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1.
Shewanella putrefaciens (Strain MR-4), a gram negative facultative marine bacterium, was grown to stationary phase under both aerobic and anaerobic conditions using lactate as the sole carbon source. Aerobically-produced cells were slightly enriched in 13C (+1.5‰) relative to the lactate carbon source, whereas those from anaerobic growth were depleted in 13C (−2.2‰). The distribution of fatty acids produced under aerobic conditions was similar to that resulting from anaerobic growth, being dominated by C16:1 ω7 and C16:0 fatty acids with a lesser amount of the C18:1 ω7 component. Low concentrations of saturated even numbered normal fatty acids in the C14 to C18 range, and iso-C15:0 were synthesized under both conditions. Fatty acids from anaerobic cultures (average δ13C=−37.8‰) were considerably depleted in 13C relative to their aerobically-synthesized counterparts (−28.8‰). The distinct differences in isotopic composition of both whole cells and individual fatty acid components result from differences in assimilation pathways. Under aerobic conditions, the primary route of assimilation involves the pyruvate dehydrogenase enzyme complex which produces acetyl-CoA, the precursor to lipid synthesis. In contrast, under anaerobic conditions formate, and not acetate, is the central intermediate in carbon assimilation with the precursors to fatty acid synthesis being produced via the serine pathway. Anaerobically-produced bacterial fatty acids were depleted by up to 12‰ relative to the carbon source. Therefore, detection of isotopically depleted fatty acids in sediments may be falsely attributed to a terrestrial origin, when in fact they are the result of bacterial resynthesis.  相似文献   

2.
Aerobic methanotrophy at ancient marine methane seeps: A synthesis   总被引：2，自引：0，他引：2  
The molecular fingerprints of the chemosynthesis based microbial communities at methane seeps tend to be extremely well preserved in authigenic carbonates. The key process at seeps is the anaerobic oxidation of methane (AOM), which is performed by consortia of methanotrophic archaea and sulphate reducing bacteria. Besides the occurrence of 13C depleted isoprenoids and n-alkyl chains derived from methanotrophic archaea and sulphate reducing bacteria, respectively, 13C depleted triterpenoids have been reported from a number of seep deposits. In order to evaluate the significance of these apparently non-AOM related molecular fossils, the biomarker inventories of one Campanian and two Miocene methane seep limestones are compared. These examples provide strong evidence that methane was not solely oxidized by an anaerobic process. Structural and carbon isotope data reveal that aerobic methanotrophy was common at some ancient methane seeps as well. The Miocene Marmorito limestone contains abundant 3β-methylated hopanoids (δ13C: −100‰). Most likely, 3β-methylated hopanepolyols, prevailing in aerobic methanotrophs, were the precursor lipids of these compounds. A series of isotopically depleted 4-methylated steranes (lanostanes; δ13C: −80‰ to −70‰) and similarly isotopically depleted 17β(H),21β(H)-32-hopanoic acid in the Miocene Pietralunga seep limestone also are derived probably from aerobic methanotrophs. Lanosterol, which is known to be produced by aerobic methanotrophs, is the most likely precursor of 4-methylated steranes. Less obvious is the origin of 8,14-secohexahydrobenzohopanes (δ13C: −110‰ to −107‰) in Late Cretaceous seep limestones. These hopanoids probably reflect early degradational products of precursor lipids locally produced by seep endemic aerobic methanotrophs.  相似文献   

