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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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
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.
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.
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
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.
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.
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.
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.
Correction for the siderite effect on Rock-Eval parameters: Application to the sediments of Lake Barombi (southwest Cameroon) Org. Geochem. (IF 2.81) Pub Date : 2018-05-25 David Sebag, Yannick Garcin, Thierry Adatte, Pierre Deschamps, Guillemette Ménot, Eric P. Verrecchia
Originally developed for use in the petroleum industry, Rock-Eval pyrolysis is a technique commonly applied to lake sediments to infer paleoenvironmental reconstructions. The standard Rock-Eval parameters provide information on the amount of total organic and inorganic carbon (TOC and MinC, respectively), and are usually interpreted as proxies for the source (aquatic or terrestrial) of the primary production of organic matter (Hydrogen Index vs Oxygen Index). Although this method usually provides valuable evidence, the common presence of siderite in tropical lake sediments can alter the primary signal of the sedimentary organic matter (SOM). Indeed, the CO2 and CO released by the pyrolysis of siderite are integral to the calculation of the SOM-related standard Rock-Eval parameters. In this study, we analyze sediments from a core collected in the Lake Barombi (southwest Cameroon) and describe the impact of siderite on standard Rock-Eval parameters. We propose a workflow that allows standard Rock-Eval parameters to be corrected, based on the analysis of thermograms. The proposed corrections provide siderite-effect-free parameters, accurately reflecting the changes in sedimentary organic matter composition.
Families of reservoired crude oils from the Cangdong Sag, Bohai Bay Basin, China Org. Geochem. (IF 2.81) Pub Date : 2018-05-19 Xiong Cheng, Zhichao Mao, Rong Mao, Zhiquan Li, Quansheng Guan, Xi Chen
The organic geochemistry of 35 crude oils from six oil fields across the Cangdong Sag, Bohai Bay Basin, was characterized and an oil family classification was carried out for the first time. Four oil families were classified based on 18 parameters related to source and depositional environment using hierarchical cluster analysis. The families have broadly similar geochemical characteristics which suggest oils derived from lacustrine source rocks deposited in a reducing environment. Family A has relatively abundant C27 steranes and C30 4-methyl-24-ethylcholestanes, high Pr/Ph, C22/C21 tricyclic terpane and C31R/C30 hopane ratios, and low ETR, C26/C25 tricyclic terpane and C35S/C34S homohopane ratios. Family A shows higher thermal maturity than families C and D, but lower than family B. The most distinct feature for Family B is the high gammacerane index; they also show slightly higher C22/C21 tricyclic terpane, C31R/C30 hopane and C29/C30 hopane, lower dibenzothiophene/phenanthrene, and higher maturity than the other families. Families C and D oils are low maturity and they are more similar to each other than families A and B based on most biomarker ratios. However, family C can be distinguished from family D by slightly higher C27/C29 steranes and C30 4Me/C29 steranes, and lower C29 steranes/C30 hopane ratios. A preliminary oil–source rock correlation suggests that the oil families originated from various intervals having different organofacies within the second member of the Kongdian Formation. Family A is more widespread than the other families, supporting extensive distribution of the source rock. Family B was only found in the southern part of Kongdong slope, which suggests limited geographical distribution of the related source rock. Families C and D show similar geographical distributions and dominate Wangguantun, Shenjiapu and Shenvsi fields, suggesting that the source kitchens are near these regions.
Branched GDGT variability in sediments and soils from catchments with marked temperature seasonality Org. Geochem. (IF 2.81) Pub Date : 2018-05-19 Min Cao, Gemma Rueda, Pedro Rivas-Ruiz, Mª Carmen Trapote, Mona Henriksen, Teresa Vegas-Vilarrúbia, Antoni Rosell-Melé
The distributions of branched glycerol dialkyl glycerol tetraethers (brGDGTs) in sediments are used as a proxy measurement to infer changes in past mean annual air temperatures (MAT). When applied in high resolution sedimentary sequences, measurement of brGDGT distributions is employed to reconstruct MAT at subdecadal time scales. In addition, brGDGT proxy estimates are also sometimes purported to be seasonally biased in environments where annual brGDGT production may not be constant during a seasonal cycle. The main aim of this study was to assess the occurrence of seasonality in the production and distribution of brGDGTs, and the sesonality bias of the derived temperature proxy. For this purpose, we examined brGDGT distributions and brGDGTs-derived MAT estimates, in surface soils and settling/suspended particulate matter over one year from two sites located in the same latitude but at different altitudes, in the Catalan Pyrenees, as well as at one site in southern Norway. These locations have marked seasonal temperature cycles, which were expected to maximize the possibility of detecting any seasonal bias in the production and compositions of brGDGTs. The results show that brGDGT abundance is heterogeneous and increases with soil humidity. The brGDGT distributions and some of the brGDGT-derived proxy measurements in soils are relatively stable throughout the year and do not change significantly in the suspended particulate matter in the river or settling particulate matter in traps. Our study shows that the impact of the seasonality of temperature on brGDGT distribution was absent in the soils studied, regardless of altitude or latitude on a catchment/regional scale. As soils are likely to contain a brGDGT signature which is representative of average environmental conditions in the catchment at least over decades, brGDGT proxy reconstructions derived from soil sources are more suitable to infer variability in environmental parameters over the same timescales (i.e. decades or longer). On shorter timescales (i.e. annual), sediment downcore variability in brGDGTs is likely to be related to changes derived from in situ production and sediment sources.
Seasonal changes in dissolved organic matter composition in Delaware Bay, USA in March and August 2014 Org. Geochem. (IF 2.81) Pub Date : 2018-05-18 Leanne C. Powers, Jenna L. Luek, Philippe Schmitt-Kopplin, Barbara J. Campbell, Cédric Magen, Lee W. Cooper, Michael Gonsior
We combined Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and mass spectrometric analysis of stable carbon isotopes, a traditional method for tracking DOM sources, to investigate the variation in the complexity of water samples collected along a salinity gradient in Delaware Bay in March and August 2014. Detailed analyses of optical properties (ultraviolet and visible (UV-Vis) absorbance and excitation-emission matrix (EEM) fluorescence spectroscopy) were carried out. Results from statistical parallel factor analysis (PARAFAC) of EEM data were also used to track and further constrain fluorescent DOM (FDOM). Optical analyses and FT-ICR-MS data showed large differences between late winter and summer conditions, and the stable carbon isotopic composition was more variable in summer than in late winter. Although terrestrial inputs of DOM are evident at low salinities in both seasons, our data suggest that a significant source of FDOM, from either in situ water column production or from benthic aquatic or semi-aquatic plant communities, contributes DOM to low and mid-salinity waters in Delaware Bay during summer but not in late winter.
Inter-molecular variations of fatty acid δD in algae and submerged plants from the north-eastern Tibetan Plateau Org. Geochem. (IF 2.81) Pub Date : 2018-05-08 Hu Liu, Hong Yang, Yunning Cao, Qin Leng, Weiguo Liu
Hydrogen isotopic compositions (δD) of fatty acids (FAs) in lake sediments have been widely used in palaeoenvironmental reconstruction, but investigations on FA δD values in algae and submerged plants from natural lakes are still limited, greatly hampering the application and interpretation of their δD values in lacustrine environments. Here, we systematically investigated δD values of short-chain (C14-C18), mid-chain (C20–C24), and long-chain (C26–C30) FAs in four algae and submerged macrophytes, two algal genera Chara and Cladophora and two angiospermous genera Potamogeton and Ruppia, that commonly occur in six lakes on the north-eastern Tibetan Plateau. We obtained the following results: (i) C20–C30 FAs in each algal and submerged plant sample have similar variation patterns in δD values, suggesting that δD values of C20–C30 FAs have consistent responses to their source water δD. In C4-like taxa (Potamogeton, Ruppia, and Chara), the C16 and C26 FAs have minor differences in their δD values, while in the C3-like Cladophora, δD values of C16 FAs are significantly more negative (avg. –48 ± 7‰) than that of the C26 FAs. We propose that different photosynthetic mechanisms in these algae and submerged plants account for the different inter-molecular δD variations between C16 and C26 FAs; (ii) δD values of C26 FAs in the carbonate-coated alga Chara are significantly correlated with the δ18O values of carbonate encrustations – such a co-variance suggests that in addition to lake water, other factors may also influence both lipid synthesis and carbonate encrustation building up in Chara, leading to a significantly positive apparent hydrogen isotope fractionation factors (εFA-W) between C26 FAs of Chara and lake water; (iii) δD values of C26 FAs from Potamogeton, Ruppia and Cladophora are correlated well with lake water δD values (R2 = 0.84, p < 0.001, n = 19), suggesting that the δD values of long-chain FAs potentially record δD variations of lake water with the apparent hydrogen isotope fractionation factors (εFA-W) of C26 FAs ranging from –153‰ to –134‰. Our results highlight the impact of different photosynthetic pathways on the inter-molecular difference of FA δD values in algae and submerged plants and illustrate both the potential and the challenge of applying their FA δD values as a palaeo-hydrology proxy.
