Directional Secular Variation Research Articles

Improvements in Archaeomagnetic Dating in Western Europe from the Late Bronze to the Late Iron Ages: An Alternative to the Problem of the Hallstattian Radiocarbon Plateau

We present a new curve of the directional secular variation of the geomagnetic field in Western Europe between 1500 bce and 200 ce. Its computation relies on a Bayesian framework. The fast secular variation during the Late Bronze and Early Iron Ages makes archaeomagnetic dating efficient with a respective precision of 150–200 and 60–100 years during these periods. The Bayesian method also provides posterior date distributions that refine the dating of reference data, especially during the period of the Hallstattian radiocarbon plateau. Archaeomagnetism becomes a valuable alternative to radiocarbon and will help to improve the archaeological chronologies.

An updated catalogue of Greek archaeomagnetic data for the last 4500 years and a directional secular variation curve

We present an updated compilation of Greek directional archaeomagnetic data for the last 4.5 millennia. The data set comprises 89 directions from archaeological artefacts and volcanic rocks. Most of the data come from the Late Bronze Age (1700-1400 BC) that is the flourishing period of the Minoan civilization in Crete, while parts of the classical (480-323 BC), Hellenistic (323-31 BC) and Roman (146 BC-330 AD) periods are also well covered. The dataset has been analysed using the Bayesian approach for curve building and a directional secular variation (SV) curve for Greece is proposed. Comparisons with regional and global model predictions show a general agreement even though some discrepancies are observed for some time intervals. The new curves together with the previously published intensity SV curve for Greece, also using the Bayesian approach, form a homogeneous set and enrich our knowledge of the full geomagnetic field vector variation in Greece during the last millennia.

Constraints of archaeomagnetic dating and field intensity determinations in three ancient tile kilns in Belgium

The aim of this study is to date by the archaeomagnetic method the last heatingcooling cycle of one Roman and two Medieval tile kilns, discovered in Belgium. The investigation demonstrates the limitations when well-documented local directional secular variation curves of the geomagnetic field in the past are used for dating and the difficulties when trying to determine field intensities from “in situ” baked clays from the kilns. The three kilns yielded very well defined ancient field directions but two possible dating solutions for each of them when no a priori time constraints are taken into account, due to field direction recurrence. As an increase of the dating accuracy and reduction of the number of dating solutions can be expected using the full field vector information, also field intensity determinations on burnt clays from the kilns were attempted. Field intensities from samples of the Roman and of one of the Medieval kilns are quite scattered. On the other hand, results obtained applying the Thellier-Thellier method and the modified method developed by Dekkers and Bohnel on sister samples from the Roman kiln agree fairly well. Rock magnetic properties reveal high variance in the kilns that point to varying spatial heating and cooling conditions in the kilns. Even well burnt material from the kilns shows irreversible changes when heated in air in the laboratory. Reliable field intensities on “in situ” baked materials from kilns themselves can therefore only be obtained when measuring sufficient number of samples from different parts of the kiln, taking into account the spatial-temporal conditions during kiln operation and cooling history. More reference intensity data is needed in our regions in order to improve dating based on directional reference data only.

Geomagnetic field variations in Western Europe from 1500BC to 200AD. Part I: Directional secular variation curve

To improve the geomagnetic field secular variation curve (SVC) of Western Europe during protohistoric times, archaeomagnetic directions of 39 archaeological kilns or hearths from France were investigated. The dating of each archaeological structure was established with archaeological or chronometric methods. Thirty-seven of these structures are dated from the first millennium BC, one from the end of the second millennium BC and the last one from the fourth millennium BC. Thermomagnetic curves, unblocking temperatures and coercivities suggest that the main carrier of the remanent magnetization is a Ti-poor titanomagnetite. Archaeodirections were obtained by alternating field and thermal demagnetizations on almost 900 specimens. The anisotropy tensor of thermoremanent magnetization was determined for 35 structures and 22 mean archaeodirections were corrected for anisotropy. The new archaeodirections are very consistent with previously published data. A new directional SVC was built using bivariate statistics with selected Western Europe data located within 1000km of Paris. Selection criteria include the number of samples, the dating reliability and the accuracy of the mean archaeodirection. Resulting secular variation between 1500BC and 0AD mainly shows large changes in declination, while inclinations are bracketed between ∼65° and ∼75°. The declinations show a strong maximum with values ∼30–35° around 800–750BC, followed by a sharp decrease to values around 0° at 500BC and close to −7° around 250BC. The main features of the secular variation from 1500BC to 0AD appear to be a dominant westward drift and two major changes around 800 and 250BC. Compared to the global and regional geomagnetic models, the new reference data are better fitted by ARCH3k_cst.1 and SCHA.DIF.3k than by ARCH3k.1 and CALS3k.4 models. The strong variation of the archaeodirection between 1000 and 500BC makes archaeomagnetism very useful for dating purposes.

