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  • The last forests on Antarctica: Reconstructing flora and temperature from the Neogene Sirius Group, Transantarctic Mountains
    Org. Geochem. (IF 3.081) Pub Date : 2018-01-12
    Rhian L. Rees-Owen, Fiona L. Gill, Robert J. Newton, Ruza F. Ivanović, Jane E. Francis, James B. Riding, Christopher H. Vane, Raquel A. Lopes dos Santos

    Fossil-bearing deposits in the Transantarctic Mountains, Antarctica indicate that, despite the cold nature of the continent’s climate, a tundra ecosystem grew during periods of ice sheet retreat in the mid to late Neogene (17–2.5 Ma), 480 km from the South Pole. To date, palaeotemperature reconstruction has been based only on biological ranges, thereby calling for a geochemical approach to understanding continental climate and environment. There is contradictory evidence in the fossil record as to whether this flora was mixed angiosperm-conifer vegetation, or whether by this point conifers had disappeared from the continent. In order to address these questions, we have analysed, for the first time in sediments of this age, plant and bacterial biomarkers in terrestrial sediments from the Transantarctic Mountains to reconstruct past temperature and vegetation during a period of East Antarctic Ice Sheet retreat. From tetraether lipids (MBT’/CBT palaeothermometer), we conclude that the mean continental summer temperature was ca. 5 °C, in agreement with previous reconstructions. This was warm enough to have allowed woody vegetation to survive and reproduce even during the austral winter. Biomarkers from vascular plants indicate a low diversity and spatially variable flora consisting of higher plants, moss and algal mats growing in microenvironments in a glacial outwash system. Abietane-type compounds were abundant in some samples, indicating that conifers, most likely Podocarpaceae, grew on the Antarctic continent well into the Neogene. This is supported by the palynological record, but not the macrofossil record for the continent, and has implications for the evolution of vegetation on Antarctica.

  • Choosing a suitable εw-p by distinguishing the dominant plant sources in sediment records using a new Pta index and estimating the paleo-δDp spatial distribution in China
    Org. Geochem. (IF 3.081) Pub Date : 2018-01-11
    Zheng Wang, Hu Liu, Yunning Cao

    The hydrogen isotopic composition of plant wax (δDwax) inherits the isotopic values of the source water and isotopic fractionations that take place in lipid biosynthesis. The δDwax values of n-alkanes or n-acids in sediment records are considered to be effective paleo-δD for precipitation proxies for different timescales. However, the vegetation effect also contributes substantially to the apparent fractionation values (εw-p) between the precipitation and plant. Therefore, a method for evaluating the dominant plant source in the sediment records is key for reconstructing a reliable paleo-δDp record. In this study, based on field data from the extremely arid Qaidam Basin and the semi-arid central Chinese Loess Plateau (CLP), a plant type index (Pta), based on the traditional Paq and n-alkane C31/C29 index, was used to evaluate the dominant plant type that contributed to the sediments. In the arid and semi-arid region, the Pta values of the aquatic plants were < 90, while the shrub Pta ranged from 90 to 180 and the grass Pta was > 180. A test on contemporary soils and surface lake sediments also showed that the Pta index could estimate the regional vegetation distribution in modern environments. We also evaluated the dominant n-alkane source for four sediment records in China during the last millennium and reconstructed a regional paleo-δDp by choosing a different εw-p. The estimated paleo-δDp values showed a good correlation with the modern regional summer δDp. We suggest that a large number of field studies are needed to improve the Pta index for quantitatively calculating the precise ratios for various plant sources. Such data would play a key role in improving hydrogen isotopic hydroclimatic research.

  • Potential recycling of thaumarchaeotal lipids by DPANN Archaea in seasonally hypoxic surface marine sediments
    Org. Geochem. (IF 3.081) Pub Date : 2017-12-26
    Yvonne A. Lipsewers, Ellen C. Hopmans, Jaap S. Sinninghe Damsté, Laura Villanueva
  • Steroid dissipation and formation in the course of farmyard manure composting
    Org. Geochem. (IF 3.081) Pub Date : 2017-12-23
    Katharina Prost, Pia Loretta Bradel, Eva Lehndorff, Wulf Amelung

    Steroids are used as biomarkers for tracing faecal material in the environment, but their dissipation behaviour in the course of composting has remained unclear. To assess their stability, we tracked the fate of Δ5-sterols, 5α-stanols, 5β-stanols, epi-5β-stanols, stanones and bile acids after 0, 7, 14, 28, 56, 112 and 168 days and after 0 and 168 days of composting cattle and horse farmyard manure, respectively. After composting, extractable steroid content decreased by 93.8-99.9% for cattle manure and by 54.0-100% for horse manure relative to the initial amount. The loss was, particularly pronounced for bile acids in the cattle manure compost. Our findings challenge the assumption that bile acids are generally more resistant towards degradation than other steroids. Contrary to the bile acids, the Δ5-sterol content did not decrease constantly, but showed a temporary increase, pointing to a delayed release from straw (β-sitosterol, stigmasterol and cholesterol) or to temporary production by fungi (cholesterol). Similarly, there was no continued loss of cholesterol transformation products (5α-cholestanol, cholestanone and epicoprostanol); they either increased temporarily or showed delayed degradation. These changes in steroid patterns complicate the identification of compost as faecal matter by way of commonly used steroid ratios and stress the importance of additional bile acid analysis in geo-archaeological research. Nevertheless, slower dissipation and smaller relative loss of epi-5β-stanols vs. 5β-stanols increased the value of r = epi-5β-stigmastanol/5β-stigmastanol + epicoprostanol/coprostanol during composting and allowed differentiation between fresh farmyard manure, and immature and mature compost.

