Mid-Cretaceous climatic fluctuations traced by calcareous nannofossils

Several regional to global episodes, known as Oceanic Anoxic Events (OAE)s occurred over the Aptian – early Turonian time interval and were characterized by excess CO2, intense volcanism, perturbed climate and altered oceanic chemistry. In order to either understand the dynamics leading to these “extreme” conditions and the ocean-atmosphere system recovery after the OAE, it is important to focus on a longer-term record thus to include the sequence of intervals of perturbation and the phases of stable conditions. For this purpose, calcareous nannoplankton is a useful tool, since it is extremely sensitive to changes in surface waters parameters like temperature and nutrient content and interacts with the C cycle through biological processes and production of calcareous oozes. We gathered new quantitative nannofossil data for the Tethys Ocean (Umbria Marche Basin, Italy) to derive climatic fluctuations and changes in ocean fertility during the late Albian – early Turonian. The new dataset was integrated with the nannofossil data collected for the Aptian – early Albian time interval to provide a compilation of temperature and surface water nutrient variations on the long-term through the Aptian – early Turonian interval in the western Tethys. The nannofossil Temperature Index (TI) outlines warm conditions for OAE 1a, followed by a cooling trend culminating soon after the N. truittii acme interval. Progressively increasing temperature (T) characterized instead the latest phases of the Aptian and warmer conditions were reached in the Albian – Cenomanian, although interrupted by relatively cooler phases, as for example in correspondence of the MCE. The highest paleo-T were reached across OAE 2. Surface water fertility (F) resulted instead to be relatively high during most of the early-middle Aptian, exception made for the N. truittii acme interval when a significant decrease occurred. The latest Aptian – early Albian interval was marked by intermediate trophism in surface waters, with intervals of higher F during black shales deposition. During the interval comprised between the OAE 1b and the OAE 1d, F was relatively high while it decreased in the Cenomanian. A distinct F pulse was detected prior to OAE 2. We correlated the TI with available paleo-T proxies: a good correlation exists with the ð18O records through the Aptian - upper Albian but it is not possible to provide a comparison for the lower-middle Albian since there are no ð18O data available. During the Cenomanian, the trends of the two paleo-T records are instead the opposite, with the TI shifted towards cooler T and displaying oscillations not evidenced by the ð18O. Further paleo-T reconstructions are available across the upper Aptian - lower Albian from TEX86 data from the proto-North Atlantic Ocean and show very good correspondence with the TI. We therefore calibrated the TI (which does not provide absolute T values) on the basis of TEX86 paleo-T and estimated the extent of T-variations.