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Some evidence for sharp changes in the archaeomagnetic intensity variation during the last 2000 years
Physics ofthe Earth and Planetary Interiors, 70(1992) 85—89 Elsevier Science Publishers B.V., Amsterdam
85
Some evidence for sharp changes in the archaeomagnetic intensity variation during the last 2000 years a Research
Y. Liritzis a and M. Kovacheva b Centre for Astronomy and Applied Mathematics, Academy of Athens, 14 Anagnostopoulou str., Athens 10673, Greece b Geophysical Institute, Bulgarian Academy of Sciences, Ac.G.Bonchev str., Block 3, Sofia 1113, Bulgaria (Received 31 May 1990; revision accepted 19 July 1991)
ABSTRACT Liritzis, Y. and Kovacheva, M., 1992. Some evidence for sharp changes in the archaeomagnetic intensity variation during the last 2000 years. Phys. Earth Planet. Inter., 70: 85—89. An assessment of archaeomagnetic intensities from Greek and Bulgarian data sets during the last 2000 years indicates sharp changes in their variation. Particular emphasis is given to the reliability of these data and the results are discussed in terms of a latitude effect.
1. Introduction Some early reports of rapid changes in the secular variation of geomagnetic intensity puzzled geophysicists (see, for example, fig. 2a of Bucha (1970), for Mexican materials). Since then, several workers have presented palaeointensity results from ancient ceramics which show dramatic short-term changes of the geomagnetic field intensity (Nagata et a!. (1963), for lavas from Japan; Sasajima (1965), for Japanese archaeological material; Shaw (1979), for French material; Champion (1980), for western USA material; Games (1980), fig. 5, for Egyptian material; Kovacheva (1980), for Bulgarian material; Burakov et al. (1982), for Soviet material; Hirooka (1983), for Japan of the last 1300 years; Walton (1983, 1984), for Greek material; Aitken et al. (1983), for Near Eastern, Egyptian and Mesopotamian material, especially in the date range 1300—500 BC; Wei et al. (1983, 1987), for China in the last 2000 years; Kono et a!. (1986), fig. 5; DuBois (1989); Sternberg (1989), for American material). A summary of most of this work has been given by Burlatskaya and Nashasova (1977) and Creer et al. (1983). 0031-9201/92/$05.00 © 1992
—
Considerable changes are also observed in the natural remanent magnetization (NRM), which is an approximate measure of the geomagnetic intensity, assuming a constant concentration of magnetic minerals from lake sediments (e.g. Creer et a!., 1983; Smith, 1985). The short-term variation in NRM or palaeodirection has been removed from other lake sediment results by application of various smoothing procedures (e.g. running means, Fourier smoothings, and construction of stacking and averaging curves), but the large-scale changes are still observable with a 500 year period (see, for example, Creer et al., 1983, Chapter 4). The origin of the sharp changes was attributed either to measurement and dating errors or local distortion of the field so that the results were considered as outliers, or to true periodic variations of the non-dipole field in the range 300—700 years. A smooth variation of the magnetic field was preferred to an oscillating variation. A!though the superimposition of quasi-periodic non-dipole field changes on the dipole variation is the accepted view, the sharp variations of significant amplitude were not easily resolved or
Elsevier Science Publishers B.V. All rights reserved
86
Y. LIRITZIS AND M. KOVACHEVA
accepted as true; they therefore merited a detai!ed and carefu! investigation, The reliable dating of specimens has been a major problem in resolving sharp geomagnetic intensity variations and has constrained the establishment and interpretation of the real nature of these changes. Therefore, the reduction of dating errors is of prime importance in the study of these rapid changes. One approach was to use samp!es from neighbouring Greek monasteries (situated in the same region) which were we!l dated by the inscriptions on the church wal!s (Aitken et a!., 1989). By using and comparing archaeointensity data from Greece and Bu!garia, this paper reassesses sharp variations of the field over a !atitudinal range of approximately 100. 2. Data The Greek data comprise nine ceramic sampIes (four of grade 1 or 2 and five of grade 3, where grades 1 and 2 are good, and grade 3 is poor), 22 tiles and ti!ebricks of grade 1 or 2 (Aitken et al., 1989) and 13 tiles and tilebricks of grade 3. The Thel!ier method (The!lier and The!her, 1959) was employed but for some of the samples, of grades A~or B~,a!ternating field (a.f.) technique of Shaw (1974) was used. The re!iabi!ity criteria have been described elsewhere
~
F~.6
fo15
H
0
F
~