  • Taphonomy and chemotaxonomy of Eocene amber from southeastern Australia
    Org. Geochem. (IF 3.081) Pub Date : 2017-12-21
    Andrew J. Coward, Chris Mays, Antonio F. Patti, Jeffrey D. Stilwell, Luke A. O'dell, Pedro Viegas

    Amber is a complex, organic polymer that offers unparalleled utility as a preservation medium, providing insights into past organisms and environments. However, under specific circumstances, this information can be compromised through alteration of the amber structure. Understanding the degradation of amber in the geosphere could improve prospecting techniques and maximise the quality and validity of chemical information from altered samples. This study analysed 114 amber samples retrieved from two new Eocene Australian deposits at Strahan, Tasmania and Anglesea, Victoria using a combination of attenuated total reflectance Fourier transform infrared and solid-state 13C cross-polarised magic angle spinning nuclear magnetic resonance spectroscopy. Results identified both Class Ib polylabdanoid and Class II cadinene-based amber. The presence of Class II amber in Australia suggests one of two possibilities: (1) a local Dipterocarpaceae source, the primary producer of Class II resins, despite the absence of this family from the Australian Eocene fossil record; or (2) a local, unidentified botanical source of cadinene-based amber. A third alternative, that Class II amber was transported to Australia from southeast Asia via ocean currents, is rejected. Taphonomic analysis revealed four mechanisms of alteration prevalent in amber across the two study regions, with evidence of oxidation and metal carboxylate formation. Both the nature and extent of these alterations were found to vary significantly between classes I and II, suggesting that amber class may play a defining role in determining the chemical pathways by which amber degrades. Of note was the high proportion of amber that exhibited no significant chemical changes despite extensive visible alteration features, suggesting the integrity of palaeobiological and palaeoenvironmental information in these samples may be preserved.

  • Assessing environmental controls on the distribution of long-chain alkenones in the Canadian Prairies
    Org. Geochem. (IF 3.081) Pub Date : 2017-12-20
    Julien Plancq, Bianca Cavazzin, Steve Juggins, Heather A. Haig, Peter R. Leavitt, Jaime L. Toney

    Long-chain alkenones (LCAs) have been used for decades to reconstruct quantitative sea-surface temperature records, but they also have a great potential for paleotemperature reconstructions in lacustrine settings. Here, we investigated how the presence and abundance of LCAs in surface sediments from 106 lakes varied with environmental conditions in lakes of the Northern Great Plains (Canadian Prairies) in southern Saskatchewan, Canada. Consistent with preliminary research, we found LCAs in 55% of surveyed lakes, with mean concentrations of 143 μg/g dry sediment, but very high concentrations (up to 2.3 mg/g dry sediment) in 7% of lakes. Statistical analyses indicate that salinity and stratification play key roles in determining LCA presence and abundance supporting previous research in Spain and the northern Great Plains, USA. Overall, the alkenone unsaturation index ( View the MathML source U 37 K ) was not correlated significantly with average summer water temperature, probably because the timing of maximum LCA production occurs during the spring season. We conclude that improved seasonal sampling is required within the study lakes to better identify the timing and habitat of haptophyte production, and allow development of environmental temperature reconstruction tools.

  • Supraglacial microbes use young carbon and not aged cryoconite carbon
    Org. Geochem. (IF 3.081) Pub Date : 2017-12-15
    Drake O. McCrimmon, Michael Bizimis, Alexandra Holland, Lori A. Ziolkowski

    Cryoconite holes are depressions in glacial ice surfaces filled with dark debris that reduce albedo. The relative contributions of combustion and microbial carbon to cryoconite carbon are currently not known. To constrain cryoconite organic carbon composition and carbon sources to microorganisms living on glacier surfaces, measurements of bulk organic carbon and microbial phospholipid fatty acids (PLFAs) from supraglacial cryoconite sediment within the ablation zones of Spencer and Matanuska glaciers in southern Alaska were coupled with radiocarbon (14C) analyses. The 14C content of bulk cryoconite organic carbon on both glaciers was depleted relative to the modern atmosphere, while the PLFAs contained carbon that was recently in equilibrium with the atmosphere. Because the bulk cryoconite material is isotopically distinct from the PLFAs, these results indicate that cryoconite organic carbon is not bioavailable to the microbes. Instead, modern carbon in the microbes suggests that carbon is being quickly cycled by them within the cryoconite. Biomarker and stable isotope analyses of cryoconite organic carbon points to fresh carbon inputs to cryoconite and indicates that combusted fossil carbon is a not a major component of cryoconite organic carbon. Trace element analyses of the bulk cryoconite show crustal inputs and no evidence for excess metals associated with recent combustions sources, indicating that the aged bulk cryoconite organic carbon is likely of rock origin (e.g., shale). This study highlights that microbes in cryoconite holes on these glaciers are fixing atmospheric carbon and not using the aged carbon surrounding them.

  • Distributions of 5- and 6-methyl branched glycerol dialkyl glycerol tetraethers (brGDGTs) in East African lake sediment: Effects of temperature, pH, and new lacustrine paleotemperature calibrations
    Org. Geochem. (IF 3.081) Pub Date : 2017-12-15
    James M. Russell, Ellen C. Hopmans, Shannon E. Loomis, Jie Liang, Jaap S. Sinninghe Damsté

    The distribution of branched glycerol dialkyl glycerol tetraethers (brGDGTs) in soils, peats, and lake sediments has been shown to correlate with mean annual air temperature (MAAT) and has provided valuable new climate reconstructions. Here we use an improved chromatographic method to quantify the fractional abundances of 5- and 6-methyl isomers in surface sediments from 65 East African lakes spanning temperatures 1.6–26.8 °C, and investigate the relationships between these fractional abundances and temperature, lake pH, and other environmental variables. We find that temperature exerts a strong control on brGDGT distributions, including the relative abundances of 5- and 6-methyl isomers, whereas other environmental variables, including lake pH, are weakly correlated to the fractional abundances of the brGDGTs. The distributions of brGDGTs in our lake sediments differ from those of soils and peats, leading to temperature offsets if soil- and peat-based brGDGT temperature calibrations are applied. We develop new calibrations for MAAT for use in lake sediment based upon the MBT́5Me and Index 1 ratios, as well as a multivariate regression of brGDGT fractional abundances on temperature using stepwise forward selection. We obtain root mean square errors (RMSE) between ∼2.1 and 2.5 °C for these calibrations, highlighting the potential for brGDGTs to provide precise temperature reconstructions using lake sediment cores. Calibrations for lake pH perform more poorly, likely due to weak correlations between pH and brGDGT distributions in East African lakes. These results indicate that quantification of 5- and 6-methyl isomers separately in lake sediment can improve paleoclimatic reconstructions.

