Abstract
Abstract. 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).
Highlights
Coccolithophores are abundant oceanic single-cell algae that produce calcite plates called coccoliths that are arranged around the cell to form an exoskeleton
It consists of a linear polarizer oriented at +90◦ placed below a quarter-wave plate oriented at +45◦ mounted in a Leica cube and placed in the upper automatic turret of the microscope
The bidirectional circular polarization” (BCP) method has a great advantage over previous methods for which it is difficult to maintain stable light (i) in time and (ii) in space since the field of view may not be uniformly illuminated
Summary
Coccolithophores are abundant oceanic single-cell algae that produce calcite plates called coccoliths that are arranged around the cell to form an exoskeleton. Slight change in one of those parameters has important consequence for the results Another limitation is that the measured light intensity is not linearly proportional to the thickness but follows a sigmoid (Beaufort et al, 2014; Bollmann, 2014), making it difficult to estimate the thickness precisely at the two ends of the calibration. Some species have coccoliths thicker than this limit: in present ocean and Pleistocene sediments, rare examples are Coccolithus pelagicus, Ceratolithus cristatus and Pontosphaera multipora, and coccoliths exceed this threshold only on limited surfaces of the thickest specimens. We propose a new method that solves those problems: the estimations are not the results of a calibration; they can be applied to crystals as thick as 1.7 μm and are not dependent on the precise tuning of the light of the microscope
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
