The "Missing Xenon Problem" is an outstanding geochemical puzzle that relates to observed concentrations of noble gases in the Earth's atmosphere. After considering addition of radiogenic Ar and loss of He, elemental ratios of noble gases in the atmosphere are similar to those in the solar system (as determined from meteorites) with the notable exception of a 10- to 20-fold deficiency in Xe (e.g., Canalus et al., 1968; Pepin, 1992; Tolstikhin and O'Nions, 1994). Thus, for several decades, it was believed that Xe missing from the atmosphere was stored in some repository of the upper geosphere; that is, a large reservoir exists near the Earth's surface that contains abundant Xe without significant enrichment of other noble gases. However, despite numerous investigations of potential Xe-rich reservoirs (e.g., ice caps, shale, siliceous ooze, etc.), no repository has been identified that can account for Xe missing from the Earth's atmosphere (Podosek et al., 1981; Wacker and Anders, 1984; Bernatowicz et al., 1984, 1985; Matsuda and Matsubara, 1989). An alternative explanation for the "Missing Xenon Problem" is that Xe was lost through a specific sequence of processes early in the evolution of our Earth (e.g., Tolstikhin and O'Nions, 1994).
Clathrate hydrates of gas ("gas hydrates") are crystalline compounds in which cages of water molecules host molecules of gas. Vast quantities of CH4 hydrate are located in pore space of sediment along continental margins and in permafrost regions (Kvenvolden, 1993; Dickens et al., 1997). Although noble gas concentrations had never been determined in samples of CH4 hydrate (experimental or natural) before this study, theoretical considerations suggest that formation of CH4 hydrate should involve significant fractionation of the noble gases such that the solid hydrate lattice will become greatly enriched in Xe relative to other noble gases (Nikitin, 1937; Barrer and Stuart, 1957; Davidson, 1973; Verkhovsky et al., 1988). Previous discussions of the "Missing Xenon Problem" have not considered the amount of Xe and other noble gases potentially stored in CH4 hydrate.
Leg 164 was devoted to understanding the composition and distribution of gas in a large gas hydrate reservoir located on the Blake Ridge off the southeast coast of the United States (Fig. 1; Paull, Matsumoto, Wallace, et al., 1996). Here we present results of a pilot study to examine the noble gas composition of natural CH4 hydrate specimens collected from boreholes drilled during this leg. Initial goals of this research were twofold: (1) to test whether expected noble gas fractionation actually occurs during the formation of natural CH4 hydrate, and (2) to assess whether the deficiency of Xe in the atmosphere can be accounted for by storage of Xe in oceanic CH4 hydrate. Although we address both issues, the main thrust of our discussion is to suggest that noble gases may provide an important means to distinguish between multiple pathways for the genesis of CH4 hydrate in marine sediment.