Carbon Burial in Salt Marshes: Impacts of Sea Level Rise and Marsh Restoration

Approximately 50% of U.S. wetlands have been lost since 1900, with destruction of coastal wetlands attributed to infilling and building, nutrient loading, disruption of sediment supply and, recently, sea level rise. Salt marshes globally are a significant carbon sink, yet their fate and continued carbon sequestration capacity is uncertain. On Cape Cod, MA, USA, many salt marshes converted into fresh wetlands when tidal flow was restricted by construction of roads and berms. As a result salt marsh restoration efforts largely focus on returning tidal flow, and success has been evaluated upon restoring ecosystem function and services, including fish and bird habitat, shoreline protection and nutrient filtering. However, understanding salt marsh vulnerability to sea level rise and return of carbon sequestration capacity are also key factors in salt marsh restoration success. In this study we evaluate carbon storage and sediment accretion rates across a restoration chronosequence (0 to 15 years post restoration), as well as at adjacent natural salt marshes. We constructed high temporal resolution records of salt marsh accretion using the constant rate of ²¹⁰Pb supply model. The power of this model is that accretion is not assumed to be constant, rather variability in sediment concentrations of ²¹⁰Pb are due to both decay and dynamic accretion rates. At natural (never restricted) sites, we observed an increase in accretion, with modern rates up to four times greater than those in the early 1900’s, suggesting that marsh growth has accelerated to catch up with sea level rise rates. Carbon burial over the same period likewise increased, driven primarily by increased accretion rates, not changes in soil carbon content. Modern accretion rates and carbon burial in restored salt marshes are similar to the natural marshes, however, there is evidence for loss of buried carbon in restored marshes, which occurred prior to the return of tidal exchange.