Organic geochemical studies of a Messinian evaporitic basin,northern Apennines (Italy) I: Hydrocarbon biological markers for a hypersaline environment

This paper describes the occurrence and significance of hydrocarbons present in two bituminous marl layers and one distinct gypsum layer from a Messinian sedimentary basin, where hypersaline conditions prevailed. Several new compounds were detected and tentatively identified: of these 20R and 20S 4α, 24-dimethyl-5α(H),14β(H),17β(H) and 20R and 20S 4β,24-dimethyI-5α(H),14β(H),17β(H) cholestanes; 4-methylspirosterenes; 4,4-dimethyl-5α(H),14β(H),17β(H) pregnanes and homopregnanes are discussed in this paper. Several of these compounds might be considered as biological markers for a (hyper)saline environment. The short side chain 4-desmethylsteranes, 5α(H),14β(H),17β(H), 5α(H),14β(H),17α(H) and 5α(H),14α(H), 17α(H) pregnanes and homopregnanes, are the most abundant compounds in the extract from the gypsum sample. It is suggested that in this case these compounds do not reflect the stage of diagenesis but are related to certain organisms exclusively occurring in hypersaline environments. In addition the very low pristane/phytane ratio, often considered as an indicator for anoxicity, could also be interpreted as a useful indicator for hypersalinity.     

The occurence of unusual C27 and C29 sterane predominances in two types of Oman crude oil

Two types of Oman crude oils reveal unusual sterane distributions. Type “A”, which is the more common (74 examples), is characterised by a predominance of C₂₉ iso- and normal-steranes and generally none or only very low relative concentrations of rearranged-steranes. The triterpanes are characterised by the predominance of the C₂₉ 17αH, 21βH norhopane over the C₃₀ 17αH, 21βH hopane and non-predominant C₂₀–C₃₀ tricyclic terpanes. The C₂₉ steranes of this type of crude were not derived from the C₂₉ sterols of land-plant origin (frequently proposed as the source of C₂₉ steranes) since there is good geological evidence that these crudes were generated from a pre-Cambrian source rock, a geological period when land-plants did not exist.The type “B” crude oil (11 known examples) is characterised by a strong predominance of C₂₇ iso-, normal- and rearranged-steranes, relatively lower concentrations of 17αH, 21βH hopanes and relatively high concentrations of C₂₀–C₃₀ tricyclic terpanes.The remarkably different biomarker characteristics of these crude oils imply that the organisms active in the depositional environment of the respective source rocks were significantly different.     

Steranes and triterpanes generated from kerogen pyrolysis in the absence and presence of minerals

Steranes and triterpanes generated from pyrolysis of immature Monterey Formation kerogen in the presence and absence of calcite, illite and montmorillonite reveal results that are both consistent and divergent with published data that reflect the use of these biological markers as maturation indicators. The extent of isomerization of biomarkers generated from pyrolysis of kerogen at 300°C for 2 hours, at C-20 in 14α(H),17α(H)-steranes, at C-22 in 17α(H),21β(H)-hopanes and of 17β(H),21β(H)-hopanes correspond to early diagenetic stages in rock extracts from sedimentary basins. Isomerization increases with heating time and, after 1000 hours, attains values which correspond to the catagenetic stage in sedimentary basins, or equivalent to that of mature oil. Stepwise pyrolysis of the kerogen indicates faster isomerization rates for steranes and triterpanes in the bitumen than for those retained in the kerogen structure, confirming earlier studies.Presence of a mineral matrix can influence the isomerization of steranes and triterpanes considerably. Comparisons with results from kerogen heated alone, for a given maturation stage, show that calcite inhibits, illite catalyzes slightly and montmorillonite has a pronounced catalytic effect on these reactions. This effect results in early isomerization of steranes and hopanes corresponding to the catagenetic stage in the presence of montmorillonite, while kerogen or kerogen with calcite held at the same temperature (300°C) and time (10 hours) only yield isomerized products which correspond to a diagenetic stage. Further, illite and montmorillonite affect various isomerization reactions differently. The fastest reaction is the isomerization at C-20 in 14α(H),17α(H)-steranes followed by that at C-22 in 17α(H),21β(H)-hopanes and the slowest is the formation of 14β(H),17β(H) steranes.These results show that maturation measurements of rock or oil samples from sedimentary basins which use biological markers have to take into account the mineral matrix effects, which have been largely ignored until present.     

