Geomagnetic intensity variation during the last 4000 years

Physics of the Earth and Planetary Interiors, 56 (1989) 49—58 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

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Geomagnetic intensity variation during the last 4000 years Martin J. Aitken, Adrian L. Ailsop, Gillian D. Bussell and Mona B. Winter Research Laboratoryfor Archaeology and the History ofArt, Oxford University, 6 Keble Roa4 Oxford OX1 3QJ (UK)

(Received November 10, 1987; revision accepted March 14, 1988)

Aitken, M.J., Alisop, Al., Bussell, G.D. and Winter, M.B., 1989. Geomagnetic intensity variation during the last 4000 years. Phys. Earth Planet. Inter., 56: 49—58. Thellier-type determinations of the ancient geomagnetic intensity are reported for a large number of samples from Greece, comprising ancient pottery and other archaeologically dated forms of baked clay. Comparison is made with results for other parts of the world of about the same latitude: the Near East, China and the western U.S.A. These data indicate that, in the time-range studied, variation of the geomagnetic dipole moment is unlikely to have been the dominant influence; it is suggested that the westward drift of a non-dipole disturbance could have been responsible.

1. Introduction In an earlier paper (Aitken et al., 1984) we reported archaeomagnetic intensity values for the Near East (i.e., Western Asia and Egypt) showing that at 1000 BC the intensity was some 60% higher than the present-day value, and gave a detailed plot showing how it had risen monotonically to that value from having been some 10% below the present-day value at 1800 BC. The results were obtained using a version of the Thellier thermal remagnetization technique on some 50 archaeologically well-dated samples of pottery fragments, ‘Royal’ bricks and funerary cones; 3 mm x 3 mm cores were extracted and a cryogenic magnetometer used for measurement. Extensive results for Greece reported here (a preliminary version was given by Aitken et al. (1983)) confirm that the high values also occurred in this adjacent region, and a broad maximum centred on 600 BC is indicated. We have also obtained results for China (Wei et al., 1987), and these too show the occurrence of a comparably high intensity. High values have also been reported for the western U.S.A. (Champion, 1980), for Hawaii (Coe et al., 1978) and for Japan (e.g., —

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Sakai and Hirooka, 1986) as well as for Bulgaria (Kovacheva and Kanarchev, 1986), Egypt (Games, 1979) and from the U.S.S.R. (see Burlatskaya, 1983). In addition, there are less comprehensive series of results in which substantially strengthened intensity has been indicated. World-wide averaging of early results (Smith, 1967) also showed strong fluctuations and the interpretation was in terms of variation in the strength of the Earth’s dipole moment (Cox, 1968). This was also the case in more recent analyses of world-wide data (Barton et al., 1979, using Burlatskaya and Nachasova, 1977; McElhinny and Senanayake, 1982). However, comparison of data sets around the world indicates that the variations were not synchronous between regions. Thus an alternative explanation is in terms of non-dipole disturbance; this would also be consistent with the rapidity of the variation indicated by detailed measurements in a single region. In this paper, we shall give the evidence for non-synchroneity and further suggest that the major maxima in each region could be the result of westward drifting of a single non-dipole disturbance; earlier interpretation of intensity data in terms of westward drift was made by Bucha (1970).

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Of course, there is also the possibility of enhanced intensity due to successive tilting of the dipole towards the regions concerned. For the mid-latitudes concerned, this alone would not be sufficient to explain the 60% increase even if the region had been over the magnetic pole, but there is the possibility it was a strengthened dipole that was tilting. However, inspection of available archaeomagnetic inclination data gives no encouragement to pursue an explanation along these lines,

