Origin of magnetic instability in sediment cores from the central North Pacific

Abstract

Previous paleomagnetic studies on deep-sea sediment cores from the central North Pacific have shown that the natural remanent magnetization (NRM) of these 'red clay' sediments was unstable below several meters depth in each core. It was also noted that the magnetic instability was related to the presence of a relatively large 1ow-coercivity component of magnetization. The purpose of this investigation was to characterize the rock magnetic properties in three select cores from this region to determine the physical .origin of the unstable magnetization. The principal findings of our investigation were as follows. (1) The ability to acquire a viscous remanent magnetization increased with depth in each core, particularly at about the level where unstable magnetization became evident. (2) The magnitude and stability of the observed NRM of the magnetically unstable section of each core can be explained by a viscous remanence acquired in the presence of the earth's magnetic field over a period of time ranging from only several weeks to several thousands of years. (3) The unstable magnetization, believed to be of viscous origin, was attributed to the presence of a magnetic mineral similar in structure and composition to maghemite. This mineral may have resulted from the low-temperature oxidation of very fine grained magnetite at about the time of deposition of these sediments. The extrapolated ages of the levels at which unstable magnetization becomes evident in the cores from this region suggest a close correspondence with the times of established upper Cenozoic climatic changes. Considerations of the alteration of the sedimentary regime resulting from the changes in climate can provide a satisfactory explanation for the observed change in magnetic properties. In a paleomagnetic study of a large suite of sediment cores from the North Pacific Opdyke and Foster [1970] reported a marked instability of magnetization that occurred with depth in most cores of 'red clay' lithology. The sediment cores in which the instability of magnetization was noted were located within a region of the central North Pacific characterized by a generally low sediment thickness and by Brunhes epoch sedimentation rates of the order of only several millimeters per thousand years (Figure 1). Both the low sedimentation rate and the small sediment thickness probably are related to the low biogenic productivity in the area [Fleming, 1957] and to the large distances from continental sediment sources. The marginal trenches surrounding much of the North Pacific basin also act as sediment traps and inhibit bottom transport of sediment to the center of the basin. The magnetic instability in these cores was characterized by ! /