NOBLE GAS FRACTIONATION IN SYNTHETIC METHANE HYDRATES

The release of methane from gas hydrates that undergo dissociation in response to global warming could act as critical, positive feedback mechanism that further exacerbates warming. On Earth, methane emissions are widespread and are most often sourced in wetlands, soils, coal, or leaky hydrocarbon reservoirs, not gas hydrates. To determine whether methane emissions can be linked to dissociating gas hydrate requires a way to distinguish this methane from other methane sources (e.g., wetlands, soils, coals, and leaky, hydrocarbon reservoirs). Traditional stable isotope measurements can only distinguish microbial from thermogenic methane. Here, we investigate the partitioning of noble gas in the methane hydrate as a potential fingerprinting method to distinguish gas released from methane hydrates from other methane populations. The two previous studies (Dickens and Kennedy, 2000, Winckler et al., 2002) of noble gas content in naturally occurring gas hydrate reported contradictory results that show vastly different mass fractionation of the noble gases, something we attribute in part to the different sampling techniques (liquid nitrogen storage and later dissociation for the former; immediate analysis for the latter). The first study showed a close air/solubility mixture of gases, while Winckler et al.’s data showed a heavily fractionated gas composition that favored the retention of the heavy noble gases with a loss of the lights. Winckler et al. initially concluded that the Dickens and Kennedy sample was contaminated with air and hypothesized that gas hydrate preferentially traps Kr and Xe in hydrate structure. To test Winckler’s hypothesis we synthesized gas hydrate under laboratory conditions in the presence of known methane and noble gas concentrations. The relative noble gas fractionation patterns observed in our experiments on synthetic hydrate step-dissociation reveals complex behavior of the noble gases, depending on the level of annealing and the sampling and preservation procedures used. The data support a unique noble gas fractionation pattern for the gas hydrates, one that we believe shows promise in evaluating modern natural gas seeps for a signature associated with gas hydrate dissociation.