2. Measurement technique As the measurement technique has been discussed elsewhere (Aitken et al., 1986), with particular emphasis on the question of reliability, we will do no more now than reiterate five aspects: (1) The duration of each heating/cooling step does not exceed 5 mm, thereby minimizing risk of mineral alteration. Because the cooling time in antiquity was probably 1 or 2 days, a correction is necessary on account of the enhanced thermoremanent magnetization (TRM) when the cooling is slow (Fox and Aitken, 1980); this correction was evaluated experimentally for one core of each sample, usually being in the range 0—13%. In a second magnetometer facility the maximum duration of heating/cooling was 30 mm and for this the corrections were in the range 0—5%. (2) Each core is orientated during mounting so that its natural remanent magnetization (NRM) direction is within 250 of that of the applied remagnetizing field. This reduces the correction necessary on account of grain anisotropy (Rogers et al., 1979), typically to <5%. If the effect of grain anisotropy is ignored, and the two directions differ by approaching 90°, errors of ±50% are possible for pottery (though less in the case of bricks for which the grain anisotropy is weaker). (3) The applied remagnetizing field is adjusted to be within 10% of the expected ancient field. Ideally, it is a null technique (as first proposed by Walton (1977); later by Tanaka and Kono (1984)) in which the moment after remagnetization stays the same in successive temperature steps. Besides making negligible the effect of any differences in —

temperature between the demagnetizing step and the remagnetizing step, this procedure avoids problems arising from the non-ideal behaviour of multidomain grains (Levi, 1977). (4) We impose very strict criteria before the result for a core is accepted as reliable (in which case it is grade 1 or 2; otherwise it is grade 3, 4 or 5). Further, at least two cores are measured for each sample and unless all cores are of acceptable reliability and give results which lie within a span of 5% the sample is rejected. (5) We aim to obtain several results per century whenever possible, so that their coherence may be judged. We favour a diversity of samples (i.e., different types of clay, different firing conditions, different burial conditions) so if any systematic error were to escape detection by the criteria it would be likely to be indicated by lack of coherence.

3. Results In the figures, the paleointensity results are represented by the ratio FA/FD, where FA is the determined ancient intensity (after correction for cooling rate and grain amsotropy) and F D is the intensity at the site resulting from a centred axial dipole of present-day strength, 8 x 1022 A m2, being calculated from the latitude, A, according to 1) 31’4 2 \1/2 F 3 cos A ,sT This representation makes first-order correction for differences in latitude, being equivalent to the VADM (virtual axial dipole moment) representation of Barton Ct al. (1979). The vertical error bars show the standard error on the mean value for the context concerned; when there is only one sample for a context this is taken as 6% and in other cases (6/N1/2)% is set as a minimum value (where N is the number of samples), to avoid unjustified optimism about accuracy. The value of 6% for a single result is derived from comparison of results for samples of the last 150 yr with observatory-based values (Aitken et a!., 1986); it is also consonant with the observed scatter in most contexts, though for one or two the standard deviation reaches 10%. Possi— —

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51 2.2 2.0

2000

1000

0

BC

DATE

1000

2000

AD

Fig. 1. Results for Greece are expressed as the ratio FA/FD, where FA is the ancient intensity and F’~is the intensity at the site owing to an axial centred dipole of present-day strength. The bulk of the results in the BC period are from Crete, Athens and Lefkandi, lying within an area spanning 10 of longitude and 30 of latitude, centred on 23.5 °Eand 370 N. For the AD period the sites span 21—24°E and 37—41°N. The ‘reference band’ has been drawn by hand (see text). The absence of horizontal bars in some cases indicates very precise dating (for the basis of the vertical error bars, see text). Data points lying outside the reference band are regarded as deviants (probably a result of archaeological misattribution) except for the points at AD 1300, which come from southern Greece; these are discussed elsewhere (Aitken et al., 1989). The short solid curve is based on values derived from the spherical harmonic fit to the historical data made by Thompson and Barraclough (1982).

ble reasons for enhanced scatter are (1) inaccurate archaeological date attribution, (2) local distortion of field during firing owing to proximity of iron objects or slag, or to the magnetism of the clay itself, and (3) inaccurate paleointensity determination despite passing our criteria. In a few cases, one sample from a context gives a value substantially different from the average of the other members of the group; such samples have been judged as deviants and are shown as a separate datum point. Except for most of the data from the western U.S.A., for which calibrated radiocarbon dates have been used, the basis of dating is either archaeological or historical. For our own results most of the samples are dated both stratigraphically and stylistically, or by inscriptions (e.g., the ‘Royal’ bricks of Mesopotamia); in general, we have only used samples which are closely dated,

usually to better than ±25 yr and worse than ±50yr in only one or two cases. From the data for Greece (Fig. 1) we have established a reference band for convenience of comparison with data from elsewhere. The minimum vertical width for this band has been set at ±5%of the value for the ratio, the band being widened where the data show more scatter so as to allow error bars to reach it (except in the case of deviants). Of course, it will be desirable eventually to employ a proper statistically based procedure. In Fig. 2 we reproduce the 500-yr average values for Europe from McEthinny and Senanayake (1982), none of our data being included. The agreement of our reference band is satisfactory, bearing in mind that the high intensity at 600 BC did not persist long enough to be reflected fully in the relevant 500-yr average. Figures 3, 5 and 6 show our data for other regions of the —

