Manganese diagenesis in temporal and permanent depositional areas of the North Sea

摘要

In hydrodynamically active systems, such as the North Sea, Mn (hydr-) oxide distributions in the sediment depend upon: (1) redox conditions, (2) sediment reworking by biota, and (3) on the hydrodynamic regime, through episodic sorting of particles. After a perturbation, pore water distributions adjust rapidly to new conditions, whereas the solid phase may exhibit transient concentration profiles for a longer period. The aim of this study was to investigate Mn diagenesis in such dynamic shelf sediments. We sampled three silty stations in the North Sea (Frisian Front, German Bight and Skagerrak), during February, May, August 1994 and November 1995. The Skagerrak site represents a very stable environment; this served as base-line for the moderately energetic Frisian Front and highly energetic German Bight sites. Profiles of solid phase Mn were transient at the Frisian Front and the German Bight, although the pore water profiles appeared at steady state. Association of the Mn oxides with organic matter, preferentially reworked by biota, was one reason for the non-steady-state depth distributions. The second reason was the hydrodynamic regime at the shallow sites which affected the vertical layering of particle size distribution and thus of the Mn associated with specific grain sizes. Mn diagenesis is stimulated in dynamic temporal deposition areas due to burial of reactive Mn oxides and organic material below the depth of oxygen penetration, thereby providing the prerequisites for sub-oxic diagenesis. (C) 2003 Elsevier Science Ltd. All rights reserved. [References: 86]