Origin of the early Albian oceanic anoxic event 1b in the western tethys (Piobbico core, central Italy) and north Atlantic (ODP Leg 171b site 1049c): calcareous nannofossil evidence.

The mid-Cretaceous greenhouse climate represents the most extreme warmth of the past 300 My, characterized by intense volcanism and increased ocean crust production. Moreover, the mid-Cretaceous is associated with oceanic anoxic events (OAEs), periods of elevated carbon buriaI in marine sediments. Calcareous nannoplankton were sensitive to changes in temperature, fertility, and chemistry of surface waters. Consequently, abundance and composition of nannofossil assemblages are used to reconstruct paleoclimatic fluctuations and the functioning of the ocean/atmosphere system. Specifically, calcareous nannofloras are used for characterization of the earliest Albian OAE1b in order to assess the role of paleoclimatic changes leading to possible perturbations of surface water masses as triggers of anoxia. Oceanic Drilling Program (ODP) Site 1049C (Blake Nose) recovered Aptian to Eocene sediments. The upper Aptian – lower Albian sequence consists of marls and calcareous marls interrupted by a prominent black shale interval (46 cm thick), that has been identified as the local expression of OAE 1b. Calcareous nannofossils were investigated every 20 cm in the upper Albian – lower Albian sediments and the early Albian OAE1b interval was sampled at high-resolution (1 cm). A similar sampling was applied to the coeval interval in the Piobbico core, drilled in the Umbria-Marche Basin (central Italy). In both sections nannofossils are well preserved and the dominant species are W.barnesiae, R. asper, B. costans, Z. erectus, D.rotatorius, L. carniolensis. The warm-water taxa (R.asper and Z.diplogrammus) increase in abundance and temperature index shows warmer conditions within OAE 1b. This result is consistent with the oxygen stable isotope record, suggesting a significant warming at the onset of OAE1b.The meso-eutrophic taxa (Z.erectus, D.rotatorius and B.costans) increase in abundance 50 cm before OAE1b in the Piobbico core. and 2 meters before OAE 1b at Site 1049C. Within black shales the fertility taxa and nutrient index are generally high although fluctuating, and a systematic relationship with the d13C excursion is not observed. Similar patterns have been documented for the Vocontian Trough (Herrle, 2003b). The nannofossil record suggests that productivity increased well before OAE1b and, consequently, cannot be the trigger of anoxia. In the western Tethys - North Atlantic area, OAE 1b is associated with a strong increase in surface-water temperatures as recorded by calcareous nannofloras. As suggested by Erbacher et al (2002), OAE1b might be regarded as a “super-sapropel”: extremely warm conditions and increased precipitation and continental runoff induced abnormal stratification of the water column with development of dysaerobic-anoxic conditions and accumulation/preservation of organic matter on the sea floor. References: Erbacher, J., Huber, B.T., Norris, R.D., Markey, M., 2001. Increased thermohaline stratification as a possible cause for an oceanic anoxic event in the Cretaceous periode. Nature 409, 325-327. Herrle, J.O., Pross, J., Friedrich, O., Kössler, P., Hemleben, C., 2003b. Forcing mechanisms for mid-Cretaceous black shale formation: evidence from the upper Aptian and lower Albian of the Vocontian basin (SE France). Palaeogeogr. Palaeoclimatolog. Palaeoecol., 190, 399-426.