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 sediment s of the central Gotland Deep is reported for the first time. The hexa gonal manganese sulfide was found in a thin sediment layer with an app roximate age of 1130 years and has an isotopic composition of delta(34 )S approximate to -13 parts per thousand vs. V-CDT. This indicates tha t the bonded sulfur is derived from the anaerobic microbial reduction of pore water sulfate. SEM analysis reveals that gamma-MnS occurs as i diomorphic crystals of about 200 mu m length and 150 mu M width, much larger than the diameters of co-existing calcian rhodochrosite globule s (similar to 6 mu M). It is suggested that formation of gamma-MnS too k place in a diagenetic environment which was locally free from reacti ve iron.