New archaeointensity data from Western Europe: an

New archaeointensity data from Western Europe: an improved description of
the 800 AD intensity bump
Nuevos datos de arqueointensidad para Europa Occidental: caracterización del máximo de
intensidad geomagnética existente en torno a 800 dC
M. Gómez-Paccard1, A. Chauvin2, P. Lanos2,3 P. Dufresne2,3 M. Kovacheva4, E. Beamud5, M. J. Hill6, S.
Blain3, A. Bouvier3, P. Guibert3 and Archaeological Working Tem*
1 Institut de Ciències de la Terra Jaume Almera, ICTJA-CSIC, Solé i Sabarís s/n, 08028 Barcelona, Spain. Tel: + 34 934095410; fax: +34 934110012.
[email protected] Géosciences-Rennes, UMR 6118, Université de Rennes 1,CNRS, Campus de Beaulieu, Bât. 15, CS 74205, 35042
Rennes, France
3 Centre de Recherches en Physique Appliquée à l’Archéologie, UMR 5060, CNRS, Université de Bordeaux 3, France
4 NIGGG, BAS, Academician Georgi Bonchev Street, Block 3, 1113 Sofia, Bulgaria
5 Laboratori de Paleomagnetisme UB-CSIC, Institut de Ciències de la Terra Jaume Almera, Solé i Sabarís s/n, 08028 Barcelona, Spain
6 Geomagnetism Laboratory, School of Environmental Sciences, University of Liverpool, Liverpool L69 7ZE, UK
* Sonia Gutiérrez-Lloret and Victor Cañavate (Área de Arqueología, Facultad de Filosofía y Letras, Universidad de Alicante, Spain), Christine
Oberlin (Centre de datation par le RadioCarbone, Université de Lyon 1, France), Christian Sapin (Laboratoire Artehis, CNRS, UMR 5594,
Université de Bourgogne, Dijon, France), Daniel Prigent (Service Départemental de l´Archéologie, CG49, Angers, France), Sébastian Jesset (Service
archéologique municipal, Orléans), Hervé Pomarèdes (INRAP : Institut national de recherche Archéologique Préventive, Direction interrégionale
Méditerranée, Nîmes).
Abstract: Archaeomagnetic data indicate that during the past 2500 years there have been periods of rapid
geomagnetic field intensity fluctuations interspersed with periods of almost constant field strength. The
occurrence and the behaviour of these changes are under discussion and the challenge now is to precisely
describe them. The aim of this study is to obtain an improved description of the sharp geomagnetic
intensity change that took place in Western Europe around 800 AD. For this purpose 13 precisely dated
early medieval Spanish pottery fragments, 4 archaeological French kilns and 3 collections of bricks used
for the construction of different French historical buildings with ages ranging between 330 and 1260 AD
have been studied. Classical Thellier experiments performed on 164 specimens, and including anisotropy
of thermoremanent magnetisation and cooling rate corrections, gave 119 reliable results. The ten new
high-quality mean archaeointensities obtained together with previously published data from Western
Europe that we deem to be the most reliable, confirm the existence of an intensity maximum of ~85 µT
centred at ~800 AD and related to intensity changes up to 25 µT per century. The new data also suggest
that a previous abrupt intensity change occurred in Western Europe around 625 AD.
Key Words: archaeomagnetism, archaeointensity, secular variation, dipole moment, Thellier, Europe.
Resumen: Los datos arqueomagnéticos existentes indican que en los últimos 2500 años ha habido
periodos de cambios rápidos de la intensidad del campo geomagnético intercalados con periodos de
intensidad más o menos constante. El reto científico actual es definir con exactitud la ocurrencia de estos
cambios bruscos y el comportamiento del campo geomagnéticos durante los mismos. El objetivo de este
trabajo es obtener una descripción precisa del cambio brusco de intensidad acontecido en torno al año
800 dC en Europa. Para ello se han estudiado 13 fragmentos de cerámica del Tolmo de Minateda
(España), 4 hornos arqueológicos y tres edificios históricos franceses de edades comprendidas entre 330
y 1260 dC. Mediante el método clásico de Thellier aplicado a 164 especímenes se han obtenido 119
arqueointensidades, incluyendo correcciones de la anisotropía de la imanación termoremanente y de la
velocidad de enfrimiento. A partir de los 119 valores de intensidad se han obtenido 10 valores medios de
alta calidad que, junto una selección de los datos más fiables europeos, permiten confirmar la existencia
de un pico de intensidad de unos 85 µT centrado en torno al año 800 dC y relacionado con cambios de
intensidad de hasta 25 µT por siglo. Los resultados sugieren también que un máximo de intensidad
abrupto ocurrió en Europa hacia el año 625 dC
Palabras clave: arqueomagnetismo, paleointensidad, variación secular, momento dipolar, Thellier.
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archeointensity data were corrected for TRM
anisotropy and cooling rate effects upon TRM
intensity. The TRM anisotropy effect for the ceramic
fragments is generally higher than for the kilns or brick
samples. In general the cooling rate correction factors
applied are not very high with a maximum value
of ~25%.
INTRODUCTION
Unravelling past changes in geomagnetic field intensity
is crucial to understand the dynamics of the
geodynamo and to investigate the link between solar
activity, 14C, the Earth´s magnetic field and climate.
Available European data indicate that during the past
2500 years there were periods of rapid intensity
fluctuations, such as the one observed around 800 AD,
interspersed with periods of little change, as seen
during Roman times.
