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Quality control of archaeomagnetic determination using a modern kiln with a complex NRM
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Physics and Chemistry of the Earth 33 (2008) 427–437 www.elsevier.com/locate/pce
Quality control of archaeomagnetic determination using a modern kiln with a complex NRM G. Catanzariti a,*, G. McIntosh a, M. Go´mez-Paccard a,b, V.C. Ruiz-Martı´nez a, M.L. Osete a, A. Chauvin b, The AARCH Scientific Team 1 a
Fı´sica de la Tierra 1, Fac. CC Fı´sicas, Universidad Complutense de Madrid, 28040 Madrid, Spain b Ge´osciences Rennes, Universite´ de Rennes 1, 35042 Rennes, Cedex, France Available online 21 February 2008
Abstract A modern (1959) brick kiln from western Spain has been studied in order to conduct a quality control test of the archaeomagnetic method in a partially heated structure. The kiln exhibits two stable natural remanent magnetisation components: a low-temperature component (150–620 °C) acquired during kiln use and a randomly oriented, high temperature component (500–680 °C) acquired during the original firing of the bricks. A detailed rock magnetic study revealed a magnetic mineralogy dominated by non-stoichiometric magnetite and hematite, both of which contribute to the characteristic remanent magnetisation. Both the direction and intensity of the characteristic magnetisation have been compared with the known values for the geomagnetic field at the site location. Irrespective of the method used to determine the mean archaeomagnetic direction (principal component analysis, great circle analysis, with or without hierarchy) the results are statistically indistinguishable from each other and from the known field direction. In all cases the direction is within 5° of the expected value. Thellier-style archaeointensity determinations have been carried out on a smaller specimen set, with the mean result consistent with the known field value. The results demonstrate the reliability of the archaeomagnetic method in determining the features of the geomagnetic field in the past, even in cases with a complicated magnetic history. Furthermore, they highlight the importance of a uniform sample distribution in order to obtain truly representative values for the direction and intensity of the ancient geomagnetic field. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Archaeomagnetism; Brick kiln; Geomagnetic field; Western Spain
1. Introduction When archaeological features have been exposed to high temperatures the natural remanent magnetisation (NRM) structure is simple. A univectorial stable component is observed which is interpreted as a thermoremanent mag*
Corresponding author. Tel.: +34 913944396; fax: +34 913944398. E-mail address: gcatanza@fis.ucm.es (G. Catanzariti). 1 The AARCH Scientific Team consists of Abrahamsen, N., University of Aarhus, Denmark; Zananiri, I, University of Bradford, UK and Institute of Geology and Mineral Exploration, Greece; Spassov, S., Centre de Physique du Globe de l’IRM, Belgium; Kostadinova, M., Bulgarian Academy of Sciences, Bulgaria; De Marco, E., Aristotle University of Thessaloniki, Greece; Winkler, U., University of Plymouth, UK; Tema, E., Universita´ degli Studi di Torino, Italy; Casas, L., Universita´ degli Studi di Napoli, Italy. 1474-7065/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.pce.2008.02.028
netisation (TRM) acquired during the last heating of the feature. However, many features suffer heating to relatively low temperatures and the NRM structure may be quite complex. Such cases may exhibit a partial TRM associated with the low-temperature heating event, a thermo-chemical remanent magnetisation (TCRM) associated with mineralogical changes and exposure to heat and a stable component associated with earlier events (e.g., firing of bricks during their production). The main aim of this study is to study the application of the archaeomagnetic method (in terms of direction and intensity) to a modern, partially heated feature where the true geomagnetic field is known. For the archaeodirection, the sensitivity of the method to the determination of the characteristic magnetisation (ChRM) direction, the calculation of mean directions and the subjectivity of the
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interpretation of complex remanences have been investigated. For the archaeointensity, Thellier–Thellier palaeointensity experiments have been carried out. A modern kiln from western Spain (Go´mez-Paccard et al., 2006) has been selected for this purpose. It was first (and last) used in 1959, for which accurate information about the mean geomagnetic field is available from both observatory and Definitive Geomagnetic Reference Field (DGRF) data. The kiln exhibits two stable NRM components, the relative importance of each varies with respect to the proximity to the heating source. Principal component analysis (PCA) of the NRM demagnetisation curve
Fig. 1. Map of Spain showing site location. Madrid and Toledo observatory also shown.
(Kirschvink, 1980) and great circle analysis (GCA) of remagnetisation circles (McFadden and McElhinny (1988)) have both been used to determine ChRM directions. The precision and accuracy of each method have been compared, along with the influence of applying a hierarchical approach when determining mean directions. 2. Archaeological context and sampling The monastery at Yuste, Caceres Province, western Spain (Fig. 1), is an important historical site where King Carlos V resided after his abdication in 1556 AD up to his death two years later. As recounted by the present Prior of the monastery, a small kiln was built in 1959 by one of the monks in order to recreate traditional ceramic production methods. It was abandoned in the same year because of a failure to produce acceptable ceramics, presumably due to the difficulties in reaching high firing temperatures. The kiln consists of a cylindrical structure about 120 cm high and 80 cm in diameter (Fig. 2), made of red, pre-fired bricks. The firing chamber occupied the upper part of the structure and the combustion chamber the lower part, whilst no remains of the grill were found in situ. For the archaeomagnetic study, five bricks were sampled from all sides of the same level of the upper part of the kiln (Fig. 2b). Standard specimens were drilled from the brick surface facing the kiln interior, with specimens labelled ‘‘A” coming from closer to the interior brick surface than those labelled ‘‘B” (see Fig. 2c). A total of 18 specimens were prepared for directional analysis. The kiln was re-sampled in order to obtain specimens for archaeointensity experiments. Four bricks were taken from lower down the kiln structure, closer to the combustion chamber than the bricks studied previously.
Fig. 2. (a) Photo of kiln, sampled levels shaded in grey, (b) distribution of sampled bricks used in archaeodirection study and (c) brick sub-sampling scheme.
G. Catanzariti et al. / Physics and Chemistry of the Earth 33 (2008) 427–437 1.40E-06
3.1. Archaeodirection
1.20E-06 1.00E-06 8.00E-07 6.00E-07 4.00E-07 2.00E-07 0.00E+00
3.2. Archaeointensity 1.20E-03
1.00E-03 2
NRM Intensity [A*m /Kg]
Archaeointensity experiments were carried out at Geosciences, Rennes (France), following the classical Thellier palaeointensity method (Thellier and Thellier, 1959). Heating was carried out from 100 °C to temperatures at which >85% of the initial NRM was lost, using between 13 and 18 steps. A laboratory field of 60 lT was applied along the specimen Z axis during TRM acquisition, with pTRM checks made every two steps. A Magnetic Measurements MMTD oven and an Agico JR5 spinner magnetometer were used throughout. TRM anisotropy and cooling rate effects were also investigated.
8.00E-04
6.00E-04
4.00E-04
2.00E-04
0.00E+00
3.3. Mineral magnetic measurements All measurements were carried out at the Universidad Complutense, Madrid, and at Geosciences, Rennes. A coercivity meter (Jasonov et al., 1998), with a maximum applied field of 500 mT, was used to measure hysteresis curves, stepwise acquisition and reverse-field acquisition of isothermal remanence (IRM). The following parameters were defined: saturation magnetisation, Ms, saturation remanence, Mr, coercivity, Bc and coercivity of remanence, Bcr. TH demagnetisation of orthogonal IRMs (Lowrie, 1990) was conducted on selected specimens. Thermomagnetic curves (room temperature to 700 °C) were measured in air using an Agico KLY3 susceptibility meter with a fitted furnace. Low-temperature IRM was measured between 5 and 300 K using a Quantum Designs MPMS SQUID magnetometer, applying a 2 T IRM at 4 K. 4. Results and discussion 4.1. Mineral magnetic properties Both K and NRM intensity are relatively low, varying between 4.34 10 8–4.44 10 6 m3 kg 1 and 6.55 10 5–7.48 10 3 A m2 kg 1 respectively. In all cases the A specimens exhibit higher values than the corresponding B specimens (Fig. 3). The magnetic properties vary within and between bricks, implying a heterogeneous distribution of magnetic phases. Ms and Mr of samples from closer to the burnt brick surface are higher than those of samples
HP.11A HP.12A HP.12B HP.13A HP.21A HP.22A HP.22B HP.23A HP.31A HP.31B HP.32A HP.33A HP.42A HP.42B HP.43A HP.52A HP.52B HP.53A HP.53B HP.61A3 HP.73A3 HP.83A1 HP.91A1 HP.91A2 HP.103A3
The archaeodirection study was carried out in the Palaeomagnetism Laboratory of the Universidad Complutense, Madrid (Spain). The NRM was measured using an AGICO JR5 spinner magnetometer; low field magnetic susceptibility (K) was measured using an AGICO KLY3 susceptibility meter. Stepwise thermal (TH) demagnetisation of NRM was carried out using a Schonsted Instruments TSD-1 thermal demagnetiser.
