AbstractWith sea level rise accelerating and sediment inputs to the coast declining worldwide, there is concern that tidal wetlands will drown. To better understand this concern, sources of sediment contributing to marsh elevation gain were computed for Plum Island Sound estuary, MA, USA. We quantified input of sediment from rivers and erosion of marsh edges. Maintaining elevation relative to the recent sea level rise rate of 2.8 mm yr−1 requires input of 32,299 MT yr−1 of sediment. The input from watersheds is only 3,210 MT yr−1. Marsh edge erosion, based on a comparison of 2005 and 2011 LiDAR data, provides 10,032 MT yr−1. This level of erosion is met by <0.1% of total marsh area eroded annually. Mass balance suggests that 19,070 MT yr−1 should be of tidal flat or oceanic origin. The estuarine distribution of 14C and 13C isotopes of suspended particulate organic carbon confirms the resuspension of ancient marsh peat from marsh edge erosion, and the vertical distribution of 14C‐humin material in marsh sediment is indicative of the deposition of ancient organic carbon on the marsh platform. High resuspension rates in the estuarine water column are sufficient to meet marsh accretionary needs. Marsh edge erosion provides an important fraction of the material needed for marsh accretion. Because of limited sediment supply and sea level rise, the marsh platform maintains elevation at the expense of total marsh area.