Molecular and stable carbon isotopic compositions of hopanoids in seep carbonates from the South China Sea continental slope

The lipid biomarkers of hopanoids in cold seep carbonates from the South China Sea continental slope were investigated by gas chromatography–mass spectrometer (GC–MS) and gas chromatography-isotope ratio-mass spectrometer (GC-ir-MS). The distribution of hopanes/hopenes shows a preference for the ‘biological’ 17β(H), 21β(H)-over the ‘geological’ 17α(H), 21β(H)-configuration. This interpretation is in agreement with the strong odd–even preference of long-chain n-alkanes in those samples, suggesting that the ββ hopanes may be the early diagenetic products of biohopanoids and the αβ, βα configurations of hopanes were mainly derived from allochthonous sources contributing to the organic matter of the carbonates. In terms of hopanoid acids, the C₃₀ to C₃₃ 17β(H), 21β(H)-hopanoid acids were detected with C₃₂ 17β(H), 21β(H)-hopanoid acid being the most abundant. However, there is a significant difference in stable carbon isotopic compostions of the C₃₂ 17β(H), 21β(H)-hopanoic acid among samples (−30.7‰ to −69.8‰). The δ¹³C values match well with the carbon isotopic compositions of SRB-derived iso-/anteiso-C_15:0 fatty acids in the samples, which strongly depend on the carbon utilization types by microbe. The most abundant compound of hopanols detected in the samples, C₃₀-17β(H), 21β(H)-hopanol, may be a good indicator of diagenetic product of type I methanotrophs. The molecular and carbon isotopic compositions of hopanoids demonstrate clearly that there is a combination contribution of both SRB and type I or type X methanotrophs to the source organism in the seep carbonates from the South China Sea continental slope.     

Chemical and carbon isotopic evolution of hydrocarbons during prograde metamorphism from 100°C to 550°C: Case study in the Liassic black shale formation of Central Swiss Alps

Hydrocarbon distributions and stable isotope ratios of carbonates (δ¹³C_car, δ¹⁸O_car), kerogen (δ¹³C_ker), extractable organic matter (δ¹³C_EOM) and individual hydrocarbons of Liassic black shale samples from a prograde metamorphic sequence in the Swiss Alps were used to identify the major organic reactions with increasing metamorphic grade. The studied samples range from the diagenetic zone (<100°C) to amphibolite facies (∼550°C). The samples within the diagenetic zones (<100 and 150°C) are characterized by the dominance of C_<20n-alkanes, suggesting an origin related with marine and/or bacterial inputs. The metamorphic samples (200 to 550°C) have distributions significantly dominated by C₁₂ and C₁₃n-alkanes, C₁₄, C₁₆ and C₁₈n-alkylcyclopentanes and to a lesser extend C₁₅, C₁₇ and C₂₁n-alkylcyclohexanes. The progressive ¹³C-enrichment (up to 3.9‰) with metamorphism of the C_>17n-alkanes suggests the occurrence of cracking reactions of high molecular weight compounds. The isotopically heavier (up to 5.6‰) C_<17n-alkanes in metamorphic samples are likely originated by thermal degradation of long-chain homologous with preferential release of isotopically light C₁ and C₂ radicals. The dominance of specific even C-number n-alkylcyclopentanes suggests an origin related to direct cyclization mechanism (without decarboxylation step) of algal or bacterial fatty acids occurring in reducing aqueous metamorphic fluid conditions. The regular increase of the concentrations of n-alkylcycloalkanes vs. C_>13n-alkanes with metamorphism suggests progressive thermal release of kerogen-linked fatty acid precursors and degradation of n-alkanes. Changes of the steroid and terpenoid distributions are clearly related to increasing metamorphic temperatures. The absence of 18α(H)-22,29,30-trisnorneohopane (Ts), the occurrence of 17β(H)-trisnorhopane, 17β(H), 21α(H)-hopanes in the C₂₉ to C₃₁ range and 5α(H),14α(H),17α(H)-20R C₂₇, C₂₉ steranes in the low diagenetic samples (<100°C) are characteristic of immature bitumens. The higher thermal stress within the upper diagenetic zone (150°C) is marked by the presence of Ts, the disappearance of 17β(H)-trisnorhopane and thermodynamic equilibrium of the 22S/(22S + 22R) homohopane ratios. The increase of the ααα-sterane 20S/(20S + 20R) and 20R ββ/(ββ + αα) ratios (from 0.0 to 0.55 and from 0.0 to 0.40, respectively) in the upper diagenetic zone indicates the occurrence of isomerization reactions already at <150°C. However, the isomerization at C-20 (R → S) reaches thermodynamic equilibrium values already at the upper diagenesis (∼150°C) whereas the epimerisation at C-14 and C-17 (αα → ββ) arrives to constant values in the lower anchizone (∼200°C). The ratios Ts vs. 17α(H)-22,29,30-trisnorneohopane [(Ts/(Ts + Tm)] and 18α(H)-30-norneohopane (C₂₉Ts) vs. 17α(H),21β(H)-30-norhopane [C₂₉Ts/(C₂₉Ts + C₂₉)] increase until the medium anchizone (200 to 250°C) from 0.0 to 0.96 and from 0.0 to 0.44, respectively. An opposite trend towards lower values is observed in the higher metamorphic samples.The occurrence of specific hydrocarbons (e.g., n-alkylcyclopentanes, cadalene, hydrogenated aromatic compounds) in metamorphic samples points to kerogen degradation reactions most probably occurring in the presence of water and under reducing conditions. The changes of hydrocarbon distributions and carbon isotopic compositions of n-alkanes related to metamorphism suggest that the organic geochemistry may help to evaluate the lowest grades of prograde metamorphism.     

