deposits, in which ceramics have been destroyed after the ceremonial use. The age of the Incoronata bricks corresponds to the age of one of the proposed archaeomagnetic jerks (Gallet et al.,
M.J. Hill et al. / Physics and Chemistry of the Earth 33 (2008) 523–533
531
Fig. 9. Histograms of archaeointensity (uncorrected data, anisotropy corrected, and cooling rate and anisotropy corrected) for all samples (INCOA + INCOB).
2003) which are characterised by a peak in intensity and a sharp directional change. Whilst it has not proved possible to obtain an estimate of the inclination of the geomagnetic field from the brick samples, a well-constrained single estimate of field intensity has been determined (Fig. 9). The archaeointensity result from Incoronata has been relocated to Mari, Syria (latitude 34.5°N) in order to compare it in Fig. 10 to the recently acquired high quality Mesopotamian dataset which includes correction for anisotropy of TRM and cooling rate (Gallet et al., 2006). The field strength at Incoronata (85 lT) is almost twice (1.8 times) that of the present day field which is broadly consistent with the peak in intensity seen in the Mesopotamian dataset as well as datasets from Greece, the Near and Middle East and Asia (see Genevey et al., 2003 and references therein) plus data from VIth and VIIth century BC ceramics from Carthage (Tunisia) from the study of Thellier and Thellier (1959). This study thus provides further evidence that the strength of the geomagnetic field was high over at least a 30° longitude area of the globe during this time. Further studies are needed, including directional and dating studies, to confirm the nature and global extent of this proposed 100
This study
80 70 60 50
Archaeointensity μT
90
Mesopotamian data
40 30 -4000
-3500
-3000
-2500
-2000 -1500 Age Year
-1000
-500
0
Fig. 10. Archaeointensity data from 0 to 4000 BC from Mesopotamia (Gallet et al., 2006) plus the result from Incoronata (taking the radiocarbon date for the age). All data relocated to Mari, Syria.
‘archaeomagnetic jerk’ and its possible impact on climate and ancient civilisation (Gallet et al., 2005, 2006). During the 2005 University of Rennes 2 excavation remains of a potter’s kiln were discovered to the south of the artificial plateau, in the slope. This supports the assumption that the bricks and most of the ceramics discovered in the pits were locally produced. Future archaeological and archaeomagnetic studies of in situ material from the kiln are planned in order to obtain both directional as well as intensity data in order to better understand and constrain the chronology of the site of Incoronata between the earlier native settlement and the later Achaean site of Metaponto. 11. Conclusions An archaeomagnetic investigation of two sets of bricks from one of the most important Greco–indigenous archaeological sites of the Central Mediterranean, the VIIIth– VIIth Century BC site of Incoronata (Metaponto, Italy) has been carried out. Rock magnetic and magnetic remanence experiments (including anisotropy of TRM, archaeointensity and cooling rate studies) indicate that the two sets of bricks are magnetically identical and have the same heating history and thus the same origin. It is thus inferred that the bricks had been reused in two different contexts: (1) mixed with stone and ceramics in deposit pits and (2) to consolidate an artificial plateau. Additionally, and importantly, this study has shown that the hypothesis of destruction by fire is no longer tenable to explain the pits which contain large quantities of ceramics along with stones and bricks previously interpreted as being oikoi. Whilst it was not possible to obtain an estimate of the inclination (the bricks did not gain their remanence whilst on one of their flat surfaces), the archaeointensity experiments (with anisotropy of TRM and cooling rate correction) give a mean intensity of 85 ± 5 lT for the field at Incoronata during the VIIth century BC. This is almost twice the present day field strength and thus provides
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further evidence that the field was strong over at least a 30° longitude area of the globe during this time. This study has demonstrated the usefulness of archaeomagnetic studies for aiding archaeological investigations as well as producing data to further understanding of the geomagnetic field. Note added in proof Since this paper was submitted a paper have been published discussing the archaeological significance of the bricks studied here (Denti and Lanos, 2008). Acknowledgments We thank Antonio di Siena, director of the Archaeological Museum of Metaponto, for kindly giving permission to sample bricks from the site of Incoronata and we thank Christine Oberlin from the Centre of radiocarbon dating of Lyon, CNRS for the dating. This study was supported by the European Network AARCH (Archaeomagnetic Applications for the Rescue of Cultural Heritage) Contract HPRN-CT-2002-00219, DyETI program n° IT67 (INSU CNRS), 2004/2005: ‘‘Variation se´culaire du Champ Ge´omagne´tique durant le 1er mille´naire BC et construction des courbes de re´fe´rence”. NERC grant NE/C51982X/1 is also acknowledged. References Centre Jean Berard and Fondazione Paestum, 1998. Siritide e Metapontino. Storie di due territori coloniali, Atti dell’incontro di studio, Policoro 31 ottobre–2 novembre 1991, Cahiers du Centre Jean Berard XX (in Italian). Chauvin, A., Garcia, Y., Lanos, Ph., Laubenheimer, F., 2000. Paleointensity of the geomagnetic field recovered on archaeomagnetic sites from France. Phys. Earth Planet. Int. 120, 111–136. Coe, R.S., Gromme´, C.S., Mankinen, E.A., 1978. Geomagnetic paleointensities from radiocarbon-dated lava flows on Hawaii and the question of the Pacific nondipole low. J. Geophys. Res. 83, 1740–1756. Day, R., Fuller, M.D., Schmidt, V.A., 1977. Hysteresis properties of titanomagnetites: grain size and composition dependence. Phys. Earth Planet. Int. 13, 260–267. Denti, M., 2000. Nuovi documenti di ceramica orientalizzante della Grecia d’Occidente. Stato della questione e prospettive della ricerca, Me´langes de l’Ecole Francßaise de Rome – Antiquite´ 112 (2000/2), pp. 781–842 (in Italian). Denti, M., 2002. Linguaggio figurativo e identita` culturale nelle piu` antiche comunita` greche della Siritide e del Metapontino. In: Moscati Castelnuovo, L. (Ed.), Identita` e prassi storicanel Mediterraneo greco, Castelnuovo, Milan, pp. 33–62 (in Italian). Denti, M., 2005. Peqiqqamsgqia figurati a rilievo nei depositi di ceramica sulla collina dell’Incoronata di Metaponto. Tracce di un’attivita` rituale?. ‘‘Siris” VI 2005, pp. 173–186 (in Italian). Denti, M., 2007. Grecs et indige`nes a` la frontie`re de l’Occident. L’occupation du territoire dans le Me´tapontin au VIIe sie`cle avant J.