HP.11A HP.12A HP.12B HP.13A HP.21A HP.22A HP.22B HP.23A HP.31A HP.31B HP.32A HP.33A HP.42A HP.42B HP.43A HP.52A HP.52B HP.53A HP.53B HP.61A3 HP.73A3 HP.83A1 HP.91A1 HP.91A2 HP.103A3
3
Mass susceptibility [m /Kg]
3. Methods
429
Fig. 3. Specimen values of (a) magnetic susceptibility and (b) NRM intensity. Black (grey) columns indicate A(B) specimens.
further away from the burnt surface. This is in agreement with the K results, since the A specimens were taken adjacent to the burnt brick surface. Hysteresis curves yield values of Bc of 10–60 mT (Fig. 4a) and show varying contributions of paramagnetic and high coercivity ferromagnetic material. The samples generally acquire most of their IRM by 300 mT, though in most cases do not approach saturation at 500 mT (Fig. 4b), a further indication of coexisting high and low coercivity magnetic phases. Bcr values range between 60 and 200 mT, the highest value observed in sample HP4_B, which also had the lowest Mr. TH demagnetisation of orthogonal IRMs (Fig. 5) show that the low coercivity phase (0–150 mT) has maximum unblocking temperatures around 550 °C, consistent with a magnetite-like composition. The high coercivity phase (300–1150 mT) has maximum Tubs near 680 °C, which suggests hematite. The low coercivity phase is relatively more important in the A than the B specimens. Sample HP_4A, from close to the burnt face of brick 4, shows a thermomagnetic curve with a broad peak between 300 and 500 °C (Fig. 6). The Curie temperature (Tc) has been estimated as being between 500 and 550 °C, lying between the maximum and minimum value of K. For
G. Catanzariti et al. / Physics and Chemistry of the Earth 33 (2008) 427–437 6.00E-02
A
HP4_
1.5E-01
4_B
HP
6.00E-02
2.00E-02
4_A
HP
4.00E-02
A HP1_
5.00E-02
1.60E-02
-5.0E-02
-2.00E-02
4.00E-02
HP1_B
1.20E-02
2
-1
2
Mr [Am Kg ]
0.00E+00
-1
2.00E-02
2
0.0E+00
HP1_B
Ms [Am Kg ]
-1
5.0E-02
2
Ms [Am Kg ]
1.0E-01
-1
2.0E-01
Mr [Am Kg ]
430
3.00E-02 8.00E-03 2.00E-02
-1.0E-01 _A
-1.5E-01
HP1
-4.00E-02
4.00E-03
1.00E-02 HP4_B
-2.0E-01 -500 -400 -300 -200 -100
0
-6.00E-02 100 200 300 400 500
0.00E+00 -200
Applied Field [mT]
-100
0
100
200
300
400
0.00E+00 500
Applied Field [mT]
Fig. 4. (a) Hysteresis curves and (b) IRM curves for representative A and B specimens. Left (right) scale refers to samples from brick HP4(HP1). Ms(Mr) = saturation magnetisation (remanence).
The magnetic properties of the bricks can be explained by a mixture of a magnetite-like phase and hematite, the relative importance of which varies as a function of distance from the burnt brick face. Closer to the burnt face the concentration of the magnetite-like phase is higher and this phase dominates. Slightly reduced Tcs and the lack of a Verwey transition in the low-temperature IRM curves (Fig. 7) suggest non-stoichiometric magnetite as a candidate for this phase. Further from the burnt face the concentration falls and hematite becomes relatively more important. Hematite is interpreted as a ‘‘primary” magnetic phase, originally present in the material used to produce the bricks or related to the brick firing process. The increase in concentration of the magnetite-like phase (as seen in K, NRM, Ms and Mr) is considered as due to the formation of a ‘‘secondary” phase, related with heating of the bricks during the lifespan of the kiln. 4.2. ChRM
Fig. 5. Thermal demagnetisation of orthogonal IRM for representative A and B specimens.
HP_4B (Fig. 6), K is much lower and a precise determination of Tc is difficult. K reaches minimum values near 650– 700 °C, which may reflect the approach to the Neel temperature (TN) of hematite of 675 °C. In the case of HP_9A and HP_9B (Fig. 6), both the A and B samples have similar curves, exhibiting a sharp peak near 550 °C, which may represent a Hopkinson peak and thus defines a Tc close to 550 °C. For the bricks from closer to the combustion chamber, the magnetisation was more homogeneous in the A and B specimens. This is consistent with being heated to higher temperatures, with the effects of heating penetrating further into the brick interior. Tcs near 550 °C suggest a magnetite-like composition, in accordance with the IRM properties previously described. Irrespective of the thermomagnetic curve shape they all show a high degree of reversibility, indicating thermal stability of the samples.
4.2.1. Component structure of NRM The initial NRM directions fall close to the present day geomagnetic north, with the exception of the five B specimens which exhibit scattered directions. Stepwise TH demagnetisation revealed two stable components: a low temperature (LT) component isolated at temperatures up to 550–620 °C, and a high temperature (HT) component between 500 and 680 °C. The LT component is better defined in the A specimens (Fig. 8a and c), comprising up to 90% of the NRM. For the B specimens, the HT component is more important, approaching 50% of the NRM, whereas the LT component is poorly defined (Fig. 8b and d). Evidence for overlapping unblocking temperatures of the LT and HT components can be found in most specimens, affecting both the A and B specimens (e.g., 42B, 53A, Fig. 8b and c). The LT component is considered the ChRM acquired during kiln usage. It is better defined in the A specimens due to their proximity to the heat source. Unblocking temperatures of 550–620 °C (Fig. 8e) suggest that this component is carried by the magnetite-like phase and by part of the hematite phase. The HT component represents a ‘‘pri-
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431
Fig. 6. Typical thermomagnetic curves. Black (grey) symbols denote heating(cooling) branches.
1.80E-02
1.00E-03
1.60E-02
1 8.00E-04
1.40E-02 1.20E-02
6.00E-04
3
1.00E-02 8.00E-03
4.00E-04 6.00E-03 4.00E-03
1 HP4_I : 0-1 cm from the kiln interior (LEFT AXIS)
2.00E-03
2 HP4_II : 4-5 cm from the brick interior (RIGHT AXIS)
IRM (2 T) [emu]
IRM (2 T) [emu]
2
2.00E-04
3 HP4_III : 8-9 cm from the kiln interior (RIGHT AXIS) 0.00E+00 0.0
50.0
100.0
150.0
200.0
250.0
0.00E+00 300.0
Temperature [K]
Fig. 7. Low-temperature IRM. Applied field = 2 T at 4 K, warming in zero field. Note different scales for HP4_II and _III. !!! There is no way to distinguish curves (especially HP4_I from HP4_III !!!!.
mary” remanence associated with the original firing of the bricks. Acquired prior to construction of the kiln, its direction is randomly distributed, but consistent within the same brick. It is more important in the B specimens, which suffered less heating during the use of the kiln. Maximum unblocking temperatures of 680 °C (Fig. 8f) show that it is carried by the hematite phase. 4.2.2. Calculating the mean ChRM direction Linear demagnetisation trajectories on the orthogonal vector projections could be identified in all specimens,
allowing PCA determination of the LT direction. For the B specimens, the trajectories were more poorly defined than for the A specimens. All specimens yielded well-defined demagnetisation planes, permitting GCA in all cases. Therefore 18 ChRM directions were determined by PCA, and 18 planes containing the ChRM direction were determined by GCA. Next, the mean ChRM direction was calculated, considering the PCA and GCA directions, individually and in combination, and by imposing a hierarchical structure defined by the five independently oriented brick samples.
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G. Catanzariti et al. / Physics and Chemistry of the Earth 33 (2008) 427–437 245
Up/W
Up/W
516 295 601
511 N
636
42A
N Horizontal plane Vertical plane
Horizontal plane Vertical plane
696
Specimen 42A
Specimen 42B
Up/W
Up/ W
680
N
N 681
Horizontal plane Vertical plane
53A
Horizontal plane Vertical plane
540 310 Horizontal plane Vertical plane
501 Specimen 53B
Specimen 53A
HP.12A
HP.31A
HP.42A
HP.12B
0.20
0.60
NRM Intensity [A/m]
NRM Intensity [A/m]
0.80
0.40
0.20
0.00
HP.31B
HP.42B
0.15
0.10
0.05
0.00 0
100
200
300
400
500
600
700
Temperature [ºC]
0
100
200
300
400
500
600
700
Temperature [ºC]
Fig. 8. (a–d) Equal-area projection and orthogonal vector projection of thermal demagnetisation of representative A and B specimens. (e–f) NRM intensity decay during TH demagnetisation of representative A and B specimens.
