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1.
The δ13C values of 23 unevenly spaced guano samples from a 17-cm long clay sediment profile in Gaura cu Musc? Cave (GM), in SW Romania, made it possible to preliminarily characterize the Medieval Warm Period summer hydroclimate regime. The beginning of the sequence (AD 990) was rather wet for more than a century, before becoming progressively drier. After a brief, yet distinct wet period around AD 1170, drier conditions, with a possible shift from C3 to a mixed C3-dominated/C4 type vegetation (2 ‰ lower δ13C values), prevailed for almost half a century before the climate became colder and wetter at the onset of the Little Ice Age, when bats left the cave. The guano-inferred wet and dry intervals from the GM Cave are mirrored by changes in the color and amount of clay accumulated in the cave. They also agree well with reconstructions based on pollen and charcoal from peat bogs and δ13C and δ18O on speleothems from other Romanian sites. Overall, these results indicate that the δ13C of bat guano can provide a sensitive record of the short-term coupling between local/regional climate and the plant–insect–bat–guano system.  相似文献   

2.
Stable isotopes of faeces contain information related to the animals feeding ecology. The use of stable isotope values from subfossil faeces as a palaeoenvironmental indicator depends on how faithfully the animal records their local environment. Here we present insectivorous bat guano δ13C and δ15N values from a precipitation gradient across the southern United States and northern Mexico to compare with local vegetation and climate. We find δ13C values to be an excellent predictor of expected C4/CAM vegetation, indicating that the bats are non-selective in their diet. Moreover, we find bat guano δ13C values to be strongly correlated with summer precipitation amount and winter precipitation ratio. We also find evidence for a significant relationship with mean annual temperature. In general, we do not find δ15N values to be related to any parameters along the climatic gradient we examined. Additionally, we measured δ13C and δ15N values of bulk guano deposited annually from 1968 to 1987 in a varved guano deposit at Eagle Creek Cave, Arizona. Neither δ13C nor δ15N values were significantly related to various local meteorological variables; however, we found δ13C values of guano to be significantly related to drought and to the North American Monsoon indicating bat guano δ13C values preserve an interpretable record of large-scale atmospheric variability.  相似文献   