Effects of legacy metabolites from previous ecosystems on the environmental metabolomics of the brine of Lake Vida, East Antarctica Org. Geochem. (IF 2.81) Pub Date : 2018-05-07 Luoth Chou, Fabien Kenig, Alison E. Murray, Christian H. Fritsen, Peter T. Doran
Lake Vida, located in an closed basin in the McMurdo Dry Valleys, East Antarctica, permanently encapsulates an interstitial anoxic, aphotic, cold (-13°C), brine ecosystem within 27+ m of ice. Metabolically active, but cold-limited, slow-growing bacteria were detected in the brine. Lake Vida brine is derived from the evaporation of a body of water that occupied the same basin prior to ∼2800 years ago. The characteristics of this body of water changed over time and, at one point, likely resembled other modern well-studied perennial ice-covered lakes of the dry valleys. We characterized the dichloromethane-extractable fraction of the environmental metabolome of Lake Vida brine in order to constrain current and ancient biogeochemical processes. Analysis of the dichloromethane-extract of Lake Vida brine by gas chromatography-mass spectrometry and comprehensive multidimensional gas chromatography-time of flight-mass spectrometry reveals the presence of legacy compounds (i.e. diagenetic products of chlorophylls and carotenoids) deriving from photosynthetic algae and anaerobic, anoxygenic photosynthetic bacteria. This legacy component dilutes the environmental signal of metabolites deriving from the extant bacterial community. The persistence of legacy metabolites (paleometabolites), apparent in Lake Vida brine, is a result of the slow turnover rates of the extant bacterial population due to low metabolic activities caused by the cold limitation. Such paleometabolites may also be preserved in other cold-limited or nutrient-depleted slow-growing ecosystems. When analyzing ecosystems with low metabolic rates, the presence of legacy metabolites must first be addressed in order to confidently recognize and interpret the environmental metabolome of the extant ecosystem.
Assessment of apolar lipids in subseafloor rocks and potential contaminants from the Atlantis Massif (IODP Expedition 357) Org. Geochem. (IF 2.81) Pub Date : 2018-05-07 Katherine A. Hickok, Tran B. Nguyen, Susan Q. Lang
International Ocean Discovery Program Expedition 357 drilled 17 boreholes across the Atlantis Massif with the goals of investigating carbon cycling and the presence of life in a zone of active serpentinization. The expedition recovered multiple lithologies including gabbros, basalts, carbonate sands, and serpentinites. A subset of contrasting lithologies were analyzed for apolar lipid content to determine if non-volatile organic molecules can be detected in the oceanic subsurface. The definitive detection and identification of abiotic and biological lipids in the subsurface of an actively serpentinizing system would be a significant step towards understanding a variety of scientific processes, including the evolution of pre-biotic chemistry, microbial habitability, and the global carbon cycle. Given the high potential for contamination during drilling, a suite of materials used in sample collection and processing were also analyzed to characterize their signatures. An n-alkane series ranging from C18 to C30 with δ13C isotopic values of –30.9‰ to –28.8‰ was present in lithologically diverse samples. Multiple lines of evidence point to the rock saw used to remove core exteriors during sample processing as the source of these compounds. Many of the other sample-handling procedures designed to reduce surface contamination were determined to be effective and could be implemented in future projects. This result highlights the value of careful prevention and characterization of contamination to allow for more accurate interpretations of complex and dynamic subsurface processes, and the importance that future reports of these compounds occurs in conjunction with thorough contamination assessments.
Production and turnover of microbial organic matter in surface intertidal sediments Org. Geochem. (IF 2.81) Pub Date : 2018-04-17 Weichao Wu, Travis Meador, Kai-Uwe Hinrichs
Benthic microorganisms play a significant role in the remineralization of sedimentary organic matter (OM). In order to constrain the growth rate, turnover and carbon metabolism of microbial communities in anoxic sandy sediments, we quantified the rate of inorganic carbon (IC) assimilation and of lipid production via a recently developed dual isotope-labelling assay that can differentiate autotrophic vs. heterotrophic production in a 22 cm sediment core from the Janssand tidal flat (Wadden Sea, Germany). Despite an extremely low concentration of total organic carbon (TOC) in the majority of samples (< 0.3%), the concentration of total fatty acids (TFAs) was in the range 4.5 to 28.4 μg/mg TOC, suggestive of a high contribution of fresh microbial and algal biomass to the TOC pool. This was corroborated by a production rate of microbial FAs, which ranged from 0.3 to 4.7 μg TFAs/gdw/yr (where dw = dry wt) and the mean turnover time of microbial FAs was 6 ± 5 yr. The enhanced production rate of iso- and anteiso-branched FAs was consistent with the presence of an active population of sulfate reducing Deltaproteobacteria. The assimilation rate of inorganic carbon (IC) into bacterial lipids was relatively low (0.16 ± 0.07 μg C/gdw/yr) in OM-lean sandy sediments (i.e. upper 17 cm), such that the IC assimilation to lipid production ratio values was typically < 0.3, indicating that heterotrophic bacteria were dominant and dark IC fixation played a minor role at the study site. The measured rates of FA production converted to anaerobic heterotrophic C demand of 0.4-1.8 µg C/gdw/d, assuming a bacterial growth efficiency of 10%. Such high biomass proportion, production rate and C demand suggested that dissolved OM must play a vital role in sustaining the active heterotrophic microbial populations in these sandy sediments.
Faulting controls on oil and gas composition in the Yingmai 2 Oilfield, Tarim Basin, NW China Org. Geochem. (IF 2.81) Pub Date : 2018-04-17 Yangyang Wang, Jianfa Chen, Xiongqi Pang, Yongfu Liu, Zheya Chen, Guangping Luo, Guoqiang Zhang, Lamei Huang
The accumulation mechanism of hydrocarbons in the Yingmai 2 Oilfield of the Tabei Uplift in the Tarim Basin is not well understood. Here, the characteristics and formation of Paleozoic faults in the Yingmai 2 Oilfield are studied with new 3D seismic data and regional geologic data. Inclusion features and geochemical parameters are used to investigate the faults controlling the oil and gas compositions. The results indicate that three fault systems developed in the Yingmai 2 Oilfield: a NNW-trending strike-slip fault system that formed in the middle Caledonian, a NW-trending strike-slip fault system that formed in the Caledonian–late Hercynian, and a NE-trending thrust fault system that formed in the late Hercynian–Indosinian, which was associated with volcanic activity. The two strike-slip fault systems were important early hydrocarbon migration pathways in the late Caledonian–early Hercynian, late Hercynian, and Himalayan accumulation periods. The NE-trending thrust faults were sealed by volcanics, and divided the originally uniform oilfield into two accumulation systems during the Himalayan. This finding is supported by the differences in the compositions of the oils and gases in the eastern (YM area) and the western (YG area) Yingmai 2 Oilfield. The oil density, iC4/nC4, Ph/nC18, and V/Ni of the oils in the reservoirs in the YM area are lower than those in the YG area, whereas the TMNr, TeMNr, 4-1/DBT, Pr/Ph, C29-diasterane/C29-regular sterane, H2S content, and the content of Himalayan hydrocarbon inclusions are higher. The hydrocarbon accumulation in the Yingmai 2 Oilfield is attributed to multiple episodes of hydrocarbon charging and suggests that the YM area has greater exploration potential than the YG area.