With respect to the hierarchical approach (as described by Lanos et al., 2005), the basic assumption is that the dispersion of data at the same level (specimen, sample, structure) is homogeneous. Therefore all data are considered with the same weight and the only sources of uncertainty are the sampling errors and/or other non-systematic errors (e.g., related with determination of directions). In the case of partial heating the NRM is complicated due to variations in the relative proportions of, and degree of overlap between, the NRM components (e.g., when comparing the A and B specimens in the present study). This does not affect GCA, while PCA requires no (or minimal) overlap. Therefore the two methods may give rise to different types of uncertainties and mixing them violates the
assumption of homogeneity. Strictly speaking, therefore, they should not be combined in the hierarchical approach. Table 1 summarises the mean ChRM directions, with and without hierarchy, using PCA, GCA and combined PCA–GCA. In all cases the mean direction is the same irrespective of whether hierarchy is applied or not, whereas the precision parameter k is higher when hierarchy is applied. For PCA, the mean directions were calculated using Fisher (1953) statistics. The precision parameter k increases when applying hierarchy to the A and the A + B specimen directions. This indicates that the hierarchical approach is effective in averaging any non-systematic errors. It can also be seen that the PCA mean directions have higher precision when considering the A specimens only. The decrease in
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433
Table 1 Mean ChRM directions Mean ChRM directions Analytical method
N
D
I
a95
k
R
b
PCA (A) PCA (A) Hier. PCA (A+B) PCA (A+B) Hier. PCA (B) GCA (A) GCA (A) Hier GCA (B) Hier. GCA (A+B) GCA (A+B) Hier. PCA (A) + GC (B) PCA (A) + GC (B) Hier. Toledo observatory direction (1959.5) DGRF direction (1959)
13 5 18 5 5 13 5 5 18 5 18 5
349.6 349.1 349.7 349.6 349.8 345.9 347.4 344.8 345.7 347.2 349.0 348.7 351.0 351.1
59.7 59.8 59.1 58.9 57.7 60.9 59.9 60.0 60.6 59.9 59.8 60.8 56.4 56.5
5.2 8.2 5.0 8.4 15.4 3.8 5.7 9.0 2.9 2.0 3.8 7.1
63.9 87.2 49.2 84.1 25.7 137.5 375.0 150.0 163.3 1500 72.8 115.7
12.812 4.954 17.655 4.952 4.845 12.960 4.996 4.990 17.951 4.999 17.801 4.965
3.4 3.5 2.8 2.6 1.4 5.2 3.9 4.8 5.0 4.0 3.6 4.6
Analytical method, PCA = principal component analysis, GCA = great circle analysis, Hier = applying hierarchy; N, number of specimens (independent samples when applying hierarchy); D, declination (°E); I, inclination (°); a95, alpha-95 confidence limit; k, precision parameter; R, length of the resultant vector; b, angular difference between mean direction and the observatory field value for 1959.
precision when B samples are considered is interpreted as being due to incomplete isolation of their ChRM direction. For GCA and PCA–GCA, mean directions were calculated following the converging remagnetisation circles method of McFadden and McElhinny (1988). This approach is based on the Fisher (1953) distribution and allows for the estimation of k, R and a95. For GCA, as with PCA, the application of hierarchy improves the precision of the mean directions. Inclusion of the B specimens improves the precision of the mean. The A and B specimens exhibit LT and HT components which both contribute to the demagnetisation plane. Therefore the analysis of the data is more homogenous and the best results are obtained by considering all of the specimens. The PCA–GCA result combines the 13 PCA A specimen directions and the five GCA B specimen planes. This gives rise to an increase in precision when compared with the PCA (A specimens) and the PCA (A + B specimens), with or without hierarchy. Whilst this suggests that results are improved by including the B specimen planes, the mixing of PCA and GCA results in a hierarchical approach violates the homogeneity of the data analysis. When comparing the PCA and GCA results, GCA appears to provide more precise mean directions. However, as stated by the authors (McFadden and McElhinny, 1988), the remagnetisation circle method tends to overestimate k and R, so that, whilst the results are internally consistent, they are not directly comparable with PCA. Therefore the increased precision of GCA (and PCA– GCA) results is, at least in part, an artefact of the method. Of more significance is the good agreement between the three approaches. Considering the best results (i.e., those with highest precision, k) obtained by PCA (A specimens, with hierarchy), GCA (A+B specimens, with hierarchy) and PCA–GCA (A + B specimens, with hierarchy), the difference between them is small and they are statistically
indistinguishable following the McFadden and Lowes (1981) test. 4.2.3. The influence of subjectivity on the determination of ChRM directions: inter-laboratory data analysis The interpretation of complex remanences may lead to a degree of subjectivity that in turn affects the determined ChRM directions. In order to see if this is important, an inter-laboratory comparison of data interpretation was carried out. Of the five bricks sampled, four that yielded specimens with a well-expressed, 2-component NRM (HP1, 3, 4 and 5) were selected. These have been used to construct a data set of 14 specimen demagnetisation data, comprising two or three A specimens and one B specimen from each brick. The data set was then sent to the participating groups, along with the a priori information that the specimens came from a partially heated kiln. Each group was charged with determining the ChRM direction of all specimens, wherever possible, using their own standard procedures and software. The ChRM directions of each group were then collected for a comparative study. Including Madrid, a total of 10 member groups participated in the experiment, and the results are summarised in Table 2. Variability in the interpretation is expressed in the method used to calculate the ChRM, the acceptance or rejection of particular specimen directions and in the calculated directions themselves. Eight of the 10 groups used PCA, whereas two used a combination of PCA and GCA. Only three groups accepted all 14 directions and one group only accepted four of the directions, with the most commonly rejected directions coming from the B specimens. Specimen directions determined by PCA are very similar, giving rise to mean directions (calculated without hierarchy) with a95 values between 3.4° and 6.4° and k between
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Table 2 Mean directions determined by the 10 participating groups Group
Method
N
D
I
a95
k
R
b
1 2 3 4 5 6 7 8 9 10
PCA + GC PCA PCA PCA PCAa PCA PCA + GC PCA PCA PCA
14 12 10 11 4 14 14 10 10 10
351.2 355.2 352.8 354.4 340.7 353.8 350.2 352.6 349.3 348.7
56.2 61.6 60.0 58.8 61.6 59.0 56.9 60.0 59.4 59.2
3.9 6.4 5.9 5.6 3.4 5.3 3.5 5.5 4.2 6.3
109 47 69 68 736 57 137 79 131 59
13.9 11.8 9.9 10.9 4.0 13.8 13.9 9.9 9.9 9.9
0.2 5.6 3.7 3.0 7.4 3.0 0.7 3.7 3.1 3.1
Group = arbitrary number assigned to the participating group; method, analytical method, PCA = principal component analysis, GCA = great circle analysis; for other definitions, see Table 1. a Mean direction was statistically different from other group means following McFadden and Lowes (1981).
47 and 736. Two groups employed GCA for specimens judged to have curved vector segments. For those specimens in which both groups used GCA, similar planes were fitted and the resulting mean directions are similar to those determined by PCA alone. Following the McFadden and Lowes (1981) test, all but one of the means are statistically indistinguishable at the 95% confidence level. The outstanding group mean value exhibits the lowest a95 and the highest precision. However, it has been calculated from four accepted specimen directions, three of which were from the same brick. Therefore the resulting mean direction is strongly biased towards the results from this brick. What these results suggest is that the calculation of ChRM directions from complex NRMs is fairly uniform within the participating groups. Whilst this may sound obvious, it is nonetheless reassuring to demonstrate that
Fig. 9. Representative archaeointensity results. Normalised NRM–TRM and vector component plots for (a, b) specimen HP10_3A3 and (c, d) HP9_1A1.
G. Catanzariti et al. / Physics and Chemistry of the Earth 33 (2008) 427–437
it is the case. The method used to calculate ChRM directions, and the criteria for acceptance or rejection of individual directions, does not strongly influence the resulting mean direction. An exception is when the distribution of accepted specimen directions is not uniform across the sampled structure. It is the non-uniform distribution that is important, rather than the data analysis itself, in that it gives rise to a mean direction that is not representative of the structure as a whole.
4.3. Archaeointensity determinations Four of the six studied specimens exhibit a single, stable NRM component. They come from bricks (HP8-10) taken lower down in the kiln than those used in the directional analysis, i.e., closer to the combustion chamber, and have probably been heated to sufficiently high temperatures to acquire a total TRM. The other two specimens, coming from bricks (HP6,7) taken higher up in the kiln, show both LT and HT components – as seen in the directional study. For these specimens the LT component has been used to calculate the archaeointensity. Two examples of NRM– TRM (Arai) diagrams are plotted together with their corresponding orthogonal vector plots (in specimen coordinates) in Fig. 9 and the results are listed in Table 3. There are no important differences between the archaeointensities determined from the single component and LT/HT component specimens, and both types yield acceptable results. For each specimen the temperature interval considered for slope computation, the number of steps (n) performed within this interval and the different factors (f, g and q) used to test data quality (see Coe et al., 1978) have been listed. The mean site intensity has been calculated using the weighting factor proposed by Prevot et al. (1985) and
435
the dispersion of the mean has been expressed as the standard deviation (sd). The TRM anisotropy tensor has been determined for four specimens, at temperatures at which at least 70% of NRM was lost. The anisotropy degree k1/k3 (where k1 and k3 are the maximum and minimum axes of the TRM anisotropy tensor) ranges between 8% and 28% and magnetic lineation dominates. However, the minimum axes are not clearly contained in the flattening (horizontal) plane of the bricks. The differences between the specimen archaeointensity values before and after TRM anisotropy correction are <6%. For this reason the site-mean intensity has been calculated before and after correction and listed in Table 3. The dependence of TRM intensity on cooling rate has been studied, following the method described in Go´mezPaccard et al. (2006). Two linear cooling times, of 24 and 48 h, were applied. For four specimens the correction factor for the 24 h cooling rate is <6%, but for the other two the correction factor is up to 13%. A combination of the low correction factor and magneto-chemical alteration during the 48 h cooling rate experiment meant that only two specimens yielded reliable correction factors. The size of the studied kiln suggests that the cooling time related to the acquisition of the TRM is most probably more similar to 24 h rather than 48 h. For this reason the 24 h correction factor has been applied to the archaeointensity determinations. Several investigations (for example, Go´mez-Paccard et al., 2006; Genevey and Gallet, 2002; Chauvin et al., 2000) illustrate the importance of cooling rate corrections in archaeointensity studies, showing correction factors that can exceed 20%. This is not the case in the present study, with the difference between the uncorrected and corrected mean intensities being less than 3%.
