Calcareous nannofossil paleofluxes as proxy of pCO2 fluctuations during the Early Cretaceous

Calcareous nannoplankton is an excellent proxy for detecting present and paste surface water conditions, being extremely sensitive to changes in temperature, fertility, salinity and chemical conditions of the oceans. We investigated a time interval of the geological history that includes the early Aptian Oceanic Anoxic Event 1a (OAE1a) (Schlanger and Jenkyns, 1976; Arthur et al., 1990), characterized by pronounced global changes in the composition and structure of the ocean-atmosphere system due to increased volcanogenic CO2 emissions associated with the emplacement of the Ontong Java and Manihiki Plateau. In order to detect the response of calcareous phytoplankton to global changes and C-cycle perturbation, quantitative analyses of nannofossil micrite were conducted in thin sections, reconstructing nannofossil absolute abundances and calcite paleofluxes in two drill sites: the Cismon core (Northern Italy) and the DSDP 463 Site in the mid-Pacific Mountains. At both sites, in the Barremian –Aptian interval, nannofossil absolute abundances and paleofluxes show a progressive drastic reduction, marked by a worldwide decrease in the rock-forming nannoconids “nannoconids decline”, that culminates with the “nannoconids crisis” just prior to the Selli event (Erba, 1994; Erba and Tremolada, 2004). At the end of the OAE1a, nannofossil biocalcification increases again, but it never reaches pre-anoxia values. In the late Aptian, nannofossil paleofluxes reach maximum values during the Nannoconus truittii acme, followed by a final decrease through the Aptian/Albian boundary interval. Calcite paleofluxes fluctuations reveal a drastic reduction in nannoplankton calcification interpreted as the adaptive response of these organisms to perturbed surface-water conditions that favoured small and less calcified forms and caused false extinction among heavily calcified nannoconids (Lazarus effect). We speculate that mid-Cretaceous nannoplankton biocalcification and nannofossil paleofluxes were strongly controlled by excess volcanogenic CO2. Despite metabolic processes, CO2 concentrations influence the ocean chemistry and the carbonate system. Excess CO2 conditions were perhaps enhanced by introduction of biolimiting metals at hydrothermal fields during construction of submarine Large Igneous Provinces.