Life in extreme Oceans: Calcareous Nannoplankton adaptations and strategies during Oceanic Anoxic Event 2

At present about one third of the carbon dioxide (CO2) released in the atmosphere from fossil fuel burning is absorbed by the oceans. The invasion of anthropogenic CO2 into the oceans increases seawater acidity and decreases carbonate ion concentration and carbonate saturation. This dramatic change of the carbonate system can have a huge impact on the marine ecosystem and, in particular, can seriously impair marine calcifiers (e.g. corals, foraminifera, coccolithophores). Coccolithophorid algae are sensitive to ocean acidification and most studies show a strong decline in growth and/or reduction in calcification rate and/or increase in coccolith malformation with increasing CO2 concentration. During the Cretaceous the Earth has already experienced extreme environmental change: the construction of Large Igneous Provinces (LIPs), forming gigantic oceanic plateaus, affected the ecosystem at global scale. LIP volcanism probably triggered global warming and enhanced primary productivity with consequent oxygen consumption and burial of massive amounts of organic matter at global scale: these episodes are known as Oceanic Anoxic Events (OAEs). We investigated calcareous nannoplankton morphological variation through one of the most interesting anoxic event of the Cretaceous, the latest Cenomanian OAE2. This episode of global anoxia is associated with the formation of the Caribbean Plateau (CP) that triggered high release of excess CO2, causing a general global warming. Recent studies of the OAE2 episode have pointed out climatic variability and fluctuations in the atmCO2 concentration. In fact, a cooling episode and CO2 drop in the early phase of OAE2 is connected with a weathering spike, followed by a new increase in CO2 and warming. We investigated OAE2 sections from Sicily, Southern France, England and Colorado. The major result that we observed is a change to tiny-dwarf coccoliths, although of different amplitude, through OAE2: calcareous nannofossil size variations follow the pCO2 fluctuations and record an increase in size when pCO2 start to decrease while dwarf coccoliths are coeval with a strong increase in CO2. 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. It seems therefore unlikely that nutrient content has controlled coccolith dwarfism during OAE2. Another selected case history is the Aptian OAE1a that is associated with the submarine construction of the Ontong Java Plateau (OJP) and triggered a disruption of the oceanic carbonate system. At the onset of the carbon isotopic anomaly, during the most profound paleoenvironmental perturbation, excess CO2 induced ocean acidification with the consequence of a temporary failure of the rock-forming nannoconids and formation of dwarf and malformed coccoliths. The inferred warmer conditions and excess CO2, during intervals of LIPs volcanism, suggest a potential role on nannoplankton calcification. Hydrothermal plumes during construction of both OJP and CP introduced biolimiting metals that fertilized the global ocean. However, some toxic metals might have disturbed the functioning of some intolerant coccolithophorid species and perhaps hampered calcification of some species. Different patterns and degree of dwarfism and malformation during OAE1a and OAE2 suggest unequal volcanic CO2 emissions (rates, pulses, amount) and/or variable combinations of CO2, climate and fertility: analogous causes (LIPs emplacement) have therefore induced only partially similar response at different times.