3.
A suite of 18 oils from the Barrow Island oilfield, Australia, and a non-biodegraded reference oil have been analysed compositionally in order to detail the effect of minor to moderate biodegradation on C5 to C9 hydrocarbons. Carbon isotopic data for individual low molecular weight hydrocarbons were also obtained for six of the oils. The Barrow Island oils came from different production wells, reservoir horizons, and compartments, but have a common source (the Upper Jurassic Dingo Claystone Formation), with some organo-facies differences. Hydrocarbon ratios based on hopanes, steranes, alkylnaphthalenes and alkylphenanthrenes indicate thermal maturities of about 0.8% Rc for most of the oils. The co-occurrence in all the oils of relatively high amounts of 25-norhopanes with C5 to C9 hydrocarbons, aromatic hydrocarbons and cyclic alkanes implies that the oils are the result of multiple charging, with a heavily biodegraded charge being overprinted by fresher and more pristine oil. The later oil charge was itself variably biodegraded, leading to significant compositional variations across the oilfield, which help delineate compartmentalisation. Biodegradation resulted in strong depletion of n-alkanes (>95%) from most of the oils. Benzene and toluene were partially or completely removed from the Barrow Island oils by water washing. However, hydrocarbons with lower water solubility were either not affected by water washing, or water washing had only a minor effect. There are three main controls on the susceptibility to biodegradation of cyclic, branched and aromatic low molecular weight hydrocarbons: carbon skeleton, degree of alkylation, and position of alkylation. Firstly, ring preference ratios at C6 and C7 show that isoalkanes are retained preferentially relative to alkylcyclohexanes, and to some extent alkylcyclopentanes. Dimethylpentanes are substantially more resistant to biodegradation than most dimethylcyclopentanes, but methylhexanes are depleted faster than methylpentanes and dimethylcyclopentanes. For C8 and C9 hydrocarbons, alkylcyclohexanes are more resistant to biodegradation than linear alkanes. Secondly, there is a trend of lower susceptibility to biodegradation with greater alkyl substitution for isoalkanes, alkylcyclohexanes, alkylcyclopentanes and alkylbenzenes. Thirdly, the position of alkylation has a strong control, with adjacent methyl groups reducing the susceptibility of an isomer to biodegradation. 1,2,3-Trimethylbenzene is the most resistant of the C3 alkylbenzene isomers during moderate biodegradation. 2-Methylalkanes are the most susceptible branched alkanes to biodegradation, 3-methylalkanes are the most resistant and 4-methylalkanes have intermediate resistance. Therefore, terminal methyl groups are more prone to bacterial attack compared to mid-chain isomers, and C3 carbon chains are more readily utilised than C2 carbon chains. 1,1-Dimethylcyclopentane and 1,1-dimethylcyclohexane are the most resistant of the alkylcyclohexanes and alkylcyclopentanes to biodegradation. The straight-chained and branched C5–C9 alkanes are isotopically light (depleted in 13C) relative to cycloalkanes and aromatic hydrocarbons. The effects of biodegradation consistently lead to enrichment in 13C for each remaining hydrocarbon, due to preferential removal of 12C. Differences in the rates of biodegradation of low molecular weight hydrocarbons shown by compositional data are also reflected in the level of enrichment in 13C. The carbon isotopic effects of biodegradation show a decreasing level of isotopic enrichments in 13C with increasing molecular weight. This suggests that the kinetic isotope effect associated with biodegradation is site-specific and often related to a terminal carbon, where its impact on the isotopic composition becomes progressively ‘diluted’ with increasing carbon number.  相似文献   

4.
Simultaneous mass spectral detection and stable carbon isotope analysis was performed on individual indigenous n-alkanes isolated from single C4 and C3 plant species and on a series of aliphatic and polycyclic aromatic hydrocarbons (PAH) produced from the combustion of these same biomass materials. The analysis technique used a combined gas chromatograph-mass spectrometer/combustion/isotope ratio mass spectrometer (GC-MS/C/IRMS). Precision (2σ) for replicate measurements of individual compounds in standard solutions using this novel configuration ranged between 0.2 and 0.5‰ for n-alkanes and 0.3 and 0.8‰ for PAH. Accuracy of the n-alkane measurements ranged between 0.1 and 0.4‰ and that of the PAH measurements ranged between 0.2 and 0.9‰. Replicate GC-MS/C/IRMS measurements on the combustion-derived n-alkene/alkane pairs were performed to within a precision of between 0.1 and 1.1‰ and the precision for the combustion PAH was similar to the standard PAH solution. No notable isotopic effects were observed when altering the temperature of the combustion process from 900 to 700°C, or as a result of the individual n-alkenes/alkanes partitioning between the gaseous and condensate fractions. Combustion-derived n-alkenes/alkanes ranged from C11 to C31, and the C4-derived n-alkenes/alkanes were approx. 8‰ more enriched in 13C than the C3-derived compounds. Both the C4 and C3-derived n-alkenes/alkanes (C20-C30) were isotopically similar to the indigenous n-alkanes and were 2-3‰ more depleted in 13C than the lower mol. wt (C1111-C19) n-alkenes/alkanes, suggesting an independent origin for the lower mol. wt compounds. Combustion-generated C4 and C3-derived 2-, 3-, and 4-ring PAH were also isotopically distinct (Δδ = 10‰). Unlike the n-alkenes/alkanes, no compound-to-compound variations were observed between the low and high mol. wt PAH. This study demonstrates that the isotopic composition of original plant biomass material is mainly preserved in the aliphatic hydrocarbons and PAH generated by its combustion. Consequently, analyses of these compounds in sediments impacted by fire occurrences may provide useful information about paleo-fire activity that may help elucidate the impact biomass burning may have had and could have on climate-biosphere interactions.  相似文献   

