Late Precambrian marbles of unusual carbon-isotope composition, southeastern British Columbia

1985 ◽  
Vol 22 (3) ◽  
pp. 324-329 ◽  
Author(s):  
Edward D. Ghent ◽  
James R. O'Neil
Keyword(s):  
British Columbia ◽  
Carbon Isotope ◽  
Isotope Composition ◽  
Isotope Effects ◽  
Large Reservoir ◽  
Kinetic Isotope ◽  
Carbon And Oxygen Isotope ◽  
Isotope Equilibrium ◽  
Mineralogical Phase ◽  
Isotope Analyses

Carbon- and oxygen-isotope analyses were made of both carbonates and graphites from several Precambrian metamorphic samples from British Columbia. The carbon-isotope data are unusual in that the δ13C values of many marbles are very high, up to 9.9, and the most positive values occur in the sillimanite-zone rocks. The δ13C values of graphite are also relatively high, and the 13C fractionations between calcite and graphite suggest that (1) the rocks attained and retained carbon-isotope equilibrium during metamorphism, and (2) the temperature of graphite equilibration in marbles from the sillimanite zone is 650–690 °C, in agreement with temperatures estimated from mineralogical phase equilibria.Possible precursor carbonates for the 13C-rich marbles in British Columbia include (1) Precambrian carbonate-secreting organisms such as algae that participate in reactions with large kinetic isotope effects, (2) carbonates that exchanged carbon isotopes with a large reservoir of organic material prior to metamorphism, and (3) travertines.

Herbivore paleodiet and paleoenvironmental changes in Chad during the Pliocene using stable isotope ratios of tooth enamel carbonate

Paleobiology ◽  
2000 ◽  
Vol 26 (2) ◽  
pp. 294-309 ◽  
Author(s):  
Antoine Zazzo ◽  
Hervé Bocherens ◽  
Daniel Billiou ◽  
André Mariotti ◽  
Michel Brunet ◽  
...  
Keyword(s):  
Carbon Isotope ◽  
Environmental Changes ◽  
Tooth Enamel ◽  
Isotope Composition ◽  
Plant Cover ◽  
Carbon Isotope Composition ◽  
Dental Morphology ◽  
Early Hominid ◽  
Carbon And Oxygen Isotope

Chad is a key region for understanding early hominid geographic expansion in relation to late Miocene and Pliocene environmental changes, owing to its location 2500 km west from the Rift Valley and to the occurrence of sites ranging in age from about 6 to 3 Ma, some of which yield fossil hominids. To reconstruct changes in herbivore paleodiet and therefore changes in the paleoenvironment, we measured the carbon and oxygen isotope composition of 80 tooth-enamel samples from three time horizons for nine families of Perissodactyla, Proboscidea, and Artiodactyla. The absence of significant alteration of in vivo isotopic signatures can be determined for carbon, thus allowing paleodietary and paleoenvironmental interpretations to be made.While the results generally confirm previous dietary hypotheses, mostly based on relative crown height, there are some notable surprises. The main discrepancies are found among low-crowned proboscideans (e.g., Anancus) and high-crowned rhinocerotids (Ceratotherium). Both species were more opportunistic feeders than it is usually believed. This result confirms that ancient feeding ecology cannot always be inferred from dental morphology or extant relatives.There is an increase in the average carbon isotope composition of tooth enamel from the oldest unit to the youngest, suggesting that the environment became richer in C4 plants with time. In turn, more C4 plants indicate an opening of the plant cover during this period. This increase in carbon isotope composition is also recorded within genera such as Nyanzachoerus, Ceratotherium, and Hexaprotodon, indicating a change from a C3-dominated to a C4-dominated diet over time. It appears that, unlike other middle Pliocene hominid sites in eastern and southern Africa, this part of Chad was characterized by very open conditions and that savanna-like grasslands were already dominant when hominids were present in the area.

scholarly journals Dependence between the photochemical age of light aromatic hydrocarbons and the carbon isotope ratios of atmospheric nitrophenols

2019 ◽  
Vol 19 (8) ◽  
pp. 5495-5509
Author(s):  
Marina Saccon ◽  
Anna Kornilova ◽  
Lin Huang ◽  
Jochen Rudolph
Keyword(s):  
Carbon Isotope ◽  
Organic Compounds ◽  
Mechanistic Model ◽  
Isotope Effects ◽  
Isotope Ratios ◽  
Kinetic Isotope Effects ◽  
Laboratory Studies ◽  
Carbon Isotope Ratios ◽  
Kinetic Isotope ◽  
Volatile Organic

