C25 and C30 biogenic alkenes in a sediment core from the upper anoxic basin of the Pettaquamscutt River (Rhode Island,U.S.A.)

Concentration profiles of five C₂₅ and C₃₀ biogenic alkenes in a sediment core collected from the upper anoxic basin of the Pettaquamscutt River have been determined. The five alkenes were identified usin gas chromatography/mass spectrometry as three isomeric C₂₅ dienes, a C₂₅ triene and a bicyclic C₃₀ diene. All five compounds exhibit subsurface concentration maxima, thought to result from either preservation of a past increase in alkene production or a current bacterial in situ production at depth. Similarities exist in the concentrations of two alkenes common to this core and a core from upper Narragansett Bay, despite significant differences in the origin and content of sedimentary organic matter (as inferred from organic carbon and δ ¹³C measurements) at each location. These observations support the proposed bacterial in situ synthesis of alkenes. Other alkenes, whose concentration in sediments had been previously correlated with the incidence of marine organic matter, were not detected in the upper basin sediments. Their absence is consistent with the range of organic carbon δ ¹³C values measured, which indicate that the component originating from marine sources is small. A comparison of organic carbon and δ ¹³C values in this core with those previously reported from a core collected in an adjoining basin indicate that the sedimentary regimes at the two sites differ despite their close proximity and similar hydrography.     

Volatile C1C7 organic compounds in an anoxic sediment core from the Pettaquamscutt River (Rhode Island,U.S.A.)

Surface sediments from an anoxic marine environment, the Upper Basin of the Pettaquamscutt River, Rhode Island, were analyzed for volatile organic compounds in the C₁C₇ range. The compounds identified included methane, ethane, alkenes (C₂C₅), carbon disulfide, cyclopentane, 3-methylpentane, methylfuran, aldehydes and ketones. Ethane, methylfuran, and most of the aldehydes and ketones showed maxima at the sediment water interface. Methane levels were very high-10–100 times greater than observed in most other surface sediments examined in this laboratory.     

Hydrocarbons of the hypersaline basins of Shark Bay,Western Australia

Shark Bay is an 8000 sq. km shallow marine embayment on the western coast of Western Australia. The waters exhibit a well established salinity gradient from oceanic to hypersaline. The chemical assemblage of sediment hydrocarbons along the salinity gradient may be classified into two distinct chemogeographic types. Firstly, oceanic sediments contain n-alkanes and a suite of highly branched and branched/cyclic C₂₅ alkenes. Hypersaline sediments are characterised by a high relative abundance of a C₂₅H₅₀ alkene together with an analogous C₂₀H₄₀ alkene and its parent C₂₀H₄₂ alkene (2,6,10-trimethyl-7-(3-methylbutyl)-dodecane). A pair of alkanes C₂₁H₄₂ and C₂₂H₄₄ increase in concentration and relative abundance with depth. The hydrocarbons of the hypersaline basins are found in only trace amounts in oceanic sediments. These chemical signals are overlain by further input indicative of the immediate biotic community.     

Carboxylic acid distribution patterns of temperate C3 and C4 crops

Agricultural grasses cover a major part of the land surface in temperate agro-ecosystems and contribute significantly to the formation of soil organic matter. Crop-derived lipids are assumed to be responsible for fast carbon turnover in soils. Differences in lipid distribution patterns between crops following C₃ and C₄ photosynthesis pathways have rarely been described, but could be useful for source apportionment of crop-derived input into soils or sediments. The distribution of long chain n-carboxylic acids (C₂₂, C₂₄, C₂₆) reveals significant differences between crop plants following either the C₃ or the C₄ photosynthetic carbon fixation pathway. The plant compartments leaves, stems and roots of C₄ plants contain relatively large proportions (> 40%) of n-C₂₄ carboxylic acid when compared to C₃ plants. These reveal larger relative proportions of n-C₂₂ and n-C₂₆ acids, whose relative abundance is subject to change between different plant compartments and during the growing season. The carboxylic acid ratio [CAR = n-C₂₄/(n-C₂₂ + n-C₂₆) carboxylic acids] provides distinct ratios for C₄ (> 0.67) and C₃ crops (< 0.67) and can thus be used as a molecular marker for the differentiation of crop plant biomass. In combination with the bulk stable carbon isotopic composition (δ¹³C) the CAR can be used as a tool for the estimation of the C₄ derived carbon proportion in soils or sediments.     

