Magnetic properties in nearshore marine sediments off southern Chile

抄録

In order to determine magnetic mineral distributions in nearshore marine sediments off southwestern Chile, chemical analysis of bottom waters and rock magnetic characterization of surface sediments were performed. The samples analyzed are terrigenous and calcareous sediments recovered with a multiple corer at five stations. Calcareous sediments occur at stations with highly dissolved oxygen catchment in the bottom waters. Concentration-dependent magnetic parameters of calcareous sediment samples show relatively low values, and their magnetic grain sizes are coarse. Higher values of concentration-dependent magnetic parameters are recognized, and fine grain magnetic minerals are distributed in silty and sandy clay sediments. Magnetic mineralogy of the sediment samples reflects not only lithology but also redox conditions in the overlying bottom waters. Thermo-magnetometry results suggest that magnetite (Fe₃O₄), hematite (αFe₂O₃), (titano)maghemite (γFe₂O₃), and goethite (αFeOOH) are common in sediment samples at all stations. Higher coercivities and total organic carbon (TOC) values are also observed at oxic stations, suggesting the presence of goethite and (titano)maghemite. In general, higher TOC contents in sediments lead anoxic conditions due to organic matter decomposition, therefore goethite and (titano)maghemite are unstable. Despite higher TOC contents in this sediment sample, such magnetic minerals can present. This implies that the magnetic minerals are kept in oxic conditions.