Using geomagnetic secular variation to separate remanent and induced sources of the crustal magnetic field

Magnetic fields originating from magnetized crustal rocks dominate the geomagnetic spectrum at wavelengths of 0.1–100 km. It is not known whether the magnetization is predominantly induced or remanent, and static surveys cannot discriminate between the two. Long‐running magnetic observatories offer a chance, in principle, of separating the two sources because secular variation leads to a change in the main inducing field, which in turn causes a change in the induced part of the short‐wavelength crustal field. We first argue that the induced crustal field, b ^I( t ), is linearly related to the local core field, B ( t ), through a symmetric, trace‐free matrix A : b ^I( t )= A B ( t ). We then subtract a core field model from the observatory annual means and invert the residuals for three components of the remanent field, b ^R( t ), and the five independent elements of A . Applying the method to 20 European observatories, all of which have recorded for more than 50 years, shows that the most difficult task is to distinguish b ^R from the steady part of b ^I. However, for nine observatories a time‐dependent induced field fits the data better than a steady remanent field at the 99 per cent confidence level, suggesting the presence of a significant induced component to the magnetization.     

The angular dependence of rotational and anhysteretic remanent magnetizations in rotating rock samples

Summary. Recent experimental work by Edwards has demonstrated that rotational remanent magnetization (RRM) is not a maximum when the alternating field is normal to the rotation axis of the sample (a rock) but is greatest when the angle is about 75°. Experiments involving the production of ARM during sample rotation gave a similar result with a maximum at about 60°. These results are explained here in terms of the response of an isotropic assembly of identical single-domain particles to a strong alternating magnetic field.     

Synfolding magnetization: detection, testing and geological applications

A correct method for determining the direction of a synfolding remanence is described, and a new test, which in a number of cases can discriminate between a true synfolding remanence and a remanence that is the sum of pre- and post-folding components, is suggested. It is shown that it is possible to reveal the folding evolution of a local object under an assumption that the acquisition of the synfolding remanence was synchronous over this object.     

Dating of thaw depths in permafrost terrain by the palaeomagnetic method: experimental acquisition of a freezing remanent magnetization

The acquisition of a freezing remanent magnetization (FRM) has been studied in controlled magnetic and thermal environments by successive freezing and thawing (−18 to +20°C) of samples of natural sediments from a frost polygon near Ny Ålesund, Spitsbergen. Successive freeze-thaw cycles cause a significant decrease in the intensity of the initially induced shock remanent magnetization (SRM), associated with directional trends towards the ambient magnetic field direction during the freezing phase. A slow increase in intensity commences after seven to 10 freeze-thaw cycles. The acquisition of a FRM in samples carrying an isothermal remanent magnetization shows a significantly smaller reduction in intensity and only minor directional variations. This result indicates that only a fraction of the magnetic grains in a natural sediment contributes to the natural remanent magnetization. Insignificant changes in lengths and directions of the principal susceptibility ellipsoid axes also indicate that magnetic fabric and remanent magnetization are carried by partly different populations of magnetic grains.
The acquisition of a FRM in nature has yet to be explored. If such a process is confirmed, however, it has the potential for obtaining age estimates of ancient thaw depths and for providing insights into material transport processes in frost polygons.     

Imaging distribution patterns of magnetic minerals by a novel high-Tc-SQUID-based field distribution measuring system: application to Permian sediments

A newly developed field distribution measuring system based on a high- T _c SQUID has been employed in the study of magnetic mineral distribution in several Permian sedimentary rocks. The instrument consists of a small, 1.4×1.4 mm sized YBaCu-oxide SQUID magnetic field sensor that is operated in a thin-walled dewar, so that the sample's surface, at room temperature, can be scanned at a distance of only ∼1.5 mm. The samples were subjected to a saturation remanence perpendicular to the surface and the scanning measurements in zero field reveal that the magnetization might be carried by only a small part of a sample, in one case associated with secondary oxide phases. High-resolution magnetic scans can aid in the interpretation of the magnetic remanence acquisition process.     

Are hydrodynamic shape effects important when modelling the formation of depositional remanent magnetization?

Recent conceptual models have attributed the weak depositional remanent magnetizations observed in natural sediments to flocculation processes in the water column. Magnetic particles included into flocs have not only to rotate themselves into alignment with the geomagnetic field but also the larger particles to which they are attached, making remanence acquisition an inefficient process. Alignment is hindered further when the magnetization vectors of the particles in any given floc partially cancel, reducing the overall magnetic torque. Existing numerical simulations of flocculation effects on depositional remanence formation have been limited to spherical bodies with translational and rotational motion acting independently of each other. In the case of non-spherical flocs, the translational and rotational motion are coupled and such bodies will describe a complex trajectory through the water column. Calculations will be presented that show the torque exerted on a non-spherical floc by the surrounding water can be orders of magnitude greater than the magnetic torque. Non-spherical flocs will, therefore, align less efficiently with the geomagnetic field and hydrodynamic effects may play an important role in controlling the magnitude of sedimentary remanence.