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  • Corrigendum to “Linking the geochemistry of crude oils and petroleum inclusions in the Ubarana and Lorena oilfields, Potiguar Basin, Brazilian Equatorial Margin” [Org. Geochem. 124 (2018) 133–150]
    Org. Geochem. (IF 2.81) Pub Date : 2018-10-17
    André Luiz Silva Pestilho, Lena Virgínia Soares Monteiro, Carlos Alberto Carbonezi, Sandra Brandão Jorge, Eugênio V. Santos Neto
    更新日期:2018-10-17
  • In situ production of core and intact bacterial and archaeal tetraether lipids in groundwater
    Org. Geochem. (IF 2.81) Pub Date : 2018-10-15
    Su Ding, Bernd Kohlhepp, Susan Trumbore, Kirsten Küsel, Kai-Uwe Totsche, Georg Pohnert, Gerd Gleixner, Valérie F. Schwab

    Groundwater ecosystems host diverse and complex microbial communities that play important roles in the biogeochemical processing of organic matter and in the maintenance of drinking water quality. Here we investigated the microbial community in suspended particulate matter (SPM) of biogeochemically distinct groundwaters (Hainich Critical Zone Exploratory) by analyzing branched and isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs) from bacteria and archaea, respectively. The contributions of those lipids derived from dead and living organisms were determined by analyses of the core lipid distributions of core and intact polar GDGTs. We compared the groundwater GDGT distributions to the ones from soils of potential recharge areas and with archaeal 16S rRNA-gene based community reconstructions to estimate their origin in these terrestrial subsurface environments and thus their potential use for evaluating soil inputs into groundwater. In soils, the relative abundance of intact polar branched GDGTs (brGDGTs) was lower than that of isoprenoid GDGTs (isoGDGTs; 2% vs 5% of total GDGTs), while the opposite trend (71% vs 22% of total GDGTs) was observed in the core lipid pools. This supports previous observations that soil brGDGT-producing bacteria might be more active and thus have higher regeneration rates than the isoGDGT-producing archaea. We found similar trends in the groundwater that might indicate higher activity (i.e., cell division) of brGDGT-producing bacteria than of isoGDGT-producing archaea. The higher relative abundance of the hexamethylated brGDGT in the groundwater SPM (mean 65 ± 9%, n = 5) than in soils (mean 16 ± 7%, n = 22) indicated an in situ origin of brGDGT-producing bacteria. Higher contributions of penta- and tetra-methylated brGDGTs, which suggested some inputs from soil bacteria, was only detected in two out of seven groundwater samples. The strong correlation between core and intact polar isoGDGTs (R2 = 0.99, n = 7) in groundwater SPM indicated low disturbance (e.g., surface inputs) and suggested indigenous archaeal communities in the groundwater. This was supported by the results from a previous 16S rRNA-gene study that detected distinct archaeal groups in soils and groundwater. This first GDGT study in groundwater demonstrated that even dynamic karstic subsurface environments host an indigenous bacterial and archaeal community that is adapted to the living conditions. Furthermore, fast recharge events are likely detectable using tetraether lipids from the soil microbial community.

    更新日期:2018-10-15
  • Source-specific biomarkers as proxies for Arctic and Antarctic sea ice
    Org. Geochem. (IF 2.81) Pub Date : 2018-10-05
    Simon T. Belt

    Over the last decade or so, certain source-specific C25 highly branched isoprenoid (HBI) lipid biomarkers have emerged as useful proxies for Arctic and Antarctic sea ice. Thus, IP25 (Ice proxy with 25 carbon atoms) and IPSO25 (Ice proxy for the Southern Ocean with 25 carbon atoms) represent binary measures of past seasonal sea ice in the Arctic and Antarctic, respectively. A further tri-unsaturated HBI (generally referred to as HBI III) appears to provide proxy evidence for the region of open water found adjacent to sea ice (i.e. the marginal ice zone (MIZ)) in both polar regions. This review provides an update on current knowledge pertaining to each proxy. The first section focuses on describing those studies that have aimed to establish the underlying features of each proxy, including source identification and spatial distribution characteristics. The second section presents some important analytical considerations pertinent to the accurate identification and quantification of HBI biomarkers. The third section describes how each HBI proxy is normally interpreted within the sedimentary record for palaeo sea ice reconstruction purposes. This includes the interpretation of individual and combined biomarker profiles such as the PIP25 index and multivariate decision tree models. A summary of all previous palaeo sea ice reconstructions based on HBIs is also given, which includes examples that clarify or reinforce our understanding of the individual or combined biomarker signatures. Some knowledge gaps and areas for future research are also briefly described.

    更新日期:2018-10-05
  • Geological controls on the heterogeneous hydrocarbon compositions of the biodegraded Grosmont Formation bitumen, Western Canada Sedimentary Basin
    Org. Geochem. (IF 2.81) Pub Date : 2018-10-03
    Barry Bennett, Stephen R. Larter

    The hydrocarbon composition of the Grosmont Formation bitumen was investigated in 51 core samples by gas chromatography–mass spectrometry (GC–MS). The bitumen composition is highly variable and is heavily to severely biodegraded. Molecular ratios based on biodegradation-resistant compounds are highly uniform laterally and vertically, even across shale barriers indicating the bitumens are genetically related. Biodegradation-resistant biomarker maturity parameters suggests the bitumen was likely sourced during the early oil window stage of petroleum generation. Biomarkers responding to biodegradation display strong lateral and vertical variations in composition with geochemical profiles based on concentration data, displaying dramatic offsets when shales are present indicating the reservoir units are compartmentalised. Considering the relationship between 25-norhopanes and hopanes, the hydrocarbon composition data indicate progressive hopane degradation, both with concomitant formation of 25-norhopanes and hopane degradation without the formation of 25-norhopanes. In wells 07-08-085-18W4 and 10-12-093-24W4, the degradation of hopanes occurs in Upper Grosmont unit 3 (UG3) without the formation of 25-norhopanes (unless 25-norhopanes have been synchronously consumed), whereas in unit UG2, hopane degradation occurs with the concomitant formation of the 25-norhopanes. Therefore, the shale compartmentalizes the reservoir creating two isolated environments whereby the biogeochemical setting contributing to 25-norhopane formation is found in the UG2 unit, whereas it is absent or inactive in unit UG3. The degradation of the 22R epimer prior to the 22S epimer in the C31–C33 17α-hopanes was recognized in well 16-05-088-19W4, which indicates that the consortia of microbes more readily biodegraded the epimer having the biological conformation.

    更新日期:2018-10-04
  • The influence of salt on dissolved organic matter from peat soils
    Org. Geochem. (IF 2.81) Pub Date : 2018-10-04
    Robert Strehse, Heike Bohne, Yosef Amha, Peter Leinweber

    Understanding the influence of salt on the release and transformations of dissolved organic matter (DOM) is crucial for developing management strategies for coastal wetlands. We hypothesize that salt causes distinct changes in certain compound classes of DOM, distinguishable from the impact of other factors such as peat origin and decomposition degree. Therefore, the DOM composition was investigated in freshwater and saltwater extracts of nine peat samples of varying origin and decomposition degree. Samples were analyzed for carbon and nitrogen concentrations and pH values, and organic matter composition by temperature-resolved pyrolysis-field ionization mass spectrometry (Py-FIMS). The data revealed larger abundances of phenols/lignin monomers, peptides, carbohydrates, (mainly heterocyclic) N-compounds/nitriles and alkylaromatics in the saltwater extracts. The thermal release curves in Py-FIMS indicated a precipitation of fractions of alkylaromatics, N–compounds/nitriles and phenols/lignin monomers and a release of lipids, alkylaromatics and free fatty acids n–C16 to n-C34 in the saltwater extracts. Coagulation/precipitation, cation exchange and, for free fatty acids, the formation of micelles seem to be key mechanisms of the salt impact. Due to the stronger direct and indirect influences of saltwater on less decomposed peat an adapted management should be directed to preferentially protect rather pristine peatlands from saltwater intrusions that can be expected as a result of rising ocean levels under climate change.

    更新日期:2018-10-04
  • ω20-Hydroxy and ω9,ω10-dihydroxy biomarker lipids in ferns from the Salviniaceae family
    Org. Geochem. (IF 2.81) Pub Date : 2018-10-02
    Klaas G.J. Nierop, Paul Brouwer, Rolande Dekker, Henriette Schluepmann, Gert-Jan Reichart
    更新日期:2018-10-02
  • Suppression of Vitrinite Reflectance by Bitumen Generated from Liptinite During Hydrous Pyrolysis of Artificial Source Rock
    Org. Geochem. (IF 2.81) Pub Date : 2018-09-19
    K.E. Peters, P.C. Hackley, J.J. Thomas, A.E. Pomerantz

    Mean random vitrinite reflectance (Ro) is the most widely accepted method to determine thermal maturity of coal and other sedimentary rocks. However, oil-immersion Ro of polished rock or kerogen samples is commonly lower than Ro values measured in samples from adjacent vitrinite-rich coals that have undergone the same level of thermal stress. So-called suppressed Ro values have also been observed in hydrous pyrolysis experiments designed to simulate petroleum formation. Various hypotheses to explain Ro suppression, such as sorption of products generated from liptinite during maturation, diagenetic formation of perhydrous vitrinite or overpressure, remain controversial. To experimentally test for suppression of vitrinite reflectance, artificial rock was prepared using silica and a calcined blend of limestone and clay with various proportions of thermally immature vitrinite-rich Wyodak-Anderson coal and liptinite-rich kerogen isolated from the oil-prone Parachute Creek Member of the Green River Formation. The samples were subjected to hydrous pyrolysis for 72 hr. at isothermal temperatures of 300°C, 330°C, and 350°C to simulate burial maturation. Compared to artificial rock that contains only coal, samples with different proportions of oil-prone kerogen show distinct suppression of calibrated Ro at 300°C and 330°C. The reflectance of solid bitumen generated during heating of the samples is lower than that of the associated vitrinite and does not interfere with the Ro measurements. These results provide the first experimental evidence that Ro suppression occurs in vitrinite mixed with liptinite-rich kerogen in a rock matrix. Although the precise chemical mechanism for Ro suppression by liptinite remains unclear, free radicals generated from solid bitumen and associated volatile products during maturation of liptinite may contribute to termination reactions that slow the aromatization and rearrangement of polyaromatic sheets in vitrinite, thus suppressing Ro. This mechanism does not preclude Ro suppression that might result from overpressure or differences in redox conditions during diagenesis.

