Abstract Stable isotope ratios ( δ 13 C) of total organic carbon were measured in surface sediments from the continental margins of the northern and western Gulf of Mexico, the north coast of Alaska and the Niger Delta. Gulf of Mexico outer-shelf isotope ratios were in the same range as has been reported for Atlantic coastal shelf sediments, −21.5 to −20‰. Off large rivers including the Mississippi, Niger and Atchafalaya (Louisiana), δ 13C values increased from terrigenous-influenced (around −24‰) to typically marine ( ∼−20‰) within a few tens of kilometers from shore. This change was accompanied by a decrease in the amount of woody terrigenous plant remains in the sediment. Alaskan continental margin samples from the cold Beaufort Sea had isotopically more negative carbon (−25.5 to −22.6‰) than did warmer-water sediments. The data indicate that the bulk of organic carbon in Recent sediments from nearshore to outer continental shelves is marine derived.