Aliphatic hydrocarbon biomarkers as indicators of organic matter source and composition in surface sediments from shallow lakes along the lower Yangtze River, Eastern China Org. Geochem. (IF 2.81) Pub Date : 2018-04-17 Yongdong Zhang, Yaling Su, Zhengwen Liu, Lingyang Kong, Jinlei Yu, Miao Jin
Aliphatic hydrocarbon biomarkers from the surface sediments of lakes along the lower reaches of the Yangtze River (Eastern China) were analyzed in order to determine the origin and composition of organic matter (OM) in the sediment, which is necessary to understand how anthropogenic environmental change in the region is impacting on OM dynamics and carbon cycles. The results indicate that OM in lake sediments is derived from biogenic sources including phytoplankton, aquatic macrophytes, bacteria and terrestrial plants, and abiotic sources of petroleum hydrocarbon contamination. The significance of each source varies greatly between lakes. Inputs of autochthonous OM to sediments were closely dependent on nutrient levels and the organisms present in the lake water column. Cyanobacterial OM input, signified by the distribution of short-chain n-alkanes, was most abundant in sediments from the most severely polluted lake, West Lake Chaohu. OM derived from diatoms, indicated by C25 highly branched isoprenoids (HBIs), was relatively abundant in sediments from lakes Gucheng and Shijiu, the two studied lakes with the lowest nutrient levels. Growth of macrophytes in lakes Gucheng, Shijiu, Gehu and Yangcheng resulted in a significant accumulation of OM from these organisms in the sediments. In contrast, the input of allochthonous OM was controlled mainly by natural and anthropogenic activities around the lakes. For example, terrestrial plant OM input, deduced from the abundance and proportion of long-chain n-alkanes, was exceptionally high in the sediment of Lake Nanyi, perhaps stemmed from frequent severe flooding in the catchment. Input of fossil fuel OM reflected direct petroleum contamination of the water or the anthropogenic petroleum burning in the catchment. In addition, the allochthonous refractory OM tends to be enriched in sediments of lakes with strong hydrodynamic process. The results of this study provided a lot of details in explaining the variation of terrestrial OM dynamics and carbon cycle in response to anthropogenic activities.
Novel alkenone-producing strains of genus Isochrysis (Haptophyta) isolated from Canadian saline lakes show temperature sensitivity of alkenones and alkenoates Org. Geochem. (IF 2.81) Pub Date : 2018-04-17 Hiroya Araie, Hideto Nakamura, Jaime L. Toney, Heather A. Haig, Julien Plancq, Takashi Shiratori, Peter R. Leavitt, Osamu Seki, Ken-ichiro Ishida, Ken Sawada, Iwane Suzuki, Yoshihiro Shiraiwa
Alkenone-producing species have been recently found in diverse lacustrine environments, albeit with taxonomic information derived indirectly from environmental genomic techniques. In this study, we isolated alkenone-producing algal species from Canadian saline lakes and established unialgal cultures of individual strains to identify their taxonomical and molecular biological characteristics. Water and sediments collected from the lakes were first enriched in artificial seawater medium over a range of salinities (5–40 ppt) to cultivate taxa in vitro. Unialgal cultures of seven haptophyte strains were isolated and categorized in the Isochrysis clade using SSU and LSU rRNA gene analysis. The alkenone distributions within isolated strains were determined to be novel compared with other previously reported alkenone-producing haptophytes. While all strains produced the typical C37 and C38 range of isomers, one strain isolated from Canadian salt lakes also produced novel C41 and C42 alkenones that are temperature sensitive. In addition, we showed that all alkenone unsaturation indices (e.g., U 37 K and U 37 K ′ ) are temperature-dependent in culture experiments, and that alkenoate indices (e.g., U 37 A , U 38 A , RIA38 and A37/A38) provide alternative options for temperature calibration based on these new lacustrine algal strains. Importantly, these indices show temperature dependence in culture experiments at temperatures below 10 °C, where traditional alkenone proxies were not as sensitive. We hypothesize that this suite of calibrations may be used for reconstructions of past water temperature in a broad range of lakes in the Canadian prairies.
Change in provenance of branched glycerol dialkyl glycerol tetraethers over the Holocene in the Baltic Sea and its impact on continental climate reconstruction Org. Geochem. (IF 2.81) Pub Date : 2018-04-12 Lisa Warden, Matthias Moros, Yuki Weber, Jaap S. Sinninghe Damsté
The Baltic Sea is an enclosed basin that experienced a number of different salinity phases during the Holocene corresponding to the establishment of a connection with the North Sea. Branched glycerol dialkyl glycerol tetraethers (brGDGTs) in surface sediments and Holocene sedimentary successions from the Gotland and Arkona Basins were analyzed to examine their potential applicability as indicators for soil organic matter input, as well as their suitability for paleoclimate reconstructions. Our results show a marked change in brGDGT distributions and the branched and isoprenoid tetraether (BIT) index. The transition of the Ancylus Lake (fresh) to the Littorina Sea (brackish) phase is revealed by a large drop in the BIT index and an increase in the MBT′5Me, which reveals a large shift in provenance of the sedimentary brGDGTs. During the lake phase, brGDGTs are presumed to be primarily produced by in situ production in the water column. During the brackish phase, in situ brGDGT production in the alkaline pore waters of the surface sediments (as revealed by their high degree of cyclization) was predominant although there was evidence of occassional input of soil-derived brGDGTs. The predominant aquatic autochthonous production of brGDGTs does not allow the use of brGDGTs for continental air temperature reconstructions but they can be used for bottom water and lake temperature reconstructions during some intervals. The results from this study demonstrate that geological changes and concomittant salinity variations can be revealed by the provenance of brGDGTs.
Determination and geochemical implication of multiple series of long-chain oxygen-bearing compounds trapped in kerogen in the Lucaogou Formation, Santanghu Basin, NW China Org. Geochem. (IF 2.81) Pub Date : 2018-04-11 Bin Cheng, Jianbing Xu, Yungan Liang, Qian Deng, Yankuan Tian, Zewen Liao
Studies of occluded oxygen-bearing compounds inside kerogen are scarce, although such studies are important in understanding early-stage transformation of organic matter. A relatively low-maturity kerogen from the late Permian mudstone of the Lucaogou Formation of the Santanghu Basin, northwestern China, was successively extracted with n-hexane, acetone and dichloromethane to release the adsorbed components, and the extracted kerogen was treated with a mild oxidation reagent H2O2 to release the occluded components. Gas chromatography–mass spectrometry analysis of the adsorbed and occluded components revealed a suite of oxygen-bearing compounds, including n-alkan-2-ones, n-aldehydes and fatty acid methyl esters. The n-alkan-2-ones were mainly detected in the adsorbed fraction and n-aldehydes in the occluded component. Comparison of the long-chain oxygen-bearing compound distributions and their individual stable carbon isotope characteristics both suggest that the oxygen-bearing compounds released by H2O2 treatment were occluded in the kerogen. The shorter-chain (C15 to C19) occluded n-aldehydes reflected similar isotopic distribution trends to the adsorbed n-alkan-2-ones, and the δ13C values of the mid-length (C21 to C25) occluded n-aldehydes were much closer to the adsorbed n-alkanes. Bacterially mediated methylation and decarbonylation of the n-aldehydes during kerogen formation may be important source(s) of n-alkan-2-ones and n-alkanes, respectively. The mid-chain ketones mainly originated from hydrothermal alteration of n-alkanes after kerogen formation. Uplift of the Lucaogou Formation in the Yuejingou section allowed surface-enhanced aerobic reactions.
