NANNOFOSSIL CARBONATE FLUXES DURING THE EARLY CRETACEOUS: PHYTOPLANKTON RESPONSE TO NUTRIFICATION EPISODES, ATMOSPHERIC CO₂ AND ANOXIA

Greenhouse episodes during the Valanginian and Aptian correlate with major perturbations in the C cycle and in marine ecosystems, carbonate crisis, and widespread deposition of Corg-rich black shales. Quantitative analyses of nannofossil micrite were conducted on continuous pelagic sections from the Southern Alps (Northern Italy), where high-resolution integrated stratigraphy allows precise dating of Early Cretaceous geological events.

Rock-forming calcareous nannofloras were quantified in smear slides and thin sections to obtain relative and absolute abundances and paleofluxes that are interpreted as the response of calcareous phytoplankton to global changes in the ocean-atmosphere system. Increased rates of volcanism during the formation of Ontong Java and Manihiki Plateaus and the Paranà-Endenteka Large Igneous Province (LIP) are proposed to have caused the geological responses associated with OAE1a and the Valanginian event, respectively.

Calcareous nannofloras reacted to the new conditions of higher pCO2 and fertility by drastically reducing calcification. The Valanginian event is marked by a 65% reduction in nannofossil paleofluxes that would correspond to a 2-3 times increase in CO2 during formation of the Paranà-Endenteka LIP. A 90 % reduction in nannofossil paleofluxes, occurred in a 1.5 m.y. long interval leading into OAE1a, is interpreted as the result of a 3 -6 times increase in CO2 produced by emplacement of the giant Ontong Java and Manihiki Plateaus. High CO2 was balanced back by accelerated biological pumping during the Valanginian episode, but not during OAE1a, suggesting persisting high levels of CO2 in the late Aptian and/or the inability of calcareous phytoplankton to absorb excess CO2 above threshold values.