Terrestrial wetlands are a highly significant carbon reservoir in North America. Forested wetlands, or swamps, are an important category of North American wetland and include boreal forested peatlands, swamps dominated by needle-leaved trees including Thuja (cedar), Picea (Spruce), Larix (Tamarack) or Taxodium (bald cypress), swamps dominated by broad-leaved trees or shrubs including Fraxinus (Ash), Ulmus (Elm), or Acer (Maple), as well as mangroves. The Second State of the Carbon Cycle Report estimates that forested wetlands may make up ~55% of the total terrestrial wetland area for North America, although estimates vary considerably due to different mapping conventions and classification systems across national and provincial borders, and also due to the ongoing impacts of land use change. Additionally, that report suggests that forested wetlands contain larger total carbon pools than non-forested wetlands, and that forested wetlands effect 53% of the estimated 123 Tg total wetland annual carbon sink for North America. Uncertainties in the sizes of the forested wetland soil carbon pools continue to be significant due in part to insufficient data on variabilities in carbon densities across diverse swamp types. Further, there are limited data on the rates of vertical accretion of swamp soils and the associated long-term rates of carbon accumulation, needed for better predicting impacts of climate warming on carbon sequestration in swamp soils. We present here a comparative synthesis of swamp soil carbon properties including bulk densities, organic carbon contents, soil thicknesses, rates of vertical accretion and rates of long-term carbon accumulation, from >200 swamp sites. We compare these properties for broad-leaf swamps (including mangroves), needle-leaf swamps, mixed swamps, and shrub-dominated swamps, and also compare across North American Ecoregions. The results show significant variability across peat-forming and mineral swamps, and indicate rates of carbon accumulation in some swamp types similar to those of northern bogs and fens.