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

Abstract

The stable isotopic composition of carbon (δ13CCH4) and hydrogen (δ2HCH4) in methane and sulfur (δ34SH2S) 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, δ13CCH4, δ2HCH4, and δ34SH2S 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 (MgSO4·7H2O). Methane experiments show less than 1‰ differences in values of δ13CCH4 between free and encaged molecules and up to 6.5‰ variations in values of δ2HCH4 between free and encaged molecules. In hydrogen-sulfide hydrates, δ34SH2S values show less than 4‰ differences between free and encaged molecules, but up to 14‰ 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 δ13CCH4 and δ2HCH4 are small and do not complicate interpretation of gas provenance. Conversely, in hydrate systems containing H2S molecules values of δ34SH2S need to be interpreted with caution. Although isotopic fractionation between free- and encaged-sulfur molecules is mild during hydrate formation, values of dissolved δ34SH2S 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 δ34SH2S values might complicate assessment of biosignatures for other planetary bodies.