3.
This study identifies isotope signatures associated with autotrophic and heterotrophic microbial communities that may provide a means to determine carbon cycling relationships in situ for acid mine drainage (AMD) sites. Stable carbon isotope ratios (δ13C) of carbon sources, bulk cells, and membrane phospholipids (PLFA) were measured for autotrophic and heterotrophic microbial enrichment cultures from a mine tailings impoundment in northern Ontario, Canada, and for pure strains of the sulfur oxidizing bacteria Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans. The autotrophic enrichments had indistinguishable PLFA distributions from the pure cultures, and the PLFA cyc-C19:0 was determined to be a unique biomarker in this system for these sulfur oxidizing bacteria. The PLFA distributions produced by the heterotrophic enrichments were distinct from the autotrophic distributions and the C18:2 PLFA was identified as a biomarker for these heterotrophic enrichments. Genetic analysis (16S, 18S rRNA) of the heterotrophic cultures indicated that these communities were primarily composed of Acremonium fungi.Stable carbon isotope analysis revealed that bulk cellular material in all autotrophic cultures was depleted in δ13C by 5.6–10.9‰ relative to their atmospheric CO2 derived carbon source, suggesting that inorganic carbon fixation in these cultures is carbon limited. Individual PLFA from these autotrophs were further depleted by 8.2–14.6‰ compared to the bulk cell δ13C, which are among the largest biosynthetic isotope fractionation factors between bulk cell and PLFA reported in the literature. In contrast, the heterotrophic bulk cells were not significantly fractionated in δ13C relative to their carbon source and heterotrophic PLFA ranged from 3‰ enriched to 4‰ depleted relative to the isotopic composition of their total biomass. These distinct PLFA biomarkers and isotopic fractionations associated with autotrophic and heterotrophic activity in this laboratory study provide potential biomarkers for delineating autotrophic and heterotrophic carbon cycling in AMD environments.  相似文献   

4.
The molecular and carbon isotopic compositions of polar isopranyl glycerol ether lipids, which are direct indicators of viable archaea, and neutral isopranyl glycerol ether lipids, which are derived from polar lipids via hydrolysis, in near-surface sediments from a methane seep in the Nankai Trough (off central Japan) were investigated. Procedures for extracting, separating and derivatizing polar and neutral ether lipids for detection using gas chromatography were first examined with one sediment sample and a cultivated methanogen. For all sediment samples, archaeol and hydroxyarchaeol were detected in both the polar and neutral ether lipid fractions. Acyclic and cyclic biphytanes were also detected in both types of lipid fractions after treatment with HI/LiAlH4 for ether cleavage and alkylation. The δ13C values of archaeol, sn-2-hydroxyarchaeol, and sn-3-hydroxyarchaeol in the sample from 0.82 m below the seafloor were lower than −100‰ relative to PDB, indicating that diverse living methanotrophic archaea are present in the seep sediments. Biphytanes released from polar ether lipids in the same sample were less depleted in δ13C (−71‰ to −36‰). The wide range of δ13C values suggests that the biphytanes were derived not only from methanotrophic but also from non-methanotrophic archaea, and that the relative contributions of the methanotrophic and non-methanotrophic archaea differed, depending on the biphytane compound. The vertical profiles and δ13C values of the neutral ether lipids were similar to those of the intact polar ether lipids, suggesting that neutral ether lipids derived from fossil archaea in the samples had mainly been lost by the time of sampling.  相似文献   

5.
We examined stable carbon isotope fractionation in biosynthesis of fatty acids of a piezophilic bacterium Moritella japonica strain DSK1. The bacterium was grown to stationary phase at pressures of 0.1, 10, 20, and 50 MPa in media prepared using sterile-filtered natural seawater supplied with glucose as the sole carbon source. Strain DSK1 synthesized typical bacterial fatty acids (C14-19 saturated, monounsaturated, and cyclopropane fatty acids) as well as long-chain polyunsaturated fatty acids (PUFA) (20:6ω3). Bacterial cell biomass and individual fatty acids exhibited consistent pressure-dependent carbon isotope fractionations relative to glucose. The observed ΔδFA-glucose (−1.0‰ to −11.9‰) at 0.1 MPa was comparable to or slightly higher than fractionations reported in surface bacteria. However, bulk biomass and fatty acids became more depleted in 13C with pressure. Average carbon isotope fractionation (ΔδFA-glucose) at high pressures was much higher than that for surface bacteria: −15.7‰, −15.3‰, and −18.3‰ at 10, 20, and 50 MPa, respectively. PUFA were more 13C depleted than saturated and monounsaturated fatty acids at all pressures. The observed isotope effects may be ascribed to the kinetics of enzymatic reactions that are affected by hydrostatic pressure and to biosynthetic pathways that are different for short-chain and long-chain fatty acids. A simple quantitative calculation suggests that in situ piezophilic bacterial contribution of polyunsaturated fatty acids to marine sediments is nearly two orders of magnitude higher than that of marine phytoplankton and that the carbon isotope imprint of piezophilic bacteria can override that of surface phytoplankton. Our results have important implications for marine biogeochemistry. Depleted fatty acids reported in marine sediments and the water column may be derived simply from piezophilic bacteria resynthesis of organic matter, not from bacterial utilization of a 13C-depleted carbon source (i.e., methane). The interpretation of carbon isotope signatures of marine lipids must be based on principles derived from piezophilic bacteria.  相似文献   

