Early Pliensbachian (Early Jurassic) C-isotope perturbation and the diffusion of the Lithiotis Fauna: Insights from the western Tethys

Abstract

High-resolution carbonate carbon-isotope stratigraphy of Lower Jurassic marine shallow-water limestones are compared with the distribution of the Lithiotis Fauna on the Trento Platform (Southern Alps, Italy). The Lithiotis Fauna is the first example of globally distributed mound-building bivalves in the geological record and experienced global diffusion in the Early Jurassic. A set of carbonate carbon-isotope excursions of 2–3‰, illustrating three distinct negative shifts followed by positive rebounds, are recorded in the isotope stratigraphy and can be correlated with 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. In the studied stratigraphic sections, the S–P Event likely triggered eutrophic conditions illustrated by the presence of organic-rich facies and by fossil associations characteristic of poorly oxygenated waters. After the eutrophic phase, the amelioration of environmental conditions was 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 coincided with the appearance of the Lithiotis Fauna that subsequently became widely distributed in the entire range of platform environments and thrived in the late Pliensbachian when metric-scale bivalve mounds were generated. During the same time, the maximum proliferation of the Lithiotis Fauna is recorded both in the Tethyan and Panthalassa regions. 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 undergoing coeval continent reorganization that led to 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.