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International audience; The tribological performances of engine oils have been shown to be enhanced by the synergistic interactions between Mo dithiocarbamates (Mo(DTC)2) with other additives, and notably Zn dithiophosphates (Zn(DTP)2). Being two key components in formulated lubricants, a detailed understanding of the mechanisms involved between these two types of additives is needed to develop engine oils with enhanced friction reduction performances, and improved fuel economy. In this context, we report here the investigation at the molecular level of the interactions between Mo and Zn complexes with DTC and DTP ligands using laboratory experiments. Our analytical approach comprised NMR spectroscopy (1H, 13C, 31P) allowing direct investigation of both homoleptic and heteroleptic Mo and Zn complexes as well as a specifically-developed HPLC-MS method for the investigation of the different DTC species formed during lubricant ageing experiments. The results showed that ligand exchange reactions between Mo(DTP)2 and Zn(DTC)2 complexes strongly favor the migration of the DTC ligands from Zn to Mo, illustrating the higher affinity of Mo for DTC ligands. In the case of binary mixtures involving Mo(DTC)2 and Zn(DTP)2 – a combination of additives frequently used in formulated lubricants – the formation of mixed complexes (Mo(DTC)(DTP)) resulting from ligand exchange reactions could be directly evidenced for the first time by the analytical methods used. These species could account, at least to some extent, for the synergistic effect of Mo(DTC)2 and Zn(DTP)2 on the friction reducing properties of engine oils. However, they were formed in significantly lower proportions than those previously reported in the literature using indirect methods

A preliminary study of the design of an electrically small plasma-based resonator using a localized surface plasmon resonance (LSPR) above 1 GHz is presented. Such a resonator is intended to be used to develop an electrically small antenna with frequency agility. This resonator consists of a plasma discharge confined in a hemispherical glass shell 3 cm in diameter on a ground plane. From 1 to 1.5 GHz, the plasma electron density required to achieve the LSPR must be between 0. $5\times 10^{11}$ and $1.4\times 10^{11}\mathrm{c}\mathrm{m}^{-3}$. Plasma losses are taken into account in this study and provide information on the required gas pressure. Typically for neon gas, the working pressure must be around 50 mTorr. A practical implementation using a miniature inductively coupled plasma (mICP) source is finally discussed.

The widely distributed aromatic di- and triterpenoids from higher plants occurring in sediments are formed by diagenetic microbial processes affecting their precursor plant lipids. These compounds and their stable isotopic composition (δ13C and δ2H) have the potential to be used for palaeoenvironmental and palaeoclimatic studies. In the present study, the isotopic composition of di- and triterpenoids aromatized to different extents has been measured to specifically examine the isotopic effects associated with the aromatization reaction. To overcome the possibility of multiple higher plant sources, as is generally the case with sedimentary lipids, the δ13C and δ2H values of aromatized di- and triterpenoids from lipid extracts recovered from conifer and angiosperm buried wood have been determined, allowing an unambiguous substrate/product relationship along the aromatization pathway to be assured. The results show that the δ13C compositions of both di- and triterpenes do not seem to be significantly affected by progressive aromatization, whereas the situation is different for δ2H values. In the case of diterpenoids related to abietic acid, a significant increase of the δ2H values by up to 86‰ with ongoing aromatization was measured. This is in contrast to what is expected for dehydrogenated compounds which should be globally more 2H-depleted than their precursor molecules (e.g., biosynthesized unsaturated fatty acids vs. their saturated precursors). This 2H enrichment of aromatized diterpenoids could indicate that they represent only minor residual intermediates, the majority having been further degraded by processes favoring the degradation of the 1H-containing substrates and having a moderately pronounced 12C/13C selectivity. With triterpenoids, preservation of the δ2H values was observed whatever the nature and extent of the aromatization process considered, which may be related to enzymatic reactions showing a limited influence on the hydrogen stable isotopic composition of the aromatized compounds.

