Okenane, a biomarker for purple sulfur bacteria (Chromatiaceae), and other new carotenoid derivatives from the 1640 Ma Barney Creek Formation

Carbonates of the 1640 million years (Ma) old Barney Creek Formation (BCF), McArthur Basin, Australia, contain more than 22 different C₄₀ carotenoid derivatives including lycopane, γ-carotane, β-carotane, chlorobactane, isorenieratane, β-isorenieratane, renieratane, β-renierapurpurane, renierapurpurane and the monoaromatic carotenoid okenane. These biomarkers extend the geological record of carotenoid derivatives by more than 1000 million years. Okenane is potentially derived from the red-colored aromatic carotenoid okenone. Based on a detailed review of the ecology and physiology of all extant species that are known to contain okenone, we interpret fossil okenane as a biomarker for planktonic purple sulfur bacteria of the family Chromatiaceae. Okenane is strictly a biomarker for anoxic and sulfidic conditions in the presence of light (photic zone euxinia) and indicates an anoxic/oxic transition (temporarily) located at less than 25 m depth and, with a high probability, less than 12 m depth.For the BCF, we also interpret renierapurpurane, renieratane and β-renierapurpurane as biomarkers for Chromatiaceae with a possible contribution of cyanobacterial synechoxanthin to the renierapurpurane pool. Although isorenieratane may, in principle, be derived from actinobacteria, in the BCF these biomarkers almost certainly derive from sulfide-oxidizing phototrophic green sulfur bacteria (Chlorobiaceae). Biological precursors of γ-carotane, β-carotane and lycopane are found among numerous autotrophic and almost all phototrophic organisms in the three domains of life. In the BCF, a paucity of diagnostic eukaryotic steroids suggests that algae were rare and, therefore, that cyanobacterial carotenoids such as β-carotene, echinenone, canthaxanthin and zeaxanthin are the most likely source of observed β-carotane. γ-Carotane may be derived from cyanobacteria, Chlorobiaceae and green non-sulfur bacteria (Chloroflexi), while the most likely biological sources for lycopane in the BCF are carotenoids of the lycopene, rhodopin and spirilloxanthin series abundant in purple sulfur bacteria.     

Evidence for euphotic zone anoxia during the deposition of Aptian source rocks based on aryl isoprenoids in petroleum,Sergipe–Alagoas Basin,northeastern Brazil

Four crude oil samples from the Sergipe–Alagoas Basin, northeastern Brazil, were analyzed using full scan gas chromatography–quadrupole mass spectrometry (GC–qMS) for biomarkers, in order to correlate them using aromatic carotenoids thereby enhancing knowledge about the depositional environment of their source rocks. The geochemical parameters derived from saturated fractions of the oils show evidence of little or no biodegradation and similar thermal maturation (Ts/(Ts + Tm) for terpanes, C₂₉ αββ/(αββ + ααα), C₂₇, and C₂₉ 20S/(20S + 20R) for steranes). Low pristane/phytane ratios and the abundance of gammacerane and β-carotane are indicative of an anoxic and saline depositional environment for the source rocks. Moreover, we identified a large range of diagenetic and catagenetic products of the aromatic carotenoid isorenieratene, including C₄₀, C₃₃, and C₃₂ diaryl isoprenoids and aryl isoprenoid derivatives with short side chains and/or additional rings. These results indicate anoxia in the photic zone during the deposition of the source rocks.     

Oligoprenyl-curcumanes and other new aromatic isoprenoids from the 1.64 billion year old Barney Creek Formation

Carbonaceous dolomites and shales of the 1.64 billion years (Ga) old Barney Creek Formation (BCF), McArthur Basin, northern Australia contain the oldest, clearly indigenous biomarkers. We describe three new series of regularly branched aryl isoprenoids with base ions at m/z 119, 161 and 175. The m/z 119 compounds were identified as a complete series of C₁₅–C₄₀ oligoprenyl-perhydro-ar-curcumenes (oligoprenyl-curcumanes). Their likely biogenic precursors are oligoprenyl-β-curcumenes that occur in a wide range of bacterial phyla.     

