New Early Pliensbachian high-resolution C-isotope record from the Trento Platform (Early Jurassic) and insights on the diffusion of the Lithiotis Fauna

In this contribution a new high-resolution carbonate carbon-isotope record and the distribution of the Lithiotis Fauna in the Lower Jurassic marine shallow-water carbonates of the Trento Platform (Southern Alps, Italy) are compared. The Lithiotis Fauna represents a unique and short-lived event in which mound-building bivalves experienced global diffusion in the Early Jurassic. The integration of isotope data with the fossil distribution allowed to explore the possible cause-effect relationships between the climate and environmental changes marked by features in the isotope record and the diffusion of the peculiar Lithiotis Fauna. A set of carbonate carbon-isotope excursions of 2–3‰, articulate in three distinct negative shifts followed by positive rebounds, are recorded in the isotope stratigraphy carried out on three stratigraphic sections and can be correlated to the global negative δ13C shift of the Sinemurian–Pliensbachian boundary event (S–P Event) and to the subsequent phase of C-isotope perturbation that characterized the lower Pliensbachian. The S-P Event triggered eutrophic conditions as illustrated by the presence of organic-rich facies and by fossil associations characteristic of poorly oxygenated waters. After the eutrophic phase, an amelioration of environmental conditions is marked by a positive ~ 3‰ rebound of the δ13Ccarb values, and by the occurrence of marine stenotypic faunas. On the Trento Platform, the stabilization of the δ13Ccarb values is coincident with the appearance of the Lithiotis Fauna that subsequently becomes widely distributed in the entire range of platform environments showing maximum abundance in the late Pliensbachian when metric-scale mounds were generated. During the same time, the maximum proliferation of the Lithiotis Fauna is recorded both in the Tethyan and Panthalassa regions together with a continent reorganization that may have led to the opening of the Hispanic Corridor, a seaway cutting through Pangea at tropical latitudes. The processes behind the opening of the Hispanic Corridor may have been responsible of the injection of CO2 into the atmosphere-ocean system that triggered the S-P Event. Hence, the reported relationships between the δ13Ccarb data and the distribution and ecological characteristics of the genera contained in the Lithiotis Fauna suggest that the S– P Event and its aftermath, possibly coupled with the opening of the Hispanic Corridor, could have set the stage for the rapid diffusion of these unusual bivalves across many parts of the globe.