Origin of polar organic sulfur compounds in immature crude oils revealed by ESI FT–ICR MS Org. Geochem. (IF 2.81) Pub Date : 2018-04-05 Weimin Liu, Yuhong Liao, Quan Shi, Chang Samuel Hsu, Bin Jiang, Ping'an Peng
Organic sulfur compounds (OSCs) are abundant in immature crude oils, including “polar” OSCs that are defined here as compounds containing one or more heteroatom(s) in addition to sulfur atoms having sufficient polarity to be analyzable by electrospray ionization (ESI) without derivatization. An understanding of the origins of polar OSCs in crude oils has been hampered by limitations in their analytical characterization. In this paper, we employed a high field (9.4 T) Fourier transform ion cyclotron resonance mass spectrometry (FT–ICR MS) coupled with ESI to study the sulfur-rich immature crude oils from the Jianghan Basin in China. The results show that the polar OSCs usually have non-sulfur counterparts, i.e., heteroatom-containing compounds without sulfur, with similar carbon number ranges, but slightly lower DBE (double bond equivalence) distributions. The similar or even identical carbon number distributions of the polar OSCs and their non-sulfur counterparts indicate their inheritance from the same precursors. The sulfur rings in polar OSCs are formed by intramolecular sulfurization that leads to the increases in DBEs compared to their non-sulfur counterparts. The results also indicate that the number of sulfur rings that can be formed in polar OSCs is largely controlled by the number of available reactive functional groups in their precursors. This paper extends our knowledge of intramolecular sulfurization during early diagenesis, especially for polar heteroatom-containing compounds.
Sediment records of long chain alkyl diols in an upwelling area of the coastal northern South China Sea Org. Geochem. (IF 2.81) Pub Date : 2018-04-03 Xiaowei Zhu, Guodong Jia, Shengyi Mao, Wen Yan
Here, Long chain alkyl diols in sediment cores were studied in the Yuedong Upwelling (YDU) area in the coastal northern South China Sea to investigate whether sea surface temperature (SST) and upwelling intensity over the last few decades can be traced by the diol-derived indices, i.e., the LDI (long chain diol index) as proxy for SST and the DI-1 and DI-2 (diol index 1 and 2) as proxies for upwelling intensity. The influence of riverine long chain alkyl diols (e.g., 1,13- and 1,15-diols), which may affect LDI-reconstructed SSTs, was largely ruled out based on the comparison between diol distributions in suspended particulate materials in the Pear River estuary and the core sediments. LDI-derived SSTs in downcore sediments matched well with local annual mean SSTs. The records of DI-2 changed in parallel with the local wind stress and inversely with SST at times of upwelling, suggesting that DI-2 as an effective proxy for upwelling intensity of YDU. The DI-1 showed a generally inverse variation pattern with DI-2, implying a major influence by SST, and it is thus not an applicable upwelling indicator for YDU. A negative relationship of DI-1 and a positive relationship of DI-2 with LDI-derived SST were observed. This occurrence might be attributed to El Niño–Southern Oscillation (ENSO), because the SST and upwelling intensity were largely modulated by ENSO, showing a warm annual mean SST during the years of enhanced El Niño and intensified summer upwelling.
Modelling leaf wax n-alkane inputs to soils along a latitudinal transect across Australia Org. Geochem. (IF 2.81) Pub Date : 2018-03-29 S. Howard, F.A. McInerney, S. Caddy-Retalic, P.A. Hall, J.W. Andrae
Leaf wax n-alkanes provide a valuable palaeoecological proxy, but their interpretation requires an understanding of the scale of temporal and spatial integration in soils. Leaf wax n-alkanes are continually deposited into soils directly from local plants as well as from more distant plants via wind or water transport. In addition, n-alkanes can persist in soils for thousands of years, and tend to decrease in age with shallower depth. To explore whether the uppermost soils reflect recent leaf fall inputs we compared surface soils and modern vegetation from 20 sites along a transect across Australia. At each site, the three most dominant plant species and a soil sample from the top 3 cm were analysed for n-alkane concentration, average chain length (ACL), proportional abundance of C33 and C29 (Norm33) and carbon preference index (CPI). Chain length distributions differ between trees and grasses, with a higher proportion of C29 in trees and C33 in grasses. Norm33 in soils correlates with proportional grass to tree cover across the transect. To model n-alkane inputs for each site, we calculated a predicted ACL, Norm33 and CPI using the dominant plants at that site, weighted by proportional species cover and n-alkane concentration. Predicted ACL, Norm33 and CPI inputs were generally higher than the soils, demonstrating that recent and local inputs do not dominate soil n-alkanes at our study sites. Thus, n-alkane distributions in surface soils do not correlate with local, current vegetation, but do correlate with proportional grass and tree cover, suggesting they provide a faithful record of large scale ecosystems structure.
Archaeal and bacterial glycerol dialkyl glycerol tetraether (GDGT) lipids in environmental samples by high temperature-gas chromatography with flame ionisation and time-of-flight mass spectrometry detection Org. Geochem. (IF 2.81) Pub Date : 2018-03-28 Sabine K. Lengger, Paul A. Sutton, Steven J. Rowland, Sarah J. Hurley, Ann Pearson, B. David A. Naafs, Xinyue Dang, Gordon N. Inglis, Richard D. Pancost
Archaeal isoprenoidal glycerol dibiphytanyl glycerol tetraether lipids (iGDGTs) and their non-isoprenoidal branched bacterial analogues (brGDGTs) have widespread applications in biogeochemistry and paleothermometry. Analysis of GDGTs usually involves separation using high performance liquid chromatography, typically coupled via atmospheric pressure chemical ionisation to mass spectrometric detection in selected ion-monitoring mode (HPLC–APCI-MS). However, reliable determination of ratios and, in particular, quantification by this technique, can be challenging due to differences in ionisation efficiencies of the various compounds. Quantification of GDGTs also relies on external calibration of the relative response to an internal standard with authenticated GDGTs, which are often not readily accessible. Here, we tested the suitability of high temperature gas chromatography with flame ionisation detection (HTGC-FID) for the determination of concentrations and tetraether lipid-based ratios in marine and terrestrial samples. For this, we identified GDGTs in environmental samples using HTGC coupled to time-of-flight mass spectrometry (HTGC–MS). Using a purified GDGT standard, we show we can quantify GDGT-0 in environmental samples by GC-FID. Some GDGT-based ratios measured by HTGC-FID exhibited a linear correlation (1:1) with ratios derived from HPLC–MS and weight-based ratios of mixtures of purified standards. However, ratios relying on minor isomers, such as TEX86 and MBT/CBT have many unresolved challenges for determination by HTGC. Detection limits were higher than for HPLC–MS. However, the advantages of employing HTGC-based methods include: (1) the independence from MS tuning-related differences in ionisation energies; (2) the potential for direct comparison with other, non-GDGT based biomarkers; and (3) a more complete insight into biomarker distributions in environmental samples by the extension of the temperature range. Quantitative elution of GDGTs from a HTGC column as demonstrated herein, will also enable their analysis by compound-specific isotope ratio mass spectrometry.
