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Coccoliths are major contributors to the particulate inorganic carbon in the ocean that is a key part of the carbon cycle. The coccoliths are a few micrometres in length and weigh a few picogrammes. Their birefringence characteristics in polarized optical microscopy have been used to estimate their mass. This method is rapid and precise because camera sensors produce excellent measurements of light. However, the current method is limited because it requires a precise and replicable set-up and calibration of the light in the optical equipment. More precisely, the light intensity, the diaphragm opening, the position of the condenser and the exposure time of the camera have to be strictly identical during the calibration and the analysis of calcite crystal. Here we present a new method that is universal in the sense that the thickness estimations are independent from a calibration but result from a simple equation. It can be used with different cameras and microscope brands. Moreover, the light intensity used in the microscope does not have to be strictly and precisely controlled. This method permits the measurement of crystal thickness up to 1.7 µm. It is based on the use of one left circular polarizer and one right circular polarizer with a monochromatic light source using the following equation: d=λπΔnarctanILRILL, where d is the thickness, λ the wavelength of the light used, Δn the birefringence, and ILR and ILL the light intensity measured with a right and a left circular polarizer. Because of the alternative and rotational motion of the quarter-wave plate of the circular polarizer, we coined the name of this method “bidirectional circular polarization” (BCP).

The rise of calcareous nannoplankton in Mesozoic oceans has deeply impacted ocean chemistry and contributed to shaping modern oceans. Nevertheless, the calcareous nannoplankton colonization of past marine environments remains poorly understood. Based on an extensive compilation of published and unpublished data, we show that their accumulation rates in sediments increased from the Early Jurassic (∼200 Ma) to the Early Cretaceous (∼120 Ma), although these algae diversified up to the end of the Mesozoic (66 Ma). After the middle Eocene (∼45 Ma), a decoupling occurred between accumulation rates, diversity and coccolith size. The time series analyzed points toward a three-phase evolutionary dynamic. An invasion phase of the open-ocean realms was followed by a specialization phase occurring along with taxonomic diversification, ended by an establishment phase where a few small-sized species dominated. The current hegemony of calcareous nannoplankton in the world ocean results from a long-term and complex evolutionary history shaped by ecological interactions and abiotic forcing.