Low 13C-13C abundances in abiotic ethane.

Distinguishing biotic compounds from abiotic ones is important in resource geology, biogeochemistry, and the search for life in the universe. Stable isotopes have traditionally been used to discriminate the origins of organic materials, with particular focus on hydrocarbons. However, despite extensive efforts, unequivocal distinction of abiotic hydrocarbons remains challenging. Recent development of clumped-isotope analysis provides more robust information because it is independent of the stable isotopic composition of the starting material. Here, we report data from a 13C-13C clumped-isotope analysis of ethane and demonstrate that the abiotically-synthesized ethane shows distinctively low 13C-13C abundances compared to thermogenic ethane. A collision frequency model predicts the observed low 13C-13C abundances (anti-clumping) in ethane produced from methyl radical recombination. In contrast, thermogenic ethane presumably exhibits near stochastic 13C-13C distribution inherited from the biological precursor, which undergoes C-C bond cleavage/recombination during metabolism. Further, we find an exceptionally high 13C-13C signature in ethane remaining after microbial oxidation. In summary, the approach distinguishes between thermogenic, microbially altered, and abiotic hydrocarbons. The 13C-13C signature can provide an important step forward for discrimination of the origin of organic molecules on Earth and in extra-terrestrial environments.

Intramolecular isotopic evidence for bacterial oxidation of propane in subsurface natural gas reservoirs.

Microbial anaerobic oxidation of hydrocarbons is a key process potentially involved in a myriad of geological and biochemical environments yet has remained notoriously difficult to identify and quantify in natural environments. We performed position-specific carbon isotope analysis of propane from cracking and incubation experiments. Anaerobic bacterial oxidation of propane leads to a pronounced and previously unidentified 13C enrichment in the central position of propane, which contrasts with the isotope signature associated with the thermogenic process. This distinctive signature allows the detection and quantification of anaerobic oxidation of hydrocarbons in diverse natural gas reservoirs and suggests that this process may be more widespread than previously thought. Position-specific isotope analysis can elucidate the fate of natural gas hydrocarbons and provide insight into a major but previously cryptic process controlling the biogeochemical cycling of globally significant greenhouse gases.

Carbon, Hydrogen and Chlorine Stable Isotope Fingerprinting for Forensic Investigations of Hexachlorocyclohexanes.

Multielemental stable isotope analysis of persistent organic pollutants (POPs) has the potential to characterize sources, sinks, and degradation processes in the environment. To verify the applicability of this approach for source identification of hexachlorocyclohexane (HCHs), we provide a data set of carbon, hydrogen, and chlorine stable isotope ratios (δ13C, δ2H, δ37Cl) of its main stereoisomers (α-, ß-, δ- and γ-HCHs) from a sample collection based on worldwide manufacturing. This sample collection comprises production stocks, agricultural and pharmaceutical products, chemical waste dumps, and analytical-grade material, covering the production time period from the late 1960s until now. Stable isotope ratios of HCHs cover the ranges from -233‰ to +1‰, from -35.9‰ to -22.7‰, and from -6.69‰ to +0.54‰ for δ2H, δ13C, and δ37Cl values, respectively. Four groups of samples with distinct multielemental stable isotope fingerprints were differentiated, most probably as a result of purification and isolation processes. No clear temporal trend in the isotope compositions of HCHs was found at the global scale. The multielemental stable isotope fingerprints facilitate the source identification of HCHs at the regional scale and can be used to assess transformation processes. The data set and methodology reported herein provide basic information for the assessment of environmental field sites contaminated with HCHs.

Determination of Bromine Stable Isotope Ratios from Saline Solutions by "Wet Plasma" MC-ICPMS Including a Comparison between High- and Low-Resolution Modes, and Three Introduction Systems.

We describe a novel method for measuring stable bromine isotope compositions in saline solutions such as seawater, brines, and formation waters. Bromine is extracted from the samples by ion exchange chromatography on anion exchange resin AG 1-X4 with NH4NO3 and measured by MC-ICP-MS in wet plasma conditions. Sample introduction through a small spray chamber provided good sensitivity and stability of the Br signal compared to direct injection (d-DIHEN) and desolvation (APEX). NH4NO3 media allowed fast (<3 min) washing of the system. Despite Ar2H(+) spectral interference on (81)Br(+), for the first time low-resolution mode (with appropriate tuning of Ar2H(+)/(81)Br(+) sensitivity) gave higher precision (81)Br/(79)Br measurements than high-resolution (HR), due to the narrowness of the (81)Br(+) plateau in HR mode and to slight mass drifting with time. Additionally, 1 µg Br is the lower amount needed for a triplicate determination of δ(81)Br by MC-ICP-MS, with reproducibility often < ± 0.1‰ (2 SD). Four HBr solutions were prepared by evaporation/condensation in order to obtain in-house reference solutions with 3‰ variations in δ(81)Br and to assess the reproducibility and accuracy of the method. Long-term (>3 years) reproducibility between ± 0.11 and ± 0.27‰ (2 SD) was obtained for the four HBr solutions, the international standard reference material NIST SRM 977 (δ(81)BrSMOB = -0.65 ± 1.1‰, 1 SD), and seawaters (synthetic and natural). The accuracy of the MC-ICP-MS method was validated by comparing the δ(81)Br obtained for these solutions with dual-inlet IRMS measurements on CH3Br gas. Finally, the method was successfully applied to 22 natural samples.