  • The application of compound-specific sulfur isotopes to the oil–source rock correlation of Kurdistan petroleum
    Org. Geochem. (IF 3.081) Pub Date : 2017-12-08
    Paul F. Greenwood, Luma Mohammed, Kliti Grice, Malcolm McCulloch, Lorenz Schwark

    The concentrations and δ34S values of thioaromatic compounds of a suite of oils from several major oil fields in Kurdistan and their corresponding regional Type II-S source rocks have been measured to investigate their source relationship. The oils of three fields (Khabbaz, Jambur, Ajeel) and the bitumen extracted from specific rock formations (Alan, Sargelu, Naokelekan, Chia Gara) showed particularly high abundances of thioaromatics consistent with a carbonate source deposited in a restricted sulfate-rich marine platform setting. The δ34S [V-CDT] values of the major organosulfur compounds (OSCs) in these petroleum samples were measured with a gas chromatograph coupled to a multi-collector inductively coupled plasma mass spectrometer. δ34S values of dibenzothiophenes and methyldibenzothiophenes were consistently in the range –4‰ to –12‰ and –9‰ to –18‰ for the oils and rocks, respectively. Separate groupings of oils and rocks were distinguishable by > 2‰ difference, given an analytical reproducibility of < 0.8‰. OSCs from rocks were consistently ∼2–4‰ depleted than in oils, reflecting a similar trend to previous bulk δ34S studies from which an initial evolution of 34S depleted H2S during diagenesis and thermal maturation had been proposed. Distinctive δ34SOSC data of the oils and rocks with particularly high thioaromatic abundances did suggest several oils–source rock relationships: the Ajeel and Jambur oils and sediments from the Chia Gara formation yielded relatively enriched δ34SOSC values, whereas consistently depleted δ34SOSC values were observed for the Khabbaz oil and Naokelekan source rocks. Results suggest that compound-specific S isotope analysis can help establish oil–source rock relationships of S-rich petroleum.

  • Early diagenetic alterations of sterol biomarkers during particle settling and burial in polluted and pristine areas of the Rio de la Plata Basin
    Org. Geochem. (IF 3.081) Pub Date : 2017-12-07
    Eric Demian Speranza, Manuel Colombo, Carlos Norberto Skorupka, Juan Carlos Colombo
  • Reconstruction of prehistoric pottery use from fatty acid carbon isotope signatures using Bayesian inference
    Org. Geochem. (IF 3.081) Pub Date : 2017-12-07
    Ricardo Fernandes, Yvette Eley, Marek Brabec, Alexandre Lucquin, Andrew Millard, Oliver E. Craig

    Carbon isotope measurements of individual fatty acids (C16:0 and C18:0) recovered from archaeological pottery vessels are widely used in archaeology to investigate past culinary and economic practices. Typically, such isotope measurements are matched with reference to food sources for straightforward source identification, or simple linear models are used to investigate mixing of contents. However, in cases where multiple food sources were processed in the same vessel, these approaches result in equivocal solutions. To address this issue, we tested the use of a Bayesian mixing model to determine the proportional contribution of different food sources to a series of different mixed food compositions, using data generated both by simulation and by experiment. The model was then applied to previously published fatty acid isotope datasets from pottery from two prehistoric sites: Durrington Walls, near Stonehenge in southern Britain and Neustadt in northern Germany. We show that the Bayesian approach to the reconstruction of pottery use offers a reliable probabilistic interpretation of source contributions although the analysis also highlights the relatively low precision achievable in quantifying pottery contents from datasets of this nature. We suggest that, with some refinement, the approach outlined should become standard practice in organic residue analysis, and also has potential application to a wide range of geological and geochemical investigations.

  • Branched GDGT production at elevated temperatures in anaerobic soil microcosm incubations
    Org. Geochem. (IF 3.081) Pub Date : 2017-12-07
    Yufei Chen, Fengfeng Zheng, Songze Chen, Haodong Liu, Tommy J. Phelps, Chuanlun Zhang

    Branched glycerol dialkyl glycerol tetraethers (bGDGTs) are unique ether-linked bacterial membrane lipids, which occur in diverse environments. Acidobacteria are considered to produce them in acidic soils and peat bogs. However, the biological source(s) of bGDGTs other than acidobacteria in alkaline soils and their response to environmental changes are obscure. We designed a 1 year incubation experiment under elevated temperature conditions (30 °C, 45 °C and 85 °C) in order to accelerate the growth of mesophilic and thermophilic bacteria that may potentially produce bGDGTs and understand the mechanisms underlying the changes in bGDGT composition. The soil we selected was alkaline (pH > 8) with a small population of acidobacteria being detected using high throughput sequencing. The medium contained nutrients and organic carbon and samples were incubated under anaerobic conditions with pH of 7.5, in order to stimulate the growth of heterotrophic and anaerobic bacteria from the soil. Quantitative polymerase chain reaction (qPCR) was employed to monitor the growth status of bacteria at the beginning and the end of the experiment. The results showed multiple lines of evidence for bGDGT biosynthesis, including: (1) increased total abundance of both PL-bGDGTs (polar lipid-derived bGDGTs, “living”) and CL-bGDGTs (core lipid bGDGTs, “dead”) at 30 °C and 45 °C, but not 85 °C; (2) changes in bGDGT distribution at 30 °C and 45 °C; and (3) increased temperature inferred by the bGDGT-based proxy at an incubation temperature at 45 °C. Estimated turnover time of both PL- and CL-bGDGTs was 1–8 months at 30 °C or 45 °C under these laboratory conditions, which agrees with previous studies. Our study suggests that raising temperature may stimulate growth of bGDGT-producing bacteria that likely includes some anaerobic heterotrophs other than acidobacteria.

