PYRITE IN MINERALIZED SPONGE TISSUE - PRODUCT OF SULFATE-REDUCING SPONGE RELATED BACTERIA

Isolated pyrite crystals are common in taphonomically mineralized spon ges tissues (automicrites). In the surrounding sediment, pyrite is abs ent or rare. The sponge tissue automicrites are often dark coloured du e to statistically distributed very fine pyrite crystals (approximatel y 1 mu m or smaller). Besides the small pyrite, larger crystals often exhibit patchy concentrations or they are arranged in rows. Pyrite for mation is probably linked with sulfate reducing symbiontic bacteria in the sponge mesohyle. Modern sponges possess mesohyle symbiontic bacte ria including sulfate reducing and further anaerobic bacteria. During early decaying processes of the sponge tissue the internal sponge spac e is becoming entirely anaerobic which favours the growth of the sulfa te reducing bacteria. The sulfate reducing process significantly incre ases the carbonate alkalinity which is necessary to form automicrite. This process may explain the rapid calcification of sponge tissue in m odern marine microbialites and ancient sponge mud mounds.