On the changes of polycyclic aromatic compounds in waterflooded oil and their implications for geochemical interpretation Org. Geochem. (IF 2.81) Pub Date : 2018-03-27 Xiangchun Chang, Yue Wang, Yaohui Xu, Jing Cui, Tao Wang
Qudi oilfield had been producing for over twenty years and became waterflooded ten years ago. Samples from nine producing wells drilled recently in the Qudi oilfield were collected during the three recovery stages spanning an eight month interval of waterflooding and were analyzed for changes in their polycyclic aromatic compounds (PACs). The results indicate that most of waterflooded oils are characterized by increased tricyclic, tetracyclic, pentacyclic and triaromatic steroids concentrations. The irregular changes in parameters based on two or fewer alkyl substituted naphthalenes (MNR, ENR, DBR, DNR-1) implies a different depletion order for the PACs because the numerators or denominators are controlled by the predominance of water washing and biodegradation over a certain time. The increase in DBTs and DBFs with water washing reflects their strong resistance to biodegradation. Lower aqueous solubility and strong resistance to biodegradation are possibly responsible for the mainly unchanged values of the three or more alkylnaphthalene-related parameters (TNR-1, TDE-1, TDE-3, TBR and TeBR), tricyclic aromatics (MPI-1 and PP-1), tetracyclic aromatics (MCI1, MCI2, 2-MC/1-MC, and BaA/(BaA + Chry)), triaromatic steroids parameters (C26-TAS/C28-TAS, C27-TAS/C28-TAS, C28-TAS 20S/(20R + 20S), and TA(I)/TA(I + II)), and heterocyclic aromatic parameters (4-MDBT/DBT, 4-MDBT/1-MDBT, and 4,6-DMDBT/(1,4 + 1,6)-DMDBT), confirming their validity for geochemical interpretation even after extensive water washing. However, pentacyclic aromatic parameters (BeP/Pe, (BeP + BaP)/Pe, BF/Pe, and BF/BeP), which are theoretically expected to be unaffected, change significantly with the advance of the waterflooding process, implying a complex alteration mechanism that needs further investigation.
Sources and distribution of tetraether lipids in sediments from the Zhejiang–Fujian coastal mud area, China, over the past 160 years: Implications for paleoclimate change Org. Geochem. (IF 2.81) Pub Date : 2018-03-23 Lilei Chen, Jian Liu, Jiasheng Wang, Gang Xu, Feng Li, Xingliang He, Yuanyuan Zhang, Li Li
Proxies based on glycerol dialkyl glycerol tetraether (GDGT) lipids in surface sediments have been successfully used to reconstruct sea surface temperature (SST), mean annual air temperature (MAAT), and soil pH in the East China Sea and its adjacent ocean margins. In this study, two cores from the Zhejiang–Fujian coastal mud area (ZFCMA) were studied in detail to investigate temporal changes in the source and distribution of tetraether lipids in the area, and the implications of such data for paleoclimate change. The results indicate that isoprenoid GDGTs (isoGDGTs) in the ZFCMA were derived mainly from Thaumarchaeota, with a minor contribution from in situ and allochthonous methanogenic archaea in upwelling areas, whereas branched GDGTs (brGDGTs) originated mainly from terrigenous bacteria and in situ organisms. However, the minor methanogen-derived and allochthonous isoGDGTs were considered to have had no effect on inferred SSTs in this study. The presence of brGDGTs produced in situ might corrupt the relationship between the methylation index of branched tetraethers or the cyclization index of branched tetraethers (MBT/CBT) and MAAT or pH, particularly in hypoxic areas. However, #ringtetra data, used here as an index to evaluate the sources of the brGDGTs, indicated that the brGDGTs buried in the sediments are predominantly derived from soil erosion. Furthermore, temporal changes in the GDGT-based proxy records indicate that climate changes and events (e.g., relating to the East Asian Monsoon, Kuroshio Current (KC)/Pacific Decadal Oscillation (PDO) intensity, and flood events), as well as anthropogenic influences (e.g., dam construction, soil conservation, fertilizer usage, and discharge of industrial waste water and domestic sewage) affect the signal in the core sediments. SSTs fluctuated over the past 160 years in the ZFCMA, with higher values in the 20th century than in the 19th century. In addition, the humidity, as inferred from the relationship between precipitation/soil water content and pH, and continental temperature in the mid–lower Yangtze River basin switched between dry–hot/dry–cool and wet–hot/wet–cool over the last 160 years. The results also indicate that PDO/KC intensity plays a key role in temperature changes and has a larger impact on terrestrial ecosystems than on ocean ecosystems in the ZFCMA and the adjacent continent.
Variations in abundance and distribution of methyltrimethyltridecylchromans (MTTCs) in sediments from saline lacustrine settings in Cenozoic lacustrine basins, China Org. Geochem. (IF 2.81) Pub Date : 2018-03-21 Kaixi Jiang, Chunming Lin, Xia Zhang, Chunfang Cai, Fei Xiao, Wenxiang He, Li Peng
This study examines the abundance and distribution of MTTCs and biomarker compositions in samples of MTTC-rich sediments obtained from Paleogene strata in the Bohai Bay, Jianghan and Western Qaidam Basins in China. Biomarker signatures, including high C35/C34 hopane ratios, high gammacerane index values, low pristane/phytane (Pr/Ph) ratios and the presence of aryl isoprenoids, indicate saline lacustrine settings and photic zone euxinia (PZE). The distributions of MTTCs indicate significant palaeosalinity differences in the depositional environments of the three sets of samples obtained from the Bohai Bay, Jianghan and Western Qaidam Basins. These sets of samples were deposited under normal to mesosaline, meso- to hypersaline and hypersaline environments, respectively. The δ-MTTC% values are positively correlated with the values of the gammacerane index, but are negatively correlated with the Pr/Ph ratios. This result demonstrates that MTTCs do not form in the upper or lower parts of stratified water columns, but may form in the euxinic portions of photic zones. Therefore, MTTC parameters can only be used to reconstruct palaeosalinity in the euxinic portions of photic zones and are not applicable to the upper or lower parts of stratified water columns. This paper infers that variations in palaeosalinity in the euxinic portions of photic zones likely play a key role in controlling the distribution and abundance of MTTCs.
Temperature effect on abundance and distribution of leaf wax n-alkanes across a temperature gradient along the 400 mm isohyet in China Org. Geochem. (IF 2.81) Pub Date : 2018-03-21 Jia Wang, Ergu Axia, Yunping Xu, Guoan Wang, Liping Zhou, Yufu Jia, Zixun Chen, Jiazhu Li
Long chain n-alkanes are plant wax biomarkers that are widely used in paleoenvironmental reconstructions. Temperature is an important influence on the composition of leaf wax n-alkanes. However, to date, understanding of changes in the abundance and distribution of leaf wax n-alkanes with temperature remains unclear. The main reason is that the interference of precipitation on leaf wax n-alkanes has not been effectively removed in most investigations conducted at natural ecosystems. We sampled 357 plants across a temperature gradient along an isohyet in China to minimize the influence of precipitation and evaluate the effect of temperature on the abundance and distribution of leaf wax n-alkanes. Most plant functional groups showed similar distributions in total n-alkane concentration (∑alk), carbon preference index (CPI) and average chain length (ACL). CPI values were not sensitive to temperature change. ∑alk values were negatively correlated with summer mean temperature (TJJA, June to August) for most plant functional groups. ACL values were always positively related to temperature, especially TJJA. However, large variability in ∑alk and ACL occurred within plants from the same site. Therefore, this study suggests that ∑alk and ACL can indicate relative environmental temperature, but they should not be used as quantitative proxies of paleo-temperature.
