Magnetic grain size and viscous remanent magnetization of pelagic clay

Abstract

Unfossiliferous pelagic clay (red clay) cores from the Pacific generally have unstable remanent magnetization except for tens of centimeters to several meters below the surface. The origin of this instability has been considered to be viscous remanent magnetization (VRM). We conducted a rock-magnetic study of a pelagic clay core from the South Pacific, which has the unstable-to-stable transition at about 1.5 m below the surface. Using the suspension method of Yoshida and Katsura (1985), we determined downcore variation of complete alignment magnetization (CAM) and magnetic moment distribution (geometric mean moment, mo, and log standard deviation, log a) assuming a lognormal distribution. Mean magnetic grain diameter was calculated from the mo using the saturation magnetization of magnetite, 4.8 x 10 s A m-l, on the assumption that all the magnetic grains are single-domain magnetites of spherical shape. This assumption is supported by thermomagnetic analyses, isothermal remanent magnetization (IRM) acquisition experiments, and the ratio of CAM to saturation IRM The grain diameter ranges from about 0.02 to 0.15 !xm and decreases with depth below about 1.5 m. The derived grain size holds even when the magnetites in the sediments suffered oxidation because the saturation magnetization of maghemite is close to that of magnetite. The frequency dependence of magnetic susceptibility showed a downward increase in the amount of superparamagnetic grains. On the other hand, the magnetic viscosity acquisition coefficient norrnalized by the CAM increases with depth. We conclude that magnetic grain size can control the magnitude of the secondary magnetization, VRM, of pelagic clay. It has been proposed that the major source of pelagic clay is atmospherically transported dust. Intensified global atmospheric circulation by the change of climate would raise the grain size of the magnetic minerals of the eolian component, which can cause the unstable-to-stable transition of the remanent magnetization.

Environmental rock‐magnetism of pelagic clay: Implications for Asian eolian input to the North Pacific since the Pliocene

“…However, their conclusions should be reconsidered since they did not correct for the paramagnetic contribution. Yamazaki and Katsura [ 1990] used X;FD to argue an increase in the magnetic grain size of pelagic clay in the South Pacific since the Pliocene. They also ignored the paramagnetic fraction; however, its influence is smaller because the ferrimagnetic component in the South Pacific pelagic clay is much larger than that of the North Pacific, and a linear relationship between X;L-X;H and X;L did not occur.…”

“…Their core covered the entire Cenozoic, but age resolution was rather poor, partly because the sedimentation rate was low and their study was not focused on Pliocene-Pleistocene changes. Yamazaki and Katsura [ 1990] carried out a rockmagnetic study of a pelagic-clay core from the South Pacific and discussed the relation between magnetic grain size and the stability of the remanent magnetization. They suggested that magnetic grain size had increased since the Pliocene.…”

Magnetic mineral diagenesis

“…Development of paleomagnetic instability at depths of a few metres below the seafloor has been reported widely for these sediments (e.g., Opdyke and Foster, 1970;Kent and Lowrie, 1974;Johnson et al, 1975;Henshaw and Merrill, 1980;Yamazaki and Katsura, 1990;Yamazaki and Ioka, 1997). The magnetic instability of pelagic red clays has been attributed to lowtemperature oxidation of stable SD magnetite, which is now known to be carried by magnetite magnetofossils (Yamazaki and Shimono, 2013), so that it forms smaller magnetically unstable SP maghemite particles (Kent and Lowrie, 1974;Johnson et al, 1975).…”

Late Quaternary paleoenvironment of the Ross Sea continental shelf, Antarctica

“…Magnetic susceptibility profiles may be used to infer content of terrigenous materials in the sediments. The frequency dependencies were calculated from the differences of measured susceptibility values between the two modes, and those values suggest the grain-sizes of magnetic minerals (Yamazaki and Katsura, 1990): X FD is the frequency dependence, X L is the magnetic susceptibility of 0.47 kHz frequency and AΉ is the magnetic susceptibility of 4.7 kHz frequency Organic carbon contents were measured at 10-20 cm intervals. Samples were stored frozen before drying at 50°C.…”