International audience; Deciphering the role of sulfur in biogeochemical cycles strongly relies on its stable isotope composition, which ultimately depends on the ability to quantitatively recover different sulfur species from geological samples. For decades most studies have been restricted to the 34S/32S composition of bulk samples, using simple methods based on the analysis of SO2 released by sample combustion combined to mass spectrometry. The wet chemistry procedures required to selectively extract specific sulfur species were generally avoided due to their tediousness and inefficiency for some complex matrices, especially when targeting organic sulfur. With the recent advent of multi-isotope studies (investigating the minor sulfur stable isotopes 33S and 36S) which rely either on the analysis of sulfur as SF6, or on the use of secondary ion or multi-collector inductively coupled plasma mass spectrometry, wet chemistry-based preparation procedures were brought back to the stage with a renewed interest in developing procedures better adapted to the investigation of specific sulfur species. Here we propose a new stepwise chemical procedure for the quantitative recovery and multi-isotope analysis of organic sulfur from both solvent soluble (total lipid extract) and insoluble (kerogen) fractions, based on a wet oxidation by sodium hypochlorite. This procedure also allows the multi-isotope analysis of inorganic sulfur species (elemental sulfur, sulfates and sulfides) in the same sample. Its application to different well-known petroleum source rocks and to an oil demonstrates its relevance for disentangling the interactions between the different sulfur pools and for shedding new light on the sulfur biogeochemical cycle.

International audience

Sulfur is a key element of biogeochemical cycles, being both an essential component of living cells and involved in major geological processes. Its four stable isotopes (32S, 34S, 33S, 36S, in order of abundance) are subject to equilibrium and kinetic fractionations, which can be used as tracers to understand sulfur cycling processes. Multi-isotope studies noticeably revealed mass-independent fractionation processes of 33S and 36S, enlightening previously unknown specific organic or inorganic (bio)(geo)chemical mechanisms. Though the analytical procedures for the determination of S-multi-isotope compositions, which requires a quantitative extraction and purification of sulfur, are well-known for mineral sulfur species, the existing procedures for the extraction of organic sulfur are either restricted to specific species or technically restrictive. Here we propose a new chemical procedure for the quantitative recovery of organic sulfur in the form of Ag2S, for multi-isotope analysis, which is also compatible with the multi-isotope analysis of inorganic sulfur species from the same samples. This procedure is based on a standard sequence of reductive attacks specific of mineral sulfur species and was validated on various samples including fresh sediments, source rocks and oils.

A recent article (Said-Ahmad, W., Amrani, A., Aizenshtat, Z., 2013. The action of elemental sulfur plus water on 1-octene at low temperatures. Organic Geochemistry 59, 82–86) is commented on in this discussion. Radical mechanisms proposed by Said-Ahmad et al. (2013) for the formation of sulfurized and oxidized organic compounds in experiments involving elemental sulfur and 1-octene under aqueous conditions, aimed at investigating the interaction of organic matter and sulfur compounds under geological conditions, are not compatible with their experimental results and other studies. In addition, results from the experiments aimed at demonstrating the role of HO radicals in the formation of oxidized organic compounds were incorrectly interpreted by the authors. In this discussion, alternative mechanisms for the formation of the sulfurized and oxygenated products reported by Said-Ahmad et al. (2013) are proposed.