5.
The Northwest Shelf of the Delaware Basin, SE New Mexico is the site of several large and productive oil and gas fields. The most productive reservoirs are located in the late Pennsylvanian Morrow and early Permian Abo formations. Production from the latter more important play is predominately from fluvial Abo red beds of the Pecos Slope Field. The oxidizing conditions implied by the reddish color of the formation require an external hydrocarbon source. To test the existing migration model for the region and constrain the location of potential hydrocarbon sources, we measured the elemental and isotopic composition of noble gases produced along with the hydrocarbons. We found the hydrocarbons to be highly enriched in radiogenic 4He, 40*Ar and nucleogenic 21*Ne [F(4He) = 44,000-250,000; 40Ar/36Ar = 400-3145; 21Ne/22Ne = 0.044-0.071]. The greatest enrichments occur in the Pecos Slope gas fields. The hydrocarbons also contain three independent nonradiogenic noble gas components each with an atmospheric isotopic composition. One component is most likely air-saturated water (ASW). The second component is enriched in the heavy noble gases [F(130Xe) > 8.5] and is derived from the hydrocarbon sources. The third component is enriched in Ne [F(20Ne) > 0.8] that we believe is degassed from sources within the reservoirs. This component is correlated with but decoupled from the dominant source of radiogenic 4He and 40*Ar. Very high concentrations of 4He (up to ∼1% by volume) in the Pecos slope reservoirs require a source external to the reservoirs, such as the underlying Precambrian basement granites and sedimentary equivalents. Structural buckles cutting through the Pecos field may act as high flux vertical pathways for the radiogenic 4He. If the hydrocarbons in the Pecos slope fields have migrated northward from the deeper Delaware Basin, as suggested by compositional trends, then perhaps the buckles also play an important role in the distribution and filling of the Pecos slope reservoirs.  相似文献   

6.
This paper reports the isotope effects in an open-system Fischer-Tropsch type (FTT) synthesis, with implications for the origin of natural abiogenic hydrocarbons. The starting form of carbon was CO2, with carbon and hydrogen isotopic compositions measured for products of catalytic hydrogenation of CO2 on iron and cobalt catalysts (FTCO2-Fe and FTCO2-Co) at 350 and 245 °C, respectively, and 10 MPa. The carbon isotopic composition of the resulting saturated hydrocarbons (alkanes) as a function of carbon number shows a positive trend for both FTCO2-Fe and FTCO2-Co, with a fractionation of 2-4‰ and 3-6‰ between CH4 and C2H6 over the Fe and Co catalysts, respectively. The unsaturated hydrocarbons (alkenes) do not show any trend. A strong kinetic isotope fractionation (>40‰) occurred between CO2 and CH4 in both experiments. The hydrogen isotope fractionation between alkanes appeared to be similar to that found in natural (thermogenic and biogenic) gases, with enrichment in deuterium of longer hydrocarbon chains; the dominant H/D fractionation occurred between CH4 and C2H6. Alkenes in the products of the FTCO2-Fe reaction are enriched in deuterium (∼50‰) and do not show any trend versus carbon number. We suggest that other than FTT reactions or a simple mixing are responsible for the occurrence of the inverse isotopic trends in both δ13C and δD found in light hydrocarbons in some terrestrial environments and meteorites.  相似文献   