-C., Actes du colloque ‘‘Pouvoir et territoire I (Antiquite´-Moyen ˆ ge)”, Saint-E ´ tienne novembre 2005, Saint-E ´ tienne, pp. 225–244 (in A French). Denti, M., Lanos, Ph., 2008. Rouges, non rougies : les briques de l’Incoronata et le proble`me de l’interpre´tation des de´poˆts de ce´rami-
que, Me´langes de l’Ecole Francßaise de Rome. Antiquite´, 119/2, 2007, pp. 489–525 (in French). Dodson, M.H., McClelland-Brown, E., 1980. Magnetic blocking temperature of single-domain grains during slow cooling. J. Geophys. Res. 85, 2625–2637. Fox, J.M.W., Aitken, M.J., 1980. Cooling-rate dependence of thermoremanent magnetisation. Nature 283, 462–463. Gallet, Y., Genevey, A., Courtillot, V., 2003. On the possible occurrence of ‘archaeomagnetic jerks’ in the geomagnetic field over the past three millennia. Earth Planet. Sci. Lett. 214, 237–242. Gallet, Y., Genevey, A., Fluteau, F., 2005. Does Earth’s magnetic field secular variation control centennial climate change? Earth Planet. Sci. Lett. 236, 339–347. Gallet, Y., Genevey, A., Le Goff, M., 2002. Three millennia of directional variations of the Earth’s magnetic field in western Europe as revealed by archaeological artefacts. Phys. Earth Planet. Int. 131, 81–89. Gallet, Y., Genevey, A., Le Goff, M., Fluteau, F., Ali Eshraghi, S., 2006. Possible impact of the Earth’s magnetic field on the history of ancient civilizations. Earth Planet. Sci. Lett. 246, 14–26. Genevey, A., Gallet, Y., 2002. Intensity of the geomagnetic field in western Europe over the past 2000 years: new data from ancient French pottery. J. Geophys. Res. 107, B11. doi:10.1029/2001JB00070. Genevey, A., Gallet, Y., Margueron, J., 2003. Eight thousand years of geomagnetic field intensity variations in the eastern Mediterranean. J. Geophys. Res. 108, B5. doi:10.1029/2001JB00161. Go´mez-Paccard, M., Chauvin, A., Lanos, Ph., Thiriot, J., Jime´nezCastillo, P., 2006. Archeomagnetic study of seven contemporaneous kilns from Murcia (Spain). Phys. Earth Planet. Int. 157, 16–32. Korte, M., Constable, C.G., 2005. Continuous geomagnetic field models for the past 7 millennia: 2. CALS7K. Geochem. Geophys. Geosyst. 6. doi:10.1029/2004GC00080. Korte, M., Genevey, A., Constable, C.G., Frank, U., Schnepp, E., 2005. Continuous geomagnetic field models for the past 7 millennia: 1. A new global data compilation. Geochem. Geophys. Geosyst. 6. doi:10.1029/2004GC00080. Kovacheva, M., 1997. Archaeomagnetic database from Bulgaria. Phys. Earth Planet. Int. 102, 145–151. Lanos, Ph., 1987. The effects of demagnetizing fields on thermoremanent magnetization acquired by parallel-sided baked clay blocks. Geophys. J. Royal Astron. Soc. 91, 985–1012. Lanos, Ph., Kovacheva, M., Chauvin, A., 1999. Archaeomagnetism, methodology and applications: implementation and practice of the archaeomagnetic method in France and Bulgaria. Eur. J. Archaeol. 2, 365–392. Orlandini, P. (Ed.), 1986. I Greci sul Basento. Mostra degli Scavi archeologici all’Incoronata di Metaponto 1971–1984. New Press, Como (in Italian). Orlandini, P., 1988. Due nuovi vasi figurati di stile orientalizzante dagli scavi dell’Incoronata di Metaponto, BdA, vol. 49, maggio-giugno, pp. 1–16 (in Italian). Orlandini, P., 1995. Incoronata, in EAA, 2o Supplemento, III, Roma, pp. 96–98 (in Italian). Orlandini, P., 1999. La colonizzazione ionica della Siritide, In: Adamesteanu, D. (Ed.), Storia della Basilicata. 1, L’antichita`, Roma-Bari, pp.197–210 (in Italian). Orlandini, P., Castoldi, M. (Eds.), 1991. Incoronata I, Ricerche archeologiche all’Incoronata di Metaponto. Scavi dell’Universita` degli Studi di Milano. Istituto di Archeologia. 1. Le fosse di scarico del saggio P. Materiali e problematiche, Milan (in Italian). Orlandini, P., Castoldi, M. (Eds.), 1992. Incoronata II, Ricerche archeologiche all’Incoronata di Metaponto. Scavi dell’Universita` degli Studi di Milano. Istituto di Archeologia. 2. Dal villaggio indigeno all’emporio greco. Le strutture e i materiali del saggio T, Milan (in Italian). Orlandini, P., Castoldi, M. (Eds.), 1995. Incoronata II, Ricerche archeologiche all’Incoronata di Metaponto. Scavi dell’Universita` degli Studi di Milano. Istituto di Archeologia. 3. L’oikos greco del saggio S. Lo scavo e i reperti, Milan (in Italian).
M.J. Hill et al. / Physics and Chemistry of the Earth 33 (2008) 523–533 Orlandini, P., Castoldi, M., (Eds.), 2000. Incoronata IV, Ricerche archeologiche all’Incoronata di Metaponto. Scavi dell’Universita` degli Studi di Milano. Dipartimento di Scienze dell’Antichita`. Sezione di Archeologia. 4. L’oikos greco del grande perirrhanterion nel contesto del saggio G., Milan (in Italian). Orlandini, P., Castoldi, M. (Eds.), 1997. Incoronata V, Ricerche archeologiche all’Incoronata di Metaponto. Scavi dell’Universita` degli Studi di Milano. Istituto di Archeologia. 5. L’oikos greco del saggio H. Lo scavo e i reperti, Milan (in Italian). Ramsey, C.B., 1995. Radiocarbon calibration and analysis of stratigraphy: the OxCal program. Radiocarbon 37 (2), 425–430. Ramsey, C.B., 1998. The role of statistical methods in the interpretation of radiocarbon dates, Actes du colloque ‘‘C14 Arche´ologie”. Revue d’Arche´ome´trie, 83–86. Rogers, J., Fox, J.W.M., Aitken, M.J., 1979. Magnetic anisotropy in ancient pottery. Nature 277, 644–646.
