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Gas hydrates are crystalline inclusion compounds made of water molecules that form cages. These cages trap low-molecular weight molecules, typically at high pressure and low temperature conditions. Gas hydrates are known to proliferate in the sediments of the continental slopes and the permafrost. The majority of these natural gas hydrates are composed of methane in the structure I (sI) lattice. The sI lattice unit cell consists of 46 waters in two cage types: two pentagonal dodecahedron (512) and six tetrakaidecahedron (51262) cages. Molecules such as methane and hydrogen sulfide form this structure (Sloan and Koh, 2007).

Interest in natural gas hydrates has largely been driven by the vast quantities of methane they are estimated to contain. Current estimates of methane trapped in gas hydrate are 2.5 × 1015 to 440 × 1015 m3 at standard temperature and pressure (Milkov, 2004; Klauda and Sandler, 2005). In addition, the possible role of gas hydrates in climate change and as a geohazard has been the focus of numerous studies (Dickens, 2001; Kennett et al., 2003; Milkov et al., 2003). Structural and compositional characterization of natural gas hydrates is important to determine gas hydrate properties including pressure-temperature stability and thermal conductivity, both from energy recovery and geohazard aspects.

Integrated Ocean Drilling Program (IODP) Expedition 311 was performed in the northern Cascadia margin (see the “Expedition 311 summary” chapter). The goal of this program was to constrain geological models for the formation of gas hydrates. This includes both field measurements and recovery of samples for laboratory characterization. The contribution of this manuscript is to provide structural and compositional characterization with nuclear magnetic resonance (NMR) and Raman spectroscopy of two recovered gas hydrate samples. Characterization of gas hydrates from this area adds to the understanding of how these systems formed and the current state of the gas hydrates in the northern Cascadia margin.