Rotational remanent magnetization (RRM) and its high temporal and thermal stability

The time and temperature stability of various types of magnetic remanence has been measured in pottery samples containing magnetite and in a clay sample containing manganese ferrite. The time decay of rotational remanent magnetization (RRM), anhysteretic remanent magnetization (ARM) and a low-field isothermal remanent magnetization (IRM) has been measured. While the decay of the last two remanences is easily measurable at about 2 and 19 per cent per decade of time, respectively, the decay of RRM is too small to be measured, being less than about 0.1 per cent per decade of time. Thermal demagnetization of thermoremanent magnetization (TRM), ARM and RRM indicates that RRM is also the most thermally stable. The implications of these experiments are that rocks which exhibit gyromagnetic effects such as RRM contain highly stable particles and therefore are likely to be most suitable for palaeomagnetism.     

Gyroremanent magnetization and the magnetic properties of greigite-bearing clays in southern Sweden

The acquisition of a gyroremanent magnetization (GRM) by single-domain (SD) greigite particles during alternating-field (AF) demagnetization is demonstrated. Previous palaeomagnetic studies failed to identify the presence of authigenic greigite in the glacio-marine clays studied. These clays formed the subject of an earlier debate about the validity of a Late Weichselian geomagnetic excursion (the Gothenburg Flip) in southern Sweden. The greigite carries a stable chemical remanent magnetization (CRM), which coexists with a detrital remanent magnetization (DRM) carried by magnetite. AF demagnetization could not isolate the primary remanence in the sediments where magnetite and greigite coexist, due to the overlapping coercivity spectra of the two minerals and the inability to determine the time lag between sediment deposition and CRM formation. Thermal demagnetization removed the CRM at temperatures below 400 C, but this method was hindered by the unconsolidated nature of the sediments and the formation of secondary magnetic minerals at higher temperatures. The results suggest that the low-coercivity DRM carried by magnetite was mistaken for a 'viscous' component in the earlier studies. Hence the former debate about the record of the Gothenburg Flip may have been based on erroneous palaeomagnetic interpretations or non-reproducible results. AF demagnetization procedures applied to samples suspected of bearing SD magnetic particles (such as greigite) should be carefully selected to recognize and account for GRM acquisition.     

The detection of single-domain greigite (Fe3S4) using rotational remanent magnetization (RRM) and the effective gyro field (Bg): mineral magnetic and palaeomagnetic applications

The intensity of rotational remanent magnetization (RRM) acquired by single-domain greigite at a rotation frequency of 5 rps was combined with measurements of anhysteretic remanent magnetization (ARM) to calculate the effective biasing field ( Bg ) that produced the RRM. Samples of single-domain greigite had Bg values between -137 and -84 μT, and a MDF_RRM of c. 80 mT. By contrast, a suite of natural and synthetic ferrimagnetic iron oxide samples, including single-domain magnetite and y Fe₃O₄ tape particles, acquired Bg values between -3 and -14 μT, and MDF_RRM ranged between 43 and 68 mT (when RRM was acquired). Multidomain magnetite did not acquire a RRM. Bg values at 5 rps were calculated from previously published data for magnetite particles of different grain sizes, which revealed a minimum Bg value of -24 μT and a MDF_RRM of 57 mT for the finest fraction (0.2-0.8 μm in diameter). In a geological example, measurements of Bg and MDF_RRM were used to detect the presence of greigite in a 4 m long Late Weichselian sediment core. Variations in inclination, declination and the intensity of the natural remanent magnetization (NRM) correlate with changes in magnetic mineralogy.     

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.     

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.     

Palaeomagnetic results from remagnetized mid-Cretaceous(Albian–Cenomanian) strata of northeastern Venezuela

We report palaeomagnetic and rock magnetic results of a sedimentary sequence (Pertigalete cement quarry) located in northeastern Venezuela. Sampling was restricted to the vicinity of the contact between the upper Cretaceous Chimana and Querecual formations. Biostratigraphic evidence reveals an upper Albian age for this formational transition. Profiles of site-averaged NRM intensity of the high-coercivity (over 30 mT) and high-temperature (over 400 °C) components appear to be related to the contact and distance from the contact. We interpret this profile as the probable outcome of overlapping thermochemical remagnetization events resulting from hydrothermal activity that was focused along the two formations. Direct spectral analyses performed on the site-averaged stable NRM intensity profile allow the separation of at least two of these remagnetization events. On the other hand, palaeomagnetic results show a considerable streaking of site mean declinations, suggesting that tectonic or structural horizontal movements around a vertical axis have occurred after NRM acquisitions. Horizontal rotation angles, plotted against stratigraphic levels for bedding-corrected data, show some features that seem to coincide with the alteration peaks isolated in the profile of site-averaged stable NRM intensities. Thus, it appears that repeated thermochemical remagnetizations with overlapping unblocking spectra, and horizontal movements around a vertical axis could have been responsible for much of the within-site dispersion. A simple three-stage reconstruction of the possible chain of thermochemical and tectonic occurrences that could lead to the present-day palaeomagnetic and rock magnetic evidence is proposed. These events, including clockwise horizontal rotations around a vertical axis, are tentatively placed in a geological time framework between middle Miocene and Pliocene times according to the main geological and geochemical evidence available.