Walter A. Hartgers

Diagenetic and catagenetic products of isorenieratene: Molecular indicators for photic zone anoxia

Abstract A wide range of novel diagenetic and catagenetic products of the diaromatic carotenoid isorenieratene, a pigment of the photosynthetic green sulphur bacteria Chlorobiaceae, has been identified in a number of sedimentary rocks ranging from Ordovician to Miocene. Compound identification is based on NMR, mass spectrometry, the presence of atropisomers, and stable carbon isotopes. Atropisomers contain an axially chiral centre which, in combination with other chiral centres, results in two or more diastereomers that can be separated on a normal GC column. Chlorobiaceae use the reverse TCA cycle to fix carbon, so that their biomass is enriched in 13C. High 13C contents of isorenieratene derivatives therefore support their inferred origins. Isorenieratene derivatives include C 40, C 33, and C 32, diaryl isoprenoids and short-chain aryl isoprenoids with additional aromatic and/or S-containing rings. C 33 and C 32 compounds are diagenetic products of C 33 and C 32 “carotenoids” formed from isorenieratene during early diagenesis through expulsion of toluene and m-xylene, respectively. Cyclisation of the polyene acyclic isoprenoid chain can proceed via an intramolecular Diels-Alder reaction, followed by aromatisation of the newly formed ring. Sulphurisation is also an important process during early diagenesis, competing with expulsion and cyclisation. Sulphur-bound isorenieratane is released during progressive diagenesis, due to cleavage of relatively weak S S and C S bonds. Cleavage of C-C bonds during aromatisation of newly formed rings and during catagenesis yields short-chain compounds. The inherent presence of a conjugated double bond system in carotenoids implies that similar diagenetic and catagenetic reactions can occur with all carotenoids. Chlorobiaceae live at or below the oxic/anoxic boundary layer and require both light and H 2S. The presence of isorenieratene or its diagenetic and catagenetic products in ancient sedimentary rocks and crude oils is therefore an excellent indication for photic zone anoxia in the depositional environment. Diagenetic and catagenetic products of isorenieratene are expected to find applications in reconstruction of palaeoenvironments and in oil-oil and oil-source rock correlation studies. Their presence in several petroleum source rocks suggests that anoxia is an important environmental parameter for the preservation of organic matter.

A molecular and carbon isotopic study towards the origin and diagenetic fate of diaromatic carotenoids.

Pyrolysates of high-molecular-weight sedimentary fractions of the Duvernay Formation (Western Canada Basin) are dominated by 1,2,3,4- and 1,2,3,5-tetramethylbenzene, which, generated via beta-cleavage, indicate the presence of diaromatic carotenoids in the macromolecular aggregates. This was substantiated by desulphurization of sulphur-rich aggregates of the polar fraction, which released (partly) hydrogenated carotenoids. Furthermore, these components were important constituents of the aromatic hydrocarbon fractions and related oils. Apart from renieratane and isorenieratane, 1H NMR analysis established the aromatic substitution pattern of the most abundant component present, which was identified as a diaromatic compound with an unprecedented 2,3,6-/3,4,5-trimethyl aromatic substitution pattern. Molecular and isotopic analyses of both soluble and insoluble fractions of organic matter revealed relationships between diagenetically-derived carotenoids found in bitumen and related oils and their precursors incorporated into high-molecular-weight fractions. Aryl isoprenoids, important components in extracts and oils, were apparently derived from thermal cracking of bound diaromatic carotenoids rather than cleavage of free carotenoids as previously suggested. Furthermore, products derived from diaromatic carotenoids were substantially enriched in 13C relative to n-alkanes of algal origin. Together with the characteristic carotenoids, this isotopic enrichment provides evidence of significant contributions from photosynthetic green sulphur bacteria (Chlorobiaceae), which fix carbon via the reversed tricarboxylic acid (TCA) cycle. In spite of the prominence of these molecular signals, the overall isotopic composition of the organic matter indicated that only a very small portion of the preserved organic carbon was derived from the biomass of photosynthetic green sulphur bacteria.

Geochemical significance of alkylbenzene distributions in flash pyrolysates of kerogens, coals, and asphaltenes

