Global and regional factors responsible for the drowning of the Central Apennine Chattian carbonate platforms

Two stratigraphic sections of Central Apennine have been analyzed to interpret the factors responsible for the Oligocene-Miocene drowning of the carbonate platforms. The Mt. La Serra section, representing the Northern sector of the Latium-Abruzzi carbonate platform and the Opi section, representing the Eastern sector of the same domain have been investigated for facies and stable isotope characteristics (Brandano et al., in press). Important positive δ¹³C shifts have been recorded at the Oligocene-Miocene boundary in both sections. These shifts range from -0.34‰ 1 m below the top of the Chattian to +0.85‰ in the Aquitanian spongolitic marls in the Mt. La Serra section, and from +0.05‰ to +0.60‰ in the Aquitanian spongolitic interval of the Opi section. These facies and Cisotope variations reflect an increased productivity of surface seawater and correspond to changes from a middle carbonate ramp to outer ramp depositional environment. This drowning event is recorded not only in Apennine platforms, but also in other Mediterranean platforms such as in southern Apulia, Sicily and Malta, and outside the Mediterranean Basin, (e.g., the Carribbean area). We propose two possible major factors responsible for this drowning event: the Mi-1 event, and the volcanic activity peak developed in the Central-Western Mediterranean during the Chattian-Burdigalian interval. The Mi-1 glacial maximum (~24-23.5 Ma) may have had a strong influence on the drowning event on a global scale, as it triggered a weathering and continental runoff increase, which sustained eutrophic conditions and water turbidity. The development of a widespread subduction-related igneous activity in the Western Mediterranean during the Latest Oligocene-Early Miocene may have had even a stronger effect on the carbonate production of the Apennine platforms. Arrival of volcanic material such as airborne ash can have increased water turbidity, reducing light penetration and, above all, producing a huge fertilization effect of the euphotic marine environments, bringing micro- and macronutrients to the surface waters. A second consequence is an increase of the SiO2 seawater content, thus favouring siliceous organisms. Last not least, it has to be mentioned that the propellants for explosive volcanism are CO2, H2O and sulphur. A raise of both atmospheric and marine CO2 concentration could have led to a greenhouse effect and, thus, to an additional increase of weathering and runoff, and also to a decrease of seawater pH and a reduction of carbonate ions concentration, favouring calcite-dominated skeletal assemblages (heterozoans) and, above all, siliceous production