    更新日期:2018-09-20
  • Two novel decamethyl henicosanes (C31H64) identified in a Maoming Basin shale, China
    Org. Geochem. (IF 2.81) Pub Date : 2018-09-19
    Jing Liao, Hong Lu, Qiao Feng, Youping Zhou, Quan Shi, Ping'an Peng, Guoying Sheng

    Two new C31 branched alkanes (botryococcanes) presumably produced by the B race of Botryococcus braunii were isolated and purified from the Maoming Basin shales using column chromatography and preparative gas chromatography and structurally characterized with HR-EI-MS and 1D and 2D NMR. Interpretation of their EI mass spectral and 1D and 2D NMR (HMBC and HSQC) data led to the firm assignments of the two alkanes as diastereoisomeric 2,3,6,7,10,12,15,16,19,20-decamethylhenicosanes (DMHs). The structural assignments were further confirmed by the close match of the measured 13C NMR chemical shifts with those predicted by Lindeman-Adams 13C chemical shift modeling. The skeletons of these two DMHs are virtually identical to that of the recently identified C33 botryococcane/botryococcanone in the same sample. It is proposed that these two DMHs share a precursor C33 botryococcene biochemically formed by condensing two farnesyl diphosphates involving an unusual cyclobutanation, a retro-Prins reaction and a tetramethylation. A photo-mediated geochemical oxidation of the double bond in the ethenyl group connected to the sole quaternary carbon C-10 is also proposed to be responsible for the formation of the co-occurring DMHs and C33 botryococcane/botryococcanone.

    更新日期:2018-09-20
  • Genomic identification of the long-chain alkenone producer in freshwater Lake Toyoni, Japan: implications for temperature reconstructions
    Org. Geochem. (IF 2.81) Pub Date : 2018-09-18
    Julien Plancq, Jill L. McColl, James A. Bendle, Osamu Seki, Jillian M. Couto, Andrew C.G. Henderson, Yohei Yamashita, Kimitaka Kawamura, Jaime L. Toney
    更新日期:2018-09-19
  • Investigating the contribution of the coarse fraction to total pyrogenic carbon stocks in forest soil
    Org. Geochem. (IF 2.81) Pub Date : 2018-09-13
    Bernardo Maestrini, Jessica R. Miesel

    Pyrogenic carbon (PyC) quantification methods based on visual identification typically measure only the PyC particles > 0.5 mm, whereas oxidative methods are usually conducted on the < 2 mm soil fraction. To reconcile the discrepancy between these two methods, we used the weak nitric acid digestion method to quantify the PyC mass in the < 2 mm soil, and in the charcoal particles > 2 mm, in a California dry mixed conifer forest spatially heterogeneous in fire severity. We found that charcoal particles > 2 mm represented 5–10% of mineral soil PyC in recently burned areas (three years post-fire in our study) and 3% in the unburnt areas (> 100 years since the last fire). We, therefore, stress the importance of considering the size fraction of the charcoal quantified.

    更新日期:2018-09-13
  • Preservation of feather fibers from the Late Cretaceous dinosaur Shuvuuia deserti raises concern about immunohistochemical analyses on fossils
    Org. Geochem. (IF 2.81) Pub Date : 2018-09-11
    Evan T. Saitta, Ian Fletcher, Peter Martin, Michael Pittman, Thomas G. Kaye, Lawrence D. True, Mark A. Norell, Geoffrey D. Abbott, Roger E. Summons, Kirsty Penkman, Jakob Vinther

    White fibers from a Late Cretaceous dinosaur Shuvuuia deserti stained positive for β-keratin antibodies in a 1999 paper, followed by many similar immunological claims for Mesozoic protein in bones and integument. Antibodies recognize protein epitopes derived from its tertiary and quaternary structure, so such results would suggest long polypeptide preservation allowing for sequencing with palaeobiological implications. However, proteins are relatively unstable biomacromolecules that readily hydrolyze and amino acids exhibit predictable instability under diagenetic heat and pressure. Furthermore, antibodies can yield false positives. We reanalyzed a Shuvuuia fiber using focused ion beam scanning electron microscopy, energy-dispersive X-ray spectroscopy, time-of-flight secondary ion mass spectrometry, and laser-stimulated fluorescence imaging, finding it to be inorganic and composed mainly of calcium phosphate. Our findings are inconsistent with any protein or other original organic substance preservation in the Shuvuuia fiber, suggesting that immunohistochemistry may be inappropriate for analyzing fossils due to issues with false positives and a lack of controls.

    更新日期:2018-09-11
  • Molecular and isotopic evaluation of the maturation history of the organic matter in an Ordovician aquiclude (Michigan Basin): Evidence for late diagenetic biodegradation
    Org. Geochem. (IF 2.81) Pub Date : 2018-09-08
    Josué J. Jautzy, Jason M.E. Ahad, Mark Jensen, Ian D. Clark

    Previous characterization of a low permeability (10–16 ≤ Kh ≤ 10–12 m.s–1) and high salinity (> 5 M of Cl–, > 10× seawater salinity) aquiclude, in Upper Ordovician-aged sediments situated on the eastern flank of the Michigan Basin, where a deep geological repository for low and intermediate level nuclear waste is proposed, suggested a microbial origin for CH4 and CO2 based on their stable isotope values. CH4 is believed to have been produced and trapped during the Paleozoic in a relatively discrete and high organic matter (OM) horizon at the shale/carbonate transition. To further investigate the efficiency of confinement of this aquiclude, here we present detailed isotopic and geochemical stratigraphic profiles of OM and biomarkers, with the objective of understanding the origin of CH4 and its relative timing of confinement. Classical diagnostic ratios of polycyclic aromatic hydrocarbons, relative abundances of hopanes and degraded hopanes coupled to dual compound-specific isotopic analysis of n-alkanes, as well as δ13C analysis on different fractions of OM were measured. A partitioning of the Ordovician sedimentary succession into two systems is observed with an upper system that is self-sourced and confined, hosting the microbially derived CH4 with evidence of late diagenetic OM biodegradation and a lower system that reveals later secondary oil migration which has overprinted the organic geochemical record. We propose a conceptual model that would explain the generation and preservation of this paleo-bioreactor over time using the maturation history reconstruction derived from this dataset. This work not only provides essential field empirical evidence of a relationship between methanogenesis and late diagenetic biodegradation, but also shows the power of dual-compound-specific isotope analyses in deciphering between different maturation processes affecting OM in natural subsurface settings.

    更新日期:2018-09-09
  • Siderophore-promoted dissolution of ferrihydrite associated with adsorbed and coprecipitated natural organic matter
    Org. Geochem. (IF 2.81) Pub Date : 2018-09-06
    Christine Poggenburg, Robert Mikutta, Patrick Liebmann, Markus Koch, Georg Guggenberger

    Acquisition of Fe(III) from hardly soluble Fe oxyhydroxides by bacteria, fungi, and plants in the soil is often facilitated by Fe(III)-chelating siderophores. While the siderophore-promoted dissolution of pure Fe oxyhydroxides and the influence of defined low molecular weight organic acids has been investigated meticulously, studies examining Fe oxyhydroxides coated with more complex adsorbed organic matter (OM) or Fe-OM coprecipitates have been sparse. This study therefore explored the impact of the composition of both adsorbed and coprecipitated OM on the siderophore-promoted dissolution of ferrihydrite (Fh) in relation to (i) surface site coverage and pore blockage, (ii) aggregation state, (iii) charge characteristics, and (iv) crystallinity. In addition to pure Fh, Fh-OM adsorption complexes and Fe-OM coprecipitates were synthesized using OM of different origin (extracellular polymeric substances of Bacillus subtilis, natural OM (NOM) extracted from soil Oi and Oa horizons). The Fe-organic associations were characterized by N2 gas adsorption, X-ray photoelectron spectroscopy (XPS), electrophoretic mobility, particle size measurements and desorption experiments. Siderophore-mediated dissolution experiments with desferrioxamine-B (DFOB) were conducted at pH 4 and 7. At pH 4, adsorbed OM provided a more effective protection against DFOB due to its larger sorption affinity. At pH 7, the surface site blockage by adsorbed OM at comparable C loadings was less effective, while less aggregated particles were more prone to dissolution. For OM coprecipitated with Fe, there was no distinct accelerating or inhibiting effect on DFOB-promoted dissolution to be found. Initial Fe dissolution rates at given soil conditions were thus affected by both the type of OM and the sorption procedure. Our results therefore corroborate the importance of considering the heterogeneous composition of soil OM when investigating the mobilization of Fe-organic associations.

    更新日期:2018-09-07
  • Leaf wax n-alkane distributions across plant types in the central Chinese Loess Plateau
    Org. Geochem. (IF 2.81) Pub Date : 2018-09-06
    Jinzhao Liu, Zhisheng An, Hu Liu
    更新日期:2018-09-06
  • Early diagenesis of triterpenoids derived from mangroves in a subtropical estuary
    Org. Geochem. (IF 2.81) Pub Date : 2018-09-06
    Ding He, Bernd R.T. Simoneit, Joshua B. Cloutier, Rudolf Jaffé
    更新日期:2018-09-06
  • Revised genetic diagrams for natural gases based on a global dataset of >20,000 samples
    Org. Geochem. (IF 2.81) Pub Date : 2018-09-03
    Alexei V. Milkov, Giuseppe Etiope

    The origin of natural gases, in particular those containing methane (CH4 or C1), ethane (C2H6 or C2), propane (C3H8 or C3) and carbon dioxide (CO2), is commonly interpreted using binary genetic diagrams of δ13C-C1 versus C1/(C2+C3), δ13C-C1 versus δ2H-C1 and δ13C-C1 versus δ13C-CO2. These diagrams are empirical, but their currently used genetic fields were proposed around 30-40 years ago based on geographically and geologically limited datasets of tens to few hundreds gas samples. As a result, many recently collected gas samples plot outside of accepted genetic fields making these genetic diagrams partly inadequate for the purpose of gas interpretation. Here, we update the genetic diagrams using geochemical and geological data on 20,621 gas samples from a variety of geographical areas (76 countries and territories on six continents) and geological habitats (conventional and unconventional petroleum reservoirs, petroleum seeps and mud volcanoes, gas hydrates, volcanic/geothermal/hydrothermal manifestations, seeps and groundwater in serpentinized ultramafic rocks, aquifers, freshwater and marine sediments, igneous and metamorphic rocks). The revision includes genetic fields for primary microbial gases from CO2 reduction and methyl-type fermentation, secondary microbial gases generated during petroleum biodegradation, thermogenic and abiotic gases. The genetic field of thermogenic gases now includes early mature (δ13C-C1 as low as -75‰) and very late mature (δ13C-C1 around -15‰) gases recently recognized in various petroleum systems. Abiotic C1 is not necessarily 13C-eniched (δ13C>-20‰) as was often considered in the past. The δ13C values of abiotic C1 can be as negative as around -50‰, although a minor component of biotic (microbial or thermogenic) C1 is often associated with abiotic gas. In addition, the diagrams display molecular and isotopic changes that accompany post-generation processes of mixing, migration, biodegradation, thermochemical sulphate reduction and oxidation. The proposed diagrams cover the vast majority of hydrocarbon-containing gases currently known to exist in nature, are the most comprehensive empirical gas genetic diagrams published to date, and thus represent an essential tool for interpretations of natural gases. Still, holistic integration of geochemical and geological data is necessary to better interpret the origin of natural gases and processes that affected their composition.