6.
As a contribution to a project on the role of aerobic heterotrophic bacteria in the organic matter turnover within cyanobacterial mats, we investigated selected lipid classes of five morphologically different microbial mats from the intertidal area of Abu Dhabi, United Arab Emirates. The mats thrive under extreme environmental conditions as they are exposed to salinity up to 200 and shadowless sunshine for nearly 12 h per day. The total extractable organic matter is characterised by heavy δ13C values of −17‰ to −10‰, suggesting CO2 limitation. The lipids reflect the biomass of the principal mat-building phototrophic and heterotrophic microorganisms. Moderate amounts of hydrocarbons, mainly cyanobacterial n-heptadecane, short chain monomethyl alkanes and diploptene were detected in all the mats. The ubiquitous n-hexadecanoic acid is dominant amongst the free and ester-bound phospholipid fatty acids (PLFAs), whereas the overall acid composition varies due to differences in the extreme environmental conditions. Additionally, the top layers of two of the five mats were analysed separately to compare their lipid compositions with those of the whole mats. As would be expected, the concentrations of cyanobacterial biomarkers exceed those found in the total mats. Noteworthy is the presence of n-heneicosadiene as predominant hydrocarbon in the top layer of the so called gelatinous mat, which thrives under the locally most extreme conditions. As the n-alkadiene was present to a much lesser extent in the intact mat, it may provide a new biomarker for a so far unknown important aerobic organism.  相似文献   

7.
The CO2 gas reservoir sandstones in the Hailaer Basin contain abundant dawsonite and provide an ideal laboratory to study whether any genetic relationship exists between dawsonite and the modern gas phase of CO2. The origins of dawsonite and CO2 in these sandstones were studied by petrographic and isotopic analysis. According to the paragenetic sequence of the sandstones, dawsonite grew later than CO2 charging at 110–85 Ma. The dawsonite δ18O value is 7.4‰ (SMOW), and the calculated δ18O values of the water present during dawsonite growth are from −11.4‰ to −9.2‰ (SMOW). This, combined with the NaHCO3-dominated water linked to dawsonite growth, suggests meteoric water being responsible for dawsonite growth. The δ13C values of gas phase CO2 and the ratios of 3He/4He of the associated He suggest a mantle magmatic origin of CO2-rich natural gas in Hailaer basin. Dawsonite δ13C values are −5.3‰ to −1.5‰ (average −3.4‰), and the calculated δ13C values of CO2 gas in isotopic equilibrium with dawsonite are −11.4‰ to −7.3‰. These C isotopic values are ambiguous for the dawsonite C source. From the geological context, the timing of events, together with formation water conditions for dawsonite growth, dawsonite possibly grew in meteoric-derived water, atmospherically-derived CO2 maybe, or at least the dominant, C source for dawsonite. It seems that there are few relationships between dawsonite and the modern gas phase of CO2 in the Hailaer basin.  相似文献   