Primary cracking of kerogens. Experimenting and modelling C1, C2–C5, C6–C15 and C15+ classes of hydrocarbons formed

Mathematical models of hydrocarbon formation can be used to simulate the natural evolution of different types of organic matter and to make an overall calculation of the amounts of oil and/or gas produced during this evolution. However, such models do not provide any information on the composition of the hydrocarbons formed or on how they evolve during catagenesis.From the kinetic standpoint, the composition of the hydrocarbons formed can be considered to result from the effect of “primary cracking” reactions having a direct effect on kerogen during its evolution as well as from the effect of “secondary cracking” acting on the hydrocarbons formed.This report gives experimental results concerning the “primary cracking” of Types II and III kerogens and their modelling. For this, the hydrocarbons produced have been grouped into four classes (C₁, C₂–C₅, C₆–C₁₅ and C₁₅₊). Experimental data corresponding to these different classes were obtained by the pyrolysis of kerogens with temperature programming of 4°C/min with continuous analysis, during heating, of the amount of hydrocarbons corresponding to each of these classes.The kinetic parameters of the model were optimized on the basis of the results obtained. This model represents the first step in the creation of a more sophisticated mathematical model to be capable of simulating the formation of different hydrocarbon classes during the thermal history of sediments. The second step being the adjustment of the kinetic parameters of “secondary cracking”.     

Spatial and temporal variation of organic carbon in the northern South China Sea revealed by sedimentary records

Three sediment cores were taken from the Pearl River estuary and adjacent northern South China Sea (SCS). These sediment cores span the time interval 1900–2000 AD. The stratigraphy of the concentration, the ratio of total organic carbon (TOC) to total nitrogen (TN) and stable isotope (δ¹³C_org) of organic carbon (OC) from three high-resolution sediment cores were analyzed. The stratigraphic profiles of OC concentration, TOC/TN ratios and δ¹³C_org for the near past 100 yrs indicate that terrestrial organic matter decreases from 68.3% to 27.4% of the TOC in the Pearl River estuary, while Dapeng Bay (offshore east of Hong Kong) apparently had throughout little terrestrial organic matter input. The highest deposited OC occurs at the Humen River mouth and the OC concentrations are higher in the outer estuary than in the inner shelf of the northern SCS. The deposited OC at the River mouth increased with time, which could be caused by the high precipitation of land-derived organic matter and the high input of terrestrial organic matter, which is likely related to the rapid urbanization and industrial development in the Pearl River Delta since the 1970s. The OC concentrations did not exhibit an obvious increase with time in most areas of the Pear River estuary and adjacent inner shelf of the SCS, but the algal-derived OC concentration inferred from the δ¹³C_org values increased with time especially from 1980 to 2000 in the outer Pearl River estuary and Dapeng Bay. This increase is presumably caused by enhanced primary marine productivity supported by higher anthropogenic nutrient inputs.     

Sedimentary record of black carbon in the Pearl River estuary and adjacent northern South China Sea

The concentrations of black carbon (BC) and δ¹³C_BC were determined in sediments of three dated cores from the Pearl River estuary (core PR-3) and adjacent northern South China Sea (cores SS-30, E2). For comparison, the total organic C (TOC) contents and δ¹³C_TOC in the sediments were also measured. Relatively higher concentrations and fluxes of BC were found in sedimentary core PR-3, taken in the Pearl River estuary. The BC concentration profiles or fluxes correlated well with fossil-fuel usage in the Pearl River Delta. Maximum BC fluxes occurred in the late 1970s to early 1980s as recorded in core PR-3, and in the 1950s (core SS-30), reflecting the maximum BC emission in the Pearl River Delta and Hong Kong region, respectively. After the 1980s, a rapid decrease of BC fluxes and a light δ¹³C_BC excursion were presumably due to improvements in combustion and pollution-control technologies and a shift of energy structure from biomass and coal to a mixture of coal, gas, oil and biomass. The fossil BC that contributed to total BC in core PR-3 increased from 20–30% to 70–80% during the last five decades. The study also shows that BC correlates well with terrestrial organic matter and that the ratio of BC to TOC is a good pollution indicator in relation to anthropogenic activities.     