    更新日期:2018-09-04
  • Depletion of 13C in residual ethane and propane during their thermal decomposition in sedimentary basins
    Org. Geochem. (IF 2.81) Pub Date : 2018-09-03
    Xinyu Xia, Yongli Gao

    Thermal decomposition of ethane and propane in sedimentary basins has long been conceptual without quantitative evidence. This work bridges the gap between 13C depletion and the conversion of ethane and propane, and confirms their thermal decompositions during the late catagenesis stage. Isotopic fractionation is rigorously investigated in a partly reversible reaction scheme representing ethane and propane decompositions. Accurate analytical solutions under isothermal conditions are derived. Non-monotonic isotopic fractionation with respect to conversion is demonstrated, which may be ignored if steady-state approximation is applied. The results imply that the depletion of 13C in ethane and propane with respect to the decrease of their molar fractions (isotopic inverse with respect to thermal maturity) results from the reversible conversion from alkane to alkyl groups. Kinetic parameters are obtained for ethane and propane decomposition in sedimentary basins. The chemical kinetic method is applicable to different isotopic fractionation systems. The results help to better understand the chemical form of deep carbon and the conversion of gaseous hydrocarbons in the geochemical carbon cycle.

    更新日期:2018-09-04
  • Origin, formation and environmental significance of des-A-arborenes in the sediments of an East African crater lake
    Org. Geochem. (IF 2.81) Pub Date : 2018-09-03
    L.G.J. van Bree, M.M. Islam, W.I.C. Rijpstra, D. Verschuren, A.C.T. van Duin, J.S. Sinninghe Damsté, J.W. de Leeuw

    Non-hopanoid pentacyclic triterpenoids occur widely in lake sediments, but their biological sources and diagenetic pathways are not fully resolved. We tentatively identified a number of des-A-arborene isomers occurring in relatively high abundance in the 25,000 year (25 kyr) sedimentary record of Lake Chala, a deep crater lake in tropical East Africa. The mono-, di- and tri-unsaturated des-A-arborenes are transformation products of isoarborinol/arborinone. These precursors have an aquatic source and are most likely biosynthesized by algae or aerobic bacteria in the epilimnion. The relatively depleted δ13C values (on average –32.3‰ ± 1.3‰) of des-A-arbor-9(11)-ene are consistent with an aquatic source (algae or aerobic bacteria). In general, isoarborinol and its microbially induced transformation products are found in present and ancient tropical lacustrine settings (typically crater lakes) with permanently anoxic bottom waters and sediments. Based on molecular mechanics calculations of des-A-arborenes, it is clear that these transformation products are not in thermodynamic equilibrium, strongly indicating that their formation is microbially mediated. Subtle temporal and spatial differences in the microbial community might therefore not only dictate the variable relative contributions of different des-A-arborenes found in the sediments of Lake Chala over the last 25 kyr, but also explain the distribution of arborane derivatives in comparable crater-lake settings elsewhere.

    更新日期:2018-09-04
  • Autochthonous organic carbon contributions to the sedimentary pool: a multi-analytical approach in Laguna Garzón
    Org. Geochem. (IF 2.81) Pub Date : 2018-08-31
    Leandro Bergamino, Lorena Rodríguez-Gallego, Andrés Pérez-Parada, Mauricio Rodríguez Chialanza, Valentina Amaral, Laura Perez-Becoña, Fabrizio Scarabino, Carolina Lescano, Camilo García-Sposito, Soledad Costa, Chad S. Lane, Adriana Tudurí, Natalia Venturini, Felipe García-Rodríguez
    更新日期:2018-08-31
  • An assessment of biomarker-based multivariate classification methods versus the PIP25 index for paleo Arctic sea ice reconstruction
    Org. Geochem. (IF 2.81) Pub Date : 2018-08-30
    Denizcan Köseoğlu, Simon T. Belt, Katrine Husum, Jochen Knies

    The development of various combinative methods for Arctic sea ice reconstruction using the sympagic highly branched isoprenoid (HBI) IP25 in conjunction with pelagic biomarkers has often facilitated more detailed descriptions of sea ice conditions than using IP25 alone. Here, we investigated the application of the Phytoplankton-IP25 index (PIP25) and a recently proposed Classification Tree (CT) model for describing temporal shifts in sea ice conditions to assess the consistency of both methods. Based on biomarker data from three downcore records from the Barents Sea spanning millennial timescales, we showcase apparent and potential limitations in both approaches, and provide recommendations for their identification or prevention. Both methods provided generally consistent outcomes and, within the studied cores, captured abrupt shifts in sea ice regimes, such as those evident during the Younger Dryas, as well as more gradual trends in sea ice conditions during the Holocene. The most significant discrepancies occurred during periods of highly unstable climate change, such as those characteristic of the Younger Dryas–Holocene transition. Such intervals of increased discrepancy were identifiable by significant changes of HBI distributions and correlations to values not observed in proximal surface sediments. We suggest that periods of highly fluctuating climate that are not represented in modern settings may hinder the performance and complementary application of PIP25 and CT-based methods, and that data visualisation techniques should be employed to identify such occurrences in downcore records. Additionally, due to the reliance of both methods on biomarker distributions, we emphasise the importance of accurate and consistent biomarker quantification.

    更新日期:2018-08-30
  • Sulfurization as a preservation mechanism for the δ13C of biomarkers
    Org. Geochem. (IF 2.81) Pub Date : 2018-08-24
    Yoav O. Rosenberg, Ilya Kutuzov, Alon Amrani

    We compiled data from studies that examined pairs of compound specific δ13C in both the free- and S-bound fractions. The studies compiled span a wide range of biomarker type, rock age (∼235Ma to 3Kyr) and paleo-environments in which the sedimentary organic matter was deposited. Despite this variability, majority of data shows that the S-bound fraction is heavier on average by 2‰ compared with the free HC. The hypotheses of different biological sources and fractionation during sulfurization or maturation of the organic matter are reexamined in light of this result. We conclude that the δ13C fractionation between these two fractions should be a result of a more generalizing mechanism, operating at an early diagenetic stage. Fractionation during biodegradation of the free biomarker fraction can offer such a mechanism. Thus, the S-bound fraction may better represent the original δ13C values of biomarkers used in paleoenvironmental studies. Future studies should confirm this suggested mechanism.

    更新日期:2018-08-26
  • Comparison of black carbon chemical oxidation and macroscopic charcoal counts for quantification of fire by-products in sediments
    Org. Geochem. (IF 2.81) Pub Date : 2018-08-24
    Richard S. Vachula, Ewerton Santos, Marcelo R. Alexandre, Yongsong Huang

    Black carbon (BC) ranges in size from submicron to millimeter scales and represents incompletely combusted or pyrolysed organic fuels (e.g., coal, petroleum, biomass etc). The content of BC in sediments or in the atmosphere has been widely used to infer fossil fuel combustion from local to regional sources. However, the assessment of natural fires (biomass burning) in the past has been conventionally performed by counting charcoal particles in different size fractions (greater than ca. 10 and 100 microns for microscopic and macroscopic charcoal, respectively) using a microscope. Because similarly sized charcoal particles can have large differences in mass, counting using microscopes may induce significant error for quantifying fire-produced BC in sediments. A popular method oxidizes BC with nitric acid to produce benzene polycarboxyclic acids (BPCAs), which can be subsequently quantified using a gas chromatograph and mass spectrometer (GC-FID and GC–MS). In this paper, we provide the first parallel comparison of charcoal and BPCA analyses to assess their ability to quantitatively record BC content in experimental sediments. We find that while qualitatively comparable, the BPCA method more accurately quantifies changes in BC content than does charcoal microscopy. Additionally, we use metadata analyses of the Global Charcoal Database to contextualize our results. Further, we explore possible applications of these two methods in tandem which could provide novel insight into paleofire characteristics.

    更新日期:2018-08-26
  • Environmental controls on pigment distributions in the freshwater microbialites of Fayetteville Green Lake
    Org. Geochem. (IF 2.81) Pub Date : 2018-08-25
    Benjamin T. Uveges, Mark A. Teece, James M. Fulton, Christopher K. Junium

    We analyzed pigments in acetone extracts of microbialites collected over a shallow depth gradient from the freshwater Fayetteville Green Lake (FGL), Fayetteville, NY. Pigment identification and quantification were achieved using reverse phase high performance liquid chromatography and ion trap multi-stage mass spectrometry (MSn). Chlorophyll a and its derivatives, the photoprotective pigment scytonemin, and the carotenoid fucoxanthin were present in all samples, β-carotene was observed in all but one sample, and minor abundances of other pigments such as chlorophyll d and lutein were observed in select samples. The concentration of scytonemin and its abundance relative to that of chlorophyll a decrease logarithmically with depth, consistent with the function of scytonemin as a UV screening pigment. As well, the increase in the concentration of chlorophyll a, b and the photosynthetic accessory carotenoids fucoxanthin and β-carotene with depth are consistent with lower irradiance at depth. The distribution and relative abundance of photosynthetic and light shielding pigments may provide a means for determining the relative water depth/incident radiation levels of ancient microbialites in which pigments or their derivatives are preserved.

    更新日期:2018-08-26
  • Effect of continuous light on leaf wax isotope ratios in Betula nana and Eriophorum vaginatum: Implications for Arctic paleoclimate reconstructions
    Org. Geochem. (IF 2.81) Pub Date : 2018-08-22
    William C. Daniels, Yongsong Huang, James M. Russell, Anne E. Giblin

    Reconstructions of climate using leaf wax D/H ratios (δDwax) require accounting for the apparent isotopic fractionation (εapp) between plant source water and waxes. There have been conflicting publications on whether plants in the Arctic growing under 24-hour continuous light fractionate less than temperate and tropical plants. In this study, we examine the effect of diurnal light (DL) versus 24-hour continuous light (CL) on the isotopic composition of leaf n-alkanes and n-acids in greenhouse experiments using two common Arctic plants (Eriophorum vaginatum, or tussock cottongrass, and Betula nana, or dwarf birch). For E. vaginatum, the δDwax values of various wax homologues were 5-11‰ more positive for CL plants relative to their DL counterparts, whereas for B. nana, CL waxes were 3-24‰ more negative, suggesting that daylight length is not a unifying control on leaf wax D/H ratios of Arctic plants. The δ13Cwax of B. nana was more negative for plants grown in continuous light compared to diurnal light, reflecting lower water-use efficiency associated with prolonged stomatal opening in the CL treatment. We modeled the impact of increasing stomatal conductance and effective flow path lengths (mimicking variable leaf morphologies) on the isotopic composition of leaf waters (δDlw) and find that variations in leaf-water enrichment may explain the variable δDwax responses seen between E. vaginatum and B. nana. We suggest that between-species differences in the δDlw response to light, and differences in the utilization of stored carbohydrates were important for governing δDwax. Our greenhouse results suggest that Arctic plant leaf waxes do not consistently display reduced εapp values as a result of 24-hour day light, providing additional support for field observations.

    更新日期:2018-08-22
  • Selective preservation among bacterial alkyl glycerol ether lipid structures during long term oxic and anoxic incubation
    Org. Geochem. (IF 2.81) Pub Date : 2018-08-22
    Arnauld Vinçon-Laugier, Cristiana Cravo-Laureau, Vincent Grossi

    Bacterial mono- and dialkyl glycerol ether lipids (MAGEs and DAGEs) are potential powerful lipid biomarkers whose respective diagenetic fate still needs to be precisely assessed. To do so, the lipid content of cultures of an anaerobic bacterium synthetizing both MAGEs and DAGEs was analyzed at different growth stages and after long-term incubation under aerobic or anaerobic conditions. The proportions of the different lipid classes (fatty acids, MAGEs and DAGEs) as well as their structural distribution did not significantly vary during anaerobic growth under optimal conditions until the stationary phase. Long-term exposure to oxygen (up to 480 days) induced the preferential degradation of fatty acids and MAGEs relative to DAGEs and of sn-1 relative to sn-2 positional isomers of MAGEs. The degradation of fatty acids and MAGEs appeared slower under anaerobic conditions and no specific structures appeared preferentially degraded. All DAGE structures were very well preserved whatever the redox conditions and the time of incubation considered (480 days and 13 years under oxic and anoxic conditions, respectively). The results highlight the excellent diagenetic stability of DAGEs and the much lower potential of preservation of MAGEs, warranting caution for an eventual use of those latter compounds as tracers of environmental or microbial community changes.

