Discrete element modeling of post-depositional remanent magnetization acquisition: numerical principles.

A discrete element model (DEM) is presented that simulates the processes considered to influence the formation of post-depositional remanent magnetization (PDRM) in marine and limnic sediments. The premise of the model is a collection of deformable particles held within a two-dimensional container and their behaviour in response to the influence of external forces. The considered external forces are gravity, magnetic torque and particle-particle plasto-elastic collisions. The calculation of the change in a given particle's velocity and position is performed according to Newton's laws of motion over extremely small time steps (fractions of a second). Under such conditions, the motion of any particle can be considered to only affect its nearest neighbours. This approach substantially increases the calculation efficiency of the DEM model but limits the time span which can be represented during one simulation. Interactions between touching particles can also included in the simulation and are represented in terms of contact forces with both normal and shear components. To simulate the short term processes of sedimentation and PDRM formation, we construct a model suspension of initially randomly distributed magnetic and non-magnetic particles settling under the influence of gravity in the presence of an external magnetic field. It is possible to modify the distributions of particle size, shape and magnetization in order to represent PDRM formation in different sedimentary settings. Additional forces can be assigned to the particles to represent naturally occurring phenomena such as van der Waals interactions, rotational Brownian motion, compaction by overlying sediment and bacterial scale bioturbation. The outcome of the individual DEM runs is evaluated by calculations of porosity, PDRM intensity, PDRM inclination and the numbers of free and locked particles within the sedimentary matrix. A series of models simulating different depositional conditions show that particle shape as well as compaction and the external magnetic field have a dominant influence on the acquisition of PDRM, whilst van der Waals forces control the formation of pore spaces within the sediment. Numerical details of the underlying physical processes and the numerical evaluations will be presented.