7.
To understand reaction pathways and isotope systematics during mineral-catalyzed abiotic synthesis of hydrocarbons under hydrothermal conditions, experiments involving magnetite and CO2 and H2-bearing aqueous fluids were conducted at 400 °C and 500 bars. A robust technique for sample storage and transfer from experimental apparatus to stable isotope mass spectrometer provides a methodology for integration of both carbon and hydrogen isotope characterization of reactants and products generated during abiogenic synthesis experiments. Experiments were performed with and without pretreatment of magnetite to remove background carbon associated with the mineral catalyst. Prior to experiments, the abundance and carbon isotope composition of all carbon-bearing components were determined. Time-series samples of the fluid from all experiments indicated significant concentrations of dissolved CO and C1-C3 hydrocarbons and relatively large changes in dissolved CO2 and H2 concentrations, consistent with formation of additional hydrocarbon components beyond C3. The existence of relatively high dissolved alkanes in the experiment involving non-pretreated magnetite in particular, suggests a complex catalytic process, likely involving reinforcing effects of mineral-derived carbon with newly synthesized hydrocarbons at the magnetite surface. Similar reactions may be important mechanisms for carbon reduction in chemically complex natural hydrothermal systems. In spite of evidence supporting abiotic hydrocarbon formation in all experiments, an “isotopic reversal” trend was not observed for 13C values of dissolved alkanes with increasing carbon number. This may relate to the specific mechanism of carbon reduction and hydrocarbon chain growth under hydrothermal conditions at elevated temperatures and pressures. Over time, significant 13C depletion in CH4 suggests either depolymerization reactions occurring in addition to synthesis, or reactions between the C1-C3 hydrocarbons and carbon species absorbed on mineral surfaces and in solution.  相似文献   

8.
Bat guano cores have been used as a source of palaeoenvironmental information to aid in the reconstruction of past climates and vegetation. We collected a 104‐cm‐long (43 cm compacted) guano core from Fern Cave, Alabama, USA, that provided a c. 6000‐year record of guano accumulation. Pollen, nutrients (C, N, P) and stable isotopes (δ13C, δ15N) were measured on the guano core with the objective of reconstructing the environmental history of the area from the mid‐Holocene to present. Our data indicate that bats have utilized Fern Cave for at least 6000 years and that Woodland Indians also utilized the cave for a short period. A 3‐cm charcoal layer was dated to 2720±30 cal. a BP and inferred to be Woodland Indian in origin from microscopic inspection and thickness. Pollen and geochemical data showed that bat diets changed in the late Holocene possibly linked to food supply and climate changes. These results demonstrate that guano cores are a useful tool of palaeoenvironmental reconstruction when other forms of palaeorecords do not exist and can add to local archaeological information.  相似文献   

9.
The natural gases in the Upper Paleozoic strata of the Ordos basin are characterized by relatively heavy C isotope of gaseous alkanes with δ 13C1 and δ13C2 values ranging mainly from ?35‰ to ?30‰ and ?27‰ to ?22‰, respectively, high δ13C excursions (round 10) between ethane and methane and predominant methane in hydrocarbon gases with most C1/(C1-C5) ratios in excess of 0.95, suggesting an origin of coal-derived gas. The gases exhibit different carbon isotopic profiles for C1-C4 alkanes with those of the natural gases found in the Lower Paleozoic of this basin, and believed to be originated from Carboniferous-Permian coal measures. The occurrence of regionally pervasive gas accumulation is distinct in the gently southward-dipping Shanbei slope of the central basin. It is noted that molecular and isotopic composition changes of the gases in various gas reservoirs are associated with the thermal maturities of gas source rocks. The abundances and δ13C values of methane generally decline northwards and from the basin center to its margins, and the effects of hydrocarbon migration on compositional modification seem insignificant. However, C isotopes of autogenetic calcites in the vertical and lateral section of reservoirs show a regular variation, and are as a whole depleted upwards and towards basin margins. Combination with gas maturity gradient, the analysis could be considered to be a useful tool for gas migration.  相似文献   