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Stea, G., 1999. Forme della presenza greca sull’arco ionico della Basilicata: tra emporı`a e apoikı`ai, Koina`. In: Castoldi, M. (Ed.), Miscellanea di studi archeologici in onore di Piero Orlandini, Milan, pp. 49–71 (in Italian). Stuiver, M., Reimer, P.J., Bard, E., Beck, J.W., Burr, G.S., Hughen, K.A., Kromer, B., McCormac, F.G., Plicht, J.V.D., Spurk, M., 1998. INTCAL98 radiocarbon age calibration, 24,000-0 cal BP. Radiocarbon 40, 1041–1083. Tauxe, L., Mullender, T.A.T., Pick, T., 1996. Potbellies, wasp-waists, and superparamagnetism in magnetic hysteresis. J. Geophys. Res. 101, 571–583. Thellier, E., Thellier, O., 1959. Sur l’intensite´ du champ magne´tique terrestre dans le passe´ historique et ge´ologique. Annal. Ge´ophys. 15, 285–376. Veitch, R.J., Hedley, G., Wagner, J.J., 1984. An investigation of the intensity of the geomagnetic field during Roman times using magnetically anisotropic bricks and tiles. Arch. Sci. Gene`ve 37, 359–373.
Physics and Chemistry of the Earth 33 (2008) 523–533 www.elsevier.com/locate/pce
Archaeomagnetic investigation of bricks from the VIIIth–VIIth century BC Greek–indigenous site of Incoronata (Metaponto, Italy) Mimi J. Hill a,*, Philippe Lanos a,1, Mario Denti b, Philippe Dufresne a,1 a
Civilisations Atlantiques et Arche´osciences, CNRS, UMR 6566, Universite´ de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France b University of Haute-Bretagne Rennes 2, Place Recteur H. Le Moal, CS 24307, 35043 Rennes Cedex, France Available online 20 February 2008
Abstract An archaeomagnetic investigation of two sets of brick fragments (in total 39) along with a radiocarbon date from one of the most important Greco–indigenous archaeological sites in the Central Mediterranean, the VIIIth–VIIth Century BC site of Incoronata (Metaponto, Italy) has been carried out in order to aid archaeological understanding of the site as well as to produce high quality archaeomagnetic data. A full suite of rock magnetic experiments have been carried out in addition to the classical Thellier method experiments with correction for anisotropy of TRM and cooling rate. The results indicate that the two sets of bricks are magnetically identical and have the same heating history and thus it is inferred the same origin. It seems that the bricks had been reused in two different contexts: (1) mixed with stone and ceramics in deposit pits and (2) used to consolidate an artificial plateau. The brick samples all contain a single component of remanence and thus, importantly, this study has shown that the hypothesis of destruction by fire is no longer tenable to explain the deposit pits previously interpreted as being storage houses or dwellings (oikoi), but is consistent with the suggestion that the pits are ritual deposits. Whilst it did not prove possible to obtain an estimate of the inclination of the geomagnetic field (since the bricks did not gain their remanence whilst on one of their flat surfaces), the archaeointensity experiments (with anisotropy of TRM and cooling rate correction) give a mean intensity of 85 ± 5 lT for the field at Incoronata during the VIIIth–VIIth century BC. This is almost twice the present day field strength and thus provides further evidence that the field was strong over at least a 30° longitude area of the globe during this time. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Archaeomagnetism; Archaeointensity; Incoronata; Greek Archaic
1. Introduction Direct measurements of the Earth’s magnetic field only go back a few hundred years. In order to extend the record further back in time the remanent magnetisation held by archaeological material, lava flows and sediments must be * Corresponding author. Present address: Geomagnetism Laboratory, University of Liverpool, Oliver Lodge Building, Oxford Street, Liverpool L69 7ZE, UK. Tel.: +44 151 794 3460. E-mail address: [email protected] (M.J. Hill). 1 Present affiliation: CNRS, Institut de Recherche sur les Arche´omate´riaux (IRAMAT, UMR 5060), Centre de Recherche en Physique Applique´e a` l’Arche´ologie (CRP2A), Universite´ Bordeaux 3 and Ge´osciences-Rennes (UMR 6118), Universite´ Rennes 1, France. Present address: Equipe Arche´omagne´tisme Campus scientifique de Beaulieu, CS 74205 35042 RENNES Cedex, France.
1474-7065/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.pce.2008.02.026
investigated. Global geomagnetic models using archaeomagnetic data, such as the model of Korte and Constable (2005), can offer important insights into geomagnetic field evolution and core mantle boundary processes. However, it is necessary to have many data with good spatial and temporal coverage in order to provide a complete field description. Due mainly to experimental difficulties there are many more directional data than intensity data, and whilst data coverage is reasonable for some locations (e.g. Bulgaria (Kovacheva, 1997) and France (Gallet et al., 2002, 2005; Chauvin et al., 2000; Genevey and Gallet, 2002)) many more high quality data are still needed (Korte et al., 2005). As well as furthering understanding of the geomagnetic field, archaeomagnetic studies can be an extremely useful archaeological tool, not only for dating purposes, but also
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for providing information on provenance and firing conditions (e.g. Lanos et al., 1999). This study presents results from an archaeomagnetic study of two sets of brick fragments (in total 39) found at two different locations at the VIIIth–VIIth century BC Greek–indigenous site of Incoronata, Metaponto, Italy. The aims of the study are twofold; firstly, to further archaeological understanding of the site and secondly, to produce high quality geomagnetic data for a time period and location for which there are currently few high quality data. 2. Archaeological context The site of Incoronata (40°220 N, 16°490 E) is located on a plateau overlooking the river Basento, about 7 km from the town of Metaponto, in the Basilicata region of Italy. It is one of the oldest Greek settlements along the Ionian coast of Southern Italy and is one of the most important Greek sites of the more ancient stage of the colonisation of the Central Mediterranean. Incoronata is at the heart of the Hellenic Mediterranean world within the future Achaean colony of Metaponto: the Metapontan, a rich area which along with Siritide is a key place for understanding the first relationships between Iron Age native communities (the Oenotres) and the Greek people who arrived at the end of the VIII century BC – beginning of the VII century BC (Centre Jean Berard and Fondazione Paestum, 1998; Denti, 2007). Excavations at Incoronata started in the 1970s under the direction of Professor Piero Orlandini of the University of Milan (see Orlandini and Castoldi, 1991, 1992, 1995, 1997, 2000 and Orlandini, 1986 for excavation reports). According to the traditional interpretation (Orlandini, 1995, 1999), the history of the site is characterised by two different phases: an Iron Age native village at the top of the plateau dating from the end of the IX century BC followed at the beginning of the VII century BC, by a settlement of Greek people coming from the Eastern Aegean which was occupied until the third quarter of that century. The nature of this ancient site is the subject of important scientific debate (Stea, 1999).