Influence of vegetation type on n-alkane composition and hydrogen isotope values from a high latitude ombrotrophic bog Org. Geochem. (IF 2.81) Pub Date : 2018-03-21 Nicholas L. Balascio, William J. D'Andrea, R. Scott Anderson, Stephen Wickler
The composition and hydrogen isotope values of leaf wax components can be powerful tools in reconstructing past climate and environments. However, interpretation of past environmental conditions from such components in sediments is complicated by species-specific influences and there is a need to better understand how vegetation type affects leaf wax composition and isotope ratios in modern environments. In this study we analyzed leaf wax (n-alkane) distributions and hydrogen isotope values of plants from a high latitude ombrotrophic bog in northern Norway. The isotopic analysis of surface water was also conducted on samples from 15 lakes along a ca. 150 km transect to contextualize the bog water isotopic composition and constrain fractionation factors among n-alkane homologues. We identified 14 different plant types growing on the bog surface, including mosses, graminoids and other herbs, sub-shrubs and a tree. n-Alkanes from the leaves of the modern plants had average chain length from 25 to 30.5, with a variety of distributions, and with the dominant compound of longer chain length (C27, C29, or C31). δD values of n-C25 to n-C33 for the vegetation samples ranged from −197‰ to −116‰, with an average of −162‰. The data also revealed that the δD values for the homologues for half of the vegetation types had ranges that were ≥20‰. Using the average isotopic value of bog water samples, −60‰, we calculated apparent fractionation factors that ranged from −66‰ to −134‰ (avg. −108 ± 22‰), similar to other sites across Europe and to a global data compilation. Our results demonstrate the range of species-specific influences on leaf wax composition and isotopic values at this site and presumably other ombrotrophic bog environments, and provide a dataset to help evaluate the influence of vegetation type on regional sedimentary leaf wax records.
Patterns of change in permanganate oxidizable soil organic matter from semiarid drylands reflected by absorbance spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry Org. Geochem. (IF 2.81) Pub Date : 2018-03-14 Carlos M. Romero, Richard E. Engel, Juliana D'Andrilli, Chengci Chen, Catherine Zabinski, Perry R. Miller, Roseann Wallander
Organic matter (OM) oxidized by slightly alkaline KMnO4, termed permanganate-oxidizable carbon (POXC), has recently emerged as a standardized indicator of active, labile carbon within soil quality frameworks. Yet, qualitative information on POXC, particularly in semiarid drylands, is very scarce. The aim of this study was to characterize POXC within three long-term field experiments in Montana, USA: (i) across a wide range of edaphic (e.g.,% clay) and management conditions (e.g., cropping intensity) (n = 148); and (ii) to identify the molecular composition of soil OM before and after KMnO4 treatment using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS). The content of POXC was significantly greater under perennial (382–685 mg/kg) or annual cropping (404–607 mg/kg) than fallow-wheat (359–543 mg/kg) systems. Soil OM changes, however, were equally or better expressed when considering soil organic carbon (SOC) concentration. The occurrence of POXC paralleled SOC (R = 0.87; P <0.001) and total nitrogen (TN) (R = 0.82; P <0.001) concentration, regardless of soil textural differences. The ESI FT-ICR MS analyses of aqueous soil extracts indicated that the KMnO4 reaction oxidized dissolved OM of diverse molecular character. OM molecular composition after KMnO4 treatment was enriched by strongly reduced chemical constituents (O/C <0.4) at greater condensed aromaticity (AI >0.67) and hydrogen saturation (aliphatic composition; H/C >1.5) across all heterogeneous groups (CcHhNnOoSs). Although POXC is a rapid assay widely used for characterizing soil OM dynamics, it may not provide a clear advantage over SOC concentration in semiarid drylands. The view of POXC as a merely labile, simple biodegradable OM fraction needs to be reconsidered.
Glacial-interglacial vegetation changes in northeast China inferred from isotopic composition of pyrogenic carbon from Lake Xingkai sediments Org. Geochem. (IF 2.81) Pub Date : 2018-03-10 Weiwei Sun, Enlou Zhang, Enfeng Liu, Jie Chang, Rong Chen, Ji Shen
Understanding the changes in monsoon intensity and ecosystem response at different timescales is crucial for the well-being of humans, yet the paleoclimatic interpretation of stable carbon isotope (δ13C) values from northeast China records is debatable. In this study, reported δ13C data from 76 surface soils in northeast China are compiled, and a δ13C record of pyrogenic carbon (δ13CPyC) from Lake Xingkai in northeast China since the last interglacial period is presented. The aim was to investigate the orbital timescale environmental implication of geological δ13CPyC data for northeast China. The results showed a distinct increase in δ13C values of surface soils, which correlated with increasing temperature of the warmest month. Higher temperature favored the expansion of C4 plants, while precipitation had only a weak correlation with δ13C values of surface soils in the region. On an orbital timescale, the δ13CPyC record from Lake Xingkai generally reflected paleovegetation change, suggesting that the abundance of C4 plants was relatively high during the warm periods, changing to almost purely C3 plants during the cold periods. Both modern and geological analyses suggest that the climatic factor determining the δ13C in northeast China was temperature of the warmest month. This is similar to the situation for mid-latitudes such as the Chinese Loess Plateau, in contrast to low latitudes such as southern China.
Amberene and 1-methylamberene, isolated and identified from Kuji amber (Japan) Org. Geochem. (IF 2.81) Pub Date : 2018-03-09 Tetsuaki Kawamura, Hiroyuki Koshino, Takemichi Nakamura, Yoshiyuki Nagasawa, Hidetaka Nanao, Masayuki Shirai, Shota Uesugi, Misa Ohno, Ken-ichi Kimura
Abundant labdatriene derivatives, named amberene [15,19,20-trinor-5,7,9-labdatriene (or 1,6-dimethyl-5-isopentyltetralin) (1)] and 1-methylamberene [15,20-dinor-5,7,9-labdatriene (or 1,1,6-trimethyl-5-isopentyltetralin) (2)] were isolated from the methanol extract of Kuji amber by HPLC. The structures were elucidated by spectral analyses including EIMS, 1D and 2D NMR and some previous assignments corrected.
n-Alkan-2-one biomarkers as a proxy for palaeoclimate reconstruction in the Mfabeni fen, South Africa Org. Geochem. (IF 2.81) Pub Date : 2018-03-08 Andrea Baker, Joyanto Routh, Alakendra N. Roychoudhury
The sub-tropical Mfabeni fen is the only continuous coastal peat deposit that documents glacial and interglacial palaeoenvironmental conditions since the late Pleistocene (ca. 47 cal kyr BP) in southern Africa. Published bulk geochemical, biomarker and leaf wax δ13C data, along with palynology and stratigraphic studies of the Mfabeni peat sequence, renders it an ideal record for testing new palaeoreconstruction proxies. In this study, we aimed to establish the proxy potential of n-alkan-2-one (n-ket) compounds by tracing their source/origin and post-depositional diagenetic change, and if they preserve or not a robust palaeoenvironment signal that complements our understanding of palaeoclimatic variations. In the Mfabeni archive the most likely source for n-kets is via microbial decarboxylation of n+1-alkanoic acids (n-FAs) and, to a lesser degree, oxidation of same chain length n-alkanes (n-alks). The n-ket average chain length (ACLket) and n-C23 and C25ket/precursor ratios displayed a statistical significant negative relationship with the n-alk aquatic plant proxy (Paq), suggesting the source of n-kets to be submerged aquatic plants during waterlogged conditions that suppressed microbial activity during the ensuing anoxic conditions. Both the mid-chain and long chain n-ket/precursor ratios displayed predominant water level fluctuation controls, with temperature as a secondary regulator. By comparing the n-ket data with published environmental and climate reconstructions from the same core, and with geomorphology and palynological studies of the Mfabeni basin, we conclude that the n-kets show promise as a palaeoclimate proxy and can be used in conjunction with other biomarker proxies to reconstruct ancient hydrological changes in sub-tropical peatlands.
Reconstructing the palaeoecosystem and palaeodepositional environment within the Upper Devonian–Lower Mississippian Bakken Formation: a biomarker approach Org. Geochem. (IF 2.81) Pub Date : 2018-03-07 Titilade Aderoju, Stephen Bend
Biomarkers have been used extensively to correlate oil families, oil–source rock relationships and source inputs, and to help identify the palaeodepositional environment for a given geological sedimentary unit. Despite their application in characterising the Bakken Formation shales, a number of biomarkers still appear unable to provide us a consistent understanding regarding heterogeneity in source organic matter and palaeoenvironment. Here, we infer the palaeodepositional environment within the Bakken Formation shales using a biomarker suite and biomarker depth profiles. We also use the occurrence and abundance of biomarkers to define organic facies and to make inferences regarding biotic interactions drawn from modern analogues. The variations and co-occurrence of biomarkers is explained by the existence of a proposed complex algal–bacterium–bacteriovore palaeoecosystem. This approach not only helps explain the depth variation in occurrence and concentration of biomarkers and the molecular composition of associated organic matter, but also advances the use of biomarker analysis and biomarker associations to determine a complex environment-sensitive palaeoecosystem.