52 2.2

2.0 1.8

~ 2000

~—

1000

0

BC

DATE

1000

2000

AD

Fig. 2. Ratio values for European region derived from averages calculated by McElhinny and Senanayake (1982), with reference band from Fig. 1 superimposed. The averages do not include the data of Fig. 1. The vertical error bars represent the 95% confidence limits 1”2) where a is the standard deviation of the N values used for the average (the span of which is indicated by obtained as ±(2 a/N the horizontal error bars). 2.2

2.0 1.8

:1:

6

2000

1000

0

BC

DATE

1000

2000

AD

Fig. 3. Results for China (Wei et al., 1987). The sites used lie between longitudes 105 and 120 °Eand latitudes 30 and 40°N, with a centroid around 113°E, 33°N. The points represented by unfilled squares are of poor reliability. The Greek reference band is superimposed.

53 2.2 2.0

2000

1000

0

1000

2000

BC DATE AD Fig. 4. Results for western U.S.A. taken from Champion (1980) and Sternberg and Butler (1978), represented by unfilled and filled squares respectively. Where dating is by radiocarbon, conversion to calendar years has been made. Error bars are as given by the authors. The Greek reference band is superimposed.

2.2 2.0 1.8

91,:

2000

1000

0

1000

2000

BC DATE AD Fig. 5. Our results for India. The sites used lie between longitudes 72 and 820 E and latitudes 22 and 29° N, with a centroid around 75°E, 24°N. The Greek reference band is superimposed.

54 2.2

2000

1500

1000

BC

500

0

DATE

Fig. 6. Results for Near East (Aitken et al., 1984). The sites used lie between longitudes 33 and 48°E and latitudes 26 and 36°N, with a centroid around 36°E, 31°N. The basis of the error bars is as for Fig. 1. The Greek reference band is superimposed.

world, and Fig. 4 shows the data of Champion (1980) and Sternberg and Butler (1978) for the western U.S.A. In all cases, the reference band is that of Fig. 1.

4. Discussion To justify the proposal of an interpretation at variance with the commonly held view that the dipole moment is the dominant influence in paleointensity change, we first briefly comment on the basis for that view, 4.1. Historical perspective The notion of strong dipole variation, with a periodicity of the order of 10000 yr, arose from the averaging of world-wide paleointensity data (Smith, 1967; Cox, 1968). It was then reinforced by the explanation that it apparently provided for the long-term variation of atmospheric 14C variation (see Olsson, 1970) in terms of geomagnetic modulation of cosmic-ray flux; a sinusoidal varia-

tion was assumed for each quantity and the corre!ation obtained was highly encouraging. Subsequently, Barton et al. (1979), from analysis of the world-wide data provided by Burlatskaya and Nachasova (1977), concurred that the simplest model was in terms of dipole change but noted that there were data (e.g., Yukutake and Tachinaka, 1969; Coe et a!., 1978) indicative of strong non-dipole contributions; they suggested that the “alleged sinusoidal variation of the dipole field... should be regarded as highly tentative”. McElhinny and Senanayake (1982) too, although they interpreted their own average values of world-wide paleointensity data in terms of dipole change, found “no clear evidence for any sinusoidal variation”. Thus it would seem that one of the principal assumptions that was used in showing that l4~ variations are the result of geomagnetic modulation is seriously in question. 4.2. The cosmogenic isotope record The other principal assumption was that the 14C variation was sinusoidal. An important sup-