  • Drimane-type compounds in source rocks and fluids from fluvial-deltaic depositional settings in the North-West Shelf of Australia
    Org. Geochem. (IF 3.081) Pub Date : 2017-12-07
    Jaime Cesar, Kliti Grice

    Fluvial-deltaic petroleum systems with gas-prone source rocks are difficult to assess because the released fluids generally lack biomarkers and this limits the implementation of traditional organic geochemistry screening. However, there are a number of compound classes yet to be investigated in order to establish novel parameters for correlation studies in fluvial-deltaic geological settings. In this study we have focused on drimane-skeleton molecules (C15-decalins), particularly the compounds 1,2,2,5,5-pentamethyl-trans-decalin and 1,1,2,5,5-pentamethyl-trans-decalin (termed herein as rearranged drimanes RD1 and RD2, respectively) and drimane (D), in source rock extracts and fluids from fluvial-deltaic petroleum systems in the Carnarvon Basin, North-West Shelf of Australia (NWS). We have calculated the ratio (RD1+RD2)/D as proposed by Ji et al. (Ji, L., He, C., Zhang, M., Wu, Y., Li, X. [2016]. Bicyclic alkanes in source rocks of the Triassic Yanchang Formation in the Ordos Basin and their inconsistency in oil-source correlation. Marine and Petroleum Geology 72, 359–373) and compared their results with samples from the NWS. Our compilation of rearranged drimane ratios for a variety of lithologies suggests clay-catalysed rearrangement of C15-decalins in siliciclastic source rocks from fluvial-deltaic depositional environments. Therefore, the mineral content seems to be a key factor controlling the isomerisation of decalins. Samples with higher clay content, such as claystone from the Brigadier Formation, have higher (RD1+RD2)/D ratios (∼6) and higher C29 diasterane / C29 sterane (∼1.1) compared to silty claystone from the Legendre Formation (∼2 and ∼0.8 respectively). The effects exerted by thermal maturity of the source rock and redox conditions of the depositional environment require further investigation, although samples from the Yanchang Fomation and La Luna Formation, where deposition occurred under more reducing conditions, exhibit lower rearranged drimane ratios (<2).

  • Sources and degradation of sedimentary organic matter in the mud belt of the East China Sea: Implications from the enantiomers of amino acids
    Org. Geochem. (IF 3.081) Pub Date : 2017-11-28
    Yan Chen, Gui-Peng Yang, Chong-Xiao Ji, Hong-Hai Zhang, Peng-Yan Zhang

    Total organic carbon (TOC), total N (TN) and amino acids (AAs, including the bacterial biomarker D-AAs) were determined from the mud belt of the East China Sea (ECS) in sediment cores. The concentration of total hydrolyzable AAs (THAAs) ranged from 3.35 to 13.44 µmol/g dry wt (dw) of the sedimentary organic matter (SOM), exhibiting a decreasing trend downcore. Major constituents of THAAs were glycine (Gly), L-serine (L-Ser), L-alanine (L-Ala), L-glutamic acid (L-Glu) and L-aspartic acid (L-Asp), whereas the D-aspartic acid (D-Asp), D-glutamic acid (D-Glu), D-serine (D-Ser) and D-alanine (D-Ala), along with non-protein AAs (γ-aminobutyric acid and β-alanine) together accounted for ca. 7% of the THAA pool. Given the C/N ratio values, the OM in three cores was predominantly of marine origin. Based on D-Ala yield, bacterial OM represented on average 18% of TOC and ca. 30% of TN. C-normalized THAAss (THAA-C%) was a sensitive indicator of SOM diagenetic alteration in the upper 40 cm of sediment. However, the degradation index (DI) and reactivity index (RI) values in the cores did not exhibit any definite trend, which may indicate that these two indicators were not sensitive in the early stage of sediment degradation. Negative correlation between the D/L ratio of Ala and THAA-C% suggested a close coupling between the extent of degradation and the accumulation and/or selective preservation of bacterial material in the sediments. In addition, correlation between clay content and the THAA-C% suggested that fine grained particles played an important role in affecting the quality of SOM.

  • Distribution and export of isoprenoid tetraether lipids in suspended particulate matter from the water column of the Western Atlantic Ocean
    Org. Geochem. (IF 3.081) Pub Date : 2017-11-22
    Sarah J. Hurley, Julius S. Lipp, Hilary G. Close, Kai-Uwe Hinrichs, Ann Pearson

    The TEX86 paleotemperature proxy is based on the distribution of archaeal glycerol dibiphytanyl glycerol tetraether (GDGT) lipids preserved in marine sediments, yet both the influence of different physiological factors on the structural distribution of GDGTs and the mechanism(s) by which GDGTs are exported to marine sediments remain unresolved. We investigated the abundance and structural distribution of GDGTs in the South-west and Equatorial Atlantic Ocean in four water column profiles spanning 48 degrees of latitude. The depth distribution of GDGTs was consistent with production by ammonia-oxidizing Thaumarchaeota; maximum GDGT concentration occurred at the base of the NO2- maximum, core GDGTs dominated the structural distribution in surface waters above the NO2- maximum, and intact polar GDGTs – potentially indicating live cells – were more abundant below the NO2- maximum. Between 0 and1000 m, > 98% of the integrated GDGT inventory was present in waters at and below the NO2- maximum. Depth profiles of TEX86 temperature values displayed local minima at the NO2- maximum, while the ratio of GDGT-2:GDGT-3 increased with depth. A model based on the results predicts an average depth of origin for GDGTs exported to sediments between ca. 80-250 m. In the model, exported TEX86 values are remarkably insensitive to change in the average depth of origin of GDGTs. However, net TEX86 values exported from the water column appear to reflect euphotic zone productivity, possibly due to the correlative intensity of organic matter remineralization providing substrates for ammonium oxidation. Predicting the influence of these regional controls on sedimentary TEX86 records requires a better understanding of the interaction between GDGT production, particle dynamics, and the depth of origin for exported organic matter.

