Calcification of cyanobacterial filaments: Girvanella and the origin of lower Paleozoic lime mud

The origin of lime mud in ancient shallow-marine carbonate platforms is one of the most vexing mysteries in sedimentary geology. Possible sources prev iously suggested include the physical and biological disintegration of anim al and algal bioclasts, mineralizing pelagic organisms, and spontaneous whi tings from either abiotic precipitation or calcification of suspended picop lankton and organic matter. The styles of occurrence of micrite-walled tube s referred to Girvanella in lower Paleozoic limestones argue that occasiona l permineralization of mucilaginous sheaths of the benthic filamentous cyan obacteria precursor could have produced lime mud when the tubes fell apart. Estimated volumes are comparable to those calculated for the modern Bahama platform. The copious growth of cyanobacteria typical in shallow-marine se ttings indicates that this mechanism could account for a considerable amoun t of lime mud, especially in the early Paleozoic.