The “van Zijl” Jurassic geomagnetic reversal revisited

We have collected two new detailed records of what could be the second oldest well documented reversal, the “van Zijl” Jurassic (∼180 Ma) reversal recorded in the thick basalt sequences of the Karoo large igneous province in Lesotho and South Africa. Sections yielded 10 and 8 independent transitional paleomagnetic directions respectively over two continuous 130 m and 160 m sequences. The corresponding VGP reversing paths share a number of consistent features and are integrated with a third record at Bushmen's Pass from Prévot et al. (2003) to yield a single reversal path with: 1‐ a large directional jump with no transitional poles between 45°S and 45°N in the first, main phase of the reversal, 2‐ a large elongated loop (multiple hairpin) to the SE with poles going down to ∼20° latitudes in the second phase of the reversal, and 3‐ a rebound before finally settling in normal polarity. We have measured relative paleo‐intensities on a set of carefully selected samples. Intensity was low prior to the directional changes and recovery to the following normal polarity was progressive. Transitional intensity is 80 to 90% lower than full polarity values. There is no significant increase in intensity at the time of transitional directional clusters. Our results are consistent with a weak non‐dipolar reversing field, with faster and larger directional secular variation as the equator is crossed by VGPs. They tend to vindicate the notion of a short duration of the directional reversal, though one cannot draw global conclusions from a record limited to essentially a single location.

Comments on directional secular variation of the geomagnetic field in late devonian volcanics of the Northern Tien Shan by M.L. Bazhenov and N.M. Levashova

The paper by M.L. Bazhenov and N.M. Levashova is devoted to the paleomagnetic study of Late Devo� nian volcanics in the Northern Tien Shan. Based on this study, the authors suggest a new hypothesis char� acterizing secular variation of the geomagnetic field in the past. Thus, the paper consists of two parts, of which the first part is the basis for the second part. Below, we present a brief analysis of each part of the paper separately.

Directional secular variation of the geomagnetic field in Late Devonian volcanics of the Northern Tien Shan

We have paleomagnetically studied sixty andesite-basalt lava flows of the Late Devonian (Frasnian) in the Northern Tien Shan. Stepwise thermal demagnetization revealed a high-temperature dualpolarity magnetization in most of the samples. The primary origin of this component was established by the positive reversal and conglomerate tests. We found a clear difference in angular dispersion of this component between the lower and upper portions of the section. In the former, the dispersion is close to the predictions of various models of geomagnetic secular variation, while the scatter in the top part of the sequence is smaller by a factor of four. We conclude that this distinction is most likely due to the geomagnetic field behavior and hypothesize that the magnitude of secular variation could vary severalfold over time intervals of 105−106 years. This is in sharp contrast with other models of secular variation, where this magnitude has been assumed to be rather time-independent (for a given latitude). We discuss probable implications and alternative interpretations of the observed data.

Archeomagnetism of Piton de la Fournaise: Bearing on volcanic activity at La Réunion Island and geomagnetic secular variation in Southern Indian Ocean

Historical and other recent lavas from Piton de la Fournaise (La Réunion Island) are studied using the large sample archeomagnetic method which provides here paleodirections of the Earth Magnetic Field with confidence cones between 0.9 and 2.5°, thus offering a precise record of the geomagnetic field and potential dating constraints. Ages of the lavas are known thanks to historical chronicles (from 1708) or 14C dating and their analysis give information about the directional secular variation (SV) in a region where observatory measurements are scarce and not available before the end of the 19th century. For the past 250yr we find a high value of inclination (−50 to −55°) with respect to the geographic latitude (21°S), connected with a very restricted directional SV. Conversely, the older volcanic units present a larger SV with magnetic inclinations of between −31° and −46°, and declinations from 16°W to 10°E. These results, which are in reasonable agreement with instrumental measurements made on ships in the vicinity of La Réunion, allow us to infer that undated lavas of the northern part of the caldera emanated from eruptions during the second half of the 1700s. Other volcanic products (e.g. Mare Longue flow, Piton Chisny cone and flows and Pointe Langevin), despite their fresh morphology, are necessarily older than about 1500AD. Further knowledge of the path of the SV in the more distant past, and therefore further archeomagnetic dating, is hampered by the lack of a precise chronology through radiometric or other similar methods. Available models of the geomagnetic field during the last millennium, which suffer from the scarcity of data in the Southern Hemisphere, are discussed in the light of our results.

