Palaeomagnetism and rock magnetism in the upper Pliocene Valle Ricca (Rome, Italy) section

A revision of the magnetostratigraphy in the Valle Ricca (Monterotondo, Rome) Plio-Pleistocene section was carried out. This section, consisting mainly of blue-grey marly clays, was previously investigated for palaeomagnetic effects by Arias et al. (1980, 1990), who detected a normal polarity interval, close to a volcanic ash layer, which was correlated to the Reunion subchron. This interpretation conflicts, however, with new biostratigraphic data and palaeomagnetic observations. In order to investigate the possible origin of these discrepancies, we performed a detailed sampling for palaeomagnetic and rock-magnetic analyses in the controversial interval around the volcanic ash layer. The present results show strong variations in the rock-magnetic parameters and suggest significant changes in the magnetic mineralogy. Two levels with a high-intensity, low-temperature, normal component of magnetization were distinguished. At these two levels, the rock-magnetic parameters are typical of greigite, and a widespread occurrence of framboids of micron-sized iron sulphide grains was found by mineralogical analyses. A high-temperature reversed component of magnetization, carried by magnetite, was isolated for the whole interval investigated. We demonstrate that the previously recognized normal polarity interval is the effect of changes in the magnetic mineralogy and it does not represent a true reversal of the Earth's magnetic field. The origin of the low-temperature normal component of magnetization is discussed in light of the present knowledge about greigite formation and remanence acquisition.