Maleimides (transformation products of chlorophylls and bacteriochlorophylls) were studied in recent sediments from the Swiss lake Rotsee and the Romanian Black Sea Shelf to investigate chlorophyll degradation, the role of oxygen in maleimide formation, and to identify their sources. Naturally occurring maleimides (i.e. “free” maleimides) and maleimides obtained after chromic acid oxidation of sediment extracts (i.e. “bound” maleimides) were analysed. 2-Methyl-maleimide (Me,H maleimide), 2,3-dimethyl-maleimide (Me,Me maleimide), 2-methyl-3-vinyl-maleimide (Me,vinyl maleimide), 2-methyl-3-ethyl-maleimide (Me,Et maleimide) and traces of 2-methyl-3- iso -butyl-maleimide (Me, i -Bu maleimide) occurred naturally in the sediment with a large predominance of the Me,Et homologue. Tetrapyrrolic pigments related to chlorophylls were the main source of maleimides, although variable contributions of other sources such as cytochromes and/or phycobilins cannot be completely ruled out. The predominant Me,Et maleimide and Me,vinyl maleimide most likely originate mainly from chlorophyll a related pigments. The same holds for Me,H maleimide, which might be formed following degradation of ring E from the tetrapyrrolic nucleus. Alternatively, Me,H maleimide and Me,Me maleimides might be formed by a recently discovered transformation pathway involving the oxidation of vinylic chlorophyll substituents and the formation of an aldehyde intermediate. 2-Methyl-3- n -propyl-maleimide (Me, n -Pr maleimide) and Me, i -Bu maleimide arising from bacteriochlorophyll related pigments traced the presence of phototrophic sulfur bacteria (Chlorobi), indicating photic zone euxinic and anoxic conditions in Rotsee during the last 150 years and throughout the Black Sea history, including the limnic phase of the Black Sea (Unit 3). Some other minor maleimides with specific alkylation pattern also originate from bacteriochlorophylls, while the source of others could not be identified. Free maleimides were mainly formed in the sediment in the absence of oxygen. Novel maleimide degradation indices are proposed to estimate the degree of OM degradation (OM freshness/degradability). These proxies are applicable on longer timescales than e.g. the chlorin index.

Laboratory experiments carried out under plausible prebiotic conditions (under conditions that might have occurred at primitive deep-sea hydrothermal vents) in water and involving constituents that occur in the vicinity of submarine hydrothermal vents (e.g., CO, H(2)S, NiS) have disclosed an iterative Ni-catalyzed pathway of C-C bond formation. This pathway leads from CO to various organic molecules that comprise, notably, thiols, alkylmono- and disulfides, carboxylic acids, and related thioesters containing up to four carbon atoms. Furthermore, similar experiments with organic compounds containing various functionalities, such as thiols, carboxylic acids, thioesters, and alcohols, gave clues to the mechanisms of this novel synthetic process in which reduced metal species, in particular Ni(0), appear to be the key catalysts. Moreover, the formation of aldehydes (and ketones) as labile intermediates via a hydroformylation-related process proved to be at the core of the chain elongation process. Since this process can potentially lead to organic compounds with any chain length, it could have played a significant role in the prebiotic formation of lipidic amphiphilic molecules such as fatty acids, potential precursors of membrane constituents.

The hypothesis of a low temperature natural “vulcanization” process as a potential mechanism for the formation of insoluble bitumen and H2S in reservoirs which have not undergone biodegradation, high thermal stress, or thermochemical sulfate reduction (TSR) has been investigated using laboratory simulation experiments. The experiments, involving elemental sulfur (32S and 34S), a low maturity oil and water at 200 °C in sealed gold tubes, clearly showed that elemental sulfur is able to induce the formation of H2S, polar compounds (resins and asphaltenes) and insoluble bitumen most likely by way of sulfur cross-linking processes. Specific organic sulfur compounds (thiophenes, dithiophenes and thienothiophenes) formed by the reaction of sulfur and hydrocarbons may be potential markers of such a process, but remain to be detected in natural samples.

