Theory and Calculation of Nontraditional Stable Isotope Fractionation

This paper conducts processing on isotope anharmonic effect with molecular dynamics method and Monte Carlo method based on path integration. It introduces the theoretical calculation method of pressure effect, and finally the nuclear volume effect and its theoretical calculation method, stressing that the nuclear volume effect is an important part of isotopic studies of heavy metals in the future. This paper makes an analysis on the equilibrium fractionation theory based on simple harmonic approximation.

Disentangle Kinetic From Equilibrium Fractionation Using Primary (δ17O, δ18O, δD) and Secondary (Δ17O, dex) Stable Isotope Parameters on Samples From the Swiss Precipitation Network

Since 1971 water isotope measurements are being conducted by the Climate and Environmental Physics Division at the University of Bern on precipitation, river- and groundwater collected at several places within Switzerland. The water samples were stored in glass flasks for later analyses with improved instrumentation. Conventional isotope ratio measurements on precipitated water from all stations of the network are well correlated as expected. However, Δ17O as well as dex is anticorrelated to these isotope ratio. The combination of these parameters allow to investigate dependencies on temperature, turbulence factor, and humidity of these values as well as to look into the importance and relative contributions of kinetic to equilibrium fractionations. We used published temperature dependent fractionation factors in combination with a simple Rayleigh model approach to investigate the importance of the meteorological parameters on the isotope ratios. A direct comparison of measured and modeled isotope ratios for primary (δ17O, δ18O and (δD) as well as secondary isotope parameters (Δ17O and dex) is shown.

Kinetic effects during ocean evaporation: observed relationship with wind speed

<p>Water vapor has a fundamental role in weather and climate, being the strongest natural greenhouse gas in the Earth’s atmosphere. The main source of water vapor in the atmosphere is ocean evaporation, which transfers a large amount of energy via latent heat fluxes. In the past, evaporation was intensively studied using stable isotopes because of the large fractionation effects involved during water phase changes, providing insights on processes occurring at the air-water interface. Current theories describe evaporation near the air-water interface as a combination of molecular and turbulent diffusion processes into separated sublayers. The importance of those two sublayers, in terms of total resistance to vapor transport in air, is expected to be dependent on parameters such as moisture deficit, temperature and wind speed. Non-equilibrium fractionation effects in isotopic evaporation models are then expected to be related to these physical parameters. In the last 10 years, several water vapor observations from oceanic expeditions were focused on the impact of temperature and wind speed effect, assuming the influence of those parameters on non-equilibrium fractionation in the marine boundary layer. Wind speed effect is expected to be small on total kinetic fractionation and was discussed at length but was not completely ruled out. With a gradient-diffusion approach (2 heights above the ocean surface) and Cavity Ring-Down Spectroscopy we have estimated non-equilibrium fractionation factors for <sup>18</sup>O/<sup>16</sup>O during evaporation, showing that the wind speed effect can be detected and has no significant impact on kinetic fractionation. Results obtained for wind speeds between 0 and 10 m s<sup>-1</sup> in the North Atlantic Ocean are consistent with the Merlivat and Jouzel (1979) parametrization for smooth surfaces (mean ε<sub>18</sub>=6.1‰). A small monotonic decrease of the fractionation parameter is observed as a function of 10 m wind speed (slope  ≅ 0.15 ‰ m<sup>-1</sup> s), without any evident discontinuity. However, depending on the data filtering approach it is possible to highlight a rapid decrease of the kinetic fractionation factor at low wind speed (≤ 2.5 m s<sup>-1</sup>). An evident decrease of fractionation factor is also observed for wind speeds above 10 m s<sup>-1</sup>, allowing to hypothesize the possible effect of sea spray in net evaporation flux. Considering the average wind speed over the oceans, we conclude that a constant kinetic fractionation factor for evaporation is a more simple and reasonable solution than a wind-speed dependent parametrization. </p><p> </p><p>Merlivat, L., & Jouzel, J. (1979). Global climatic interpretation of the deuterium‐oxygen 18 relationship for precipitation. Journal of Geophysical Research: Oceans, 84(C8), 5029-5033.</p>

Monitoring of stable isotopes (δ2H, δ18O) in precipitations of Moscow city (Russia): comparison for 2005–2014 and 1970–1979 periods

