Carbon export from fringing saltmarsh shoreline erosion overwhelms carbon storage across a critical width threshold

Saltmarshes are carbon storage hotspots and help to offset anthropogenic carbon emissions; however, marshes are threatened by sea-level rise, erosion, and human development. Recent efforts to constrain the saltmarsh carbon cycle have focused on the processes of carbon burial and sequestration with respect to sea-level rise and global warming. Simultaneously, many marshes that fringe the margins of estuaries and barrier islands are eroding, which releases old carbon from the saltmarsh and transports it into the estuary, and that process should be included in marsh carbon budgets. Additionally, if marshes cannot transgress the upland at a rate that balances shoreline retreat, then the marsh will narrow, thus reducing the area available for carbon storage. Here, we present the development of a box model that incorporates both carbon storage and carbon export via shoreline erosion to estimate the annual carbon budget of saltmarsh sediments. We test the model using field data collected at a fringing marsh within the Rachel Carson National Estuarine Research Reserve in North Carolina. The shoreline erosion rates along the fringing marsh are variable and the model output shows that the stretch of marsh that is retreating 0.76 m yr^-1 switched to a carbon source in 1930, while another portion of the marsh that is retreating more slowly (0.65 m yr^-1) will switch to a source in 2021. The model indicates that the carbon budget of a saltmarsh is highly sensitive to the rate of shoreline retreat and that rapidly-eroding marshes may already be net sources of carbon. These results underscore the importance of conserving existing marshes, mitigating shoreline erosion, and considering shoreline erosion in the design of saltmarsh restoration projects.