Abstract. Concepts were developed to establish relationships between the stable carbon isotope ratios of nitrophenols in the atmosphere and the photochemical processing of their precursors, light aromatic volatile organic compounds. These concepts were based on the assumption that nitrophenols are formed dominantly from the photo-oxidation of aromatic volatile organic compounds (VOCs). A mass balance model as well as various scenarios based on the proposed mechanism of nitrophenol formation were formulated and applied to derive the time-integrated exposure of the precursors to processing by OH radicals (∫[OH]dt) from ambient observations made between 2009 and 2012 in Toronto, Canada. The mechanistic model included the possibility of isotopic fractionation during intermediate steps, rather than only during the initial reaction step. This model takes kinetic isotope effects for the reaction of the precursor VOC with the hydroxyl radical and their respective rate constants into account, as well as carbon isotope ratio source signatures. While many of these values are known, there are some, such as the kinetic isotope effects of reactions of first- and second-generation products, which are unknown. These values were predicted in this study based on basic principles and published laboratory measurements of kinetic carbon isotope effects and were applied to the mechanistic model. Due to the uncertainty of the estimates based on general principles, three scenarios were used with different values for isotope effects that were not known from laboratory studies. Comparison of the dependence between nitrophenol carbon isotope ratios and ∫[OH]dt with published results of laboratory studies and ambient observations was used to narrow the range of plausible scenarios for the mechanistic model. The results also suggests that mass-balance-based models do not adequately describe the dependence between nitrophenol carbon isotope ratios and ∫[OH]dt.

Isotopic Composition of Plant Carbon Correlates With Water-Use Efficiency of Wheat Genotypes

1984 ◽  
Vol 11 (6) ◽  
pp. 539 ◽  
Author(s):  
GD Farquhar ◽  
RA Richards
Keyword(s):  
Water Use Efficiency ◽  
Carbon Isotope ◽  
Water Use ◽  
Isotope Composition ◽  
Isotope Effects ◽  
Isotope Analysis ◽  
Wheat Genotypes ◽  
C3 Species ◽  
Breeding Programmes ◽  
Use Efficiency

Variation in carbon-isotope composition among and between wheat genotypes was correlated with variation in water-use efficiency in separate pot experiments conducted in spring-summer and in winter. In the main, winter experiment, the water-use efficiencies ranged from 2.0 to 3.7 mmolC/mol H2O (means of four replicates) while the corresponding isotope effects for leaf material ranged from 1.0225 to 1.0194. 13C was more abundant in grain than in leaves and stems. It is suggested that carbon-isotope analysis may be a useful tool in selection for improved water-use efficiency in breeding programmes for C3 species.

Atypical diagenetic effects on strontium-isotope composition of Early Jurassic belemnites, Queen Charlotte Islands, British Columbia, Canada

2007 ◽  
Vol 44 (2) ◽  
pp. 181-197 ◽  
Author(s):  
Darren R Gröcke ◽  
Stephen P Hesselbo ◽  
Duncan J Findlay
Keyword(s):  
British Columbia ◽  
Isotope Composition ◽  
Early Jurassic ◽  
Striking Similarity ◽  
Geochemical Data ◽  
Screening Methods ◽  
Element Abundances ◽  
Queen Charlotte Islands ◽  
Diagenetic Alteration ◽  
Carbon And Oxygen Isotope

In this study, strontium-, carbon- and oxygen-isotope data are reported from Early Jurassic belemnites from Yakoun River, Queen Charlotte Islands (QCI), British Columbia. Assuming that the established ammonite correlations between Canada and Europe are accurate, the seawater 87Sr/86Sr curve between Europe and QCI should be similar. However, the Rarenodia planulata and Phymatoceras crassicosta ammonite biozones from Yakoun River record 87Sr/86Sr values that are lower than expected. This shift is interpreted to be a result of diagenetic alteration by later hydrothermal fluid (87Sr-depleted). In general, the diagenetic samples have more negative δ13Cbel and δ18Obel values, and higher Mn concentrations, but show no difference in Fe concentration. A δ13Cbel curve from diagenetically screened samples exhibits only slightly depleted values with respect to a similar curve generated from Europe. A similar curve for δ18Obel shows a striking similarity with that generated from Europe. This study illustrates that diagenetic effects on Sr-isotope composition can lower values from the expected seawater 87Sr/86Sr curve, thus potentially producing erroneous correlations between ammonite biostratigraphic schemes. Traditional screening methods for diagenesis, such as cathodoluminescence and trace-element abundances, were inadequate for evaluating diagenesis in the Yakoun River sections. Other such studies must also consider alternative and (or) a combination of methods in evaluating geochemical data from belemnites.