Source of sediments and metal fractionation in two Chinese estuarine marshes

The Changjiang and the Jiulong Estuaries, located in eastern and southeastern China, respectively, have different geomorphologic and tidal processes as well as anthropogenic development in their associated watersheds. Sediments in the Changjiang estuary mostly consist of SiO₂, CaO and MgO (mean percentages of 63.9, 4.34 and 2.35%), whereas sediments from the Jiulong estuary mostly consist of Al₂O₃, Fe₂O₃ and organic matter (mean percentages 19.2, 6.82 and 4.14%). The Jiulong estuarine sediments contain more than twice the concentrations of Pb, Zn, Cu, than those from the Changjiang estuary. In the Jiulong estuary, these heavy metals are associated with carbonates and organic matter, whereas in the Changjiang estuary, they are associated with residual fractions or clay. Sediments from the Changjiang estuary, mostly sediments with little organic matter, do not efficiently sequester anthropogenic-derived trace metals. In contrast, sediments from the Jiulong estuary consist of a mixture of fluvial and marine matter which can sequester heavy metals contributed by larger landscapes with industrial and municipal wastewater.     

Variations in distributions of C25 highly branched isoprenoid (HBI) alkenes in the diatom, Haslea ostrearia: influence of salinity

C₂₅ HBI alkenes of the diatom, Haslea ostrearia, have been examined in experiments in which the algae were cultured for up to 10 days at 14–15°C at nine different salinities (15 to 40 per mil). H. ostrearia proved to be an osmotolerant organism and growth was observed at all salinities. After 10 days growth at the lowest and highest salinities of 15 and 40 per mil, the concentrations of 2,10,14-trimethyl-6-methylene-7-(3-methylpent-1-enyl)pentadec-9-ene, which was the only HBI present in any of the samples, averaged 1.2±0.7 and 2.1±0.7 pg cell⁻¹, respectively. These were slightly lower than the HBI concentrations at 25 to 35 per mil (2.8±0.5 pg cell⁻¹). The data indicate that although salinity has an influence on HBI production in H. ostrearia, factors other than salinity are probably more important in controlling the large variations of HBI alkene concentrations and distributions found in sedimentary environments.     

Spatial and Temporal Variability of Sediment Organic Matter Recycling in Two Temperate Eutrophicated Estuaries

This paper deals with the spatial and seasonal recycling of organic matter in sediments of two temperate small estuaries (Elorn and Aulne, France). The spatio-temporal distribution of oxygen, nutrient and metal concentrations as well as the organic carbon and nitrogen contents in surficial sediments were determined and diffusive oxygen fluxes were calculated. In order to assess the source of organic carbon (OC) in the two estuaries, the isotopic composition of carbon (δ ¹³C) was also measured. The temporal variation of organic matter recycling was studied during four seasons in order to understand the driving forces of sediment mineralization and storage in these temperate estuaries. Low spatial variability of vertical profiles of oxygen, nutrient, and metal concentrations and diffusive oxygen fluxes were monitored at the station scale (within meters of the exact location) and cross-section scale. We observed diffusive oxygen fluxes around 15 mmol m^?2 day^?1 in the Elorn estuary and 10 mmol m^?2 day^?1 in the Aulne estuary. The outer (marine) stations of the two estuaries displayed similar diffusive O₂ fluxes. Suboxic and anoxic mineralization was large in the sediments from the two estuaries as shown by the rapid removal of very high bottom water concentrations of NO _x ^? (>200 μM) and the large NH₄ ⁺ increase at depth at all stations. OC contents and C/N ratios were high in upstream sediments (11–15 % d.w. and 4–6, respectively) and decreased downstream to values around 2 % d.w. and C/N ≤ 10. δ ¹³C values show that the organic matter has different origins in the two watersheds as exemplified by lower δ ¹³C values in the Aulne watershed. A high increase of δ ¹³C and C/N values was visible in the two estuaries from upstream to downstream indicating a progressive mixing of terrestrial with marine organic matter. The Elorn estuary is influenced by human activities in its watershed (urban area, animal farming) which suggest the input of labile organic matter, whereas the Aulne estuary displays larger river primary production which can be either mineralized in the water column or transferred to the lower estuary, thus leaving a lower mineralization in Aulne than Elorn estuary. This study highlights that (1) meter scale heterogeneity of benthic biogeochemical properties can be low in small and linear macrotidal estuaries, (2) two estuaries that are geographically close can show different pattern of organic matter origin and recycling related to human activities on watersheds, (3) small estuaries can have an important role in recycling and retention of organic matter.     