8.
Inorganic gases are commonly seen in eastern China and occasionally in southern China from the shallow water columns above hot and cold springs. The gases contain 68% to nearly 100% CO2, with δ13CCO2 and δ13C1 values in the range of −1.18‰ to −6.00‰ and −19.48‰ to −24.94‰, respectively. All of the 34 large inorganic CO2 and one inorganic methane accumulations discovered in China are distributed in eastern parts of the country, from both onshore and continental shelf basins. No commercial inorganic gas accumulation has been found in central and western China. This is a review of the occurrence and geochemical characteristics of inorganic gas accumulations in Chinese sedimentary basins. A detailed study of gas samples collected from four representative inorganic CO2 pools and one possible inorganic methane pool indicates that inorganic alkane gases typically show δ13C1 values greater than −10‰ versus PDB (mostly −30‰), with a positive stable carbon isotope sequence of δ13C1 < δ 13C2 < δ13C3 < δ 13C4. In contrast, the δ13C1 values of biogenic alkane gases are lighter than −30‰, with a negative isotope sequence (i.e. δ13C1 > δ13C2 > δ 13C3 > δ13C4). Inorganic gases also tend to show less negative δ13CCO2 values (−10‰) than biogenic gases (<−10‰).  相似文献   

9.
This study describes the biogeochemical cycling of seston in Grand Traverse Bay, Lake Michigan. Seston was characterized by carbon and nitrogen elemental and isotopic abundances. Fluorescence, temperature, light transmittance, and concentrations of dissolved inorganic nitrogen were also determined. PCBs were analyzed from surface (10 m) seston and ΣPCB was calculated by summing all of the congeners quantified in each sample. The vertical and seasonal trends in the δ13C values of seston exhibited a broad range from −30.7 to −23.9‰. Low δ13C values that occur concurrently with a peak in fluorescence below the thermocline reflect uptake of 13C depleted respiratory CO2 and/or the accumulation of 13C depleted lipids by phytoplankton. High δ13C values late in the season likely result from a reduction in photosynthetic fractionation associated with a decrease in the CO2 pool. Seasonal δ15N values of seston were high in the spring and declined through August. The δ15N values of seston reflect a balance between fractionation during assimilation of NH4+ or NO3− and degradative processes. The seston ΣPCB and fluorescence were both high in the spring and subsequently declined, suggesting that the concentrations of PCBs in seston were associated with labile material derived from primary productivity. The strong seasonal trends in the organic geochemical characteristics of seston and concentrations of PCBs emphasize the complex nature of particle cycling in aquatic environments.  相似文献   

10.
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   

11.
Stable isotope ratios of oxygen and carbon were determined for CO2 in soil gas in the vicinity of the massive sulfide deposit at Crandon, Wisconsin with the objective of determining the source of anomalously high CO2 concentrations detected previously by McCarthy et al. (1986). Values of δ13C in soil gas CO2 from depths between 0.5 and 1.0 m were found to range from −12.68‰ to −20.03‰ (PDB). Organic carbon from the uppermost meter of soil has δ13C between −24.1 and −25.8‰ (PDB), indicating derivation from plant species with the C3 (Calvin) type of photosynthetic pathway. Microbial decomposition of the organic carbon and root respiration from C3 and C4 (Hatch-Slack) plants, together with atmospheric CO2 are the likely sources of carbon in soil gas CO2. Values of δ18O in soil-gas CO2 range from 32 to 38‰ (SMOW). These δ18O values are intermediate between that calculated for CO2 gas in isotopic equilibrium with local groundwaters and that for atmospheric CO2. The δ18O data indicate that atmospheric CO2 has been incorporated by mixing or diffusion. Any CO2 generated by microbial oxidation of organic matter has equilibrated its oxygen isotopes with the local groundwaters.The isotopic composition of soil-gas CO2 taken from directly above the massive sulfide deposit was not distinguishable from that of background samples taken 1 to 2 km away. No enrichment of the δ13C value of soil-gas CO2 was observed, contrary to what would be expected if the anomalous CO2 were derived from the dissolution of Proterozoic marine limestone country rock or of Paleozoic limestone clasts in glacial till. Therefore, it is inferred that root respiration and decay of C3 plant material were responsible for most CO2 generation both in the vicinity of the massive sulfide and in the “background” area, on the occasion of our sampling. Interpretation of our data is complicated by the effects of rainfall, which significantly reduced the magnitude of the CO2 anomaly. Therefore, we cannot rule out the possible mechanism of carbonate dissolution driven by pyrite oxidation, as proposed by Lovell et al. (1983) and McCarthy et al. (1986). Further work is needed on seasonal and daily variations of CO2 concentrations and stable isotope ratios in various hydrogeologic and ecologic settings so that more effective sampling strategies can be developed for mineral exploration using soil gases.  相似文献   