10.
Despite its location on sediment-free basalt, vent fluids from the Main Endeavour Field (MEF) contain chemical species that indicate fluids have interacted with sediments during circulation. We report on the distribution and isotopic abundances of organic compounds (C1-C3 alkanes and alkenes, benzene and toluene) in fluids collected from the Main Endeavour Field (MEF) in July, 2000, to understand the processes that regulate their abundances and characterize fluid sources. Aqueous organic compounds are derived from the thermal alteration of sedimentary organic matter and subsequently undergo further oxidation reactions during fluid flow. Fluid:sediment mass ratios calculated using ΣNH4 concentrations indicate that the sediments are distal to the MEF, resulting in a common reservoir of fluids for all of the vents. Following the generation from sediment alteration, aqueous organic compounds undergo secondary alteration reactions via a stepwise oxidation reaction mechanism. Alkane distributions and isotopic compositions indicate that organic compounds in MEF fluids have undergone a greater extent of alteration as compared to Middle Valley fluids, either due to differences in subsurface redox conditions or the residence time of fluids at subsurface conditions. The distributions of the aromatic compounds benzene and toluene are qualitatively consistent with the subsurface conditions indicated by equilibration of aqueous alkanes and alkanes. However, benzene and toluene do not achieve chemical equilibrium in the subsurface. Methane and CO2 also do not equilibrate chemically or isotopically at reaction zone temperatures, a likely result of an insufficient reaction time after addition of CO2 from magmatic sources during upflow. The organic geochemistry supports the assumption that the sediments with which MEF fluids interact has the same composition as sediments present in Middle Valley itself, and highlight differences in subsurface reaction zone conditions and fluid flow pathways at these two sites.  相似文献   

11.
Organic carbon isotopes in sediments have been frequently used to identify the source of organic matter.Here we present a study of organic δ~(13)C on two sediment profiles influenced by guano from Guangjin and Jinqing islands in the Xisha Archipelago,South China Sea.Organic matter from ornithogenic coral sand sediments has two main sources,guano pellets and plant residues,and their organic δ~(13)C(δ~(13)C_(OM)) are significantly different.Organic carbon δ~(13)C_(guano) is much higher thanδ~(13)C_(plants),and δ~(13)C_(OM)of bulk samples is intermediate.Based on a two-end-member mixing model,the proportions of guano-and plant-derived organic matter in the bulk samples were reconstructed quantitatively.The results showed that seabirds began to inhabit the islands around approximately1200-1400 AD,and that guano pellets have been an important source of soil organic matter since then.With the accumulation of guano-derived nutrients,plants began to develop prosperously on the islands in the last 200 years,which is reflected by the significant increase of plant-derived organic matter in the upper sediment layer.However,guano-derived organic matter decreased greatly in recent decades,indicating a rapid decrease in seabird population.Our results show that organic δ~(13)C can be effectively used to quantitatively determine different source contributions of OM to bulk ornithogenic coral sand sediments.  相似文献   

12.
Batina, M. C. & Reese, C. A. 2010: A Holocene pollen record recovered from a guano deposit: Round Spring Cavern, Missouri, USA. Boreas, 10.1111/j.1502‐3885.2010.00186.x. ISSN 0300‐9483. Bat guano deposits have been the subject of relatively few palynological studies. The studies to date have focused on the most modern samples only, and therefore the full potential of bat guano as an archive for fossil pollen analysis remains unknown. In this paper we report the results of pollen analysis on an 88‐cm vertical profile of fossil bat guano from Round Spring Cavern, Missouri. Four AMS radiocarbon dates were in sequence and showed that the guano pile was active between 8155 and approximately 550 cal. yr BP. Pollen is well preserved throughout the profile, with most samples containing less than 10% indeterminable pollen. Wind‐pollinated taxa are better represented than insect‐pollinated taxa, and pollen concentrations range from 842 to 371 660 grains cm?3. As expected, the dominant vegetation types represented by the pollen assemblage are oak and pine, in accordance with the region's oak–hickory–pine forest. However, there are discrepancies between the guano pollen record and the results of a lake pollen study conducted in southern Missouri. We propose several possible factors that may be influencing the guano pollen record and provide direction for further investigation into the palaeoecological potential of bat guano.  相似文献   