Structure proof and significance of stereoisomeric 28,30-bisnorhopanes in petroleum and petroleum source rocks

Two C₂₈H₄₈-pentacyclic triterpanes were isolated from Monterey shale. X-ray crystallography of a crystal containing both compounds proved their structures as 17β,18α,21α(H)-28,30-bisnorhopane and 17β,18α,21β(H)-28,30-bisnorhopane. Several differences are found between 28,30-bisnorhopanes and the regular hopanes. Unlike the regular hopane epimers, for practical purposes the three epimeric 28,30-bisnorhopanes [17α,21β(H)-, 17β,21α(H)-, and 17β,21β(H)-]cannot be distinguished by their mass spectra. Special conditions are needed to separate them by gas chromatography. The diagenetically first-formed epimer is thought to be 17α,21β(H)- because it predominates in immature shales. The order of thermodynamic stability is 17β,2lα(H) < > 17α,21β(H) > 17β,21β(H), and all three epimers are present in petroleum. 25,28,30-Trisnorhopanes can be analyzed in similar fashion and are found to have similar thermodynamic characteristics. The percent of the ring D/E cis epimer of 28,30-bisnorhopane and/or 25,28,30-trisnorhopane is a useful maturation parameter similar to the 20S/20R sterane ratio. Evidence indicates 25-demethylation of 28,30-bisnorhopane to 25,28,30-trisnorhopane during advanced stages of biodegradation. Hence, percent ring $\text{D}\text{E}$cis 25,28,30-trisnorhopane has an application to maturation assessment in heavily biodegraded oils.     

Applications of steranes,terpanes and monoaromatics to the maturation,migration and source of crude oils

Novel biological marker parameters are applied to problems of geochemical correlation of crude oils in the McKittrick Field, California. An attempt is described to distinguish four diagenetic parameters; namely, source input, source maturation, migration and ‘in reservoir’ maturation. The tools include the absolute concentration of steranes, terpanes and paraffins (_{n + iso}) in combination with internal ratios of individual biomarkers such as primary/secondary terpanes, 17α(H)-trisnorhopane/18α(H)-trisnorhopane II (both maturation specific), 5β/5α-steranes, 5β-steranes/17α(H)-hopanes and rearranged steranes/5α-steranes (all migration oriented), 5α/5α-steranes and a number of terpane ratios of partially unknown chemical structure (source input specific).Among other new correlation parameters are: two series of mass chromatograms (m/e 253 and 239), signaling monoaromatized steranes, a series of presumably rearranged steranes (m/e 259), and a series of methylhopanes (m/e 205).The results obtained on the molecular level exceed the degree of information obtainable from organic geochemical ‘bulk’ parameters such as yields of saturates, aromatics, sulfur compounds and C¹³/C¹² ratios by far; however, both types of parameters are mutually supporting. All conclusions are consistent with subtle stratigraphie and overall geologic prerequisites.     