    更新日期:2018-08-22
  • Geochemical and petrographic evidence for the co-occurrence of races B and L of Botryococcus braunii in the Maoming oil shales, China
    Org. Geochem. (IF 2.81) Pub Date : 2018-08-16
    Hong Lu, Yujiao Zhang, Qiao Feng, Liming Ji, MingZhen Zhang, Ping'an Peng, Guoying Sheng, Dehan Liu

    Around 30 Maoming oil shale samples were collected and analyzed geochemically to investigate their biomarker compositions. C31 and C33 botryococcanes were abundant in some samples whilst high contents of lycopanes (C39 and C40) and monoaromatic lycopane derivative with concomitant occurrence of botryococcanes were also detected in other samples. These indicate that the fossils of the races B and L of Botryococcus braunii co-exist in the Maoming sediments thus providing an excellent opportunity to explore the morphological differences of two races of B. braunii. Different preparation and observation methods were carried out for better and detailed morphological observations enabling the fossil characters between race B and L of B. braunii to be recognized. Better observations were obtained using white fluorescence transmitted light under UV-induced fluorescence microscopy because yellow fluorescence was avoided. Race B of B. braunii has polygonal cup mouths and is often covered by an envelope. However, race L has a relatively smaller colony size and round cup mouth. The co-occurrence of race B and L of B. braunii in Maoming sediments possibly implies a tropical to subtropical climate during the Late Eocene–Oligocene period in the study area.

    更新日期:2018-08-17
  • Dissolved organic carbon compounds in deep-sea hydrothermal vent fluids from the East Pacific Rise at 9°50’N
    Org. Geochem. (IF 2.81) Pub Date : 2018-08-14
    Krista Longnecker, Stefan M. Sievert, Sean P. Sylva, Jeffrey S. Seewald, Elizabeth B. Kujawinski

    Deep-sea hydrothermal vents are unique ecosystems that may release chemically distinct dissolved organic matter to the deep ocean. Here, we describe the composition and concentrations of polar dissolved organic compounds observed in low and high temperature hydrothermal vent fluids at 9°50’N on the East Pacific Rise. The concentration of dissolved organic carbon was 46 µM in the low temperature hydrothermal fluids and 14 µM in the high temperature hydrothermal fluids. In the low temperature vent fluids, quantifiable dissolved organic compounds were dominated by water-soluble vitamins and amino acids. Derivatives of benzoic acid and the organic sulfur compound 2,3-dihydroxypropane-1-sulfonate (DHPS) were also present in low and high temperature hydrothermal fluids. The low temperature vent fluids contain organic compounds that are central to biological processes, suggesting that they are a by-product of biological activity in the subseafloor. These compounds may fuel heterotrophic and other metabolic processes at deep-sea hydrothermal vents and beyond.

    更新日期:2018-08-14
  • Net fractionation of hydrogen isotopes in n-alkanoic acids from soils in the northern boreal forest
    Org. Geochem. (IF 2.81) Pub Date : 2018-08-11
    Aleesha Bakkelund, Trevor J. Porter, Duane G. Froese, Sarah J. Feakins

    Plant-derived n-alkyl lipids are well-preserved in sedimentary archives, and their stable hydrogen isotope ratio (δDwax) is a proxy for precipitation δD and climate. Net fractionation of H isotopes between plant source water and n-alkyl lipids (εwax/water) is the largest uncertainty for interpreting this proxy and depends on plant type and environment. Although popular proxies, n-alkanoic acids (n-acids) are less frequently calibrated in modern environments than n-alkanes. We constrain the net fractionation (εwax/MAP – Mean Annual Precipitation) for the northern boreal forest based on n-C24,26,28 acids in topsoils from a 13-site network in Yukon, Alaska and Northwest Territories (60-68°N). n-Acid homologue distributions for common boreal plants (gymnosperm trees, shrubs, forbs, C3 grasses and mosses) show that soil n-acids are dominated by mosses, but with a reduced carbon preference index compared to fresh mosses, possibly owing to post-depositional degradation. Regionally averaged εwax/MAP values are –93±10‰, –101±11‰ and –95±11‰ for n-C24,26,28 acids, respectively, which are similar to values reported from low and high latitude regions. We find that εwax/MAP for n-C26,28 acids (but not n-C24) is significantly (p≤0.05) smaller with increasing latitude and decreasing mean annual temperature, factors that covary across our network. Since latitude is typically known for ancient deposits, we parsed the dataset to obtain latitude-specific εwax/MAP values for 60-64°N (–107±8‰ for n-C26; and –101±8‰ for n-C28) and 65-68°N (–94±11‰ for n-C26; and –89±10‰ for n-C28). These soil-derived values are relevant for hydroclimate reconstructions using fossil n-acids in paleosols from a comparable paleoflora and latitude.

    更新日期:2018-08-13
  • The impact of oxygen exposure on long-chain alkyl diols and the long chain diol index (LDI) – a long-term incubation study
    Org. Geochem. (IF 2.81) Pub Date : 2018-08-10
    Sophie Reiche, Sebastiaan W. Rampen, Denise J.C. Dorhout, Jaap S. Sinninghe Damsté, Stefan Schouten

    In recent years, long chain alkyl diols (LCDs) have been increasingly used to study and reconstruct past sea surface temperatures using the long chain diol index (LDI), which is based on changes in the distribution of 1,15-LCDs. However, the impact of diagenesis on LCDs and the LDI is still poorly constrained. Here we studied the impact of oxygen exposure on LCDs and the LDI, by aerobically incubating biomass of the LCD-producing alga Nannochloropsis oculata for 271 days. The concentrations of extractable free- and bound, residually ester-bound and residually-bound glycosidic ether- or amide-bound saturated fatty acids and LCDs were determined. A significant impact of oxygen exposure was observed for C14, C16 and C18 saturated fatty acids and the C20:5 polyunsaturated fatty acid as their concentration decreased significantly over time, irrespective of their mode of binding. LCDs, in contrast, increased significantly in concentration over incubation time, e.g. up to a 30-fold increase in concentrations for residually ester-bound LCDs at day 125 compared to concentrations at the beginning of the experiment. This increase in concentration most likely represents a release of LCDs from the insoluble biopolymer algaenan due to the impact of oxygen exposure. Values of the LDI differed strongly depending on the mode of occurrence of LCDs in the biomass. However, despite the large changes in concentration of LCDs in response to oxygen exposure, the LDI remained relatively stable after prolonged degradation. This suggests that oxygen exposure may not have a substantial impact on the LDI of extractable LCDs used for its determination.

    更新日期:2018-08-11
  • Duxite - fossil resin of Miocene age
    Org. Geochem. (IF 2.81) Pub Date : 2018-08-04
    Martina Havelcová, Vladimír Machovič, Ivana Sýkorová, Ladislav Lapčák, Alexandra Špaldoňová, Karel Mach, Zdeněk Dvořák

    A series of Miocene fossil resin from the northwest part of the Czech Republic, called duxite, has been analyzed by elemental, microscopic, gas chromatography (GC/MS), pyrolysis-gas chromatography, Fourier Transform infrared (FTIR), and Raman techniques. The set of samples consisted of museum, contemporary and artificially altered samples. The results of GC/MS revealed fine variances in chemical structure among the samples, which could be attributed to the geologic paleosituation during resin deposition, as was verified by alteration of a sample under different conditions. Sesquiterpenes, including α-cedrene and cuparene, were identified in sample extracts and sample pyrolysates together with diterpenoid members abietanes, pimaranes, and dehydroabietanes. The distribution and intensity of functional groups of FTIR spectra also confirmed that the duxite samples were fossilised exudates from a member of the Cupressaceae conifer family. Raman spectra supported this record indicating aromatic character of duxite and higher maturity of the samples.. The chemical composition indicated that duxite is a member of Class IV resins of the fossil resin classification system. Members of this group do not have a polymeric structure. This nonpolymerizable behaviour was confirmed by their excellent solubility in an organic solvent and the low softening point of the material. Our results therefore provide a valuable insight into the duxite-producing process and its potential for evaluating the geological environment and diagenetic conditions.

    更新日期:2018-08-05
  • Radically different lignin composition in Posidonia species may link to differences in organic carbon sequestration capacity
    Org. Geochem. (IF 2.81) Pub Date : 2018-08-01
    Joeri Kaal, Oscar Serrano, José C. del Río, Jorge Rencoret

    There is considerable variability in the ability of seagrass ecosystems to sequester organic carbon (Corg) in their sediments, which act as natural carbon sinks contributing to climate change mitigation. In this work, we studied the chemistry of two Posidonia seagrass species aiming to elucidate whether differences in chemical composition might explain differences in their Corg sequestration capacity. Pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) and Thermally assisted Hydrolysis and Methylation (THM) GC-MS data showed a remarkable difference in phenolic compound patterns between P. oceanica and P. australis bulk plants and individual organs (leaves, sheaths, roots and rhizomes). The lignin of P. australis generates a series of p-hydroxyphenyl (H), guaiacyl (G) and syringyl (S) products that are typical of herbaceous plants, whereas P. oceanica is particularly rich in p-hydroxybenzoic acid (pBA) derivatives. The structural characteristics of the lignins were further investigated by two-dimensional Nuclear Magnetic Resonance (2D-NMR) and Derivatization Followed by Reductive Cleavage (DFRC), focusing on sheath tissues. The analyses confirmed important differences in the lignin content (19.8% in P. australis and 29.5% in P. oceanica) and composition between the two species; intriguingly, the cell-walls of P. oceanica sheaths were highly enriched in pBA, a component that was completely absent in P. australis. 2D-NMR and DFRC further revealed that pBA was esterified to the lignin, acylating the γ-OH of the lignin side-chain. Interestingly, P. oceanica lignin presented an extremely high degree of p-hydroxybenzoylation in both guaiacyl (73%) and syringyl (61%) lignin units, the highest p-hydroxybenzoylation degree reported in plant lignins to date. It is tempting to conclude that the higher soil Corg storage capacity of P. oceanica ecosystems might be related to the higher abundance of pBA-rich lignin and its recalcitrant nature.