Fatty acid geochemistry of a 200 m sediment core from Lake Biwa,Japan. Early diagenesis and paleoenvironmental information

Fatty acids (FAs), β- and ω-hydroxy acids, α,ω-dicarboxylic acids and n-alkanes were studied in a 200 m sediment core taken from Lake Biwa, Japan. FAs showed bimodal distribution with peaks at C₁₆ and C₂₂-C₂₈. Their distribution patterns clearly changed with depth from lower molecular weight (C₁₂-C₁₉) predominance to higher molecular weight (C₂₀-C₃₂) predominance in the upper 20 m interval. Analyses of related compounds (β- and ω-hydroxy acids and α, ω-dicarboxylic acids) suggest that β- and ω-oxidative degradation of C₁₂-C₁₉ FAs has occurred in the sediments.The ratio of bound C₁₂-C₁₉ to unbound FAs increases with depth in the upper 0–1 m sediments, suggesting that unbound FAs are more labile. However, the ratio varies significantly in deeper sections and may be associated with water temperature.In the sediments deeper than 20 m in depth, C₁₂-C₁₉ FAs gradually decrease. On the other hand, higher molecular weight FAs (HFAs: C₂₀-C₃₂), which were probably derived from terrestrial plants, increase in concentration from 20 m to 100 m, suddenly decrease at 100 m and show progressively lower concentration in deeper sediments. These fluctuations are interpreted in relation to paleolimnological changes of the lake and the drainage basin. ω-Hydroxy C₂₀-C₃₀ acids and C₂₀-C₃₀α, ω-dicarboxylic acids show a distribution pattern similar to HFAs. Branched chain FAs, ω-hydroxy acids and C₉-C₁₉α,ω-dicarboxylic acids show a major peak around 3–15 m in depth. This peak is probably caused by increased bacterial activity in the water column and surface sediments in the past, which may be associated with an increase in primary production of the lake.     

Carotenoid-derived alkanes in oils from northwestern China

Carotenoid pigments are ubiquitous components of the biosphere but have rarely been reported in geological samples. Oils from the Kelamayi oilfield of northwestern China have been shown by GC-MS analysis to contain abundant carotenoid-derived alkanes including β- and γ-carotanes, 1,1,3-trimethyl-2-alkylcyclohexanes and a compound tentatively identified as saturated lexene. In addition, a seepage oil contained a possible C₄₀ alkene with a structure similar to that of β-carotane except for a carbon-carbon double bond in one of the cyclohexyl rings. The distributions of biomarker compounds in these oils are very similar to those observed in the few other reported samples that contain abundant carotenoid-derived compounds, indicating that the Kelamayi source rock was probably deposited under very specific environmental conditions. The distribution of carotenoid-derived compounds, particularly the ratio of γ-carotane to β-carotane, appears to be sensitive to both thermal maturation and biodégradation. This ratio increases with maturation and decreases with increasing biodegradation.     

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.     

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.     

Sterols of a contemporary lacustrine sediment

C₂₆, C₂₇, C₂₈ and C₂₉ sterols, including Δ⁵-, Δ²²- and Δ^5,22-sterols and both 5α- and 5β-stanols, have been identified in the contemporary lacustrine sediment of Rostherne mere (Cheshire, England). Amounts of total sterols decrease from ca. 400 ppm extracted sediment dry weight for the 0–7 cm sediment section to ca. 40 ppm for the corresponding 18–30 cm section. 5α-stanols are of far greater abundance than the 5α-stanol isomers. The carbon number distributions of unsaturated and saturated sterols and the increase in stanol: Δ⁵-sterol ratio with sediment depth provide indirect evidence for the operation of a sterol hydrogenation process. C₂₉ sterols preponderate at lower sediment depths, suggesting a predominantly higher plant input, whereas C²⁷ sterols are more abundant in the surface sediment.     

