Coastal ecosystems are highly regarded for their potential to combat climate change through storage of organic carbon (carbon bound within organic matter like plant roots), which consequently reduces the carbon available to transform into planet-warming carbon dioxide.

Researchers from William & Mary’s Virginia Institute of Marine Science have, for the first time, assessed a more complete picture, quantifying the seaside erosion of backbarrier lagoon and peat deposits along Virginia’s Atlantic Coast. Published recently in Nature Communications, the new study counters the traditional understanding about vegetated ecosystems dominating the coastal storage of carbon.

In addition, the study determines that the erosion and landward migration of the barrier islands along the Virginia Eastern Shore not only leads to an extremely rapid rate of carbon erosion — releasing 26.1 Gigagrams of organic carbon annually — but also entails potential trouble for the continued ability of the entire system (with all its salt marshes, seagrass beds and lagoons) to function as a net sink for carbon.

An informational graphic shows what 21.6 Gg of organic carbon equals

“We’re finding that along certain coastlines, carbon is not buried as long as we expected and thus, relying on coastal wetlands to store carbon is not a permanent solution to mitigating carbon emissions,” said lead author of the study Mary Bryan Barksdale, a School of Marine Science Ph.D. student. “Coastal carbon budgets that ignore carbon storage and erosion in deeper and unvegetated sediment overestimate the strength of the coastal carbon sink.”

The role of a neglected ecosystem in coastal carbon budgets

The existing body of research focuses primarily on carbon storage in salt marshes, with an emphasis on the role of marsh plants.

However, this study finds that the sand, silt, and clay found at the bottom of unvegetated lagoons are less carbon-dense than marshes but play an outsized role in carbon storage — as indicated by their contribution of more than 80% of the carbon eroded along the beach and shoreface of migrating barrier islands — because of their thickness and ubiquity across the landscape.

These new findings counter many common assumptions about carbon burial in coastal habitats, with implications for the assessment of blue carbon for research and carbon market purposes.

Erosion exceeds accumulation

Leveraging existing data on carbon accumulation rates of the backbarrier landscape (area that is landward of the barrier islands), these researchers were able to compare average annual carbon accumulation (or storage) rates to average annual carbon erosion rates through time.

What they found was quite alarming: in recent decades, carbon erosion along the seaside of these islands has begun outpacing carbon accumulation in adjacent barrier and backbarrier ecosystems by about 30%.

This finding suggests that the chain of islands and backbarrier mashes and lagoons along Virginia’s Eastern Shore have recently flipped from a carbon sink to a carbon source — now emitting more carbon than they are capturing per year. Though, by how much depends on the (as yet unknown) ultimate fate of the carbon eroded along the beach and shoreface. Nevertheless, this comparison reveals that the carbon burial power of the Virginia Barrier system is much less powerful than previously thought.

Barrier islands: a dynamic landform

The study’s findings are specific to open ocean coastlines where transgression — the rapid retreat of the shoreline due to sea level rise and storms — is occurring.

Study co-author Chis Hein, an expert on barrier systems, notes that “Coastal barriers are among the — if not the — most dynamic landforms on Earth. In their natural state, they are constantly undergoing reworking by waves, wind and currents; they shape and reform in response to storm impacts; and, at any time, there are barriers undergoing long-term phases of progradation, elongation, accretion, erosion, breaching or migration.”

Recognizing this constant change, carbon budgets that cross traditional ecosystem boundaries (such as marshes, lagoons and the islands themselves) — and assess changes in carbon stocks on all sides of these islands — are crucial for establishing the degree to which coastal landscapes can mitigate climate change through carbon sequestration.

To quantify changes that have taken place over the past 150 years, the research team integrated several methods including sediment coring and geochemical and geospatial analysis, painting a comprehensive picture of the movement of carbon across this dynamic landscape.

The team took new, deep sediment cores (up to 62 feet long) from the beachface of seven of the 10 Virginia barrier islands that are currently migrating or eroding. They specifically cored through backbarrier marsh and lagoon muds which formed in the island backbarriers, but had been overrun by the migrating islands and left exposed to waves along the beach and shoreface.

They geochemically analyzed these new cores for carbon content and conducted stratigraphic (or geologic layering) analysis to determine the thicknesses of marsh and lagoon units.

Finally, a combination of island-specific carbon densities and geospatial analysis yielded a timeseries of annualized carbon erosion rates for each island as well as rates for the entire island chain.

Implications of the impermanence of coastal carbon storage

The study highlights a critical issue: the impermanence of carbon stored in coastal environments. While these ecosystems overall are efficient at sequestering carbon, their vulnerability to erosion and sea-level rise means that their role as long-term carbon sinks may be overestimated.

This finding challenges the current understanding of coastal carbon budgets, which often focus on vegetated ecosystems and overlook non-vegetated lagoon sediments such as those that are now being eroded along Virginia’s barrier beaches.

“The finding that deep carbon is simply eroding away is a serious challenge to our hopes and expectations that wetlands can sequester enough carbon to help offset emissions,” said study co-author Associate Professor Matt Kirwan. “On the other hand, if so much carbon is stored in sandy sediments, maybe this carbon isn’t lost forever.”

The team’s findings underscore the need for integrated coastal management strategies that consider the dynamic nature of these environments. Blue-carbon markets in any rapidly changing coastal landscape, like these transgressive barrier systems, must consider the full picture by factoring in all sites of carbon accumulation and erosion, including unvegetated and deep sediments, as well as their rates of change. Only by following this approach can the extent be determined to which conservation of these carbon-rich systems poses a long-term solution to climate change.