The intensity of the geomagnetic field in the late-Archaean: new measurements and an analysis of the updated IAGA palaeointensity database

Abstract We firstly present the results of a detailed palaeointensity study performed on 54 samples from 9 volcanic units of late Archaean age (2724-2772 Ma) from the Pilbara Craton, Western Australia. These results were severely affected by magnetomineralogical alteration occurring during the laboratory heating process necessitating the application of a correction procedure. The correction allowed results from three lavas to pass strict selection criteria but we deem that only one of these exhibits sufficient internal consistency to be considered moderately reliable. It yields a virtual dipole moment of 47±6 ZAm2which is 60% of the present-day value. We combine this determination with a filtered dataset from the updated IAGA (International Association of Geomagnetism and Aeronomy) palaeointensity database, PINT08. Directional secular variation has recently been shown to have changed fundamentally since the Archaean, probably as a consequence of inner core growth since that time. However, here we argue that it is still unclear whether this evolution was accompanied by a single long timescale change in average poloidal field intensity. While the distribution of Precambrian palaeointensity determinations as a whole is significantly lower than that for the last 300 Myr, we show that this finding largely reflects data from the Proterozoic aeon. The distribution of more ancient measurements from the late Archaean-earliest Proterozoic is indistinguishable from that of the last 300 Myr which might suggest that a ‘Proterozoic dipole low’ period existed between two periods of higher field intensity. Were this pattern of long-term geomagnetic intensity variation to be supported by the addition of new data in the future, then it could indicate a related three-stage evolution in core dynamics, namely: vigorous thermal convection caused by high core-mantle heat flux early in the Earth’s history, weaker thermal convection later as the heat flux fell, and finally, strong compositional convection since the inner core nucleated.

Archaeomagnetic dating of a High Middle Age likely iron working site in Corroy-le-Grand (Belgium)

Archaeological burnt materials and structures provide unique records of direction and intensity of the Earth’s magnetic field in the past, elements that can be absolutely determined applying the archaeomagnetic method. At present, such records within Europe are irregular in both space and time. Presented here is the archaeomagnetic investigation of three kilns that were discovered during a preventive excavation of an archaeological site considered of High Middle Age in Corroy-le-Grand (Belgium) and that are assumed to be related to iron working activities. Archaeological context dating points to kiln operation between the second half of the 10th century until the 12th century AD. As the site is not far from Paris, declination and inclination of the characteristic remanent magnetisation of the kilns were compared with the standard directional secular variation curve for France in order to propose archaeomagnetic dates for the cessation of kiln operation by using probability densities [Lanos, Ph., 2004. Bayesian inference of calibration curves, application to archaeomagnetism. In: Buck, C.E., Millard, A.R. (Eds.), Tools for Constructing Chronologies: Crossing Disciplinary Boundaries. Lecture Notes in Statistics. Springer Verlag, London, pp. 43–82; Lanos, Ph., Le Goff, M., Kovacheva, M., Schnepp, E., 2005. Hierarchical modelling of archaeomagnetic data and curve estimation by moving average technique. Geophysical Journal International 160 (2), 440–476]. This confirms the presumed archaeological age and resulted in more precise time constraints for the last kiln operation. Rock magnetic techniques, proposed by Spassov and Hus [Spassov, S., Hus, J., 2006. Estimating baking temperatures in a Roman pottery kiln by rock magnetic properties: implications of thermochemical alteration for archaeointensity determinations. Geophysical Journal International 167, 592–604], were applied to examine the suitability of the burnt materials from the kilns for archaeointensity determinations and to increase the success rate of the Thellier–Thellier double heating technique. An average value for the field intensity of 69.4 ± 2.5 μT was estimated from 10 specimens from a single kiln, which corresponds reasonably well with published data for Western Europe.

