Biogenic and Geochemical Properties of Intertidal Biosedimentary Deposits Related to Mytilus Beds

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Abstract
Biofacies development, sedimentology, and geochemical properties of intertidal Mytilus beds in the German Wadden Sea have been studied in vertical profiles in situ and in box core samples. Characteristic features of both the live mussel beds and the underlying sediment sequences were detected. The deposition of faeces, pseudofaeces, and trapped fine‐grained material results in the formation of well‐defined biosedimentary mud layers of several centimetres thickness. The initial stages of this deposition process were observed in a young, growing Myrilus colony over a 17‐month period. In an old mussel bed, being in state of decline, both reworking of dead shells and erosion of the biosedimentary mud layers were recognisable. However, deeper parts of the sediment column reveal shell horizons rich in “pairs” of dead Mytilus shells, i.e., corresponding right and left valves found close together. These represent subrecent records of former Mytilus colonisation. Historically, they point to long‐term occurrence of mussel beds at typical localities, in the framework of fluctuation and patchiness patterns, in the more recent past. On the other hand, mud banks at tidal flat margins adjacent to channels suggest a high preservation potential of biosedimentary mud layers in the low‐water level environment. Geochemical properties of these muddy sediments corroborate their biodepositional origin.