In situ anaerobic degradation of petroleum alkanes in marine sediments: preliminary results

The degradation of acyclic petroleum hydrocarbons was studied during a 24-month experiment in Mediterranean coastal sediments (Gulf of Fos). Sediment cores entirely contaminated with oil (Arabian Light Crude Oil) were incubated in situ. The use of conservative tracers of sediment's particles reworking (luminophores) allowed the distinction of the reworked layer from the anoxic deeper sediments. Using the 17α,21βC₃₀ hopane (C₃₀H) as an inert internal reference, we could demonstrate that, after 24 months of experiment, acyclic petroleum hydrocarbons can be degraded under natural anaerobic conditions. The reactivity of individual alkanes appeared to depend on their chemical structure. To cite this article: D. Massias et al., C. R. Geoscience 335 (2003).To cite this article: D. Massias et al., C. R. Geoscience 335 (2003).     

Thermodynamics and behavior of γ-Mg2SiO4 at high pressure: Implications for Mg2SiO4 phase equilibrium

Raman spectra of γ-Mg₂SiO₄ taken to 200 kbar were used to calculate entropy and heat capacity at various P-T conditions. These new thermodynamic data on γ-MgSiO₄, similar data on MgSiO₃ perovskite (pv), previous data on β-MgSiO₄ and MgO (mw), and previous volumetric data of all phases were used to calculate the phase boundaries in the Mg₂SiO₄ phase diagram. Our resulting slope for the β→γ transition (50±4 bar K^-1) is in excellent agreement with recent multi-anvil studies. The slopes for the β→pv+MgO and γ→pv+MgO are-7±3 and -25±4 bar K^-1, respectively, and are consistent with our CO₂ laser heated diamond anvil studies. These slopes result in a β-γ-MgO+pv triple point at approximately 229 kbar and 2260 K for the iron free system.     

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.     

A geochemical investigation of crude oils and source rocks from Biyang Basin, China

China has a number of petroliferous lacustrine sedimentary basins of varying salinity and age (mainly Eocene). A geochemical investigation has been undertaken on several oils and source rocks from the Eocene lacustrine Biyang Basin. The distributions of n-alkanes, isoprenoids, steranes, and terpanes have been studied and used to characterize the sedimentary environment of deposition, maturity, biodegradation and undertake possible correlations. The ratios of C₃₀-hopane/gammacerane, 4-methyl-steranes/regular steranes, steranes/hopanes, C₂₁ tricyclic/C₃₀ hopane are proposed to be indicative of the depositional environment whereas ß-carotane appears to be a source related indicator. The geochemical data obtained in this study suggest that the major source rocks in the Biyang Basin were deposited in a saline/hypersaline depositional environment.     

Biogeochemistry of particulate organic matter from lakes of different trophic levels in Switzerland

Biomarker compositions of particulate organic matter (POM) from the oligotrophic Lake Brienz and the eutrophic Lake Lugano (both Switzerland) are compared, in order to obtain information about organic matter (OM) production and transformation processes in relation to water column stratification. Eutrophic conditions in Lake Lugano are reflected by enhanced alkalinity, elevated total organic carbon (TOC) and chlorin contents compared with Lake Brienz. Lower δ¹³C values of dissolved inorganic carbon (DIC) in Lake Lugano reflect enhanced OM respiration in the water column.Differences in OM dynamics between both lakes, as well as seasonal variations, are evidenced by TOC-normalised concentration profiles of total fatty acids (FAs) and total neutrals. In Lake Brienz, the results reflect the relative contributions of primary productivity and refractory, allochthonous OM to POM, governed by particle load and interflows due to density stratification. The depth trends at Lake Lugano are a result of high primary productivity, water column stratification and associated particle load in the upper layers, as well as microbially induced degradation close to the chemocline and greater preservation under anoxic conditions. Minor differences exist with regard to the OM composition. In both lakes, FA distributions and the composition of n-alkanols indicate a predominant autochthonous OM source (algae, zooplankton, bacteria). Inputs of OM from diatoms are reflected in highly-branched isoprenoid (HBI) alkenes, 16:1 n-FAs and 24-methylcholesta-5,22-dien-3β-ol (either epibrassicasterol or brassicasterol). Differences in relative proportions of n-C₁₆ vs. n-C₁₈ FAs and alkanols, respectively, as well as in the percentages of C₂₇, C₂₈ and C₂₉ sterols relative to the sum of sterols are related to differences in the abundances of chrysophytes, diatoms and green algae within the euphotic zone of both lakes as well as in bacterial activity and soil in-wash. High relative proportions of cholesterol in the autumn samples, most pronounced at Lake Lugano, were attributed to an increased input from zooplankton grazing in the water column.Differences in OM degradation processes are reflected in slightly higher chlorin index values and higher relative proportions of saturated vs. unsaturated n-FAs in Lake Lugano. Higher contents of branched chain FAs, 16:1ω7 n-FA, and enhanced 18:1ω7/18:1ω9 n-FA ratios suggest enhanced bacterial biomass in the water column of Lake Lugano close to the chemocline. Increasing proportions of saturated n-FAs and n-alkanols with increasing water depth, most distinct in the autumn for both lakes, argue for intensified bacterial activity and degradation of OM during autumn. High relative contents of sterols and low n-alkanol concentrations in POM close to the chemocline at Lake Lugano during spring are interpreted to reflect higher primary productivity in the photic zone, OM export to the deeper parts and enhanced degradation rates of more labile constituents (i.e. C₁₃–C₂₀ n-alkanols), as compared to Lake Brienz.     

