Coccolithopores are phytoplanktonic algae which produce a calcitic exoskeleton (coccosphere) and are extremely important primary producers playing a direct role on the equilibrium of the organic and inorganic carbon cycle. Laboratory experiments on living coccolithophores documented decreased biocalcification and partial production of deformed/malformed coccoliths in some species as a consequence to increased surface-water acidification associated with elevated CO2 concentrations. A similar response is registered during geological intervals of super-greenhouse climate and profound environmental perturbations. Specifically, malformed coccoliths were identified during the early Aptian Oceanic Anoxic Event (OAE) 1a characterized by emission of large amount of CO2, widespread organic matter burial in oxygen-depleted oceans, paralleled by a decrease in total nannofossil carbonate paleofluxes. The OAE 1a was also accompanied by a geologically rapid warming and input of biolimiting metals in the oceans associated with multiple volcanic phases of the Ontong Java Plateau (OJP). The late Aptian was instead characterized by intervals of intense sub-aerial volcanism of the Kerguelen Plateau (KP) province and resulted to be coeval with interludes of relatively cool conditions. The aim of this study is to reconstruct the biogenic carbonate production of calcareous nannoplankton during the Aptian and detect if and how biocalcification of coccolithophores was affected by fluctuating pCO2 prior, during and after the construction of the OJP and KP. Specifically, we present quantitative analyses of nannofossil micrite in thin sections and reconstructed nannofossil 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 culminated 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 only during the Nannoconus truittii acme, followed by a final collapse across the Aptian/Albian boundary interval. The variations in nannoplankton carbonate production are interpreted as the adaptive response to perturbed surfacewater conditions that favoured small and less calcified forms and caused false extinctions among heavily calcified nannoconids. The correlation between reduced biocalcification rates and intervals of intense volcanism, suggests 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 and provide possible scenarios of paleoCO2 concentrations delivered by OJP and KP volcanism and interplay with climate changes.
|Titolo:||Biogenic carbonate paleofluxes as proxy for pCO2 during the Aptian|
BOTTINI, CINZIA (Primo)
ERBA, ELISABETTA (Ultimo)
|Parole Chiave:||biogenic carbonate fluxes; nannofossils; Aptian|
|Settore Scientifico Disciplinare:||Settore GEO/01 - Paleontologia e Paleoecologia|
|Data di pubblicazione:||lug-2015|
|Appare nelle tipologie:||01 - Articolo su periodico|