These data include vertical profiles of concentrations and stable carbon isotope values of dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), total organic carbon (TOC) from sediment cores collected at methane seeps and reference sites. Sediment porosity and dissolved sulfate and methane concentrations are also presented. Methane seeps in Astoria Canyon, Barkley Canyon, Hydrate Ridge, and Bullseye Vent, along the Cascadia Margin, were sampled. These data were collected over two decades and ...