Silurian bryozoans from Gotland fossilized in galena and sphalerite

Abstract

Marls and limestones from the Lower Visby Marl of Gotland, close to the Llandovery/Wenlock boundary, contain rare specimens of bryozoans fossilized in galena or sphalerite. One species (possibly two) are mineralized by galena, and one by sphalerite out of about 20 bryozoan species (and many more species from other animal groups) in the same rock. The mechanisms of mineralization seem to occur in several stages, possibly involving microbial activity. The lead content of the sediment also seems to be extremely high. These observations raise taphonomic, palaeontological, sedimentological and geochemical problems, which are discussed: 1) the origin of the lead (ordinary sea water?), 2) origin of the high metal (especially lead) content of the rock (accumulation through green algae?), 3) the mechanism of mineralization through replacement of the bryozoan skeletal calcite (involving changes in pH, and other processes in the pore water?) and 4) why galena was only deposited on one (or two) species and sphalerite in another (nucleation initiated by microbes on species specific matrix proteins?).