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Polar fractions of three immature sulphur-rich sediments and four sulphur-rich oils, all of Miocene age, were studied using two selective chemolytic methods, namely methyl lithium/methyl iodide, which selectively cleaves polysulphide bonds, and Raney nickel or nickel boride which cleave both mono-and polysulphide bonds. In this way the amount of polysulphide-linked vs mono- and polysulphide-linked carbon skeletons was assessed. Steranes, phytane and gammacerane are bound by polysulphide linkages in large amounts (30% or more) in the immature Vena del Gesso sediment whereas squalane, isorenieratane and C35 hopane carbon skeletons are hardly bound at all by polysulphide-bonds only. The amounts of polysulphide-bound carbon skeletons are much lower in samples from the Northern Apennines and the Monterey Formation and consist predominantly of n-alkanes. Phytane and steranes are relatively more polysulphide-linked than β-carotane and n-alkanes in the oils. This phenomenon may be explained in part by the number of carbon-sulphur bonds by which they are bound to the macromolecular moieties, since it is statistically unlikely that moieties bound by several sulphur-bonds are linked exclusively by polysulphide-bonds. Other explanations, however, such as differences in the timing and nature of the sulphur incorporation processes have also to be considered. |
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