    更新日期:2018-08-02
  • Depositional environment of the Late Santonian lacustrine source rocks in the Songliao Basin (NE China): Implications from organic geochemical analyses
    Org. Geochem. (IF 2.81) Pub Date : 2018-08-01
    Xiaoning Tong, Jianfang Hu, Dangpeng Xi, Mengbo Zhu, Jianzhong Song, Ping'an Peng

    The Songliao Basin (SLB) located in northeastern China is one of the largest Cretaceous continental sedimentary basins in the world. The SLB is filled with sediments deposited in the Upper Jurassic, the Lower Cretaceous and the Upper Cretaceous epochs. The Nenjiang Formation (K2n) is subdivided into five members, where Member 2 (K2n2) was deposited in the late Santonian in the lower part of the Nenjiang Formation. This member is characterized by a thick succession of organic-rich source rocks. However, the complexity of the depositional environment raises questions about the specific factors that drove this accumulation of organic material. Here, we present data on the total organic carbon (TOC) contents and their stable carbon isotope values (δ13Corg), as well as biomarker data from 50 outcrop samples collected from the Yuewangcheng (YWC) section, in the southeastern SLB. According to the variations of the bulk organic parameters (TOC and δ13Corg) and biomarker indices, the profile could be divided into three stages (stages I–III). The distribution of biomarkers (n-alkanes, steranes and hopanes) and δ13Corg values indicate that the organic matter (OM) in the Lower K2n2 is derived largely from algae and macrophytes, with a minor input from bacteria and land plants. The water column was stratified, as indicated by the presence of gammacerane. Variations in the pristane/phytane (Pr/Ph) and aryl isoprenoids relative to phenanthrene ratio (A-i/P) suggest that bottom waters were anoxic during Stage I (31.5–26.0 m) and Stage II (26.0–16.3 m), with the anoxic layer impinging on the euphotic zone and a relative oxic environment at Stage III (16.3–0 m). This brackish environment persisted in the water over the interval represented by the section, as reflected by methytrimethyltridecyl chromans (MTTCs) Index (MTTCI) and α-MTTC/γ-MTTC ratios. Marine transgressions, with subsequent seawater incursions, can be detected with the presence of 24-n-propyl-cholestanes and 24-iso-propyl-cholestanes in the Lower K2n2 sediments, especially during Stage I. These seawater incursions are closely correlated with anoxic conditions and the deposition of organic-rich source rocks.

    更新日期:2018-08-02
  • Steroid fingerprints: efficient biomarkers of human decomposition fluids in soil
    Org. Geochem. (IF 2.81) Pub Date : 2018-08-01
    Barbara von der Lühe, Jago J. Birk, Lorna Dawson, Robert W. Mayes, Sabine Fiedler

    The decomposition of animal and human cadavers can lead to comprehensive chemical and biochemical changes in soil, which can provide helpful information for the analysis of archaeological and crime scenes. The current study focused specifically on the concentration and distribution patterns of steroids in soil with the aim of assessing their suitability for demonstrating the presence of human decomposition products. Soil samples were collected from a forensic site where a human corpse had been lying on the soil surface for 18 days. Total organic carbon (TOC) and steroid concentrations were analysed in soil samples taken from beneath the body at the time the corpse was removed and also one year later. High concentrations of TOC and steroids were detected close to the soil surface at the time the body was removed, and the concentration of cadaver-derived cholesterol was considerably higher than that of the plant-derived sitosterol. The soil beneath the cadaver contained higher concentrations of faeces-derived 5β-stanol coprostanol than the control soil. Different concentrations and distribution patterns of steroid compounds in the soil upon removal of the corpse and one year later indicate that significant transformation, degradation and translocation processes had taken place during this period of time. The depth distribution of steroids beneath a decomposing body along with changes in the human steroidal fingerprints in soil over time were found to provide information that could help resolve forensic and archaeological issues. The combined analysis of tissue steroids (cholesterol), the reduction product of cholesterol in soil (5α-cholestanol) as well as faecal steroids (coprostanol, 5β-stigmastanol and their conversion products epicoprostanol and epi-5β-stigmastanol) represent excellent additions to the forensic toolbox for determining whether a soil of interest has ever been affected by a human cadaver or not.

    更新日期:2018-08-02
  • Universal Biomarker Analysis: Aromatic hydrocarbons
    Org. Geochem. (IF 2.81) Pub Date : 2018-07-30
    Clifford C. Walters, Frank C Wang, Meytal B. Higgins, Marlene E. Madincea

    Universal Biomarker Analysis is a GC×GC-ToFMS method that provides absolute quantification and non-targeted pseudo MS-MS for all resolved hydrocarbon components in petroleum and rock extracts. This is achieved by merging two chromatographic datasets obtained under field ionization (FI), which provides molecular ions, and electron impact (EI), which provides diagnostic fragment ions. We recently demonstrated this technique for the analysis of saturated biomarkers (Walters et al., 2018. Org. Geochem. 115, 57). Here, we show its application to aromatic biomarkers. Universal Biomarker Analysis is compared to conventional GC-MS for frequently analyzed aromatic hydrocarbons, such as the alkylated naphthalenes and biphenyls, mono- and tri-aromatic steroidal hydrocarbons to illustrate where this new method provides demonstrated advantages. The technique is well suited for the analysis of rare species not commonly analyzed in targeted GC-MS/MS analyses. This is illustrated by the tentative identification of A/B-ring C26-C29 diaromatic steroidal hydrocarbons in a crude oil, compounds that have been previously identified as minor components of immature rock extracts, and indications of previously unknown C26 methyl-triaromatic steroidal hydrocarbons.

    更新日期:2018-07-31
  • TEX86 in sinking particles in three eastern Atlantic upwelling regimes
    Org. Geochem. (IF 2.81) Pub Date : 2018-07-29
    Eunmi Park, Jens Hefter, Gerhard Fischer, Gesine Mollenhauer

    Seasonal variations in fluxes of isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs) and the estimated temperatures based on TEX86 were examined in sinking particles collected using moored sediment traps in the eastern Atlantic upwelling regions. In the equatorial Guinea Basin, GDGT fluxes show a correlation with opal fluxes, implying that GDGTs are mainly transported via aggregation with diatoms. The flux-weighted T E X 86 H temperatures derived from particles collected both at 853 m and 3,921 m depth correspond to the water temperature (24.1 °C) of ca. 50 m depth, where nitrate concentration starts to increase. This suggests that nutrient concentrations may affect the depth habitat of Thaumarchaeota, thereby influencing the TEX86-derived water temperatures. In the coastal upwelling off Namibia, T E X 86 H temperatures are similar to satellite-derived sea surface temperature (SST) during the warm season, but the record derived from the trap is delayed relative to the SST by approximately 26 days. Warm biases, however, occur during the cold season. Higher TEX86 values have been found within the oxygen minimum zones (OMZs) in the water column of coastal upwelling regions. Thus, contributions from GDGTs produced in OMZs might explain the warmer temperature estimates during the cold season in regions where OMZs are pronounced. This scenario could explain the observed warm bias off Namibia. We, therefore, suggest that in the eastern Atlantic upwelling systems, nutrient depth distribution and GDGTs produced in OMZs can be potential environmental factors influencing TEX86 in sinking particles. In paleoenvironmental records of TEX86, non-thermal signals have to be considered on regional scales.

    更新日期:2018-07-30
  • Environmental petroleum pollution analysis using ramped pyrolysis-gas chromatography–mass spectrometry
    Org. Geochem. (IF 2.81) Pub Date : 2018-07-27
    Meredith Evans Seeley, Qing Wang, Hernando Bacosa, Brad E. Rosenheim, Zhanfei Liu
    更新日期:2018-07-27
  • Assessing the metabolism of sedimentary microbial communities using the hydrogen isotopic composition of fatty acids
    Org. Geochem. (IF 2.81) Pub Date : 2018-07-19
    Sandra M. Heinzelmann, Laura Villanueva, Yvonne A. Lipsewers, Danielle Sinke-Schoen, Jaap S. Sinninghe Damsté, Stefan Schouten, Marcel T.J. van der Meer

    The hydrogen isotopic composition of fatty acids (FAs) has previously been shown to reflect the metabolism of microorganisms in pure culture, but has rarely been tested in the environment. Here, we report the abundance and hydrogen isotopic composition of polar lipid derived FAs extracted from surface sediments of the saline Lake Grevelingen (The Netherlands), at two different stations and during two seasons with oxic bottom water conditions during spring and hypoxic to anoxic conditions during late summer. These data are compared with the bacterial diversity revealed by 16S rRNA gene amplicon sequencing. All measured FAs were depleted in deuterium relative to the bottom water by 103‰ to 267‰. FAs associated with heterotrophic bacteria (i-15:0 and ai-15:0) showed the smallest fractionation (–103‰ to –185‰) while those derived from pelagic photoautotrophic phytoplankton (20:5) showed the largest fractionation (–230‰ to –267‰). Overall, the hydrogen isotope fractionation reflected in the majority of the more commonly occurring FAs (14:0, 16:0, 16:1ω7) is relatively large (–172‰ to –217‰). Together with the high relative abundance of the 20:5 FA, this suggests a substantial contribution from dead pelagic biomass settling from the water column to the sedimentary polar lipid derived FA pool and not from the in situ microbial communities. Therefore, the majority of the isotope signal in the fatty acids from surface sediments might not represent the general metabolism of the active sedimentary communities. Therefore, the input of pelagic biomass into sedimentary environments may bias the information contained in the hydrogen isotopic composition of FAs.

    更新日期:2018-07-20
  • Improved chromatography reveals multiple new bacteriohopanepolyol isomers in marine sediments
    Org. Geochem. (IF 2.81) Pub Date : 2018-07-18
    Stephanie Kusch, Sunita R. Shah Walter, Jordon D. Hemingway, Ann Pearson

    Bacteriohopanepolyols (BHPs) are characterized by a large structural diversity, although methodological constraints have limited investigations of the occurrence of isomers among composite BHPs in environmental samples. Here, we describe a novel chromatography method that uses three serial Phenomenex Kinetex C18 columns to successfully resolve new structural isomers of BHPs commonly observed in marine sediment samples. The investigated samples consistently contain a high diversity of BHP isomers, but their relative abundances differ significantly between samples. These differences in relative abundance have potential to reflect different environmental conditions such as depositional setting or redox conditions. The improved BHP resolution and baseline separation of the new method is promising for accurate quantification and future environmental proxy development and compound-specific isotope work.

    更新日期:2018-07-19
  • Quantification of lipid biomarkers in sedimentary contexts: comparing different calibration methods
    Org. Geochem. (IF 2.81) Pub Date : 2018-07-17
    Antonio V. Herrera-Herrera, Carolina Mallol

    The analysis of lipid biomarkers has gained increasing importance within environmental and archaeological fields because they are representative of particular plant and animal sources. Accurate quantification of such compounds is essential to obtaining significant information. In this paper, accuracy in the quantification of n-alkanes (as model compounds) in sedimentary contexts was studied using three different approaches: proportions of areas with regards to an internal standard, instrumental calibration and matrix-matched calibration . Alkanes were extracted by ultrasonic assisted solid-liquid extraction and analyzed by gas chromatography coupled to mass spectrometry. Instrumental calibration curves were obtained with determination coefficients (R2) higher than 0.9900. The matrix effect was studied (in the range between -100% and 140%) and matrix-matched calibration curves with R2 above 0.9904 were obtained. Matrix matched calibration provided the best results (percentages in the range 10-130%) when compared with instrumental calibration (1-142%) and no calibration (2-274%). Four sediment samples from an archaeological Neanderthal site were analyzed with the three quantification methods and a Student́s t−test demonstrated that the results obtained were significantly different: Concentrations were non-uniformly under- or overestimated, and the highest peak and the alkane profile varied with the different methods. According to these data, matrix-matched calibration should be the preferred method for quantification since results are more accurate with only a small amount of extra work. This method requires that control sediment samples are collected at the site under consideration.