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porting data set (Tauber, 1970) has now been changed and the 14C measurements reported by Becker and Kromer (1986) and by Stuiver et a!. (1986) indicate a substantially higher atmospheric concentration at 9000 yr BP and beyond that would fit the sinusoidal model. Belief in a geomagnetically driven sinusoidal component can only be retained if it is assumed that climatic changes towards the end of the last ice age have had a strong influence: this may, of course, have been the case but the need for a further assumption does weaken the argument. Without reliance on any hypothetical periodicity, Barton et al. (1979) derived values for the dipole moment corresponding to observed 14C concentrations, on the assumptions that the ‘4C variations are solely of geomagnetic origin, and that the production and decay of ‘4C are instantaneously balanced; in fact, owing to reservoir mixing there is a substantial lag of concentration variation behind production variation as well as substantial attenuation. From fig. 3 of Barton et al. (1979) we see that half of the required increase in dipole moment has occurred by 3500 yr BP, which calibrates to around 1850 BC in calendar years (Pearson and Stuiver, 1986). From fig. 2 of McElhinny and Senanayake (1982), we see that this would fit well with the presumed change in dipole moment were it not for the 1000-yr lag suggested by Barton et al. on the basis of Houtermans (1966); other inconsistencies of timing were also noted by Barton et al. Thus, although the 14C evidence for dipole moment variation is persuasive, it is not conclusive, Recently, in discussion of ‘°Be measurements from the Camp Century ice core, Beer et al. (1988) have demonstrated consistency of the 14C record with the indications from 10Be. However, although they noted that their high-latitude ~ record was not ideal for studying dipole moment variations, they considered that “the ‘°Bedata do not give support to the hypothesis that the observed slow 14C decrease was due to a gradual geomagnetic field change”. Thus the evidence from the cosmogemc record is by no means so emphatic that alternatives to the hypothesis of dipole variation should not be considered. —

4.3. Geomagnetic evidence: limitations in averaging Spherical harmonic analysis of geomagnetic data from the last few centuries (e.g., Thompson and Barraclough, 1982) gives direct indication of change in dipole moment; however; this is not of a magnitude, or over a time period, comparable with the variation that is under discussion. Otherwise the geomagnetic evidence is from the averaging of world-wide data, with support from consonance of the findings with expectations based on the present-day field. As the authors involved point out, the geographic coverage of the averaging is far from adequate; McElhinny and Senanayake (1982) used a total of 1200 results but 750 of these came from one-eighth of the Earth’s surface (the European region). There is also risk of interference by a reinforcement syndrome: once the broad pattern of dipole variation had been proposed by Cox (1968) there would have been a tendency for experimentalists to scrutinize very closely those results not fitting the pattern, effectively applying stricter discard criteria in consequence. 4.4. Non-dipole interpretation: indications of westward drift On account of the foregoing, rather than interpret the results presented in the figures in terms of non-dipole modification of a dipole-dominated variation, we follow an alternative approach. The dominant feature of the reference band derived from Fig. 1 is the maximum at 600 BC when the intensity ratio reached 1.65 ±0.1; this is followed by a shallow minimum at AD 100, with tenuous indications of a weaker maximum later. For China (Fig. 3) there is a comparable pattern but occurring earlier by about 600 yr. There is a comparable pattern too for the western U.S.A. (Fig. 4); the data are sparse but it is apparent that the maxima occur 800—1000 yr later than for China. It is possible that these variations in timing result from separate non-dipole disturbances which, by chance, give similar patterns. An alternative interpretation is that the main influence was a single non-dipole disturbance drifting westwards. That the patterns are not exactly —

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56 TABLE I

Dates of paleointensity maxima and implied rates of westward drift Date range China Greece West U.S.A. China Greece West U.S.A.

1400-1000 BC 800- 400 BC 600- 400 BC 1 500 AD 500-1000 AD 1100-1300 AD

Implied drift rate (°long a~)

>

0.1-0.4 ~ 3

~

0.2-1.4

Central longitudes for sampling areas are: Greece 23°E, China wes . . .