Unusual C35 and C36 alkenones in a paleoceanographic benchmark strain of Emiliania huxleyi

Our specimen of the cultured Emiliania huxleyi strain (CCMP1742, also known as NEPCC55a) that provides the benchmark for -based paleothermometry has started producing, for reasons yet unclear, major amounts of three new alkenones identified as ω15,22-C₃₅ methyl ketone, ω15,22-C₃₆ ethyl ketone and ω16,23-C₃₆ methyl ketone. Comparison of these structures with those established now by the same OsO₄ derivatization method applied to the di-unsaturated C₃₇, C₃₈ and C₃₉ alkenones typically found in this organism provides insight into the possible pathway for their biosynthesis. Isothermal batch culture experiments also show the content and composition of these new compounds change systematically and quite significantly in cells when subjected to environmental conditions such as nutrient depletion, variation in light availability and prolonged darkness. Alkenones of similarly unusual short-chain length are evident in suspended particulate materials from present day surface waters in the Ligurian Sea (Mediterranean) and in two different Holocene time horizons (Unit I and Unit II deposits) in Black Sea sediments. However, the positions of the double bonds are different from those that we now report in our culture, implying a different biosynthetic sequence. These alkenones are most likely derived from another, as yet unknown, haptophyte species. If this other organism accounts for all documented occurrences of these compounds in natural samples, then either it has a capacity for growth over a remarkably wide salinity range or surface water salinity in the early Holocene Black Sea may not have been as low as is currently believed.     

The lignin geochemistry of marine sediments from the southern Washington coast

Lignin oxidation products and stable carbon isotope distributions are used to investigate the sources, transport, and chemical stability of land-derived organic matter in dated cores of modern sediment from the southern Washington State continental shelf and slope. There is no evidence for significant chemical alteration of lignin compounds in these sediments for time periods of up to 400 yr. Gymnosperm woods and nonwoody angiosperm tissues account for most of the land-derived organic matter in the deposits. These land plant remains have an average δ¹³C of approximately ?25.5% and are concentrated in a narrow band of silty sediment which extends northward from the Columbia River mouth along the mid-shelf. Marine organic matter having an approximate δ¹³C of ?21.5%, strongly predominates in most other shelf and slope environments. Net fluxes of land-derived organic matter into the surface 5 cm of the cores vary directly with sediment accumulation rates. Net fluxes of marine organic material into the surface sediments are highest in environments which favor the preservation of organic matter, but correspond to less than 1% of the primary productivity in the overlying waters.     

Identification and occurrence of novel C36–C54 3,4-dialkylthiophenes with an unusual carbon skeleton in immature sediments

A series of novel long-chain 3,4-dialkylthiophenes (C₃₆–C₅₄) was identified in a number of sediments ranging from Pleistocene to Cretaceous. The identifications were based on mass spectral characterisation, desulphurisation and mass spectral data of synthesised model compounds. These organic sulphur compounds are probably formed by sulphur incorporation into mid-chain dimethylalkadienes with two methylenic double bonds. These putative precursor lipids are unprecedented and may be considered rather unusual. The distribution of 3,4-dialkylthiophenes in sediments varies considerably with the depositional palaeoenvironment, indicating that these compounds have a potential as molecular markers reflecting changes in palaeoenvironment.     