Carbon, sulfur, and oxygen isotope evidence for a strong depth gradient and oceanic oxidation after the Ediacaran Hankalchough glaciation

In order to understand spatial variations of stable isotope geochemistry in the Quruqtagh basin (northwestern China) in the aftermath of an Ediacaran glaciation, we analyzed carbonate carbon isotopes (δ¹³C_carb), carbonate oxygen isotopes (δ¹⁸O_carb), carbonate associated sulfate sulfur (δ³⁴S_CAS) and oxygen isotopes (δ¹⁸O_CAS), and pyrite sulfur isotopes (δ³⁴S_py) of a cap dolostone atop the Ediacaran Hankalchough glacial diamictite at four sections. The four studied sections (YKG, MK, H and ZBS) represent an onshore-offshore transect in the Quruqtagh basin. Our data show a strong paleobathymetry-dependent isotopic gradient. From the onshore to offshore sections, δ¹³C_carb values decrease from −2‰ to −16‰ (VPDB), whereas δ¹⁸O_carb values increase from −4‰ to −1‰ (VPDB). Both δ³⁴S_CAS and δ³⁴S_py show stratigraphic variations in the two onshore sections (MK and YKG), but are more stable in the two offshore sections (H and ZBS). δ¹⁸O_CAS values of onshore samples are consistent with terrestrial oxidative weathering of pyrite. We propose that following the Hankalchough glaciation seawater in the Quruqtagh basin was characterized by a strong isotopic gradient. The isotopic data may be interpreted using a three-component mixing model that involves three reservoirs: deep-basin water, surface water, and terrestrial weathering input. In this model, the negative δ¹³C_carb values in the offshore sections are related to the upwelling of deep-basin water (where anaerobic oxidation of dissolved organic carbon resulted in ¹³C-depleted DIC), whereas sulfur isotope variations are strongly controlled by surface water sulfate and terrestrial weathering input derived from oxidative weathering of pyrite. The new data provide evidence for the oceanic oxidation following the Hankalchough glaciation.     

Early diagenesis of fatty acids in lacustrine sediments—I. Identification and distribution of fatty acids in recent sediment from a freshwater lake

Fatty acids have been isolated and quantitatively determined from a 1.5 m sediment core of Lake Suwa, a eutrophic lake in the central districts of Japan.The fatty acids identified by combined gas chromatography-mass spectrometry were straight-chain saturated (C₁₂ to C₃₄), monounsaturated with even carbon number (C₁₆ to C₂₄) and branched-chain (iso, anteiso, 10-methyloctadecanoic) acids. The concentrations of the higher molecular weight (? C₂₀) saturated fatty acids remained nearly constant throughout the core, suggesting a high degree of preservation of those acids, whereas the monounsaturated and the lower molecular weight saturated fatty acids indicated a great decrease in concentration with depth to an approximately 20cm level. It is suggested that the microbial activity in sediments causes a significant reconstruction of the fatty acid distribution during early diagenesis.     

