Calcareous nannoplankton response to Oceanic Anoxic Event 2 (Cenomanian-Turonian, Late Cretaceous)

The Cenomanian/Turonian boundary interval was marked by an extreme environmental change coinciding with Oceanic Anoxic Event 2 (OAE 2; ~94 Ma) characterized by global warming, accelerated hydrological cycle, enhanced production and bu­rial of organic matter and high concentrations of CO₂. The causes are linked to Large Igneous Provinces (LIPs) activity, which released vast amounts of greenhouse gases into the ocean-atmosphere system as well as toxic metals responsible of altered chemistry and structure of the oceans. Also, the OAE 2 experienced a significant brief period of cooling (ca. 40 kyr) occurred known as the Plenus Cold Event (PCE), firstly identified and described in the Eastbourne section (Sussex, England). In its complex, the OAE 2 represents a key case study to understand the dynamics of oceanic process as well as the response of biota to climate change.

In this work we present data on calcareous nannofossils from the Eastbourne section, which is a reference section for OAE 2, with the general aim to implement our understanding of the effects of OAE 2 perturbation on calcareous nannoplankton. Specifically, 44 samples were analysed for morphometry of Eprolithus floralis and assemblage composition. For each sample nannofossil temperature and nutrient indices were also calculated.

We selected Eprolithus floralis because is a nannolith taxon, heavily calcified and with a reconstructed preference of cooler waters. Previous morphometric investigations focused on coccolith taxa (i.e. Biscutum constans, Discorhabdus ignotus, Zeugrhabdotus erectus and Watznaueria barnesiae) predominantly associated with fertility fluctuations in surface waters.

Results show that E. floralis underwent significant size variations across OAE 2 with a decrease in the total diameter during OAE 2 of ca. 0.5 μm. Minimum sizes are reached in the latest part of OAE 2 (chemostratigraphic peak B), with specimens ca. 1 μm smaller than in pre- and post-OAE 2 intervals.

Two distinct E. floralis morphotypes were separated, one with spiky and one with rounded elements. The latter morphogroup, more abundant during OAE 2, has a relatively smaller (ca. 0.2 μm) total average diameter.

The diaphragm diameter also shows size changes but with opposite trends, thus specimens have larger diameter during OAE 2 except for peak B which is characterized by the smallest values.

Our findings indicate that morphometric fluctuations are unrelated to abundance. Moreover, although E. floralis is as a cold-water species, its abiundance is unrelated to temperature fluctuations across OAE 2, including the PCE.

The main size patterns of E. floralis are partially similar to those of B. constans, D. ignotus and Z. erectus possibly suggesting that common environmental stressors affected all these species. We speculate that also E. floralis used the strategy to reduce the average size to cope with excess CO₂ and/or toxic metals. Moreover, we do see a change in the dominance of the morphotypes probably suggesting that those with rounded shape and smaller size were better adaptable to/ less stressed by the OAE 2 perturbation.