The “schizohaline” environment: Its sedimentary and diagenetic fabrics as exemplified by Late Paleozoic rocks of Bear Island,Svalbard

Environments characterized by fluctuating hypersaline to fresh-water conditions are defined as “schizohaline” and examples are given of situations in which “schizohaline” environments can arise. Fabrics diagnostic for the occurrence of both hyper- and hyposalinity have been recorded in Late Paleozoic rocks from Bear Island (74°30′N, 19°E): evaporite nodules, length-slow chalcedony and very finely crystalline penecontemporaneous dolomite are considered as indicators of hypersaline conditions while euhedral limpid dolomite crystals, coarse poikilitic sparry calcite and microspar calcite are thought to have originated under the hyposaline regime. Zoned dolomite crystals and euhedral crystals of authigenic mega-quartz replacing sulphates have also been recorded in the same rocks; their diagnostic importance for the schizohaline environment is, however, uncertain.Internal structural pores of some crinoid and bryozoan skeletal grains of biosparites from Bear Island are either empty or infilled with very finely crystalline dolomite. Expulsion of Mg²⁺ ions from the magnesian-calcite skeletons and either: (1) “poisoning” of the micro-environment of the pores; or (2) formation of stagnant “micro-sabkha” conditions in the pores are proposed as an explanation for these phenomena.     

The occurrence of bisnorhopane,trisnorhopane and 25-norhopanes as free hydrocarbons in some Australian shales

The triterpenoid hydrocarbons of some West Australian shales have been examined by GC-MS. In addition to the common 17α(H),21β(H)-hopanes, 17β(H),21β(H)-hopanes and 17β(H),21α(H)-moretanes, 28,30-bisnorhopane, 25,28,30-trisnorhopane and 25-norhopanes were identified in the organic extracts. In contrast, pyrolysates of the solvent-extracted sediments contained only the common hopane and moretane series, indicating that 28,30-bisnorhopane, 25,28,30-trisnorhopane and 25-norhopanes are not bonded to kerogen, but rather are present in the sediments as free hydrocarbons.     

Organic matter preservation in the carbonate matrix of a recent microbial mat – Is there a ‘mat seal effect’?

Phototrophic mats (microbial mats with a phototrophic top layer) are complex systems in terms of microbial diversity, biogeochemical cycles and organic matter (OM) turnover. It has been proposed that these mats were a predominant life form in Proterozoic shallow water settings, prior to the emergence of bioturbating organisms in the Ediacaran–Cambrian transition. For most of the Precambrian, microbial mats were not only quantitative important carbon fixing systems, but also influenced the transfer and transformation of OM before it entered the geosphere. The profound alteration of compound inventories during transit through microbial mats, implying substantial consequences for OM preservation in the Proterozoic, was recently proposed as a “mat-seal effect” [Pawlowska et al. (2012) Geology 41, 103–106]. To obtain a better understanding of the early diagenetic fate of primary produced OM in microbial mats, we studied a recent calcifying mat from a hypersaline lake in Kiritimati, which showed in the deeper mat layers a maximum ¹⁴C_carbonate age of ∼1500 years. We particularly focused on OM entrapped in the carbonate matrix, because of the better potential of such biomineral-encapsulated OM to reach the geosphere before degradation (and remineralization). Our data indicate that selective preservation is important in phototrophic mats. While a diagenetic transformation of lipid fatty acids (FAs) was evident, their fatty acyl-derived hydrocarbon moieties were not introduced into protokerogen, which was instead mainly comprised of cyanobacterial and/or algal biomacromolecules. Our data support the proposed major impact of the “mat-seal effect” on OM turnover and preservation; i.e. the suppression of biosignatures derived from the upper mat layers, while signals of heterotrophic microbes thriving in deeper mat layers become preferentially preserved (e.g. high hopane/sterane ratios). This mechanism may have broad consequences for the interpretation of biomarkers from Proterozoic shelf environments, because biosignatures of phototrophic mat dwellers as well as planktonic signals may have become heavily biased by the production and turnover of OM in microbial mat systems.     

