Concentrations of organic carbon (excluding belowground plant material) decreased logarithmically with depth in the sediment of a Spartina alterniflora marsh near Drum Inlet, N.C. A similar decrease was measured in nearby areas of unvegetated dredge spoil and in dredge spoil planted with Spartina. Both spoil areas accumulated organic carhon, but the annual rate was higher where Spartina was present (range, 80.3–96.8 g C m−2). The organic carbon in the top 13 cm of the natural marsh sediment has a turnover time, based on this rate of accumulation, of 3.7 to 4.5 years.