12.
Lime mortar and plaster were sampled from Roman, medieval and early modern buildings in Styria. The historical lime mortar and plaster consist of calcite formed in the matrix during setting and various aggregates. The stable C and O isotopic composition of the calcite matrix was analyzed to get knowledge about the environmental conditions during calcite formation. The δ13Cmatrix and δ18Omatrix values range from −31 to 0‰ and −26 to −3‰(VPDB), respectively. Obviously, such a range of isotope values does not represent the local natural limestone assumed to be used for producing the mortar and plaster. In an ideal case, the calcite matrix in lime mortar and plaster is isotopically lighter in the exterior vs. the interior mortar layer according to the relationship δ18Omatrix = 0.61 · δ13Cmatrix − 3.3 (VPDB). Calcite precipitation by uptake of gaseous CO2 into alkaline Ca(OH)2 solutions shows a similar relationship, δ18Ocalcite = 0.67 · δ13Ccalcite − 6.4 (VPDB). Both relationships indicate that the 13C/12C and 18O/16O values of the calcite reflect the setting behaviour of the lime mortar and plaster. Initially, CO2 from the atmosphere is fixed as calcite, which is accompanied by kinetic isotope fractionation mostly due to the hydroxylation of CO2 (δ13Cmatrix ≈  −25‰ and δ18Omatrix ≈ −20‰). As calcite formation continued the remaining gaseous CO2 is subsequently enriched in 13C and 18O causing later formed calcite to be isotopically heavier along the setting path in the matrix. Deviations from such an ideal isotopic behaviour may be due to the evolution of H2O, e.g. evaporation, the source of CO2, e.g. from biogenic origin, relicts of the natural limestone, and secondary effects, such as recrystallization of calcite. The results of the field and experimental study suggest that isotope values can be used as overall proxies to decipher the origin of carbonate and the formation conditions of calcite in the matrix of ancient and recent lime mortar and plaster. Moreover, these proxies can be used to select calcite matrix from historical lime mortar and plaster for 14C dating.  相似文献   

13.
Variations in the carbon isotopic composition (δ13C) of pristane, phytane, n-heptadecane (n-C17), C29 ααα 20R sterane, and aryl isoprenoids provide evidence for a diverse community of algal and bacterial organisms in organic matter of the Upper Ordovician Maquoketa Group of the Illinois Basin. Carbon isotopic compositions of pristane and phytane from the Maquoketa are positively covariant (r = 0.964), suggesting that these compounds were derived from a common source inferred to be primary producers (algae) from the oxygenated photic zone. A variation of 3‰ in δ13C values (−31 to −34‰) for pristane and phytane indicates that primary producers utilized variable sources of inorganic carbon. Average isotopic compositions of n-C17 (−32‰) and C29 ααα 20R sterane (−31‰) are enriched in 13C relative to pristane and phytane (−33‰) suggesting that these compounds were derived from a subordinate group of primary producers, most likely eukaryotic algae. In addition, a substantial enrichment of 13C in aryl isoprenoids (−14 to −18‰) and the identification of tetramethylbenzene in pyrolytic products of Maquoketa kerogen indicate a contribution from photosynthetic green sulfur bacteria to the organic matter. The presence of anaerobic, photosynthetic green sulfur bacteria in organic matter of the Maquoketa indicates that anoxic conditions extended into the photic zone.The δ13C of n-alkanes and the identification of an unusual suite of straight-chain n-alkylarenes in the m/z 133 fragmentograms of Ordovician rocks rich in Gloeocapsomorpha prisca (G. prisca) indicate that G. prisca did not contribute to the organic matter of the Maquoketa Group.  相似文献   