Identification of C24 and C25 lanostanes in Tertiary sulfur rich crude oils from the Jinxian Sag, Bohai Bay Basin, Northern China

Unusual short chain lanostanes (C₂₄ and C₂₅) and C₃₀ lanostane were identified in sulfur rich crude oils from the Jinxian Sag, Bohai Bay Basin, northern China. Besides the regular steranes (C_27-30), a series of 4-methyl steranes (C₂₂₋₂₃, C₂₇₋₃₀), 4,4-dimethyl steranes (C₂₂₋₂₄, C₂₈₋₃₀), short chain steranes (C₂₃₋₂₆), abundant pregnanes (C₂₁₋₂₂) and androstanes (C₁₉₋₂₀), together with sulfur containing steroids (20-thienylpregnanes and thienylandrostanes) were detected in the aliphatic and branched-cyclic hydrocarbon fraction of these crude oils. A literature survey of some long chain sterane analogues (e.g., A-nor-steranes, norcholestanes, C₃₀ steranes, lanostanes) and pregnanes seems to point to a sponge and/or dinoflagellate source. 4-Methyl, 4,4-dimethyl steroids and lanosterols (4,4,14-trimethyl steroids as the basic skeleton of lanostanes) can be derived from methanotrophic bacteria. Thus, a biological origin from a prokaryotic methylotroph can be used to explain the common source of abundant short chain steranes (C_23-26), 4-methyl (C_22-23) and 4,4-dimethyl steranes (C_22-24), as well as lanostanes (C_24-25 and C₃₀ analogues) in our oil samples. Generally, the steroids appear to have been extensively sulfurized with sulfur substitution at the C-22 position in the side chain during the early stage of diagenesis, which was readily subject to attack by bacterial degradation (enzymatic cleavage) and/or abiotic oxidation. As a consequence, short chain sterane analogues (e.g., abundant pregnanes and androstanes in this study) and short chain lanostanes (C₂₄−C₂₅) might later be released through cleavage of weak C-S bonds at the C-22 carbon in the sulfurized steroids and lanostane sulfides. Finally, the formation of the short chain C₂₄−C₂₅ lanostanes and distinctive occurrence of short chain steranes in this study can be well explained by microbial biodegradation of sulfurized lanostanoids and steroids in the reservoir.     

Volatile C1C8 organic compounds in sediments from the Peru upwelling region

Volatile C₁C₈ organic compounds were analyzed in seven cores recovered from a transect across the Peru Shelf Upwelling Region. Compounds detected in ng/g dry weight quantities included methane, ethane, propane and other C₄C₇ alkanes as well as functionalized compounds including alkenes, furans and aldehydes. Strong correlations were observed between some groups of compounds with similar structures. Maxima in two groups of compounds were observed: (1) C₁, C₂, C₃, furans, and butanals (plus sporadic alkanes) as nitrate levels decreased below about 2–4 μM in the subsurface, and (2) C₄C₆ alkenes together with predominantly branched and cycloalkanes at the sediment-water interface in cores recovered from oxygenated bottom waters. The levels of C₁C₇ alkanes in these sediments are comparable to levels found in DSDP cores buried to depths of less than 600 m (or not exposed to geothermal temperatures exceeding about 30°C).     

Influence of physiology and climate on δD of leaf wax n-alkanes from C3 and C4 grasses