Incoronata contains one of the biggest known deposits of VII century BC orientalizing ceramics in the Mediterranean. Within pits huge quantities of broken pieces of large ceramic vessels have been found mixed with stones and in some places brick fragments. The ceramics (of so-called ‘‘colonial” type) are manufactured locally or have been imported from various Aegean centres. There are large vases, storage jars (pithoi), amphorae and wine drinking vessels; many decorated, and often painted with mythical subjects (Orlandini, 1988; Denti, 2000, 2002). Additionally, a number of locally produced large ritual vases decorated in relief (perirrhanteria) have also been found (Denti, 2005). The pits excavated at Incoronata have been interpreted as oikoi, which are dwellings or storage rooms for prestigious artefacts assigned for sale or exchange with inland native people. They would have been built with stone foundations, adobe bricks (unbaked clay bricks dried in the sun) and incannucciata (cane structures). This type of site should have been destroyed by the arrival of the new Achaean colonists that founded Metaponto city at the end of the VII century BC. One argument supporting this hypothesis is the presence of brick fragments in some of the deposits (Fig. 1a and photo on p. 54 of Orlandini, 1986), which were interpreted by Orlandini as adobe bricks that had become baked due to a (destructive) fire. However, on the basis of recent archaeological observations made by the team from the University of Rennes 2 (directed by M. Denti) it is suggested that the pits are in fact a series of ritual deposits and that the site was not destroyed by fire. By investigating the magnetic remanence properties of the brick fragments excavated by Orlandini in 1981 from trench N, this study aims to determine their heating history and see if there is any evidence for a fire. In 2003–2004, the University of Rennes 2 team discovered a large artificial plateau built with compacted earth mixed with brick fragments. This is interpreted as terracing to regularise and consolidate the narrowing of the natural plateau which occurs at this point. A large trench at the south of the artificial plateau (9 7 m) was excavated in
Fig. 1. Brick fragments found: (a) in the 1981 Orlandini University of Milan excavation from trench N and (b) within the artificial plateau from the University of Rennes 2, 2003 excavation.
M.J. Hill et al. / Physics and Chemistry of the Earth 33 (2008) 523–533
2004, approximately 50 m away from trench N. The brick fragments found within the artificial plateau look extremely similar to those recovered in the Orlandini excavations (Fig. 1). In order to test if both sets of bricks could have the same origin the magnetic mineralogy and remanence characteristics have been investigated.
a
3. Age of the bricks
b
The bricks found in trench N by Orlandini from the supposed oikoi will be contemporaneous in age or earlier than the archaeological artefacts with which they were found. These are Oenotre (native) or Greek ceramics from the VIIIth to the third quarter of the VIIth century BC. An AMS radiocarbon date has been obtained from charcoal fragments found during the 2004 excavation of the artificial plateau (sector 1, trench 3). The uncalibrated 14 C age obtained by the radiocarbon dating centre in Lyon (Lyon-3120(Poz)) is 2520 ± 35 BP. Using atmospheric data from Stuiver et al. (1998) OxCal v3.5 (available from http://www.rlaha.ox.ac.uk/oxcal/oxcal_h Ramsey, 1995, 1998) gives a calibrated age of 800–510 BC at the 95% confidence level confirming that the site is Archaic (Fig. 2). This gives an independent age estimate in addition to the archaeological dating evidence for the artificial plateau. 4. Archaeomagnetic sampling Thirty nine brick fragments from the two different locations were sampled for archaeomagnetic investigation (Fig. 3). As the brick fragments were not in situ (they had all been displaced) it was not possible to orient them to geographic coordinates. However, it is sometimes possible to determine the past inclination of the geomagnetic field if the bricks all acquired their remanence whilst on a flat surface (e.g. Lanos et al., 1999). The first sample set, named INCOA, consists of 15 bricks fragments from the artificial plateau. Eleven were found during the 2004 exca-
Radiocarbon determination
2800BP
Lyon-3120 (Poz) : 2520±35BP
2700BP
Calibration Curve OxCal v3.5
2600BP 2500BP
68.2% probability 790BC (13.9%) 750BC 690BC (10.0%) 660BC 650BC (44.3%) 540BC 95.4% probability 800BC (95.4%) 510BC
525
Drill core
Brick
Fig. 3. Sampling: (a) cartoon showing how the samples were drilled with orientation to a straight edge where possible and (b) photograph showing some of the brick samples with drill holes.
vation in sector 1, trench 1C US1. The other four (samples A14–A17) were found lying along the edge of a nearby trench dug in 2003. The second set of 24 brick fragments, named INCOB, are from the supposed oikoi excavated in 1981 from trench N by Orlandini. Twelve bricks came from the collection stored at Metaponto museum and 12 bricks were collected on site in 2004. The bricks collected on site were from the 1981 Orlandini excavation which had been discarded in a pile as they were not as well preserved as the ones kept for storage in the museum. Samples were drilled using an electric water-cooled rock drill. The samples were drilled perpendicular to the flat surface, interpreted as the top or bottom of the brick. Where possible, samples were oriented to a straight edge as shown in Fig. 3. It was possible to orient 23 of the 39 bricks (8 from INCOA and 15 from INCOB). In the laboratory the cores were cut to produce a sample for remanence measurements and another for rock magnetic experiments (apart from sample B12 where there was only enough material for remanence measurement). Twenty of the 39 samples were extremely friable, so the cores for remanence measurements were coated in water glass prior to any experimentation.
2400BP 2300BP
5. Experimental methods
2200BP
Probability Density
1000CalBC
800CalBC
600CalBC
400CalBC
Calibrated date
Fig. 2. Calibrated radiocarbon age determination for the charcoal sample from the 2004 excavation, sector 1, trench 3. The terminus post quem for the use of the wood is 800 BC at the 95% confidence level.