Sedimentary records of human activity and natural environmental evolution in sensitive ecosystems: A case study of a coral nature reserve in Dongshan Bay and a mangrove forest nature reserve in Zhangjiang River estuary, Southeast China Org. Geochem. (IF 2.81) Pub Date : 2018-03-02 Dongyi Li, Yonghang Xu, Yunhai Li, Jianjun Wang, Xijie Yin, Xiang Ye, Aijun Wang, Lian Wang
Total organic carbon (TOC), total nitrogen (TN), δ13Corg, δ15N, and grain size were measured in two sediment cores from reserves adjacent to Dongshan Bay, China. The aims were to identify organic matter (OM) sources and to evaluate the combined impact of human activity and natural environmental processes over the last ∼100 yr. In a coral reserve far from the estuary of Zhangjiang River, the average sediment accumulation rate (SAR) was 0.51 cm/yr from ca. 1839-2015, and the values of δ13Corg and C/N (atomic ratio of TOC to total N) ranged from -21.9‰ to -23.7‰ and from 5.7 to 8.1, respectively. In a mangrove forest reserve near the estuary, SAR averaged 0.49 cm/yr from ca. 1891-1985 and 2.41 cm/yr from ca. 1985-2015, and the δ13Corg and C/N values ranged from -22.7‰ to -26.3‰ and from 9.3 to 21.7, respectively. By combining a comparison plot of C/N and δ13Corg and a binary mixing model (based on δ13Corg and Norg/Corg, respectively), the major OM sources were assigned and their sedimentary contributions estimated. The sedimentary OM in the coral reserve had mainly an offshore marine authigenic signature with an average terrigenous contribution of <27%. In the mangrove forest reserve, the sedimentary OM was a mixture of terrestrial and marine sources, with an average terrigenous contribution of >56%. While input from the Zhangjiang River declined from the 1980s, urban development, deforestation and land reclamation contributed to an overall increase in the total mass accumulation rate (MAR). The coral was also severely damaged by the advent of poaching around the same time. These two factors collectively resulted in a rapid decrease in OM content and OC accumulation rate (CARorg) of the sediments until the establishment of the coral reserve in 1997. Changes in terrain and climate and the construction of water conservancy facilities were traditionally the primary factors responsible for changes in the sedimentary environment of the mangrove forest reserve. Since the 1980s, however, the expansion of the mangrove forest along with human activity has led to rapid increases in MAR and CARorg; whilst, protective measures and a favourable climate have also promoted the growth of the mangrove forest.
Effect of simulated diagenesis on the compositions, chemical stability and sorption properties of natural and engineered organic matter with different mineral contents Org. Geochem. (IF 2.81) Pub Date : 2018-03-02 Lanfang Han, Ziying Wang, Marco Keiluweit, Yan Yang, Ke Sun, Zheyun Zhang, Baoshan Xing
Assessing the diversity of lipids formed via Fischer-Tropsch-type reactions Org. Geochem. (IF 2.81) Pub Date : 2018-03-02 Helge Mißbach, Burkhard C. Schmidt, Jan-Peter Duda, Nils Keno Lünsdorf, Walter Goetz, Volker Thiel
Lipid biomarkers are commonly used for tracking life through Earth’s history and are also gaining in importance in the search for extraterrestrial life. However, some lipids may also be formed in-situ via abiotic Fischer-Tropsch-type (FTT) reactions. These processes have been considered as a source of prebiotic organic matter. Here we report on a FTT synthesis experiment performed under hydrothermal conditions, focusing on more complex, previously undescribed FTT products that may potentially mimic biological signals. The experiment was carried out in stainless steel reactors by heating aqueous solutions of oxalic acid mixed with montmorillonite to 175 °C for 3 days. Organic extracts of the products and extraction residues were analyzed by gas chromatography-mass spectrometry (GC-MS) and combustion-infrared detection, attenuated total reflectance Fourier transform infrared spectroscopy and pyrolysis GC-MS, respectively. FTT reactions led to a number of biomarker-like lipids such as linear and methyl-branched alkanes and alkanols as well as n-alkanoic acids. However, FTT reactions a priori produce unimodal chain length distributions and isomeric mixtures of methyl-branched compounds, as opposed to biolipids which typically show preferences of individual homologs and/or isomers. Experimental maturation of an immature kerogen demonstrated how these biological signatures may pass into unimodal distributions similar to FTT products by thermal overprint. Therefore, discrimination of biologically derived compounds from FTT organics may become increasingly problematic with ongoing thermal maturation. FTT extraction residues contained organic material, possibly in form of an organic polymer. However, it remains to be tested whether these residual organics can evolve into more complex macromolecular networks.
Fatty acids and other biomarkers in two Early Jurassic concretions and their immediate host rocks (Lias δ, Buttenheim clay pit, Bavaria, Germany) Org. Geochem. (IF 2.81) Pub Date : 2018-03-02 Volker Thiel, Michael Hoppert
Two carbonate- and phosphate-rich concretions from Early Jurassic strata at Buttenheim (Bavaria, Germany) and their immediate claystone host rocks were analysed to study the mechanisms of biomarker preservation in concretions. Superimposed on a common sedimentary background signal, distinctive biomarkers reflect microbial processes involved in concretion formation. When normalized to the HCl-insoluble insoluble rock portion, concretions show ∼1.5 to 3 times higher TOC values as their host rocks, but identical absolute quantities of most extractable hydrocarbons. In contrast, n-fatty acids and α,ω-dicarboxylic acids are by an order more abundant as compared to the host rocks, suggesting an enhanced preservation or selective accumulation mechanism for carboxylic acids in the concretions. Further, linear and iso-/anteiso-fatty acids with unusually short carbon chains (as low as C8) are exclusively observed in the concretions and are interpreted as biodegradation products of lipids derived from in-situ bacteria. Likewise, strongly enhanced neohop-13(18)-enes and hopanoic acids with more ‘immature’ isomer distributions indicate an in-situ contribution of microbial biomass during formation of the concretions. Pyrolysis released ample additional saturated short-chain n-fatty acids from the kerogens of the concretions, but not from the host rocks. We hypothesize that these compounds are remnants of locally accumulated OM rich in fatty acyl moieties (e.g. plant polymers or triglyceride storage lipids) whose hydrolysis products were initially precipitated as fatty acid salts (‘adipocere’). During shallow burial, these substances may have delivered a steadily flowing energy source for sedimentary microorganisms, with their anaerobic decomposition supplying both, carbonate and metal ions. In concert with other factors, this may have promoted carbonate precipitation and thus, the formation of the Buttenheim concretions.
Different temperature dependence of the bacterial brGDGT isomers in 35 Chinese lake sediments compared to that in soils Org. Geochem. (IF 2.81) Pub Date : 2018-02-20 Xinyue Dang, Weihua Ding, Huan Yang, Richard D. Pancost, B. David A. Naafs, Jiantao Xue, Xiao Lin, Jiayi Lu, Shucheng Xie
During the last decade, the distribution of branched glycerol dialkyl glycerol tetraethers (brGDGTs) in lacustrine sediments has been widely used to reconstruct past variations in lake temperature. A prerequisite for the application of brGDGTs to lacustrine paleoclimate reconstructions is to understand the sources of brGDGTs in lake systems and the processes that influence their distribution. In this study, we investigated the distribution of brGDGTs in core-top sediments from 35 lakes across China, with a broad mean annual air temperature (MAAT) range, but a constrained pH range, to explore the effect of temperature. The results reveal a contrasting response of MBT'5ME and MBT'6ME to temperature in lake environments compared to that in soils. The sedimentary distributions of 5- and 6-methyl brGDGTs exhibit different relationships with temperature, with most of the latter being correlated to MAAT while the former responding to temperature by only hexamethylated compounds. In both global and Chinese soils, most 6-methyl brGDGTs have no relationship with MAAT but the distribution of 5-methyl brGDGTs is correlated with MAAT. The different behaviors suggest that communities producing 5- or 6-methyl brGDGTs might be different in lakes and soils. In addition, in lakes from cold regions (MAAT < 5 °C), the brGDGT distributions correlate only with warm season temperatures (April to October) but exhibit no correlation with cold seasons, suggesting a seasonal bias in brGDGT production in these lakes. This bias towards the warm season is not found in lakes from warmer regions (MAAT > 5 °C). Based on these results we propose new temperature calibrations for paleotemperature reconstructions in Chinese alkaline lakes.
