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Please use this identifier to cite or link to this item: http://repositorio.ufba.br/ri/handle/ri/5292

Title: New historical archeointensity data from Brazil: Evidence for a large regional non-dipole field contribution over the past few centuries
Other Titles: Earth and Planetary Science Letters
Authors: Hartmann, Gelvam André
Genevey, Agnès
Gallet, Yves
Trindade, Ricardo Ivan Ferreira da
Goff, Maxime Le
Najjar, Rosana
Etchevarne, Carlos Alberto
Afonso, Marisa Coutinho
Keywords: archeomagnetism;archeointensity;geomagnetic secular variation;historical geomagnetic field;Southeast Brazil;South Atlantic Magnetic Anomaly
Issue Date: Jun-2011
Abstract: We report new archeointensity data obtained from the analyses of baked clay elements (architectural and kiln brick fragments) sampled in Southeast Brazil and historically and/or archeologically dated between the end of the XVIth century and the beginning of the XXth century AD. The results were determined using the classical Thellier and Thellier protocol as modified by Coe, including partial thermoremanent magnetization (pTRM) and pTRM-tail checks, and the Triaxe protocol, which involves continuous high-temperature magnetization measurements. In both protocols, TRM anisotropy and cooling rate TRM dependence effects were taken into account for intensity determinations which were successfully performed for 150 specimens from 43 fragments, with a good agreement between intensity results obtained from the two procedures. Nine sitemean intensity values were derived from three to eight fragments and defined with standard deviations of less than 8%. The site-mean values vary from ~25 μT to ~42 μT and describe in Southeast Brazil a continuous decreasing trend by ~5 μT per century between ~1600 AD and ~1900 AD. Their comparison with recent archeointensity results obtained from Northeast Brazil and reduced at a same latitude shows that: (1) the geocentric axial dipole approximation is not valid between these southeastern and northeastern regions of Brazil, whose latitudes differ by ~10°, and (2) the available global geomagnetic field models (gufm1 models, their recalibrated versions and the CALSK3 models) are not sufficiently precise to reliably reproduce the nondipole field effects which prevailed in Brazil for at least the 1600–1750 period. The large non-dipole contribution thus highlighted is most probably linked to the evolution of the South Atlantic Magnetic Anomaly (SAMA) during that period. Furthermore, although our dataset is limited, the Brazilian archeointensity data appear to support the view of a rather oscillatory behavior of the axial dipole moment during the past three centuries that would have been marked in particular by a moderate increase between the end of the XVIIIth century and the middle of the XIXth century followed by the well-known decrease from 1840 AD attested by direct measurements.
Description: Acesso restrito: Texto completo. p. 66-76.
URI: http://www.repositorio.ufba.br/ri/handle/ri/5292
ISSN: 0012-821X
Appears in Collections:Artigos Publicados em Periódicos (PPGA)

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