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Rationale: When isotope composition is measured in dual‐inlet mode with an isotope ratio mass spectrometer (IRMS), reference gas may be gradually enriched in heavy isotopes due to preferential loss of light ones from the bellows over time. Quantifying the degree of isotopic enrichment of the reference gas is imperative for high‐precision isotopic analysis (i.e. at per meg level). Methods: O2 and CO2 leaking experiments were performed with the dual‐inlet system of an IRMS (Thermo Fisher® MAT 253 Plus). During each experiment, the drop of gas pressure in the bellows with time was recorded and isotope ratios of residual gas at various time intervals were analyzed. Results: Isotopic enrichment of residual O2 gas could be as large as 1‰ for δ18O when a large fraction (>75%) of initial gas was lost. The evolution of isotope compositions of the remaining gas can be well described by a pressure‐dependent Rayleigh fractionation equation. When the pressure in the bellows is within 10–50 mBar, the isotope fractionation factor (α18O) for O2 leaking ranges from 0.99911 to 0.99982 and the characteristic relationship of α17O and α18O is from 0.5123 to 0.5124. Conclusions: Isotope fractionation associated with capillary leaking from bellows is pressure‐dependent. We recommend that the reference gas should be reloaded frequently, especially after a measurement with a low analyzing pressure for the analysis of small amounts of sample. [ABSTRACT FROM AUTHOR]

It is expected that information on the source, reaction pathway, and reaction kinetics of an organic compound can be obtained from its position-specific isotope compositions or intramolecular isotope distribution (Intra-ID). To retrieve the information, we could use its predicted equilibrium Intra-ID as a reference for understanding the observed Intra-IDs. Historically, observed, apparently close-to-equilibrium carbon Intra-ID has prompted an open debate on the nature of biosystems and specifically the pervasiveness of reversible biochemical reactions. Much of the debate remains unresolved, and the discussion has not clearly distinguished between two states of equilibrium: (1) the equilibrium among the corresponding bond-breaking and bond-forming positions in reactant and product and (2) the equilibrium among all carbon positions within a compound. For an organic molecule with multiple carbon positions, equilibrium carbon Intra-ID can be attained only when a specific reaction is in equilibrium and the sources of each position are also in equilibrium with each other. An observed Intra-ID provides limited information on if the sources and pathways are both unconstrained. Here, we elaborate on this insight using examples of the observed Intra-IDs of hydroxyl-bearing minerals, N2O , and acetic acid. Research effort aiming to calibrate position-specific equilibrium and kinetic isotope fractionation factors for defined processes will help to interpret observed Intra-IDs of a compound accurately and fully. [ABSTRACT FROM AUTHOR]

Isotope fractionation during the evaporation of silicate melt and condensation of vapor has been widely used to explain various isotope signals observed in lunar soils, cosmic spherules, calcium-aluminum-rich inclusions, and bulk compositions of planetary materials. During evaporation and condensation, the equilibrium isotope fractionation factor (α) between high-temperature silicate melt and vapor is a fundamental parameter that can constrain the melt’s isotopic compositions. However, equilibrium α is difficult to calibrate experimentally. Here we used Mg as an example and calculated equilibrium Mg isotope fractionation in MgSiO3 and Mg2SiO4 melt-vapor systems based on first-principles molecular dynamics and the high-temperature approximation of the Bigeleisen-Mayer equation. We found that, at 2500 K, δ25Mg values in the MgSiO3 and Mg2SiO4 melts were 0.141 ± 0.004 and 0.143 ± 0.003‰ more positive than in their respective vapors. The corresponding δ26Mg values were 0.270 ± 0.008 and 0.274 ± 0.006‰ more positive than in vapors, respectively. The general α-T equations describing the equilibrium Mg α in MgSiO3 and Mg2SiO4 melt-vapor systems were: αMgl-Mgg=1+5.264×105T21m-1m′ and αMgl-Mgg=1+5.340×105T21m-1m′, respectively, where m is the mass of light isotope 24Mg and m′ is the mass of the heavier isotope, 25Mg or 26Mg. These results offer a necessary parameter for mechanistic understanding of Mg isotope fractionation during evaporation and condensation that commonly occurs during the early stages of planetary formation and evolution. [ABSTRACT FROM AUTHOR]