  • Fractionation of dibenzofurans during subsurface petroleum migration: based on molecular dynamics simulation and reservoir geochemistry
    Org. Geochem. (IF 3.081) Pub Date : 2017-11-20
    Meijun Li, Xiaoqiang Liu, T.-G. Wang, Weidong Jiang, Ronghui Fang, Lu Yang, Youjun Tang

    Dibenzofurans, oxygen-containing heterocyclic aromatics, are ubiquitous in crude oils. Their geochemical implications in petroleum geochemistry have been studied in numerous previous publications. Here, we present the distribution of dibenzofuran and its methylated homologues, and calculate the polarity and solubility of methyldibenzofuran isomers based on the Connolly surfaces and molecular dynamics simulation. The results show that the geo-chromatographic fractionation effect is evident due to absorption onto the water/silica matrix carrier bed during the oil migration process. Two potential molecular indicators relative to dibenzofurans, i.e. the total content of dibenzofurans and the relative abundances of 1-methyldibenzofuran to 4-methyldibenzofuran, are proposed for tracing the oil migration orientation and filling pathways. These two indicators were applied successfully in Paleozoic marine carbonate and Tertiary lacustrine sandstone reservoirs. The variations of source input, depositional environment, and maturity for oils from same source bed/kitchen appear to have no significant effect on these two indicators. It suggested that these two dibenzofuran indicators are potentially effective molecular tracers for orientation and filling pathways in oil migration.

  • Synthesis of 26-methyl cholestane and identification of cryostanes in mid-Neoproterozoic sediments
    Org. Geochem. (IF 3.081) Pub Date : 2017-11-16
    Pierre Adam, Philippe Schaeffer, Jochen J. Brocks

    The biomarker distributions of mid-Neoproterozoic sediments (800 and 717 million yr, Ma) from various locations are characterized by the occurrence, besides cholestane isomers, of a novel series of C28 steranes, whereas classical C-24 alkylated steranes are absent. These unusual C28 steranes, termed cryostanes, seem to be restricted to pre-Snowball Earth sediments. We report their conclusive identification as 26-methyl cholestanes based on comparison of gas chromatographic (GC) behavior and mass spectrum of the last eluting cryostane isomer with those of a 26-methyl 5α (H),14 α (H),17 α (H),20R-cholestane standard obtained by way of synthesis. Sterols methylated at C-26 seem to be restricted to some demosponges and, based on molecular clock estimates, demosponges may have emerged in the time interval 800 to 700 Ma. A sponge origin of cryostanes is thereby conceivable, making cryostanes the oldest molecular markers for animals. However, other biological sources need to be explored, including the wide variety of eukaryotic protists.

  • Evidence for major input of riverine organic matter into the ocean
    Org. Geochem. (IF 3.081) Pub Date : 2017-11-14
    Xiaoyan Cao, George R. Aiken, Kenna D. Butler, Thomas G. Huntington, William M. Balch, Jingdong Mao, Klaus Schmidt-Rohr

    The changes in the structure of XAD-8 isolated dissolved organic matter (DOM) samples along a river (Penobscot River) to estuary (Penobscot Bay) to ocean (across the Gulf of Maine) transect and from the Pacific Ocean were investigated using selective and two dimensional (2D) nuclear magnetic resonance (NMR) spectroscopy coupled with elemental and carbon isotope analysis. The results provide important insights into the nature of relatively stable structures in the river-to-ocean continuum and the enigma of the fate of terrestrial DOM in the marine system. First, lignin and carboxyl-rich alicyclic molecules (CRAMs), which are indistinguishable from mass spectrometry, were clearly differentiated with NMR spectroscopy. NMR unambiguously showed that CRAMs persisted along the river-to-ocean transect and in the Pacific Ocean, while lignin residues dramatically decreased in abundance from the river to the coastal ocean and from the Pacific Ocean. The results challenge a previous conclusion that lignin-derived compounds are refractory and can accumulate in the coastal ocean. The loss of terrestrial plant-derived aromatic compounds such as lignin and tannin residues throughout the sequence of riverine, coastal, and open ocean DOM extracts could also partially explain the change in carbon stable isotope composition from riverine DOM (δ 13C -27.6‰) to ocean DOM (δ 13C -23.0‰) extracts. The observation, from advanced NMR, of similar CRAM molecules in XAD-8 isolated DOM samples from the Penobscot River to the Penobscot Bay and from the ocean refutes a previous conclusion that XAD-isolated DOM from seawater and river are distinctly different. The alicyclic structural features of CRAMs and their presence as the major structural units in DOM extracts from the Penobscot River to Gulf of Maine transect, together with the deduced old 14C age of CRAMs in the ocean, imply that terrestrial CRAMs may persist on timescales long enough to be transported into the ocean.