    更新日期:2018-07-18
  • Origin of condensates and natural gases in the Almond Formation reservoirs in southwestern Wyoming, USA
    Org. Geochem. (IF 2.81) Pub Date : 2018-07-17
    Mei Mei, K.K. (Adry) Bissada, Thomas B. Malloy, L. Mike. Darnell, Zhengfan Liu

    Condensates and natural gases are significant resources that frequently occur together in many petroleum systems. Unraveling their origins is crucial for effective petroleum exploration and exploitation. This is often a challenge because light crudes are volatile, deficient in biomarkers, and commonly altered. The origin of condensates and natural gases in the Upper-Cretaceous Almond Formation in southwestern Wyoming was unraveled through use of improved analytical and interpretative geochemical-approaches constrained by regional geologic perspectives and basin modeling. Here, an improved GC-MS/MS method allowed simultaneous determination of saturate and aromatic biomarkers, diamondoids, and organo-sulfur compounds in the whole crudes. Furthermore, basin-specific calibration of diamondoid-based thermal-maturity parameters, through correlation of measured indices for diamondoids in rock extracts versus measured reflectance of vitrinite from the same rocks, allowed reliable interpretation of maturity of the most likely source rocks. Modified interpretation schemes for C7-hydrocarbon distributions, and compositions of C1-C30 alkanes, aromatic and organosulfur compounds, and δ13C1-C5 together indicated that the condensate and natural gas from each well are likely co-generated and thermogenic products from non-marine source rocks. Except methane is partially biogenic. The bulk thermogenic petroleum are too mature to have originated in the rocks adjacent to the producing reservoirs. The geochemical, geologic, and modeling interpretations collectively indicate that the condensates and associated natural gases have most likely generated, expulsed, and migrated from the downdip, highly mature, near-shore/transitional humic source rocks of the Almond Formation/Mesaverde Group in the Washakie Basin and Great Divide Basin at > 1.3% Ro and ∼30 Ma.

    更新日期:2018-07-18
  • Comparison of n-alkane molecular, carbon and hydrogen isotope compositions of different types of plants in the Dajiuhu peatland, central China
    Org. Geochem. (IF 2.81) Pub Date : 2018-07-17
    Bingyan Zhao, Yiming Zhang, Xianyu Huang, Ruya Qiu, Zhiqi Zhang, Philip A. Meyers

    Long chain n-alkanes preserved in peat deposits have been widely used to reconstruct paleoenvironmental and paleoecological changes. However, our knowledge of how the leaf wax molecular and isotopic compositions of peat sequences might respond to changes in plant communities is limited. This study investigates the n-alkane molecular, carbon and hydrogen isotopic compositions of plant species collected from the Dajiuhu peatland, central China. The results show that forb and graminoid plants growing in peatlands have a relatively higher carbon preference index of long-chain n-alkanes than shrubs, ferns and aquatic plants. The δ13C values of individual n-alkanes generally decrease with increasing chain lengths, and different plant groups have similar δ13C values of C31 and C33n-alkanes. An exception is Sphagnum, which has more negative δ13C values of C23 and C25 than those of its longer chain length homologues. Relative to peat pore water δD values in different seasons and different peat depths, the Dajiuhu plants normally exhibit relatively large hydrogen isotope fractionations, and the fractionations gradually increase from graminoid to forb to shrub. Taken together, these results highlight the influence of different kinds of vegetation on the collective molecular and isotopic ratios of n-alkanes in peat deposits.

    更新日期:2018-07-18
  • High abundance of alkylated diamondoids, thiadiamondoids and thioaromatics in recently discovered sulfur-rich LS2 condensate in the Tarim Basin
    Org. Geochem. (IF 2.81) Pub Date : 2018-07-11
    Guangyou Zhu, Ying Zhang, Zhiyao Zhang, Tingting Li, Nannan He, Kliti Grice, Yuan Neng, Paul Greenwood

    Molecular and stable sulfur isotopic (δ34S) analysis of petroleum recently discovered in the Bachu uplift of the Tarim Basin (China) was conducted to characterize the oil and assist future regional petroleum correlation studies. Sulfur rich condensate from the Lower Ordovician – Upper Cambrian reservoir rock in the LS2 well showed abundant aromatic hydrocarbons and organic sulfur compounds. Extensive distributions of alkylated caged hydrocarbons (i.e., diamondoids) and caged sulfur compounds (thiadiamondoids), including many additional isomers not previously detected in petroleum, were resolved by GC×GC-MS analysis. The thiadiamondoid products and thioaromatics (i.e., benzothiophenes, dibenzothiophenes), also detected in LS2, products were attributed to a major thermochemical sulfate reduction (TSR) event, which resulted in elevated H2S (3.66%) and dry gas in the condensate. TSR might also have contributed to the generally heavier δ34S values of the benzothiophenes (+26 to +28 ‰) compared to dibenzothiophenes (+20 to +27 ‰). The LS2 reservoir has no sulfate evaporites and the temperature (144 °C) is relatively low for TSR, implying that it is a secondary reservoir for migrated TSR-derived gas, possibly derived from deeper and hotter (>200 °C) Cambrian strata.

    更新日期:2018-07-12
  • Spatial and temporal shifts in fluvial sedimentary organic matter composition from a High Arctic watershed impacted by localized slope disturbances
    Org. Geochem. (IF 2.81) Pub Date : 2018-07-09
    David M. Grewer, Melissa J. Lafrenière, Scott F. Lamoureux, Myrna J. Simpson

    Arctic warming may induce slope failure in upland permafrost soils. These landslide-like events, referred to as active layer detachments (ALDs), redistribute soil material into hydrological networks during spring melt and heavy rainfall. In 2011, 2013 and 2014, fluvial sediments from the West River at the Cape Bounty Arctic Watershed Observatory were sampled where ALDs occurred in 2007–2008. Two ALD-impacted subcatchments were examined exhibiting either continuing disturbance or short-term stabilization. Solid-state 13C nuclear magnetic resonance (NMR) spectroscopy and targeted biomarker analysis via gas chromatography–mass spectrometry were used to investigate shifts in organic matter (OM) composition. Additionally, radiocarbon ages were determined using accelerator mass spectrometry. Biomarker concentrations and O-alkyl carbon assessed via NMR were both lower in sediments nearest the active disturbance and increased in sediments downstream where other aquatic inputs became more dominant. This suggests immobilization of recalcitrant OM near the ALD and the sustained transport of labile ALD-derived OM further downstream. Shifts toward older radiocarbon dates along the river between 2011 and 2014 suggest the continued transport of permafrost-derived OM downstream. The stabilizing subcatchment revealed high O-alkyl carbon via NMR and increased concentrations of unaltered terrestrial-derived biomarkers indicative of enhanced OM accumulation following ALD activity. The relatively young radiocarbon ages from these sediments suggest accumulation from contemporary sources and potential burial of the previously dispersed ALD inputs. Within the broader context of Arctic climate change, these results portray a complex environmental trajectory for thaw-released permafrost-derived OM and highlight uncertainty in the relationship between lability and persistence upon release by permafrost disturbance.

    更新日期:2018-07-10
  • A novel method to measure the 13C composition of intact bacteriohopanepolyols
    Org. Geochem. (IF 2.81) Pub Date : 2018-07-09
    Jordon D. Hemingway, Stephanie Kusch, Sunita R. Shah Walter, Catherine A. Polik, Felix J. Elling, Ann Pearson

    We present a novel method to measure the 13C/12C isotope ratio (reported as δ13C) of individual, intact bacteriohopanepolyols (BHPs) using semi-preparative ultrahigh pressure liquid chromatography (UPLC) followed by high temperature gas chromatography–isotope ratio mass spectrometry (HT-GC–IRMS). The method is reproducible, as indicated by the precision of δ13C values for bacteriohopanetetrol (BHT) extracted from R. palustris biomass and analyzed across an order-of-magnitude range of IRMS peak areas (δ13C = –33.4 ± 0.6‰ VPDB, n = 94, ± 1σ). To show that this method successfully separates individual BHPs from complex environmental matrices, we report δ13C values for BHT and the BHT stereoisomer (BHT-II) from a ca. 2.9 Ma, organic-rich Mediterranean Sea sediment sample. These analyses suggest that intact BHP δ13C measurements can greatly improve the interpretation of environmental signals by alleviating the need for side-chain cleavage which reduces BHP source-specificity.

    更新日期:2018-07-09
  • The composition and radiolysis impact on aromatic hydrocarbons in sedimentary organic matter from the Mulga Rock (Australia) uranium deposit
    Org. Geochem. (IF 2.81) Pub Date : 2018-07-03
    Paul F. Greenwood, Chao Shan, Alex I. Holman, Kliti Grice

    Our interest in the effect of ionizing radiation on sedimentary organic matter (OM) has extended to the molecular distributions and stable isotopic values of aromatic hydrocarbons. Thermally immature sediments (%Ro = 0.26; Tmax < 421 °C) spanning a 173–5280 ppm radiolytic gradient from the Mulga Rock Uranium Deposit (Western Australia) contained high abundances of vascular plant sourced aromatic (and aliphatic) terpenoids. Terpenoids with 2–5 rings and various levels of aromatisation were detected in the aromatic fractions of these sediments. The extent of aromatisation increased with sedimentary U/radiolytic levels, attributed to free-radical promoted reactions including dehydrogenation and oxidation of aliphatic precursors. The sediment with the highest U content (5280 ppm) showed only diterpenoids and low MW PAHs, reflecting complete radiolytic removal of higher MW terpenoids. A larger number and variety of terpenoid products were detected from the other sediments (U ≤ 1700 ppm) from which several radiolytically promoted reaction pathways were identified. Cadinene, for instance, was subject to competing hydrogenation and dehydrogenation reactions which gave cadinane and progressively aromatised diterpenoid analogues, respectively. Pentacyclic terpane precursors were also progressively aromatised and partially converted to des-A-triterpenoid or C-ring cleaved triterpenoid (tetracyclic) products. These reactions are typical of the natural diagenetic transformation of plant terpenoids, with the main effect of the high radiolytic levels an apparent hastening of diagenetic processes. The δ13C values of several aromatised products indicated a slight 13C enrichment with increasing aromatisation, likely due to preferential abstraction of H from 12C.

    更新日期:2018-07-04
  • A novel heterocyst glycolipid detected in a pelagic N2-fixing cyanobacterium of the genus Calothrix
    Org. Geochem. (IF 2.81) Pub Date : 2018-06-30
    Nicole J. Bale, Ellen C. Hopmans, Denise Dorhout, Lucas J. Stal, Michele Grego, Judith van Bleijswijk, Jaap S. Sinninghe Damsté, Stefan Schouten

    Previous studies have shown that heterocyst glycolipids (HGs) are unique markers for N2-fixing heterocystous cyanobacteria. In this study, the HGs of a marine pelagic Calothrix sp. CCY1611 isolated from the tropical western North Atlantic were analyzed by ultra-high pressure liquid chromatography–high resolution mass spectrometry and it was shown that this organism contains an unusual C28 triol HG with a methylated C6 sugar (methyl-HG28 triol) head group. Gas chromatography–mass spectrometry analysis of the sugar released from the novel HG by acid methanolysis revealed that the sugar is likely 6-O-methyl-β-D-glucopyranose. We propose that this methyl-HG28 triol is a potential biomarker for pelagic members of the genus Calothrix.