Effect of a dolerite intrusion on triterpane stereochemistry and kerogen in Rundle oil shale,Australia

Stereochemical changes of triterpanes present in extracts from an immature oil shale sequence intruded by a 3-m dolerite sill have been studied by gas chromatography-mass spectrometry (GC-MS). The steric configuration of the hopanes was observed to change from one dominated by the thermally less stable 17β(H), 21β(H) configuration at some distance from the intrusion, to one dominated by the thermally more stable 17α(H), 21β(H) and 17β(H), 21α(H) configurating in the immediate vicinity of the intrusion. In addition, severe alteration of the kerogen appeared to have taken place as a result of the contact metamorphism, and high concentrations of extractable organic matter were observed below the intrusion. Characterization of the kerogens by Curie-point pyrolysis has enabled the effects of the intrusion on the shales to be monitored.     

Selective enrichment of steroid and triterpenoid hydrocarbons from crude oil using gel permeation chromatography for stable carbon isotope analysis

Gel permeation chromatography (GPC) using a high performance liquid chromatography (HPLC) system was studied for the separation and enrichment of steroid and hopanoid hydrocarbons from crude oil for stable carbon isotope analysis. A crude oil sample was pretreated using silica gel chromatography and 5A molecular sieve to remove polycyclic aromatic hydrocarbons and n-alkanes. The GPC behavior of both the pretreated saturated hydrocarbon fraction of the oil and standard steroid [5α(H), 14α(H), 17α(H) C₂₇–C₂₉ steranes], hopanoid [17α(H) C₂₇ trisnorhopane, 17α(H), 21β(H) C₂₉–C₃₂ hopanes] and triterpenoid [18α(H)-oleanane, gammacerane] mixtures were examined. The results indicate that 17α(H), 21β(H) hopanes as well as steranes could be enriched efficiently using GPC and that they could be obtained without removing n-alkanes from the oil saturated hydrocarbon fraction. The GPC behavior of steroid and triterpenoid hydrocarbons was controlled by molecular size and shape.     

Several unexpected marine sterols in a freshwater sediment

Some unexpected stanols, 22,23-methylene-23,24-dimethyl-5α(H)-cholestan-3β-ol (gorgostanol and the related stenol), 24,26-dimethyl-5α(H)-cholestan-3β-ol (aplystanol) and most probably 24-isopropyl-5α(H)-cholest-22-en-3β-ol, have been found in the sediment of a eutrophic pond. These compounds possess side chains which have until now been considered to belong exclusively to sterols of marine origin. The presence of these stanols in a freshwater sediment suggests that considering their side chain structures as specific marine input markers can lead to erroneous conclusions. The vertical distribution of these compounds along a 6 m core shows that they arise from organisms which still exist except for aplystanol which was found only in the sections below 5 m.     

Organic Geochemistry of the Cenomanian–Turonian Bahloul Formation Petroleum Source Rock,Central and Northern Tunisia