The sulfur isotopic composition (δ34S) of petroleum is believed to be affected mainly by sulfur incorporation reactions into the sedimentary organic matter during the early diagenesis. However, secondary processes could affect the original δ34S of oil under the effect of thermal maturity or of the microbial activity of biodegraded reservoirs. In this study, the different processes that may affect the δ34S of in-reservoir oils were assessed based on the sulfur content and isotopes of a series of oil and core samples coming from various reservoirs of the Lower Cretaceous Mannville Group, Western Canada Sedimentary Basin (WCSB). Based on the molecular study, these samples appear to have reached various levels of maturity and biodegradation, ranging from 0 to 6.5 on the biodegradation scale of Peters and Moldowan. In addition, mixing of organic matter coming from different source rocks was identified based on the comparison with extensive correlation studies performed in the WCSB. Investigation of the δ34S shows a trend that seems a priori correlated to the level of biodegradation. However, a careful interpretation of molecular and sulfur isotope data leads to the conclusion that the observed δ34S variations have rather to be ascribed to contributions of oils generated by various source rocks. Alternatively, variations of δ34S could neither be related to maturity differences nor to kinetic effects during organic sulfur compounds biodegradation. In the case of some specific core samples showing a common origin based on biomarker study, δ34S variations might not be related to different sources but to secondary sulfur incorporation/exchange processes occurring within the reservoir. These processes would involve reduced sulfur species from bacterial sulfate reduction formed in situ or migrated into Mannville reservoirs. This hypothesis is supported by laboratory experiments showing sulfur exchange/incorporation under plausible conditions for shallow reservoirs.

Thermochemical sulfate reduction (TSR) is usually associated with significant sour gas (H 2 S) generation and calcite precipitation in deep and hot carbonate reservoirs as a result of reaction between sulfate minerals (primarily anhydrite) and petroleum fluids at elevated temperature (beginning at ∼110-140°C). Thermal enhanced oil recovery (TEOR) techniques are widely applied during production to increase heavy oil recovery in mature fields by lowering oil viscosity. The reservoir rock may reach high temperature (150 °C < T< 300 °C) during steam and hot water flooding for secondary oil recovery that induces TSR. Experimental (pyrolysis in gold tubes) and numerical (0D fluid-rock interaction numerical model: ArXim) simulations were undertaken with a simple petroleum system assimilated to n-alkane model compounds in the presence of sulfate to assess the feasibility of "induced TSR" and to tentatively estimate the sulfur risk in terms of H 2 S and organosulfur compound (OSC) formation during steam and hot water stimulated enhanced recovery operations. The oxidation of organic matter (by reaction with both S° and sulfate) can be easily reproduced under laboratory conditions, as confirmed by the presence of organic sulfur in the soluble and insoluble residue. For the experiments performed in the presence of sulfate, higher amounts of H 2 S, polar compounds and organic residue are produced, and aromatic fractions are enriched in condensed aromatics (sulfur-containing and non-sulfur-containing). Although sulfate reduction from 3% to 5% is deduced from a sulfur balance calculation, no secondary carbonate is detected using X-ray diffraction (XRD). Numerical simulations suggest no limiting thermodynamic factor for the precipitation of carbonate under our experimental conditions and indicate the replacement of sulfate by carbonate. According to numerical data, the lack of carbonate can be explained by its slow growth and/or high energy barrier of nucleation and/or insufficient residence time in laboratory experiments.

A novel series of sedimentary aromatic lycopane derivatives, cyclolycopatrienes, has been found in an outcrop sample from the Messel oil shale. The predominant monoaromatic compound 4 from this hydrocarbon series has been isolated using reversed phase HPLC and its structure determined using 1D and 2D 1H NMR spectroscopy, as well as from identification of the RuO4 degradation products. From examination of its structure, related to that of lycopane, and from its stable carbon isotopic composition, this compound and related isomers can be considered as new proxies for Botryococcus braunii race L in sediments deposited under freshwater and/or brackish conditions.

The effects of weathering on the organic matter within a narrow sediment interval from surficial to 10 m depth belonging to the Lower Toarcian shales of the Paris basin, were studied at a molecular level. Among the compositional changes observed, the most striking feature is a severe breakdown of the metalloporphyrins (Ni and VO) at the surface, as evidenced by quantitative data. In addition, this degradation of the geoporphyrins is accompanied by substantial modification of their composition with preferential degradation of porphyrins with exocyclic rings over polyalkylporphyrins. These quantitative and qualitative changes are thought to result from exposure of the sediment to weathering conditions. More specifically, abiotic oxidation is likely to be the main weathering process responsible for this superficial alteration of the sedimentary porphyrins. This is particularly suggested by oxidation experiments carried out on a mixture of sedimentary nickel and vanadyl porphyrins where oxidation of the former complexes occurs at a faster rate. Thus, geoporphyrins may be used as molecular indicators to evaluate the degree of oxidation of weathered geological samples containing this kind of molecule.

