Post‐deformational growth of late diagenetic greigite in lacustrine sediments from southern Italy

Abstract

Paleomagnetic, rock magnetic and micro‐textural analyses from a Middle Pleistocene lacustrine sequence in the southern Apennines (Italy) indicate the presence of greigite and magnetite as the main magnetic minerals at different stratigraphic levels. In all cases a normal polarity characteristic remanent magnetization (ChRM) was observed, in agreement with an 40Ar–39Ar age of 0.712 ± 0.018 Ma from volcaniclastic sanidine. After correction for bedding tilt, ChRM directions carried by greigite do not coincide with the expected geocentric axial dipole field direction at the site latitude, whereas the magnetite ChRMs directions do. The data indicate that the greigite magnetization was acquired after tilting and after lock‐in of the magnetite ChRM. The estimated delay for the remanence carried by greigite with respect to deposition is ∼300 ka. Scanning electron microscope analyses indicate that alteration of detrital volcanic minerals has occurred and that authigenic greigite is generally present in agglomerates and around volcanic grains. This observation is consistent with a late diagenetic origin of greigite due to anoxic conditions and availability of dissolved sulfide associated with decomposition of organic matter in the paleolake. Documentation of a late diagenetic magnetization confirms that care should be taken when using greigite‐bearing sediments for magnetostratigraphic and tectonic studies.