Table 3 Archaeointensity results Spm
NRM (A/m)
K (10 5)
Tmin–Tmax (°C)
n
f
g
q
F (lT)
HP6.1A3 HP7.3A3 HP8.3A1 HP9.1A1 HP9.1A2 HP10.3A3
0.14 0.15 0.8 1.6 0.8 13
17 23 86 69 47 888
100–580 100–580 100–560 100–530 100–530 100–530
14 14 13 12 12 12
0.62 0.99 0.90 0.84 0.92 0.99
0.85 0.86 0.83 0.67 0.74 0.87
12.80 27.70 151.80 24.02 25.25 43.36
43.3 43.3 56.0 45.7 46.0 42.1
Fpo (lT) Fpocr 24 h (lT) Fm ± sd (lT)
48.7 47.4 ± 5.8 44.9 ± 4.4
F1959-DGRF (lT)
Fe (lT)
53.8 43.1 44.1 42.0
DM (24 h) (%)
Alt (24 h) (%)
DM (48 h) (%)
10.3 12.9 0.2 5.0 5.3 2.3
0.6 0.0 2.6 3.1 3.6 1.2
18.6 21.2 1.4 0.7 0.4 1.8
Alt (48 h)(%) 7.4 1.3 3.8 2.4 2.0 1.9
43
Spm, specimen; NRM, natural remanent magnetisation intensity; K, initial susceptibility; Tmin–Tmax, temperature interval used for slope calculation; n, number of data points within this temperature interval; f, NRM fraction used in slope calculation; g, gap factor; q, quality factor; F, specimen intensity; Fe, anisotropy corrected specimen intensity; Fm ± sd, anisotropy corrected mean intensity and standard deviation; Fpo, anisotropy corrected weighted mean intensity; DM (24 h/48 h), cooling rate correction for a cooling time of 24 h/48 h; alt (24 h/48 h), magneto-chemical evolution of the magnetisation during the 24 h/48 h cooling rate cycle; Fpocr, anisotropy and cooling rate corrected weighted mean intensity and standard deviation; F1959-DGRF, mean DGRF intensity value at site location for year 1959.
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4.4. Comparison with observatory/model data The kiln was first (and last) used in 1959, a year for which observatory and DGRF data are available for Spain. The values for declination and inclination at the site location are listed in Table 1. Unsurprisingly, the observatory and model data are in close agreement. DGRF intensity value is given in Table 3. The kiln mean directions (PCA, GCA and PCA–GCA) are close to the observatory and DGRF directions, with angular differences between them of <5°. For PCA and PCA–GCA, the differences lie within the a95 confidence limits of the means. For GCA the difference sometimes exceeds the a95 limits, but it should be noted that the a95 is probably under-estimated by the GCA method (McFadden and McElhinny, 1988), so that the difference may not be significant. All together, it is suggested that the mean ChRM direction yielded by the kiln is in agreement with the known field direction. Irrespective of the method used to determine the ChRM direction (PCA, GCA or PCA–GCA, with or without hierarchy), the mean direction is within 5° of the expected value. This seems an acceptable accuracy for a structure displaying complex partial remanences. The DGRF intensity value for Yuste is 43 lT, which is slightly lower than the archaeointensity estimations (47.4 ± 5.8 lT with TRM anisotropy and cooling rate effect correction). The difference is not significant when the dispersion is taken into account. It is worth pointing out that two archaeointensity determinations were excluded from the calculation of the fully corrected mean because their TRM anisotropy was not measured. Had their inclusion been possible, the mean would have been lower and thus closer to the model value. To conclude, the archaeointensity estimation is found to be consistent with the expected/known field intensity. Partially heated structures can be considered as suitable candidates for intensity determinations, within the usual limits imposed by the method. 5. Conclusions A modern kiln (1959) with a complex NRM structure has been studied in order to compare the archaeomagnetic signal (both direction and intensity) with the observatory and model geomagnetic field at the time of kiln usage. The kiln is made from pre-fired bricks whose magnetic properties are dominated by non-stoichiometric magnetite and hematite. The NRM consists of two stable components: a ChRM acquired during kiln use and a randomly oriented component associated with the original firing of the bricks. The former dominates in those parts of the bricks that have been heated to higher temperatures (the A specimens), and is carried by both magnetic phases. The latter is more important in the less heated parts (the B specimens), and is predominantly carried by hematite. The mean direction obtained for the kiln was the same irrespective of the method used to determine the individual
ChRM directions (PCA, GCA or combined PCA–GCA). Applying a hierarchical approach to calculate the mean leads to an increase in precision when compared with a non-hierarchical approach. This is consistent with the basic premise of hierarchy, which acts to average non-systematic errors at the same level (specimen, sample, structure). Although by combining PCA and GCA results considerations of data homogeneity are compromised, this does not significantly influence the mean direction of the kiln studied here. Comparisons of the mean directions with the DGRF and observatory directions show that they are not significantly different, with an accuracy of <5° in all cases. Combining the results of PCA for A and B specimens reduces the precision of the mean direction. This is probably due to the incomplete isolation of the ChRM in the B specimens when calculating the best-fit PCA direction. The subjectivity implied by the analysis methods does not strongly influence the results obtained. Much more important is the distribution of samples. In order to determine truly representative directions it is necessary to sample and analyse all parts of the structure. Thellier-type archaeointensity experiments yielded successful results, despite the complex nature of the NRM. TRM anisotropy and cooling rate corrections were found to be small, leading to modification of the archaeointensity values of <3%. The resulting site-mean intensity was in close agreement with the DGRF intensity. These results show that whilst a complex NRM history may give rise to an archaeomagnetic direction with a relatively large degree of dispersion, the true field value can still be determined. As such the use of a95 (or k) as a sole description of reliability may be overly restrictive. Applying hierarchy in the statistical treatment of the data gives a more precise direction because errors are conveniently averaged. But special care should be taken when comparing directions obtained from the parts of the structure subjected to different temperatures, especially when PCA is used to determine directions obtained from samples heated to low temperatures. Acknowledgements This work has been carried out through the support of European Union contract No. HPRN-CT-2002-00219 (Archaeomagnetic Applications for the Rescue of Cultural Heritage, AARCH). We would like to thank Ildefonso Ramirez Gonzalez for his support and archaeological knowledge. References Chauvin, A., Garcia, Y., Lanos, Ph., Laubenheimer, F., 2000. Palaeointensity of the geomagnetic field recovered on archaeomagnetic sites from France. Phys. Earth Planet. Int. 120, 111–136. Coe, R., Gromme´, C., Mankinen, E., 1978. Geomagnetic paleointensities from radiocarbon-dated lava flows on Hawaii and the question of the Pacific nondipole low. J. Geophys. Res. 83, 1740–1756.
G. Catanzariti et al. / Physics and Chemistry of the Earth 33 (2008) 427–437 Fisher, R.A., 1953. Dispersion on a sphere, Proc. Roy. Soc. London A 217, 295. 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), 2285. Go´mez-Paccard, M., Catanzariti, G., Ruiz-Martı´nez, V.C., McIntosh, G., Nun˜ez, J.I., Osete, M.L., Lanos, Ph., Chauvin, A., Tarling, D.H., Bernal-Casasola, D., Thiriot, J.archaeological working group, 2006. A catalogue of Spanish archaeomagnetic data. Geophys. J. Int. 166, 1125–1143. 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. 57 (1–2). Jasonov, P.G., Nurgaliev, D.K., Burov, D.V., Heller, F., 1998. A modernized coercivity spectrometer. Geologica Carpathica 49 (3), 224–225. Kirschvink, J., 1980. The least-squares line and plane and the analysis of paleomagnetic data. Geophys. J. Royal Astr. Soc. 62, 699–718.