The desulfurization of fuels (for example,diese loil) is a major concern of the petroleum industry worldwide because of the consequences for human health and the environment of high levels of sulfur dioxide emissions. In this context, a major problem encountered in refineries is the elimination of organosulfur components resistant to hydrodesulfurization processes. Although it has been shown that a substantial fraction of the organic sulfur compounds remaining after hydrodesulfurization is composed of mono-andpolyalkylated dibenzothiophenes (mainly 4,6-dialkyldibenzothiophenes),[1] the lack of knowledge concerning the structures of residual highly resistant organic sulfur compounds still appears to be the greatest handicap in progressing towards higher degrees of hydrodesulfurization.We report here the identification of a series of novel, terpenoid-derived, non-dibenzothiophenic polycyclic sulfides from highly desulfurized diesel oils. These 1,1,4a,6-tetramethyl-9-alkyl-1,2,3,4,4a,9b-hexahydrodibenzothiophenes1(9-alkylHDBT) were isolated from the aromatic fraction after oxidation to the corresponding sulfones 2 (9alkyl-HDBTO2).[2–4] The 9-ethyl homologue was unambiguously identified by mass spectrometry and NMR studies.

The reduction of allylic thiols to alkenes by hydrogen sulfide in aqueous solutions, a novel reaction that may explain reduction processes widely occurring in natural environments, has been discovered. Its mechanism has been studied and suggested to follow an S(RN)1-like pathway involving radical intermediates undergoing 1,4 hydrogen shifts. [reaction: see text]

The Be’eri sulfur mine (Israel) is a unique deposit mainly composed of sandstone intercalated with biogenic mats and possessing organic matter exceptionally depleted in 13C. Molecular and isotopic studies of free and bound biomarkers were performed to unravel the source of the organic matter co-occurring with sulfur in this deposit and to propose a paleoenvironmental model of bacterial life in a type of extreme environment. They showed that the biomarkers are all extremely 13C-depleted and almost exclusively composed of hopanoids and biphytane derivatives of bacterial origin, notably methanotrophic bacteria and acidophilic archaea. δ13C values of individual components and of bulk organic carbon are in the −80% to −90% range and are among the lowest values ever measured for hopanoids. Organic matter in the sandstone and the mats differ mainly by the occurrence of 3-methylated hopanoids in the mats, which may reflect either different bacterial populations or different conditions of growth. These data demonstrate that the complete biomass of this deposit primarily derives from methanotrophic hopanoid-synthesizing bacteria consuming methane having seeped toward the surface, and that all other organisms—apparently only archaea and bacteria—must have been thriving on methane-derived carbon (methane, CO2, biomass of methanotrophic bacteria). Unambiguous evidence for photosynthetic organisms in the environment of deposition could not be found. The Be’eri sulfur deposit is thus a fossil remain of an exclusively bacterial ecosystem fueled by methane as sole carbon source and having developed in an interstitial aqueous medium within the sandstone. Elemental sulfur from the deposit probably originates from the oxidation of hydrogen sulfide seeping along with methane, which could have been oxidized either abiotically or biologically by sulfur-oxidizing Beggiatoa-like bacteria and archaea. Further oxidation of elemental sulfur might explain the high acidity of the deposit. The oxidizing conditions now prevailing in the Be’eri deposit were revealed by the occurrence of degraded, oxidized, or thiophenic hopanoid structures. Some of them, unambiguously characterized by synthesis, were also obtained by heating hopenes with elemental sulfur, thus suggesting that the latter could play a role, as dehydrogenating and oxidizing agent, in the transformations undergone by organic matter in the Be’eri deposit.

