Origin of perylene in ancient sediments and its geological significance

The distributions of polycyclic aromatic hydrocarbons (PAHs) in sediments of three Upper Triassic to Middle Jurassic sedimentary sequences from the Northern Carnarvon Basin, Australia have been investigated. Perylene was found to be a major PAH component in the top Lower to base Middle Jurassic sediments that are immature or at low maturity. Its depth/age profiles are not related to the combustion-derived PAHs that have been believed to be produced during ancient vegetation fires before deposition. This suggests a diagenetic origin for perylene. The concentration of perylene in the sediments is proportional to the amount of terrestrial input, decreasing with distance from the source of land sediments. Its carbon isotope composition is slightly heavier than higher-plant derived PAHs, but still in the range of the terrestrially sourced PAHs including higher-plant PAHs and combustion-derived PAHs as suggested previously. Fungi are proposed to be the major precursor carriers for perylene in sediments based on the facts that (1) perylenequinone structures have been previously suggested to be the natural precursors for perylene; (2) perylenequinone pigments exist in many fungal bodies; (3) fungi have played an important role during geological processes.     

A geochemical reconstruction of oil generation in the Barrow Sub-basin of Western Australia

A suite of crude oils and petroleum source rock extracts from the Barrow Sub-basin of Western Australia have been analysed for biological marker compounds by capillary GC-MS, and for volatile hydrocarbons by whole oil capillary GC. These analyses were used to calculate values for twenty-three biomarker parameters in order to assess aspects of source type, maturity, migration and biodegradation of the hydrocarbons.The crude oils had a source in the Upper Jurassic Dingo Claystone formation. These hydrocarbons accumulated in the reservoir sands and in some cases were biodegraded. Several accumulation and biodegradation episodes have been recognised while the basin continued to subside, which resulted in a suite of oils showing marked differences in composition.     

Tetracyclic diterpenoid hydrocarbons in some Australian coals,sediments and crude oils

Tetracyclic diterpenoid hydrocarbons (diterpanes) based on the ent-beyerane, phyllocladane and ent-kaurane skeletons have been identified in the hydrocarbon extracts of some Australian coals, sediments and crude oils. Structures were assigned to the geological diterpanes by comparison with synthetically prepared reference compounds. Studies of a sample suite consisting of low-rank coals and sediments indicate that the ratios of C-16 epimers of phyllocladane and ent-kaurane are maturity dependent, and that the relative proportion of the thermodynamically preferred 16β (H)-compounds increases with increasing thermal maturity. Thermodynamic equilibrium for the interconversion reactions is attained in sediments before the onset of crude oil generation.The most likely natural product precursors for the tetracyclic diterpanes are considered to be the tetracyclic diterpene hydrocarbons which occur widely in the leaf resins of conifers. Tetracyclic diterpanes have been identified in sediments and coals of Permian age or younger, suggesting that these compounds are markers for both modern and extinct families of conifers. In particular, phyllocladane is proposed as a marker for the Podocarpaceae family of conifers.     

Volcanic emissions and atmospheric pollution:A study of nanoparticles

The influence of emissions of an active volcano on the composition of nanoparticles and ultrafine road dust was identified in an urban area of the Andes.Although many cities are close to active volcanoes,few studies have evaluated their influence in road dust composition.Air quality in urban areas is significantly affected by nonexhaust emissions(e.g.road dust,brake wear,tire wear),however,natural sources such as volcanoes also impact the chemical composition of the particles.In this study,elements from volcanic emissions such as Si>Al>Fe>Ca>K>Mg,and Si-Al with K were identified as complex hydrates.Similarly,As,Hg,Cd,Pb,As,H,Cd,Pb,V,and salammoniac were observed in nanoparticles and ultrafine material.Mineral composition was detected in the order of quartz>mullite>calcite>kaolinite>illite>goethite>magnetite>zircon>monazite,in addition to salammoniac,a tracer of volcanic sources.The foregoing analysis reflects the importance of carrying out more studies relating the influence of volcanic emissions in road dust in order to protect human health.The road dust load(RD₁₀)ranged between 0.8 and 26.8 mg m^?2in the city.     

Mineralogical Characterization of Graphite Deposits from Thodupuzha-Kanjirappally Belt, Madurai Granulite Block, Southern India

Graphite from deposits occurring in the high-grade metamorphic rocks and their larteritized equivalents of the Thodupuzha-Kanjirappally Belt in Madurai Granulite Block, southern India is structurally fully ordered (crystallite size, Lc₍₀₀₂₎ ranging from 469 to 749 Å), possess high degree of graphitization (DG value ranging from 105 to 267 Å) and reflect crystallization at high temperature (700±100°C). Raman spectra of graphite display profiles corresponding to high crystallinity and high structural ordering. The high temperature crystallinity characteristics of graphite were not obliterated during retrogression of granulites to amphibolite facies gneisses. Preliminary carbon stable isotope results show a spread in isotope values from —11.8 to —26.8 %, which suggest more than one sources for carbon. The lighter carbon isotope values are suggestive of biogenic origin, whereas the heavier ones are probably fluid precipitated graphite.     

