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Methyl bromide (CH3Br) emitted from plants constitutes a natural source of bromine to the atmosphere, and is a component in the currently unbalanced global CH3Br budget. In the stratosphere, CH3Br contributes to ozone loss processes. Studies of stable isotope composition may reduce uncertainties in the atmospheric CH3Br budget, but require well-constrained isotope fingerprints of the source end members. Here we report the first measurements of stable bromine isotopes (δ81Br) in CH3Br from abiotic plant emissions. Incubations of both KBr-fortified pectin, a ubiquitous cell-stabilizing macromolecule, and of a natural halophyte (Salicornia fruticosa), yielded an enrichment factor (ε) of −2.00±0.23‰ (1σ, n =8) for pectin and −1.82±0.02‰ (1σ, n =4) for Salicornia (the relative amount of the heavier 81Br was decreased in CH3Br compared to the substrate salt). For short incubations, and up to 10% consumption of the salt substrate, this isotope effect was similar for temperatures from 30 up to 300°C. For longer incubations of up to 90h at 180°C the δ81Br values increased from −2‰ to 0‰ for pectin and to −1‰ for Salicornia. These δ81Br source signatures of CH3Br formation from plant matter combine with similar data for carbon isotopes to facilitate multidimensional isotope diagnostics of the CH3Br budget. [ABSTRACT FROM AUTHOR]

Improved sensitivity in the analysis of stable chlorine isotopes of organochlorines (δ37Cl-OCl) has been established using sealed tube combustion in conjunction with thermal ionization mass spectrometry (TIMS). TIMS of chlorine isotopes was performed on < 85 nmol of Cl with an achievable precision of < 0.25‰ for pure inorganic chloride samples and 0.46‰ for chloride liberated from organochlorines (OCls). This makes possible significant reductions in the overall sample size requirement, as compared to the techniques of gas source stable isotope ratio mass spectrometry (SIRMS). Yields in excess of 99% were demonstrated in the dechlorination of <0.14 μmol 1,1, l- trichloro-2,2-bis(p-chlorophenyl)ethane (DDT), and the overall yield, including purification of liberated chloride, was 86-97%. The accuracy of TIMS in the measurement of chlorine isotopes derived from OCls was con- finned by analysis of a DDT sample previously analyzed with SIRMS. Using the described method for TIMS, the DDT sample gave a bulk chlorine isotope ratio of δ37Cl -4.42 ± 0.46‰ (1σ). The reported value from SIRMS analysis is -4.34 ± 0.25‰ indicating the conformity of the two methods. [ABSTRACT FROM AUTHOR]

The East Siberian Arctic Shelf (ESAS) has a large inventory of buried carbon and methane (CH4) as part of and below subsea permafrost. Warming of this system by geothermal heat and by overlying seawater throughout the Holocene, recently ammended by Anthropocene warming, has brought subsea permafrost towards the point of thaw. There have been extensive observations of strongly elevated CH4 concentrations in ESAS seawater over the recent decade. However, there are still large uncertainties from what part of the subsea system the released methane originates, and also about the large-scale geographical variations. This study presents methane observations from the 2014 SWERUS-C3 cruise along an east-west transect of the outer East Siberian Sea (ESS). Surface ocean methane concentrations were regularly above the atmospheric equilibrium concentration, with seawater concentrations in a ESS hotspot region reaching up to 460 nM. The observations include 13C stable isotope observation in an active ebullition site. Understanding the sources and spatial distribution of the methane releases will aid in understanding the composition and functioning of the subsea permafrost/methane system and help us move toward predictive capacity of how the methane evasions will develop into the future. [ABSTRACT FROM AUTHOR]

Chlorinated ethenes (CEs) are ubiquitous groundwater contaminants, yet there remains a need for a method to efficiently monitor their in situ degradation. We report here the first field application of combined stable carbon and chlorine isotope analysis of tetrachloroethene (PCE) and trichloroethene (TCE) to investigate their biodegradation in a heavily contaminated aquifer. The two-dimensional Compound Specific Isotope Analysis (2D-CSIA) approach was facilitated by a recently developed gas chromatography-quadrupole mass spectrometry (GCqMS) method for δ37Cl determination. Both C and Cl isotopes showed evidence of ongoing PCE transformation. Applying published C isotope enrichment factors (ϵC) enabled evaluation of the extent of in situ PCE degradation (11-78%). We interpreted C and Cl isotopes using a numerical reactive transport model along a 60-m flow path. It revealed that combined PCE and TCE mass load was dechlorinated by less than 10%, and that cis-dichloroethene was not further dechlorinated. Furthermore, the 2D-CSIA approach allowed estimation of Cl isotope enrichment factors ϵCl (-7.8 to -0.8‰) and characteristic ϵCl/ϵC values (0.42-1.12) for reductive PCE dechlorination at this field site. This investigation demonstrates the benefit of 2D-CSIA to assess in situ degradation of CEs and the applicability of Cl isotope fractionation to evaluate PCE and TCE dechlorination. [ABSTRACT FROM AUTHOR]