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Drilling on northern Cascadia was aimed at understanding the biogeochemical factors controlling the distribution and concentration of gas hydrates in an accretionary margin setting. To constrain the distribution and concentration of gas hydrate in this region, a transect of four sites (Sites U1325, U1326, U1327, and U1329) was drilled during Integrated Ocean Drilling Program (IODP) Expedition 311 (Fig. F1) (see the "Expedition 311 summary" chapter). A fifth site (Site U1328) was drilled south of the transect to target a cold vent with active fluid and gas flow where gas hydrate occurs near the seafloor. The four transect sites encompass the earliest gas hydrate occurrence on the westernmost accreted ridge (Site U1326) and extends to the eastward limit of gas hydrate occurrence (Site U1329). These sites complement previous drilling in central Cascadia during Ocean Drilling Program (ODP) Leg 204 (Fig. F1) (Tréhu et al., 2006). Several of the objectives in Expedition 311 and Leg 204 address fundamental questions pertaining to the carbon cycle and fluid transport in this accretionary margin, as these processes play fundamental roles in unraveling the source and flux of hydrocarbons to the gas hydrate stability zone (GHSZ). The isotopic characterization of the dissolved inorganic carbon (DIC) pool is critical to address these objectives, which include establishing carbon sources and metabolic paths for organic carbon decomposition, as well as allowing for reconstruction of the history of the development of this hydrate-bearing province based on analyses of carbonate deposits.