Rock magnetic experiments were carried out using the Variable Field Translation Balance (MMVFTB) at the University of Liverpool. Isothermal remanent (IRM) acquisition and back field coercivity experiments were carried out as well as hysteresis loops and Curie curves in order to characterise mineralogy. The classical Thellier method (Thellier and Thellier, 1959) with partial thermal remanent magnetisation (pTRM) checks and correction for anisotropy of TRM
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a
ments were carried out when approximately 30% of the NRM remained (at temperatures 450, 510 or 540 °C). Remanence acquisition is influenced by the rate of cooling (e.g. Fox and Aitken, 1980; Dodson and McClellandBrown, 1980). Cooling times used in the Thellier experiments (1.5 h) will be quicker than the cooling rate when the sample acquired its natural remanence. In order to determine the influence, and correct for cooling rate, experiments were carried out on each sample after the Thellier experiment had finished. The samples were heated and cooled four times in a field of 60 lT in custom built ovens each time imparting a full laboratory TRM. The samples were cooled twice with a quick cooling rate (1.5 h) and twice were slowly cooled in order to compare the slow and fast cooled TRM and check for alteration during the experiment (see Go´mez-Paccard et al., 2006 for methodological details). A linear cooling time of around 24 h was used to approximate the natural slow cooling rate. Whilst it is realised that this is an approximation of the natural cooling time, it has been demonstrated that the choice of
Normalised Magnetisation cv
and cooling rate has been used to determine both the directions and the archaeointensity. Samples were heated and cooled in a MMTD oven and remanence was measured with a Molspin spinner magnetometer at the Universite´ Rennes 1. The heating steps used were 100, 150, 200 and then 240 up to 570 °C in 30 °C step (or until less than 10% of the original remanence remained). Samples were heated twice at each temperature step in a laboratory field of 60 lT, the first time with the field on in the +Z sample direction (along the long axis of the sample), and the second time the samples were rotated by 180° (field in the Z direction). PTRM checks were carried out at least after every other temperature step. The manufacturing process of fired archaeological material often results in magnetic anisotropy (Rogers et al., 1979). It is therefore necessary to determine and correct for this effect. Anisotropy of TRM was determined by heating the sample in six perpendicular orientations, inducing a remanence in the ±X, ±Y, and ±Z sample directions (Veitch et al., 1984; Chauvin et al., 2000). These experi-
90
B08 40
-80
-60
-40
-10 -20 0
20
40
60
80
-60
IRM 10-3 Am2/kg
Magnetisation 10-3 Am2/kg
140 10 8 6 4 2 0
-110
0
20
40
60
1 0.8 0.6 0.4 0.2 0 0
80
100 200 300 400 500 600 700 Temperature °C
Field mT
-160 Field mT
B01
20
-80
-60
-40
0 -20 0 -20
20
40
60
80
Normalised Magnetisation cv
Magnetisation 10-3 Am2/kg
40
IRM 10-3 Am2/kg
60
b
4 3 2 1 0 0
-40
20
40
60
1 0.8 0.6 0.4 0.2
80
0 0
100 200 300 400 500 600 700
Field mT
Temperature °C
-60 Field mT
c
200
A03
100
-80
-60
-40
0 -20 0 -100 -200 -300 -400
20
40
60
80
Normalised Magnetisation df
300 IRM 10-3 Am2/kg
Magnetisation 10-3 Am2/kg
400 30 25 20 15 10 5 0 0
20
40 Field mT
60
80
1 0.8 0.6 0.4 0.2 0 0
100 200 300 400 500 600 700 Temperature °C
Field mT
Fig. 4. Examples of the three types of rock magnetic behaviour. From left to right hysteresis loop, IRM acquisition and Curie curve (solid (dashed) line heating (cooling) curve).
M.J. Hill et al. / Physics and Chemistry of the Earth 33 (2008) 523–533
527
seen in both the hysteresis and IRM curves is likely to be hematite. There is no evidence for a high coercivity component in the rest of the samples with the hysteresis and IRM acquisition curves saturating by 20 mT (Fig. 4b and c). Hysteresis loops for these samples indicate low coercivity (less than 10 mT) and a ratio of saturation remanence to saturation magnetisation (Mrs/Ms) ranging from 0.06 to 0.18, which is well below the expected value for a single population of single domain grains of magnetite (Day et al., 1977). Maximum Curie points for all samples are between 500 and 600 °C. Generally the Curie curves show an almost linear decrease in magnetisation with increasing temperature (Fig. 4a and b) indicating a distributed range of Curie temperatures. Seven samples have a discernable low
slow cooling rate is not critical (Genevey et al., 2003; Go´mez-Paccard et al., 2006). 6. Rock magnetism A total of 38 samples (all samples bar B12) were investigated. Fig. 4 shows examples of the different types of rock magnetic behaviour found. Ten samples (4 from INCOA and 6 from INCOB) exhibit wasp waisted (restricted) hysteresis loops indicating the presence of two populations of grains (Tauxe et al., 1996) (Fig. 4a). The IRM acquisition curves for these samples also indicate the presence of two grain populations with magnetisation increasing rapidly at low fields and then increasing more slowly up to the maximum field of 80 mT. The high coercivity component
Table 1 Location
Sample
NRM A/m
MAD
DANG
t1
t2
N
f
g
q
b
F
bF
Fa
INCOA
01 02 03 04 05 06 07 08 11 12 13 14 15 16 17
3.0 6.1 18.0 2.5 2.9 9.7 1.7 4.2 11.0 4.3 13.0 13.0 1.8 4.6 8.5
1.7 2.3 3.2 3.2 3.4 3.2 2.3 3.8 2.7 2.8 2.8 4.6 3.2 2.1 2.5
1.0 1.4 0.8 1.3 2.3 0.3 1.4 1.4 3.0 0.6 1.2 1.6 2.5 1.2 0.6
150 200 240 150 100 100 150 150 240 150 150 150 150 100 100
480 540 570 540 540 540 570 480 570 570 570 570 570 480 540
11 12 12 13 14 14 14 11 12 14 14 13 14 12 14
