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

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 × 10²² Am², 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.