The isotopic characterization of carbon in the dissolved inorganic carbon (DIC) pool is fundamental for a wide array of scientific studies directly related to gas hydrate research. Here we present the DIC data from pore fluid samples recovered from the northern Cascadia accretionary margin during Integrated Ocean Drilling Program (IODP) Expedition 311. Comparison of these results with data obtained from offshore central Cascadia during Ocean Drilling Program Leg 204 provides clues on carbon cycling processes that control methane inventories and fluxes. Microbial methane production preferentially incorporates the light carbon isotope. As sediment ages, more of the original CO (sub 2) is converted to methane, leaving behind an isotopically heavier residual DIC. This DIC is progressively enriched in (super 13) C below 200 meters below seafloor with increasing distance from the deformation front. In the shallower sections, minima in downcore (super 13) C (sub DIC) profiles coincide with the sulfate-methane transition (SMT) zone. Here the (super 13) C (sub DIC) values provide information on the metabolic pathways that consume sulfate and reveal that anaerobic oxidation of methane (AOM) is not the dominant reaction at all sites drilled. There appears to be no simple correlation between the extent of AOM, depth of the SMT, and thickness of the gas hydrate occurrence zone along the transect drilled in northern Cascadia.