A new method to determine the amount and moment distribution of fine magnetic grains in sediments is proposed. The method is applied to a dilute suspension made of the sediment. To apply this method, the magnetic grains must be in a non-interacting state. This condition can be confirmed by the magnetic relaxation pattern in a magnetic field-free space. The dilute suspension in a non-interacting state shows monotonic decrease of the magnetization with an exponential form, independent of whether the concentration changes. On the contrary, the concentrated suspension in an interacting state shows either little decrease of magnetization or else a 'polarity' change after a quick decrease of magnetization. These two states are separated by the 'critical concentration', which is attributed to the characteristics of the source sediment.The fine magnetic grain assemblage of a non-interacting dilute suspension in a magnetic field attains an equilibrium magnetization state under the combined effects of the magnetic torque and thermal agitation (Brownian motion). This magnetization is called the 'equilibrium magnetization', and is proportional to the concentration of the suspension in a steady field. The measurement of the equilibrium magnetization in a weak magnetic field (less than 1.1 X lO-'T) has been performed by the use of a cryogenic magnetometer equipped with a flux-compensated coil. The measured values of equilibrium magnetization may be explained by classical dilute paramagnetic gas theory. A least-squares fitting with this theory demonstrates the grain moment distribution to be log-normal and provides an estimate of the amount of magnetic grains present. The amount and the moment distribution are given by the complete alignment magnetization (CAM) and by the geometric mean moment (mG) with log-standard deviation Two types of deep-sea sediments have been examined by this method. The magnetic characteristics obtained are CAM=4.5x 10-3A m2 kg-', rnG=3.7X10-'6Am2, a=3.8 for a reddish-brown clay and CAM= 1.3x10-3Am2kg-', mG=1.6X10-'6Am2, a=4.3 for a calcareous ooze. When compared with SIRM values for consolidated sediment samples, these results suggest that most magnetic grains of both the sediments are in single-domain (SD) and pseudo-single-domain (PSD) states.(log 4.

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.

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