Differences in reactivities of sedimentary hopane diastereomers when heated in the presence of clays

Mixtures of hopane diastereomers obtained by fractionation of the organic extract from an immature oil shale have been heated in the presence of clay-containing substrates. In experiments conducted at 250°C with an extracted source rock as the substrate, the relative amounts of 17β(H),21β (H)-hopanes were found to decrease with respect to the moretanes and 17α(H),21β(H)-hopanes in a manner parallelling that observed with increasing maturity in sediments. In this case however, the change was shown to be due to the selective removal of the 17β(H),21β(H)-hopanes, rather than conversion of these compounds into the other diastereomers. In order to assess whether the use of elevated temperatures was enhancing processes other than those which operate in natural systems, a second experiment was conducted in which the sample of immature hopanes was heated at 75°C with the very catalytically active substrate aluminum montmorillonite. In this experiment also, the changes in hopane composition was shown to be due to selective removal of 17β(H),21β(H)-hopanes rather than conversion into the corresponding compounds in the other two series of diastereomers. These results suggest that the observed relative depletion of 17β(H),21β(H)-hopanes in sedimentary rocks of increasing maturity may similarly be due to removal by selective catalytic processes, and not to interconversion processes associated with isomerisation at C-17 and C-21 as had previously been believed.     

Incorporation of U,Pb and Rare Earth Elements in Calcite through Crystallisation from Amorphous Calcium Carbonate: Simple Preparation of Reference Materials for Microanalysis

Uncertainty for elemental and isotopic measurements in calcite by LA‐ICP‐MS is largely controlled by the homogeneity of the reference materials (RMs) used for calibration and validation. In order to produce calcite RMs with homogeneous elemental and isotopic compositions, we incorporated elements including U, Pb and rare earth elements into calcite through heat‐ and pressure‐induced crystallisation from amorphous calcium carbonate that was precipitated from element‐doped reagent solution. X‐ray absorption spectra showed that U was present as U(VI) in the synthesised calcite, probably with a different local structure from that of aqueous uranyl ions. The uptake rate of U by our calcite was higher in comparison with synthetic calcite of previous studies. Variations of element mass fractions in the calcite were better than 12% 2RSD, mostly within 7%. The ²⁰⁷Pb/²⁰⁶Pb ratio in the calcite showed < 1% variations, while the ²³⁸U/²⁰⁶Pb ratio showed 3–24% variations depending on element mass fractions. Using the synthetic calcite as primary RMs, we could date a natural calcite RM, WC‐1, with analytical uncertainty as low as < 3%. The method presented can be useful to produce calcite with controlled and homogeneous element mass fractions and is a promising alternative to natural calcite RMs for U‐Pb geochronology.     

The effects of thermal maturity on distributions of dimethylnaphthalenes and trimethylnaphthalenes in some Ancient sediments and petroleums

The distributions of di- and trimethylnaphthalenes in two sedimentary sequences from Western Australia have been examined by capillary gas chromatography and combined gas chromatography-mass spectrometry. A general decrease was observed in the relative proportions of αα-dimethylnaphthalenes and ααβ-trimethylnaphthalenes with increasing thermal maturity. Similar trends were also observed for six crude oils which have very different ratios of ethylcholestane epimers indicating very different thermal histories. These results suggest that changes in the relative abundances of certain methyl substituted naphthalenes may be useful indicators of thermal maturity of sedimentary organic matter, and the use of a number of isomer ratios is illustrated.     

Alkylbiphenyls in ancient sediments and petroleums

C₁ and C₂-alkylbiphenyls have been analysed in crude oils and sediment extracts by capillary gas chromatography after preliminary isolation procedures involving TLC and HPLC. The abundance of 2-methylbiphenyl relative to 3-methylbiphenyl has been measured in samples from two sedimentary sequences. 2-Methylbiphenyl decreases in relative abundance with increasing thermal maturity of the sediments in both sedimentary sequences. These isomeric methylbiphenyls are potentially useful as thermal maturity indicators for crude oils.     

GC-MS characterisation of C27 and C28 triterpanes in sediments and petroleum

Earlier studies have shown that an unusual C₂₇ triterpane is abundant in sediments from the Norwegian Continental Shelf and the North Sea. This compound was assigned the tentative structures 24,28,30-trisnormoretane or 25,28,30-trisnormoretane, but we have now shown from detailed retention time measurements and a reinterpretation of the mass spectral data that its correct structure is 17α(H),18α (H),21β(H)-25,28,30-trisnorhopane. Two other triterpanes, 25,28,30-trisnormoretane and 28,30-bisnormoretane, have also been identified as minor constituents of extracts of sediments from the North Sea. Possible origins for these compounds are discussed.