scholarly journals Evidence for a major missing source in the global chloromethane budget from stable carbon isotopes

2018 ◽  
Author(s):  
Enno Bahlmann ◽  
Frank Keppler ◽  
Julian Wittmer ◽  
Markus Greule ◽  
Heinz Friedrich Schöler ◽  
...  
Keyword(s):  
Carbon Isotope ◽  
Carbon Isotopes ◽  
Stable Carbon Isotopes ◽  
Tropical Rainforest ◽  
Isotope Effects ◽  
Stratospheric Ozone ◽  
Mixing Ratios ◽  
Promising Tool ◽  
Kinetic Isotope ◽  
Phase Out

Abstract. Chloromethane (CH3Cl) is the most important natural input of reactive chlorine to the stratosphere, contributing about 16 % to stratospheric ozone depletion. Due to the phase out of anthropogenic emissions of chlorofluorocarbons, CH3Cl will largely control future levels of stratospheric chlorine. The tropical rainforest is commonly assumed to be the strongest single CH3Cl source, contributing over half of the global annual emissions of about 4000 to 5000 Gg (1 Gg = 109 g). This source shows a characteristic carbon isotope fingerprint, making isotopic investigations a promising tool for improving its atmospheric budget. Applying carbon isotopes to better constrain the atmospheric budget of CH3Cl requires sound information on the kinetic isotope effects for the main sink processes e.g. the reaction with OH and Cl in the troposphere. We conducted photochemical CH3Cl degradation experiments in a 3500 L smog chamber to determine the carbon isotope fractionation (ε) for the reaction of CH3Cl with OH and Cl. For the reaction of CH3Cl with OH, we determined a ε of (−11.2 ± 0.8) ‰ (n = 3) and for the reaction with Cl we found a ε of (−10.2 ± 0.5) ‰ (n = 1) being five to six times smaller than previously reported. Our smaller isotope effects are strongly supported by the lack of any significant seasonal covariation in previously reported tropospheric δ13C(CH3Cl) values with the OH driven seasonal cycle in tropospheric mixing ratios. Applying these new fractionation factors to the global CH3Cl budget using a simple two hemispheric box model, we derive a tropical rainforest CH3Cl source of (670 ± 200) Gg a−1, which is considerably smaller than previous estimates. A revision of previous bottom up estimates, using above ground biomass instead of rainforest area, strongly supports this lower estimate. Finally, our results suggest a large unknown tropical CH3Cl source of (1230 ± 200) Gg a−1.

scholarly journals Shale gas plays, Neuquén Basin, Argentina: chemostratigraphy and mud gas carbon isotopes insights

2016 ◽  
Vol 46 (suppl 1) ◽  
pp. 181-196 ◽  
Author(s):  
Héctor Adolfo Ostera ◽  
Roberto García ◽  
Daniel Malizia ◽  
Pablo Kokot ◽  
Leonel Wainstein ◽  
...  
Keyword(s):  
Carbon Isotope ◽  
Isotope Composition ◽  
Isotope Analysis ◽  
Geochemical Data ◽  
Complex Processes ◽  
Vaca Muerta ◽  
Neuquen Basin ◽  
Neuquén Basin ◽  
Isotope Analyses ◽  
Mixing Of Gases

ABSTRACT: In order to enhance the knowledge of shale objectives from Vaca Muerta and Los Molles Formations in the Neuquén Basin, Argentina, chemostratigraphic and mud gas carbon isotope analyses were performed in two wells from Agua del Cajón and Salitral oilfields (ADC-1016 and NqSa-1148). Geochemical data show restricted levels in both cases to perforate and produce. In ADC-1016 well, Lower Los Molles Formation looks like the most suitable play to be produced. At El Salitral oilfield (NqSa-1148), the best remarkable Vaca Muerta-Quintuco objectives are associated with authigenic elements, in limited horizons. Enhancement of the Quintuco reservoir by deep circulating fluids (thermobaric reservoir) is suggested. Carbon isotope analysis reveals complex processes that affected the gas composition. Addition of microbial methane, biodegradation of ethane-propane and mixing of gases has been recognized. Isotope reversals and presumed water reforming of hydrocarbons have been registered associated with overpressure for Lower Los Molles Formation in the ADC-1016 well, which is pointed out as the most promising shale play in the area. Vaca Muerta gases at Agua del Cajon ADC- 1016 well are associated with the homonymous source. El Salitral 1148 well shows that primary isotope composition in gases from Vaca Muerta shale play and Quintuco reservoir could be associated with a Lower Los Molles source, an aloctonous charge related with the main structures of the area.

