A re-evaluation of the Italian historical geomagnetic catalogue: implications for paleomagnetic dating at active Italian volcanoes

Abstract

Paleomagnetism is proving to represent one of the most powerful dating tools of volcanics emplaced in Italy during the last few centuries/millennia. This method requires that valuable proxies of the local geomagnetic field (paleo) secular variation ((P)SV) are available. To this end, we re-evaluate the whole Italian geomagnetic directional dataset, consisting of 833 and 696 declination and inclination measurements, respectively, carried out since 1640AD at several localities. All directions were relocated via the virtual geomagnetic pole method to Stromboli (38.8 N, 15.2 E), the rough centre of the active Italian volcanoes. For declinationonly measurements, missing inclinations were derived (always by pole method) by French data (for period 1670– 1789), and by nearby Italian sites/years (for periods 1640– 1657 and 1790–1962). Using post-1825 declination values, we obtain a 0.46±0.19 yr−1 westward drift of the geomagnetic field for Italy. The original observation years were modified, considering such drift value, to derive at a driftcorrected relocated dataset. Both datasets were found to be in substantial agreement with directions derived from the field models by Jackson et al. (2000) and Pavon-Carrasco et al. (2009). However, the drift-corrected dataset minimizes the differences between the Italian data and both field models, and eliminates a persistent 1.6 shift of 1933–1962 declination values from Castellaccio with respect to other nearly coeval Italian data. The relocated datasets were used to calculate two post-1640 Italian SV curves, with mean directions calculated every 30 and 10 years before and after 1790, respectively. The curve comparison suggests that both available field models yield the best available SV curve to perform paleomagnetic dating of 1600–1800AD Italian volcanics, while the Italian drift-corrected curve is probably preferable for the 19th century. For the 20th century, the global model by Jackson et al. (2000) yields more accurate inclination values, while the declinations from our drift-corrected curve seem to better represent the local field evolution, at least for the first half of the century.