Sediment colour analysis from digital images and correlation with sediment composition

Sediment colour records can be extracted from digital images of sediment core surfaces, which provide the spatial resolution needed to measure colour in laminated sediments. Digital cameras sample colour in three broad wavelengths, red, green and blue, which are subsequently translated into the CIE L*a*b* colour space. Methods to extract calibrated colour values are discussed in this paper. L*, a* and b* values are correlated with geochemical analyses of cm-scale bulk sediment samples. The sediments are from a suite of laminated and homogenous sections containing organic matter, carbonate, biogenic opal and lithogenic material in variable proportions. Total organic carbon (TOC) content has the dominant effect on sediment colour. Results show that there is a strong correlation with lightness (L*) for TOC values between 0.5 and 10\%, but that sediment lightness becomes saturated at higher TOC concentrations. Biogenic opal content cannot be resolved using the L*a*b* colour space. Biogenic opal in itself has a light colour but it tends to occur in darker coloured sediments because of a positive correlation between opal and TOC content. Carbonate content in the measured sections is generally less than 25\%, at which values its effect on colour is obscured by the other sediment components.