Total organic carbon (TOC) determination, Rock‐Eval pyrolysis, extractable organic matter content (EOM) fractionation, gas chromatography (GC) and gas chromatography–mass spectrometry (GC–MS) analyses, were carried out on 79 samples from eleven outcrop cross sections of the Bahloul Formation in central and northern Tunisia. The TOC content varied between 0.23 to 35.6%, the highest average values (18.73%, 8.46% and 4.02%) being at the east of the study area (at Ain Zakkar, Oued Bahloul and Dyr Ouled Yahia localities, respectively). The Rock‐Eval maximum pyrolysis temperature (Tmax) values in the 424–453°C range delineated a general east‐west trend increase in the organic matter (OM) maturity. The disparity in hydrogen index (HI) values, in the range 114–824 mg hydrocarbons (HC) g^?1 TOC, is relevant for the discrepancy in the level of OM preservation and maturity among localities and samples. The n‐alkane distributions, maximizing in the C17 to C20 range, are typical for a marine planktonic origin, whereas pristine/phytane (Pr/Ph) average values in the 1–2 range indicate an oxic to suboxic depositional environment. Pr/n‐C17 and Ph/n‐C18 ratios in the 0.38–6.2 and 0.68–3.25 range, respectively, are consistent with other maturity indicators and the contribution of specific bacteria to phytol as a precursor of isoprenoids. The thermal maturity varies between late diagenesis to main‐stage of petroleum generation based on the optic and the cis‐trans isomerisation of the C29 sterane [20S/(20S+20R) and 14β(H),17β(H)/(14β(H),17β(H)+14α(H),17α(H)), respectively] and the terpane [18α(H)22,29,30‐Trisnorneohopane/(18α(H)22,29,30‐Trisnorneohopane+17α(H)22,29,30‐Trisnorhopane): Ts/(Ts+Tm)] ratios. The Bahloul OM is represented by an open marine to estuarine algal facies with a specific bacterial contribution as revealed by the relative abundance of the ααα‐20R C27 (33–44%), C28 (22–28%) and C29 (34–41%) steranes and by the total terpanes/total steranes ratio (1.2–5.33). These results attested that the Bahloul OM richness was controlled both by an oxygen minimum zone induced by high productivity and restricted circulation in narrow half graben structures and around diapirs of the Triassic salt.     

Stable isotope and Sr2+/Ca2+ evidence of diagenetic dedolomitization in a schizohaline environment: Cenomanian of northern Israel

The Cenomanian—Turonian Upper Judea Group of Israel comprises shallow-marine hypersaline dolomites passing laterally into ‘basinal’ limestones and chalks. Chertbearing diagenetic dedolomites characterize the transitional zone. The restriction of dedolomites to a narrow zone, their light δ¹³C (up to ?11%), and their low Sr²⁺ concentration (<16 ppm) all suggest that the dedolomites were formed diagenetically through exposure of this zone to fresh meteoric waters. This type of partly changing environments was referred to as schizohaline by Folk and Siedlecka (1974).     

Sterols in interstitial waters of marine sediments

In order to attempt to elucidate the nature of biogeochemical processes occurring at the water-sediment interface, sterols have been analysed in near bottom sea and interstitial waters collected in the eastern and western intertropical Atlantic ocean. Free and esterified sterol concentrations range from 0.2 to 82 μg l^?1 and are much higher than those found in overlying sea water, which range from 0.2 to 1.7 μg l^?1 for the dissolved fraction and from 0.01 to 0.07 μg l^?1 for the particulate fraction. Cholest-5-en-3β-ol and 24-ethylcholest-5-en-3β-ol are the dominant sterols in sea and interstitial waters. The variability encountered for the relative importance of minor sterols such as 24-methylcholesta-5,24(28)-dien-3β-ol and stanols, 5α-cholest-22(E)-en-3β-ol, 5α-cholestan-3β-ol and 24-ethyl-5α-cholestan-3β-ol in interstitial water and their variation with depth is discussed in terms of diversity of inputs and bacterial activity. For sediments cored off the Mauritanian coast, a productive area characterized by an intense upwelling, the chemical signatures observed in interstitial water through stanol/stenol ratios occur at levels of very high heterotrophic aerobic bacterial biomass estimations. The study of the sterol composition of interstitial water could constitute a valuable tool in appreciating the intensity of chemical and biological processes occurring in the first few metres of recent marine sediments.     

Estimation of maximum temperature of mudstone by two kinetic parameters; epimerization of sterane and hopane

A geochemical method for estimation of the maximum temperature of mudstones is proposed. The extents of epimerization of the sterane and the hopane are used. The temperature function is:

$\text{T}{}_{\mathrm{max.}}\text{(°C) =}\text{6060}\text{15.0?In(dU}\text{dU}\text{dV)}\text{?273}$

where U = ln (1 ? α/0.54), V = ln(1 ? β/0.61), α = 20S-/20S- + 20R-24-ethyl-5α(H), 14α(H), 17α(H)-cholestane(C₂₉-sterane) and β = 22S-/22S- + 22R-17α(H), 21β(H)-bishomohopane(C₃₂-hopane). The value of dU/dV can be obtained from the tangent to the evolution curve in the UversusV. This temperature function is applicable to the temperature analysis in the range of 50°C–150°C.     

