MICROBIALITE EPISODES IN THE JURASSIC-CRETACEOUS OF CENTRAL-SOUTHERN ITALY: A TALE OF MAJOR PALEOCEANOGRAPHIC DISRUPTIONS PUNCTUATING THE LONG-TERM EVOLUTION OF THE TETHYS OCEAN.

Composite floral and faunal assemblages of the marine realm are known to produce distinctive types of carbonate sediments as well as peculiar stratigraphic architectures by secreting mineralized skeletons or inducing carbonate precipitation. Carbonate factories may be thus grossly grouped into three different classes based on relative modes of carbonate production and environmental setting: a) the tropical factory dominated by photo-autotrophic, k-strategist organisms; b) the temperate-to-cool water factory dominated by heterotrophic, r-competitor organisms; and c) the mound factory dominated by microbial communities. Detailed investigations on biofacies, sedimentary and paleoecologic features of rapidly evolving shallow-water carbonate successions have demonstrated that microbial assemblages thrived throughout the Mesozoic in environmentally-stressed carbonate platforms which were basically unhealthy for k-strategist and, although to a lesser extent, for r-competitor organisms. In addition, there are several lines of evidence calling for a close genetic relationship between the instauration of stressed environmental conditions and large-scale blooms of marine cyanobacteria in the greenhouse tropical seas during several intervals of the Mesozoic characterized by sustained volcanic activity and/or massive methane release from gas hydrates. Marine cyanobacteria typically form pioneer communities which are actually restricted to extreme environments; furthermore they are imputed to represent typical dysaster taxa thriving during the onset and development of global paleoenvironmental disruptions in the oceanic realm.. As a consequence, the paleoecologic significance of fossil microbial communities is increasingly evaluated in the geologic studies focusing on metazoan extinctions, drowning events and biocalcification crises, especially during OAEs. Results presented herein focus on the geologic reconstruction of the environmental scenarios which regulated the installation, development and demise of several microbial carbonate factories in the Jurassic-Cretaceous from the Umbria-Marche Basin, the transition between the Apulia Platform and the Ionian basin, as well as the Apennine Platform (central-southern Italy). All of these paleogeographic domains were part of the Mesozoic Mediterranean Tethys. Detailed sedimentologic and paleoecologic investigations of selected, biostratigraphically constrained microbial key-markers have been carried out both in field exposures and thin sections. Widespread deposition of microbial-rich successions have been identified during discrete intervals of the Jurassic-Cretaceous in shallow- and deep-water settings (namely: Early Pliensbachian, Early Toarcian, Bajocian p.p.-Callovian p.p., Late Valanginian, late Early Aptian and Early-Middle Turonian). These shallow-water microbial-rich intervals typically correlate with sharp facies changes globally recorded by deep-water pelagic successions thus suggesting the occurrence of obtrusive paleoenvironmental processes driving major perturbations in the hydrosphere-biosphere system. Additional geochemical data which are increasingly available for the investigated successions have been coupled to an event stratigraphic approach (including the evaluation of the role exherted by geodinamics on the regulation of major bio-sedimentary cycles in the Jurassic-Cretaceous oceans) to perform: 1) a paleoceanographically and paleobiologically constrained scenario for the onset and development of cyanobacteria episodes; 2) a chronostratigraphically constrained framework with which to correlate the observed record in a regional and/or global perspective. The strict chronostratigraphic correlation among the most relevant microbialite episodes in the studied Mesozoic record and the peaks of CO2 concentration through time is particularly meaningful in terms of causal and effect relationships. It is assumed here that repeated environmental collapses through the Mesozoic promoted the flourishment of microbial associations in the oceans and enabled them to rapidly replace chlorozoan and/or foramol ones at the onset of major paleoceanographic disruptions affecting the ecologic evolution of carbonate platforms. Microbialites appear to be basically coeval with the beginning of relevant perturbations of the global carbon cycle, mostly coinciding with negative spikes of d13C curve, prior to the deposition of the black shales linked to the various Mesozoic OAEs, at the very onset of the related drownings of shallow-water factories. From this viewpoint, cyanobacteria may be considered as dysaster taxa which may have cooled hyper-greenhouse episodes and acted as major sinks for excess CO2 delivered to the oceans by way of outpouring at large igneous provinces (LIPs) and/or oxidation of destabilized methane gas-hydrates. Noteworthy, such a peculiar paleobiogeological role of cyanobacteria may have been shared by further dysaster taxa, such as several brachiopods, as in the cases of the Early Toarcian (Soaresirhynchia), Late Valanginian (Peregrinella) and late Early Aptian (Orbirhynchia) events at least.