0.80 0.82 0.87 0.79 0.89 0.85 0.85 0.85 0.84 0.85 0.87 0.77 0.80 0.86 0.89
0.88 0.91 0.73 0.91 0.92 0.87 0.92 0.88 0.85 0.92 0.85 0.84 0.92 0.88 0.85
53.8 118.0 19.8 55.0 65.5 52.2 77.6 27.7 68.1 123.0 21.2 29.2 62.4 63.4 28.4
0.013 0.006 0.032 0.013 0.012 0.014 0.010 0.027 0.010 0.006 0.035 0.022 0.012 0.012 0.027
94.7 103.4 94.4 101.2 93.9 100.5 90.9 89.8 98.7 106.6 79.3 93.7 91.9 92.3 91.2
1.3 0.7 3.0 1.3 1.1 1.4 0.9 2.4 1.0 0.7 2.8 2.1 1.1 1.1 2.5
92.3 1.1 91.3 0.6 91.1 2.9 102.9 1.3 95.8 1.1 97.5 1.4 92.8 1.0 No result 97.4 1.0 91.0 0.7 86.6 3.1 105.9 2.7 90.7 1.0 91.7 1.1 92.1 2.5
82.7 87.1 85.0 92.9 88.9 87.3 82.5
01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
2.2 2.4 2.0 3.8 3.4 2.9 2.1 5.8 1.8 4.7 3.0
3.9 2.8 2.9 2.3 3.2 2.0 3.3 2.2 3.0 2.6 1.6 7.1 3.4 2.5 3.0 2.8 2.5 2.5 2.3 2.8 2.6 2.6 2.4 1.4
2.1 1.5 2.2 1.1 0.4 0.3 1.7 1.5 0.6 1.2 0.3 2.4 1.9 0.9 0.9 0.1 3.5 0.6 0.6 0.2 1.0 0.8 1.5 1.2
100 100 100 100 100 100 100 100 150 150 100 150 150 150 100 100 100 150 100 100 100 150 100 100
540 510 540 480 570 540 540 510 540 540 480 540 540 510 540 540 480 570 540 540 480 540 570 480
14 13 14 12 14 14 14 13 13 13 12 13 13 12 14 14 12 14 14 14 12 13 15 12
0.85 0.83 0.83 0.83 0.86 0.84 0.85 0.84 0.78 0.82 0.85 0.86 0.81 0.78 0.85 0.83 0.81 0.83 0.86 0.88 0.88 0.84 0.88 0.84
0.92 0.90 0.92 0.90 0.92 0.92 0.92 0.90 0.91 0.91 0.90 0.87 0.91 0.90 0.91 0.92 0.90 0.92 0.92 0.92 0.89 0.90 0.93 0.89
62.5 59.0 23.3 42.3 38.1 35.7 42.8 41.3 54.2 65.8 57.1 28.6 28.4 44.9 57.2 51.3 38.8 38.3 113.2 45.4 54.9 54.5 72.3 45.4
0.013 0.013 0.033 0.018 0.020 0.022 0.018 0.018 0.013 0.011 0.013 0.026 0.026 0.016 0.014 0.015 0.019 0.020 0.007 0.018 0.014 0.014 0.011 0.017
99.7 89.7 95.2 84.8 93.9 96.5 88.4 104.2 95.6 101.2 92.2 101.1 107.1 96.7 97.7 93.7 90.3 96.4 92.8 95.1 79.7 96.5 95.9 86.0
1.3 1.1 3.1 1.5 1.9 2.1 1.6 1.9 1.3 1.1 1.2 2.6 2.8 1.5 1.3 1.4 1.7 1.9 0.7 1.7 1.1 1.3 1.1 1.4
94.7 90.9 93.1 84.6 94.0 94.2 89.2 97.0 93.0 86.3 89.1 100.8 103.3 92.1 97.4 91.5 85.2 94.4 90.2 97.0 78.8 95.5 91.5 86.4
85.8 86.6 83.8 82.5 84.9 85.0 81.2 91.7 87.3 81.3 84.2 87.4 93.5 87.0 87.8 76.6 81.0 83.0 79.5 89.3 76.8 88.1 83.6 83.6
INCOB
4.7 3.2 3.2 2.4 2.6 3.3 3.9 5.0 3.6 4.7 2.9 3.8
bFa
1.3 1.1 3.1 1.5 1.9 2.1 1.7 1.8 1.3 1.0 1.3 2.7 2.7 1.4 1.3 1.4 1.6 1.9 0.6 1.7 1.1 1.3 1.1 1.4
Fa+crc
88.4 82.4 76.8 96.8 78.5 86.4 83.9
Sample results giving location; brick sample number; natural remanent magnetisation NRM; maximum angular deviation MAD and deviation angle DANG in degrees; t1 and t2 the minimum and maximum temperatures (°C) defining the primary component of remanence and the archaoeointensity estimate; N the number of data points; f, g, and q the archaeointensity quality parameters of Coe et al. (1978) and b is the ratio of the error in the slope to the slope of the best straight line in the NRM/TRM plot; F is the archaeointensity estimate and bF the error in F; Fa is the anisotropy corrected archaeointensity estimate and Fa+crc is the anisotropy and cooling rate corrected archaeointensity estimate, all in lT.
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M.J. Hill et al. / Physics and Chemistry of the Earth 33 (2008) 523–533
temperature Curie point (189–315 °C) as shown in Fig. 4a. Four samples from INCOA have natural remanent magnetisation (NRM) greater than the other samples (>10 A/ m as opposed to a mean of 3.7 A/m for the other samples) (see first column of Table 1 and Fig. 4c). These samples have a more pronounced high temperature Curie point and likely contain a greater proportion of this magnetic grain population compared to the other samples. The heating and cooling curves for all samples are very similar, indicating that the magnetic minerals are stable to heating up to 700 °C. The rock magnetic behaviour of the Incoronata bricks is similar to that seen in other studies of bricks, tiles and pottery (e.g. Chauvin et al., 2000; Genevey and Gallet, 2002). Titanomagnetites, with various amounts of titanium, aluminium substituted magnetite or stable substituted maghaemite are likely to be the main magnetic minerals plus a high coercivity mineral such as haematite, is certainly present in type Fig. 4a samples. The bricks from the two locations have very similar rock magnetic properties; however, samples with the strong NRM and type Fig. 4c behaviour were only found amongst the INCOA bricks.
7. Anisotropy of TRM Alteration checks carried out as part of the anisotropy experiment indicate that little or no alteration occurred. The check differs from the first remanence acquisition step by less than 5% for all but seven samples, with the biggest difference being 15% for sample A03. One of the magnetometer measurements for sample A08 was found to be anomalous (experimental error), hence this sample has been excluded from further consideration. The degree of anisotropy defined as the ratio of the maximum and minimum axes of the anisotropy tensor varies from 4% to 46%, but generally it is less than 20% (Fig. 5). There is no preferred direction of anisotropy with samples showing both lineation and foliation. There is no way to distinguish the two sets of brick fragments using anisotropy of TRM. 8. Remanence results All brick samples (from both locations) are dominated by a stable component of remanence heading towards the origin with a viscous component that is removed by 150 °C (Fig. 6). In three cases the characteristic direction
INCOA
INCOB
1.4
INCOA + INCOB
Lineation
1.3
INCOA INCOB
1.2
1.1
1 1
1.1
1.2
1.3
1.4
Foliation
Fig. 5. Histograms showing the degree of anisotropy for INCOA, INCOB and INCOA + INCOB and a Flinn diagram of lineation against foliation.
