Eroding US Atlantic and Gulf marshes export more carbon than new marshes bury

Why it matters

Salt marshes are seen as lasting carbon sinks, but this work suggests that edge loss turns part of that stored carbon into a flow to the sea. The authors argue this pathway belongs in coastal carbon budgets, greenhouse gas inventories and management plans. The burial side is too uncertain to pin down the net result.

What they did

The team used yearly satellite maps of wetland change from 1985 to 2022 to find where marsh turned to water, either abruptly or gradually. They estimated how deep the soil eroded by comparing elevations of stable marsh and stable water at five representative sites. They multiplied eroded volume by soil carbon content and a fraction of carbon that enters the ocean. They also estimated burial in newly formed marsh, ran 10,000 Monte Carlo simulations, and compared the results with river and marsh outwelling carbon fluxes.

Key findings

• Erosion mobilizes about 0.66 Tg C per year (68% confidence interval 0.46 to 0.91), while burial in new marshes is about 0.22 Tg C per year (0.09 to 0.59).
• Net export is 0.38 Tg C per year, but its 68% confidence interval runs from -0.012 to 0.753, so the sign is not certain.
• The erosion flux equals 5.7% of riverine organic carbon input and 24.2% of marsh outwelling, with regional ranges of 3.3 to 13.8% and 6.0 to 28.6%.
• The Gulf Coast accounts for 67% of the erosion flux, and jumps in the time series follow Hurricanes Katrina and Ida.
• Burial rate, soil carbon and eroded soil depth add the most uncertainty.

Limitations

• Burial estimates are highly uncertain, and they ignore mudflat instability and long-term decomposition. This limits conclusions about net flux.
• Map accuracy was checked only for 2006 to 2010 against imperfect reference data and then assumed constant, so area uncertainty may be underestimated.
• Erosion depth comes from five sites scaled to whole regions, and LiDAR elevation bias was not corrected. A rough correction lowered the flux by 15%. Riverbank erosion and double counting with river fluxes were not fully treated.

Glossary

• Outwelling: Export of dissolved and particle carbon from stable marshes to the coastal ocean through porewater exchange.
• Mobilization fraction: Share of eroded soil carbon that enters the coastal ocean instead of being reburied nearby.
• Blue carbon: Carbon stored in coastal ecosystems such as salt marshes.
• Monte Carlo simulation: Repeated calculation with randomly drawn inputs to show how input uncertainty spreads into the result.