The effect of hydrosulphurization on the carbon isotopic composition of free and S-bound lipids

1-Decene was subjected to hydrosulphurization under phase-transfer conditions at room temperature. Compound-specific isotope analysis revealed that 1-decene becomes enriched in 13C with increasing yields of sulphurization whilst the sulphur compounds formed are initially depleted in 13C compared to the starting substrate. The average carbon isotopic fractionation effect of the reaction on the two carbon atoms involved is ca. −20%o. Sedimentary sulphur compounds, however, do not show this fractionation effect indicating that either a different sulphurization reaction with different isotope effects has occurred or that functionalized lipids may have been fully sulphurized

Restricted utility of aryl isoprenoids as indicators of photic zone anoxia

In a North Sea oil, the carotenoid derivatives -carotene, -isorenieratane, and isorenieratane were identified, together with a series of aryl isoprenoids with a 2,3,6-trimethyl substitution pattern for the aromatic ring. The 13C values of -carotene and -isorenieratane are similar, whereas isorenieratane is ca. 15% heavier. This suggests that -isorenieratane is not derived from -isorenieratane biosynthesised by Chlorobiaceae, but from aromatisation of -carotene. This was confirmed by laboratory aromatisation of partially hydrogenated -carotene, which yielded -isorenieratane as the main product. The aryl isoprenoids, which can be formed by C---C bond cleavage of both isorenieratane and -isorenieratane, have a mixed isotopic signature in the oil. These results indicate that mere identification of aryl isoprenoids, without determination of their 13C values, cannot be used to assess the presence of Chlorobiaceae, and, thus, photic zone anoxia in the depositional environment

The combined application of organic sulphur and isotope geochemistry to assess multiple sources of palaeobiochemicals with identical carbon skeletons

Five immature sediments from a Messinian evaporitic basin, representing one evaporitic cycle, were studied using molecular organic sulphur and isotope geochemistry. It is shown that a specific carbon skeleton which is present in different 'modes of occurrence' ('free' hydrocarbon, alkylthiophene, alkylthiolane, alkyldithiane, alkylthiane, and sulphur-bound in macromolecules) may have different biosynthetic precursors which are possibly derived from different biota. It is demonstrated that the mode of occurrence and the carbon isotopic composition of a sedimentary lipid can be used to 'reconstruct' its biochemical precursor. This novel approach of recognition of the suite of palaeobiochemicals present during the time of deposition allows for identification of the biological sources with an unprecedented specificity

Recognition of paleobiochemicals by a combined molecular sulphur and isotope geochemical approach

Study of organic matter in immature sediments from a Messinian evaporitic basin shows that consideration of structures, modes of occurrence, and carbon isotopic compositions of free and sulfur-bound carbon skeletons allow identification of biochemical precursors. Detailed information concerning biotic communities present during deposition of sediments can be retrieved in this way. Moreover, unprecedented biochemicals were recognized; these extend the horizon of biomarker geochemistry

Recognition of paleobiochemicals by a combined molecular sulphur and isotope geochemical approach

Study of organic matter in immature sediments from a Messinian evaporitic basin shows that consideration of structures, modes of occurrence, and carbon isotopic compositions of free and sulfur-bound carbon skeletons allow identification of biochemical precursors. Detailed information concerning biotic communities present during deposition of sediments can be retrieved in this way. Moreover, unprecedented biochemicals were recognized; these extend the horizon of biomarker geochemistry

Quantitative assessment of mono- and polysulphide-linked carbon skeletons of S-rich macromolecular aggregates present in bitumens and oils

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 -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

Identification and geochemical significance of cyclic di- and trisulphides with linear and acyclic isoprenoid carbon skeletons in immature sediments

Homologous series (C₁₅-C₂₄) of novel 3-n-alkyl-1,2-dithianes and 3-n-alkyl-6-methyl-1,2-di-thianes have been identified in immature sediments. The identification of these compounds was based on comparison of mass spectra and Chromatographie data with those of synthesized 3-methyl-6-tridecyll, 2-dithiane. In addition, 4-methyl-3-(3,7,11-trimethyldodecyl)-1,2-dithiane, 4-(4,8,12-trimethyltridecyl)-1,2-dithiane, 5-methyl-4-(3,7,11-trimethyldodecyl)-1,2,3-trithiepane, and a 1,2-dithiane possessing a pentakishomohopane carbon skeleton were tentatively assigned on the basis of mass spectral characteristics, selective chemolysis, and desulphurisation. The occurrence of these cyclic di-and trisulphides with linear, acyclic isoprenoid and hopanoid carbon skeletons in thermally immature sediments indicates that inorganic polysulphides are incorporated into functionalised lipids during the early stages of diagenesis

Quantitative assessment of mono- and polysulphide-linked carbon skeletons of S-rich macromolecular aggregates present in bitumens and oils

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 -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