Macromolecular fractions, isolated from the solvent extract of sulfur-rich Recent (Siders Pond, USA; Lake Cadagno, Switzerland; Walvis Bay, Namibia) and immature sediments (Gibellina, Messinian of Sicily; Vena del Gesso, Messinian of Italy), were investigated by chemical degradation using sodium ethanethiolate/methyliodide. This mild reagent which cleaves polysulfide bonds to yield methylsulfides has the advantage over other methods of leaving intact other functionalities (like double bonds) and preserving sulfur atoms at their incorporation site. The method is, therefore, well-suited to the molecular level investigation of sulfur-rich macromolecules from Recent sediments containing highly functionalized polysulfide-bound subunits. In Recent anoxic sulfur-rich sediments, the release of various methylthioethers clearly demonstrates that intermolecular sulfurization of organic matter does occur at the earliest stages of diagenesis. Steroids and phytane derivatives are the major sulfurized lipids, a feature also observed in more mature sulfur-rich sediments. Several phytene derivatives, such as cis and trans 1-methylthiophyt-2-enes, as well as methylthiosteroids, including 5α- and 5β-3-(methylthio)-cholest-2-enes, were identified by comparison with synthesized standards. Steroid methylthioenolethers are released from polysulfide-bound steroid enethiols present in the macromolecular fractions. The latter, which correspond to thioketones, can be considered as intermediates in the reductive sulfurization pathway leading from steroid ketones to polysulfide-bound saturated steroid skeletons and are characterized for the first time in the present study. Thus, it could be shown that the major part of the polysulfide-bound lipids occurring in Recent sediments is apparently the result of sulfurization processes affecting carbonyls (aldehydes and ketones). The unsaturated methylthioethers obtained from Recent sediments were not present in more mature evaporitic samples, which suggests that polysulfide-bound unsaturated thiols are intermediates formed during the first sulfurization steps occurring soon after deposition and that they are rapidly transformed by various processes taking place during early diagenesis, notably to yield their saturated counterparts.

Substantial amounts of alcohols occur in the desulfurization products of sulfur-rich nonpolar macromolecular fractions (NPMF) isolated from two crude oils and a sediment extract. These macromolecularly bound oxygenated compounds have been investigated in detail. Released straight chain components may have a hydroxy functionality at any position of the carbon skeleton and without any isomer predominance. Furthermore, the carbon-number distributions are very similar for the different alcohol isomers in each case and resemble those of the aliphatic hydrocarbons released by desulfurization. Thus, released hydrocarbons and alcohols likely originate from common functionalized precursors, most probably from polyunsaturated lipids of biological origin. Furthermore, they may derive from polyunsaturated components formed by elimination reactions on functionalized precursors which incorporated oxygen in free or already bound form at an early stage of diagenesis. The presence of hydroxyl functionalities at every position in the carbon skeleton suggests that double bond isomerization probably occurred in linear components prior to oxygen incorporation. Similarly, 2-hydroxy stanols released by desulfurization most likely result from oxygen uptake into Δ 2 -sterenes during diagenesis. The presence of mid-chain hydroxylated phytanols in the degradation products with OH-group mainly at the tertiary positions indicates that they result (at least partially) from oxygen incorporation into unsaturated phytane skeletons. Additional functionalities in the oxygenated substances, such as double bonds, aldehydes or allylic alcohols, may have served as substrates for reactions with sulfur species, resulting in sulfur-rich cross-linked macromolecular structures. The type of oxygen groups present in the macromolecules could be partially assigned by sequential chemical degradation experiments. The results indicate that free OH-functions are not abundant. Part of the oxygen is present as carboxylic acid, ester and ketone functions. However, the nature of a substantial part of the oxygen species remains unclear.

