Paleomagnetic secular variation at Vulcano (Aeolian Islands) during the last 135 kyr

Abstract

Paleosecular variation (PSV) of the Earth’s magnetic field during the last 135 kyr has been investigated in lavas, scoriae and pyroclastic rocks of Vulcano (Aeolian Islands). About 1000 samples have been collected at 77 sites from 25 distinct volcanic units, whose age is either known from published isotopical data or constrained on the grounds of statigraphical relationships. Magnetic mineralogy investigation shows that Ti-magnetite is the main ferromagnetic mineral. At most sites, secondary magnetization components are either absent or easily removed by stepwise thermal or alternating field demagnetization. The mean site direction of the characteristic remanent magnetization is usually well-defined, since the semi-angle of confidence is greater than 5° at only four sites. The mean paleomagnetic direction over the last 135 kyr (D=9.4°, I=53.2°, α95=3.5°) differs from the geocentric axial dipole (GAD) at Vulcano (D=0°, I=57.8°) and might be interpreted as the effect of a long-term, non-axial-dipolar component. The PSV record from Vulcano agrees well with those from the lacustrine sediments of Lago Grande di Monticchio (100 kyr BP) and Lago di Mezzano (30 kyr BP), located in the Italian peninsula [Brandt et al., Quat. Sci. Rev. 18 (1999) 961–976]. The inclination anomaly ΔI found at Vulcano corresponds to about half of the shallowing observed in the sediments of the two lakes and the declination anomaly ΔD may be used to tie the declination values, derived from azimuthally unoriented cores, to the geographical reference system. In order to find the optimum site to be used as reference for PSV studies in Italy, the angular values of the Earth’s magnetic field measured at the 113 repeat stations of the Italian Geomagnetic Network [Coticchia et al., Boll. Geod. Sci. Aff. 40 (2001) 277–291] have been analyzed with the relocation via pole method [Noel and Batt, Geophys. J. Int. 102 (1990) 753–756]. The Viterbo station (lat. 42°27′N, long. 12°02′E) proved the best, since the mean error is 0.3° for both declination and inclination, wherever the original PSV site is located in Italy. A preliminary, composite PSV curve for the last 30 kyr BP is thus proposed, merging and relocating to Viterbo the data from Vulcano and the curve from Lago di Mezzano, corrected for the GAD deviation found at Vulcano.