scholarly journals Clumped isotope effects of thermogenic methane formation: insights from pyrolysis of hydrocarbons

2021 ◽  
Author(s):  
Guannan Dong ◽  
Hao Xie ◽  
Michael Formolo ◽  
Michael Lawson ◽  
Alex Sessions ◽  
...  
Keyword(s):  
Isotope Effects ◽  
Methane Formation ◽  
Clumped Isotope ◽  
Thermogenic Methane

scholarly journals Clumped isotope effects during OH and Cl oxidation of methane

2017 ◽  
Vol 196 ◽  
pp. 307-325 ◽  
Author(s):  
Andrew R. Whitehill ◽  
Lars Magnus T. Joelsson ◽  
Johan A. Schmidt ◽  
David T. Wang ◽  
Matthew S. Johnson ◽  
...  
Keyword(s):  
Isotope Effects ◽  
Oxidation Of Methane ◽  
Clumped Isotope

scholarly journals Equilibrium clumped-isotope effects in doubly substituted isotopologues of ethane

2017 ◽  
Vol 197 ◽  
pp. 14-26 ◽  
Author(s):  
Michael A. Webb ◽  
Yimin Wang ◽  
Bastiaan J. Braams ◽  
Joel M. Bowman ◽  
Thomas F. Miller
Keyword(s):  
Isotope Effects ◽  
Clumped Isotope

scholarly journals Kinetic isotope effects of <sup>12</sup>CH<sub>3</sub>D  + OH and <sup>13</sup>CH<sub>3</sub>D  + OH from 278 to 313 K

2016 ◽  
Vol 16 (7) ◽  
pp. 4439-4449 ◽  
Author(s):  
L. M. T. Joelsson ◽  
J. A. Schmidt ◽  
E. J. K. Nilsson ◽  
T. Blunier ◽  
D. W. T. Griffith ◽  
...  
Keyword(s):  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
State Theory ◽  
Atmospheric Methane ◽  
Isotopic Signatures ◽  
Budget Analysis ◽  
Kinetic Isotope ◽  
Methane Budget ◽  
Source Signal ◽  
Clumped Isotope

Abstract. Methane is the second most important long-lived greenhouse gas and plays a central role in the chemistry of the Earth's atmosphere. Nonetheless there are significant uncertainties in its source budget. Analysis of the isotopic composition of atmospheric methane, including the doubly substituted species 13CH3D, offers new insight into the methane budget as the sources and sinks have distinct isotopic signatures. The most important sink of atmospheric methane is oxidation by OH in the troposphere, which accounts for around 84 % of all methane removal. Here we present experimentally derived methane + OH kinetic isotope effects and their temperature dependence over the range of 278 to 313 K for CH3D and 13CH3D; the latter is reported here for the first time. We find kCH4/kCH3D = 1.31 ± 0.01 and kCH4/k13CH3D = 1.34 ± 0.03 at room temperature, implying that the methane + OH kinetic isotope effect is multiplicative such that (kCH4/k13CH4)(kCH4/kCH3D) = kCH4/k13CH3D, within the experimental uncertainty, given the literature value of kCH4/k13CH4 = 1.0039 ± 0.0002. In addition, the kinetic isotope effects were characterized using transition state theory with tunneling corrections. Good agreement between the experimental, quantum chemical, and available literature values was obtained. Based on the results we conclude that the OH reaction (the main sink of methane) at steady state can produce an atmospheric clumped isotope signal (Δ(13CH3D) = ln([CH4][13CH3D]/[13CH4][CH3D])) of 0.02 ± 0.02. This implies that the bulk tropospheric Δ(13CH3D) reflects the source signal with relatively small adjustment due to the sink signal (i.e., mainly OH oxidation).

THE RADIOLYSIS OF ETHANOL: IV. DEUTERATED ETHANOLS IN THE LIQUID AND GAS PHASES

1965 ◽  
Vol 43 (5) ◽  
pp. 1484-1492 ◽  
Author(s):  
J. J. J. Myron ◽  
G. R. Freeman
Keyword(s):  
Liquid Phase ◽  
Gas Phase ◽  
Vapor Phase ◽  
Isotope Effects ◽  
Methane Formation ◽  
Gas Phases ◽  
Product Distributions ◽  
Γ Rays ◽  
The Difference ◽  
Α Particles

The value of G(–ethanol) in the vapor phase is nearly double that in the liquid phase. Part of the difference appears to be due to the recombination of radicals in liquid cages. Ethanol molecules, on the average, break into smaller fragments in the gas than in the liquid phase radiolysis. The isotopic compositions of the hydrogen produced from various deuterated ethanols are consistent with the suggestion that the reaction[Formula: see text]occurs to a significant extent in the liquid but not in the gas phase. This reaction probably involves the shift of a hydrogen atom along a hydrogen bond. The reaction[Formula: see text]does not occur to an appreciable extent in the liquid phase. In the liquid phase the relative contributions of the three different groups in the ethanol molecule to hydrogen production are in the order [Formula: see text] A similar trend occurs in the gas, although the contributions of the three groups are more nearly equal in this phase. Isotope effects, in the range kH/kD = 2.2–3.9 per bond, occur in the methane formation mechanism. The isotope effects are somewhat smaller in the liquid than in the vapor phase and somewhat smaller in the inhibited than in the uninhibited systems. A comparison of product distributions in the liquid and gas radiolyses of several compounds by γ-rays and by α-particles indicates that L.E.T. effects can also occur in the gas phase.

