Comments on the paper “bioturbation: minimal effects on the magnetic fabric of some natural and experimental sediments”, by Brooks B. Ellwood

Abstract

Anisotropy of magnetic susceptibility (AMS) and remanent magnetization in semi-consolidated sediments reflect some ordered, spatial distributions of magnetic mineral grains, caused by forces acting during or shortly after deposition. Any preferred grain orientation is likely to be modified by biological disturbances, and in this paper by E1lwood [1] three different sediment types were investigated in order to elucidate the effect of bioturbation on magnetic fabric characteristics. We find his interpretation and conclusions derived from the tidal flat experiment (Sapelo Island) rather unsatisfactory, and offer hereby an alternative interpretation. The magnetic fabric in the established magnetite horizon gradually change towards what may be inferred to represent a primary depositional magnetic fabric, characterized by Q-values below 2/3 (oblate ellipsoids) associated with closely grouped, almost vertical distributions of kmin directions (minimum axis of the susceptibility ellipsoid). Mean azimuths of the flat-lying kma x axis all strike N-S, parallel to the shore line. Ellwood has inferred that this evidently post-depositionai reorientation of magnetite grains developed by subsurface long-axis grain alignment caused by water flowing horizontally through the (biological) expanded sediment. The apparent depositional (primary) characteristics of the magnetic fabric, is concluded to be secondary, improved during initial expansion followed by grain settling and dewatering after passage of organisms. In addition to