A novel series of di- to pentacyclic polyprenoid sulfides ( 6, 7, 15, 16 ) which are structurally related to the recently identified highly cyclised regular polyprenoid aromatic ( 1, 2, 3 ) and saturated hydrocarbons (5) and to the widely distributed tricyclopolyprenanes (e.g., 10 ) have been characterized in a wide range of sediments. Their distributions, extending from C 13 to C 30 , are clearly dominated by the C 15 , C 20 , C 25 and C 30 homologues, each being represented by several isomers. Two C 30 isomers ( 6a and 6b ), which are generally predominant, were isolated from a sample from Semecourt (Paris basin, France) and their structure established by NMR studies. Interestingly, these two isomers are the sole sulfides present in several samples. The structure of two C 20 isomers ( 7a, 7b ) was confirmed by synthesis of standards. These new sulfides clearly derive from precursors resulting from the incomplete cyclization of regular polyprenoids (e.g., 4 ). The presence of functionalities both on the side chain and, probably in the last ring, allowed intramolecular sulfur incorporation by a process similar to that leading to the formation of low molecular weight sulfides in sediments. Detection of these new polyprenoid sulfides in recent sediments indicates that the sulfurization step occurs at an early stage of diagenesis. The hydrocarbon skeletons of the lower di-, tri- and tetracyclic sulfides may derive from widespread terpenoids present in various organisms. The predominant pentacyclic homologues, however, have a rare and specific carbon skeleton which can not be related to any known biological precursor. The fact that these lower homologues always co-occur with the pentacyclic sulfides points to a common and specific origin for the whole series. Their origin seems, however, distinct from that of the related monoaromatic and saturated regular polycyclic polyprenoid hydrocarbons ( 1-3, 5 ) which are absent from the samples where sulfides occur. The carbon isotopic composition of two C 30 pentacyclic homologues ( 6a, 6b ) could be determined in two samples and suggests that their polyprenoid precursors originate either from primary producers or from heterotrophic organisms living in the oxic part of the water column.

The possibility of light induced formation of sedimentary organo-sulfur compounds has been investigated with simulation experiments. These experiments, using mono- and diunsaturated hydrocarbons and reduced sulfur species, in particular H 2 S and elemental sulfur, were conducted under mild conditions (sunlight, low temperature) likely to occur in the photic zone of anoxic environments. These experiments showed an efficient sulphurization of the organic matter under sunlight conditions leading to the formation of organo-sulfur compounds, such as thiophenes and sulfur cross-linked macromolecules, very similar to those occurring in sediments deposited in anoxic environments. These results show the potential for photochemically induced sulfurization of organic matter in environments with photic zone anoxia. Such a process would be especially favored in evaporitic environments with a shallow anoxic boundary extending well into the photic zone.

Novel aromatic carotenoid derivatives 7 and 8 have been identified by synthesis of reference compounds in a H2/PtO2-hydrogenated fraction from the organic extract of a lake sediment (lake Cadagno, Switzerland). They derive from unreported carotenoids presumably biosynthesized by purple sulfur photosynthetic bacteria (Chromatiaceae) and represent potential biomarkers to establish the occurrence of photic zone anoxia in ancient ecosystems.