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Lanos, Ph., Le Goff, M., Kovacheva, M., Schnepp, E., 2005. Hierarchical modelling of archaeomagnetic data and curve estimation by moving average technique. Geophys. J. Int. 160, 440–476. Lowrie, W., 1990. Identification of ferromagnetic minerals in a rock by coercivity and unblocking temperature components. Geophys. Res. Lett. 17, 159–162. McFadden, P.L., Lowes, J., 1981. The discrimination of mean directions drawn from Fisher distributions. Geophys. J. R. astr. Soc. 67, 19–33. McFadden, P.L., McElhinny, M.W., 1988. The combined analysis of remagnetization circles and direct observations in palaeomagnetism. Earth Planet. Sci. Lett. 87, 161–172. Prevot, M., Mankinen, E.A., Coe, R.S., Gromme, C.S., 1985. The Steens Mountain (Oregon) geomagnetic polarity transition; 2, Field intensity variations and discussion of reversal models. J. Geophys. Res. 90 (12), 10417–10448. Thellier, E., Thellier, O., 1959. Sur l’intensite´ du champ magne´tique terrestre dans le passe´ historique et ge´ologique. Ann. Ge´ophys. 15, 285–376.
Physics and Chemistry of the Earth 33 (2008) 427–437 www.elsevier.com/locate/pce
Quality control of archaeomagnetic determination using a modern kiln with a complex NRM G. Catanzariti a,*, G. McIntosh a, M. Go´mez-Paccard a,b, V.C. Ruiz-Martı´nez a, M.L. Osete a, A. Chauvin b, The AARCH Scientific Team 1 a
Fı´sica de la Tierra 1, Fac. CC Fı´sicas, Universidad Complutense de Madrid, 28040 Madrid, Spain b Ge´osciences Rennes, Universite´ de Rennes 1, 35042 Rennes, Cedex, France Available online 21 February 2008
Abstract A modern (1959) brick kiln from western Spain has been studied in order to conduct a quality control test of the archaeomagnetic method in a partially heated structure. The kiln exhibits two stable natural remanent magnetisation components: a low-temperature component (150–620 °C) acquired during kiln use and a randomly oriented, high temperature component (500–680 °C) acquired during the original firing of the bricks. A detailed rock magnetic study revealed a magnetic mineralogy dominated by non-stoichiometric magnetite and hematite, both of which contribute to the characteristic remanent magnetisation. Both the direction and intensity of the characteristic magnetisation have been compared with the known values for the geomagnetic field at the site location. Irrespective of the method used to determine the mean archaeomagnetic direction (principal component analysis, great circle analysis, with or without hierarchy) the results are statistically indistinguishable from each other and from the known field direction. In all cases the direction is within 5° of the expected value. Thellier-style archaeointensity determinations have been carried out on a smaller specimen set, with the mean result consistent with the known field value. The results demonstrate the reliability of the archaeomagnetic method in determining the features of the geomagnetic field in the past, even in cases with a complicated magnetic history. Furthermore, they highlight the importance of a uniform sample distribution in order to obtain truly representative values for the direction and intensity of the ancient geomagnetic field. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Archaeomagnetism; Brick kiln; Geomagnetic field; Western Spain
1. Introduction When archaeological features have been exposed to high temperatures the natural remanent magnetisation (NRM) structure is simple. A univectorial stable component is observed which is interpreted as a thermoremanent mag*
Corresponding author. Tel.: +34 913944396; fax: +34 913944398. E-mail address: gcatanza@fis.ucm.es (G. Catanzariti). 1 The AARCH Scientific Team consists of Abrahamsen, N., University of Aarhus, Denmark; Zananiri, I, University of Bradford, UK and Institute of Geology and Mineral Exploration, Greece; Spassov, S., Centre de Physique du Globe de l’IRM, Belgium; Kostadinova, M., Bulgarian Academy of Sciences, Bulgaria; De Marco, E., Aristotle University of Thessaloniki, Greece; Winkler, U., University of Plymouth, UK; Tema, E., Universita´ degli Studi di Torino, Italy; Casas, L., Universita´ degli Studi di Napoli, Italy. 1474-7065/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.pce.2008.02.028
netisation (TRM) acquired during the last heating of the feature. However, many features suffer heating to relatively low temperatures and the NRM structure may be quite complex. Such cases may exhibit a partial TRM associated with the low-temperature heating event, a thermo-chemical remanent magnetisation (TCRM) associated with mineralogical changes and exposure to heat and a stable component associated with earlier events (e.g., firing of bricks during their production). The main aim of this study is to study the application of the archaeomagnetic method (in terms of direction and intensity) to a modern, partially heated feature where the true geomagnetic field is known. For the archaeodirection, the sensitivity of the method to the determination of the characteristic magnetisation (ChRM) direction, the calculation of mean directions and the subjectivity of the
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interpretation of complex remanences have been investigated. For the archaeointensity, Thellier–Thellier palaeointensity experiments have been carried out. A modern kiln from western Spain (Go´mez-Paccard et al., 2006) has been selected for this purpose. It was first (and last) used in 1959, for which accurate information about the mean geomagnetic field is available from both observatory and Definitive Geomagnetic Reference Field (DGRF) data. The kiln exhibits two stable NRM components, the relative importance of each varies with respect to the proximity to the heating source. Principal component analysis (PCA) of the NRM demagnetisation curve
Fig. 1. Map of Spain showing site location. Madrid and Toledo observatory also shown.
(Kirschvink, 1980) and great circle analysis (GCA) of remagnetisation circles (McFadden and McElhinny (1988)) have both been used to determine ChRM directions. The precision and accuracy of each method have been compared, along with the influence of applying a hierarchical approach when determining mean directions. 2. Archaeological context and sampling The monastery at Yuste, Caceres Province, western Spain (Fig. 1), is an important historical site where King Carlos V resided after his abdication in 1556 AD up to his death two years later. As recounted by the present Prior of the monastery, a small kiln was built in 1959 by one of the monks in order to recreate traditional ceramic production methods. It was abandoned in the same year because of a failure to produce acceptable ceramics, presumably due to the difficulties in reaching high firing temperatures. The kiln consists of a cylindrical structure about 120 cm high and 80 cm in diameter (Fig. 2), made of red, pre-fired bricks. The firing chamber occupied the upper part of the structure and the combustion chamber the lower part, whilst no remains of the grill were found in situ. For the archaeomagnetic study, five bricks were sampled from all sides of the same level of the upper part of the kiln (Fig. 2b). Standard specimens were drilled from the brick surface facing the kiln interior, with specimens labelled ‘‘A” coming from closer to the interior brick surface than those labelled ‘‘B” (see Fig. 2c). A total of 18 specimens were prepared for directional analysis. The kiln was re-sampled in order to obtain specimens for archaeointensity experiments. Four bricks were taken from lower down the kiln structure, closer to the combustion chamber than the bricks studied previously.
Fig. 2. (a) Photo of kiln, sampled levels shaded in grey, (b) distribution of sampled bricks used in archaeodirection study and (c) brick sub-sampling scheme.
G. Catanzariti et al. / Physics and Chemistry of the Earth 33 (2008) 427–437 1.40E-06
3.1. Archaeodirection
1.20E-06 1.00E-06 8.00E-07 6.00E-07 4.00E-07 2.00E-07 0.00E+00
3.2. Archaeointensity 1.20E-03
1.00E-03 2
NRM Intensity [A*m /Kg]
Archaeointensity experiments were carried out at Geosciences, Rennes (France), following the classical Thellier palaeointensity method (Thellier and Thellier, 1959). Heating was carried out from 100 °C to temperatures at which >85% of the initial NRM was lost, using between 13 and 18 steps. A laboratory field of 60 lT was applied along the specimen Z axis during TRM acquisition, with pTRM checks made every two steps. A Magnetic Measurements MMTD oven and an Agico JR5 spinner magnetometer were used throughout. TRM anisotropy and cooling rate effects were also investigated.
8.00E-04
6.00E-04
4.00E-04
2.00E-04
0.00E+00
3.3. Mineral magnetic measurements All measurements were carried out at the Universidad Complutense, Madrid, and at Geosciences, Rennes. A coercivity meter (Jasonov et al., 1998), with a maximum applied field of 500 mT, was used to measure hysteresis curves, stepwise acquisition and reverse-field acquisition of isothermal remanence (IRM). The following parameters were defined: saturation magnetisation, Ms, saturation remanence, Mr, coercivity, Bc and coercivity of remanence, Bcr. TH demagnetisation of orthogonal IRMs (Lowrie, 1990) was conducted on selected specimens. Thermomagnetic curves (room temperature to 700 °C) were measured in air using an Agico KLY3 susceptibility meter with a fitted furnace. Low-temperature IRM was measured between 5 and 300 K using a Quantum Designs MPMS SQUID magnetometer, applying a 2 T IRM at 4 K. 4. Results and discussion 4.1. Mineral magnetic properties Both K and NRM intensity are relatively low, varying between 4.34 10 8–4.44 10 6 m3 kg 1 and 6.55 10 5–7.48 10 3 A m2 kg 1 respectively. In all cases the A specimens exhibit higher values than the corresponding B specimens (Fig. 3). The magnetic properties vary within and between bricks, implying a heterogeneous distribution of magnetic phases. Ms and Mr of samples from closer to the burnt brick surface are higher than those of samples
HP.11A HP.12A HP.12B HP.13A HP.21A HP.22A HP.22B HP.23A HP.31A HP.31B HP.32A HP.33A HP.42A HP.42B HP.43A HP.52A HP.52B HP.53A HP.53B HP.61A3 HP.73A3 HP.83A1 HP.91A1 HP.91A2 HP.103A3
The archaeodirection study was carried out in the Palaeomagnetism Laboratory of the Universidad Complutense, Madrid (Spain). The NRM was measured using an AGICO JR5 spinner magnetometer; low field magnetic susceptibility (K) was measured using an AGICO KLY3 susceptibility meter. Stepwise thermal (TH) demagnetisation of NRM was carried out using a Schonsted Instruments TSD-1 thermal demagnetiser.
