Particle flocculation: One possible control on the magnetization of deep‐sea sediments

Abstract

Anisotropy of magnetic susceptibility measurements have been used to describe the orientations of magnetic grains in sediments from the South Atlantic Ocean. It has been observed that magnetic grain long axis alignment is normal to the expected bottom current direction in the Vema Channel. Traction transport is the inferred mechanism for this alignment. It is proposed here that one possible explanation for this process is sediment aggregation into large floccules (>16 µm), which are of low density due to entrapped water and can therefore become elogated and aligned due to shear produced by bottom currents. The resulting poorly compacted bottom sediment compresses after accumulation of small amounts of overburden. Below ∼ 2 meters the compressed magnetic fabric, residing in the larger magnetic grains, is consistent with expected current deposited primary sedimentary fabric.The process of particle aggregation, current controlled deposition, compaction, and particle dewatering and breakup can explain the remanent magnetic characteristics observed in some deep‐sea sediments. A DRM inclination error of ∼11°, found at the top of core CH115‐61, disappears with depth in the core; RM inclinations steepen to −50° consistent with an axial geocentric magnetic field inclination for the site. It is suggested that orientations of very small magnetic grains are initially constrained within large floccules and that a DRM inclination error is produced during deposition. Later, floccule disaggregation and dewatering allows realignment of these very small grains along the geomagnetic field direction.