  • Rapid and economical quantification of black carbon in soils using a modified benzene polycarboxylic acids (BPCA) method
    Org. Geochem. (IF 3.081) Pub Date : 2017-11-14
    Mireia Llorente, María-Belén Turrión, Bruno Glaser

    Black carbon, the ubiquitous stable product of incomplete combustion, is a potential sink for atmospheric CO2 and, therefore, a contributor to the Earth’s radiative heat balance. Accurate information about the distribution of black carbon in soils is important for climate change projections, given that the black carbon content of soils varies widely across the globe. In response to this issue, an accurate, rapid and cost-effective method to assess BC is needed. The use of benzene polycarboxylic acids (BPCA) as molecular markers of black carbon has the advantage of specifically tracking the polyaromatic backbone of the black carbon continuum. However, this method has three major disadvantages: (a) it is expensive; (b) it is very time-consuming; and (c) some of the literature states that care has to be taken when applying the BPCA method to organic-rich matrices. In response to these disadvantages, the aims of the present study were: (a) to check the validity of a new shorter and economical method for black carbon quantification based on BPCA as molecular markers; and (b) to study the correlation between BPCA formation during the analysis and the soil sample’s OC content. The proposed method for black carbon quantification is based on the BPCA method developed by Glaser et al. (1998), but uses a simple direct elemental analysis (EA) of C content in the residue obtained after a strong digestion instead of the molecular determination by gas chromatography with flame ionisation detector (GC-FID). This simplification of the method represents a great cost reduction (in time and expense) providing a rapid and accurate method for large soil sample sets.

  • Geochemical composition of oils in the Dunga Field, western Kazakhstan: Evidence for a lacustrine source and a complex filling history
    Org. Geochem. (IF 3.081) Pub Date : 2017-11-13
    H.I. Petersen, D. Cumming, E. Dujoncquoy

    The onshore Mangyshlak Basin, located in western Kazakhstan along the eastern coastline of the Caspian Sea, is a prolific hydrocarbon province and home to numerous oil and gas fields. The primary reservoirs are Jurassic clastic sandstones, but the Dunga Field contains hydrocarbons in Lower Cretaceous (Valanginian, Barremian, Aptian, Albian) clastic and carbonate reservoirs. The general perception is that these petroleum accumulations were charged from marine source rocks of likely Middle Triassic age. A detailed organic geochemical investigation of oils from the Cretaceous reservoirs of Dunga Field suggests that all were generated from the same lacustrine source rock facies (organofacies C) in mature kitchens in the offshore Mangyshlak Basin, followed by a complex filling history. The oils are characterised by relatively high wax content, low sulfur content (mainly < 0.2 wt%), typical lacustrine tricyclic terpane ratios (high T26/T25, high T24/T23, low T22/21) and low H31R/H30 hopane ratios. Maturity of the source rock that generated the oils would tentatively correspond to a VR of ∼0.9–1.1%Ro. The oils are non-biodegraded, but some minor water washing might have occurred. Diamondoid data suggest that the Barremian reservoir contains a mixed oil pool composed of the non-cracked lacustrine oil and another charge of oil that is cracked from 79% to 83% and likely also to be slightly fractionated. This requires a complex filling history of the Dunga Field involving two hydrocarbon charges, likely from two different fetch areas. One charge of non-cracked oil migrated into all four reservoirs whereas another charge was ‘hoteled’ and cracked before remigration to the Barremian reservoir only and mixed with the non-cracked oil.

  • Degradation changes in plant root cell wall structural molecules during extended decomposition of important agricultural crop and forage species
    Org. Geochem. (IF 3.081) Pub Date : 2017-11-13
    Kathryn E. White, Frank J. Coale, James B. Reeves

    Little is known regarding changes in cell wall structural molecules (lignin, cellulose and hemicellulose) as plant roots decompose, despite their importance for soil organic matter (OM) formation. The objectives of this study were to quantify changes in root composition during 270 d incubations of ten important grain and forage crops utilizing forage fiber analysis and to characterize the changes in cell wall composition and structure using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Large, species-dependent variation was observed in the extent of root tissue decomposition over time, ranging from 82.5% of initial mass for alfalfa to 21.5% for switchgrass. Fiber analysis revealed that initial rapid decomposition increased lignin concentration and cellulose concentration while hemicellulose declined, while all three moieties degraded proportionally thereafter. Similar trends were found in the ratios between the DRIFTS diagnostic peaks for lignin, cellulose and the carbonyls of hemicellulose and wax components. Spectra illustrated changes during decomposition, particularly in more extensively decomposed roots. Features potentially indicative of suberin preservation were found in the region between 2800 cm-1 and 3000 cm-1. Examination of the region between 1000 cm-1 and 1300 cm-1 revealed possible change in hemicellulose structure. The results illustrate the effect of differences in cell wall composition and structure during root decomposition and expand understanding of the role of roots in soil OM dynamics. Variability in root degradation and change in cell wall composition among species demonstrate that characterization of a broad range of individual species is necessary to predict root contributions to soil C.

  • Severe drought-influenced composition and δ13C of plant and soil n-alkanes in model temperate grassland and heathland ecosystems
    Org. Geochem. (IF 3.081) Pub Date : 2017-11-11
    Kavita Srivastava, Guido L.B. Wiesenberg

    Drought events are predicted to increase under future climate change. In temperate ecosystems, plants are capable of resisting drought due to their hydrophobic wax layer, in which n-alkanes are important constituents. In soils, plant-derived n-alkanes are comparatively resistant to degradation. To improve understanding of the significance of n-alkanes in plant-soil systems during a severe drought period (104 days), we investigated bulk organic carbon (Corg) concentration, total lipid extract (TLE) concentration, n-alkane molecular ratios such as average chain length (ACL), carbon preference index (CPI) and chain length ratios of different n-alkane compounds, in addition to the compound-specific isotope composition (δ13C) of n-alkanes in model temperate grassland and heathland plant-soil systems. Although plant communities of two (heathland) and four (grassland) species were available, only one representative species per biome was accessible for the current study. Heathland plants and soil revealed significantly higher concentrations of Corg and TLE compared with grassland. TLE and alkane composition responded quickly during the first drought phase (0 – 40 days). This indicates that plants were actively utilizing C and produced more n-alkanes in order to withstand drought, which was confirmed by increased (2 – 3‰) δ13C values for n-alkanes in shoot biomass. However, during later drought phases all the parameters remained constant for plants and soils. This suggests limited change in biosynthesis and cycling of plant lipids such as n-alkanes during intense drought. Surprisingly, during the first drought phase, increased ACL and CPI ratios in soil demonstrated a rapid input of plant-derived long chain n-alkanes to soil, which was not expected due to the decadal residence time of alkanes in soil. The study enabled tracing of plant metabolic response in terms of alkane biosynthesis under different phases of drought and rapid cycling of alkanes in the plant-soil system.

