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, 1082, 1084, aned 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.
Reprinted with permission from Terra Scientific Publishing Company.