The Archaeointensity of the Earth's Magnetic Field Retrieved From Pampean Ceramics (South America)

Abstract

Absolute intensity determinations using the Coe variant of the Thellier method have been carried out on some selected pottery fragments collected in the wetlands of the lower Paraná (Pampean region, Argentina) in order to construct the first archaeointensity master curve for South America. Associated radiometric ages range between 1640 1 70 and 730 1 70 BP. Twenty-one samples (five fragments) out of 46 studied (eight fragments) provided successful absolute intensity determinations. The fragment-mean archaeointensity values obtained in this study range from 21.9 1 2.3 to 42.6 1 5.4 mT, with corresponding virtual axial dipole moments (VADMs) ranging from 4.0 1 0.5 to 8.1 1 1.0 x 10 22 Am 2 . This corresponds to a mean VADM of (6.4 1 1.8) ¥ 10 22 Am 2 . The synthetic record retrieved from southern Argentina and Brazil consists of 17 mean archaeointensities distributed between approximately AD 700 and AD 1700. The data set shows several distinct periods of fluctuations of quite large intensity. However, most data are concentrated into a relatively narrow time period from AD 950 to AD 1300. Three general features may be detected: the time intervals from about AD 950 to 1130 and 1350 to 1480 are characterized by quite monotonic increases of geomagnetic intensity, while some decrease is observed from AD 1150 to 1280. These variations may be speculatively correlated to climate changes over multi-decadal time scales. Important differences are observed between the data and the geomagnetic field predictions derived from recently reported global models, which reinforces the importance of regional reference curves for dating purposes.

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

“…They are abundant for Europe (Genevey and Gallet, 2002;Schnepp and Lanos, 2005;Gómez-Paccard et al, 2006), but particularly poor in South America (Goguitchaichvili et al, , 2012Hartmann et al, 2011). Even so, these data represent about 2 per cent of the worldwide database (GEOMAGIA50.v3 (http://geomagia.gfz/potsdam.de, Donadini et al, 2006;Korhonen et al, 2008).).…”

“…The regional data compilation includes results from Peru (Shaw et al, 1996), Argentina (Goguitchaichvili et al, , 2012 and Brazil (Hartmann et al, 2010(Hartmann et al, , 2011. There are some more data from South America but most of them do not meet any commonly acceptable criteria.…”

Archaeomagnetic field intensity evolution during the last two millennia

“…Corrections using the ATRM@350°C tensor were applied to five specimens (RSLG1-06-10, RSJB-10-39, RSJB-10-40, Several authors have attempted to avoid the time-consuming anisotropy correction either by imparting the laboratory field along the ancient field (e.g. Rogers et al, 1979;Aitken et al, 1981) or by averaging intensities acquired in mutually orthogonal directions by different specimens of a given fragment (Goguitchaichvili et al, 2012). The first method has been successfully tested extensively.…”

“…The works of Nagata et al (1965), Kitazawa and Kobayashi (1968), Lee (1975) and Kono et al (1986) were performed before the tests and corrections used now routinely in archeointensity experiments were established. They do not present pTRM checks, pTRM tail checks, cooling-rate and anisotropy corrections and were consequently discarded ( anisotropy corrections were tentatively circumvented by averaging the intensity estimates of six sister specimens positioned orthogonal to each other (Goguitchaichvili et al, 2012).…”

“…Since then, few archeomagnetic studies have been performed in the region for the past two millennia with a 14-years gap between 1996 and 2010. Recently, this kind of work has been revived and in the last five years the number of studies has doubled Goguitchaichvili et al, , 2012Roperch et al, 2014 focusing dominantly on archeological material from Brazil, Argentina and Chile and also on volcanic lavas from Chile. Previous studies in Brazil were performed first in Northeast Brazil , and then in Southeast Brazil , deliberately occupying different latitudes in order to assess the influence of non-dipolar components related to the arrival of the…”