Nannofossil carbonate paleofluxes as proxy for pCO2 during the Aptian

The Aptian (~121 to ~113 Ma) has been characterized by super-greenhouse climate and profound environmental perturbations, including the early Aptian Oceanic Anoxic Event (OAE 1) a, an episode of widespread organic matter burial in oxygen-depleted oceans. The OAE 1a is thought to be related to the emplacement of the Ontong Java Plateau which probably introduced in the atmosphere a large amount of CO2 with consequent impact on biota, climate and ocean chemistry. The major perturbation of the carbon cycle is mirrored by the carbon isotopic record which shows a negative shift at the beginning of OAE 1a followed by a positive excursion which persisted also after the event. The aim of this study is to detect if and how the biogenic carbonate production of calcareous nannofossil during the Aptian was affected by high pCO2. Calcareous nannoplankton is in fact extremely sensitive to changes in physical and chemical conditions of the oceans including CO2 concentration. Laboratory experiments on living coccolithophores indicate that coccolith type, abundance and degree of mineralization depend on chemical-physical-trophic conditions of water as well as on pCO2. The geological record has revealed the presence of dwarf/malformed coccoliths during OAE 1a interpreted to be the response of some coccolithophore species to increased surface-water acidification. Here, we intend to focus on the Aptian interval and present quantitative analyses of nannofossil micrite in thin sections and, specifically, we reconstructed nannofossil absolute abundances and calcite paleofluxes in three drill sites: the Cismon core (Northern Italy), Piobbico core (Central Italy) and DSDP Site 463 in the mid-Pacific Mountains. The data obtained revealed a drastic reduction in nannoplankton calcification starting in the latest Barremian related to a decrease in the rock-forming nannoconids, “nannoconids decline”, that culminates with the “nannoconids crisis” just prior to OAE 1a. At the end of OAE 1a, nannofossil biocalcification increased again, but it never reached pre-anoxia values. In the late Aptian, nannofossil paleofluxes reached high values during the Nannoconus truittii acme, followed by a final decrease through the Aptian/Albian boundary interval. These variations in carbonate production can be interpreted as the adaptive response of calcareous nan noplankton to perturbed surface-water conditions that favoured small and less calcified forms and caused false extinction among heavily calcified nannoconids (Lazarus effect). We conclude that, despite metabolic processes, CO2 concentrations influence the ocean chemistry and the carbonate system. In particular, the correlation between reduced biocalcification rates and intervals of intense volcanism, suggest that mid-Cretaceous nannoplankton biocalcification and nannofossil paleofluxes were strongly controlled by excess volcanogenic CO2. Following this observation, we propose calcite paleofluxes as proxy for reconstructing past atmospheric CO2 in the oceans and provide possible scenarios of CO2 concentrations.