M.E.L. Kohnen

Recognition of Paleobiochemicals by a Combined Molecular Sulfur 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.

Di- or polysulphide-bound biomarkers in sulphur-rich geomacromolecules as revealed by selective chemolysis

Abstract Three types of sulphur-rich high-molecular-weight material in the alkylsulphide, the polar, and the asphaltene fractions isolated from the bitumen of an immature bituminous shale from the Vena del Gesso basin (Italy) were desulphurised using Raney Ni and were treated with MeLi/MeI, a chemical degradation method which cleaves selectively and quantitatively di- or polysulphide linkages. The products formed were characterised by gas chromatography-mass spectrometry. Raney Ni desulphurisation revealed that these S-rich macromolecules are in substantial part composed of sulphur-linked biomarkers with linear, branched, isoprenoid, steroid, hopanoid, and carotenoid carbon skeletons. MeLi/Mel treatment provided evidence that a major part of the total amount of macromolecularly bound biomarkers are linked via di- or polysulphide moieties to the macromolecular network. Since the di- or polysulphide linkages are attached at specific positions of the bound biomarkers it is proposed that they are formed by intermolecular incorporation reactions of HSx− into low-molecular-weight functionalised biological lipids during early diagenesis. The different properties (solubility and molecular weight) of the sulphur-rich macromolecules in the alkylsulphide, the resin, and the asphaltene fractions can be explained simply by differences in degree of sulphur cross-linking.

Organic sulfur compounds formed during early diagenesis in Black Sea sediments

Abstract Sediments from Units I and II of the Black Sea were analyzed to assess the early diagenetic formation of organic sulfur compounds (OSC). A series of isomeric C28-2,4-dialkylthiophenes was found at low concentrations in surface sediments. OSC with C25-highly branched isoprenoid (RBI) skeletons were not found in surface sediments even though there was a rapid decrease in the concentrations of C25-HBI alkenes in the same sediments, nor were OSC found which contained carbon skeletons derived from C37–C39 alkenes which are also abundant in the sediment. As depth increased in Unit I, concentrations of the C28-2,4-dialkylthiophenes decreased while C25- and C30-HBI thiolanes appeared at the bottom of Unit I and increased in concentration into Unit II. Treatment of macromolecular material from sediment extracts with Raney nickel yielded phytane as the dominant hydrocarbon as well as series of C25- and C30-HBI hydrocarbons, β-carotane, and isorenieratane. These latter two components were generally absent from the free hydrocarbon fractions. These results indicate that sulfur incorporation into functionalized lipids can occur during the very early stages of sedimentary diagenesis, even at the sediment-water interface. On the other hand, the rapid loss of C25-HBI alkenes in surface sediments could not be accounted for by sulfur quenching, and other potential OSC-precursors, such as C37-C39 alkenes, also apparently did not incorporate sulfur into readily-analyzable material.

Origin and diagenetic transformations of C25 and C30 highly branched isoprenoid sulphur compounds : further evidence for the formation of organic sulphur compounds during early diagenesis

Abstract A number of C25 and C30 highly branched isoprenoid (HBI) sulphur compounds (E.G., thiolanes, 1-oxo-thiolanes, thiophenes, and benzo[ b ]thiophenes) with 2,6,10,14-tetramethyl-7-(3-methylpentyl)pentadecane and 2,6,10,14,18-pentamethyl-7-(3-methylpentyl)nonadecane carbon skeletons were identified in sediments, ranging from Holocene to Upper Cretaceous. These identifications are based on mass spectral characterisation, desulphurisation, and, in some cases, by comparison of mass spectral and relative retention time data with those of authentic standards. The presence of unsaturated C25 and C30 HBI thiolanes in a Recent sediment from the Black Sea (age 3–6 × 10 3 a) strongly supports their formation during early diagenesis. The co-occurrence of HBI polyenes (C25 and C30) and unsaturated HBI thiolanes (C25 and C30) possessing two double bonds less than the corresponding HBI polyenes, in this Recent sediment, testifies to the formation of unsaturated HBI thiolanes by a reaction of inorganic sulphur species with double bonds of the HBI polyenes. Furthermore, a diagenetic scheme for HBI sulphur compounds is proposed based on the identification of HBI sulphur compounds in sediment samples with different maturity levels.

Restricted utility of aryl isoprenoids as indicators for 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.