scholarly journals Dependence between the Photochemical Age of Light Aromatic Hydrocarbons and the Carbon Isotope Ratios of Atmospheric Nitrophenols

2018 ◽  
Author(s):  
Marina Saccon ◽  
Anna Kornilova ◽  
Lin Huang ◽  
Jochen Rudolph
Keyword(s):  
Mass Balance ◽  
Carbon Isotope ◽  
Mechanistic Model ◽  
Isotope Effects ◽  
Isotope Ratios ◽  
Kinetic Isotope Effects ◽  
Oh Radicals ◽  
Laboratory Studies ◽  
Carbon Isotope Ratios ◽  
Kinetic Isotope

Abstract. Concepts were developed to establish relationships between the stable carbon isotope ratios of nitrophenols in the atmosphere and photochemical processing of their precursors, light aromatic volatile organic compounds. The concepts are based on the assumption that nitrophenols are formed dominantly from the photo-oxidation of aromatic VOC. A mass balance model as well as various scenarios based on the proposed mechanism of nitrophenol formation were formulated and applied to derive the time integrated exposure of the precursors to processing by OH-radicals (∫[OH]dt) from ambient observations taken between 2009 and 2012 in Toronto, Canada. The mechanistic model included the possibility of isotopic fractionation during intermediate steps, rather than during the initial reaction step alone. This model included knowledge of kinetic isotope effects of the precursor VOC with the hydroxyl radical and their respective rate constants, as well as isotope ratio source signatures. While many of these values are known, there were some, such as the kinetic isotope effects of reactions of the intermediate compounds, which were unknown. These values were predicted based on basic principles and published laboratory measurements of normal kinetic carbon isotope effects and were applied to the mechanistic models. Due to uncertainty of the estimates based on general principles three scenarios were used with different values for isotope effect that were not known from laboratory studies. Comparison of the dependence between nitrophenol carbon isotope ratios and ∫[OH]dt with published results of laboratory studies and ambient observations was used to narrow the range of plausible scenarios of the mechanistic model and to eliminate the mass balance based model as useful formulation of a the dependence between nitrophenol carbon isotope ratios and ∫[OH]dt.

scholarly journals NO<sub>x</sub> cycle and tropospheric ozone isotope anomaly: an experimental investigation

2013 ◽  
Vol 13 (4) ◽  
pp. 9443-9483 ◽  
Author(s):  
G. Michalski ◽  
S. K. Bhattacharya ◽  
G. Girsch
Keyword(s):  
Steady State ◽  
Isotopic Composition ◽  
Isotope Composition ◽  
Dominant Role ◽  
Isotope Effects ◽  
No Oxidation ◽  
Rate Coefficients ◽  
Mixing Ratios ◽  
Kinetic Isotope ◽  
Terminal Atom

Abstract. The oxygen isotope composition of nitrogen oxides (NOx) in the atmosphere may be a useful tool for understanding the oxidation of NOx into nitric acid/nitrate in the atmosphere. A set of experiments were conducted to examine changes in isotopic composition of NOx due to O3-NOx photochemical cycling. At low NO2/O2 mixing ratios, NO2 becomes progressively and nearly equally enriched in 17O and 18O over time until it reaches a steady state with Δ17O values of 40.6 ± 1.9‰ and δ18O values of 84.2 ± 4‰, relative to the isotopic composition of the O2 gas. As the mixing ratio increases, isotopic exchange between O atoms and O2 and NOx suppresses the isotopic enrichments. A kinetic model simulating the observed data shows that the isotope effects during ozone formation play a more dominant role compared to kinetic isotope effects during NO oxidation or exchange of NO2. The model results are consistent with the data when the NO + O3 reaction occurs mainly via the transfer of the terminal atom of O3. The model predicts that under tropospheric concentrations of the three reactants, the timescale of NOx isotopic equilibrium ranges from hours (ppbv mixing ratios) to days/weeks (pptv) and yields steady state Δ17O and δ18O values of 46‰ and 115‰ respectively with respect to Vienna Standard Mean Ocean Water. Interpretation of tropospheric nitrate isotope data can now be done with the derived rate coefficients of the major isotopologue reactions at various pressures.

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