Record of a previously unidentified short geomagnetic event from an upper Miocene sedimentary sequence, and preferred path of the transitional VGPs

Abstract

Numerous short features in the ocean magnetic anomaly patterns (25–100 nT amplitude, 8–25 km wavelength: tiny wiggles) have been identified in several independent magnetic profiles, and have been modelled either as short polarity intervals (‘cryptochrons’) or as palaeointensity fluctuations (Cande & Kent 1992). In the last few years, several authors (e.g. Tauxe et al. 1994; Lowrie & Lanci 1994; Hartl, Tauxe & Constable 1993) have identified, on high-resolution magnetostratigraphic sections, short polarity intervals, correlated with cryptochrons deriving from the ocean floor. The record of a previously undetected short normal-polarity event, lasting about 11 kyr, from an upper Tortonian-lower Messinian sedimentary sequence (e.g. Compagnoni et al. 1992) in central Italy (42.0°N, 13.0°E), is reported here. The north virtual geomagnetic polar (VGP) path of the R-N transition appears to be strongly confined to a meridian band passing over the Americas about 90° away from the site longitude, as reported in recent years for a large number of reversals. This short feature is lacking in the corresponding ocean-floor magnetic anomaly patterns, probably because of the difficulties of resolving polarity intervals as short as this one in ocean magnetic profiles.