A magnetization process of sediments: laboratory experiments on post-depositional remanent magnetization

Abstract

Palaeomagnetic studies require a theory of magnetization mechanism of sediments and a method of estimating magnetic field intensity from their remanences. This paper establishes a physical basis for the generation of the remanence in deep-sea and lake sediments experimentally.Redeposition experiments have been carried out under centrifugal force in weak magnetic fields. The centrifuging method produces post-depositional remanent magnetization (post-DRM) in the compacted sediment, and its remanence and susceptibility are compatible with those of natural sediments and reconstituted materials of other redepositional experiments. Three properties of the post-DRM have been deduced from the experiments: (1) the efficiency of acquisition of post-DRM decreases with increase in density during the compaction process, (2) the total post-DRM is equal to the sum of the partial post-DRM (addition law), and (3) time is not a substantial factor for alignment of the magnetic particles. These results lead to the conclusions that the magnetic particles do not rotate steadily but in a series of steps, and that the density change is the crucial factor giving rise to the post-DRM.A mathematical formula representing the remanence record in sediments is proposed on the basis of the experimental results and the model. The principal equation is expressed as an integral of the product of three parameters over time when sediments have been compacted; the field intensity variation, characteristic function of the sediment and the time derivative of the density change.

Possibility of reconstruction of the past geomagnetic field from homogeneous sediments.

“…The intensity drop shows a convex decrease and the following recovery a concave increase. These features are not only observed in natural deep-sea sediments (OPDYKE et al, 1966;HARISON and SOMAYAJULU, 1966;KOBAYASHI et al, 1971) but also explained by some models (LOvLIE, 1976;NIITSUMA, 1977;OTOFUJI and SASAJIMA, 1981). The present model is applied to the magnetization data of the core KH 68-4-20 (KOBAYASHI et al, 1971).…”

“…YASKAWA (1974) postulated only an increase of viscosity due to compaction as a factor determining the depth of fixing of magnetic grains. OTOFUJI and SASAJIMA (1981) demonstrated that a magnetic particle rotation does not occur steadily but in a halting fashion and that the rotation is dominantly controlled by a maximum friction with matrix grains but not by the viscosity.…”

“…The degree of compaction was replaced with a water content of a sample (PAYNE and VEROS 1982). (c) A specimen is compressed using an apparatus (HAMANO,1980;OTOFUJI and SASAJIMA, 1981). A sediment was compacted with various degrees which correspond to depths in a natural sediment.…”

Magnetobiostratigraphy of Planktonic Foraminiferal Datums: Deep Sea Drilling Project Leg 94, North Atlantic

“…2, 3). It is not certain at what depth below the sediment/water interface the remanent magnetization is locked in (Denham and Chave, 1982;Tucker, 1980;Otofuji and Sasjima, 1981). Because of bioturbation, the magnetic minerals will not remain stably oriented in the upper few centimeters of the sediment, so the remanence is most probably set at or below the mixed layer.…”

Paleomagnetism of a core from Osaka Bay.

“…This demonstrates that magnetization of sediment under suitable conditions can be a recorder of the geomagnetic field. Thus, the study of paleofield using sediments has made progress on the basis of fundamental investigations of magnetizing process (IRvING and MAJOR, 1964;TUCKER, 1980;HAMANO, 1980;OTOFUJI and SASAJIMA, 1981).…”