We measured hydrogen isotope compositions (δD) of high-molecular-weight n-alkanes (C₂₇-C₃₃) from grasses grown in greenhouses and collected from the US Great Plains. In both cases, n-alkanes from C₄ grasses are enriched in D by more than 20‰ relative to those from C₃ grasses. The apparent enrichment factor (ε_C29-GW) between C₂₉n-alkane and greenhouse water is −165 ± 12‰ for C₃ grasses and −140 ± 15‰ for C₄ grasses. For samples from the Great Plains, δD values of C₂₉n-alkanes range from −280 to −136‰, with values for C₄ grasses ca. 21‰ more positive than those for C₃ grasses from the same site. Differences in C₃ and C₄ grass n-alkane δD values are consistent with the shorter interveinal distance in C₄ grass leaves, and greater back-diffusion of enriched water from stomata to veins, than in C₃ grass leaves. Great Plains’ grass n-alkane isotopic ratios largely reflect precipitation δD values. However, the offset or apparent fractionation between n-alkanes and precipitation is not uniform and varies with annual precipitation and relative humidity, suggesting climatic controls on lipid δD values. The dryer sites exhibit smaller absolute apparent fractionation indicative of D-enrichment of source waters through transpiration and/or soil evaporation. To explore the relationship between climate and n-alkane δD values, we develop three models. (1) The ‘direct analog’ model estimates δD_C29 values simply by applying the apparent enrichment factors, ε_C29-GW, observed in greenhouse grasses to precipitation δD values from the Great Plains. (2) The ‘leaf-water’ model uses a Craig-Gordon model to estimate transpirational D-enrichment for both greenhouse and field sites. The transpiration-corrected enrichment factors between C₂₉ and bulk leaf-water, ε_C29-GW, calculated from the greenhouse samples (−181‰ for C₃ and −157‰ for C₄) are applied to estimate δD_C29 values relative to modeled bulk leaf-water δD values. (3) The ‘soil- and leaf-water’ model estimates the combined effects of soil evaporation, modeled by analogy with a flow-through lake, and transpiration on δD_C29 values. Predictions improve with the addition of the explicit consideration of transpiration and soil evaporation, indicating that they are both important processes in determining plant lipid δD values. D-enrichment caused by these evaporative processes is controlled by relative humidity, suggesting that important climatic information is recorded in leaf wax n-alkane δD values. Calibration studies such as this one provide a baseline for future studies of plant-water-deuterium systematics and form the foundation for interpretation of plant wax hydrogen isotope ratios as a paleo-aridity proxy.     

Kinetics of microbial sulfate reduction in estuarine sediments

Kinetic parameters of microbial sulfate reduction in intertidal sediments from a freshwater, brackish and marine site of the Scheldt estuary (Belgium, the Netherlands) were determined. Sulfate reduction rates (SRR) were measured at 10, 21, and 30 °C, using both flow-through reactors containing intact sediment slices and conventional sediment slurries. At the three sites, and for all depth intervals studied (0-2, 2-4, 4-6 and 6-8 cm), the dependence of potential SRR on the sulfate concentration followed the Michaelis-Menten rate equation. Apparent sulfate half-saturation concentrations, K_m, measured in the flow-through reactor experiments were comparable at the freshwater and marine sites (0.1-0.3 mM), but somewhat higher at the brackish site (0.4-0.9 mM). Maximum potential SRR, R_max, in the 0-4 cm depth interval of the freshwater sediments were similar to those in the 0-6 cm interval of the marine sediments (10-46 nmol cm⁻³ h⁻¹ at 21 °C), despite much lower in situ sulfate availability and order-of-magnitude lower densities of sulfate-reducing bacteria (SRB), at the freshwater site. Values of R_max in the brackish sediments were lower (3.7-7.6 nmol cm⁻³ h⁻¹ at 21 °C), probably due to less labile organic matter, as inferred from higher C_org/N ratios. Inflow solutions supplemented with lactate enhanced potential SRR at all three sites. Slurry incubations systematically yielded higher R_max values than flow-through reactor experiments for the freshwater and brackish sediments, but similar values for the marine sediments. Transport limitation of potential SRR at the freshwater and brackish sites may be related to the lower sediment porosities and SRB densities compared to the marine site. Multiple rate controls, including sulfate availability, organic matter quality, temperature, and SRB abundance, modulate in situ sulfate-reducing activity along the estuarine salinity gradient.     

Generation and migration of light hydrocarbons (C2C7) in sedimentary basins

Sixty-five samples from selected source bed-type shale sequences from three exploration wells were analysed for yield and detailed composition of light hydrocarbons(C₂C₇) by a new hydrogen stripping/capillary gas chromatographic technique. In spite of low maturation levels (0.35–0.55% vitrinite reflectance), significant generation of ethane and propane was recognized in a Jurassic source bed sequence bearing hydrogen-poor kerogens. Light hydrocarbon generation in another and mature Jurassic source rock sequence is controlled by kerogen quality. Associated with a change from hydrogen-poor to hydrogen-rich kerogens, yields of total and most individual hydrocarbons exhibit orders-of-magnitude increases. At the same time, iso/n-alkane ratios for butanes, pentanes and heptanes decrease significantly. A study of an interbedded marine/nonmarine coal-bearing sequence of Upper Carboniferous age from the Ruhr area, West Germany, revealed that a marine shale unit in comparison to the adjacent coal seam is more prolific in generating n-alkanes of increasing molecular size.A case history for migration of light hydrocarbons by means of diffusion through shales is presented. In two shallow core holes in Campanian/Maastrichtian shales in West Greenland, upward diffusion of ethane to pentane range hydrocarbons is an active process within the near-surface 3 m interval. Diffusive losses within this interval amount to 99.8% for propane, 85.6% for n-butane and 38.9% for n-pentane.     

