There are at least three sources of noble gases to our eight gas samples collected from dissociation of gas hydrate specimens recovered from the Blake Ridge. One of these sources is air, which is a significant component of six samples. A second source is possibly air-saturated seawater where noble gases have been fractionated during hydrate formation. The other source(s) is largely unconstrained with available data but suggests that there are multiple pathways for gas hydrate formation on the Blake Ridge. Relative to air (and air-saturated seawater), other sources of noble gases are significantly enriched in Xe. Xenon depletion in residual gas of certain gas fields may be related to sequestering of Xe during hydrate formation. However, if our samples are at all representative of oceanic gas hydrate, there is insufficient Xe in hydrates to account for "missing Xe" in the atmosphere.
Major problems with our pilot study were twofold: (1) there was significant introduction of noble gases from air to six of our samples during shipping and storage, and (2) we could not collect samples of residual gas (i.e., free gas or dissolved gas associated with gas hydrate; see Dickens et al., 1997) appropriate for noble gas analyses.
With hindsight, these problems could have been minimized. Samples released during gas dissociation experiments could be collected directly into pre-evacuated Cu tubes with only minor changes to the dissociation chamber. Samples of residual gas also could be collected from the pressure core sampler (PCS; Dickens et al., 1997). Unfortunately, PCS operations on Leg 164 precluded collecting in situ gas at greater than 1 atm, because gas was collected in a simple bubble chamber (Dickens et al., 1997). The bubble chamber should be modified so that gas volumes can be compressed to increase pressure and samples can be collected directly into pre-evacuated Cu tubes.
After dissociation of solid CH4 hydrate with an initial composition of CH4 · 6H2O (Handa, 1986), the volumetric ratio of gas to water should be ~225 at STP. All gas dissociation experiments gave gas to water ratios significantly less than 225 (Table 2). This suggests that partial release of gas from the clathrate structure occurred prior to gas dissociation experiments. In addition to gas sampling, therefore, future investigations may wish to consider how storage of hydrate in a container with liquid nitrogen and rapid pressure drops during core recovery affect noble gas compositions of CH4 hydrate.