The first 150 million years history of calcareous nannoplankton: Biosphere–geosphere interactions

Abstract

The Mesozoic evolution of calcareous nannoplankton is here compared to global changes in the Earth system to derive possible causal links between biosphere–geosphere interactions. The Late Triassic appearance of nannofossils represents an innovation that affected the marine carbonate system, the global carbon cycle and the ocean–atmosphere CO 2. Nannofossil biodiversity generally increases through the Jurassic and Cretaceous, with development of approximately 30 coccolith–nannolith families. Speciations, (mass) extinctions, diversifications and turnovers correlate with global changes in the geosphere, hydrosphere, and atmosphere suggesting that the evolutionary patterns are intimately linked to environmental modifications. Coccolithophore biocalcification involves conversion of Ca 2+ and CO 3 2− ions into CaCO 3 when high fluxes of Ca 2+ and inorganic C from the external environment into the cell are maintained. Seawater CO 2 availability and nutrient type and abundance are also important for the functioning of photosynthesis and calcification. The role of coccolith secretion with respect to photosynthesis remains uncertain. Current dating of events suggests that pCO 2 and chemistry of the oceans (Ca 2+, Mg / Ca ratio) are instrumental in coccolith and particularly nannolith appearance and development, whereas climate and sea level apparently play a secondary role. Availability of nutrients and biolimiting metals certainly influence nannoplankton evolution. It is suggested here that there is a causal link between levels of CO 2 and calcite secretion by nannoplankton. Namely, when pCO 2 in the ocean–atmosphere system is minimum, production of heavily calcified coccoliths/nannoliths might provide sufficient CO 2 inside the cell to sustain photosynthesis. On the contrary, excess carbon dioxide makes calcification less indispensable or even inhibits calcite production: smaller and less calcified coccoliths become dominant. Natural variations in atmospheric CO 2 are essentially triggered by igneous activity that also controls the ocean chemistry and nutrient cycling. In particular, the seawater Mg / Ca ratio is related to seafloor spreading and hydrothermal processes. High production of ocean crust results in high Ca 2+ concentrations, low Mg 2+ concentrations and low Mg / Ca ratio, and vice versa. Accordingly, the Phanerozoic is subdivided into intervals of “calcite” and “aragonite” seas. The appearance of calcareous nannoplankton and its maximum diversification indeed correlate with the shift from the Paleozoic–Triassic “aragonite” sea to the Jurassic–Paleogene “calcite” sea and the core of the latter, respectively. Although there is a logical connection between pCO 2 and Ca 2+ concentration and nannofossil diversity, species richness alone cannot measure the productivity—production of coccolithophores through time. Absolute abundances are necessary to interpret and model the distribution of calcareous nannofossils relative to the environment and quantify positive and negative feedbacks of coccolithophore biocalcification. Improved chronology of paleobiological and geological events will be crucial for the understanding of evolutionary processes.