Magnetic mineralogy changes in the Pleistocene marine sequence of Montalto di Castro (central Italy) and influences on the magnetic anisotropy

Abstract

The magnetic mineralogy of a Pleistocene sedimentary sequence drilled near Montalto di Castro (Latium, Italy) shows an extreme variability in a 40 m thick stratigraphic interval. Sedimentary facies in this interval also show rapid and strong changes in the granulometry and colours of the deposits, reflecting deposition in a coastal environment under a variety of energy and redox conditions. Rock magnetism analyses of the fine-grained sediments from this stratigraphic interval indicate the presence, in variable proportions, of several magnetic minerals. A new semi-quantitative best-fit method has been specifically developed for the estimation of the relative percentage contribution of the various magnetic minerals to the isothermal remanent magnetization. The variability in the magnetic mineralogy throughout the whole sequence produces remarkable changes in the anisotropy of magnetic susceptibility of the deposits. The interval with the major sedimentological and magnetic mineralogy changes is characterized by a fairly isotropic magnetic fabric, unexpected for sediments. This anomalous magnetic fabric is probably due to the balance between the inverse magnetic fabric produced by antiferromagnetic goethite and the normal magnetic fabric due to the paramagnetic matrix and other magnetic minerals.