A summary of Brunhes paleomagnetic field variability recorded in Ocean Drilling Program cores

The Ocean Drilling Program (ODP) has recovered many long sediment sequences from around the World that contain medium to high-resolution paleomagnetic records of Brunhes age. These records have provided an important new global view of geomagnetic field variability during ‘stable’ magnetic (dipole) polarity that could not be recovered by conventional terrestrial or deep-sea piston coring. The ODP paleomagnetic records of directional variability can be routinely recovered and used regionally as a very high-resolution relative chronostratigraphic tool (±200 yr resolution). ODP paleomagnetic records of Brunhes age relative paleointensity have dramatically improved our understanding of global geomagnetic field intensity. These records document the global-scale pattern of paleointensity and demonstrate its use as a high-resolution relative chronostratigraphic tool that under ideal conditions could have a precision of ±500 yr . ODP paleomagnetic records and associated oxygen isotope chronostratigraphies have also greatly improved our understanding of the number and ages of Brunhes geomagnetic field excursions, and their relationship to normal directional secular variation and paleointensity variability. Although significant progress has been made though ODP paleomagnetic studies of Brunhes-aged sediments, it is clear that more work is needed to further resolve and define the detailed global pattern of Brunhes PSV and its relationship to excursions and true polarity reversals.

Hierarchical versus stratification statistical analysis of archaeomagnetic directions: the secular variation curve for Hungary

SUMMARY Extended by 26 new data, the present archaeomagnetic data set for Hungary consists of 185 couples of inclination and declination distributed between BC 350 and AD 1800. We analysed these data by separately using a stratification and a hierarchical statistical method for unravelling the directional secular variation of the ‘local’ geomagnetic field. For the calculations we applied weighted summation and the moving window technique. In order to have a reasonably quantity of data in every window we selected 100 yr windows with 75 per cent overlaps between consecutive windows. Our calculations ended up with two, almost identical models of secular variation, despite of the widely different weightings that are required in the two approaches. Nevertheless, of the two solutions the one produced by the hierarchical modelling is preferred because the correct statistics in archaeomagnetism are of a hierarchical type.

High-resolution magnetostratigraphy of late quaternary sediments from Lake Baikal, Siberia: timing of intracontinental paleoclimatic responses

Sediment cores retrieved from 6 locations in Lake Baikal were subjected to a paleomagnetic study in order to establish detailed age models based on correlations of relative paleointensity records. Additional data were provided by calibrated accelerator mass spectrometry (AMS) 14C dating, as well as by documentation of geomagnetic excursions like Laschamp at ∼42 ka and Iceland Basin at ∼185 ka. Few intervals were affected by diagenetic features like selective reductive dissolution of magnetite and greigite mineralization (Demory et al., 2005-this issue), and those that were left out of paleointensity records. These records were tuned to the well-dated paleomagnetic record from ODP Site 984 Channell [Channell, J.E.T., 1999. Geomagnetic paleointensity and directional secular variation at Ocean Drilling Program (ODP) site 984 (Bjorn Drift) since 500 ka: comparisons with ODP site 983 (Gardar drift). J. Geophys. Res., B: [Solid Earth], 104 (10):22, 937-22, 951]. The complex shape of the resulting depth/age curves highlights the need for a high-resolution age model. We focused on the climatic boundary between marine isotopic stage (MIS) 7 and 6 where the Iceland Basin paleomagnetic excursion is clearly documented in the North Atlantic Channell et al.[Channell, J.E.T., Hodell, D.A., Lehman, B., 1997. Relative geomagnetic paleointensity and d 18O at ODP Site 983 (Gardar Drift, North Atlantic) since 350 ka. Earth Planet. Sci. Lett., 153 (1–2), 103-118] and in Lake Baikal Oda et al. [Oda, H., Nakamura, K., Ikehara, K., Nakano, T., Nishimura, M., Khlystov, O., 2002. Paleomagnetic record from Academician Ridge, Lake Baikal: a reversal excursion at the base of marine oxygen isotope stage 6. Earth Planet. Sci. Lett., 202 (1), 117-132] ; present study). During this period, we provide evidence for a return to cold conditions in the Lake Baikal region simultaneous to the sea surface cooling, but earlier than the global ice volume change observed in North Atlantic planktonic and benthic δ 18O records, respectively. The classical strategy of age model reconstruction, based on direct correlation of the climatic record from Lake Baikal sediments with the marine δ 18O reference curves is shown here to be unreliable. Moreover, this strategy does not consider (i) the nonlinearity of the age model in Lake Baikal sediments and (ii) the time lags between the global ice volume change and sea surface cooling observed in δ 18O marine records. Finally, the “Baikal 200” compilation of the paleointensity records established in this study provides a 200-ka-long synthetic paleomagnetic record for Central Eurasia.