13.
With a production of 208.2 m3/d, heavy oil was produced by drill stem test (DST) from three shallow reservoirs in Sand Group Nos. Ⅰ and Ⅲ of the Neogene Guantao Formation (NgⅠ and NgⅢ) and the Eogene Dongying Formation (Ed) in an exploratory well Ban-14-1 within the Qianmiqiao region, Bohai Bay Basin, northern China. Based on the GC and GC-MS data of the NgⅠ and NgⅢ heavy oil samples, all n-alkanes and most isoprenoid hydrocarbons are lost and the GC baseline appears as an evident "hump", implying a large quantity of unresolved complex mixture (UCM), which typically revealed a result of heavy biodegradation. However, there still is a complete series of C14-C73 n-alkanes in the high-temperature gas chromatograms (HTGC) of the heavy oil, among which, the abundance of C30- n-alkanes are drastically reduced. The C35-C55 high molecular weight (HMW) n-alkanes are at high abundance and show a normal distribution pattern with major peak at C43 and an obvious odd-carbon-number predominance with CPI37-55 and  相似文献   

14.
The isotopic composition of dissolved inorganic carbon （DIC） in estuarine environments has been studied for its significant role in determining the isotopic composition of inorganic/organic matter and its applications to the study of various natural processes. In this paper, based on the stable isotope geochemical characteristics of dis- solved inorganic carbon in the Jiulong River Estuary, the following conclusions are drawn： （1） δ13CDIC values are mainly controlled by the mixing ratio of fresh water and sea water; （2）δ13Cphytoplankton values are linearly related to the δ13CDIC values; （3） δ13CpoM values for the Jiulong River Estuary are affected by anthropogenic pollution signifi- cantly; and （4） the comprehensive analysis of δ13Cphytoplankton, δ13CpoM and δ13CDIc shows that along with increasing salinity, the proportion of POM derived from the degradation of phytoplanktons gradually increases.  相似文献   

15.
Compound-specific isotope analysis has become an important tool in environmental studies and is an especially powerful way to evaluate biodegradation of hydrocarbons. Here, carbon isotope ratios of light hydrocarbons were used to characterise in-reservoir biodegradation in the Gullfaks oil field, offshore Norway. Increasing biodegradation, as characterised, for example, by increasing concentration ratios of Pr/n-C17 and Ph/n-C18, and decreasing concentrations of individual light hydrocarbons were correlated to 13C-enrichment of the light hydrocarbons. The δ13C values of C4 to C9n-alkanes increase by 7-3‰ within the six oil samples from the Brent Group of the Gullfaks oil field, slight changes (1-3‰) being observed for several branched alkanes and benzene, whereas no change (<1‰) in δ13C occurs for cyclohexane, methylcyclohexane, and toluene. Application of the Rayleigh equation demonstrated high to fair correlation of concentration and isotope data of i- and n-pentane, n-hexane, and n-heptane, documenting that biodegradation in reservoirs can be described by the Rayleigh model. Using the appropriate isotope fractionation factor of n-hexane, derived from laboratory experiments, quantification of the loss of this petroleum constituent due to biodegradation is possible. Toluene, which is known to be highly susceptible to biodegradation, is not degraded within the Gullfaks oil field, implying that the local microbial community exhibits rather pronounced substrate specificities. The evaluation of combined molecular and isotopic data expands our understanding of the anaerobic degradation processes within this oil field and provides insight into the degradative capabilities of the microorganisms. Additionally, isotope analysis of unbiodegraded to slightly biodegraded crude oils from several oil fields surrounding Gullfaks illustrates the heterogeneity in isotopic composition of the light hydrocarbons due to source effects. This indicates that both source and also maturity effects have to be well constrained when using compound-specific isotope analysis for the assessment of biodegradation.  相似文献   