Biomarker evidence for Botryococcus and a methane cycle in the Eocene Huadian oil shale,NE China

The saturated and unsaturated hydrocarbons of two samples (HD-19 and HD-21) from the same section of the Middle Eocene lacustrine Huadian oil shale in NE China were identified and shown to be mainly from algal and bacterial sources. Comparison of the two samples provided an opportunity to explore the contribution from telalginite to the hydrocarbon profiles. Cells identified from microscopy as Botryococcus in the telalginite of HD-21 were confirmed as belonging to the L race of B. braunii from the presence of monoaromatic lycopane derivatives and small amounts of several lycopadienes. Lycopane was abundant and was probably derived from biohydrogenation of lycopadienes and related lipids on the basis of δ¹³C values. Hopane distributions showed a dominance of those with the biological 17β,21β-stereochemistry, as expected for an immature shale, with low amounts of 17β,21α-hopanes (moretanes) and 17α,21β-hopanes. Two hopenes were also abundant and assigned as C₂₉ and C₃₀ neohop-13(18)-enes, which occurred together with the C₂₉ and C₃₀ hop-17(21)-enes. These had depleted carbon isotope values (−43.7‰ to −50.8‰), indicative of production by methane oxidizing bacteria (methanotrophs). The high proportion of hopanoids with carbon numbers < C₃₂ indicates extensive post-depositional diagenetic alteration of bacteriohopanepolyols as well as a direct input of C₃₀ hopanoids. The data clearly indicate that there was active utilization of methane in this lacustrine depositional setting, but isoprenoid hydrocarbon biomarkers for methanogens, such as pentamethylicosane (PMI) and squalane, were in surprisingly low abundance. It is possible that these bacterial contributions were present as polar lipids. The origins of an unusual C₃₈ isoprenoid alkane assigned as bipristane are uncertain, but may be from methanogens. Steranes and sterenes were relatively minor components, but abundant diasterenes and 4-methyldiasterenes were present, reflecting significant conversion of the original lipid composition by way of clay-catalysed diagenesis. The biomarker data suggest that the bottom waters in the original depositional environment had low O₂ content, but the sediments were probably neither sulfidic nor strongly reducing. The high content of organic matter in the shale likely reflects both high (but fluctuating) productivity due to eutrophic conditions in the overlying water and good preservation in the sediments.     

南海北部冷泉碳酸盐岩中系列藿烷酸的检出及意义

对南海东沙海域九龙甲烷礁冷泉碳酸盐岩的藿烷酸系列化合物进行了研究, 结果显示冷泉碳酸盐岩中含有17β(H), 21β(H)-30-藿烷酸至17β(H), 21β(H)-33-藿烷酸系列化合物. 其中site1样品TVG3-C2(ANME-2古菌种群为主)中的17β(H), 21β(H)-32-藿烷酸化合物亏损13C(-69.8‰), 且其同位素值较接近iso-/aiso-C_15:0脂肪酸(-75.2‰~-90.0‰), 说明其母质摄入了甲烷来源的碳.尽管site2(TVG13-C3和TVG14-C2)和site3(TVG8-C5)自生碳酸盐岩中藿烷酸化合物都表现出了非渗漏特征(-30.7‰~-40.3‰, ANME-1古菌种群为主), 但这些样品中藿烷酸的δ13C值同样较接近硫酸盐还原菌来源的iso-/anteiso-C_15:0的δ13C值(-32.5‰~-49.8‰).由此说明大多数存在于以ANME-1种群为主的渗漏环境中的硫酸盐还原菌并没有参与到甲烷厌氧氧化作用中, 而导致藿烷酸化合物和硫酸盐还原菌来源的生物标志物并不明显亏损13C, 并且δ13C值相近.