Cenomanian–Turonian carbon isotope stratigraphy of the Western Canadian Sedimentary Basin

The Cenomanian–Turonian boundary was characterized by distinctive positive carbon isotope excursions that were related to the formation of widespread oceanic anoxia. High-resolution geochemical proxies (TOC, CaCO₃, δ¹³C_org, and δ¹³C_carb) obtained from bulk rock, planktic foraminifers, and inoceramids from four marine marlstone-dominated stratigraphic sections in the Western Canada Sedimentary Basin (WCSB) were used to establish a regional carbon isotope stratigraphic framework and to investigate paleoenvironmental variability in four different depositional settings. Compared to background δ¹³C_org, (<−27‰) and δ¹³C_carb (<2‰) values which were correlative to stable isotope excursions during Oceanic Anoxic Event (OAE) II worldwide, the δ¹³C_org (>24‰), and δ¹³C_carb (>4‰) derived from inoceramid prisms in the studied sections within WCSB, were elevated during the Late Cenomanian–Early Turonian. During this interval, TOC and CaCO₃ values which increased sporadically to >40% and 7%, respectively, were not consistent enough to be used for stratigraphic correlations. Based on the δ¹³C_org excursions, two bentonite beds were regionally correlated across this portion of the Western Interior Seaway (WIS). The eruption associated with the “Red” bentonite occurred approximately coeval with the maximum δ¹³C_org-excursion during OAE II in the Neocardioceras juddii Zone, whereas the “Blue” bentonite coincides with the termination of OAE II in the latest Watinoceras devonense zone. During the Late Cenomanian–Early Turonian in the WCSB, benthic foraminifers were sparse or totally absent, indicating the existence of fully anoxic bottom-water conditions. Planktic foraminifera were common in the well-oxygenated surface waters. A benthic oxic zone characterized by several agglutinated species occurs in the eastern part of the WSCB at the beginning of OAE II in the Sciponoceras gracile zone. The termination of the OAE II in the WCSB coincides with the first occurrence of small ammonites (Subprionocyclus sp.) in the western part of the basin.     

Molecular and isotopic stratigraphy in an ombrotrophic mire for paleoclimate reconstruction

A 40 cm deep Sphagnum-dominated peat monolith from Bolton Fell Moss in Northern England was systematically investigated by lipid molecular stratigraphy and compound-specific δ¹³C and δD analysis using gas chromatography (GC), GC-mass spectrometry (GC-MS), GC-combustion-isotope ratio-MS (GC-C-IRMS) and GC-thermal conversion-IRMS (GC-TC-IRMS) techniques. ²¹⁰Pb dating showed the monolith accumulated during the last ca. 220 yr, a period encompassing the second part of Little Ice Age. While the distributions of lipids, including n-alkan-1-ols, n-alkan-2-ones, wax esters, sterols, n-alkanoic acids, α,ω-alkandioic acids and ω-hydroxy acids, display relatively minor changes with depth, the cooler climate event was recorded in the concentrations of n-alkanes and organic carbon, CPI values of n-alkanes and n-alkanoic acids, and the ratio of 5-n-alkylresorcinols/sterols. Superimposed on the fossil fuel effect, the relatively cooler climate event was also recorded by δ¹³C values of individual hydrocarbons, especially the C₂₃n-alkane, a major compound in certain Sphagnum spp. The δD values of the C₂₉ and C₃₃n-alkanes correlated mainly with plant composition and were relatively insensitive to climatic change. In contrast the C₂₃n-alkane displayed variation that correlated strongly with recorded temperature for the period represented by the monolith, agreeing with previously reported deuterium records in tree ring cellulose spanning the same period in Scotland, Germany and the USA, with more negative values occurring during the second part of Little Ice Age. These biomarker characteristics, including the compound-specific δ¹³C and δD records, provide a new set of proxies of climatic change, potentially independent of preserved macrofossils which will be of value in deeper sections of the bog where the documentary records of climate are unavailable and humification is well advanced.     

