Archaeomagnetic evidence of a likely earlier occupation of “El Caracol” lava flow (Zacapu Malpaís, Western Mesoamerica)

A comprehensive archaeomagnetic investigation was carried out on seven ceramic fragments recovered at the "El Caracol" lava flow in the Zacapu Malpaís –Michoacan state, Mexico –. A full set of magnetic-mineralogy experiments (encompassing thermomagnetic curves, hysteresis loops, backfield curves, and isothermal remanent magnetization acquisition measurements) were carried out. The experimental methodology also considers magneto-chemical alteration detection, cooling rate, and remanence anisotropy effects to ensure reliable archaeointensity determinations. Five of the seven ceramic fragments – a pipe (potsherd I), vessel's supports (potsherd II and V), a vessel's wall-fragment (potsherd III), and a pipe's nozzle (potsherd VII) – yielded archaeointensity results within the established acceptance criteria to guarantee quality data. Thermal demagnetization of potsherds I, II, V, and VII shows between two and three magnetization components: one associated with potsherd's fabrication, the second with sample's reheating, and the third one, when present, with a viscous component. For each of the first two components, an intensity value was calculated. Potsherds I, II, III, and VII yielded archaeointensity values between 30 and 40 µT, while potsherd V had a much lower value <20 µT. Although secular variation models do not predict such low intensities, similar values have been reported for the center and south of Mexico. Archaeomagnetic dating was carried out with two regional Paleosecular Variation Curves. The "El Caracol" lava flow is an area with human occupation reported from AD 1200. The results obtained in this work, on the contrary, envision the possibility of an early occupation of the site.