HP.11A HP.12A HP.12B HP.13A HP.21A HP.22A HP.22B HP.23A HP.31A HP.31B HP.32A HP.33A HP.42A HP.42B HP.43A HP.52A HP.52B HP.53A HP.53B HP.61A3 HP.73A3 HP.83A1 HP.91A1 HP.91A2 HP.103A3
3
Mass susceptibility [m /Kg]
3. Methods
429
Fig. 3. Specimen values of (a) magnetic susceptibility and (b) NRM intensity. Black (grey) columns indicate A(B) specimens.
further away from the burnt surface. This is in agreement with the K results, since the A specimens were taken adjacent to the burnt brick surface. Hysteresis curves yield values of Bc of 10–60 mT (Fig. 4a) and show varying contributions of paramagnetic and high coercivity ferromagnetic material. The samples generally acquire most of their IRM by 300 mT, though in most cases do not approach saturation at 500 mT (Fig. 4b), a further indication of coexisting high and low coercivity magnetic phases. Bcr values range between 60 and 200 mT, the highest value observed in sample HP4_B, which also had the lowest Mr. TH demagnetisation of orthogonal IRMs (Fig. 5) show that the low coercivity phase (0–150 mT) has maximum unblocking temperatures around 550 °C, consistent with a magnetite-like composition. The high coercivity phase (300–1150 mT) has maximum Tubs near 680 °C, which suggests hematite. The low coercivity phase is relatively more important in the A than the B specimens. Sample HP_4A, from close to the burnt face of brick 4, shows a thermomagnetic curve with a broad peak between 300 and 500 °C (Fig. 6). The Curie temperature (Tc) has been estimated as being between 500 and 550 °C, lying between the maximum and minimum value of K. For
G. Catanzariti et al. / Physics and Chemistry of the Earth 33 (2008) 427–437 6.00E-02
A
HP4_
1.5E-01
4_B
HP
6.00E-02
2.00E-02
4_A
HP
4.00E-02
A HP1_
5.00E-02
1.60E-02
-5.0E-02
-2.00E-02
4.00E-02
HP1_B
1.20E-02
2
-1
2
Mr [Am Kg ]
0.00E+00
-1
2.00E-02
2
0.0E+00
HP1_B
Ms [Am Kg ]
-1
5.0E-02
2
Ms [Am Kg ]
1.0E-01
-1
2.0E-01
Mr [Am Kg ]
430
3.00E-02 8.00E-03 2.00E-02
-1.0E-01 _A
-1.5E-01
HP1
-4.00E-02
4.00E-03
1.00E-02 HP4_B
-2.0E-01 -500 -400 -300 -200 -100
0
-6.00E-02 100 200 300 400 500
0.00E+00 -200
Applied Field [mT]
-100
0
100
200
300
400
0.00E+00 500
Applied Field [mT]
Fig. 4. (a) Hysteresis curves and (b) IRM curves for representative A and B specimens. Left (right) scale refers to samples from brick HP4(HP1). Ms(Mr) = saturation magnetisation (remanence).
The magnetic properties of the bricks can be explained by a mixture of a magnetite-like phase and hematite, the relative importance of which varies as a function of distance from the burnt brick face. Closer to the burnt face the concentration of the magnetite-like phase is higher and this phase dominates. Slightly reduced Tcs and the lack of a Verwey transition in the low-temperature IRM curves (Fig. 7) suggest non-stoichiometric magnetite as a candidate for this phase. Further from the burnt face the concentration falls and hematite becomes relatively more important. Hematite is interpreted as a ‘‘primary” magnetic phase, originally present in the material used to produce the bricks or related to the brick firing process. The increase in concentration of the magnetite-like phase (as seen in K, NRM, Ms and Mr) is considered as due to the formation of a ‘‘secondary” phase, related with heating of the bricks during the lifespan of the kiln. 4.2. ChRM
Fig. 5. Thermal demagnetisation of orthogonal IRM for representative A and B specimens.
HP_4B (Fig. 6), K is much lower and a precise determination of Tc is difficult. K reaches minimum values near 650– 700 °C, which may reflect the approach to the Neel temperature (TN) of hematite of 675 °C. In the case of HP_9A and HP_9B (Fig. 6), both the A and B samples have similar curves, exhibiting a sharp peak near 550 °C, which may represent a Hopkinson peak and thus defines a Tc close to 550 °C. For the bricks from closer to the combustion chamber, the magnetisation was more homogeneous in the A and B specimens. This is consistent with being heated to higher temperatures, with the effects of heating penetrating further into the brick interior. Tcs near 550 °C suggest a magnetite-like composition, in accordance with the IRM properties previously described. Irrespective of the thermomagnetic curve shape they all show a high degree of reversibility, indicating thermal stability of the samples.
4.2.1. Component structure of NRM The initial NRM directions fall close to the present day geomagnetic north, with the exception of the five B specimens which exhibit scattered directions. Stepwise TH demagnetisation revealed two stable components: a low temperature (LT) component isolated at temperatures up to 550–620 °C, and a high temperature (HT) component between 500 and 680 °C. The LT component is better defined in the A specimens (Fig. 8a and c), comprising up to 90% of the NRM. For the B specimens, the HT component is more important, approaching 50% of the NRM, whereas the LT component is poorly defined (Fig. 8b and d). Evidence for overlapping unblocking temperatures of the LT and HT components can be found in most specimens, affecting both the A and B specimens (e.g., 42B, 53A, Fig. 8b and c). The LT component is considered the ChRM acquired during kiln usage. It is better defined in the A specimens due to their proximity to the heat source. Unblocking temperatures of 550–620 °C (Fig. 8e) suggest that this component is carried by the magnetite-like phase and by part of the hematite phase. The HT component represents a ‘‘pri-
G. Catanzariti et al. / Physics and Chemistry of the Earth 33 (2008) 427–437
431
Fig. 6. Typical thermomagnetic curves. Black (grey) symbols denote heating(cooling) branches.
1.80E-02
1.00E-03
1.60E-02
1 8.00E-04
1.40E-02 1.20E-02
6.00E-04
3
1.00E-02 8.00E-03
4.00E-04 6.00E-03 4.00E-03
1 HP4_I : 0-1 cm from the kiln interior (LEFT AXIS)
2.00E-03
2 HP4_II : 4-5 cm from the brick interior (RIGHT AXIS)
IRM (2 T) [emu]
IRM (2 T) [emu]
2
2.00E-04
3 HP4_III : 8-9 cm from the kiln interior (RIGHT AXIS) 0.00E+00 0.0
50.0
100.0
150.0
200.0
250.0
0.00E+00 300.0
Temperature [K]
Fig. 7. Low-temperature IRM. Applied field = 2 T at 4 K, warming in zero field. Note different scales for HP4_II and _III. !!! There is no way to distinguish curves (especially HP4_I from HP4_III !!!!.
mary” remanence associated with the original firing of the bricks. Acquired prior to construction of the kiln, its direction is randomly distributed, but consistent within the same brick. It is more important in the B specimens, which suffered less heating during the use of the kiln. Maximum unblocking temperatures of 680 °C (Fig. 8f) show that it is carried by the hematite phase. 4.2.2. Calculating the mean ChRM direction Linear demagnetisation trajectories on the orthogonal vector projections could be identified in all specimens,
allowing PCA determination of the LT direction. For the B specimens, the trajectories were more poorly defined than for the A specimens. All specimens yielded well-defined demagnetisation planes, permitting GCA in all cases. Therefore 18 ChRM directions were determined by PCA, and 18 planes containing the ChRM direction were determined by GCA. Next, the mean ChRM direction was calculated, considering the PCA and GCA directions, individually and in combination, and by imposing a hierarchical structure defined by the five independently oriented brick samples.
