CALCAREOUS NANNOFOSSIL PALEOFLUXES AS PROXY FOR PALEOCO2 DURING THE APTIAN

The Cretaceous was characterized by intervals of super-greenhouse climate and profound environmental perturbations, including Oceanic Anoxic Events (OAE)s being episodes of widespread organic matter burial in oxygen-depleted oceans. The early Aptian OAE 1a is thought to be related to the emplacement of the Ontong Java Plateau (OJP) which probably introduced in the atmosphere a large amount of CO₂ with consequent impact on biota, global warming and ocean chemistry. The formation of the Kerguelen Plateau in the late Aptian is, conversely, associated to a prolonged cooling coeval with enhanced atmospheric CO₂.

We quantified biogenic carbonate production of calcareous nannoplankton and reconstructed environmental stress influencing biocalcification. Laboratory experiments on coccolithophores indicate that coccolith type, abundance and degree of mineralization depend on chemical-physical-trophic conditions of seawater as well as on pCO₂. Dwarf/malformed coccoliths were documented during OAE 1a and interpreted as the species-specific response to surface-water acidification under excess CO₂. A remarkable crisis in nannofossil calcite production correlates with OAE 1a, when highest CO₂ concentrations of the Aptian were reached under the most intense phase of OJP construction. During the late Aptian, nannofossil calcite fluxes were characterized by fluctuations, suggestive for variable paleoCO₂. An interval of lower CO₂ concentrations corresponds to the late Aptian N. truittii Acme that presumably testifies lowered enough values to promote a larger proliferation of nannoconids. In the latest Aptian, further injections of CO₂ lead to the final collapse of nannoconids and suppressed nannofossil paleofluxes.

In the Aptian, nannoplankton biocalcification was ruled by complex ocean/atmosphere dynamics resulting from the balance between phases of volcanic emissions and CO₂ uptake via ocean adsorption, weathering and organic matter burial.