Distribution of long chain alkyl diols along a south-north transect of the northwestern Pacific region: Insights into a paleo sea surface nutrient proxy Org. Geochem. (IF 2.81) Pub Date : 2018-02-15 Jong-Ku Gal, Jung-Hyun Kim, Kyung-Hoon Shin
Long chain alkyl diols (LCDs) have previously been used to develop several diol indices as a proxy for upwelling or stratification conditions and sea surface temperatures. However, there is still a lack of data on the distribution of LCDs in the Pacific Ocean, which calls into question to what extent these diol indices can serve as a proxy in the Pacific Ocean. Hence, we investigated the distribution of LCDs in suspended particulate matter (SPM) samples collected in July 2015 along a south-north transect ranging from the East Sea of Korea to the Bering Sea in the northwestern Pacific region. Our results showed that both saturated and unsaturated C28 and C30 1,14-diols were dominant in most SPM samples investigated. The principal component analysis (PCA) results of the SPM data revealed that the C28:1 and C30:1 1,14-diols were positively associated with nutrient concentrations. However, only the C28:1 1,14-diol in addition to the C28 1,14-diol was positively associated with nutrient concentrations in the global surface sediment data set previously published, while the C30:1 1,14-diol was related to sea surface temperature (SST). Based on these observations, we defined a new diol index, i.e., the nutrient diol index (NDI), which included the C28 1,14-diol as well as the C28:1 1,14-diol in the numerator and all the 1,13-, 1,14-, and 1,15-diols, excluding the C32 1,15-diol, in the denominator. The NDI was positively associated with the ocean atlas values of surface water phosphate and nitrate concentrations for the SPM and surface sediment data sets. Accordingly, the results of our study showed that the NDI has potential as a quantitative nutrient proxy for estimating sea surface phosphate (or nitrate) changes. Further studies should be undertaken in order to assess the applicability of the NDI as a paleo-nutrient proxy for confidently reconstructing surface water nutrient variations in the past, by analyzing more SPM, surface sediments, and sediment cores collected from various oceanic settings.
Biodegradation in the Nkh 3-4 reservoir at Vankor Field (West Siberia Basin, Russia) is strongly controlled by rock properties Org. Geochem. (IF 2.81) Pub Date : 2018-02-12 Nikolay V. Oblasov, Ivan V. Goncharov, Vadim V. Samoilenko, Ger W. van Graas
The Vankor Field, located in the West Siberia Basin, Russia, contains oils at various levels of biodegradation. A study of reservoir oils and rock extracts from the Lower Cretaceous sandstones in the Nkh 3-4 interval reveals that increasing biodegradation when moving from the gas-oil contact (GOC) towards the oil-water contact (OWC) is strongly controlled by rock properties. Biodegradation is most advanced where the OWC is located in the high permeability reservoir interval. High permeability rocks also allow for more extensive upward mixing of biodegraded oil. Changes in the oil and gas fractions due to biodegradation also affect the gas-oil ratio as dry gas is less soluble in heavy oil. This explains the presence of reservoir oil saturated with gas near the OWC. The latter also can be partly due to the admixture of methane through methanogenic processes at the OWC. Upward movement of microbial gas bubbles enhances the mixing of biodegraded oil in the high permeability reservoir intervals.
Determination of position-specific carbon isotope ratios of propane from natural gas Org. Geochem. (IF 2.81) Pub Date : 2018-02-12 Yun Li, Lin Zhang, Yongqiang Xiong, Shutao Gao, Zhiqiang Yu, Ping'an Peng
Experimental investigation of the role of rock fabric in gas generation and expulsion during thermal maturation: anhydrous closed-system pyrolysis of a bitumen-rich Eagle Ford Shale Org. Geochem. (IF 2.81) Pub Date : 2018-02-12 Deyong Shao, Geoffrey S. Ellis, Yanfang Li, Tongwei Zhang
Gold-tube pyrolysis experiments were conducted on miniature core plugs and powdered rock from a bitumen-rich sample of Eagle Ford Shale to investigate the role of rock fabric in gas generation and expulsion during thermal maturation. The samples were isothermally heated at 130, 300, 310, 333, 367, 400, and 425 °C for 72 h under a confining pressure of 68.0 MPa, corresponding to six levels of induced thermal maturity: pre-oil generation (130 °C/72 h), incipient oil/bitumen generation (300 and 310 °C/72 h), early oil generation (333 °C/72 h), peak oil generation (367 °C/72 h), early oil cracking (400 °C/72 h), and late oil cracking (425 °C/72 h). Experimental results show that gas retention coupled with compositional fractionation occurs in the core plug experiments and varies as a function of thermal maturity. During the incipient oil/bitumen generation stage, yields of methane through pentane (C1–C5) from core plugs are significantly lower than those from rock powder, and gases from core plugs are enriched in methane. However, the differences in C1–C5 gas yield and composition decrease throughout the oil generation stage, and by the oil cracking stage no obvious compositional difference in C1–C5 gases exists. The decrease in the effect of rock fabric on gas yield and composition with increasing maturity is the result of an increase in gas expulsion efficiency. Pyrolysis of rock powder yields 4–16 times more CO2 compared to miniature core plugs, with δ13CCO2 values ranging from –2.9‰ to –0.6‰, likely due to carbonate decomposition accelerated by reactions with organic acids. Furthermore, lower yields of gaseous alkenes and H2 from core plug experiments suggest that the rock fabric plays a role in promoting hydrogenation reactions of alkenes.
Carbon isotope ratios of coccolith–associated polysaccharides of Emiliania huxleyi as a function of growth rate and CO2 concentration Org. Geochem. (IF 2.81) Pub Date : 2018-02-11 Elise B. Wilkes, Renee B.Y. Lee, Harry L.O. McClelland, Rosalind E.M. Rickaby, Ann Pearson
The calcite plates, or coccoliths, of haptophyte algae including Emiliania huxleyi are formed in intracellular vesicles in association with water–soluble acidic polysaccharides. These coccolith–associated polysaccharides (CAPs) are involved in regulating coccolith formation and have been recovered from sediment samples dating back to ∼180 Ma. Paired measurements of the carbon isotopic compositions of CAPs and coccolith calcite have been proposed as a novel paleo–pCO2 barometer, but additional proxy validation and development are still required. Here we present culture results quantifying carbon isotopic offsets between CAPs and other cellular components: bulk organic biomass, alkenones, and calcite. E. huxleyi was grown in nitrate–limited chemostat experiments at growth rates (µ) of 0.20–0.62/d and carbon dioxide concentrations of 10.7–17.6 µmol/kg. We find that CAPs are isotopically enriched by 4.5‰ to 10.1‰ relative to bulk organic carbon, exhibiting smaller isotopic offsets at faster growth rates and lower CO2 concentrations. This variability suggests that CAPs record a complementary signature of past growth conditions with different sensitivity than alkenones or coccolith calcite. By measuring the isotopic compositions of all three molecules and minerals of self-consistent origin, the ratio µ/[CO2(aq)] may be reconstructed with fewer assumptions than current approaches.
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