The Fate and Stability of Eroding Wetland Soil Carbon in a Subsiding Deltaic Coastal Plain

Coastal wetlands can respond to rapid rates of relative sea level rise via wetland submergence and/or erosion, which occur when wetlands are unable to vertically accrete to keep pace with sea level rise. As coastal wetlands erode, previously sequestered organic carbon is exposed to oxygen-rich estuarine water. This transition in redox from anaerobic to aerobic condition can trigger increased mineralization rates of decades to centuries'-old soil carbon. Barataria Bay, Louisiana has one of the highest coastal wetland land loss rates in the United States, primarily due to eustatic sea level rise coupled with coastal subsidence. Marsh-edge erosion rates measured over the past two years are on the order of 1.5 meters per year. Meter long soil cores were obtained from vegetated wetland sites and sectioned into 11 intervals to investigate aerobic and anaerobic mineralization rates with depth. In surface soils, organic carbon mineralization rates averaged 16 times greater than anaerobic mineralization rates. In deeper, older soils, the aerobic mineralization rate was still an order of magnitude greater than the anaerobic rate, suggesting a significant portion of this older, soil carbon is readily cycling back to the atmosphere after erosion followed by mineralization by microorganisms. These results have consequences for increased atmospheric CO2 concentrations in the future, as stable coastlines worldwide will be subjected to Barataria-bay levels of sea level rise in the next 50-75 years.