M.J. Hill et al. / Physics and Chemistry of the Earth 33 (2008) 523–533
has been defined from 200 or 240 °C as the samples do not start to demagnetise until higher temperatures and the low temperature data is a little noisy (see Fig. 6 sample A03 and Table 1). The shape of the demagnetisation curves are similar to the shape of the Curie curves indicating that the magnetic grains dominating the thermomagnetic signature hold a stable remanence (Figs. 4 and 6). The maximum angular deviation (MAD) and the deviation angle (DANG) are very similar for both sample sets (see Table 1) with the mean MAD 2.9° and mean DANG 1.3°. The MAD for sample 12 is the largest at 7° due to the fact that the sample is small and has an anomalous shape and was therefore not easy to orient in the magnetometer. The directional results corrected for anisotropy of TRM are plotted on a stereoplot in Fig. 7. It can be seen that the directions do not consistently fall into one of the areas expected if the bricks had been placed on one of their flat surfaces (flat, on edge or upright) when acquiring their remanence. Analysing the results assuming that the bricks did in fact acquire their remanence whilst on any of the three flat surfaces, whichever position maximises the inclination (e.g. see Lanos et al., 1999), does not reduce the scatter in the data. Restricting the analysis to only those samples which were oriented to a straight edge of the brick
X
Y 75˚ 55˚
Fig. 7. Directional results plotted on a stereonet. INCOA (INCOB) are depicted by black (grey) symbols. Solid (open) symbols indicate a positive (negative) vertical component. If the bricks gained their remanence whilst flat, it is expected that the directions would fall within the band defined by inclination of 55–75°. Similarly the bands close to the X and Y axes indicate areas that the directions are expected to be if the bricks gained their remanence whilst on edge or upright.
1
1
0.8
0.6
+X
0.4 0.2
100
200
300
400
500
0
1
600
Temperature C
8
0.6
100
200
300
400
500
600
+X 3
Temperature C
0.8
-8
0.4
0
+Y
NRM
0.8
NRM
0.2
4 0
0.6 0.4
A03
0.2
0.4
1.2
0
1
0.6
NRM
NRM
0.8
1.2
529
A02
0.2
-3
0 0
0.1
0.2
0.3
0.4
0.5
0.6
+Y +Z
0
+Z
0.7
0
0.1
0.2
0.3
TRM
0.4
0.5
0.6
0.7
0.8
TRM
1
1 0.8
0.8
1.2
1.2
0.4 0.2
1
NRM
NRM
+X 0.6
0
0
0
100
200
300
400
500
0.8
600
NRM
NRM
0.8
Temperature C
0.6 1
0.4
B19
0.2
0.4 0.2
1
2
+X
0.6
+Y
1 0
100
200
300
400
500
600
Temperature C
0.6
1
-1
+Y
0.4 -1
B11
0.2 -1 +Z
0 0
0.1
0.2
0.3
0.4
TRM
0.5
0.6
0.7
0.8
+Z
0 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
TRM
Fig. 6. Examples of results from the Thellier experiment. NRM/TRM plots with pTRM checks and demagnetisation of NRM with temperature, NRM and TRM normalised to the initial NRM. Orthoganal vector plots (OVP) in sample coordinates, solid (open) symbols represent the vertical (horizontal) component of remanence, scale is in A/m.
M.J. Hill et al. / Physics and Chemistry of the Earth 33 (2008) 523–533
also does not improve matters. The directions are in fact completely scattered leading to the conclusion that the bricks did not acquire their remanence whilst on a flat surface and it is therefore not possible to determine the inclination of the geomagnetic field at the time of remanence acquisition. The brick samples from the two locations behave in a similar manner and it is not possible to distinguish the results from the two locations. 9. Archaeointensity results High quality archaeointensity results have been obtained from all samples with little evidence for alteration occurring during the experiment as shown in the NRM/ TRM diagrams (Fig. 6). All results are listed in Table 1 along with the Coe et al. (1978) quality parameters. The fraction of NRM used (f) ranges between 0.77 and 0.89 and the quality parameter q ranges from 20 up to 123 (mean value 53). The archaeointensity estimates for INCOA and INCOB are very similar and range from 79 to 106 lT. The mean archaeointensity per sample set is 95 lT for both INCOA and INCOB (Table 2). After correction for anisotropy of TRM (Tables 1 and 2) the mean archaeointensity reduces by a few micro Tesla and the scatter about the mean reduces slightly. The results of the cooling rate experiment are shown in Fig. 8. All samples gave a positive cooling rate correction (between 2% and 16%) as expected for single domain acting grains (e.g. Dodson and McClelland-Brown, 1980). The alteration check varies between 7% and +1%. A negative check indicates that the evaluated cooling rate correction is likely to be a minimum estimate. All samples have been corrected for cooling rate and the corrected archaeointensities are listed in Table 1. The mean archaeointensity (Table 2) is reduced by 9% to 85 lT, a value almost twice that of the present day field (calculated using IGRF 2005). 10. Discussion The only difference seen in the magnetic behaviour of the brick samples from the two locations is that some of Table 2 Mean archaeointensity results per location (INCOA and INCOB) and combined mean results Samples
N
F lT
SD
Raw data
INCOA INCOB ALL
15 24 39
95 95 95
7 6 6
Anisotropy corrected
INCOA INCOB ALL
14 24 38
94 92 93
5 5 5
Anisotropy and cooling rate corrected
INCOA INCOB ALL
14 24 38
86 85 85
5 4 5
Where N is the number of brick samples and F and SD are the archaeointensity and standard deviation respectively.
18 16
INCOA 14
INCOB Cooling Rate Correction %
530
12 10 8 6 4 2 0
-10
-8
-6
-4 -2 Alteration Check %
0
2
Fig. 8. Percentage cooling rate correction versus percentage alteration.
the bricks from INCOA have a higher NRM. This is most probably due to natural variation in the material used to manufacture the bricks. There is therefore nothing to magnetically distinguish the two sets of bricks strongly suggesting that all the bricks have the same origin. It is certainly the case that all the bricks gained their remanence in the same field (presumably at the same time) as indicated by the similarity in the archaeointensity results. This along with the fact that no complete bricks, only fragments, have been found suggests that the bricks have been discovered in a secondary context. The remanent magnetisation is single component (with a small viscous component) therefore the last heating event was to 570 °C (maximum heating step used) or higher. This remanence was most likely acquired during deliberate heating such as in a kiln during their manufacture or if the bricks themselves formed part of a kiln or fire place. The directional results indicate that the bricks were not lying on a flat edge (all on the same one or in any combination) when they acquired their remanence, contrary to what has been found in other studies (e.g. Lanos, 1987; Lanos et al., 1999). Instead, the bricks may have been stacked at an angle inside the kiln or fire place. The bricks were baked at high temperature, homogeneously, without any reheating to lower temperatures as would be expected if they had been in a fire. It can thus be inferred that the bricks found with the ceramic deposits in trench N are not adobe bricks from an oikoi that had been destroyed by fire as suggested by Orlandini (1986). Instead it seems likely that the brick fragments were mixed with stone and systematically broken ceramics in the deposit pits. A possible reason is that the pits are ritual deposits, in which ceramics have been destroyed after the ceremonial use. The age of the Incoronata bricks corresponds to the age of one of the proposed archaeomagnetic jerks (Gallet et al.,
M.J. Hill et al. / Physics and Chemistry of the Earth 33 (2008) 523–533
531
Fig. 9. Histograms of archaeointensity (uncorrected data, anisotropy corrected, and cooling rate and anisotropy corrected) for all samples (INCOA + INCOB).