the same does not rule this out—it cannot be expected that the disturbance will remain unmodified as it moves, nor that all other disturbances are absent. An extension of the westward drift interpretation is to consider the second maximum in each region as the second time round of the disturbance which caused the first. In Table I we list the dates of successive appearances together with the rates of drift that are implied; inevitably, given the poor definition of some of the maxima, these rates are no more than rough estimates. However, none are unacceptably different from other estimates of drift, such as the value of 0.20 a1 derived by Bullard et a!. (1950) from observatory measurements of this century, a similar value derived by Bucha (1970) from earlier intensity data, and the values derived by various Soviet archaeomagnetic workers (see Burlatskaya, 1983) from longitudinal movement of the last inclination maximum. However, the persistence of the disturbance is substantially longer than the usually accepted few hundred years (indicated by archaeomagnetic measurements of the present millennia; see Aitken and Weaver (1964)) though in line with the conclusions drawn from recent directional measurements on a Mexican stalagmite by Latham (1989). The limited data we have for India (Fig. 5), approximately midway between Greece and China, are too sparse to do more than indicate that the intensity was high in that region at 900 BC; this is not inconsistent with expectation from the westward drift hypothesis. Closer still to the reference region is the Near East (Fig. 6), and there we see —

that there was a tendency for the rise to the first maximum to occur sooner than in Greece. The samples for the Near East come from a region whose central point is some 170 further east than that for the Greek samples. For the samples in the range 1200—1000 BC the horizontal distance from the reference band is 100—200 yr, corresponding to a westward drift rate would of 0.17_0.090 a1. ofAna alternative interpretation be in terms systematic dating error m one or other of the regions. In the period covered by Fig. 6 the chronological system on which the Greek samples is based is derived from archaeological linkages with the Near East. Although it has been recently suggested (Betancourt, 1987) that earlier than 1300 BC the Greek chronology should be lengthened there is no argument about reliability of the linkages in later centuries. 4.5. Other data The results for Egypt obtained by Games (1979) are in agreement with those of Fig. 6 up to 1300 BC, but around 1000 BC they show intensities that are lower by a factor of more than two. The technique used was a non-thermal one and the samples were sun-dried bricks; we have to presume that during the period of discordance the technique of brickmaking had been changed to one that was adverse to obtaining reliable results by this technique. As regards the results of Sakai and Hirooka (1986) for Japan (some 230 east of the region sampled in China for Fig. 3), although there is some concordance with China and there are high values, there is no minimum at 400—1 BC. At face value one has to assume that a secondary localized disturbance was affecting Japan. Finally, there are the results for Bulgaria, about 50 north of the centre of the area sampled in Greece. Referring to Fig. 6 of Kovacheva and Kanarchev (1986), one sees that there were high values there too and that our Greek results can be accommodated within the 95% confidence limits shown. The results from Hawaii of Coe et a!. (1978) provide an example of high intensity from low latitude. The data points around 5000—4000 yr BP indicate an intensity that is nearly twice the —

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world-wide average of McElltinny and Senanayake (1982). Deviation from the world-wide average is also evident in the results from the U.S.S.R. (see Burlatskaya, 1983) as well as from the analyses of Yukutake and Tachinaka (1969).

5. Summary In the foregoing we have presented a comprehensive archaeointensity data set for Greece and made comparison with other regions. The dominant feature of high intensity was present in these also but, where defined, the pattern was displaced in time. This excludes a straightforward explanation solely in terms of an enhanced dipole moment. Going a step further we suggest that the cause could be a long-lived non-dipole disturbance which is drifting westward, though we accept that the evidence for westward drift is far from conclusive. We note the general acceptance that observed intensity variation is dominated by influence of the dipole moment. The strongest evidence for this is from dorrelation with variations in atmospheric ‘4C concentration; however, excluding the analyses based on sinusoidal models (which lack basis in observational data), there are significant inconsistencies of timing. There is no compelling evidence of dipole primacy from iOBe measurements.

Acknowledgements We are highly appreciative of the helpful collaboration of Hector Catling, Roger Moorey, Mervyn Popham, Sinclair Hood, Ken Wardle and Vassos Karageorgliis. The samples from Greece were obtained with the kind permission of the Inspector of Antiquities of the Greek Archaeological Service. The work was supported in part by the Science-based Archaeology Committee of the U.K. Science and Engineering Research Council and the Natural Environment Research Council. Earlier members of our team who contributed substantially were P.A. Alcock, J.M.W. Fox, V. Jones, J. Rogers and C.J. Shaw.

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