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G. Catanzariti et al. / Physics and Chemistry of the Earth 33 (2008) 427–437 245
Up/W
Up/W
516 295 601
511 N
636
42A
N Horizontal plane Vertical plane
Horizontal plane Vertical plane
696
Specimen 42A
Specimen 42B
Up/W
Up/ W
680
N
N 681
Horizontal plane Vertical plane
53A
Horizontal plane Vertical plane
540 310 Horizontal plane Vertical plane
501 Specimen 53B
Specimen 53A
HP.12A
HP.31A
HP.42A
HP.12B
0.20
0.60
NRM Intensity [A/m]
NRM Intensity [A/m]
0.80
0.40
0.20
0.00
HP.31B
HP.42B
0.15
0.10
0.05
0.00 0
100
200
300
400
500
600
700
Temperature [ºC]
0
100
200
300
400
500
600
700
Temperature [ºC]
Fig. 8. (a–d) Equal-area projection and orthogonal vector projection of thermal demagnetisation of representative A and B specimens. (e–f) NRM intensity decay during TH demagnetisation of representative A and B specimens.
With respect to the hierarchical approach (as described by Lanos et al., 2005), the basic assumption is that the dispersion of data at the same level (specimen, sample, structure) is homogeneous. Therefore all data are considered with the same weight and the only sources of uncertainty are the sampling errors and/or other non-systematic errors (e.g., related with determination of directions). In the case of partial heating the NRM is complicated due to variations in the relative proportions of, and degree of overlap between, the NRM components (e.g., when comparing the A and B specimens in the present study). This does not affect GCA, while PCA requires no (or minimal) overlap. Therefore the two methods may give rise to different types of uncertainties and mixing them violates the
assumption of homogeneity. Strictly speaking, therefore, they should not be combined in the hierarchical approach. Table 1 summarises the mean ChRM directions, with and without hierarchy, using PCA, GCA and combined PCA–GCA. In all cases the mean direction is the same irrespective of whether hierarchy is applied or not, whereas the precision parameter k is higher when hierarchy is applied. For PCA, the mean directions were calculated using Fisher (1953) statistics. The precision parameter k increases when applying hierarchy to the A and the A + B specimen directions. This indicates that the hierarchical approach is effective in averaging any non-systematic errors. It can also be seen that the PCA mean directions have higher precision when considering the A specimens only. The decrease in
G. Catanzariti et al. / Physics and Chemistry of the Earth 33 (2008) 427–437
433
Table 1 Mean ChRM directions Mean ChRM directions Analytical method
N
D
I
a95
k
R
b
PCA (A) PCA (A) Hier. PCA (A+B) PCA (A+B) Hier. PCA (B) GCA (A) GCA (A) Hier GCA (B) Hier. GCA (A+B) GCA (A+B) Hier. PCA (A) + GC (B) PCA (A) + GC (B) Hier. Toledo observatory direction (1959.5) DGRF direction (1959)
13 5 18 5 5 13 5 5 18 5 18 5
349.6 349.1 349.7 349.6 349.8 345.9 347.4 344.8 345.7 347.2 349.0 348.7 351.0 351.1
59.7 59.8 59.1 58.9 57.7 60.9 59.9 60.0 60.6 59.9 59.8 60.8 56.4 56.5
5.2 8.2 5.0 8.4 15.4 3.8 5.7 9.0 2.9 2.0 3.8 7.1
63.9 87.2 49.2 84.1 25.7 137.5 375.0 150.0 163.3 1500 72.8 115.7
12.812 4.954 17.655 4.952 4.845 12.960 4.996 4.990 17.951 4.999 17.801 4.965
3.4 3.5 2.8 2.6 1.4 5.2 3.9 4.8 5.0 4.0 3.6 4.6
Analytical method, PCA = principal component analysis, GCA = great circle analysis, Hier = applying hierarchy; N, number of specimens (independent samples when applying hierarchy); D, declination (°E); I, inclination (°); a95, alpha-95 confidence limit; k, precision parameter; R, length of the resultant vector; b, angular difference between mean direction and the observatory field value for 1959.
precision when B samples are considered is interpreted as being due to incomplete isolation of their ChRM direction. For GCA and PCA–GCA, mean directions were calculated following the converging remagnetisation circles method of McFadden and McElhinny (1988). This approach is based on the Fisher (1953) distribution and allows for the estimation of k, R and a95. For GCA, as with PCA, the application of hierarchy improves the precision of the mean directions. Inclusion of the B specimens improves the precision of the mean. The A and B specimens exhibit LT and HT components which both contribute to the demagnetisation plane. Therefore the analysis of the data is more homogenous and the best results are obtained by considering all of the specimens. The PCA–GCA result combines the 13 PCA A specimen directions and the five GCA B specimen planes. This gives rise to an increase in precision when compared with the PCA (A specimens) and the PCA (A + B specimens), with or without hierarchy. Whilst this suggests that results are improved by including the B specimen planes, the mixing of PCA and GCA results in a hierarchical approach violates the homogeneity of the data analysis. When comparing the PCA and GCA results, GCA appears to provide more precise mean directions. However, as stated by the authors (McFadden and McElhinny, 1988), the remagnetisation circle method tends to overestimate k and R, so that, whilst the results are internally consistent, they are not directly comparable with PCA. Therefore the increased precision of GCA (and PCA– GCA) results is, at least in part, an artefact of the method. Of more significance is the good agreement between the three approaches. Considering the best results (i.e., those with highest precision, k) obtained by PCA (A specimens, with hierarchy), GCA (A+B specimens, with hierarchy) and PCA–GCA (A + B specimens, with hierarchy), the difference between them is small and they are statistically
indistinguishable following the McFadden and Lowes (1981) test. 4.2.3. The influence of subjectivity on the determination of ChRM directions: inter-laboratory data analysis The interpretation of complex remanences may lead to a degree of subjectivity that in turn affects the determined ChRM directions. In order to see if this is important, an inter-laboratory comparison of data interpretation was carried out. Of the five bricks sampled, four that yielded specimens with a well-expressed, 2-component NRM (HP1, 3, 4 and 5) were selected. These have been used to construct a data set of 14 specimen demagnetisation data, comprising two or three A specimens and one B specimen from each brick. The data set was then sent to the participating groups, along with the a priori information that the specimens came from a partially heated kiln. Each group was charged with determining the ChRM direction of all specimens, wherever possible, using their own standard procedures and software. The ChRM directions of each group were then collected for a comparative study. Including Madrid, a total of 10 member groups participated in the experiment, and the results are summarised in Table 2. Variability in the interpretation is expressed in the method used to calculate the ChRM, the acceptance or rejection of particular specimen directions and in the calculated directions themselves. Eight of the 10 groups used PCA, whereas two used a combination of PCA and GCA. Only three groups accepted all 14 directions and one group only accepted four of the directions, with the most commonly rejected directions coming from the B specimens. Specimen directions determined by PCA are very similar, giving rise to mean directions (calculated without hierarchy) with a95 values between 3.4° and 6.4° and k between
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Table 2 Mean directions determined by the 10 participating groups Group
Method
N
D
I
a95
k
R
b
1 2 3 4 5 6 7 8 9 10
PCA + GC PCA PCA PCA PCAa PCA PCA + GC PCA PCA PCA
14 12 10 11 4 14 14 10 10 10
351.2 355.2 352.8 354.4 340.7 353.8 350.2 352.6 349.3 348.7
56.2 61.6 60.0 58.8 61.6 59.0 56.9 60.0 59.4 59.2
3.9 6.4 5.9 5.6 3.4 5.3 3.5 5.5 4.2 6.3
109 47 69 68 736 57 137 79 131 59
13.9 11.8 9.9 10.9 4.0 13.8 13.9 9.9 9.9 9.9
0.2 5.6 3.7 3.0 7.4 3.0 0.7 3.7 3.1 3.1
Group = arbitrary number assigned to the participating group; method, analytical method, PCA = principal component analysis, GCA = great circle analysis; for other definitions, see Table 1. a Mean direction was statistically different from other group means following McFadden and Lowes (1981).
47 and 736. Two groups employed GCA for specimens judged to have curved vector segments. For those specimens in which both groups used GCA, similar planes were fitted and the resulting mean directions are similar to those determined by PCA alone. Following the McFadden and Lowes (1981) test, all but one of the means are statistically indistinguishable at the 95% confidence level. The outstanding group mean value exhibits the lowest a95 and the highest precision. However, it has been calculated from four accepted specimen directions, three of which were from the same brick. Therefore the resulting mean direction is strongly biased towards the results from this brick. What these results suggest is that the calculation of ChRM directions from complex NRMs is fairly uniform within the participating groups. Whilst this may sound obvious, it is nonetheless reassuring to demonstrate that
Fig. 9. Representative archaeointensity results. Normalised NRM–TRM and vector component plots for (a, b) specimen HP10_3A3 and (c, d) HP9_1A1.
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it is the case. The method used to calculate ChRM directions, and the criteria for acceptance or rejection of individual directions, does not strongly influence the resulting mean direction. An exception is when the distribution of accepted specimen directions is not uniform across the sampled structure. It is the non-uniform distribution that is important, rather than the data analysis itself, in that it gives rise to a mean direction that is not representative of the structure as a whole.
