A digital image analysis approach to measurement of the conodont colour alteration index (CAI): a case study from the Moravo-Silesian Zone, Czech Republic

Abstract

Presented is a simple non-destructive method for quantifying conodont colour based on digital processing of conodont images taken by a digital camera attached to a binocular microscope operating under incident light, using the RGB colour space. The digital colour data was calibrated to a spectral reflectance of the standard colour background. The results from the Moravo-Silesian Zone (MSZ) of the Bohemian Massif, Czech Republic, demonstrate an excellent relationship existing between the digital intensities of the red (R), green (G) and blue (B) colours and CAI in the range from CAI 3 to CAI 6. The R/B colour ratio proved the best proxy of CAI. The R/B ratio decreases with increasing CAI from values of about 1.6 (CAI3) to about 1.0 (CAI5) and then slightly increases to 1.15 (mean for CAI 5.5) and 1.26 (mean for CAI6). It is closely related to CAI (R2 = 0.8974) within CAI 3 to CAI 5 interval. The R/B ratio is similar in concept to the slope of the reflectance curve obtained from the spectral reflectance of conodonts by previous authors, as well as to the shift from yellow and brown colours towards greyscale conodont colours with increasing thermal maturation according to the original definition of CAI. Regional distribution of the CAI and R/B ratio from the MSZ suggests that the digital conodont colour data is closely related to vitrinite reflectance and illite crystallinity data.