Organic matter geochemistry and petrography of Late Cretaceous (Cenomanian-Turonian) organic-rich shales from the Belle Fourche and Second White Specks formations,west-central Alberta,Canada

Organic-rich mudstones with up to 10 wt% TOC from the upper portion of the Belle Fourche Formation and the lower part of the Second White Specks Formation in the Western Canada Sedimentary Basin were evaluated as source rocks. Both geochemistry and organic petrography indicate an open marine paleoenvironment with deposition of Type II kerogen based on the predominance of marine alginite and amorphous organic matter (OM), limited amounts of terrigenous vitrinite and inertinite macerals, the presence of marine fossils, and the low ratio of TOC to total sulfur (∼1.26). The prevalence of short-chain n-alkanes (n-C₁₃ to n-C₁₉), a predominance of C₂₈ αββ(H)-20S steranes, and small concentrations of oleanane confirm the dominantly algal and planktonic origin of OM. Alternating oxic to anoxic paleoenvironmental sedimentary conditions are proposed based on common bioturbation, abundant inoceramid prisms, and good organic richness. Biomarker distributions are consistent with intermittent anoxia, without unequivocal evidence for water column stratification or hypersalinity. The thermal maturity measured in seven sediment cores by different methods consistently indicates a westward increase in maturity according to vitrinite reflectance, T_max, and hopane and sterane biomarkers. Two cores are thermally immature (∼0.42 %Ro), one is early mature (∼0.65 %Ro), and four cores are within the oil window (∼0.78 to 0.89 %Ro). All thermally mature cores retain good to very good hydrocarbon potential (248 mg HC/g rock) and are dominantly oil-prone and minor gas-prone based on their maceral compositions. The upper Belle Fourche and lower Second White Specks Formations represent potential targets for unconventional light shale oil production.     

Variations in structures and distributions of C25 highly branched isoprenoid (HBI) alkenes in cultures of the diatom, Haslea ostrearia (Simonsen)

The highly branched isoprenoid (HBI) alkenes of three batch cultures of the diatom Haslea ostrearia have been examined and the occurrence of C₂₅ tri-, tetra- and pentaenes confirmed. Growth of cultures outside at ambient temperature, under natural sunlight, in May (6 days) and June (10 days) 1995 and monitoring of HBI concentrations in samples collected daily and stored frozen, revealed that in May greater concentrations of HBIs (ca 6500 fg cell⁻¹) were produced than in June and at a much earlier stage of growth. Furthermore, in May the HBIs were more unsaturated (predominantly a tetraene rather than a triene). The reasons for these differences are at present unknown and future investigations of the effect of irradiance, temperature, salinity and other environmental variables are required. Such experiments may allow the various HBI distributions in the alga and in sediments to be better understood and, like some other polyunsaturated lipids, HBI alkenes may become useful environmental and palaeoenvironmental indicators. Two previously unreported alkenes, 2,6,10,14-tetramethyl-7-(3-methylpent-4-enyl)pentadec-2,5,9,13-ene (GC retention index, 2201_DB-1) and 2,6,10,14-tetramethyl-7-(3-methylene pent-4-enyl)pentadec-2,5,9,13-ene (GC RI, 2248_DB-1) from a batch culture of Haslea ostrearia grown in October/November 1993 and stored frozen for over a year, have also been isolated and characterised by ¹³C and ¹H NMR, epoxide derivatisation and mass spectrometry. Such structural characterisation studies of HBI alkenes should also aid our understanding of the geochemical fate of these widely distributed hydrocarbons.