IMPACTS OF GEOMORPHIC CHANGE ON THE CARBON BUDGETS OF ESTUARINE FRINGING MARSHES AND TRANSGRESSIVE BARRIER ISLANDS: INSIGHTS FROM A SIMPLE BOX MODELLING APPROACH

Saltmarshes are considered carbon sinks given that carbon storage increases as the marsh grows vertically to keep pace with rising sea level. However, marshes are situated in dynamic environments, such as fringing the edges of estuaries and barrier islands, and are disappearing globally. Marsh carbon assessments currently focus on burial and storage and exclude export from shoreline erosion. Additionally, barrier island transgression, which can either increase or reduce marsh area, has not been placed into context with the coastal carbon budget. Here, we present box models of fringing marsh and transgressive barrier island carbon budgets that include erosion and transgression. The fringing marsh model has a carbon storage term that scales with marsh area and a carbon export term that is a function of the shoreline retreat rate. Carbon export occurs as the marsh shoreline erodes and carbon storage capacity is reduced if the marsh cannot migrate landward. The barrier island model builds on the marsh model with an additional term for shoreface export from ravinement of old marsh. Both models compare storage and export in order to: (1) assess if a site is a carbon sink or source and (2) calculate the timing and width of the transition from sink to source. Carbon budgets were assessed at sites in North Carolina that include a fringing marsh with upland development (Carrot Island), an eroding barrier island that cannot transgress (Onslow Beach), and an undeveloped transgressive barrier island fronting a wide lagoon (Core Banks). At Carrot Island, the less erosive section of the marsh will remain a sink until 2021, while the more erosive stretch became a source in 1930. Along Onslow Beach, budgets were examined at two sites with different morphologies and erosion rates. Both are currently carbon sources to the coastal ocean; however, one had been a sink until a washover fan deposited during Hurricanes Fran and Bonnie buried a large portion of the backbarrier marsh. The site at Core Banks is currently a carbon sink and will remain a sink for centuries. Expansive backbarrier marsh, relatively low erosion and transgression rates, and a wide lagoon allow for sustained carbon storage. These results indicate that geomorphic change must be considered in coastal carbon budgets as well as in the design of marsh restoration and conservation projects.