Variation of the Earth?s magnetic field strength in South America during the last two millennia: New results from historical buildings of Buenos Aires and re-evaluation of regional data

AVTO GOGUITCHAICHVILI; JUAN MORALES; DANIEL SCHAVELZON; CARLOS VASQUEZ; CLAUDIA S. G. GOGORZA; DANIEL LOPONTE; AUGUSTO RAPALINI

PHYSICS OF THE EARTH AND PLANETARY INTERIORS

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2015 vol. 245 p. 15 - 25

0031-9201

The causes of the systematic decay of the Earth?sMagnetic Field strength since eighteen century have been a matter of debateduring the last decade. It is also well known that such variations may havecompletely different expressions under an area characterized with strongmagnetic anomalies, such as the South Atlantic Magnetic Anomaly. To fullyunderstand these atypical phenomena, it is crucial to retrieve the pastevolution of Earth?s magnetic field beyond the observatory records. We reporton detailed rock-magnetic and archeointensity investigations from somewell-studied historical buildings of Buenos Aires city, located at the heart ofthe South Atlantic Magnetic Anomaly. Samples consist of bricks, tiles, fireplacesand pottery, which are considered as highly suitable materials forarchaeointensity studies. The dating is ascertained by historical documentscomplemented by archeological constraints. Eighteen out of 26 analyzed samplesyield reliable absolute intensity determinations. The site-mean archaeointensityvalues obtained in this study range from 28.5 to 43.5 microT, withcorresponding virtual axial dipole moments (VADMs) ranging from 5.3 to 8.04 x 1022Am2. Most determinations obtained in the present study are inremarkable agreement with the values predicted by the time varying field model CALS10k.1b(Korte et al., 2011). For the older periods the recently available SHA.DIF.14model (Pavón-Carrasco et al., 2014) seems to have greater resolution. South American archaeointensity database now includesabsolute intensities from 400 to 1930 AD based on 63 selected archaeointensitydeterminations. The data set reveals several distinct periods of quite largefluctuations of intensity. However, most data are concentrated into arelatively narrow interval from AD 1250 to AD 1450. At the beginning of therecord, values between 400 AD and 830 AD match well with ARCH3k.1 model. Somegeneral features may be detected: the time intervals from about AD 400 to 950and 1150 to 1280 are characterized by a quite monotonic decrease of geomagneticintensity, while some increase is observed from AD 950 to AD 1250. In contrast,a systematic intensity decay is detected from 1550 to 1930 in excellentagreement with the model prediction. No firm evidence of correlation betweenthe climate changes over multi-decadal time scales and geomagnetic intensitywas found for South America.