Blue carbon to the waveside: carbon erosion along migrating barrier islands


Personal history
During regular visits to the Virginia Barrier Islands over the past decade, co-author Christopher Hein (Associate Professor at the Virginia Institute of Marine Science, William & Mary) encountered turbid waters lapping onto the beach and light balls of dried marsh peat skittering across the sand. What was going on? The dynamism of these undeveloped barrier islands and their natural response to sea-level rise hold the answer. Due to sea-level rise, barrier islands migrate upslope along the continental shelf, maintaining their subaerial position with the help of storms and overwash processes. What Chris was noticing was the result of decades of migration: the eventual exposure of formerly backbarrier deposits along the beach and shoreface. Salt marshes, once protected by the oceanfront barrier islands, were buried by the migrating landforms and ultimately exhumed along the seaward shoreline, where they are subjected to erosive wind-wave forces. The observable result: sediment-laden waters on windy days, bouncing peat balls, and a muddy bank of marsh soils and lagoon sediments extending into the waves. But the result we were most interested in? The quantity of carbon being eroded along the entire eroding/migrating barrier chain. We ultimately wanted to discover what implications this massive migration had on the carbon storage capacity of the system as a whole.

We took nearly a dozen cores (some up to 9 meters deep) along 7 of the central 10 islands. We analyzed the cores for carbon density and stratigraphy, and combined those geochemical and geologic results with geospatial data to calculate a timeseries of annualized carbon erosion rates for each island and for the island chain as a whole. We compared these data to previously published carbon accumulation rates for the entire backbarrier of these islands[1-3] and what we found was quite surprising. Detailed below are our major findings, presented within the context of the current state of the field.
Climate change mitigation
Annual carbon burial in coastal environments like salt marshes occurs rapidly compared to terrestrial systems [4]. These coastal carbon “hotspots” can mitigate climate change by drawing down atmospheric carbon during photosynthesis and sequestering that carbon in soils for many millennia.
In fact, carbon markets around the United States center around conservation and remediation of wetlands based on their powerful carbon uptake and storage systems. To accurately capture the carbon values of these systems (and to understand the extent to which these ecosystems can balance continuing carbon emissions), we need to account for and carefully measure all carbon gain and loss terms.
Recent work has drawn attention to the difference between short-term accumulation and long-term burial, highlighting uncertainty around the preservation potential of carbon stored in coastal environments[5] . However, three general oversights continue to weaken these efforts. Here, I’ll present the work described in “Shoreface carbon erosion counters blue carbon accumulation in transgressive barrier-island systems” side-by-side with the oversights that our work addresses.
Landscape-Scale Carbon Budgets
Firstly, carbon budgets that track gains and losses typically consider losses on an ecosystem-level, with less attention to carbon stored across multiple adjacent systems, especially those with large lateral carbon fluxes[6]. Our work adds to recent progress in quantifying lateral fluxes across adjacent ecosystems, but targets a site of rapid and large carbon loss that has been neglected by the majority of coastal carbon researchers: the shoreface of transgressing (landward moving) open-ocean coastlines. The Virginia Barrier Islands, an undeveloped 112-km long barrier chain along the U.S. Atlantic Coast, are migrating landward due to sea-level rise and storm impacts at an average annual rate of 4.3 meters per year (1851-2017)[7] . We find that the combined annual erosion of salt marsh peat and lagoon sediments along the beach and shoreface of 10 central Virginia Barrier Islands drives a loss of carbon on the order of 26 Gigagrams per year. Without considering this lateral flux of eroded carbon along the transgressing shoreline, the Virginia Barrier Island as a system would be considered a robust net sink for carbon, with its hundreds of square kilometers of salt marsh and recently restored seagrass beds. As it stands however, and detailed more below, this system may no longer act as a net carbon sink.