16.
Peats in a sediment core from Ruoergai bog, which has a cold and moist plateau climate with major source input from herbaceous plants, have been studied by GC–MS in order to understand the composition and diagenetic processes of lipids in this depositional environment. Long chain components (C21–C35) predominate in the n-alkanes, n-alk-1-enes, n-fatty acids, n-alkan-2-ones and n-alkanols with a maximum of C31, C27, C22 or C24, C23 or C25 and C22, respectively. A herbaceous origin for these long chain compounds is suggested, and this is supported by their stable carbon isotopic compositions. Diterpenoid hydrocarbons with abietane, pimarane and kaurane skeletons, some of which have not been reported often in modern sediments, are prominent and are derived from higher plants. Several triterpenoid ketones and alcohols with oleanane or lupane skeletons, and a series of des-A-triterpenoid hydrocarbons which have not been reported often in modern sediments are also present, and are assigned to a higher plant source. Hopanoids, including their alkanes, alkenes, ketones, alcohols and esters, are abundant and of bacterial origin. Steroid ketones and alkanols are dominated by C29 homologues. C28 and C29 steroids are derived mainly from higher plants, whereas the C27 component is assigned to a microbial source. The presence of short-chain n-alkanes with no odd-even carbon predominance, bacterially derived fatty acids (C14, C15, iso- and unsaturated acids), n-alkan-2-ones, des-A-triterpenoid hydrocarbons, hopanoids and some steroid ketones indicate that intense microbial reworking of the organic matter has taken place in this depositional environment. The chemical and biochemical conversions of some cyclic alkenes to alkanes, such as tricyclic diterpenoids, tetracyclic terpenoids and steroid ketones, are also evident with depth. The dominance of C20 components in the diterpenoid hydrocarbons may reflect an oxidizing or reducing depositional condition.  相似文献   

17.
Anammox, the microbial anaerobic oxidation of NH4+ by NO2− to produce N2, is recognised as a key process in the marine, freshwater and soil N cycles, and has been found to be a major sink for fixed inorganic N in the ocean. Ladderane lipids are unique anammox bacterial membrane lipids used as biomarkers for such bacteria in recent and past environmental settings. However, their fate during diagenesis and early catagenesis is not well constrained. In this study, hydrous pyrolysis experiments were performed on anammox bacterial biomass and the generated aliphatic hydrocarbons, present in oil generated at 220–365 °C, were analysed. A unique class of hydrocarbons was detected, and a representative component was isolated and rigorously identified using 2D nuclear magnetic resonance (NMR) spectroscopy. It consisted of C24 to C31 branched long chain alkanes with two internal ethyl and/or propyl substituents. The alkanes were generated above 260 °C, with maximum generation at 320 and 335 °C. Their stable carbon isotopic values were depleted in 13C, similar to carbon isotope values of the original anammox lipids, indicating that they were thermal products generated from lipids of anammox bacterial biomass. A range of sediments from different geological periods where anammox may have been an important process was screened for the presence of these compounds as possible catagenetic products. They were not detected, either because the concentration was too low, or the sediments screened were too immature for them to have been generated, or because the artificially produced products of anammox lipids may not reflect the natural diagenetic and catagenetic products of ladderane lipids.  相似文献   

18.
《Applied Geochemistry》2003,18(10):1641-1651
Compound-specific H isotope analysis has been used to monitor bioremediation of petroleum hydrocarbons. However, the success of this approach requires a full evaluation of the isotopic effects resulting from evaporation, because light petroleum hydrocarbons undergo both biodegradation and evaporation under natural conditions. The authors determined the H isotope fractionation of common volatile petroleum hydrocarbons, including the C10–C14 n-alkanes, MTBE (tert-butyl methyl ether), and BTEX (benzene, toluene, ethylbenzene, p-xylene and o-xylene) during progressive vaporization under simulated experimental conditions. A decrease in δD values for n-alkanes of up to 33.3‰ and up to 44.5‰ for BTEX compounds when 99% of these substances had evaporated was observed. The results also show that H isotope fractionation increases with n-alkane chain length. Such fractionation patterns are interpreted in terms of competition between the decreased intermolecular binding energy in D-enriched species, and the isotope effect due to the mass difference. In contrast to hydrocarbons, methanol and ethanol show H isotopic enrichment during vaporization, indicating that H-bonding, when present in organic molecules, plays a controlling role on the vapor pressure of different isotope species.  相似文献   

