Calcareous nannoplankton response to the latest Cenomanian Oceanic Anoxic Event 2

Oceanic Anoxic Event (OAE)2 (latest Cenomanian, ca. 94 Ma) represents a major palaeoceanographic phenomenon that took place during an interval of extreme global warmth. High-resolution climate records reveal considerable changes in temperature, carbon cycling, and ocean chemistry during this climatic perturbation. In particular, a transient fall in temperature has been detected in the English Chalk on the basis of invasion of boreal faunas and bulk oxygen-isotope excursions: it was called Plenus Cold Event (PCE) characterized by two cooler water intervals punctuated by an intervening warmer episode.

Being calcareous nannoplankton sensitive to changes in surface water conditions, the nannofossil record is ideal to implement our understanding of the paleoceanographic perturbations occurring during OAE 2, and specifically the climatic variability. Previous studies documented how some calcareous nannofossil species, in particular B. constans, underwent size reduction during OAE 2 in response to increased pCO₂ and trace metal concentrations with a temporary size recovery in correspondence of the PCE.

Here, we integrate the nannofossil morphometric datum by reconstructing calcareous nannofossil abundance and assemblage composition in the same three sections (Eastbourne, UK; Clot Chevalier, France; Novara di Sicilia, Italy) previously studied for morphometries. In addition, we performed morphometrics of B. constans and nannofossil quantitative analyses on Tarfaya (Morocco) section. Correlation between the four studied sites is achieved by biostratigraphy and carbon-isotope stratigraphy (δ¹³C_carb and δ¹³C_org).

Nannofossil temperature and nutrient index are calculated to detect if variations in temperature and/or nutrients a) are coeval and similar in the four sections; b) depend on the latitude and/or the paleoceanographic context; c) during the PCE are registered in all studied sections; d) correlate with the nannofossil size changes.

Results highlight some similarities in the fluctuations of the relative abundance of the cold-water species E. floralis in Eastbourne and Clot Chevalier around the PCE interval not mirrored at lower latitudinal sites possibly suggesting that the inflow of cooler waters did not reach the lower latitudes.