14.
Groundwaters in the confined aquifers of the Chianan and Ilan coastal plains of Taiwan are rich in dissolved methane (CH4). Serious endemic “blackfoot disease”, which occurred in the Chianan plain, especially during AD1950-1970, has been demonstrated to have arisen from drinking highly reducing groundwater with abnormal arsenic and humic substance levels. In order to explore the origin of CH4 and its hydrological implications, stable carbon isotope ratios (δ13C) and radiocarbon (14C) ages of exsolved CH4, dissolved inorganic carbon (DIC), and sedimentary biogenic sediments from a total of 34 newly completed water wells at 16 sites were determined. The main results obtained are as follows: (1) The δ13CCH4 (−65‰ to −75‰) values indicate that, except for one thermogenic sample (δ13CCH4=38.2‰) from the Ilan plain, all CH4 samples analyzed were produced via microbially mediated CO2 reduction. Many δ13CDIC values are considerably greater than −10‰ and even up to 10‰ due to Rayleigh enrichment during CO2 reduction. (2) Almost all the 14C ages of CH4 samples from the shallow aquifer (I) (<60 m depth) are greater than the 14C ages of coexisting DIC and sediments, suggesting the presence of CH4 from underlying aquifers. (3) The 14C ages of coexisting CH4, DIC and sediments from aquifer (II) of the Chianan plain are essentially equal, reflecting in-situ generation of CH4 and DIC from decomposition of sedimentary organic matter and sluggishness of the groundwater flow. On the other hand, both CH4 and DIC from each individual well of the relatively deep aquifers (III) and (IV) in the Chianan plain are remarkably younger than the deposition of their coexisting sediments, indicating that current groundwaters entered these two aquifers much later than the deposition of aquifer sediments. (4) Each CH4 sample collected from the Ilan plain is older than coexisting DIC, which in turn is distinctly older than the deposition of respective aquifer sediments, demonstrating the presence of much older CO2 and CH4 from underlying strata.  相似文献   

15.
Carbon isotope chemostratigraphy has been used for worldwide correlation of Precambrian/Cambrian (Pc/C) boundary sections, and has elucidated significant change of the carbon cycle during the rapid diversification of skeletal metazoa (i.e. the Cambrian Explosion). Nevertheless, the standard δ13C curve of the Early Cambrian has been poorly established mainly due to the lack of a continuous stratigraphic record. Here we report high-resolution δ13C chemostratigraphy of a drill core sample across the Pc/C boundary in the Three Gorge area, South China. This section extends from an uppermost Ediacaran dolostone (Dengying Fm.), through a lowermost Early Cambrian muddy limestone (Yanjiahe Fm.) to a middle Early Cambrian calcareous black shale (Shuijingtuo Fm.). As a result, we have identified two positive and two negative isotope excursions within this interval. Near the Pc/C boundary, the δ13Ccarb increases moderately from 0 to + 2‰ (positive excursion 1: P1), and then drops dramatically down to − 7‰ (negative excursion 1: N1). Subsequently, the δ13Ccarb increases continuously up to about + 5‰ at the upper part of the Nemakit–Daldynian stage. After this positive excursion, δ13Ccarb sharply decreases down to about − 9‰ (N2) just below the basal Tommotian unconformity. These continuous patterns of the δ13C shift are irrespective of lithotype, suggesting a primary origin of the record. Moreover, the obtained δ13C profile, except for the sharp excursion N2, is comparable to records of other sections within and outside of the Yangtze Platform. Hence, we conclude that the general feature of our δ13C profile best represents the global change in seawater chemistry. The minimum δ13C of the N1 (− 7‰) is slightly lower than carbon input from the mantle, thus implying an enhanced flux of 13C-depleted carbon just across the Pc/C boundary. Hence, the ocean at that time probably became anoxic, which may have affected the survival of sessile or benthic Ediacaran biota. The subsequent δ13C rise up to + 5‰ (P2) indicates an increase of primary productivity or an enhanced rate of organic carbon burial, which should have resulted in lowering pCO2 and following global cooling. This scenario accounts for the cause of the global-scale sea-level fall at the base of the Tommotian stage. The subsequent, very short-term, and exceptionally low δ13C (− 9‰) in N2 could have been associated with the release of methane from gas hydrates due to the sea-level fall. The inferred dramatic environmental changes (i.e., ocean anoxia, increasing productivity, global cooling and subsequent sea-level fall with methane release) appear to coincide with or occur just before the Cambrian Explosion. This may indicate synchronism between the environmental changes and rapid diversification of skeletal metazoa.  相似文献   

