Carbon- and oxygen-isotope records of mid-Cretaceous Tethyan pelagic sequences from the Umbria–Marche and Belluno Basins (Italy)

The late Albian–early Turonian time interval was marked by major environmental changes at regional to global scales. Oceanic Anoxic Events (OAE) 1d and OAE2 and the mid-Cenomanian Event(MCEI) are all associated with δ13C anomalies that indicate perturbations of the global carbon cycle. In thisstudy, new carbon- and oxygen-isotope records are presented from four Italian Tethyan sections: Furlo, Le Brecce and Monte Petrano (Umbria–Marche Basin) and Cismon (Belluno Basin), deposited in pelagicsettings characterized by the alternation of nannofossil-planktonic foraminiferal limestones, radiolarian-richlevels and shales. The δ13C records exhibit a ~1‰ positive excursion in δ13Ccarbcorresponding with OAE1dand a much larger positive anomaly (~3–4‰) in δ13Corgcorresponding with OAE2 (Bonarelli Level). Be-tween the strata that record these two events a double-spiked minor (~0.7‰) positive excursion in δ13Ccarbmarks the MCEI. Locally, after deposition of the MCEI but before formation of the Bonarelli Level, sedi-mentation shifted to alternate dysoxic/anoxic and well-oxygenated conditions as represented by lithologicalrhythms of black shales/black chert bands and whitish limestones. δ18O data indicate a progressive decreasein values from the upper Albian up to the Bonarelli Level (uppermost Cenomanian), suggesting an overallwarming trend. Carbon-isotope stratigraphy, calibrated against biostratigraphy, provides a robust frameworkfor dating and correlation, and allows identification of hiatuses in the studied sections. In particular, the high-resolution δ13C profiles through the Bonarelli Level highlight the presence of hiatuses of variable extent that systematically affect the middle to upper part of the characteristic carbon-isotope excursion and part of the following interval in all of the studied sections. Locally, a hiatus may also be present at the base of theBonarelli Level. Correlations with carbon-isotope reference curves from other localities allow the durationof the missing intervals to be estimated as between 160 and 510kyr in the middle to latter part of OAE2 andthe succeeding interval, showing that the Bonarelli Level extends no higher than the uppermost Cenomanian,even though the isotopic definition of the oceanic anoxic event indicates continuation into the early Turonian.The origin of these stratigraphic gaps can be attributed to physical and/or chemical processes operating at the sea floor. Complete recovery from the peculiar physico-chemical conditions that characterized the deeperparts of the Tethys Ocean during OAE2 took at least 1million years.