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Current underground nuclear explosion (UNE) detection strategies rely heavily on atmospheric noble gas sampling of radioxenon. However, discriminating nuclear weapons testing programs from civilian sources is difficult due to highly variable atmospheric radioxenon backgrounds and processes affecting subsurface transport of parent radionuclides. Here, we aim to study the transport of gases produced by subsurface explosions as novel stable signatures for underground nuclear explosion (UNE) monitoring. These gases may be produced in large quantities with distinct molecular ratios, which will be impacted by subsurface transport processes. To demonstrate how ratios of gases produced by explosions can change during transport in geomaterials,more » we conducted laboratory benchtop experiments on the transport of carbon dioxide (CO₂) and hydrogen (H₂) gases through variably saturated zeolitic tuff, which is abundant at the historic US testing site. We observed that zeolitic tuff sorbs substantial quantities of CO₂ while allowing H₂ to transport more freely, leading to changes in the molecular ratios of the two gases along the transport pathway. Gas uptake in the dry zeolitic tuff core was 72.3% for CO₂, compared with 53.4% for xenon and 7.6% for H₂. The presence of 20% water saturation disrupted the CO₂ sorption process, though to a lesser extent than observed for noble gases, with a 36.7% drop in xenon sorption compared with a 21.9% drop for CO₂. These results represent the first observations of zeolite sorption altering explosive gas ratios during transport through geomedia relevant to nuclear proliferation monitoring.« less

Signatures of mass-independent fractionation (MIF) of sulfur in Archean sulfide and sulfate minerals are widely thought to record an anoxic early Earth’s atmosphere. While experiments of ultraviolet irradiation of SO₂ produce significant sulfur mass-independent fractionation (S-MIF) in reaction products (elemental sulfur and residual sulfur dioxide), they have not been able to reproduce the isotope patterns, in particular Δ³⁶S/Δ³³S ratios, observed in the geologic rock record. Studies that focused on organic sulfur gases and hazes in Archean did not report organosulfur aerosol photoproducts as major contributors to Archean S-MIF chemistry. Here we show, for the first time, that photochemical reactions ofmore » SO₂ in the presence of gaseous hydrocarbons (CH₄, C₂H₂, and C₂H₄) produce haze-like organosulfur aerosols bearing S-MIF with variable Δ³⁶S/Δ³³S ratios. The isotope trends for the organosulfur photoproducts produced in our experiments suggest that in addition to elemental sulfur, organosulfur compounds—in particular methanesulfonic acid—are a key component of S-MIF signals from the atmosphere to the ocean and sediments with possible links to Archean atmosphere warmed by a methane greenhouse.« less

Methane clumped isotope compositions signify the relative natural abundances of rare, doubly substituted isotopic species of methane (¹³CH₃D and ¹²CH₂D₂) and have emerged as a new isotopic tool to trace the sources, sinks, and lifecycles of methane in the environment. Such measurements can identify equilibration (or reequilibration) temperatures if found to be in isotopic equilibrium or non-equilibrium processes (e.g., kinetically controlled reactions or mixing) if not in isotopic equilibrium. Naturally occurring thermogenic methane—formed by the thermally activated breakdown of larger organic molecules—has been found to have clumped isotope compositions consistent with equilibrium at reasonable gas formation temperatures in some settingsmore » and non-equilibrium processes occurring during either formation, migration, storage, or extraction in others. To explore the potential controls on the isotopic composition of thermogenic methane, we conducted isothermal time-series ethane pyrolysis experiments at 550 and 600 °C to measure methane and ethane ¹³C/¹²C and D/H fractionations and methane clumped isotope compositions (resolved ¹³CH₃D and ¹²CH₂D₂). We explore the effects of modifying the initial clumped isotope composition of ethane and the addition of water vapor to pyrolysis experiments. We observe that ethane and methane ¹³C/¹²C are controlled by kinetic isotope effects and Rayleigh distillation processes. In contrast, ethane and methane D/H and methane clumped isotope compositions appear to be controlled by a combination of these processes and hydrogen isotope exchange. The hydrogen isotope exchange processes lead to isotopic equilibrium as reaction completion is approached for both D/H (ethane/methane) and methane clumped isotope compositions. Here, we develop a chemical model based on a mass balance approach that accounts for inheritance vs. hydrogen-abstraction formation pathways for singly and doubly substituted isotopologues of ethane and methane that is compared to the experimental data. The model allows the determination of carbon and hydrogen kinetic isotope effects associated with ethane cracking and hydrogen abstraction reactions that, where applicable, we compare to prior theoretical constraints. From the comparison of the model to the experimental data, we infer that the kinetically controlled ethane and methane bulk isotope compositions and methane clumped isotope compositions are controlled by kinetic isotope effects (both primary and secondary) associated with both C–C bond and C–H bond cleavage reactions. Specifically, the methane clumped isotope compositions likely result from a combination of clumped isotope effects associated with ethane breakdown and/or assembly of methane isotopologues (expressed in terms of γ-factor parameters ≠ 1) and combinatorial effects that arise probabilistically. We discuss our experimental results in the context of recent pyrolysis experiments and observations of naturally occurring thermogenic methane. We consider a proposal consistent with observations from nature that the hydrogen isotope exchange reactions that promote equilibration of methane isotopic molecules at or near formation temperature may be facilitated by free radicals generated by pyrolysis reactions. In this framework, isotope exchange effectively ceases when pyrolysis effectively ceases locking in compositions that can be consistent with peak formation temperatures.« less