Resumo:
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.