(Aitken et a!., 1983, 1989). Grade 3 data, although poor, genera!ly fo!low the trend of grades 1 and 2 within a ±10%band around their mean variation (Liritzis, 1990). Intensity values from individual cores within a samp!e were averaged, as were results with very c!ose dates, whose error bars overlapped, all of the resu!ts being considered of equal weight. The Bu!garian data comprise 79 intensity values of grades A or B derived from the revised catalogue of Kovacheva (1991). The original values represent the weighted averages at the sites, F~av±U, taken from the revised catalogue. The weights of the individual experimental results have been explained by Kovacheva and Kanarchev (1986). The dating accuracy of all the data used is ±25 years at worse; most of them (about 60%) are in the range of ±1—10 years. The dating is secured by archaeological methods (coins, inscriptions, sty!e and paral!els) and, in a few cases, by the archaeomagnetic resu!ts. The archaeointensity errors are about ±5% per site average. In averaging adjacent dates, and when they are supp!ied with their attached errors as in the present work, the standard deviation of the mean (o) is taken as —
°~
2
~(F—F) N(N— 1)
E
D
C
B
A
1 2 3 4 5 6 7 ~, 9 10 11 212) 13 14 15 16 17 18 19 YEARS AD (x10 Fig. 1. Variation of the normalized archaeointensity values (FA/FD) during the last 2000 years for Greek and Bulgarian data. •, 35—37.5~N; +, 37.5—41.2°N; ~, 41.2—43.7°N; 0, 43.7—44.2°N.
EVIDENCE FOR ARCHAEOMAGNETIC INTENTSITY VARIATION CHANGES
where F is the average value of the intensity ratios and N is the number of averaged data. The data set is !imited to the last 2000 years because the samples have a better dating control and there are smaller time intervals between samples (Kovacheva, 1991). 3. Results and discussion
The archaeointensity data (FA) are presented as ratios, FA/FD, where FD is the intensity at the site’s latitude (A) due to an axially centred geomagnetic dipole value for a present-day moment of 8 x 1022 A m2, according to the formula FD = 31(4 3 cos2A)’~2~sT(Fig. 1). This norma!ization makes first-order correction for differences in latitude. Four latitudinal zones are used: (1) 35—37.5 °N,abbreviated L for low; (2) 37.5— 41.2 °N, abbreviated M for middle; (3) 41.2— 43.7 °N,abbreviated, H for high; and (4) 43.7— 44.2 °N,abbreviated H * for higher latitude. The choice of bands of approximately 2.50 (about 270 km) is to include possible local disturbances such as those found between southern and northern Greece around 1300 AD (Aitken et a!., 1989). ‘Reference bands’ are drawn by hand to include the data points and their error bars. The following remarks can be made. (1) An overall oscillating character in the vanation is evident for the four latitudinal zones. The sharp changes appear to occur in consecutive 200—300 year intervals and are commensurate with the non-dipole part of the recent (last 400 years) geomagnetic field. (2) Any averaging (smoothing) of geomagnetic data for the purpose of further data processing must be applied with great caution. Production of artificial data points (from interpolation) may induce erroneous conclusions, (3) Minima! extrema values reach approximately the same absolute FA values at about 1000, 1500 and 1850 AD. (4) For 200—650 AD the values for M, H and H * follow a similar trend but are shifted, with H preceding M by about 80—100 years. Coincidence of the two bands cannot be excluded, if error bars are considered, and if more data points were —
87
available. A sharp change occurs in the period 100—200 AD, the field changing by about 50%. High field values occurring in the Christian era (around 0 AD) have already been reported (see references in the Introduction). (5) During the period 900—1400 AD the overall trend is similar. The H latitude values are higher than the L values at around 1000 AD. If a shift is considered between intensity values of different latitudes, then peak (D) of the M zone corresponds to peak (D) of the H zone, and peak (C) of the L zone corresponds to (C) of the M zone and (C) of the H zone. The H * values follow the trend of H values. This shift amounts to about 150 years. (6) A similar trend is observed between the L and H zones for 1550—1650 AD and between M and H zones for around 1500 AD and around 1780—1900 AD. (7) A sharp change occurring at 1600—1800 AD is in agreement with modelled observatory and archaeomagnetic data (Barraclough, 1974; Thompson and Barraclough, 1982; Aitken et a!., 1989; Liritzis, 1990). (8) The mean decreasing trend of intensity is depicted as a solid line and resembles those for Japan (Sasajima, 1965), France (Thellier and Thellier, 1959) and the USSR (Burlatskaya, 1983). (9) The observed shift may imply a latitudinal drift of an oscillating and drifting source, which grows and decays in a periodic manner.
