The origin of insoluble organic matter in sediments is still a matter of debate. Detailed characterisation at the molecular level by pyrolytic and chemolytic methods can be of great help in resolving this question. The application of isotope-ratio-monitoring gas chromatography-mass
spectrometry (irm-GC-MS) in combination with these methods allows the determination of the 13C-content of pyrolysis and chemolysis products released from the kerogen and provides a new tool to determine the origin of kerogen. (Collister et al., 1992; Eglinton, 1994; Hartgers et al., 1994).
Off line pyrolysis (400°C, 1 h) was performed on several extracted sediments and kerogens representing a range of depositional environments and ages (Precambrian-Tertiary). The pyrolysis products were separated with column chromatography (Al2O3 and SiO2-Ag+) by elution with hexane, hexane/DCM (7:1,v/v) and MeOH/DCM (1:1,v/v). Four fractions were obtained: an alkane, an alkene, an aromatic and a polar fraction. The first three fractions were analysed by gas chromatography (GC), gas chromatography-mass spectrometry (GC-MS) and irm-GC-MS. In addition, the bulk stable carbon isotope composition of the kerogens, the pyrolysis residues and the polar fractions were measured. Pyrolysis yields varied between 10 and 25 wt %.
The stable carbon isotopic composition of the residual organic matter left after pyrolysis was always heavier (0.4 - 0.9 ) than that of the starting material indicating that isotopically light material was released. Indeed, the stable carbon isotopic compositions of the n-alkanes, n-alkenes, alkylbenzenes, alkylthiophenes and the polar fraction were in all cases lighter than the stable carbon isotopic composition of the kerogens. Irm-GC-MS revealed that the stable carbon isotopic compositions of the n-alkanes are quite similar to those of the n-alkenes. This suggests that the
n-alkanes and the n-alkenes may have a common origin (possibly an algal biopolymer). The isoprenoid alkanes, when present, also have similar stable carbon isotopic compositions but differ from the values of the n-alkanes and n-alkenes. These isoprenoids could be derived from an isoprenoid algaenan similar to that biosynthesised by the freshwater algae Botryococcus braunii (Metzger and Casadevall, 1992). The analysis of the aromatic fraction of the pyrolysates, containing alkylbenzenes and alkylthiophenes, showed a wide range of isotopic values for the compounds analysed, which suggest multiple origins for these compounds (Sinninghe Damsté et al., 1989).
Our results show that the structure and stable carbon isotopic composition of kerogens investigated are composed of isotopically heavy material which could not be thermally released and of isotopically lighter material, possibly algaenans, bound lipids and organic sulphur compounds which yield multiple pyrolysis products.
Collister, J.W., Summons, R.E., Lichtfouse, E. & Hayes, J.M., Org. Geochem. 19, 265-276 (1992).
Eglinton, T.I., Org. Geochem. 21, 721-735 (1994).
Hartgers, W.A et al., Org. Geochem. 22, 703-725. (1994).
Metzger, P. & Casadevall, E. , Phytochemistry 31, 2341-2349 (1992).
Sinninghe Damsté, J.S., Eglinton, T.I., de Leeuw, J.W. & Schenck, P.A., Geochim. Cosmochim. Acta 53, 873-889 (1989).