Recent achievements in archaeomagnetism in Hungary

Over the last six years the Hungarian archaeomagnetic database has been increased by the addition of 60 new directional results. At present there are a total of 159 archaeomagnetic directions with age estimates falling between BC 350 and AD 1800. Using this more extended data set a new model of the directional secular variation (DSV) was derived for Hungary (Budapest), which is considered an improvement relative to that known six years ago. The main features of the model are an almost sinusoidal variation of the inclination between 58° and 72° and a more complicated variation of the declination between −15° and 25° with several changes of sign. Comparison of this new model DSV with those derived by Daly & Le Goff for other European ‘world sites’ shows general agreement as well as important differences, such as the occurrence of negative declinations during early medieval times, which apparently are not present in the DSV for regions west of Hungary.

Holocene paleomagnetic records from the St. Lawrence Estuary, eastern Canada: centennial- to millennial-scale geomagnetic modulation of cosmogenic isotopes

Two long Holocene piston cores (MD99-2220 and MD99-2221) were raised from the St. Lawrence Estuary, eastern Canada because of the expanded Holocene sediment sequence this location provides. A u-channel-based paleomagnetic study, supported by an accelerator mass spectrometry (AMS) 14C chronology, rock-magnetic and sedimentological data, indicates that these sediments provide a paleomagnetic directional secular variation (PSV) and relative paleointensity (RPI) proxy records for the last ∼8500 cal BP. Sedimentation rates vary from ∼1.2 to 4.2 m/kyr. Comparison of inclination and declination features with other North American Holocene PSV records are generally consistent and temporal offsets are within chronological uncertainties. The normalized natural remanent magnetization intensity record, a RPI proxy, from the postglacial sediments of core 2220 compares favorably with North American and European Holocene RPI records at millennial and even some centennial timescales. Comparisons between core 2220 RPI proxy and the 10Be flux record from the Greenland Summit (GISP2) ice core [Finkel and Nishiizumi, J. Geophys. Res. 102 (1997) 26699–26706] and a 14C production rate record [Bond et al., Science 294 (2001) 2130–2136] suggest that geomagnetic modulation may control the millennial- and even some centennial-scale variability within cosmogenic isotope records. This implies that the core 2220 RPI record reflects changes in global-scale geomagnetic field at these timescales.

Three millennia of directional variation of the Earth’s magnetic field in western Europe as revealed by archeological artefacts

Directional secular variation of the geomagnetic field over the last 2000 years has been defined in western Europe from numerous archeomagnetic studies. However, the number of archeomagnetic results for older periods is much more limited. For this reason, we present new data obtained from fired archeological structures found in two French sites (Loupiac and Aspiran) dated within the first millennium b.c. (latest Bronze-earliest Iron Age transition, ∼850–700 b.c. and Iron Age, ∼525–475 b.c., respectively). From a compilation of archeomagnetic results from western Europe (Great Britain, Italy and France) and northern Africa (Tunisia), we propose a directional secular variation curve for western Europe that covers the entire first millennium b.c. This curve exhibits a large clockwise motion with rapid changes during the first half of the millennium, while the last four centuries b.c. are characterized by weak variations.

Geomagnetic paleointensity and directional secular variation at Ocean Drilling Program (ODP) Site 984 (Bjorn Drift) since 500 ka: Comparisons with ODP Site 983 (Gardar Drift)

Normalized remanence (relative geomagnetic paleointensity) records for the last 500 kyr can be matched between two Iceland Basin Ocean Drilling Program (ODP) sites (984 and 983) and correlated with other high‐resolution records. Directional secular variation is, however, not easily correlated between sites due to inadequate recording, at these mean sedimentation rates (12–15 cm/kyr), of the characteristic high frequency variability of the directional record. Both sites record the Iceland Basin event at ∼186–189 ka in which the characteristic magnetization component rotates through 180° and back, coincident with a paleointensity low that lasted about 3 kyr. Other Brunhes Chron geomagnetic excursions appear to be manifest as intervals of higher‐amplitude secular variation during lows in paleointensity. There is a tendency for paleointensity lows to correlate with peak interglacials in the oxygen isotope records; however, this does not translate into a correlation between paleointensity and percent carbonate or between paleointensity and magnetic concentration or grain size parameters.