4. Conclusions The present evidence indicates that the sharp changes in the geomagnetic field are probably real. The observed period of about 200—300 years has been quoted in some earlier spectral analyses (Liritzis, 1986; Xanthakis and Liritzis, 1991), and a general decrease is noted for the last 2000 years. The oscillating variation is superimposed on the longer-term smoother variation of the main dipole field. Similar periodic patterns existing in the radiocarbon (14C) variation in the atmosphere may reflect these geomagnetic changes.
88
Acknowledgements We are grateful to many archaeologists for help during sampling and to Dr. R. Jones for reading and improving the style of the paper. y . Liritzis is indebted to Professor M. Aitken and his team at Oxford for their hospitality and for the measurements. He also appreciates financial assistance from Oxford University, as well as from Dr. D.F.O. Russe!l and The Russell Trust, the Academy of Athens and the European Science Foundation, for funding of various stages of the archaeomagnetic project.
References Aitken, M.J., Alcock, P.A., Bussell, G.D. and Shaw, C.J., 1983. Palaeointensity studies on archaeological material from the Near East. In: K.M. Creer, P. Tucholka and C.E. Barton (Editors), Geomagnetism of Baked Clays and Recent Sediments. Elsevier, Amsterdam, pp. 122—127. Aitken, MJ., Alisop, A.L., Bussell, G.D., Liritzis, Y. and Winter, M.B., 1989. Geomagnetic intensity measurements using bricks from Greek churches of the first and second millennia AD. Archaeometry, 31: 77—87. Barraclough, D.R., 1974. Spherical harmonic analyses of the geomagnetic field for eight epochs between 1600 and 1910. Geophys. J.R. Astron. Soc, 36: 497—513. Bucha, V., 1970. Influence of the Earth’s magnetic field on radiocarbon dating. In: I.U. Oisson (Editor), Nobel Symposium, Vol. 12. Radiocarbon, Variations and Absolute Chronology. Wiley, New York, pp. 501—511. Burakov, K.S., Burlatskaya, S.P., Nashasova, I.Ye and Chelidze, Z.A., 1982. The geomagnetic field in the Caucasus over the past 2000 years. Geomagn. Aeron., 22(3): 439—440. Burlatskaya, S.P., 1983. Archaeomagnetic investigations in the USSR. In: K.M. Creer, P. Tuchoilca and C.E. Barton (Editors), Geomagnetism of Baked Clays and Recent Sediments. Elsevier, Amsterdam, pp. 127—137. Burlatskaya, S.P. and Nashasova, I.E., 1977. Archaeomagnetic determinations of geomagnetic field elements. World Data. Materials of the World Data Centre, Soviet Geophysical Committee, Academy of Sciences of the USSR, Moscow, 112 pp. (in Russian with English summary). Champion, D.E., 1980. Holocene geomagnetic secular vanation in the western United States; implications for the global geomagnetic field. U.S. GeoL Surv. Open-File Rep. 80-824. Creer, KM., Tucholka, P. and Barton, C.E. (Editors), 1983. Geomagnetism of Baked Clays and Recent Sediments. Elsevier, Amsterdam, 306 pp.
