Magnetic Mineralogy in the Brunhes / Matuyama Boundary: Pleistocene of the Buenos Aires Province

Abstract

The magnetic mineral in loess and paleosols in a geological sequence of La Plata, Argentina, 34º 54´ 14´´ S and 58º 02' 00´´ W were studied by applying Mössbauer spectroscopy and rock magnetism. Hematite (antiferromagnetic) is by far the dominant iron-bearing magnetic component in both loess and paleosol natural samples. However, SIRM and coercitivity values correspond to ferrimagnetic magnetite. The samples were also subjected to magnetic separation. The magnetically enriched loess samples show a remarkable increase in the content of magnetite. The enhancement of non-magnetic Fe3+ and a decrease of Fe2+ mineral phases in the paleosol layer seems to be a consequence of the process of pedogenesis which also has given rise to a dissolution of magnetic iron oxides by weathering. Consequently the enhancement of SIRM and mass susceptibility values were obtained in horizons less affected by pedogenesis. The increase of magnetite phase in the fraction below 0.062 mm, suggests that the wind could have been the main carrier of magnetic minerals, causing the mayor increasing of the observed magnetic parameters.