The isotopic composition of oxygen (δ18O) and hydrogen (δ2H) of atmospheric precipitation in Moscow in 2005–2014 was studied by sampling single precipitations (a total of 842 measurements after rejection of unreliable samples). A comparison is made with similar studies carried out by VSEGINGEO and IVP RAS for the IAEA-WMO GNIP network in 1969–1979, when monthly average samples were taken by the accumulation method (61 pair determinations of deuterium and oxygen-18, mainly in 1975–1979). The 2005–2015 series is reduced to a similar form for the 1969–1979 series, recalculated through the data on the volume of precipitation at the nearest meteorological station. It was found that in the last decade there has been a significant change in the equation of the local line of meteoric waters, which for the first period had the form δ2H = 6.09×δ18O – 23.0 ‰ (R2 = 0.87), and is currently described by the relation δ2H = 6.93×δ18O – 11.3 ‰ (R2 = 0.944). There is also a decrease in depletion of the average values of δ2H and δ18O, compared with the first observation period, which apparently reflects the course of climatic changes. At the same time, within each of the periods, a negative (albeit relatively small) slope of the line of approximation of chronological data is noted. Deuterium excess naturally changes seasonally, taking negative values in summer, primarily due to non-equilibrium fractionation during evaporation. In 2005–2014, the temperature dependence of the isotopic composition of precipitation changed significantly in comparison with 1969–1979, which makes it impossible to reconstruct the composition of precipitation in the past from meteorological observations.

Theoretical Estimates of Equilibrium Carbon and Hydrogen Isotope Effects in Microbial Methane Production and Anaerobic Oxidation of Methane

Microbial production and consumption of methane are widespread in natural and artificial environments, with important economic and climatic implications. Attempts to use the isotopic composition of methane to constrain its sources are complicated by incomplete understanding of the mechanisms of variation in methane's isotopic composition. Knowledge of the equilibrium isotope fractionations among the large organic intracellular intermediates in the microbial pathways of methane production and consumption must form the basis of any exploration of the mechanisms of isotopic variation, but estimates of these equilibrium isotope fractionations are currently unavailable. To address this gap, we calculated the equilibrium isotopic fractionation of carbon (¹³C/¹²C) and hydrogen (D/H) isotopes among compounds in anaerobic methane metabolisms, as well as the abundance of multiple isotope substitutions ("clumping," e.g., ¹³C--D) in these compounds. The Density Functional Theory calculations employed the M06-L/def2-TZVP level of theory and the SMD implicit solvation model, which we have recently optimized for large organic molecules and tested against measured equilibrium isotope fractionations. The computed ¹³beta and ²beta values decrease with decreasing average oxidation state of the carbon atom in the molecules, resulting in a preference for enrichment of the molecules with more oxidized carbon in ¹³C and D. Using the computed $\beta$ values, we calculated the equilibrium isotope fractionation factors in the prominent methanogenesis pathways (hydrogenotrophic, methylotrophic and acetoclastic) and in the pathway for anaerobic oxidation of methane (AOM) over a temperature range of 0-700 degrees Celsius. Our calculated equilibrium fractionation factors compare favorably with experimental constrains, where available, and we used them to investigate the relation between the apparent isotope fractionation during methanogenesis and AOM and the thermodynamic drive for these reactions. We show that a detailed map of the equilibrium fractionation factors along these metabolic pathways allows an evaluation of the contribution of equilibrium and kinetic isotope effects to apparent isotope fractionations observed in laboratory, natural and artificial settings. The comprehensive set of equilibrium isotope fractionation factors calculated in this study provides a firm basis for future explorations of isotope effects in methane metabolism.

Theoretical Estimates of Equilibrium Carbon and Hydrogen Isotope Effects in Microbial Methane Production and Anaerobic Oxidation of Methane

Microbial production and consumption of methane are widespread in natural and artificial environments, with important economic and climatic implications. Attempts to use the isotopic composition of methane to constrain its sources are complicated by incomplete understanding of the mechanisms of variation in methane's isotopic composition. Knowledge of the equilibrium isotope fractionations among the large organic intracellular intermediates in the microbial pathways of methane production and consumption must form the basis of any exploration of the mechanisms of isotopic variation, but estimates of these equilibrium isotope fractionations are currently unavailable. To address this gap, we calculated the equilibrium isotopic fractionation of carbon (¹³C/¹²C) and hydrogen (D/H) isotopes among compounds in anaerobic methane metabolisms, as well as the abundance of multiple isotope substitutions ("clumping," e.g., ¹³C--D) in these compounds. The Density Functional Theory calculations employed the M06-L/def2-TZVP level of theory and the SMD implicit solvation model, which we have recently optimized for large organic molecules and tested against measured equilibrium isotope fractionations. The computed ¹³beta and ²beta values decrease with decreasing average oxidation state of the carbon atom in the molecules, resulting in a preference for enrichment of the molecules with more oxidized carbon in ¹³C and D. Using the computed $\beta$ values, we calculated the equilibrium isotope fractionation factors in the prominent methanogenesis pathways (hydrogenotrophic, methylotrophic and acetoclastic) and in the pathway for anaerobic oxidation of methane (AOM) over a temperature range of 0-700 degrees Celsius. Our calculated equilibrium fractionation factors compare favorably with experimental constrains, where available, and we used them to investigate the relation between the apparent isotope fractionation during methanogenesis and AOM and the thermodynamic drive for these reactions. We show that a detailed map of the equilibrium fractionation factors along these metabolic pathways allows an evaluation of the contribution of equilibrium and kinetic isotope effects to apparent isotope fractionations observed in laboratory, natural and artificial settings. The comprehensive set of equilibrium isotope fractionation factors calculated in this study provides a firm basis for future explorations of isotope effects in methane metabolism.