    更新日期:2018-07-01
  • Methylated naphthalenes as indicators for evaluating the source and source rock lithology of degraded oils
    Org. Geochem. (IF 2.81) Pub Date : 2018-06-30
    Kenta Asahina, Noriyuki Suzuki

    Methylated naphthalenes in oils/condensates are relatively resistant to bio- and thermal degradation and can be useful compounds for evaluating the source organic matter and source rock lithology. Assuming that cadinene, abietic acid, and cholesterol are major precursors of methylated naphthalenes in oils and condensates, laboratory hydrous pyrolysis experiments for these possible precursors were performed to understand the precursor/products relationship. The major dimethylnaphthalenes (DMNs) and trimethylnaphthalenes (TMNs) in pyrolyzates of cadinene and abietic acid were 1,2,5-TMN and 1,6- and 1,5-DMNs, whereas those in pyrolyzates of cholesterol were 2,3,6-TMN and 1,3- and 1,7-DMNs. The experimental results suggested that the dimethylnaphthalene ratio (DMR) of [1,5- + 1,6-DMNs]/[1,3- + 1,7-DMNs], and the trimethylnaphthalene ratio (TMR) of 1,2,5-TMN/2,3,6-TMN, can be potential indicators of source organic matter. The relative abundance of TMNs in the pyrolyzates was controlled largely by the maturity level and presence or absence of montmorillonite catalyst. The relative abundance of total TMNs to total MNs (ΣTMNs/ΣMNs ratio) can be a novel indicator for evaluating the clay-catalytic effect in source rocks. A lower ΣTMNs/ΣMNs ratio is related to clay-poor source rocks, such as coal, carbonates, and siliceous rocks, whereas a higher ΣTMNs/ΣMNs ratio suggests shaly (clay-rich) source rocks. The DMR, TMR, and ΣTMNs/ΣMNs ratios of various oils and condensates are very consistent with their source organic matter and source rock lithology, indicating that these indicators are practically useful.

    更新日期:2018-07-01
  • The Structural and Dynamical Role of Water in Natural Organic Matter: a 2H NMR and XRD Study
    Org. Geochem. (IF 2.81) Pub Date : 2018-06-25
    Raju Nanda, U. Venkateswara Reddy, Geoffrey M. Bowers, Mark Bowden, R. James Kirkpatrick

    Natural organic matter (NOM) is an important component in many near-surface geochemical environments, and its properties are greatly affected by the incorporation of water. Because of its importance, the macroscopic behavior and effects of water in NOM and soil organic matter (SOM) have been extensively studied using a wide range of experimental and computational methods. The molecular scale structural and dynamical behavior of water in these materials, however, is less well understood. This paper presents a variable temperature 2H NMR and XRD study of water in Suwannee River NOM and its fulvic acid (FA) and humic acid (HA) fractions that provides new insight into the dynamical behavior of structurally different types of water and exchangeable hydrogen environments in NOM. The results provide a basis for future studies of more complex natural organic materials and the interaction of organic materials with mineral surfaces. Room temperature 2H NMR spectra of samples hydrated in 2H2O and then dehydrated distinguish2H2O molecules that are in rapid reorientational motion (correlation times, νc, > 105 Hz), 2H exchanged onto carboxylic sites of the NOM that do not undergo rapid reorientation at frequencies > ∼103 Hz, and 2H exchanged onto phenolic and possibly other alcohol sites of the NOM that undergoes rapid but anisotropic dynamical reorientation. For samples exposed to water and not dried, the XRD results collected at temperatures from 173 to 298K show the formation of ice-1h in samples exposed to 100% relative humidity (R.H.) but not in samples exposed to 43% R.H. 2H NMR of those samples collected at temperatures from 313K to 173K show the presence of multiple sites. Near room temperature, the spectra contain a narrow resonance for mobile water undergoing rapid isotropic motion, and a broader symmetrical resonance probably due to a combination of more dynamically restricted water molecules and 2H exchanged onto phenolic and alcohol functional groups undergoing rapid anisotropic motion. The 43% R.H. samples also yield a broader quadrupole-dominated resonance for 2H exchanged onto functional groups of the NOM. With decreasing temperature, the resonances for dynamically restricted water molecules and 2H exchanged onto phenolic and alcohol functional groups become broader, reflecting a decreasing rate of exchange between the water molecules and functional groups and a decreasing rate of reorientation of the 2H2O molecules. The formation of ice-1h is directly reflected in the 2H spectra of the 100% R.H. samples as a resonance with a quadrupole coupling constant (QCC) of ∼ 180 kHz. For the 43% R.H. samples, there is also a broad, poorly resolved resonance with typical QCCs of ∼ 180 kHz for which the relative signal intensity increases with decreasing temperature. This signal represents 2H2O molecules that are not crystallized in ice-1h but have greatly reduced reorientation frequencies at low temperature and a hydrogen bonding network with hydrogen bond strengths similar to but somewhat weaker than ice-1h. Such molecules are also likely to be present in the 100% R.H. samples. At both R.H.s, some of the 2H2O molecules do not freeze and retain their isotropic motion down to 173K, the lowest temperature investigated.

    更新日期:2018-06-27
  • Thermal alteration of biomarkers in the presence of elemental sulfur and sulfur-bearing minerals during hydrous and anhydrous pyrolysis
    Org. Geochem. (IF 2.81) Pub Date : 2018-06-25
    Liangliang Wu, XinYan Fang, Shuhuan Ji, Ansong Geng

    Although elemental sulfur and sulfur-bearing minerals are not the main constituents of sedimentary rock, they are still important for the formation and destruction of biomarkers. In this study, a bitumen of Sichuan basin mudstone with abundant biomarkers was separately pyrolyzed (under both hydrous and anhydrous conditions) with elemental sulfur (S0) and sulfur-bearing minerals (including pyrite, ferrous sulfate, and ferric sulfate) at various temperatures (300, 330 and 350 °C). The results show that the effects of different forms of sulfur on the evolution of biomarkers vary. Pyrite (FeS2) had only a slight influence on the characteristics of the biomarkers during anhydrous and hydrous pyrolysis. On the other hand, the presence of S0, ferrous sulfate (FeSO4) and ferric sulfate (Fe2(SO4)3) promoted the thermal cracking of the biomarkers and changed the biomarker distributions under anhydrous conditions. The extent of biomarker thermal alterations decreased in the following order: S0 > Fe2(SO4)3 > FeSO4 > FeS2. Additionally, the presence of water seemed to promote the effects of the sulfur additive on the changes in biomarker compositions, but this did not change their raking in terms of influence. The elemental sulfur alteration of the biomarkers increased with pyrolysis temperature (simulated maturity) and the abundance of elemental sulfur in the sample. The results obtained offer new insights into how biomarkers evolve when elemental sulfur and sulfur-bearing minerals are present.

    更新日期:2018-06-27
  • The effect of biodegradation on adamantanes in reservoired crude oils from the Bohai Bay Basin, China
    Org. Geochem. (IF 2.81) Pub Date : 2018-06-20
    Xiong Cheng, Dujie Hou, Changgui Xu

    The effect of biodegradation on adamantanes has been studied based on a suite of genetic related crude oils with increasing degree of biodegradation from the Miaoxi Depression, Bohai Bay Basin, China. Selectivity during biodegradation was observed for adamantanes. The relative susceptibility to biodegradation of individual adamantane compounds was determined based on their compositional changes. Adamantanes with more alkyl substituents are generally more resistant to biodegradation, but exceptions also occur. The position of alkylation has great control on the susceptibility to biodegradation of adamantanes. However, the more thermally stable compounds are not always more readily biodegraded, suggesting that the susceptibility of adamantanes is not directly controlled by thermodynamic stability. Selective biodegradation of adamantanes may be related to their stereochemical structures and demethylation could have occurred. Microbial alteration of adamantanes has negligible effect on methyladmantane index, dimethyladamantane index 1, dimethyladamantane index 2, trimethyladamantane index1 and trimethyladamantane index 2, suggesting that they could be used to evaluate the thermal maturity of biodegraded crude oils. In contrast, the ethyladmantane index was significantly altered and thus is not valid for maturity assessment for biodegraded oils.

    更新日期:2018-06-22
  • Differences in long-term vs short-term carbon and nitrogen sequestration in a coastal river delta wetland: implications for global budgets
    Org. Geochem. (IF 2.81) Pub Date : 2018-06-19
    R.D. DeLaune, J.R. White, T. Elsey-Quirk, H.H. Roberts, D.Q. Wang
    更新日期:2018-06-20
  • Influence of land use on distribution of soil n-alkane δD and brGDGTs along an altitudinal transect in Ethiopia: Implications for (paleo)environmental studies
    Org. Geochem. (IF 2.81) Pub Date : 2018-06-15
    Andrea Jaeschke, Janet Rethemeyer, Michael Lappé, Stefan Schouten, Pascal Boeckx, Enno Schefuß

    The combined use of plant wax n-alkane δD values and branched glycerol dialkyl glycerol tetraether lipid (brGDGT) distributions provides a novel approach for paleoaltitude reconstruction. Studies from East Africa revealed, however, inconsistent results between the proxy estimates and altitudinal parameters. Here, we explore these proxies in soils of different land use (forest, cropland and pasture) along an altitudinal transect in the Jimma zone of the southwest Ethiopian highlands to better understand environmental and plant-specific factors controlling the isotopic composition and distribution of n-alkanes and brGDGTs. The hydrogen isotope composition of individual n-alkanes does not unambiguously reflect the altitude effect on precipitation δD, but seems largely influenced by the specific land use. Only forest soil-derived n-C27 and n-C29 alkane δD values exhibit a significant linear relationship with altitude (r -0.87, p < 0.05), likely reflecting the most stable ecosystem. The resulting lapse rate of -17‰/1000 m is comparable with that of local precipitation in the southwest Ethiopian highlands. In addition, the linear correlation of the average chain length (ACL) and δ13C values of forest soil n-alkanes suggests a physiological adaptation of the specific plant type waxes to altitude-induced environmental changes in the study area. The distribution of brGDGTs also reveals a significant linear correlation with altitude (r -0.97, p < 0.01), reflecting the decrease in temperature with higher elevation, independent of land use. In addition, brGDGT-based mean annual air temperature (MAT) estimates of 19.5°C to 14.0°C and temperature lapse rate of -6°C/1000 m are in good agreement with direct measurements in the Jimma zone. In contrast to previous studies from East Africa, our results show that both soil n-alkane δD values and brGDGT-based MAT distributions track present day altitude effects on local environmental gradients in the southwest Ethiopian highlands.

