The characterisation of sedimentary organic matter in carbonates with Fourier-transform infrared (FTIR) spectroscopy

Abstract

The characterisation of the insoluble macromolecular fraction of sedimentary organic matter preserved in carbonate sediments (also known as kerogen; Durand B, Kerogen: Insoluble Organic Matter from Sedimentary Rocks. Editions Technip, Paris, 1980) allowed for the identification of fossil organisms that were not otherwise preserved. To perform rapid analyses on organic matter, capable of determining not only its crude constituents but also its geochemical composition and thermal maturity, we tested a new approach with Fourier-transform infrared (FTIR) spectroscopy on the Calcare di Base (CdB). These carbonate sediments have previously been characterised through molecular fossils and other organic markers (Guido et al., Palaeogeography, Palaeoclimatology, Palaeoecology 255:265-283, 2007); the authors demonstrated that the CdB was deposited via heterotrophic bacterial metabolism. Considering the very low organic-matter content, it was necessary to concentrate the organic compounds for our analyses. The infrared spectra showed stretching aliphatic bands (νCHali) and deformation bands indicative of methyl (δCH3) and methyl-methylene [δ(CH2+CH 3)] groups. The spectra also displayed the band assigned to carbonyl and/or carboxyl groups (νC=O). In addition, the band assigned to the skeletal vibration of more than four methylene groups [γ(CH2) 4] was observed, and the νC-O vibration was also recorded. The ratio of carbonyl to aliphatic groups (νC=O/νCHali) permitted us to distinguish the marine vs. continental inputs, whereas the ratios for chain length (δCH2/δCH3) and the degree of branching [ν(CH2)4/νCHali] allowed for the determination of the thermal maturity of organic matter. The data confirmed the organic geochemical analyses performed by Guido et al. (Palaeogeography, Palaeoclimatology, Palaeoecology 255:265-283, 2007), demonstrating that the FTIR is a reliable approach for the recognition of the origin and thermal maturity of the organic matter preserved in carbonate sediments. © 2011 Springer Berlin Heidelberg.