Anharmonic effect on the equation of state (EoS) for NaCl

We find clear intrinsic anharmonicity in the NaCl-B1 phase by examining the equation of state (EoS) based on previous ultrasonic velocity data for pressures up to 0.8 GPa and temperatures up to 800 K. The experimental EoS for this phase shows that its specific heat at constant volume (C _V ) is significantly smaller than that based on a harmonic model. Also, the sign of $\left( {{{\partial C_{V} } \mathord{\left/ {\vphantom {{\partial C_{V} } {\partial P}}} \right. \kern-0pt} {\partial P}}} \right)_{T} ,$ which is normally negative in the quasi-harmonic approximation, is unexpectedly positive. The thermodynamic Grüneisen parameter (γ), which has frequently been assumed to be a single-variable function of molar volume, shows not only volume dependence but also negative temperature dependence. To understand these features of C _V and γ, we introduce a thermodynamic model including positive quartic anharmonicity. To make an anharmonic model advancing the ordinarily quasi-harmonic approximation model, we introduce two parameters: anharmonic characteristic temperature (θ _a ) and its volume derivative. In the anharmonic model, the value of C _V is calculated along an isochore using classical statistical mechanics and a harmonic quantum correction. At high temperatures, the decrease in C _V from the Dulong-Petit limit is related to the value of T/θ _a . For infinitely large θ _a , the system is approximately quasi-harmonic. The temperature dependence of γ is related to C _V by the thermodynamic identity $\left( {{{\partial C_{V} } \mathord{\left/ {\vphantom {{\partial C_{V} } {\partial \ln V}}} \right. \kern-0pt} {\partial \ln V}}} \right)_{T} = C_{V} \left( {{{\partial \gamma } \mathord{\left/ {\vphantom {{\partial \gamma } {\partial \ln T}}} \right. \kern-0pt} {\partial \ln T}}} \right)_{V} + \gamma \left( {{{\partial C_{V} } \mathord{\left/ {\vphantom {{\partial C_{V} } {\partial \ln T}}} \right. \kern-0pt} {\partial \ln T}}} \right)_{V}.$ Even though our modification of the quasi-harmonic approximation is simple, our anharmonic model succeeds in reproducing the experimental γ and C _V simultaneously for the NaCl-B1 phase.     

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.     

Sources of neutral lipids in a temperate intertidal sediment

The compositions of sterol, alcohol and ketone fractions from an oxic intertidal surface sediment from Corner Inlet. Victoria, Australia, have been examined by capillary gas chromatographymass spectrometry and related to the lipids of diatoms cultured from the sediment and to lipids of the sea-grass Zostera muelleri. Of the more than thirty sterols in the sediment most appear to derive from diatoms, including the major sterol 24-methylcholesta-5.22E-dien-3β-ol. Small amounts of 24-ethylcholesterol probably derive from Zostera, with a minor diatom contribution. 5α-stanols, both fully saturated and with a side-chain double bond, represented ca. 14% of the total sterols: a significant proportion of these are probably derived from marine invertebrates. These organisms also contribute C₂₆ sterols. most of the cholesterol and possibly small amounts of Δ-sterols. The data suggest that in situ biohydrogenation of stenols was not a major process in sterol transformation in the surface oxic sediment. Alcohols ranged from C₁₂ to C₃₀ and showed an unusual abundance of unsaturated components with Δ⁹-16:1 being the major constituent. The long-chain alcohols probably originate largely from Zostera but the shorter-chain. C₁₂-C₂₀. alcohols are at least partly derived from wax esters of unknown origin. Diatoms do not contribute significant amounts of alcohols. Ketones were not major constituents of the sediment and consisted mainly of 6.10.14-trimethylpentadecan-2-one and a series of n-alkan-2-ones ranging from C₁₆ to C₃₃. The latter showed a similar distribution to that of the n-alkanes but the correspondence was not sufficient to substantiate a product-precursor relationship. Very long-chain C₃- C₃₉ unsaturated methyl and ethyl ketones, which may originate from the marine unicellular alga Emiliania huxleyi. were present in low concentration.