A novel hexane-soluble nonpolar macromolecular fraction (NPMF) has been found to occur in substantial amounts (up to 32%) in sulfur-rich crude oils and a rock extract. It is highly aliphatic and has a molecular weight culminating at several thousand mass units, as proven by spectroscopic and molecular weight studies. C-S bond hydrogenolysis of NPMF with Raney nickel as a catalyst yields high proportions of aliphatic hydrocarbons in which long linear, acyclic polyisoprenoid and carotenoid chains usually predominate (except in one case) over polycyclic structures, such as steroids and hopanoids. Hence, NPMF consists mainly of macromolecules composed of low molecular weight hydrocarbon subunits cross-linked with sulfide bridges. Use of deuterated Raney nickel indicated in one case (Rozel Point oil) that the long chains and some hopanoids are multiattached to the macromolecular network, whereas other structural subunits, such as steroids or gammacerane, are essentially monoattached. Detailed structural determinations of the hydrocarbon “building blocks” of NPMF give information on their origin and the mode of formation of these macromolecules in the subsurface. Indeed, most of the building blocks can be related to algal (e.g., long linear chains, steroids, β-carotene, and related carotenoids) or bacterial (e.g., acyclic and monocyclic carotenoids, long-chain acyclic isoprenoids) precursors which essentially exist in living organisms as monounsaturated or polyunsaturated species or are easily transformed into such species by diagenetic processes (e.g., steroids). It appears that these alkenes or polyenes become selectively trapped into a macromolecular network by reaction with inorganic sulfur species produced by bacteria in a kind of natural, low-temperature, vulcanization process. This process could start at early diagenesis already in the water column or eventually continue in the bottom sediment. Although its exact nature is yet unknown, it seems likely that the cross-linking reaction can be initiated by the cleavage of sulfur species in a radical type mechanism. The alkanes formed upon desulfurization of NPMF usually represent much higher amounts than the free alkanes of the samples and show a dramatically different composition. They may deliver very useful, complementary information in studies related to source and palaeoenvironment.

Novel aromatic carotenoid derivatives 7 and 8 have been identified by synthesis of reference compounds in a H2/PtO2-hydrogenated fraction from the organic extract of a lake sediment (lake Cadagno, Switzerland). They derive from unreported carotenoids presumably biosynthesized by purple sulfur photosynthetic bacteria (Chromatiaceae) and represent potential biomarkers to establish the occurrence of photic zone anoxia in ancient ecosystems.

Five series of crude oil samples exposed to atmospheric conditions have been analysed at the molecular level, each series comprising several samples originating from the same crude oil but altered to different extents. The aim of our investigation was to compare the specific impact of abiotic oxidation to other alteration processes such as biodegradation, evaporation and water washing. Bulk analyses revealed that increasing alteration is accompanied by an increase in oxygen content which parallels a relative increase of the proportions, as well as of the molecular weights of the macromolecular constituents of the bitumens. Gas chromatographic–mass spectrometric analyses of polar fractions showed the presence of oxygen-containing compounds (steroid ketones, benzothiophenic acids and sulfones) which result from oxidation of petroleum lipids. The hypothesis that part of these oxygenated compounds results from abiotic oxidation processes rather than from biodegradation is supported, notably, by the fact that oxygen incorporation generally occurred without any diastereomeric discrimination. This is also supported by simulation experiments performed on petroleum lipids, which showed that abiotic oxidation induces cleavage reactions affecting C–C and C–S bonds which may intervene in the transformation of geomacromolecules in the environment by degradation (“depolymerization”). Thus abiotic oxidation may play a major role in the fate of petroleum pollutants in the environment by transforming lipidic organic matter from petroleum into more water soluble and, therefore, more biodegradable constituents. However, these can be more toxic to the environment as the water-soluble fraction may be easily taken up by biota.

Asphaltene samples extracted from archeological and geological bitumens from the Middle East, France, and Spain were studied by sulfur K- and L-edge X-ray absorption near-edge structure (XANES) spectroscopy in combination with isotopic analyses (d 13 C and dD). Within each series, the samples were genetically related by their d 13 C values. The gross and elemental composition and the dD values were used to characterize the weathering state of the samples. Sulfur K- and L-edge XANES results show that in all the samples, dibenzothiophenes are the dominant forms of sulfur. In the least oxidized asphaltenes, minor species include disulfides, alkyl and aryl sulfides, and sulfoxides. With increasing alteration the proportion of oxidized sulfur (sulfoxides, sulfones, sulfonates and sulfates) increases, whereas the disulfide and sulfide content decreases. This evolution is observed in all the series, regardless of the origin of the asphaltenes. This work illustrates the advantages of XANES spectroscopy as a selective probe for determining sulfur speciation in natural samples. It also shows that S K- and L-edge XANES spectroscopy are complementary for identifying the oxidized and reduced forms of sulfur, respectively. Copyright © 1999 Elsevier Science Ltd