History of the oceanic carbonate cycle based on calcareous nannofossil absolute abundance and flux in the past 190 Myr
Résumé
Calcareous nannoplankton is the main pelagic carbonate producer representing half of the deep oceans calcium carbonate. Thus it is a main actor in the modern carbonate cycle. Nowadays, the oceanic carbonate cycle is considered to rely extensively on pelagic production (Cretan ocean in Zeebe and Westbroek, 2003). Calcareous nannofossils firstly appeared at the end of the Triassic and prior to this major oceanic event, carbonate production was restricted on epicontinental platforms (Neritan ocean in Zeebe and Westbroek, 2003). The transition from a neritic-supported to a pelagic-supported oceanic carbonate production is related to the evolution of calcareous nannofossils. Unfortunately, this transition is poorly constrained in time influencing the comprehensive mechanisms at its origin. Here we present a compilation of calcareous nannofossil absolute abundances and fluxes from the Early Jurassic (~190 Ma) to the end of the Pleistocene (~0.12 Ma). Absolute abundance quantification has been made using random settling methods and sedimentation rates are based on cyclostratigraphy or biostratigraphy based on Gradstein et al. (2012).
The absolute abundance and flux records show an increase from the Early Jurassic to the Early Cretaceous followed by a plateau, interrupted by short time decrease until the Pleistocene. Apparently, the transition from Neritan to Cretan oceans was a slow transitional event all along the Jurassic whereas an effective pelagic production only started in the Early Cretaceous. The evolution of the calcareous nannofossil production in the Meso-Cenozoïc interval is a complex interplay between nannofossil evolution and environmental conditions such as sea level, temperature, nutrients concentration and ocean chemistry. Nevertheless, on a long-term perspective, there is an intriguing opposite relationship between atmospheric CO2 concentrations and calcareous nannofossil fluxes that need to be further explored.
