The contemporary coastal ocean, characterized by abundant nutrients and high primary productivity, is generally seen as a significant CO2 sink at the global scale. However, mechanistic understanding of the coastal ocean carbon cycle remains limited, leading to the unanswered question of why some coastal systems are sources while others are sinks of atmospheric CO2. Here we proposed a distinct physical-biogeochemical setting, Ocean-dominated Margin (OceMar), in order for better shaping the concept of the coastal ocean carbon study. OceMars, in contrast to previously recognized River-dominated Ocean Margins, are characterized by dynamic interactions with the open ocean, which may provide nonlocal CO2 sources thereby modulating the CO2 fluxes in OceMars. Using the basin areas of the largest marginal seas of the Pacific and the Atlantic, the South China Sea and the Caribbean Sea as examples of OceMars, we demonstrated that such external CO2 sources controlled the CO2 fluxes.