Flocculation mechanism of the acquisition of remanent magnetization by sedimentary rocks

Coagulation of particles into aggregates during their deposition in a reservoir is numerically simulated with regard for Brownian motion, Van der Waals forces, gravitation, Stokes friction, and magnetostatic interaction, and the effect of this process on the depositional magnetization (DRM) is estimated. Clusters obtained due to random aggregation of smaller clusters have a loose and branching structure. The average fractal dimension of the clusters is d = 1.83 +/- 0.23. In the process of coagulation, magnetic particles do not form chains or clusters, as was supposed in a number of preceding works, but become rather uniformly distributed among nonmagnetic particles, which provides an additional argument in favor of the fact that chains of magnetite particles in marine sediments are of biogenic origin rather than a result of mutual attraction of magnetic particles due to magnetostatic interaction. The deposition process is shown to obey a kind of the principle of scale invariance: the number of clusters and their average number of particles do not change if the basin depth H and the surface concentration of initial material c(0) simultaneously change (provided that temperature and the initial particle size r are constant) in such a way that Hc(0) = const. Coagulation is the most important factor forming the bottom layer structure and the magnetization of the suspension at a relatively high concentration c(0) typical of redeposition conditions, lakes, and shelf seas. Coagulation virtually does not influence oceanic sediments because of the smallness of c(0).