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

Abstract

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 data set, consisting of 833 and 696 declination and inclination (respectively) measurements carried out since 1640 AD at several localities. All directions were relocated via virtual geomagnetic pole method to Stromboli (38.8° N, 15.2° E), rough centre of the active Italian volcanoes. For declination-only 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-1805 declination values, we obtain a 0.46 ± 0.19 °/yr westward drift of the geomagnetic field for Italy. Original observation years were modified considering such drift value to derive a drift-corrected relocated data set. Both data sets 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 data set 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 data sets were used to calculate two post-1640 Italian SV curves, with mean directions calculated every 30 and 10 years before and after 1790, respectively. Curve comparison suggests that the regional model by Pavon-Carrasco et al. (2009) yields the best available SV curve to perform paleomagnetic dating of 1600–1800 AD Italian volcanics, while the Italian drift-corrected curve is probably preferable for the XIX century. For the XX 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.