2003) which are characterised by a peak in intensity and a sharp directional change. Whilst it has not proved possible to obtain an estimate of the inclination of the geomagnetic field from the brick samples, a well-constrained single estimate of field intensity has been determined (Fig. 9). The archaeointensity result from Incoronata has been relocated to Mari, Syria (latitude 34.5°N) in order to compare it in Fig. 10 to the recently acquired high quality Mesopotamian dataset which includes correction for anisotropy of TRM and cooling rate (Gallet et al., 2006). The field strength at Incoronata (85 lT) is almost twice (1.8 times) that of the present day field which is broadly consistent with the peak in intensity seen in the Mesopotamian dataset as well as datasets from Greece, the Near and Middle East and Asia (see Genevey et al., 2003 and references therein) plus data from VIth and VIIth century BC ceramics from Carthage (Tunisia) from the study of Thellier and Thellier (1959). This study thus provides further evidence that the strength of the geomagnetic field was high over at least a 30° longitude area of the globe during this time. Further studies are needed, including directional and dating studies, to confirm the nature and global extent of this proposed 100
This study
80 70 60 50
Archaeointensity μT
90
Mesopotamian data
40 30 -4000
-3500
-3000
-2500
-2000 -1500 Age Year
-1000
-500
0
Fig. 10. Archaeointensity data from 0 to 4000 BC from Mesopotamia (Gallet et al., 2006) plus the result from Incoronata (taking the radiocarbon date for the age). All data relocated to Mari, Syria.
‘archaeomagnetic jerk’ and its possible impact on climate and ancient civilisation (Gallet et al., 2005, 2006). During the 2005 University of Rennes 2 excavation remains of a potter’s kiln were discovered to the south of the artificial plateau, in the slope. This supports the assumption that the bricks and most of the ceramics discovered in the pits were locally produced. Future archaeological and archaeomagnetic studies of in situ material from the kiln are planned in order to obtain both directional as well as intensity data in order to better understand and constrain the chronology of the site of Incoronata between the earlier native settlement and the later Achaean site of Metaponto. 11. Conclusions An archaeomagnetic investigation of two sets of bricks from one of the most important Greco–indigenous archaeological sites of the Central Mediterranean, the VIIIth– VIIth Century BC site of Incoronata (Metaponto, Italy) has been carried out. Rock magnetic and magnetic remanence experiments (including anisotropy of TRM, archaeointensity and cooling rate studies) indicate that the two sets of bricks are magnetically identical and have the same heating history and thus the same origin. It is thus inferred that the bricks had been reused in two different contexts: (1) mixed with stone and ceramics in deposit pits and (2) to consolidate an artificial plateau. Additionally, and importantly, this study has shown that the hypothesis of destruction by fire is no longer tenable to explain the pits which contain large quantities of ceramics along with stones and bricks previously interpreted as being oikoi. Whilst it was not possible to obtain an estimate of the inclination (the bricks did not gain their remanence whilst on one of their flat surfaces), the archaeointensity experiments (with anisotropy of TRM and cooling rate correction) give a mean intensity of 85 ± 5 lT for the field at Incoronata during the VIIth century BC. This is almost twice the present day field strength and thus provides
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further evidence that the field was strong over at least a 30° longitude area of the globe during this time. This study has demonstrated the usefulness of archaeomagnetic studies for aiding archaeological investigations as well as producing data to further understanding of the geomagnetic field. Note added in proof Since this paper was submitted a paper have been published discussing the archaeological significance of the bricks studied here (Denti and Lanos, 2008). Acknowledgments We thank Antonio di Siena, director of the Archaeological Museum of Metaponto, for kindly giving permission to sample bricks from the site of Incoronata and we thank Christine Oberlin from the Centre of radiocarbon dating of Lyon, CNRS for the dating. This study was supported by the European Network AARCH (Archaeomagnetic Applications for the Rescue of Cultural Heritage) Contract HPRN-CT-2002-00219, DyETI program n° IT67 (INSU CNRS), 2004/2005: ‘‘Variation se´culaire du Champ Ge´omagne´tique durant le 1er mille´naire BC et construction des courbes de re´fe´rence”. NERC grant NE/C51982X/1 is also acknowledged. References Centre Jean Berard and Fondazione Paestum, 1998. Siritide e Metapontino. Storie di due territori coloniali, Atti dell’incontro di studio, Policoro 31 ottobre–2 novembre 1991, Cahiers du Centre Jean Berard XX (in Italian). Chauvin, A., Garcia, Y., Lanos, Ph., Laubenheimer, F., 2000. Paleointensity of the geomagnetic field recovered on archaeomagnetic sites from France. Phys. Earth Planet. Int. 120, 111–136. Coe, R.S., Gromme´, C.S., Mankinen, E.A., 1978. Geomagnetic paleointensities from radiocarbon-dated lava flows on Hawaii and the question of the Pacific nondipole low. J. Geophys. Res. 83, 1740–1756. Day, R., Fuller, M.D., Schmidt, V.A., 1977. Hysteresis properties of titanomagnetites: grain size and composition dependence. Phys. Earth Planet. Int. 13, 260–267. Denti, M., 2000. Nuovi documenti di ceramica orientalizzante della Grecia d’Occidente. Stato della questione e prospettive della ricerca, Me´langes de l’Ecole Francßaise de Rome – Antiquite´ 112 (2000/2), pp. 781–842 (in Italian). Denti, M., 2002. Linguaggio figurativo e identita` culturale nelle piu` antiche comunita` greche della Siritide e del Metapontino. In: Moscati Castelnuovo, L. (Ed.), Identita` e prassi storicanel Mediterraneo greco, Castelnuovo, Milan, pp. 33–62 (in Italian). Denti, M., 2005. Peqiqqamsgqia figurati a rilievo nei depositi di ceramica sulla collina dell’Incoronata di Metaponto. Tracce di un’attivita` rituale?. ‘‘Siris” VI 2005, pp. 173–186 (in Italian). Denti, M., 2007. Grecs et indige`nes a` la frontie`re de l’Occident. L’occupation du territoire dans le Me´tapontin au VIIe sie`cle avant J.-C., Actes du colloque ‘‘Pouvoir et territoire I (Antiquite´-Moyen ˆ ge)”, Saint-E ´ tienne novembre 2005, Saint-E ´ tienne, pp. 225–244 (in A French). Denti, M., Lanos, Ph., 2008. Rouges, non rougies : les briques de l’Incoronata et le proble`me de l’interpre´tation des de´poˆts de ce´rami-
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