LIEBEZEIT, G., BÖTTCHER, M.E., GEHLKEN, P.-L., GERDES, G., GIANI, L., HEINZE, A., MEDERER, J., SCHNETGER, B. & SEGL, M. (2007):
Methane-derived carbonate concretions in modern intertidal flats.
A large number of carbonate concretions have been found in the tidal flat area of the drowned village Otzum behind the backbarrier island of Langeoog, Lower Saxonian Wadden Sea, southern North Sea. From field observations they are associated with the former settlement having, however, formed in intertidal flat sediments. The majority occur as horizontal plates although in a few instances vertical positions have also been observed. The latter appear to be related to mud cracks.
The concretions consist of about 40 – 50 % calcium carbonate (essentially low-magnesium calcite and aragonite), the remainder being quartz and traces of chlorite, illite, kaolinite and feldspar. Iron, magnesium, strontium, sulphur and phosphorus are partly associated with the carbonate fraction although dissolution of the latter elements during carbonate removal cannot be ruled out. Organic carbon contents are typically low.
Carbonate carbon has a 13C down to -35‰ vs. V-PDB, indicating that oxidation of organic matter or methane contributed significantly to the dissolved carbonate species fixed in the authigenic calcium carbonate (low Mg Cc) lattice. Oxygen isotope ratios indicate formation from only slightly diluted sweater solutions.
Microfacies analysis suggests that the secondary carbonates formed within primary matrices characteristic of both sandy as well as muddy tidal flats. The primary fabrics of the sand-flat type are composed of rounded quartz grains of fine to medium sand size. Included are various biogenic compounds such as diatom and mollusc shells characteristic of tidal flats, but also botanical macro-remains. The primary pore spaces are almost entirely occupied by carbonate crystals. In the carbonate precipitates various diatom tests have left moulds. In the mud-flat concretion, biogenic compounds increase in number and diversity. Some of these are also furnished with organic coatings characteristic of biofilms. Remaining primary pore spaces as well as secondary pores are relatively more frequent within this type of concretion. Pyrite framboids are also present occasionally.