scholarly journals Iron-Manganese System for Preparation of Radiocarbon Ams Targets: Characterization of Procedural Chemical-Isotopic Blanks and Fractionation

Radiocarbon ◽  
1997 ◽  
Vol 39 (3) ◽  
pp. 269-283 ◽  
Author(s):  
R. Michael Verkouteren ◽  
Donna B. Klinedinst ◽  
Lloyd A. Currie
Keyword(s):  
Sample Size ◽  
Isotope Effects ◽  
Isotopic Fractionation ◽  
Chemical Yield ◽  
Standard Reference Materials ◽  
Methane Formation ◽  
Fractionation Factor ◽  
Beam Geometry ◽  
C Fibers ◽  
Counting Statistics

We report a practical system to mass-produce accelerator mass spectrometry (AMS) targets with 10–100 μg carbon samples. Carbon dioxide is reduced quantitatively to graphite on iron fibers via manganese metal, and the Fe-C fibers are melted into a bead suitable for AMS. Pretreatment, reduction and melting processes occur in sealed quartz tubes, allowing parallel processing for otherwise time-intensive procedures.Chemical and isotopic (13C, 14C) blanks, target yields and isotopic fractionation were investigated with respect to levels of sample size, amounts of Fe and Mn, pretreatment and reduction time, and hydrogen pressure. With 7-day pretreatments, carbon blanks exhibited a lognormal mass distribution of 1.44 μg (central mean) with a dispersion of 0.50 μg (standard deviation). Reductions of 10 μg carbon onto targets were complete in 3–6 h with all targets, after correction for the blank, reflecting the 13C signature of the starting material. The 100 μg carbon samples required at least 15 h for reduction; shorter durations resulted in isotopic fractionation as a function of chemical yield. The trend in the 13C data suggested the presence of kinetic isotope effects during the reduction. The observed CO2-graphite 13C fractionation factor was 3–4% smaller than the equilibrium value in the simple Rayleigh model. The presence of hydrogen promoted methane formation in yields up to 25%.Fe-C beaded targets were made from NIST Standard Reference Materials and compared with graphitic standards. Although the 12C ion currents from the beads were one to two orders of magnitude lower than currents from the graphite, measurements of the beaded standards were reproducible and internally consistent. Measurement reproducibility was limited mainly by Poisson counting statistics and blank variability, translating to 14C uncertainties of 5–1% for 10–100 μg carbon samples, respectively. A bias of 5–7% (relative) was observed between the beaded and graphitic targets, possibly due to variations in sputtering fractionation dependent on sample size, chemical form and beam geometry.

Clumped isotope constraints on equilibrium carbonate formation and kinetic isotope effects in freezing soils

2018 ◽  
Vol 235 ◽  
pp. 402-430 ◽  
Author(s):  
Landon K. Burgener ◽  
Katharine W. Huntington ◽  
Ronald Sletten ◽  
James M. Watkins ◽  
Jay Quade ◽  
...  
Keyword(s):  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Carbonate Formation ◽  
Kinetic Isotope ◽  
Clumped Isotope

Farmed calcite δ18O, δ13C, and Δ47 at Ascunsă cave, Romania

2021 ◽  
Author(s):  
Virgil Dragusin ◽  
Vasile Ersek ◽  
Alvaro Fernandez ◽  
Roxana Ionete ◽  
Andreea Iordache ◽  
...  
Keyword(s):  
Air Temperature ◽  
Saturation Index ◽  
Isotope Effects ◽  
Monitoring Program ◽  
Drip Water ◽  
Water Ph ◽  
Calcite Saturation ◽  
Calcite Deposition ◽  
Clumped Isotope ◽  
Kinetic Fractionation

&lt;p&gt;Ascuns&amp;#259; cave (Romania) is the subject of a monitoring program since 2012. While the cave air temperature was very stable around 7&amp;#176;C for most of the time, it experienced in 2019 a 3&amp;#176;C rise, and remained high until the present.&lt;/p&gt;&lt;p&gt;We present here &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O, &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C, and clumped isotope results from calcite farmed at two drip points inside the cave (POM X and POM 2). POM X has a slower drip rate than POM 2 and deposits calcite more continuously. Calcite deposition has been shown to depend on cave air CO&lt;sub&gt;2&lt;/sub&gt; concentration, which controls the drip water pH and, further, the calcite saturation index.&lt;/p&gt;&lt;p&gt;In 2019, &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O values at both sites quickly shifted to lower values as a response to the increase in temperature. At POM X, values were situated between approximately -7.2&amp;#8240; and -7.6&amp;#8240; before this transition, whereas in 2019 they shifted to -7.8&amp;#8240; - -8.0&amp;#8240;. At POM 2, where values were generally lower, they shifted from -7.5&amp;#8240; to -7.8&amp;#8240; to -8.0&amp;#8240;.&lt;/p&gt;&lt;p&gt;Clumped isotope temperature estimates mostly agree, within measurement error, with measured cave temperature. This agreement is notable given that strong offsets are commonly observed in mid-latitude caves, reflecting kinetic fractionation effects. However, intervals with deviations from cave temperature are also observed, suggesting variations in isotopic disequilibrium conditions with time.&lt;/p&gt;&lt;p&gt;Here we will discuss these isotope changes in relation to cave air temperature and CO&lt;sub&gt;2&lt;/sub&gt; concentration, drip water isotope values and elemental chemistry, as well as in relation to drip rates, in order to improve our understanding of calcite precipitation and isotope effects in caves.&lt;/p&gt;

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