Net lateral carbon export from eroding United States Atlantic and Gulf Coast marshes

Coastal marshes store large amounts of organic carbon in their soils, but erosion can mobilize this carbon to the coastal ocean, creating uncertainty in carbon budgets and climate mitigation efforts. Here we show that erosion of coastal marshes along the United States Atlantic and Gulf Coasts represents a substantial lateral transfer of organic carbon (0.66 Tg C yr−1; 68% confidence interval, 0.46–0.91 Tg C yr−1). Using long-term satellite observations of wetland change and elevation-based estimates of eroded soil depth, we quantified carbon mobilized by abrupt and gradual marsh erosion from 1985 to 2022 and compared it with carbon burial (0.22 Tg C yr−1; 68% confidence interval is 0.09–0.59 Tg C yr−1) in newly formed marshes. We found that erosion exports more carbon than is buried in newly formed marshes (net export = 0.38 Tg C yr−1), with strong spatial and temporal variability. The erosion flux is equivalent to 5.7% of riverine organic carbon inputs and 24.2% of marsh outwelling, with regional values ranging from 3–14% and 6–29%, respectively. These results demonstrate that marsh erosion is an important pathway in coastal carbon cycling and should be incorporated into coastal carbon accounting and management frameworks. Coastal marshes along the United States Atlantic and Gulf Coasts mobilize substantial quantities of buried coastal carbon, representing a lateral flux to the coastal ocean as high as 0.66 Tg carbon per year, based on an erosion and burial carbon flux estimates from 1985−2022.