Premise of an Oceanic Revolution: Understanding the origin of pelagic calcification

In the modern ocean, calcareous plankton has a key role in the biological carbon pump and carbonate counter pump. Their rate of bioproductivity directly influences the exchange of CO2 between the ocean and the atmosphere as well as the alkalinity of the ocean. Before their emergence, the carbonate production was limited to shallow seas with the consequence of occasional times of extreme carbonate saturation states. Several important pelagic calcyfiers emerged in the Late Triassic, such as the coccolithophores. Their emergence eventually created a carbonate sink in the deep open ocean, changing the ocean to as we know it today. No impact of their emergence can be observed in the calcium stable isotope record in the Late Triassic, as was previously expected. Instead, changes in the Ca and Sr isotopic composition is observed right before the emergence of calcareous nannofossils. The isotopic anomaly is coeval with a major (~100 m) sea-level fall and possibly directly linked to the erosion of carbonate platforms and evaporites in the low stand. This weathering may have increased the concentration of Ca ions in the ocean and thus favored the emergence of calcareous plankton.

The aim of this project is to investigate the link between the weathering and erosion of the carbonate platforms and evaporites with the sea-level fall and isotopic anomalies. To do this Mg isotopes will be analyzed to control the impact of evaporites in the isotopic record. Li isotopes will be analyzed to control silicate weathering vs. carbonate weathering. Study and analyze Upper Triassic evaporites for their Ca, Sr, Mg and Li isotopic composition.

The emergence of tiny organisms that builds a shell from carbon and calcium in the open ocean, was fundamental for the establishment of our modern oceanic carbon pump. These organisms act as giant CO2 buffer and changes in their bioactivity directly affect the CO2 exchange between the oceans and atmosphere as well as the acidification of the ocean. Even though the conservation of this function is vital to preserve life under water and a key in the carbon cycles as a whole, little is known about their emergence. In extension, one could say we do not know how our modern oceans came to be. This project aims to study what set the stage and allowed the change to happen. Rocks and salt that formed ~210 million years ago will be studied for their magnesium and lithium isotopic composition. The hypothesis to be confirmed or ruled out, is if a major sea-level fall exposed certain types of rocks and the weathering and dissolution of these rocks changes the chemistry of the ocean. It is possible that such a chemistry change, set the right conditions in the ocean for the tiny organisms to evolve, flourish and populate the open ocean.