Abstract The distribution of C 0 -C 5 alkylbenzenes in flash pyrolysates of forty-seven immature kerogens and coals from different geographical locations and of different ages were studied using gas chromatography (GC) in combination with mass spectrometry (MS) in order to decipher the origin of aromatic moieties in macromolecular matter. All possible structural isomers of the alkylated benzenes were determined, and, in some cases, absolute yields were calculated. Sulphur-rich (Type II-S) kerogens yield higher absolute amounts of alkylbenzenes in comparison to Type I, II, and III kerogens. The variations in internal distribution patterns of C 2 -C 4 alkylbenzenes were analyzed using multivariate analysis techniques (principal component analysis; PCA). Major variations in alkylbenzene distributions were due to an increased abundance of specific alkylbenzenes, which are related to specific precursor moieties in the macromolecular structure assuming that they are mainly formed via β-cleavage. Alkylbenzenes possessing “linear” carbon skeletons are enhanced in flash pyrolysates of Guttenberg and Estonian Kukersite kerogens (Type I) and are proposed to be derived from linear precursors which have undergone cyclization/aromatization. Relatively high amounts of 1,2,3,4- and 1,2,3,5-tetramethylbenzenes were found in flash pyrolysates of Womble and Duvernay kerogens (Type II) which are likely to be derived from macromolecularly bound diaromatic carotenoids. The relatively high abundance of 1,2,3-trimethylbenzene and 1,3-/1,4-dimethylbenzene in pyrolysates of Monterey kerogens (Type II-S) is proposed to be indicative of the presence of bound nonaromatic carotenoids (e.g., β,β-carotene) which have undergone aromatization and/or loss of methyl groups upon diagenesis. 1-methyl-4-isopropylbenzene, which appears in relatively high amounts in flash pyrolysates of Walcott Chuar kerogen (Type II) and Catalan coals (Type III), is thought to be derived from a heteroatom-bound precursor. These results demonstrate that the structure of monoaromatic moieties in kerogen is biologically controlled. The internal distribution patterns of alkylbenzenes of flash pyrolysates of kerogens and asphaltenes show a marked resemblance, the latter showing a slight enhancement of alkylbenzenes with longer carbon skeletons. These observations support the idea that asphaltenes are structurally related to kerogen and that aromatic moieties in asphaltenes are bound by fewer intermolecular bridges.

Sulfur-binding in recent environments: II. Speciation of sulfur and iron and implications for the occurrence of organo-sulfur compounds

Speciation of iron and sulfur species was determined for two recent sediments (La Trinitat and Lake Ciso) which were deposited in environments with a high biological productivity and sulfate-reducing activity. In sediments from calcite ponds of La Trinitat an excess of reactive iron species (iron monosulfides, iron hydroxides) results in a depletion of reactive sulfur which is accompanied by a virtual absence of organo-sulfur compounds, both in low (LMW) and high molecular-weight (HMW) fractions. Small amounts of phytanyl and highly branched isoprenoid (HBI) thiophenes in the extract demonstrate that these molecules exhibit a higher reactivity towards reduced sulfur species as compared to detrital iron. Euxinic sediments from Lake Ciso are characterised by an excess of reduced sulfur species which can rapidly trap reactive iron. High concentrations of H2S results in the formation of organo-sulfur compounds which were encountered in both LMW and HMW fractions. The major part of the organic sulfur is bound to the carbohydrate portion of woody tissues, whose presence was revealed by a specific alkylthiophene distribution in the flash pyrolysate and by Li/EtNH2 desulfurisation of the kerogen which resulted in the solubilisation of the sulfur-enriched hemicellulose fraction. Relatively high amounts of sulfurised C25 HBI compounds in the sediment extract of Lake Ciso reflect the incorporation of sulfur into algal derived organic matter upon early diagenesis. The combined approach of the speciation of iron and sulfur species and the molecular analysis of sedimentary fractions demonstrates that abiotic sulfur binding to organic matter occurs at the earliest stages of diagenesis under specific depositional conditions (anoxic, stratified water column) in which an excess of reduced sulfur species relative to the amount of reactive iron is a controlling factor.

Curie-point pyrolysis of sodium salts of functionalized fatty acids

Abstract Selected sodium salts of functionalized fatty acids were subjected to Curie-point pyrolysis and their pyrolysis products subsequently analyzed by gas chromatography/mass spectrometry in order to study the thermal dissociation mechanisms of lipid moieties in bio- and geomacromolecules. Pyrolysis of the sodium salts of hydroxy and keto acids yields in each case a methylketone as the major product, which is generated via a concerted, six-membered ring rearrangement triggered by the functionality in the alkyl chain. The thermal dissociation of alkylbenzene and alkylthiophene moieties proceeds mainly via hydrogen abstraction at the α- and γ-carbon atoms, followed by β-scission. The ratios of the main product pairs, i.e. styrene/toluene and vinylthiophene/2-methylthiophene, are largely controlled by radical mobility during pyrolysis. The major fragmentation pathway of monounsaturated alkyl chains is β-cleavage adjacent to the double bond. The identification of both Z and E isomers of alkenes in the pyrolysates demonstrates that the initial stereospecifity of the double bond is not retained during pyrolysis. The formation of linear alkylbenzenes from non-phenylic precursor moieties (e.g. diunsaturated fatty acids) shows that specific alkylbenzenes in Curie-point pyrolysates are not always derived from monoaromatic precursors. The addition of organic or mineral matter suppresses these secondary processes. The extension of the model compound results to the recognition of specific types of pyrolysis products and their relevance to the reconstruction of structures of lipid moieties in natural biomacromolecules in kerogens and coals are discussed.