Geomagnetic long-term secular variations in Italian Lower Cretaceous shallow-water carbonates

SUMMARY The remanent magnetic properties of an 88 m bore core are unrelated to either the dolomite content or the sedimentological textures and are considered to be carried primarily by biogenetic magnetite that was cemented in during very early diagenesis. Individual readings represent time intervals of c. 720±32 yr and, after 40 mT partial demagnetization, they provide an almost continuous record of averaged geomagnetic secular variations over a period of some 3.17 Myr. The magnitude of directional secular variation is twice that of the present day, despite being smoothed, and the secular variations appear to grade into polarity transitions, suggesting no diVerence in their mechanisms. The rates of change in direction between subjacent levels in the core have a log-normal distribution which extends smoothly beyond 90° and has a median value of 13°/700 yr, the same as for unsmoothed European secular variation during the last 2000 yr. The intensity of remanence, after 40 mT partial demagnetization, appears to provide a reasonable approximation to geomagnetic field intensity. This tends to be weaker when the direction is moving faster, reflecting averaging, but is unrelated to the distance of the vector from the mean direction; that is, it depends on the rate of change and not on the virtual pole latitude. The virtual poles, after correction for tectonic rotations about horizontal and vertical axes, have latitudes that form a strongly platykurtic Fisherian distribution, while their longitudes have a circular distribution on which are superimposed two Gaussian peaks, 180° apart. This bore core thus provides detailed information of smoothed geomagnetic secular variation in the Lower Cretaceous (127±3 Ma) which shows clear regularities in behaviour, some related to changes in the Earth’s orbital parameters.

Last 160 Ka Paleomagnetic Directional Secular Variation Record from Core MD972151, Southwestern South China Sea

The ground-based magnetic signature of particle precipitation fromthe ring current to low latitude regions during the recovery phase of a geomagnetic storm on 23 December,1995,has been inferred by studying photometric 630.0nm nightglow at a low latitude station,Kolhapur(7.5°N,144.7°E; Geomagnetic,dip latitude 10.6° N) and magnetograms(H,D andZ)of nearby low latitude stations.The maximum optical emission occurredsimultaneously with the maximum positive(north ward)excursions in theH trace of low latitude magnetograms associated with negative H excursions at high latitude observatories.Generally,an increase in Z componentoccurred at all low latitude stations.A large intensity increase of aboutseven times in 630.0nm emission was observed at around 2:15 hrs I.S.T on23 December,1995.The 630.0nm enhancement maximised at the time ofhigh Dst or Kp index during the night.In addition,the ionosonde experiment conducted at Ahmedabad(13.8°N,144.7°E;Geomagnetic)showed theenhancement of peak electron densities in the F-region and sudden lowering of the F layer in the ionosphere.A possible explanation of the effect isgiven in terms of emergence of energetic neutral atoms(Hydrogen or Oxygen) from high energy protons in a typical charge exchange process in thering current depositing energy into the thermosphere-ionosphere system,also possibly due to changes in the composition of the thermosphere-ionosphere during magnetic storms that would enhance the 630.0nm emissions.

The direction of geomagnetic field in Belgium since roman times and the reliability of archaeomagnetic dating

Local geomagnetic secular variation curves are needed for improving models of the palaeofield changes and to increase the accuracy of archeomagnetic dating. Archaeomagnetic results from 34 baked structures are given as a first trial to construct directional secular variation curves for Belgium for the period from Post-Roman until the 18th century A.D. Often regular and irregular directional deviations are noticed in the walls of kilns. Large deviations of the remanent magnetization direction found in a big, circular, medieval pottery kiln can be explained by a combination of wall movement and magnetic distortion effects as the main origin. Two quicklime kilns, with unclear relationship to other features of the archaeological sites, were archaeomagnetically dated applying the British and French secular variation master curves for inclination and declination as a reference. This example highlights some of the difficulties encountered when applying existing secular variation master curves for dating.