Y. LIR1TZ1S AND M. KOVACHEVA
DuBois, R.L., 1989, Archaeomagnetic results from southwest UnitedStates and Mesoamerica and comparison with some other areas. Phys. Earth Planet. Inter., 56: 18—33. Games, K.P., 1980. Short period fluctuations in the Earth’s magnetic field. Nature, 277: 600—601. Hirooka, K., 1983. Results from Japan. In: K.M. Creer, P. Tuchoika and C.E. Barton (Editors) Geomagnetism of Baked Clays and Recent Sediments. Elsevier, Amsterdam, pp. 150-157. Kovacheva, M., 1980. Summarized results of the archaeomagnetic investigations of the geomagnetic field for the last 8000 yrs in south-eastern Europe. Geophys. J.R. Astron. Ko M. 1991. Revised catalogue of Bulgarian archaemagnetic data (in preparation). Kovacheva, M. and Kanarchev, M., 1986. Revised archaeointensity data from Bulgaria. J. Geomagn. Geoelectr., 38: 1297—1310. Kono, M., Ueno, N. and Onuki, Y., 1986. Palaeointensity of the geomagnetic field obtained from pie-Inca potsherds near Cajamarca, northern Peru. J. Geomagn. Geoelectr., 38: 1339—1348. Liritzis, Y., 1986. Maximum entropy and power spectrum analyses of geomagnetic intensity variations from archaeomagnetic data: Emphasis on the 200 year period. Earth, Moon, Planets, 34: 235—249. Liritzis, Y., 1990. Greek archaeomagnetic intensities: Some aspects of reliability and geophysical implications. Earth, Moon, Planets, 47: 1—13. Nagata, T., Arai, Y. and Mamose, K., 1963. Secular variation of the geomagnetic total force during the last 5000 years. J. Geophys. Res., 68: 5277—5281. Sasajima, S., 1965. Geomagnetic secular variation revealed in baked earth in west Japan (part 2): Changes of the field intensity. J. Geomagn. Geoelectr., 17: 413—416. Shaw, J., 1974. A new method for determining the magnitude of the palaeomagnetic field. Geophys. J.R. Astron. Soc., 39: 107—116. Shaw, J., 1979. Rapid changes in the magnitude of the palaeomagnetic field. Geophys. J.R. Astron. Soc., 39: 107—116. Smith, G., 1985. Late glacial palaeomagnetic field variations from France. PhD thesis, Department of Geophysics, Edinburgh University. Steinberg, R.S., 1989. Archaeomagnetic palaeointensity in the American southwest during the past 2000 years. Phys. Earth Planet. Inter., 56: 1—17. Thellier, E. and Thellier, 0., 1959. Sur l’intensité du champ magnétique dans Ic passé historique et géologique. Ann. Géophys. 15: 295—376. Thompson, R. and Barraclough, D.R., 1982. Geomagnetic secular variation based on spherical harmonic and cross validatory analysis of historical and archaeomagnetic data. J. Geomagn. Geoeiectr., 34: 245—263. Walton, D., 1983. Existing archaeointensity data for Greece. In: K.M. Creer, P. Tucholka and C.E. Barton (Editors), Geomagnetism of Baked Clay and Recent Sediments. Elsevier, Amsterdam, pp. 104—105.
EVIDENCE FOR ARCHAEOMAGNETIC INTENTSITY VARIATION CHANGES
Walton, D., 1984. Re-evaluation of Greek archaeomagnitudes. Nature, 310: 740—743. Wei, Q.Y., Li, T.C., Chao, G.Y., Chang, W.S., Wang, S.P. and Wei, S.F., 1983. Results from China. In: K.M. Creer, P. Tucholka and C.E. Barton (Editors), Geomagnetism of Baked Clay and Recent Sediments. Elsevier, Amsterdam, pp. 138—150. Wei, Q.-Y., Zhang, W.-X., Li, D.-J., Aitken, M.J., Bussell,
89
G.D. and Winter, M., 1987. Geomagnetic intensity as evaluated from ancient Chinese pottery. Nature, 328: 330— 333. Xanthakis, J. and Liritzis, Y., 1991. Geomagnetic field variation as inferred by archaeomagnetism in Greece and palaeomagnetism in British lake sediments since 7000 BC. Acad. Athens PubI. (in press).
