Rock magnetism of sediments in the Angola-Namibia upwelling system with special reference to loss of magnetization after core recovery

A rock magnetic study was performed on sediment cores from four sites in th e South Atlantic off the western coast of Africa, which were taken during t he Ocean Drilling Program Leg 175 (Sites 1078, 1082, 1084, and 1085). The s ites are within the Angola-Namibia upwelling system, and the sediments have a high total-organic-carbon content. Concentration of ferrimagnetic minera ls at these sites is very low, and the magnetic susceptibility is dominated by paramagnetic and diamagnetic minerals. Severe and rapid loss of remanen t magnetization occurred during storage of the cores, with less than 10% of the initial intensity remaining a few months after core recovery. The loss of magnetization may prevail in organic-rich sediments. Changes of magneti c properties with time were examined using samples that were kept frozen be fore the experiment. Hysteresis parameters and the ratio of ARM (anhysteret ic remanent magnetization) to SIRM (saturation isothermal remanent magnetiz ation) indicate increases in the average magnetic grain size with the decay of magnetization, which suggests preferential dissolution of finer magneti c minerals. Loss of low-coercivity magnetic minerals with time was estimate d from the decrease of S ratios. Low-temperature magnetometry revealed the presence of magnetite in the sediments even after the completion of sulfate reduction. Magnetization attributable to magnetite decreased with the loss of magnetization. This suggests the transformation of magnetite into non-m agnetic phases, which is consistent with the decrease of S ratios.