Influences of salinity and temperature on the stable isotopic composition of methane and hydrogen sulfide trapped in pressure-vessel hydrates

The stable isotopic composition of carbon (delta C-13(CH4)) and hydrogen (delta H-2(CH4)) in methane and sulfur (delta S-34(H2S)) in hydrogen sulfide can be used to infer the source of volatile molecules encaged in gas hydrates. Differentiation of methane and hydrogen sulfide from microbial and thermal origins provides valuable information for hydrocarbon exploration and for climatic models assessing the role of gas hydrates during climate change. In astrobiological studies, delta C-13(CH4), delta H-2(CH4), and delta S-34(H2S) values will be critical in deciphering the origin of methane and hydrogen sulfide molecules if gas hydrates are detected within the cryosphere of Mars or associated with ice-covered oceans on Europa or Enceladus. It is challenging, however, to apply isotope systematics to hydrate-forming systems due to complex influences on nucleation and decomposition under varying conditions of salinity, pressure, and temperature. Few laboratory studies have evaluated the effect of hydrate formation, on isotopic composition of free, encaged, and dissolved gas molecules. In this study, pressure-vessel hydrates were nucleated under conditions inferred for marine continental margins and terrestrial permafrost: low temperatures, moderate pressures, saturation of methane and/or hydrogen sulfide saturation, and varying concentration of sodium chloride (NaCl) and magnesium sulfate heptahydrate (MgSO4 center dot 7H(2)O). Methane experiments show less than 1 parts per thousand differences in values of delta C-13(CH4) between free and encaged molecules and up to 6.5 parts per thousand variations in values of delta H-2(CH4) between free and encaged molecules. In hydrogen-sulfide hydrates, delta S-34(H2S) values show less than 4 parts per thousand differences between free and encaged molecules, but up to 14 parts per thousand differences between dissolved and free molecules and between dissolved and encaged molecules. Results presented here indicate that shifts found for free and encaged values of delta C-13(CH4) and delta H-2(CH4) are small and do not complicate interpretation of gas provenance. Conversely, in hydrate systems containing H2S molecules values of delta S-34(H2S) need to be interpreted with caution. Although isotopic fractionation between free-and encaged-sulfur molecules is mild during hydrate formation, values of dissolved delta S-34(H2S) are substantially fractionated and necessitate careful examination of sulfur isotopic values. Because dissolved H2S could potentially be recycled by oxidation and reduction processes during hydrate formation events, use of delta S-34(H2S) values might complicate assessment of biosignatures for other planetary bodies. (C) 2013 Elsevier Ltd. All rights reserved.