85
Some evidence for sharp changes in the archaeomagnetic intensity variation during the last 2000 years a Research
Y. Liritzis a and M. Kovacheva b Centre for Astronomy and Applied Mathematics, Academy of Athens, 14 Anagnostopoulou str., Athens 10673, Greece b Geophysical Institute, Bulgarian Academy of Sciences, Ac.G.Bonchev str., Block 3, Sofia 1113, Bulgaria (Received 31 May 1990; revision accepted 19 July 1991)
ABSTRACT Liritzis, Y. and Kovacheva, M., 1992. Some evidence for sharp changes in the archaeomagnetic intensity variation during the last 2000 years. Phys. Earth Planet. Inter., 70: 85—89. An assessment of archaeomagnetic intensities from Greek and Bulgarian data sets during the last 2000 years indicates sharp changes in their variation. Particular emphasis is given to the reliability of these data and the results are discussed in terms of a latitude effect.
1. Introduction Some early reports of rapid changes in the secular variation of geomagnetic intensity puzzled geophysicists (see, for example, fig. 2a of Bucha (1970), for Mexican materials). Since then, several workers have presented palaeointensity results from ancient ceramics which show dramatic short-term changes of the geomagnetic field intensity (Nagata et a!. (1963), for lavas from Japan; Sasajima (1965), for Japanese archaeological material; Shaw (1979), for French material; Champion (1980), for western USA material; Games (1980), fig. 5, for Egyptian material; Kovacheva (1980), for Bulgarian material; Burakov et al. (1982), for Soviet material; Hirooka (1983), for Japan of the last 1300 years; Walton (1983, 1984), for Greek material; Aitken et al. (1983), for Near Eastern, Egyptian and Mesopotamian material, especially in the date range 1300—500 BC; Wei et al. (1983, 1987), for China in the last 2000 years; Kono et a!. (1986), fig. 5; DuBois (1989); Sternberg (1989), for American material). A summary of most of this work has been given by Burlatskaya and Nashasova (1977) and Creer et al. (1983). 0031-9201/92/$05.00 © 1992
—
Considerable changes are also observed in the natural remanent magnetization (NRM), which is an approximate measure of the geomagnetic intensity, assuming a constant concentration of magnetic minerals from lake sediments (e.g. Creer et a!., 1983; Smith, 1985). The short-term variation in NRM or palaeodirection has been removed from other lake sediment results by application of various smoothing procedures (e.g. running means, Fourier smoothings, and construction of stacking and averaging curves), but the large-scale changes are still observable with a 500 year period (see, for example, Creer et al., 1983, Chapter 4). The origin of the sharp changes was attributed either to measurement and dating errors or local distortion of the field so that the results were considered as outliers, or to true periodic variations of the non-dipole field in the range 300—700 years. A smooth variation of the magnetic field was preferred to an oscillating variation. A!though the superimposition of quasi-periodic non-dipole field changes on the dipole variation is the accepted view, the sharp variations of significant amplitude were not easily resolved or
Elsevier Science Publishers B.V. All rights reserved
86
Y. LIRITZIS AND M. KOVACHEVA
accepted as true; they therefore merited a detai!ed and carefu! investigation, The reliable dating of specimens has been a major problem in resolving sharp geomagnetic intensity variations and has constrained the establishment and interpretation of the real nature of these changes. Therefore, the reduction of dating errors is of prime importance in the study of these rapid changes. One approach was to use samp!es from neighbouring Greek monasteries (situated in the same region) which were we!l dated by the inscriptions on the church wal!s (Aitken et a!., 1989). By using and comparing archaeointensity data from Greece and Bu!garia, this paper reassesses sharp variations of the field over a !atitudinal range of approximately 100. 2. Data The Greek data comprise nine ceramic sampIes (four of grade 1 or 2 and five of grade 3, where grades 1 and 2 are good, and grade 3 is poor), 22 tiles and ti!ebricks of grade 1 or 2 (Aitken et al., 1989) and 13 tiles and tilebricks of grade 3. The Thel!ier method (The!lier and The!her, 1959) was employed but for some of the samples, of grades A~or B~,a!ternating field (a.f.) technique of Shaw (1974) was used. The re!iabi!ity criteria have been described elsewhere
~
F~.6
fo15
H
0
F
~