  • Lipoxygenase-induced autoxidative degradation of terrestrial particulate organic matter in estuaries: A widespread process enhanced at high and low latitude
    Org. Geochem. (IF 3.081) Pub Date : 2017-10-31
    Marie-Aimée Galeron, Olivier Radakovitch, Bruno Charrière, Frédéric Vaultier, John K. Volkman, Thomas S. Bianchi, Nicolas D. Ward, Patricia M. Medeiros, Henrique O. Sawakuchi, Suzanne Tank, Philippe Kerhervé, Jean-François Rontani

    There exists a substantial amount of research on abiotic (e.g. photochemical) degradation pertaining to organic matter (OM) in the marine realm. While recent research has shown its importance in the degradation of terrestrial particulate OM (TPOM), the mechanisms involved in the induction of autoxidation in estuaries remain unclear. In this study, we propose for the first time the involvement of lipoxygenase (LOX) activity in the induction of autoxidation in mixed waters. The observation of unusual profiles of palmitoleic acid oxidation products and the presence of jasmonic acid in suspended particulate matter (SPM) collected close to the Rhône River, as well as in samples from the Mackenzie and Amazon rivers, is attributed to strong LOX activity. We show the role played by salinity in the induction of this LOX activity and provide an explanation for the differences in estuarine autoxidation level. At high latitude, lower temperatures and irradiance favor photooxidative damage to higher plant debris and, consequently, hydroperoxide production. High hydroperoxide content strongly contributes to LOX activation in mixed waters. The high resulting LOX activity enhances alkoxyl radical production and thus autoxidation. On the contrary, at low latitude, photooxidative effects are limited, and riverine autoxidation is favored. The higher hydroperoxide content of TPOM may, as a consequence, thereby also contribute to a high level of LOX activity and autoxidation in estuaries. In temperate zones, land and riverine photooxidative and autoxidative damage is limited, unlike estuaries where we observed significant LOX-induced and autoxidative damage.

  • Naphthenic acids in oil sands process waters: Identification by conversion of the acids or esters to hydrocarbons
    Org. Geochem. (IF 3.081) Pub Date : 2017-09-21
    Michael J. Wilde, Steven J. Rowland

    Concerns over the toxicity associated with ‘naphthenic’ acids (NA) within oil produced waters and oil sands process waters (OSPW), whether justified or not, have increased the need for the structural elucidation of NA. Certainly, oil sands mining operations, such as those in Alberta, Canada, result in the production of large volumes of OSPW contaminated with NA. Monitoring NA, predicting the toxicities and accounting for the toxicity of residual NA after clean-up treatments, have all been hindered by the lack of NA identifications. Recently, a method involving the conversion of the esters of petroleum NA to the corresponding hydrocarbons before analysis by comprehensive two-dimensional gas chromatography-mass spectrometry (GC×GC-MS), resulted in the most comprehensive study of bicyclic NA from petroleum to date. Here, we present results of the analysis by GC×GC-MS of NA extracted from OSPW after conversion of the acids or ester derivatives to the corresponding hydrocarbons. The identifications presented include novel alicyclic, aromatic and sulphur-containing hydrocarbons and thus, by inference, of the corresponding acids. This supports and significantly extends, previous identifications of OSPW NA as their methyl esters and can now be used to better inform environmental monitoring programs and toxicity studies.

  • Occurrence and significance of mono-, di- and anhydrosaccharide biomolecules in Mesozoic and Cenozoic lignites and fossil wood
    Org. Geochem. (IF 3.081) Pub Date : 2017-11-22
    Leszek Marynowski, Michał Bucha, Justyna Smolarek, Małgorzata Wendoeff, Bernd R.T. Simoneit

    Mono-, di- and polysaccharides are common constituents of living organisms, but their occurrence and state of preservation in geological materials have only rarely been considered. Here, we present the monosaccharide, and for the first time the di- and anhydrosaccharide, identifications and distributions in Middle Miocene lignite and Middle Jurassic fossil wood samples. Detritic lignites contain fructose and glucose as dominant monosaccharides, and sucrose and trehalose as important disaccharides. Xylites contain monosaccharides (arabinose, arabinofuranose, glucose, and minor xylose and fructose), saccharols (erythritol, arabitol and mannitol), and also some disaccharides. The Middle Jurassic fossil wood samples contain glucose, glucofuranose and levoglucosan. The high content of holocellulose (up to 55 wt%) and co-occurrence of characteristic monosaccharides as arabinose, xylose and mannose in xylites suggest that not only cellulose, but also hemicellulose was preserved in samples as old as 13 Ma. Compounds like trehalose and mannitol appear to be products of wood-degrading fungi. Surprisingly, glucose, the most stable monosaccharide, and levoglucosan can occur in much older organic matter (ca. 168 Ma) as products from cellulose degradation, and possibly a remnant from wildfire burning of wood, respectively. Our findings confirm that saccharides can be preserved under favorable conditions in sedimentary organic matter of the Mesozoic to the Cenozoic eras, and can be used as specific biomarkers of cellulose and hemicellulose degradation, fungal metabolism, and wildfire events. However, we cannot exclude the possibility that at least part of the saccharides may be preserved in sedimentary rocks as the free compounds, common in plants and microorganisms.