Rationale: Although being out of equilibrium, biomolecules in organisms have the potential to approach isotope equilibrium locally because enzymatic reactions are intrinsically reversible. A rigorous approach that can describe isotope distribution among biomolecules and their apparent deviation from equilibrium state is lacking, however. Methods: Applying the concept of distance matrix in graph theory, we propose that apparent local isotope equilibrium among a subset of biomolecules can be assessed using an apparent fractionation difference (|Δα|) matrix, in which the differences between the observed isotope composition (δ') and the calculated equilibrium fractionation factor (1000lnβ) can be more rigorously evaluated than by using a previous approach for multiple biomolecules. We tested our |Δα| matrix approach by re‐analyzing published data of different amino acids (AAs) in potato and in green alga. Results: Our re‐analysis shows that biosynthesis pathways could be the reason for an apparently close‐to‐equilibrium relationship inside AA families in potato leaves. Different biosynthesis/degradation pathways in tubers may have led to the observed isotope distribution difference between potato leaves and tubers. The analysis of data from green algae does not support the conclusion that AAs are further from equilibrium in glucose‐cultured green algae than in the autotrophic ones. Conclusions: Application of the |Δα| matrix can help us to locate potential reversible reactions or reaction networks in a complex system such as a metabolic system. The same approach can be broadly applied to all complex systems that have multiple components, e.g. geochemical or atmospheric systems of early Earth or other planets. [ABSTRACT FROM AUTHOR]

Presents a study of the oxygen-isotope values of two massive sulphate mineral deposits which show large isotopic anomalies despite being formed in surface environments. Origination of deposits in the central Namib desert and the sulphate-bearing Miocene volcanic ash-beds in North America; Questions regarding the source of the oxygen isotope; Possibility of the chemical origin of variable isotope anomalies on planets other than Earth.

Abstract: To improve our knowledge about the geochemical and environmental aftermath of Neoproterozoic global glaciations, we analyzed stable isotopes (δ 13C, δ 18O, δ 34S) and elemental concentrations (Ca, Mg, S, Sr, Fe, and Mn) of the ∼10-m-thick Zhamoketi cap dolostone atop the Tereeken diamictite in the Quruqtagh area, eastern Chinese Tianshan. Available chemostratigraphic data suggest that the Tereeken diamictite is probably equivalent to the Marinoan glaciation. Our new data indicate that organic and carbonate carbon isotopes of the Zhamoketi cap dolostone show little stratigraphic variations, averaging −28.2‰ and −4.6‰, respectively. In contrast, sulfur isotopes show significant stratigraphic variations. Carbonate associated sulfate (CAS) abundance decreases rapidly in the basal cap dolostone and δ 34SCAS composition varies between +9‰ and +15‰ in the lower 2.5 m. In the overlying interval, CAS abundance remains low while δ 34SCAS rises ∼5‰ and varies more widely between +10‰ and +21‰. The range of δ 34Spy of the cap dolostone overlaps with that of δ 34SCAS, but direct comparison shows that δ 34Spy is typically greater than δ 34SCAS measured from the same samples. Hypotheses to explain the observations must account for both the remarkable sulfur isotope enrichment of pyrites and the inverse fractionation. We propose that CAS and pyrite were derived from two isotopically distinct reservoirs in a chemically stratified basin or a basin with a sulfate minimum zone. In this model, CAS was derived from shallow, oxic surface waters with moderate sulfate concentration and depleted in 34S due to the post-glacial influx of sulfur from continental weathering. In contrast, pyrite was derived from anoxic bottom waters (or a sulfate minimum zone) with low sulfate concentration and 34S enrichment due to long-term syn-glacial sulfate reduction. The rapid shift in CAS abundance and sulfur isotope composition within the cap dolostone is interpreted to reflect the mixing of the two reservoirs after initial deglaciation. Comparison with other post-Marinoan cap carbonates shows significant spatial heterogeneity in δ 34SCAS, which together with strong temporal variation in δ 34SCAS, points to generally low sulfate concentrations in post-Marinoan oceans. [Copyright &y& Elsevier]