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

Abstract

A rock magnetic study was performed on sediment cores from four sites in the South Atlantic off the western coast of Africa, which were taken during the Ocean Drilling Program Leg 175 (Sites 1078(Sites , 1082(Sites , 1084(Sites , and 1085. The sites are within the Angola-Namibia upwelling system, and the sediments have a high total-organic-carbon content. Concentration of ferrimagnetic minerals at these sites is very low, and the magnetic susceptibility is dominated by paramagnetic and diamagnetic minerals. Severe and rapid loss of remanent 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 magnetic properties with time were examined using samples that were kept frozen before the experiment. Hysteresis parameters and the ratio of ARM (anhysteretic remanent magnetization) to SIRM (saturation isothermal remanent magnetization) indicate increases in the average magnetic grain size with the decay of magnetization, which suggests preferential dissolution of finer magnetic minerals. Loss of low-coercivity magnetic minerals with time was estimated 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-magnetic phases, which is consistent with the decrease of S ratios.

A Brunhes-Matuyama polarity transition record from anoxic sediments in the South Atlantic (Ocean Drilling Program Hole 1082C)

“…Remanent magnetization of the Site 1082 sediments, as well as other cores of ODP Leg 175, was very weak, 10 −3 to 10 −5 A/m, which was an order of magnitude larger before the loss of magnetization (Yamazaki et al, 2000). Magnetic susceptibility of the Site 1082 sediments is dominated by paramagnetic materials.…”

“…We thus consider these sediments may preserve paleomagnetic field behavior. Severe and rapid loss of remanent magnetization occurred during storage of the Site 1082 cores, with less than 10% of the initial intensity remaining a few months after core recovery (Yamazaki et al, 2000). The loss of magnetization was found commonly in the sediment cores taken during ODP Leg 175.…”

“…The loss of magnetization was found commonly in the sediment cores taken during ODP Leg 175. A rock magnetic study of Yamazaki et al (2000) indicated the presence of magnetite in the sediments even after the completion of sulfate reduction, and magnetization attributable to magnetite decreased with the loss of magnetization. It is hence considered that the transformation of magnetite into non-magnetic phases after core recovery is responsible for the loss of magnetization, although contribution of iron-sulfides like greigite is not clear.…”

“…We consider that relative paleointensity information has still been preserved in the sediments because NRM intensity variations before and after the loss of magnetization were almost parallel (Yamazaki et al, 2000), and that dissolution of magnetic minerals destroys both NRM intensity and the normalizers, ARM and SIRM. The dissolution can cause magnetic grainsize changes (Yamazaki et al, 2000), which would affect both ARM and IRM but in a different way. But the similarity of the records normalized by ARM and IRM suggests that the effect in relative paleointensity estimation was small if any.…”

Magnetic properties of surficial sediments in Lake Ogawara on the Pacific coast of northeastern Japan: spatial variability and correlation with brackish water stratification

“…Rapid loss of magnetization during storage has been reported for organic-rich lake sediment cores (Oldfield et al 1992) and for anoxic marine sediments Roberts et al 1999;Yamazaki et al 2000). Such modifications of magnetic properties are known as "storage diagenesis" (Oldfield et al 1992).…”