New archaeomagnetic data recovered from the study of celtiberic remains from central Spain (Numantia and Ciadueña, 3rd-1st centuries BC). Implications on the fidelity of the Iberian paleointensity database

Physics of the Earth and Planetary Interiors

Chauvin

Lanos

et al. 2021

Despite progress in the knowledge of secular variation during the first millennium BCE in Europe, data coverage remains poor at the earliest periods, especially in some regions as in the Central Mediterranean area. This study presents three new directional and six new intensity data between the 13 th and the 4 th centuries BCE on archaeological kilns, pottery and baked clay fragments from South Italy and France. Archaeodirections were determined after thermal demagnetizations and archaeointensities using the Thellier-Thellier protocol with corrections for the anisotropy and cooling rate effects. The new data confirm the large deviation of the direction from a Geocentric Axial Dipole field, the high geomagnetic field strength and the fast secular variation observed in Europe during the earliest half of the first millennium BCE. Another characteristic of this period is a difference of ~25° between the longitudes of the virtual geomagnetic poles inferred from European and Middle East data.This unusual behaviour can be mainly related to the Levantine Iron Age anomaly (LIAA) and its expansion from the Middle East to Europe. However, the review of the global directional database shows that almost all virtual geomagnetic poles, 96% of them coming from Europe, the Middle East, East Asia, North America and Hawaii, are 10-25° away from the rotation axis towards North Russia between 1000 and 600 BCE. The calculation of a mean global VGP curve suggests that the North geomagnetic pole followed a clockwise motion during this period with a dipole tilt up to around 14°. This study shows that a dipole axis tilt may have played an important role in the rapid secular variation in western Eurasia, although part of this variation may also be related to non-dipole fields associated with the LIAA.

Section: Methodsmentioning

confidence: 99%

How did the dipole axis vary during the first millennium BCE? New data from West Europe and analysis of the directional global database

Physics of the Earth and Planetary Interiors

Chauvin

Lanos

et al. 2021

“…Potteries are generally anisotropic and palaeointensity can sometimes be biased up to a factor of two (e.g. Hervé et al, 2017;Osete et al, 2016). The effects can also be high in burnt walls and soils (Palencia-Ortas et al, 2017;Molina-Cardin et al, 2018).…”

Section: Challenges In Palaeointensity Determinationmentioning

confidence: 99%

Critical analysis of the Holocene palaeointensity database in Central America: Impact on geomagnetic modelling

Physics of the Earth and Planetary Interiors

Perrin

Alva‐Valdivia

et al. 2019

Thanks to its rich archaeological heritage, Central America is a key region to recover the past secular variation of the geomagnetic field. This article presents 13 new palaeointensity data on Epiclassic (650-900 CE) pottery sherds from Central Mexico. Archaeointensities were determined using the Thellier-Thellier protocol with anisotropy and cooling rate corrections. Average results between 25 and 42 µT reveals a fast secular variation in the second half of the first millennium CE but are not in agreement with global geomagnetic models that predict a higher geomagnetic field strength. To check the reasons of this discrepancy, we compiled all intensity data over the last millennia published in Central America. The Bayesian curve calculated from 194 data covering the last 4 millennia highlights a rapid succession of oscillations of the geomagnetic field strength between 20 and 80 µT. But a critical analysis of the dataset shows a large influence of data quality, 74% of them having a poor cooling unit consistency and experimental quality. The small number of specimens per cooling unit and the anisotropy correction absent or incorrectly made increase the scatter between data, whereas the absence of cooling rate correction biases the dataset towards higher palaeointensity. Discarding these data results in a lower secular variation by removing most extreme values and several intensity oscillations. The weaknesses of the dataset are likely the main reason of the limitations of global models in Central America. Pending the acquisition of new high-quality data, archaeomagnetic dating seems premature in Central America.

“…We preferentially sampled these types because they are, at least partly, red‐colored (Figure c). Archaeointensity experiments are generally more successful on these potteries than on the grey‐black one, because they are less sensitive to mineralogical alteration (Osete et al, ).…”

Section: Archaeological Context and Samplingmentioning

confidence: 99%

Secular Variation of the Intensity of the Geomagnetic Field in Mexico During the First Millennium BCE

Geochem Geophys Geosyst

Perrin

Alva‐Valdivia

et al. 2019

In Western Eurasia, the first millennium BCE is characterized by the fastest secular variation of the Earth Magnetic Field observed over the last millennia and by a geomagnetic anomaly centered on the Middle East. On the global scale, the variation of the dipolar field during this period remains poorly constrained because of the lack of data in other geographical areas. Here, we presented 23 new mean archaeointensity data on ceramic sherds dated between 1500 BCE and 200 CE from Chalcatzingo archaeological site in Central Mexico. Archaeointensities were determined using the classical Thellier-Thellier protocol with corrections for TRM anisotropy and cooling rate effects. Our work doubles the number of high-quality archaeointensity data in Mexico during the considered period. Using a Bayesian approach, a new secular variation curve was calculated at Mexico City between 1500 BCE and 200 CE after selection of Mexican archaeointensity data. After a period of oscillations of the intensity between 20 and 40 μT from 1500 to 300 BCE, the curve shows a large maximum`~65 μT in the second century BCE. The corresponding VADM varied between~4.0 and~11.0 × 10 22 Am 2 , which highlights further that the intensity of the geomagnetic field could vary at regional scale over a larger range as previously thought. However, this amplitude variation may be overestimated, as it does not take into account the fast directional variation observed at this time.Plain Language Summary Archaeological baked clays are the best material to reconstitute the past secular variation of the Earth magnetic field, because they acquired a generally stable thermoremanent magnetization usually parallel and proportional to the ambient field at the time of their baking. Global modeling of the geomagnetic field requires a spatial and temporal distribution of data as homogeneous as possible, which is still not the case yet. In spite of its rich archaeological heritage, high-quality archaeointensity data are still very few in Mexico. The present study focuses on pottery sherds dated between 1500 BCE and 200 CE from Chalcatzingo archaeological site in Central Mexico. We obtained 21 new mean archaeointensities, which almost doubles the high-quality Mexican dataset for this period. The new secular variation Mexican curve exhibits oscillations of the intensity between 20 and 40 μT from 1500 to 300 BCE, before a large maximum~65 μT in the second century BCE. This work will help to have a better knowledge of the variation of the dipolar geomagnetic field during the first millennium BCE, when the fastest secular variation over the last millennia was reported in the Middle East ("geomagnetic spikes").