16.
Equilibrium 2H/1H fractionation factors (αeq) for various H positions in alkanes, alkenes, ketones, carboxylic acids, esters, alcohols, and ethers were calculated between 0 and 100 °C using vibrational frequencies from ab initio QM calculations (B3LYP/6-311G**). Results were then corrected using a temperature-dependent linear calibration curve based on experimental data for Hα in ketones (Wang et al., 2009). The total uncertainty in reported αeq values is estimated at 10–20‰. The effects of functional groups were found to increase the value of αeq for H next to electron-donating groups, e.g. OR, OH or O(CO)R, and to decrease the value of αeq for H next to electron-withdrawing groups, e.g. (CO)R or (CO)OR. Smaller but significant functional group effects are also observed for Hβ and sometimes Hγ. By summing over individual H positions, we estimate the equilibrium fractionation relative to water to be −90‰ to −70‰ for n-alkanes and around −100‰ for pristane and phytane. The temperature dependence of these fractionations is very weak between 0 and 100 °C. Our estimates of αeq agree well with field data for thermally mature hydrocarbons (δ2H values between −80‰ and −110‰ relative to water). Therefore the observed δ2H increase of individual hydrocarbons and the disappearance of the biosynthetic δ2H offset between n-alkyl and linear isoprenoid lipids during maturation of organic matter can be confidently attributed to H exchange towards an equilibrium state. Our results also indicate that many n-alkyl lipids are biosynthesized with δ2H values that are close to equilibrium with water. In these cases, constant down-core δ2H values for n-alkyl lipids cannot be reliably used to infer a lack of isotopic exchange.  相似文献   

17.
We examined the utilization of carbon and nitrogen in two common Red Sea coral species (Stylophora pistillata and Favia favus), differing in colony morphology and polyp size, along a depth gradient down to 60 m. We describe the changes in C/N ratios and in the stable isotope composition of carbon and nitrogen of coral’s tissue and algal symbionts. We also measured the carbon isotopic composition of the lipid fraction extracted from both coral tissue and algal symbionts in order to reveal the changes in the carbon source utilized by the host coral for lipid synthesis.The results show that for both species, δ13C decreases by 7–8‰ in animal tissue, algal symbionts and in the lipid fractions as depth increases. However, in contrast to previous reports, the difference between δ13C values of coral tissue and algal symbionts does not increase with depth. δ15N values of coral tissue and algal symbionts in both species do not correlate with depth suggesting that the heterotrophic capacity of these corals does not increase with depth. δ13C values of tissue lipids were depleted by an average of 3.5‰ compared to δ13C of the entire tissue at all depths. δ13C values of algal lipids were depleted by an average of 2‰ compared to δ13C of the entire zooxanthellae at all depths, indicating high efficiency of carbon recycling between the two symbiotic partners along the entire gradient. The depletion of lipids is attributed to the fractionation mechanism during lipid synthesis. In addition, for both species, δ13C values of algal lipids were enriched compared with δ13C of tissue lipids. In S. pistillata, the difference between δ13C values of tissue lipids and algal lipids increased linearly with depth, indicating a change in the sources of carbon utilized by the coral for lipid synthesis below 20 m from an autotrophic to a heterotrophic source. However, in F. favus, this average difference was 4 times larger compared to shallow S. pistillata and was constant along the entire depth gradient, suggesting that F. favus uses heterotrophically-acquired carbon for lipid synthesis regardless of depth. Overall, F. favus exhibited enriched δ13C and δ15N values compared to S. pistillata along the entire gradient. We attribute these differences to both morphological differences (i.e. colony morphology, tissue thickness and polyp size) between the two species and to a higher heterotrophy/autotrophy ratio in F. favus at all depths. The C/N ratio in S. pistillata tissue decreased with increasing water depth whereas in F. favus it remained constant. This reflects a higher heterotrophic capacity in the large polyped F. favus, at all depths.  相似文献   

