Carbon- and oxygen-isotope records of mid-Cretaceous Tethyan pelagic sequences from the Umbria – Marche and Belluno Basins (Italy)
Gambacorta, G.; Jenkyns, H. C.; Russo, F.; Tsikos, H.; Wilson, P. A.; Faucher, G.; Erba, E.
Newsletters on Stratigraphy Volume 48 Number 3 (2015), p. 299 - 323
published: Aug 1, 2015
ArtNo. ESP026004803005, Price: 29.00 €
The late Albian – early Turonian time interval was marked by major environmental changes at regional to global scales. Oceanic Anoxic Events (OAE) 1d and OAE 2 and the mid-Cenomanian Event (MCE I) are all associated with δ13C anomalies that indicate perturbations of the global carbon cycle. In this study, 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 pelagic settings characterized by the alternation of nannofossil-planktonic foraminiferal limestones, radiolarian-rich levels and shales. The δ13C records exhibit a ∼ 1 ‰ positive excursion in δ13Ccarb corresponding with OAE 1d and a much larger positive anomaly (∼ 3–4 ‰) in δ13Corg corresponding with OAE 2 (Bonarelli Level). Between the strata that record these two events a double-spiked minor (∼ 0.7 ‰) positive excursion in δ13Ccarb marks the MCE I. Locally, after deposition of the MCE I but before formation of the Bonarelli Level, sedimentation shifted to alternate dysoxic/anoxic and well-oxygenated conditions as represented by lithological rhythms of black shales/black chert bands and whitish limestones. δ18O data indicate a progressive decrease in values from the upper Albian up to the Bonarelli Level (uppermost Cenomanian), suggesting an overall warming trend. Carbon-isotope stratigraphy, calibrated against biostratigraphy, provides a robust framework for 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 the Bonarelli Level. Correlations with carbon-isotope reference curves from other localities allow the duration of the missing intervals to be estimated as between 160 and 510 kyr in the middle to latter part of OAE 2 and the 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 deeper parts of the Tethys Ocean during OAE 2 took at least 1 million years.