    更新日期:2018-06-15
  • The enigmatic structure of the crenarchaeol isomer
    Org. Geochem. (IF 2.81) Pub Date : 2018-06-15
    Jaap S. Sinninghe Damsté, W. Irene C. Rijpstra, Ellen C. Hopmans, Mimi J. den Uijl, Johan W.H. Weijers, Stefan Schouten
    更新日期:2018-06-15
  • Use of palmitoleic acid and its oxidation products for monitoring the degradation of ice algae in Arctic waters and bottom sediments
    Org. Geochem. (IF 2.81) Pub Date : 2018-06-06
    Jean-François Rontani, Rémi Amiraux, Catherine Lalande, Marcel Babin, Hak-Ryul Kim, Simon T. Belt

    Degradation of palmitoleic acid (C16:1ω7), the main fatty acid component of sea ice-associated (sympagic) diatoms, was monitored in Arctic sea ice at the beginning of ice melting and in the underlying sinking particles and superficial bottom sediments. In sea ice, degradation of sympagic algae involved biotic oxidation induced by 10S-DOX-like lipoxygenase of unknown salinity-stressed attached bacteria, while photo- and autoxidation were limited. In the water column, strong hydratase and Z/E isomerase activity were observed. Hydration of unsaturated fatty acids seems to be a detoxification strategy, which is essential for bacterial survival when associated with free fatty acid-rich environments such as ice algae. In contrast, Z/E isomerisation of palmitoleic acid was attributed to the release of Fe2+ ions during radical-induced damage of the active site of the bacterial 10S-DOX-like lipoxygenase and Z/E isomerases. Due to the poor physiological state of their attached bacteria resulting from salinity stress in brine channels or toxicity of free ice algae fatty acids, sympagic algae appeared to be only very weakly biotically degraded within the water column. In bottom sediments, free radicals resulting from 10S-DOX-like lipoxygenase activity induced a strong autoxidation of the ice algal material. The presence in bottom sediments of a significant proportion of oxidation products resulting from 10S-DOX-like lipoxygenase activity attested to the strong contribution of sea ice-derived OM released during the early stages of ice melt prior to deposition in the sediments. However, on the basis of the highest fatty acid photooxidation state observed in these sediments, an additional contribution of highly photooxidized material (ice algal material released at the end of ice melting or open water phytoplankton) seems likely. The degradation of hydroperoxides, resulting from biotic and abiotic degradation of palmitoleic acid, appeared to involve: (i) homolytic cleavage of the peroxyl group affording the corresponding hydroxy- and oxoacids, (ii) reduction to the corresponding hydroxyacids by peroxygenases, (iii) heterolytic proton-catalysed cleavage and (iv) conversion to allylic 1,4-diols by diol synthases and hydroperoxide isomerases.

    更新日期:2018-06-07
  • Linking the geochemistry of crude oils and petroleum inclusions in the Ubarana and Lorena oilfields, Potiguar Basin, Brazilian Equatorial Margin
    Org. Geochem. (IF 2.81) Pub Date : 2018-06-05
    André Luiz Silva Pestilho, Lena Virgínia Soares Monteiro, Carlos Alberto Carbonezi, Sandra Brandão Jorge, Eugênio V. Santos Neto

    A study of petroleum inclusions based on ultraviolet fluorescence petrography, microspectrophotometry and biomarker analyses was carried out on samples from two wells of the onshore Lorena and the offshore Ubarana oilfields, Potiguar Basin, Brazil. This study aimed to evaluate the contribution and compositional variability of oils associated with different source rocks through the geochemical correlation of petroleum inclusions and crude oils. Three groups of inclusions were identified: (i) a group from the Lorena reservoir (Pendência Formation); (ii) a second from the Ubarana siliciclastic reservoir (Açu Formation); and (iii) a third from the carbonates adjacent to the Ubarana reservoir (Ponta do Mel Formation, PML). Fluorescence microspectrophotometry shows that the three groups of inclusions have different API gravity oils, variable proportion of saturate, aromatic, resins and asphaltenes and are not biodegraded. Moreover, Pendência and PML inclusions suggest multiple pulses of hydrocarbon migration, while Açu inclusions record a homogeneous pulse. Biomarkers indicate that the Pendência and Açu inclusion oils correlate with reservoir oil extracts. The Pendência inclusion oils are similar to the lacustrine oils of the Umbuzeiro graben (Pendência Formation), whereas the Açu inclusion oil displays closer biomarker signature to the Areia Branca trend of mixed oils (Alagamar Formation source rock). However, the oil from fluid inclusions of the PML has characteristics of the Alagamar Formation lacustrine organic facies. The absence of mixed oils in the PML suggests an earlier lacustrine oil migration through the carbonate fractures prior to the main infill of mixed oils in the Ubarana oilfield.

    更新日期:2018-06-06
  • Wax lipids in fresh and charred anatomical parts of the Celtis australis tree: Insights on paleofire interpretation
    Org. Geochem. (IF 2.81) Pub Date : 2018-05-31
    Margarita Jambrina-Enríquez, Antonio V. Herrera-Herrera, Carolina Mallol
    更新日期:2018-06-01
  • Geochemical characterization of lacustrine and marine oils from off-shore Brazilian sedimentary basins using negative-ion electrospray Fourier transform ion cyclotron resonance mass spectrometry (ESI FTICR-MS)
    Org. Geochem. (IF 2.81) Pub Date : 2018-05-30
    Ygor dos Santos Rocha, Rosana Cardoso Lopes Pereira, João Graciano Mendonça Filho

    Lacustrine and marine crude oils from different off-shore Brazilian basins were analyzed using a 7.2 Tesla LTQ FTICR-MS instrument. The samples were analyzed via electrospray ionization in the negative ion mode focusing on the polar compounds, i.e., nitrogen-, sulfur-, and oxygen-containing (NSO) compounds. We also employed a combination of other geochemical methods, such as GC-FID and GC-MS analyses, to characterize and assess the depositional environments of the different oil families. The results indicate that lacustrine oils tend to be enriched in Nx compounds, while marine oils show preference for Ox compounds. The dominant heteroatomic classes in crude oils are N1, followed by O1, O2, and N1O1 with remarkable differences in their distributions between marine and lacustrine, strongly suggesting the control by the kerogen type of the heteroatomic compounds found in these crude oils. Considerable differences in the DBE distribution of the main classes analyzed between the crude oils allowed an efficient geochemical characterization regarding their origin. The use of negative ESI FTICR-MS as a geochemistry tool can provide additional information beyond that obtained with currently employed geochemical methods, resulting in the full comprehension of crude oil composition.

    更新日期:2018-05-31
  • Cyclopentanones and 2-cyclopenten-1-ones as major products of hydrous pyrolysis of immature organic-rich shales
    Org. Geochem. (IF 2.81) Pub Date : 2018-05-30
    Chunqing Jiang, Julito Reyes, Lloyd R. Snowdon, Marina Milovic, Rachel Robinson, Shunxin Zhang, Derek Armstrong, Denis Lavoie

    Cyclopentanone is an important platform chemical for various industrial syntheses. It has been recently reported as a major product in biomass cellulose-derived biofuels but has rarely been reported in geochemical research. We report here the abundant occurrence of cyclopentanone, 2-cyclopenten-1-one and their alkyl derivatives in pyrolysates of organic-rich shales. Closed system pyrolyses at temperatures from 290 °C to 350 °C have been carried out on thermally immature organic-rich shale samples of Ordovician and Devonian age from Canada. GC–MS analyses of the pyrolysates show that cyclopentanones, 2-cyclopenten-1-ones and phenols dominate over hydrocarbons in the low temperature hydrous pyrolysates, and their abundance relative to hydrocarbons decreases with increasing thermal maturation. However, these low molecular weight cyclic ketones are only minor components compared with hydrocarbons in anhydrous pyrolysates, indicating that water has played an important role in the formation of these oxygen-containing compounds. In addition, n-alkan-2-ones in the range of C6−C35 are also detected in the hydrous pyrolysates, but they are at much reduced concentrations compared with the cyclic ketones. The potential precursors in the shale and the likely chemical reaction mechanism for the formation of these cyclic ketones are discussed in detail in the paper.

    更新日期:2018-05-31
  • What is the meaning of Hydrogen-to-Carbon ratio determined in Archean organic matter?
    Org. Geochem. (IF 2.81) Pub Date : 2018-05-30
    Frédéric Delarue, Sylvie Derenne, Kenichiro Sugitani, François Baudin, Frances Westall, Barbara Kremer, Romain Tartèse, Adriana Gonzalez, François Robert

    The search for hydrocarbon molecular biomarkers in Archean metasediments is of prime importance for deciphering the early evolution of life. Suitable criteria are required to identify promising targets for further molecular biomarkers. Possible criteria include the Hydrogen-to-Carbon (H/C) atomic ratio used as a proxy of the aliphatic content of the kerogen matrix. However, H/C ratio values exhibit large variation in Archean kerogens and their significance remains poorly understood. In this study, we thus investigate the significance of the H/C ratios of Archean kerogens by combining elemental analyses, Nanoscale Secondary Ion Mass Spectrometry (NanoSIMS), Rock-Eval pyrolysis and Raman spectroscopy. First, NanoSIMS investigations show the H/C ratio of kerogen can be compromised by residual minerals. In addition, Rock-Eval pyrolysis underlines the fact that thermal cracking of Archean kerogens does not just release hydrocarbon covalently linked to the macromolecular network but also a complex mixture of organic pools distinguished by their thermal maturity. Therefore, the H/C ratio alone cannot be used to probe the preservation of aliphatic compounds bound to kerogen since it can be biased by the presence of (i) residual bitumen, as well as (ii) refractory organic matter in secondary hydrothermal veins whose syngenecity is debatable. Rock-Eval pyrolysis then provides a useful and complementary method to check the significance of H/C atomic ratio as a proxy for hydrocarbon preservation in Archean kerogens.

    更新日期:2018-05-31
  • Variable apparent hydrogen isotopic fractionation between sedimentary n-alkanes and precipitation on the Tibetan Plateau
    Org. Geochem. (IF 2.81) Pub Date : 2018-05-25
    Juzhi Hou, Qian Tian, Mingda Wang

    The hydrogen isotope ratio values of sedimentary leaf wax n-alkanes (δDwax) have been considered a reliable proxy for precipitation isotope ratios (δDP) across various transects on a global basis. Records of δDwax have been used to infer past changes in hydrology, climate and altimetry, assuming nearly constant apparent hydrogen isotopic fractionation (εwax-P) between sedimentary leaf wax components and precipitation. Here, we present the results of an analysis of δDwax values from the surface sediments of 22 lakes across a precipitation and vegetation gradient on the Tibetan Plateau. The δDwax values correlated significantly with mean annual precipitation δD (r 0.90) and summer precipitation δD (r 0.91), suggesting that δDwax can record δDP in very dry regions. However, εwax-P varies with annual precipitation as well as with relative humidity. Calculation using Craig-Gordon isotope models showed that the variable εwax-P values likely resulted from intensive soil evaporation and plant evapotranspiration in this relatively arid region. This implies that variable isotopic fractionation should be taken into consideration when reconstructing paleohydrology, paleoclimate and paleoaltimetry.

    更新日期:2018-05-27
Some contents have been Reproduced with permission of the American Chemical Society.
Some contents have been Reproduced by permission of The Royal Society of Chemistry.
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