The challenge now is to precisely describe these
rapid changes and to understand how such behaviour
can arise. This study provides new paleointensity data
from the study of 13 precisely dated early medieval
Spanish pottery fragments from el Tolmo de Minateda
(TM) with ages ranging from 750 to 900 AD. In
addition, four French kilns and three collections of
bricks used in the construction of historical French
buildings with ages ranging between 330 and 1290 AD
have been studied (Fig. 1).
FIGURE 2. Thellier archaeointensity results.
The unblocking temperatures observed range
between 410ºC and 620ºC. Together with rockmagnetic experiments (hysteresis experiments, IRM
acquisition curves and thermomagnetic curves) they
indicate that the main magnetic carriers in the studied
samples are magnetite and Ti-poor titanomagnetites.
DISCUSSION AND CONCLUSION
From the 119 reliable archaeointensity estimations at
specimen level, ten new high-quality archaeointensities
were obtained for Western Europe. They were derived
from the analysis of at least two specimens per
fragment and between two and four fragments per age
interval or between 4 and 37 independent samples per
structure/building. Consistent values were obtained
from the different material studied (Fig.3).
FIGURE 1.Location of the archaeological/historical sites.
RESULTS
Four specimens of about 1 cm x 1 cm per pottery
fragment were prepared and packed into salt pellets.
Standard cores were drilled from the three collections
of bricks. Large oriented blocks were taken from the
mediaeval
kilns.
Samples
were
dated
by
thermoluminescence and/or archaeological context.
Thellier experiments were attempted on 164 specimens
(52 salt pellets and 112 standard cores). 119 specimens
of them yield reliable absolute intensity determinations.
All accepted results correspond to well-defined single
component of magnetization going toward the origin
and high quality NRM-TRM results (Fig. 2). The
A compilation of high-quality archeointensities
available for Western Europe between 200 and 1400
AD was performed (Fig. 3). Only data obtained from
Thellier or Thellier-derived experiments including
pTRM checks were considered (Donadini et al., 2009).
Our new results are in very good agreement with
previous archeointensity values (Fig. 3).
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FIGURE 3. Geomagnetic field intensity changes in Europe. (a) New archaeointensity data obtained in this study and selected previous results for
western Europe and northern Morocco (Genevey et al., 2008); (b) selected archaeointensity data available from Bulgaria, Greece and Italy (Genevey
et al., 2008). In (c) and (d) the mean Bayesian curved derived from the selected data is shown in black together with the 95% error envelope (grey
area). In (e) and (f) the rates of change derived from the Bayesian curve.
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improved description of the geomagnetic intensity
changes that took place in this region between the 4th
and the 13th centuries AD. The results confirm that an
intensity maximum of about 85 µT occurred in Europe
at around 800-900 AD and indicate that this sharp
intensity change is related to very rapid intensity
changes as fast as 25 µT per century. The results also
suggest that a previous abrupt intensity change took
place in Europe around 650 AD.
The Bayesian approach (Lanos, 2004) was used in
order to obtain the geomagnetic field intensity variation
described by the dataset compiled (Fig. 3). The results
indicate approximately constant values between 300
and 500 AD (around 65 µT) followed by an increase of
the archaeointensity up to ~85 µT around 650 AD
followed by an important decrease between 650 AD
and 750 AD. However, this period of high intensities is
poorly defined and the results are only based on one
intensity value. High intensities (of about 85 µT) are
reached again very quickly around 800 AD. Rapid
intensity changes as fast as 25 µT per century are
observed for this period. Finally, an intensity decrease
is observed between 800 AD up to 1400 AD.
AGRADECIMIENTOS
Financial support was given by a CSIC JAE-Doc
postdoctoral research contract (MGP), by the
CGL2008-02203/BTE,
CGL2011-24790,
and
CGL2010-15767/BTE research projects from the
Spanish ministry of Science and Innovation
High-quality data from Bulgaria, Italy and Greece
for the same period have been also compiled and
treated by Bayesian modeling (Figs 3b, d). The results
indicate very similar intensity changes to those
observed in western Europe, with the occurrence of
two intensity bumps (up to ~75 µT) at ages around 650
and 950 AD. The results also suggest that the duration
of these periods of high intensities may be of less than
two centuries and that extreme intensity variation as
fast as 20 µT per century occurred in Eastern Europe
during historical times.
REFERENCIAS
Donadini, F., Korte, M. and Constable, C.G.. (2009):
Geomagnetic field for 0-3 ka: 1. New data sets for
global modeling. Geochemistry Geophysics
Geosystems,
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doi:10.1029/2008GC002295.
Genevey, A., Gallet, Y., Constable, C.G., Korte, M.,
and Hulot, G., 2008. ArcheoInt: An upgraded
compilation of geomagnetic field intensity data for
the past ten millennia and its application to the
recovery of the past dipole moment. Geochem.
Geophys.
Geosyst,
9,
Q04038,
doi:10.1029/2007GC001881.
Lanos, P. (2004): Bayesian inference of calibration
curves: Application to archeoamgetnism. En: Tools
for
Constructing
Chronologies,
Crossing
Disciplinary Boundaries (C: E: Buck and A. R.
Millard, eds.). Springer, London, 43-82.
CONCLUSION
New archaeointensities from the study of 13 precisely
dated early medieval Spanish pottery fragments, four
French kilns and three collections of bricks used for the
construction of three French historical buildings with
ages ranging between 330 and 1290 AD have been
obtained. The new intensities -corrected for TRM
anisotropy and cooling rate effects- can be considered
as reliable markers of past geomagnetic field intensity.
Together with a selection of the previous data that we
deem to be the most reliable for Europe, they enable an
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