The Characterization of Sedimentary Organic Matter in Carbonates with Fourier Transformed-Infrared (FT-IR) Spectroscopy

The characterization of the insoluble macromolecular fraction of sedimentary organic matter (also know as kerogene; Durand, 1980), preserved in the carbonate sediments, allows the identification of organisms that are not preserved. To perform rapid analyses on organic matter, capable to recognize not only its crude presence but also its geochemical composition and thermal maturity, we tested a new approach with Fourier Transformed-InfraRed (FT-IR) spectroscopy on the Calcare di Base (CdB). These carbonate sediments were already characterized through molecular fossils and other organic markers (Guido et al., 2007). The Authors demonstrated that the CdB was deposited via heterotrophic bacterial methabolism. Considering the very low organic matter content, it was necessary to concentrate the organic compounds for our analyses. The infrared spectra show stretching aliphatic bands (ƒÞCHali), and deformation bands of methyl (ƒÔCH3) and methyl-methylene [ƒÔ(CH2 + CH3)] groups. The spectra also display the band assigned to carbonyl and/or carboxyl groups (ƒÞC=O). In addition, the band assigned to the skeletal vibration of more than four methylene groups [ƒ×(CH2)4] has been observed. The ƒÞC-O vibration is also recorded. The ratio carbonyl to aliphatic groups (ƒÞC=O/ƒÞCHali) permitted to distinguish the marine vs continental inputs, whereas the chain length (ƒÔCH2/ƒÔCH3) and the degree of branching [ƒÞ(CH2)4/ƒnƒÞCHali] allowed to evaluate the thermal maturity of the organic matter. The data confirmed the organic geochemical analyses performed by Guido et al. (2007) demonstrating that the FT-IR is a reliable approach for the recognition of the origin and thermal maturity of the organic matter preserved in carbonates sediments.