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 mu T from 1500 to 300 BCE, the curve shows a large maximum `similar to 65 mu T in the second century BCE. The corresponding VADM varied between similar to 4.0 and similar to 11.0 x 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 mu T from 1500 to 300 BCE, before a large maximum similar to 65 mu T 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).