Calcareous nannofossil paleofluxes: proxies for pCO2 fluctuations through the Aptian

The Cretaceous has been punctuated by episodes of widespread organic matter burial in oxygen-depleted oceans known as Oceanic Anoxic Events (OAEs) (Schlanger & Jenkyns, 1976). In particular, the Aptian (~121 to ~113 Ma) has been characterized by a super-greenhouse climate and profound environmental perturbations, including the early Aptian OAE 1a, thought to be related to the emplacement of the Ontong Java Plateau. The aim of this work is to study variations in biogenic carbonate production of calcareous nannofossils through the Aptian in order to detect if and how it has been affected by high pCO2. Calcareous nannoplankton is an excellent proxy for reconstructing present and paste surface water conditions, being extremely sensitive to changes in physical and chemical conditions of the oceans (pH). Studies on living form have attested a direct response of these organisms to changes in CO2 concentrations (e.g. Krug et al. 2010). Production of dwarf/malformed coccoliths has also been documented in the geological record through OAE 1a (Erba et al. 2010). Here we present quantitative analyses of nannofossil micrite in thin sections, reconstructing 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. In each of the three sites, nannofossil absolute abundances and paleofluxes show a progressive drastic reduction starting in the latest Barremian. This decrease is due to a worldwide decrease in the rock-forming nannoconids, “nannoconids decline”, that culminates with the “nannoconids crisis” just prior to OAE 1a (Erba, 1994; Erba & Tremolada, 2004). At the end of OAE 1a, nannofossil biocalcification increases again, but it never reaches pre-anoxia values. In the late Aptian, nannofossil paleofluxes reach high 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 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. REFERENCES Erba, E. (1994). Nannofossils and superplumes: The early Aptian “nannoconid crisis”. Paleoceanography, 9: 483-501. Erba, E. & Tremolada, F. (2004). Nannofossil carbonate fluxes during the Early Cretaceous: phytoplankton response to nutrification episodes, atmospheric CO2 and anoxia. Paleoceanography, 19: 1008. Erba, E., Bottini, C., Weissert, H.J., Keller, C.E. (2010). Calcareous Nannoplankton response to surface-water acidification around Oceanic Anoxic Event 1a. Science 329: 428–432. Krug, S.A., Schulz, K.G., Riebesell, U. (2010). Effects of CO2-induced changes in seawater carbonate chemistry speciation on Coccolithus braarudii: a conceptual model of coccolithophorid sensitivities. Biogeosciences Discuss., 7: 8763–8778. Schlanger, S.O. & Jenkyns, H.C. (1976). Cretaceous oceanic anoxic events: causes and consequences. Geol. Mijnb., 55: 179–184.