18.
Black and white dolomite crystals (mm to cm width) of different isotopic composition are associated with Triassic diapirism in central Tunisia, as well as with evaporite minerals and clays. The white dolomites occur mostly in the Jabal Hadifa diapir near the contact with Cretaceous limestones, whereas the smaller black dolomites occur in the Jabal Hamra diapir. The former dolomite has a narrow range of δ18O and δ13C values (− 3.83‰ to − 6.60‰ VPDB for δ18O; − 2.11‰ to − 2.83‰ VPDB for δ13C), whereas the latter dolomite has a wider range and more depleted values (− 4.92‰ to − 9.97‰ for δ18O; − 0.55‰ to − 6.08‰ for δ13C). However, the 87Sr / 86Sr ratios of most of the samples are near Triassic seawater values. Dolomite formation is due to at least two different fluids. The main fluid originated from deeper hydrothermal or basinal sources related to the Triassic saliferous rocks and ascended through faults during the diapiric intrusion. The second, less important fluid source is related to meteoric water originating from Cretaceous rocks.  相似文献   

19.
The stable carbon isotopic compositions of free and sulfur (S)-bound biomarkers derived from algae, (cyano)bacteria, archaea and higher plants and total organic carbon (TOC) during the first phase of the late Cenomanian/Turonian oceanic anoxic event (OAE) were measured in black shales deposited in the southern proto-Atlantic Ocean in the Cape Verde basin (DSDP Site 367) to determine the response of these organisms to this major perturbation of the global carbon cycle resulting from widespread burial of marine organic matter. The average positive isotope excursions of TOC and biomarkers varied from 5.1‰ to 8.3‰. The δ13C values were cross correlated to infer potential common sources of biomarkers. This revealed common sources for C31 and C32 hopanes but no 1:1 relationship for pristane and phytane. The correlation of δ13CTOC with the δ13C value of sulfur (S)-bound phytane is the strongest. This is because S-bound phytane is derived from phytol that originates from all marine primary producers (algae and cyanobacteria) and thus represents a weighted average of their carbon isotopic compositions. The δ13C values of S-bound phytane and C35 hopane were also used to estimate pCO2 levels. Before the OAE burial event, pCO2 levels are estimated to be ca. 1300 ppmv using both biomarkers and the independent maximum Rubisco fractionation factors. At times of maximum organic carbon burial rates during the OAE, reconstructed pCO2 levels are estimated to be ca. 700 ppmv. However, compared to other C/T OAE sections the positive isotope excursion of S-bound phytane is also affected by an increased production during the OAE. When we compensate for this, we arrive at pCO2 levels around 1000 ppmv, a reduction of ca. 25%. This indicates that burial of organic matter can have a large effect on atmospheric CO2 levels.  相似文献   

20.
13C depleted materials of three types are encountered in the south-eastern Mediterranean Coastal Plain of Israel and Northern Sinai. Sub-surface coarse carbonates associated with elemental sulfur have gd13C values of −50‰. Organic matter (humic substances and lipid soluble) in the Be'eri sulfur quarries have δ13C values of −80 to −90‰. Dissolved bicarbonate in groundwaters show δ13C values of −13 to −17‰.It is suggested that all these values can be explained by biogenic methane leaking from sub-surface reservoir which is oxidized under different depositional and environmental conditions.  相似文献   

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