(Aitken et a!., 1983, 1989). Grade 3 data, although poor, genera!ly fo!low the trend of grades 1 and 2 within a ±10%band around their mean variation (Liritzis, 1990). Intensity values from individual cores within a samp!e were averaged, as were results with very c!ose dates, whose error bars overlapped, all of the resu!ts being considered of equal weight. The Bu!garian data comprise 79 intensity values of grades A or B derived from the revised catalogue of Kovacheva (1991). The original values represent the weighted averages at the sites, F~av±U, taken from the revised catalogue. The weights of the individual experimental results have been explained by Kovacheva and Kanarchev (1986). The dating accuracy of all the data used is ±25 years at worse; most of them (about 60%) are in the range of ±1—10 years. The dating is secured by archaeological methods (coins, inscriptions, sty!e and paral!els) and, in a few cases, by the archaeomagnetic resu!ts. The archaeointensity errors are about ±5% per site average. In averaging adjacent dates, and when they are supp!ied with their attached errors as in the present work, the standard deviation of the mean (o) is taken as —
°~
2
~(F—F) N(N— 1)
E
D
C
B
A
1 2 3 4 5 6 7 ~, 9 10 11 212) 13 14 15 16 17 18 19 YEARS AD (x10 Fig. 1. Variation of the normalized archaeointensity values (FA/FD) during the last 2000 years for Greek and Bulgarian data. •, 35—37.5~N; +, 37.5—41.2°N; ~, 41.2—43.7°N; 0, 43.7—44.2°N.
EVIDENCE FOR ARCHAEOMAGNETIC INTENTSITY VARIATION CHANGES
where F is the average value of the intensity ratios and N is the number of averaged data. The data set is !imited to the last 2000 years because the samples have a better dating control and there are smaller time intervals between samples (Kovacheva, 1991). 3. Results and discussion
The archaeointensity data (FA) are presented as ratios, FA/FD, where FD is the intensity at the site’s latitude (A) due to an axially centred geomagnetic dipole value for a present-day moment of 8 x 1022 A m2, according to the formula FD = 31(4 3 cos2A)’~2~sT(Fig. 1). This norma!ization makes first-order correction for differences in latitude. Four latitudinal zones are used: (1) 35—37.5 °N,abbreviated L for low; (2) 37.5— 41.2 °N, abbreviated M for middle; (3) 41.2— 43.7 °N,abbreviated, H for high; and (4) 43.7— 44.2 °N,abbreviated H * for higher latitude. The choice of bands of approximately 2.50 (about 270 km) is to include possible local disturbances such as those found between southern and northern Greece around 1300 AD (Aitken et a!., 1989). ‘Reference bands’ are drawn by hand to include the data points and their error bars. The following remarks can be made. (1) An overall oscillating character in the vanation is evident for the four latitudinal zones. The sharp changes appear to occur in consecutive 200—300 year intervals and are commensurate with the non-dipole part of the recent (last 400 years) geomagnetic field. (2) Any averaging (smoothing) of geomagnetic data for the purpose of further data processing must be applied with great caution. Production of artificial data points (from interpolation) may induce erroneous conclusions, (3) Minima! extrema values reach approximately the same absolute FA values at about 1000, 1500 and 1850 AD. (4) For 200—650 AD the values for M, H and H * follow a similar trend but are shifted, with H preceding M by about 80—100 years. Coincidence of the two bands cannot be excluded, if error bars are considered, and if more data points were —
87
available. A sharp change occurs in the period 100—200 AD, the field changing by about 50%. High field values occurring in the Christian era (around 0 AD) have already been reported (see references in the Introduction). (5) During the period 900—1400 AD the overall trend is similar. The H latitude values are higher than the L values at around 1000 AD. If a shift is considered between intensity values of different latitudes, then peak (D) of the M zone corresponds to peak (D) of the H zone, and peak (C) of the L zone corresponds to (C) of the M zone and (C) of the H zone. The H * values follow the trend of H values. This shift amounts to about 150 years. (6) A similar trend is observed between the L and H zones for 1550—1650 AD and between M and H zones for around 1500 AD and around 1780—1900 AD. (7) A sharp change occurring at 1600—1800 AD is in agreement with modelled observatory and archaeomagnetic data (Barraclough, 1974; Thompson and Barraclough, 1982; Aitken et a!., 1989; Liritzis, 1990). (8) The mean decreasing trend of intensity is depicted as a solid line and resembles those for Japan (Sasajima, 1965), France (Thellier and Thellier, 1959) and the USSR (Burlatskaya, 1983). (9) The observed shift may imply a latitudinal drift of an oscillating and drifting source, which grows and decays in a periodic manner.
