Technical Note: Weight approximation of single coccoliths inferred from retardation estimates using a light microscope equipped with a circular polariser – (the CPR Method)

Abstract

Abstract. A new method for weight estimates of single coccoliths using the birefringence of calcite is described. The weight estimates of 364 Holocene coccoliths using this new method are in good agreement with published volumetric estimates. A robust calibration method based on the measurement of a calibration target of known retardation enables the comparison of data between different imaging systems. Therefore, the new method overcomes the shortcomings of the error prone empirical calibration procedure of a previously reported method based on birefringence of calcite. In addition, the new method includes the application of a circular polariser that eliminates the extinction pattern in crossed polarised light. This imaging method allows for the first time the imaging of complete coccoliths on a light microscope at maximum interference colours without moving any mechanical part of the microscope. Therefore, it greatly simplifies the identification of coccolithophore species on the light microscope as well as the calculation of the area and thus weight of a coccolith.