Secular variation of the intensity of the geomagnetic field in Mexico over the last 3500 years
Résumé
Thanks to its rich archaeological heritage, Mexico is a key area to investigate the secular variation of the geomagnetic field. This communication presents first the acquisition of new archaeointensity data on pottery sherds from Chalcatzingo site (Morelos, Central Mexico) dated in Preclassic (1500-500 BCE) and Epiclassic (600-900 CE) archaeological periods. Archaeointensities were determined using the Thellier-Thellier protocol with anisotropy and cooling rate corrections and calculated with at least three accepted specimens per sherd. We obtained 23 new average data for the Preclassic and 13 for the Epiclassic period. After this study, the Mexican database contains now more than 250 archaeointensity data over the last 3500 years. Their range values between 15 and 90 µT and their temporal distribution suggest a large variation amplitude and fast rate of variation up to many dozens of µT per century. The curve calculated using a Bayesian approach from all data highlights a rapid succession of oscillations of the intensity. But a critical analysis of the dataset shows a large influence of data quality, around 70% 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 low-quality data results in a lower secular variation by removing most extreme values and several intensity oscillations. The maximum secular variation rate is observed at the end of the first millennium BCE, during which the intensity increased from 30 to 65 µT between 300 and 100 BCE. The ~250 nT/year rate is similar to the fastest one observed in Western Europe. There is no evidence of the “geomagnetic spike” seen in Texas on relative sedimentary data at the beginning of the first millennium BCE. On the contrary, the lowest values (~15-20 µT) observed over the last 3500 years correspond to this period.