  • Spatial and temporal variability in the nutritional quality of basal resources along a temperate river/estuary continuum
    Org. Geochem. (IF 3.081) Pub Date : 2017-11-21
    Nicole B. Richoux, Leandro Bergamino, Sydney Moyo, Tatenda Dalu

    The nature and quality of basal resources within aquatic food webs are complex and have the potential to shift over space and time. We used fatty acid analysis to assess variations in the nutritional structure of the suspended and basal pools along an entire river system, and to assess the contributions of vascular plant (i.e., mainly of terrestrial origin, but could include aquatic macrophytes) vs algal (i.e., aquatic origin) sources to the suspended particulate matter (SPM) pools. Samples were collected in a temperate South African river on four occasions between September 2012 and June 2013. We found orderly patterns in the fatty acid composition of the basal resources at the sites during most seasons. Regardless of site or season, the benthic algal pools (epiphyton, epipelon and epilithon) were the most nutritionally rich resources based on essential fatty acid contents and diatom indices. During early and late spring, proportions of essential fatty acids in the epiphyton decreased downstream where increased light was available, consistent with predictions from the light:nutrient hypothesis (but inconsistent with epilithon and epipelon results). There were substantial changes in vascular plant contributions to the SPM pools along the river, but the same patterns were not produced in all seasons. Of all the river models considered, the data were more consistent with the riverine productivity model, which particularly emphasises the importance of autotrophic production in rivers. Our study provides new detail on the complexity of basal resource nutritional quality and how it can shift along a lotic system over time.

  • Seasonal and canopy height variation in n-alkanes and their carbon isotopes in a temperate forest
    Org. Geochem. (IF 3.081) Pub Date : 2017-11-14
    Yeon Jee Suh, Aaron F. Diefendorf

    The stable carbon isotopic composition (δ13C) of terrestrial leaf wax components, such as n-alkanes (δ13Calk), is used extensively to address questions about past changes in vegetation, climate and the carbon cycle. To interpret sedimentary δ13Calk values, characterization of the environmental and biological controls on carbon isotopic fractionation during plant metabolism is required, especially for fractionation that occurs during photosynthesis (ɛleaf-CO2 or Δleaf) and n-alkane biosynthesis (εalk-leaf). Although much is understood about controls on ɛleaf-CO2, little is known about seasonal or within canopy variation in n-alkane composition and εalk-leaf. To address the gap, we sampled 5 common tree species (Acer rubrum, Acer saccharum, Juniperus virginiana, Sassafras albidum and Ulmus americana) from buds to senescing leaves from within a single temperate forest in southwestern Ohio, USA. We measured n-alkane concentration, δ13Calk, leaf biomass δ13C (δ13Cleaf), δ13C of atmospheric CO2 within the canopy, luminous flux and specific leaf area (SLA). In angiosperms, δ13Calk values were low in the buds, increased substantially (4 to 6‰) to the highest values in young leaves and then decreased (1 to 3‰) in mature leaves. Seasonal variation in δ13Calk values generally tracked changes in δ13Cleaf, but the fractionation between leaf and n-alkanes (εalk-leaf) was variable, with lower values in buds (−6.7 ± 1.1‰) than in young leaves (−4.9 ± 2.3‰). Later in the growing season, εalk-leaf values stabilized and ranged between −2.5 and −6.5‰, with a mean of −4.3 ± 1.4‰ in angiosperms. Stabilization of δ13Calk values coincided with settling of SLA and increase in average chain length (ACL). For the conifer, temporal variation in δ13Calk values was smaller (ca. 2‰) compared with the angiosperms, and εalk-leaf values slightly increased when n-alkane production increased. Average εalk-leaf value in the conifer at the end of the growing season was −5.0 ± 0.7‰. We speculate that εalk-leaf variation in both angiosperms and the conifer reflects changes in the source of carbon for plant metabolism as well as the timing of wax synthesis. We also tested canopy effects on n-alkane concentration and carbon isotopic fractionation at different heights and extent of canopy closure. n-Alkane concentrations were higher at the top of the canopy by 2 to 8 times than in the lower canopy. The εleaf-CO2 values were more negative at the lower two thirds of the canopy than the upper one third of the canopy by 2 to 4‰ in two species with closed canopies. The εalk-leaf values did not vary with height and extent of canopy closure. To further explore the link between canopy leaves and leaf litter, we compared δ13Calk values of mature canopy leaves with those in leaf litter collected during senescence, and the offsets were within ca. 1‰. The results confirm indications that leaf litter δ13Calk values reflect those of the upper canopy leaves due to not only high leaf biomass in the upper canopy but also to high n-alkane production.

  • Improved Method for Simultaneous Determination of Saturated and Aromatic Biomarkers, Organosulfur Compounds and Diamondoids in Crude Oils by GC-MS/MS
    Org. Geochem. (IF 3.081) Pub Date : 2017-11-10
    Mei Mei, K.K. (Adry) Bissada, Thomas B. Malloy, L. Mike Darnell, Ewa B. Szymcyk

    An improved GC-MS/MS method was developed for simultaneous determination of various types of molecular markers in whole crudes without the need for laborious group-type separation and discrete analyses of the diverse fractions. The ability to simultaneously identify and accurately quantify the numerous saturated and aromatic biomarkers, the organosulfur compounds (OSC) and the diamondoids in a single step was attained through use of expanded multiple reaction monitoring (MRM) transitions in specific time windows. The optimization was achieved through improving GC separation and selective choice of precursor and product ions, collision energies, dwell time, and time windows in the MRM settings. The method was tested using four crude oils of diverse types, densities and maturities: a heavy crude from Sicily (18° API), an intermediate-density crude from the Arabian Gulf (33° API), a light black oil from California (39° API), and a light condensate from Wyoming (53° API). The improvements were validated by checking accuracy and precision using three spiking levels of standard compounds into the oils. Comparison of the results of the single-step GC-MS/MS method with the results from traditional multi-step approaches indicates that the single-step GC-MS/MS approach achieved higher resolution, higher specificity and sensitivity, higher accuracy and precision for calculation of the common biomarker indices. Additionally, the single-step approach precluded the loss of light-end components during sample preparation and minimized co-elution problems in molecular analyses.

Some contents have been Reproduced with permission of the American Chemical Society.
Some contents have been Reproduced by permission of The Royal Society of Chemistry.
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