CYANOBACTERIAL KEY TO THE GENESIS OF MICRITIC AND PELOIDAL LIMESTONESIN ANCIENT SEAS

The origin of micritic and peloidal limestones comprising the bulk of many ancient marine carbonate deposits represents a major unsolved pro blem of carbonate sedimentology. Our studies of such limestones from a sequence of Late Jurassic open marine sediments exposed in central Po land revealed them as products of in situ calcified mats of benthic co ccoid cyanobacteria. Remains of the cyanobacteria are visible in scann ing electron microscope (SEM) images as characteristic patterns closel y resembling the common mucilage sheaths of modern entophysalidacean a nd/or pleurocapsalean cyanobacteria comparable to those we found produ cing micritic and peloidal microbialites in Lake Van, Turkey. We sugge st, by analogy, that many subtidal micritic and peloidal limestones co mmon in the marine sedimentary record might be products of similar in situ calcified cyanobacterial microbiota. Such an intensive calcificat ion of marine cyanobacteria could have proceeded only in environments more than modern seawater supersaturated with respect to calcium carbo nate minerals. Advection of excess alkalinity, originating from deeper , anaerobic or dysaerobic zones to shallow water areas is proposed as the main factor enhancing colonization of extensive sea bottom areas b y the alkaliphilic cyanobacteria and promoting their in vivo calcifica tion.