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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2020-28
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2020-28
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  04 Feb 2020

04 Feb 2020

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A revised version of this preprint is currently under review for the journal BG.

Technical Note: A universal method for measuring the thickness of microscopic calcite crystals, based on Bidirectional Circular Polarization

Luc Beaufort1, Yves Gally1, Baptiste Suchéras-Marx1, Patrick Ferrand2, and Julien Duboisset2 Luc Beaufort et al.
  • 1Aix Marseille Univ, CNRS, IRD, INRAE, Coll. France, CEREGE, Aix-en-Provence, France
  • 2Aix Marseille Univ, CNRS, Centrale Marseille, Institut Fresnel, Marseille, France

Abstract. The coccoliths are major contributors to the particulate inorganic carbon in the ocean that is a key part of the carbon cycle. The coccoliths are few microns in length and weigh few picograms. Their birefringence characteristics in polarized optical microscopy has been used to estimate their mass. This method is rapid and precise because camera sensors produce excellent measurement of light. However, current method is limited because it requires a precise and replicable set up and calibration of the light in the optical apparatus. 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 results from a simple equation. It can be used with different cameras and microscope brand. Moreover, the light intensity used in the microscope does not have to be strictly and precisely controlled. This method permits to measure crystal thickness up to 1.7&htinsp;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 = λ/πΔn arctan (√(ILR/ILL))
where d is the thickness, λ the wavelength of the light used, Δn the birefringence, ILR and ILL are the light intensity measured with a right and a left circular polarizer. Because of the alternative and rotative motion of the quarter-wave plate of the circular polarizer, we coined the name of this method Bidirectional Circular Polarization (BCP).

Luc Beaufort et al.

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Luc Beaufort et al.

Luc Beaufort et al.

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Short summary
The coccoliths are major contributors to the particulate inorganic carbon in the ocean. They are extremely difficult to weight because they are too small to be manipulated. We propose a universal method to measure thickness and weight of fine calcite using polarizing microscopy that does not require fine-tuning of the light nor a calibration process. This method named bidirectional circular polarization uses 2 images taken with 2 directions of a circular polarizer.
The coccoliths are major contributors to the particulate inorganic carbon in the ocean. They are...
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