Early diagenesis of bacteriohopanepolyol derivatives: Formation of fossil homohopanoids

Abstract Diagenetic pathways of bacteriohopanepolyol derivatives are proposed based on the concentrations and 13C contents of homohopanes, homohop-17(21)-enes, benzohopanes, hopanoid thiophenes and sulphides, and macromolecularly S-bound homohopanes present in the extracts of twelve composite one metre samples from a 120 m core recovered from the Upper Cretaceous Jurf ed Darawish Oil Shale (Jordan). A large part (>80–95%) of the pentakishomohopane skeleton occurs in a S-bound form. This reveals the selective preservation of the C35 hopane skeleton by sulphur sequestration and provides a theoretical basis for the homohopane index as an indicator of anoxia in past depositional environments. A smaller part (>50–80%) of the total extended hopane skeletons (C31–C35) occurs in a S-bound form. Of the non-sulphur-containing hopanoids the homohop-17(21)-enes dominate. These latter components show a gradual increase of 22S epimers with depth (45–52%) towards the thermodynamic equilibrium (52–53%) as calculated by molecular mechanics. Molecular mechanic calculations indicate that this increase can be explained by either isomerisation of 22R hop-17(21)-enes or by isomerisation of double bonds of homohopenes formed by dehydration of bacteriohopanepolyols “en passant” isomerising the chiral centre at C-22. A combination of these two pathways is also possible and provides an explanation for different δ13C values of pairs of 22R and 22S epimers. Isomerisation of 17β,21β(H)-homohopane to 17α,21,β(H)-homohopane carbon skeletons occurs for all compound classes in a very narrow depth span (ca. 20 m) and is probably induced by small differences in thermal history. Compound-specific carbon isotope analyses indicated that the series of homohop-17(21)-enes have in some cases significant differences in 13C content, indicating that at least two different sources have contributed to this series of components. Differences with macromolecularly S-bound C35 hopane skeletons and free C31 hopanes 13C contents are in some cases even larger. These data show that the diagenetic pathways of bacteriohopanepolyol derivatives are more complex than previously recognized and reveal that multiple precursor bacteriohopanepolyol derivatives prone to different diagenetic pathways have to be envisaged to account for the differences observed.

Sulphur-bound steroid and phytane carbon skeletons in geomacromolecules: Implications for the mechanism of incorporation of sulphur into organic matter

Sulphur-bound steroid and phytane moieties in macromolecules present in the polar fractions of six immature samples (both crude oils and sediment extracts) have been analyzed using S-selective chemolysis methods and analytical pyrolysis. The identifications of the methylthioethers released from the macromolecule-containing fractions after MeLi/MeI treatment are based on comparison of mass spectral data and chromatographic data with those for synthesized methylthioethers. Evidence is presented that di- or polysulphide linkages are present in geomacromolecules in both sediments and oils and that the location of di- or polysulphide linkages in macromolecularly S-bound moieties is the same as that of monosulphide linkages. Macromolecularly S-bound phytanyl moieties are chiefly bound with S linkages located at the tertiary positions of their carbon skeletons, which indicates that the S incorporation mechanism(s) involve(s) intermediate carbocations. The macromolecularly S-bound steroids are bound with S linkages located mainly at C-2, C-3, C-4, or C-5 of their carbon skeletons, which indicates that the S incorporation took place into sterenes or steradienes—the dehydration products of stanols and stenols, respectively. However, it remains possible that the macromolecularly S-bound steroids with an axial S linkage at C-3 are, in part, resulting from a SN2 reaction of inorganic S species with steryl esters or stanols.

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

Abstract Homologous series (C 15 -C 24 ) 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.

Variations in the content and composition of organic matter in sediments underlying active upwelling regimes: a study from ODP Legs 108, 112 and 117

Abstract An overview is presented of organic geochemical studies on sediments underlying the active upwelling cells off Northwest Africa (ODP Leg 108; Site 658), off Peru (Leg 112; Sites 679, 681, 684, 686) and off Oman (Leg 117; Sites 723, 725, 728). The investigated sediments are all characterized by high organic carbon contents (0.5–9% Corg), being the sedimentary expression of an increased primary bioproductivity induced by upwelling currents of deep nutrient-rich waters. The organic matter is predominantly of marine origin with variable admixtures of a terrigenous component; the highest relative contribution is found at Site 658. The extractable lipids are composed of a wide variety of compounds, of which the most characteristic and abundant compounds were quantified. Long-chain unsaturated ketones usually dominate. Steroids, alkanediols and their corresponding keto-ols are also present in high concentrations, although strong variations in their abundances were noted. In addition, organic sulphur compounds are omnipresent and reveal strong variations in their distribution, but do not make a major contribution to the total amount of extractable lipids.

Identification and occurrence of novel C36-C54 3,4-dialkylthiophenes with an unusual carbon skeleton in immature sediments

Abstract A series of novel long-chain 3,4-dialkylthiophenes (C36–C54) was identified in a number of sediments ranging from Pleistocene to Cretaceous. The identifications were based on mass spectral characterisation, desulphurisation and mass spectral data of synthesised model compounds. These organic sulphur compounds are probably formed by sulphur incorporation into mid-chain dimethylalkadienes with two methylenic double bonds. These putative precursor lipids are unprecedented and may be considered rather unusual. The distribution of 3,4-dialkylthiophenes in sediments varies considerably with the depositional palaeoenvironment, indicating that these compounds have a potential as molecular markers reflecting changes in palaeoenvironment.