Drilling on Hydrate Ridge (Fig. F1) during Ocean Drilling Program (ODP) Leg 204 aimed at understanding the biogeochemical factors controlling the distribution and concentration of gas hydrates in an accretionary margin setting. Several of these objectives address fundamental questions pertaining to the carbon cycle and fluid transport in this accretionary margin in an effort to document the source and flux of hydrocarbons to the gas hydrate stability zone. 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 (Claypool et al., this volume), as well as allowing for reconstruction of the history of the development of this gas hydrate–bearing province based on analyses of carbonate deposits.