Identification of C2C4 alkylated benzenes in flash pyrolysates of kerogens, coals and asphaltenes☆

Abstract Pseudo-Kovats indices were determined for all positional isomers of C2ue5f8C4 alkylated benzenes using a non-polar capillary column (CP Sil-5). These indices were applied in combination with mass spectral data to identify these compounds in complex mixtures of a relatively non-polar nature generated upon flash pyrolysis of macromolecular sedimentary organic matter (kerogens, asphaltenes and coals). Alkylbenzene distribution patterns were constructed by integration of appropriate mass chromatograms. Generally, toluene is the major pyrolysis product. Other specific alkylated benzenes (e.g. 1,2,3,4-tetramethylbenzene and 1-methyl-4-isopropylbenzene) were found to be enhanced in flash pyrolysates of different samples. Flash pyrolysates of asphaltene and kerogen fractions isolated from the same oil shale exhibit similar alkylbenzene distributions.

A molecular isotopic study of 13C-enriched organic matter in evaporitic deposits : recognition of CO2-limited ecosystems

Abstract Biomarkers were analysed in a surface sediment from La Trinitat saltern ponds and a Miocene marl from the Northern Appenines. Both are characterised by enriched δ 13 C values for bulk organic matter and for biomarkers, i.e. heavier than −20‰. Exceptions are long-chain C 22 –C 34 n -alkanes and C 16 –C 20 fatty acids which have values typically lighter than −20‰, suggesting that they are not derived from aquatic organisms but from surrounding terrestrial vegetation. The enriched δ 13 C values for biomarkers of different algae and bacteria suggest that the local (microbial mat) ecosystem may have been CO 2 -limited.

Sulphur-binding in recent environments. I. Lipid by-products from Ni2B desulphurization

Selected geochemically relevant model compounds were subjected to the nickel boride desulphurization method. Poor hydrogenating properties of this agent were observed in the case of sterically hindered double bonds. Apart from its desulphurizing properties, a significant number of functionalities present in lipid compounds were transformed on treatment with nickel boride. Ketones were reduced to their alcohols whereas aldehydes were partly converted to their ketals. Although alcohols and esters were unaffected by this agent, a nearby unsaturation (i.e., position α) resulted in the loss of the alcohol group and cleavage of the ester bond, respectively. The significance of these results was demonstrated by desulphurization experiments on polar fractions from two recent sediment extracts. The main compound among the apolar products obtained, phytane, was found to be quantitatively derived from phytol and not, as may be presumed, from its macromolecularly sulphur-bound precursor. The results of this work show that compounds generated by nickel boride desulphurization of polar fractions from lipid extracts, particularly from recent sediments, are not necessarily derived from their sulphur-bound precursors. Conclusions with regard to the palaeoenvironmental significance of the hydrocarbons formed, therefore, should be made with caution.

Bacterial and algal markers in sedimentary organic matter deposited under natural sulphurization conditions (Lorca Basin, Murcia, Spain)

Abstract Free lipids, sulphur-bound lipids present in macromolecular fractions and kerogen pyrolysis products have been studied in shales and early diagenetic carbonates replacing gypsum from a Messinian sequence of Lorca Basin. The high abundance of phytane, 2,3-dimethyl-5-(2,6,10-trimethylundecyl)thiophene, mid-chain C 20 isoprenoid thiophenes and bithiophenes and bis-O-phytanyl and O-phytanyl-O-sesterterpanyl glycerol ethers indicates that the organic matter in all these samples was deposited under hypersaline conditions. The isopranyl glycerol ethers are essentially found in the sulphur-bound macromolecular matter which contrasts with the low concentrations of these compounds as free lipids. However, the distributions of these isopranyl glycerols is paralleled by the occurrence of free phytanic acid (shales and laminated carbonate) and 3,7,11,15,19-pentamethyleicosanoic acid (only in the laminated carbonate). The bacterial inputs are represented by 2-hydroxytetracosanoic acid, n -octadec-11-enoic acid, hopanoic acids and the distributions of iso - and anteiso -C 15 and C 17 homologues and minor amounts of iso -C 16 and anteiso -C 14 . These branched fatty acids are characteristic of sulphate-reducing bacteria. The relative proportions of the iso - and anteiso -compounds in the total fatty acid distributions are correlated with the proportion of reduced sulphur in the sediments.

Sedimentary evidence for a diaromatic carotenoid with an unprecedented aromatic substitution pattern

A novel diaryl isoprenoid (C40H66), possessing a 3,4,5-/2,3,6-trimethyl aromatic substitution pattern, which is attributed to a presently unknown strain of photosynthetic green sulfur bacteria, has been identified in a carbonaceous sedimentary rock.