Theoretical Estimates of Equilibrium Carbon and Hydrogen Isotope Effects in Microbial Methane Production and Anaerobic Oxidation of Methane

Microbial production and consumption of methane are widespread in natural and artificial environments, with important economic and climatic implications. Attempts to use the isotopic composition of methane to constrain its sources are complicated by incomplete understanding of the mechanisms of variation in methane's isotopic composition. Knowledge of the equilibrium isotope fractionations among the large organic intracellular intermediates in the microbial pathways of methane production and consumption must form the basis of any exploration of the mechanisms of isotopic variation, but estimates of these equilibrium isotope fractionations are currently unavailable. To address this gap, we calculated the equilibrium isotopic fractionation of carbon (¹³C/¹²C) and hydrogen (D/H) isotopes among compounds in anaerobic methane metabolisms, as well as the abundance of multiple isotope substitutions ("clumping," e.g., ¹³C--D) in these compounds. The Density Functional Theory calculations employed the M06-L/def2-TZVP level of theory and the SMD implicit solvation model, which we have recently optimized for large organic molecules and tested against measured equilibrium isotope fractionations. The computed ¹³beta and ²beta values decrease with decreasing average oxidation state of the carbon atom in the molecules, resulting in a preference for enrichment of the molecules with more oxidized carbon in ¹³C and D. Using the computed $\beta$ values, we calculated the equilibrium isotope fractionation factors in the prominent methanogenesis pathways (hydrogenotrophic, methylotrophic and acetoclastic) and in the pathway for anaerobic oxidation of methane (AOM) over a temperature range of 0-700 degrees Celsius. Our calculated equilibrium fractionation factors compare favorably with experimental constrains, where available, and we used them to investigate the relation between the apparent isotope fractionation during methanogenesis and AOM and the thermodynamic drive for these reactions. We show that a detailed map of the equilibrium fractionation factors along these metabolic pathways allows an evaluation of the contribution of equilibrium and kinetic isotope effects to apparent isotope fractionations observed in laboratory, natural and artificial settings. The comprehensive set of equilibrium isotope fractionation factors calculated in this study provides a firm basis for future explorations of isotope effects in methane metabolism.

Theoretical Estimates of Equilibrium Carbon and Hydrogen Isotope Effects in Microbial Methane Production and Anaerobic Oxidation of Methane

Microbial production and consumption of methane are widespread in natural and artificial environments, with important economic and climatic implications. Attempts to use the isotopic composition of methane to constrain its sources are complicated by incomplete understanding of the mechanisms of variation in methane's isotopic composition. Knowledge of the equilibrium isotope fractionations among the large organic intracellular intermediates in the microbial pathways of methane production and consumption must form the basis of any exploration of the mechanisms of isotopic variation, but estimates of these equilibrium isotope fractionations are currently unavailable. To address this gap, we calculated the equilibrium isotopic fractionation of carbon (¹³C/¹²C) and hydrogen (D/H) isotopes among compounds in anaerobic methane metabolisms, as well as the abundance of multiple isotope substitutions ("clumping," e.g., ¹³C--D) in these compounds. The Density Functional Theory calculations employed the M06-L/def2-TZVP level of theory and the SMD implicit solvation model, which we have recently optimized for large organic molecules and tested against measured equilibrium isotope fractionations. The computed ¹³beta and ²beta values decrease with decreasing average oxidation state of the carbon atom in the molecules, resulting in a preference for enrichment of the molecules with more oxidized carbon in ¹³C and D. Using the computed $\beta$ values, we calculated the equilibrium isotope fractionation factors in the prominent methanogenesis pathways (hydrogenotrophic, methylotrophic and acetoclastic) and in the pathway for anaerobic oxidation of methane (AOM) over a temperature range of 0-700 degrees Celsius. Our calculated equilibrium fractionation factors compare favorably with experimental constrains, where available, and we used them to investigate the relation between the apparent isotope fractionation during methanogenesis and AOM and the thermodynamic drive for these reactions. We show that a detailed map of the equilibrium fractionation factors along these metabolic pathways allows an evaluation of the contribution of equilibrium and kinetic isotope effects to apparent isotope fractionations observed in laboratory, natural and artificial settings. The comprehensive set of equilibrium isotope fractionation factors calculated in this study provides a firm basis for future explorations of isotope effects in methane metabolism.