The importance of unvegetated sediments in coastal carbon storage
Relatedly, vegetated coastal ecosystems like salt marshes and seagrass beds have garnered significantly more research focus than those considered “bare”[6]. However, recent attention on the in situ primary production occurring in lagoons and mudflats, by photo-synthesizers like algal mats, microphytobenthos and phytoplankton communities, has revealed that these “bare” ecosystems play a much larger role in coastal carbon storage than previously thought[8]. Our work not only underscores that the exclusion of unvegetated units within the coastal landscape leads to large oversights in carbon accounting, but it reveals a surprising comparison between the carbon storage capacity of vegetated and unvegetated ecosystems. We find that while lagoons have a carbon density approximately one third of that of marsh peat, the lagoons are more pervasive both laterally and to-depth (approximately 6.6 times thicker than salt marsh peat) across the Virginia Barrier Island system and thus store much more total carbon. Consequently, the total carbon erosion rate is dominated by carbon lost solely from lagoon sediments, with only approximately 20% derived from the salt-marsh peat.

By comparing our most modern-day average annual carbon erosion rate (1994-2017) with the average annual carbon accumulation rate in all the salt marsh, seagrass beds and lagoons behind these islands (1984-2020)[1-3], we discovered a imbalanced flux of carbon. This imbalance tilts toward a net loss of carbon—approximately 30% more that that of the annual accumulation rate—suggesting that the Virginia Barrier Islands could recently have flipped from a net sink to a net source, depending on the as-yet unknown fate of the carbon once eroded. Nevertheless, this comparison reveals that the carbon burial power of the Virginia Barrier system is much lower than previously thought.

Erosion to the Depth of Closure Along Open-Ocean Coasts
Thirdly, these works often focus on losses of carbon in shallow soil depths due to wave-drive erosion and biological processes like microbial respiration[9]. Ignoring loss terms that occur to depths below one meter again leads to potentially large overestimates of the magnitude of the coastal carbon sink. We measured lagoon units extending down to 9.7 meters with all sediment and carbon fully erosional during island transgression.
Final implications
All-in-all, our work challenges several paradigms within the coastal carbon field, namely the adherence to single-ecosystem, vegetated-ecosystem, and shallow-ecosystem studies that overstate the power of the coastal carbon sink along transgressing coastlines. This work ultimately sheds light on the ephemeral nature of blue carbon stored near or along open-ocean shores and underscores the need for comprehensive, landscape-scale carbon budgets.
Sources
[1]Smith, A. J. et al. Compensatory mechanisms absorb regional carbon losses within a rapidly shifting coastal mosaic. Ecosystems https://doi.org/10.1007/s10021-023-00877-7 (2023).
[2]Hutchings, J. A. et al. Carbon deposition and burial in estuarine sediments of the contiguous United States. Global Biogeochem. Cy. 34, (2020).
[3]Nichols, M. M. Sediment accumulation rates and relative sea-level rise in lagoons. Mar. Geol. 88, 201–219 (1989).
[4] Mcleod, E. et al. A blueprint for blue carbon: toward an improved understanding of the role of vegetated coastal habitats in sequestering CO2. Front Ecol. Environ. 9, 552–560 (2011).
[5] Van de Broek, M. et al. Long-term organic carbon sequestration in tidal marsh sediments is dominated by old-aged allochthonous inputs in a macrotidal estuary. Global Change Biol 24, 2498–2512 (2018).
[6]Baustian, M. M., Stagg, C. L., Perry, C. L., Moss, L. C. & Carruthers, T. J. B. Long-term carbon sinks in marsh soils of coastal Louisiana are at risk to wetland loss. J. Geophys. Res. Biogeosci. 126 (2021).
[7] Mariotti, G. & Hein, C. J. Lag in response of coastal barrier-island retreat to sea-level rise. Nat. Geosci. 15, 633–638 (2022).
[8]Lin, W., Wu, J. & Lin, H. Contribution of unvegetated tidal flats to coastal carbon flux. Glob. Chang. Biol. 26, 3443–3454 (2020)
[9]IPCC 2014, 2013 Supplement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories: Wetlands, Hiraishi, T., et al. (eds). (IPCC, 2014).
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