19.
The carbon geochemistry of serpentinized peridotites and gabbroic rocks recovered at the Lost City Hydrothermal Field (LCHF) and drilled at IODP Hole 1309D at the central dome of the Atlantis Massif (Mid-Atlantic Ridge, 30°N) was examined to characterize carbon sources and speciation in oceanic basement rocks affected by long-lived hydrothermal alteration. Our study presents new data on the geochemistry of organic carbon in the oceanic lithosphere and provides constraints on the fate of dissolved organic carbon in seawater during serpentinization. The basement rocks of the Atlantis Massif are characterized by total carbon (TC) contents of 59 ppm to 1.6 wt% and δ13CTC values ranging from −28.7‰ to +2.3‰. In contrast, total organic carbon (TOC) concentrations and isotopic compositions are relatively constant (δ13CTOC: −28.9‰ to −21.5‰) and variations in δ13CTC reflect mixing of organic carbon with carbonates of marine origin. Saturated hydrocarbons extracted from serpentinites beneath the LCHF consist of n-alkanes ranging from C15 to C30. Longer-chain hydrocarbons (up to C40) are observed in olivine-rich samples from the central dome (IODP Hole 1309D). Occurrences of isoprenoids (pristane, phytane and squalane), polycyclic compounds (hopanes and steranes) and higher relative abundances of n-C16 to n-C20 alkanes in the serpentinites of the southern wall suggest a marine organic input. The vent fluids are characterized by high concentrations of methane and hydrogen, with a putative abiotic origin of hydrocarbons; however, evidence for an inorganic source of n-alkanes in the basement rocks remains equivocal. We propose that high seawater fluxes in the southern part of the Atlantis Massif likely favor the transport and incorporation of marine dissolved organic carbon and overprints possible abiotic geochemical signatures. The presence of pristane, phytane and squalane biomarkers in olivine-rich samples associated with local faults at the central dome implies fracture-controlled seawater circulation deep into the gabbroic core of the massif. Thus, our study indicates that hydrocarbons account for an important proportion of the total carbon stored in the Atlantis Massif basement and suggests that serpentinites may represent an important—as yet unidentified—reservoir for dissolved organic carbon (DOC) from seawater.  相似文献   

20.
The Pecos River, situated in eastern New Mexico and western Texas, receives water from a drainage area of 91 000 km2. There are primarily two major water inputs, namely snowmelt from winter storms in the headwater region of the southern Rocky Mountains and runoff from warm-season monsoonal rainfall in the lower valley. The Pecos River suffers from high levels of total dissolved solids (TDS >5000 mg L−1) under normal flow conditions. This not only poses serious problems for agricultural irrigation and safe drinking water supply, but also results in a permanent loss of biodiversity. This study examines changes in stream flow and water chemistry of the Pecos River over the last 70 a to better understand the long-term variability in stream salinity and the role of agricultural practices in salt transfer. A TDS record from the lower Pecos River near Langtry (Texas) back to 1935 was extracted to show a distinct pattern of decadal variability similar to the Pacific Decadal Oscillation (PDO), in which stream salinity is overall above average when the PDO is in positive (warm) phase and below average when the PDO is in negative (cold) phase. This is due to: (1) the dissolved salts contributed to the river are largely from dissolution of NaCl and CaSO4-bearing minerals (e.g., halite and gypsum) in the upper basin, (2) the amount of the dissolved salts that reach the lower basin is mainly determined by the stream flow yield in the upper basin and (3) the stream flow yield from the upper basin is positively correlated with the PDO index. This further attests that large-scale climatic oscillation is the major source of long-term changes in stream flow and salinity of the Pecos River. On the other hand, there is also a strong indication that the rate of salt export has been affected by reservoir operations and water diversions for agricultural practices.  相似文献   

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