AmericanChemicalSociety.com

Although it is generally accepted that sediments record past variations of the geomagnetic field, there is still no consensus on how sediments get magnetized. They contain tiny grains that are permanent magnets which can rotate freely when suspended in water. These will tend to turn into the direction of the applied field just as compass needles do. Because of the viscous drag of the water, the magnetic grains will frequently only be partially aligned, so the net magnetization is not at saturation but is usually proportional to the strength of the magnetic field. The net magnetization of such particles, if locked in place is called a depositional remanent magnetization (DRM). Sediments are also subject to post-depositional modification through the action of organisms, compaction, diagenesis and the acquisition of secondary magnetizations all of which will affect the magnetization. This magnetization is called post-depositional remanent magnetization or pDRM. Our study focused on the former. Although it is theoretically possible to get both the direction and intensity of the geomagnetic field from sedimentary records, the mechanism is not fully understood. Previous workers have postulated that a process of clumping of sedimentary particles known as flocculation could play an important role in DRM. Flocs are porous, loose and highly fragile aggregates of microscopic clay particles and their behavior in a viscous medium is likely to be different than single particles of magnetic minerals. In order to understand the role of flocculation in sediment magnetization, we carried out a set of experiments whereby mud was redeposited in the laboratory under different conditions of the applied magnetic field intensities. We varied the degree to which the sediment flocculated by varying the salt content of the water - a process that is quite effective in changing the size of the flocs, but does not change the inherent magnetic properties of the sediments. The magnetic field was oriented at an angle with respect to the horizontal of 45°. We analyzed our results in terms of a simple numerical model of flocculation, incorporating both magnetic and hydrodynamic torques to explain the experimental data. We found that at small floc sizes the DRM was likely to be acquired in a non-linear fashion with respect to the applied field strengths – a finding in direct conflict with the fundamental assumption in routine studies of relative paleointensity in sediments. On the other hand, the sediments were able to record the directions accurately. With increasing floc sizes sediments tended to retain a record of the intensity that was linearly related to the applied field or a direction parallel to the applied field, but were not able to do both at the same time. Also, we found that large flocs, comprising the majority of the magnetic particles in the sediments, are essentially randomly oriented and probably do not contribute significantly towards the net DRM and any bulk normalizing parameter may be unsuitable if the depositional environment has changed over the depositional period. Our results pertain directly to the use of relative paleointensity records in sediments for understanding past geomagnetic field behavior. They urge caution and strongly suggest that additional studies are necessary to understand the processes of sedimentary magnetization.