4. Conclusions The present evidence indicates that the sharp changes in the geomagnetic field are probably real. The observed period of about 200—300 years has been quoted in some earlier spectral analyses (Liritzis, 1986; Xanthakis and Liritzis, 1991), and a general decrease is noted for the last 2000 years. The oscillating variation is superimposed on the longer-term smoother variation of the main dipole field. Similar periodic patterns existing in the radiocarbon (14C) variation in the atmosphere may reflect these geomagnetic changes.
88
Acknowledgements We are grateful to many archaeologists for help during sampling and to Dr. R. Jones for reading and improving the style of the paper. y . Liritzis is indebted to Professor M. Aitken and his team at Oxford for their hospitality and for the measurements. He also appreciates financial assistance from Oxford University, as well as from Dr. D.F.O. Russe!l and The Russell Trust, the Academy of Athens and the European Science Foundation, for funding of various stages of the archaeomagnetic project.
References Aitken, M.J., Alcock, P.A., Bussell, G.D. and Shaw, C.J., 1983. Palaeointensity studies on archaeological material from the Near East. In: K.M. Creer, P. Tucholka and C.E. Barton (Editors), Geomagnetism of Baked Clays and Recent Sediments. Elsevier, Amsterdam, pp. 122—127. Aitken, MJ., Alisop, A.L., Bussell, G.D., Liritzis, Y. and Winter, M.B., 1989. Geomagnetic intensity measurements using bricks from Greek churches of the first and second millennia AD. Archaeometry, 31: 77—87. Barraclough, D.R., 1974. Spherical harmonic analyses of the geomagnetic field for eight epochs between 1600 and 1910. Geophys. J.R. Astron. Soc, 36: 497—513. Bucha, V., 1970. Influence of the Earth’s magnetic field on radiocarbon dating. In: I.U. Oisson (Editor), Nobel Symposium, Vol. 12. Radiocarbon, Variations and Absolute Chronology. Wiley, New York, pp. 501—511. Burakov, K.S., Burlatskaya, S.P., Nashasova, I.Ye and Chelidze, Z.A., 1982. The geomagnetic field in the Caucasus over the past 2000 years. Geomagn. Aeron., 22(3): 439—440. Burlatskaya, S.P., 1983. Archaeomagnetic investigations in the USSR. In: K.M. Creer, P. Tuchoilca and C.E. Barton (Editors), Geomagnetism of Baked Clays and Recent Sediments. Elsevier, Amsterdam, pp. 127—137. Burlatskaya, S.P. and Nashasova, I.E., 1977. Archaeomagnetic determinations of geomagnetic field elements. World Data. Materials of the World Data Centre, Soviet Geophysical Committee, Academy of Sciences of the USSR, Moscow, 112 pp. (in Russian with English summary). Champion, D.E., 1980. Holocene geomagnetic secular vanation in the western United States; implications for the global geomagnetic field. U.S. GeoL Surv. Open-File Rep. 80-824. Creer, KM., Tucholka, P. and Barton, C.E. (Editors), 1983. Geomagnetism of Baked Clays and Recent Sediments. Elsevier, Amsterdam, 306 pp.
Y. LIR1TZ1S AND M. KOVACHEVA
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