Calcareous Nannofossil adaptations and life-strategies during Oceanic Anoxic Event 2: go dwarf, don't die

The environmental changes associated with anthropogenic CO2 emissions will greatly influence the ecosystem functioning of future oceans: the oceans take up anthropogenic-atmospheric CO2 with the consequence of a strong reduction in carbonate ion concentrations leading to ocean acidification. In this scenario, international research is facing the considerable challenge to predict if and how the oceans will be able to respond/adapt to the rapid environmental change and particularly to the acidification of surface waters. The Earth has already experienced extreme environmental change and the geological record shows that one of the most pronounced perturbations of the Cretaceous is the so-called Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE2). OAE2 induced major changes in the marine environment and severe disturbances of the biosphere. The forcing functions behind such phenomena are still problematic: it has been hypothesis that abnormal volcanism triggered global warming and enhanced primary productivity with consequent oxygen consumption at global scale. OAE2 is thought to be linked to the emplacement of the Caribbean Plateau that acted as a natural source of CO2. The environmental perturbation recorded during OAE2 models with 4 main steps: 1. OAE2 onset correlates with a major volcanic episode, causing global warming and a rise in CO2. 2. Then a weathering spike, followed by a cooling episode and CO2 drop in the interval of δ13C peak A, under persisting volcanic emissions. 3. At δ13C peak B, a major volcanic activity is associated with an increase in pCO2. 4. The end of OAE2 is marked by the decrease of carbon isotopic values after δ13C peak C although temperatures remain relative warm. Calcareous nannoplankton, as primary producers, influence the marine carbon cycle and, indirectly, the climate system: they contribute to biological processes, such as photosynthesis and to biomineralization; furthermore nannoplankton are sensitive to chemical - physical - trophic parameters providing the potential for monitoring environmental variations. Here we present morphometric data of 4 nannofossil species during OAE2 interval from sections along an ideal S-N transect through Sicily, France and England. The major result is a size-shift to tiny-dwarf coccoliths of different amplitudes. There is a species - specific response in all the sections: B. constans displays the most pronounced reduction in size, D. rotatorius records a well expressed reduction in size too as opposed to Z. erectus which diminishes in size to a lesser extent. In all the sections, the mean size of W. barnesiae is within the holotype and normal range size. Furthermore, we observe that coccolith size variations follow the main steps of OAE2 perturbation: coccolith sizes return close to normal values around δ13C peak A, where minimum pCO2 and a cooling phase are reconstructed. On the contrary, a major reduction in size is recorded around δ13C peak B, coeval to an increase in CO2. On the other hand the end of the C isotopic excursion doesn’t correlate with a return to coccolith normal sizes, suggesting a protraction of anomalous conditions. The record of paleofertility during OAE2 is not straightforward: mid-latitude localities seem to have been affected by a decrease in nutrient availability, whereas, in the Atlantic tropical waters, the nutrient content increased. Comparing these data with our morphometric results, we notice a lack of repetitive pattern: specifically, intervals characterized by higher nutrient contents do not show dwarf coccoliths. It seems therefore unlikely that nutrients content have controlled coccoliths dwarfism during OAE2. We speculate that during OAE2, excess CO2 played a fundamental role in nannoplankton calcification and that coccolith dwarfism might be useful to reconstruct ocean CO2 concentration. In the analyzed sections dwarf coccoliths occur at levels with metal peaks, perhaps recording a species-specific intolerance to metal toxicity.