Low-temperature magnetic properties of pelagic sediments (Ocean Drilling Program Site 805C): Tracers of maghemitization and magnetic mineral reduction

Magnetic properties of pelagic sediments from the western equatorial Pacific (Ocean Drilling Program (ODP) Site 805C) have been measured at low temperatures to investigate changes in magnetic mineralogy and the distribution of bacterial magnetite with depth. A saturation isothermal remanent magnetization (M-rs) was given to the samples by applying a strong magnetic field (2.5 T) at 20 K after cooling both in the absence (zero field cooling, ZFC) and presence of a 2.5 T magnetic field (field cooling, FC). The thermal demagnetization of both M-rs(ZFC) and M-rs(FC) was measured from 20 to 300 K. No pronounced Verwey transition was observed in the data from samples shallower than the iron redox boundary. Below the boundary the Verwey transition is visible for all samples studied. We interpret this behavior as reflecting low-temperature oxidation (maghemitization) of primary magnetite and the subsequent dissolution of maghemite coatings below the iron redox boundary. The low-temperature oxidation may occur both at the water-sediment interface and deeper in the suboxic zone of sediments as a result of bacteria-mediated processes, including the reduction of nitrate compounds. M-rs(ZFC) and M-rs(FC) thermal demagnetization curves of all samples diverge for the entire temperature range measured. The divergence may be caused by the low-temperature stabilization of spontaneous magnetization vectors along with exchange interaction in the magnetite-maghemite system. Together, maghemitization and magnetic reduction diagenesis appear to play a dominant role in controlling the low-temperature properties of the sediment studied. As a result, low-temperature magnetic studies may provide more information on differences in reduction diagenesis than on the distribution of bacterial magnetite in natural sediments. The changes in effective oxidation state below the iron redox boundary detected by low-temperature methods may be useful in the study of some paleoenvironmental processes.