Evaluating optical lightness as a proxy for carbonate content in marine sediment cores

Optical Lightness is increasingly being used as a proxy for the carbonate content of marine sediment. We compared three measurements of optical lightness, gray scale, brightness and L*, to carbonate content to test the reliability of these measurements as carbonate content proxies. In five piston cores from isotope stages 1 though 6 (0- similar to 130 ka), all measures of optical Lightness are reasonable proxies of relative, but not absolute, changes in carbonate content. In contrast, in the upper sediments of ODP Hole 997A, which an Holocene to Pliocene in age, lightness values in only the top 10 m of the 180 m analyzed exhibit a strong correlation to carbonate content. To better understand which factors control the optical lightness of sediments, we analyzed samples in which carbonate content remained constant while the composition of three common clay minerals - chlorite, illite and kaolinite was varied. Kaolinite, even at a concentration of 20%, did not significantly change optical lightness. However, 20% illite reduced optical lightness by 30%, and 20% chlorite reduced Lightness by 20%. We conclude that regional and temporal changes in the composition of the non-carbonate sediment fraction may significantly alter the relationship between lightness and carbonate content. We recommend that care must be taken when applying these measures to sediment sequences which span long time intervals, or to sediment samples taken over a broad geographical areas. (C) 1999 Elsevier Science B.V. All rights reserved.