Characterization of fine magnetic grains in sediments by the suspension method

Abstract

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.

Cautionary note on magnetic grain‐size estimation using the ratio of ARM to magnetic susceptibility

“…, the frequency dependence of magnetic susceptibility [Mullins and Tite, 1973;Bloemendal et al, 1985], the magnetic hysteresis parameters [Day, 1977], thermal demagnetization of low-temperature iso-thermal remanent magnetization (IRM) [Hunt et al, 1995], and the suspension method [Yoshida and Katsura, 1985]. In this paper, we demonstrate that the influence of magnetic interactions on ARM/z can be significant in natural sediments.…”

Variations in Fe3O4 and CaCO3 Contents in Deep-Sea Cores from the Western Equatorial Pacific.

“…experiments have shown that variations in magnetic grain size in the two cores are negligibly small (SUEISHI et al, 1979), and that the magnetic materials are mostly composed of single domain or pseudosingle domain-sized magnetite particles (YOSHIDA and KATSURA, 1985). Therefore, it can be concluded that the magnetite content of the sediments at the two coring sites has a similar variation in the past 2.2 million years, despite the different sedimentation rates.…”

“…However, the rock magnetic diagnosis suggested that magnetite of submicron-sizes is a major magnetic constituent in the sediments (YOSHIDA and KATSURA, 1985). It can, therefore, be concluded that the majority of magnetic minerals contained in these deep-sea sediments are of bacterial origin.…”

Comments on “Field-Strength Dependence of Directional Dispersion in Magnetizations of Sediment Samples: A Statistical Model and Paleomagnetic Evidence” by Hyodo and Itota

“…First, I tried to give an order estimate for ,c,, by using the data on the particle moment distribution for deep-sea sediments of calcareous ooze (Yoshida, 1984), because unfortunately there is no data available for samples themselves used by Hyodo and Itota (1993) or other shallow marine or lake sediments. The details of principle and techniques of the experimental method can be found in Yoshida and Katsura (1985). The data gave the arithmetic summation of particle moments Nm, which is equivalent to "Complete Alignment Magnetization" defined by Yoshida and Katsura (1985), so that by dividing Nra by the natural remanent magnetization (NRM) intensity we can obtain an order estimate for Kp of 10-3 -10-2.…”

“…The details of principle and techniques of the experimental method can be found in Yoshida and Katsura (1985). The data gave the arithmetic summation of particle moments Nm, which is equivalent to "Complete Alignment Magnetization" defined by Yoshida and Katsura (1985), so that by dividing Nra by the natural remanent magnetization (NRM) intensity we can obtain an order estimate for Kp of 10-3 -10-2. Such a small Kp value means that the directions of particle moments are nearly randomly distributed in the state of natural remanence, although the moments are collectively aligned along the ancient field direction.…”