First occurrence and stable isotope composition of authigenic gamma-MnS in the central Gotland Deep (Baltic Sea)

The occurrence of authigenic gamma-MnS in organic matter-rich sediments of the central Gotland Deep is reported for the first time. The hexagonal manganese sulfide was found in a thin sediment layer with an approximate age of 1130 years and has an isotopic composition of delta(34)S approximate to -13 parts per thousand vs. V-CDT. This indicates that the bonded sulfur is derived from the anaerobic microbial reduction of pore water sulfate. SEM analysis reveals that gamma-MnS occurs as idiomorphic crystals of about 200 mu m length and 150 mu M width, much larger than the diameters of co-existing calcian rhodochrosite globules (similar to 6 mu M). It is suggested that formation of gamma-MnS took place in a diagenetic environment which was locally free from reactive iron.