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

Abstract

A B S T R A C TRecent 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.

Natural remanent magnetization acquisition in bioturbated sediment: General theory and implications for relative paleointensity reconstructions

“…An essential condition for the validity of equation is that flocs behave as discrete bodies with negligible reciprocal interaction forces. For example, can be identified with the amplitude of hydrodynamic torques from small vortices created by magnetic flocs sinking in the water column [ Heslop , ]. In this case, equation , along with an appropriate description of flocculation dynamics [ Shcherbakov and Sycheva , ], governs DRM acquisition.…”

“…In this case, equation , along with an appropriate description of flocculation dynamics [ Shcherbakov and Sycheva , ], governs DRM acquisition. Using the calculations of Heslop [], equation yields for 8 µm flocs with an aspect ratio of 1.1 sinking in a 50 µT field, and for 12 µm flocs. This example illustrates the well‐known problem of nonlinear DRM acquisition by smaller flocs [ Tauxe et al ., ].…”

“…The two main NRM acquisition processes considered by these investigations are the depositional remanent magnetization (DRM), which is acquired during and shortly after deposition, and a delayed, so‐called postdepositional remanent magnetization (PDRM). DRM acquisition models focus on flocculation of settling particles in the water column, particle rolling at the point of deposition [ Griffiths et al ., 1960; Jezek et al ., ; Bilardello et al ., ], and resuspension/reflocculation near the sediment‐water interface [ Van Vreumingen , ; Katari and Tauxe , ; Tauxe et al ., ; Heslop , ; Shcherbakov and Sycheva , ]. On the other hand, PDRM is defined as a remanent magnetization that is acquired upon magnetic particle rotation against the yield strength of sediment until it is fully locked inside the consolidating layer [ Shcherbakov and Shcherbakova , ; Roberts et al ., ].…”

Geomagnetic Excursions and Secular Variations

“…Similar clockwise loops for London, Paris, and Rome are indicative of regionally coherent secular variation. If deposition occurs in the presence of even a minor hydro/aerodynamic disturbance, the perturbation will exceed the geomagnetic torque and magnetic particles will not be ideally aligned with the geomagnetic field (e.g., Heslop, 2007). During the twentieth century, increased electromagnetic interference from cultural sources has meant that observatories in all cities shown have been moved to more magnetically quiet locations.…”

On the mechanism of formation of depositional remanent magnetization

“…Also, we will avoid problems related to the hydrodynamic torques developing during the settling and flocculation of grains. Attempts to treat this difficult topic are presented by Chermous and Shcherbakov [1980], Heslop [2007], and Mitra and Tauxe [2009]. Finally, no chemical and biological processes will be taken into account though we fully appreciate that their role also can be critical.…”