4.3. Archaeointensity determinations Four of the six studied specimens exhibit a single, stable NRM component. They come from bricks (HP8-10) taken lower down in the kiln than those used in the directional analysis, i.e., closer to the combustion chamber, and have probably been heated to sufficiently high temperatures to acquire a total TRM. The other two specimens, coming from bricks (HP6,7) taken higher up in the kiln, show both LT and HT components – as seen in the directional study. For these specimens the LT component has been used to calculate the archaeointensity. Two examples of NRM– TRM (Arai) diagrams are plotted together with their corresponding orthogonal vector plots (in specimen coordinates) in Fig. 9 and the results are listed in Table 3. There are no important differences between the archaeointensities determined from the single component and LT/HT component specimens, and both types yield acceptable results. For each specimen the temperature interval considered for slope computation, the number of steps (n) performed within this interval and the different factors (f, g and q) used to test data quality (see Coe et al., 1978) have been listed. The mean site intensity has been calculated using the weighting factor proposed by Prevot et al. (1985) and
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the dispersion of the mean has been expressed as the standard deviation (sd). The TRM anisotropy tensor has been determined for four specimens, at temperatures at which at least 70% of NRM was lost. The anisotropy degree k1/k3 (where k1 and k3 are the maximum and minimum axes of the TRM anisotropy tensor) ranges between 8% and 28% and magnetic lineation dominates. However, the minimum axes are not clearly contained in the flattening (horizontal) plane of the bricks. The differences between the specimen archaeointensity values before and after TRM anisotropy correction are <6%. For this reason the site-mean intensity has been calculated before and after correction and listed in Table 3. The dependence of TRM intensity on cooling rate has been studied, following the method described in Go´mezPaccard et al. (2006). Two linear cooling times, of 24 and 48 h, were applied. For four specimens the correction factor for the 24 h cooling rate is <6%, but for the other two the correction factor is up to 13%. A combination of the low correction factor and magneto-chemical alteration during the 48 h cooling rate experiment meant that only two specimens yielded reliable correction factors. The size of the studied kiln suggests that the cooling time related to the acquisition of the TRM is most probably more similar to 24 h rather than 48 h. For this reason the 24 h correction factor has been applied to the archaeointensity determinations. Several investigations (for example, Go´mez-Paccard et al., 2006; Genevey and Gallet, 2002; Chauvin et al., 2000) illustrate the importance of cooling rate corrections in archaeointensity studies, showing correction factors that can exceed 20%. This is not the case in the present study, with the difference between the uncorrected and corrected mean intensities being less than 3%.
Table 3 Archaeointensity results Spm
NRM (A/m)
K (10 5)
Tmin–Tmax (°C)
n
f
g
q
F (lT)
HP6.1A3 HP7.3A3 HP8.3A1 HP9.1A1 HP9.1A2 HP10.3A3
0.14 0.15 0.8 1.6 0.8 13
17 23 86 69 47 888
100–580 100–580 100–560 100–530 100–530 100–530
14 14 13 12 12 12
0.62 0.99 0.90 0.84 0.92 0.99
0.85 0.86 0.83 0.67 0.74 0.87
12.80 27.70 151.80 24.02 25.25 43.36
43.3 43.3 56.0 45.7 46.0 42.1
Fpo (lT) Fpocr 24 h (lT) Fm ± sd (lT)
48.7 47.4 ± 5.8 44.9 ± 4.4
F1959-DGRF (lT)
Fe (lT)
53.8 43.1 44.1 42.0
DM (24 h) (%)
Alt (24 h) (%)
DM (48 h) (%)
10.3 12.9 0.2 5.0 5.3 2.3
0.6 0.0 2.6 3.1 3.6 1.2
18.6 21.2 1.4 0.7 0.4 1.8
Alt (48 h)(%) 7.4 1.3 3.8 2.4 2.0 1.9
43
Spm, specimen; NRM, natural remanent magnetisation intensity; K, initial susceptibility; Tmin–Tmax, temperature interval used for slope calculation; n, number of data points within this temperature interval; f, NRM fraction used in slope calculation; g, gap factor; q, quality factor; F, specimen intensity; Fe, anisotropy corrected specimen intensity; Fm ± sd, anisotropy corrected mean intensity and standard deviation; Fpo, anisotropy corrected weighted mean intensity; DM (24 h/48 h), cooling rate correction for a cooling time of 24 h/48 h; alt (24 h/48 h), magneto-chemical evolution of the magnetisation during the 24 h/48 h cooling rate cycle; Fpocr, anisotropy and cooling rate corrected weighted mean intensity and standard deviation; F1959-DGRF, mean DGRF intensity value at site location for year 1959.
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4.4. Comparison with observatory/model data The kiln was first (and last) used in 1959, a year for which observatory and DGRF data are available for Spain. The values for declination and inclination at the site location are listed in Table 1. Unsurprisingly, the observatory and model data are in close agreement. DGRF intensity value is given in Table 3. The kiln mean directions (PCA, GCA and PCA–GCA) are close to the observatory and DGRF directions, with angular differences between them of <5°. For PCA and PCA–GCA, the differences lie within the a95 confidence limits of the means. For GCA the difference sometimes exceeds the a95 limits, but it should be noted that the a95 is probably under-estimated by the GCA method (McFadden and McElhinny, 1988), so that the difference may not be significant. All together, it is suggested that the mean ChRM direction yielded by the kiln is in agreement with the known field direction. Irrespective of the method used to determine the ChRM direction (PCA, GCA or PCA–GCA, with or without hierarchy), the mean direction is within 5° of the expected value. This seems an acceptable accuracy for a structure displaying complex partial remanences. The DGRF intensity value for Yuste is 43 lT, which is slightly lower than the archaeointensity estimations (47.4 ± 5.8 lT with TRM anisotropy and cooling rate effect correction). The difference is not significant when the dispersion is taken into account. It is worth pointing out that two archaeointensity determinations were excluded from the calculation of the fully corrected mean because their TRM anisotropy was not measured. Had their inclusion been possible, the mean would have been lower and thus closer to the model value. To conclude, the archaeointensity estimation is found to be consistent with the expected/known field intensity. Partially heated structures can be considered as suitable candidates for intensity determinations, within the usual limits imposed by the method. 5. Conclusions A modern kiln (1959) with a complex NRM structure has been studied in order to compare the archaeomagnetic signal (both direction and intensity) with the observatory and model geomagnetic field at the time of kiln usage. The kiln is made from pre-fired bricks whose magnetic properties are dominated by non-stoichiometric magnetite and hematite. The NRM consists of two stable components: a ChRM acquired during kiln use and a randomly oriented component associated with the original firing of the bricks. The former dominates in those parts of the bricks that have been heated to higher temperatures (the A specimens), and is carried by both magnetic phases. The latter is more important in the less heated parts (the B specimens), and is predominantly carried by hematite. The mean direction obtained for the kiln was the same irrespective of the method used to determine the individual
ChRM directions (PCA, GCA or combined PCA–GCA). Applying a hierarchical approach to calculate the mean leads to an increase in precision when compared with a non-hierarchical approach. This is consistent with the basic premise of hierarchy, which acts to average non-systematic errors at the same level (specimen, sample, structure). Although by combining PCA and GCA results considerations of data homogeneity are compromised, this does not significantly influence the mean direction of the kiln studied here. Comparisons of the mean directions with the DGRF and observatory directions show that they are not significantly different, with an accuracy of <5° in all cases. Combining the results of PCA for A and B specimens reduces the precision of the mean direction. This is probably due to the incomplete isolation of the ChRM in the B specimens when calculating the best-fit PCA direction. The subjectivity implied by the analysis methods does not strongly influence the results obtained. Much more important is the distribution of samples. In order to determine truly representative directions it is necessary to sample and analyse all parts of the structure. Thellier-type archaeointensity experiments yielded successful results, despite the complex nature of the NRM. TRM anisotropy and cooling rate corrections were found to be small, leading to modification of the archaeointensity values of <3%. The resulting site-mean intensity was in close agreement with the DGRF intensity. These results show that whilst a complex NRM history may give rise to an archaeomagnetic direction with a relatively large degree of dispersion, the true field value can still be determined. As such the use of a95 (or k) as a sole description of reliability may be overly restrictive. Applying hierarchy in the statistical treatment of the data gives a more precise direction because errors are conveniently averaged. But special care should be taken when comparing directions obtained from the parts of the structure subjected to different temperatures, especially when PCA is used to determine directions obtained from samples heated to low temperatures. Acknowledgements This work has been carried out through the support of European Union contract No. HPRN-CT-2002-00219 (Archaeomagnetic Applications for the Rescue of Cultural Heritage, AARCH). We would like to thank Ildefonso Ramirez Gonzalez for his support and archaeological knowledge. References Chauvin, A., Garcia, Y., Lanos, Ph., Laubenheimer, F., 2000. Palaeointensity of the geomagnetic field recovered on archaeomagnetic sites from France. Phys. Earth Planet. Int. 120, 111–136. Coe, R., Gromme´, C., Mankinen, E., 1978. Geomagnetic paleointensities from radiocarbon-dated lava flows on Hawaii and the question of the Pacific nondipole low. J. Geophys. Res. 83, 1740–1756.
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